Thursday, November 1, 2012

The Wonderful World of Color

During the final days of Engineering Verification, we took a three-color (gri) dither sequence - that is, a set of 9 offset exposures in each of these three bands to produce a color image.  The object was the Bubble Nebula (NGC 7635), and I posted a single band image in a previous blog entry.  Now that we are working on the pipeline, it is our intention to produce a nice color image to advertise the capability of pODI.

Well, Ralf Kotulla, our commissioning working group observer from U. Wisconsin, beat us to it.  He took the 27 images, used swarp to align and combine them, and produced this nice color image.

Since we used g, r, and i, and since H-alpha, which dominates the nebular emission, is in the r band, if we had assigned g, r, and i to the colors blue, green, and red, the nebular would have been bright green.  Ralf decided to mess with our minds, and so he assigned g, i, and r, to blue, green, and red, respectively.  Thus, the nebula is reddish, but the stars tend to be green.

We are working on an "official" version of this, and we'll post it when it's ready, but this gives you and idea of what nice images pODI will produce.  Thanks, Ralf.


Wednesday, October 31, 2012

A comet

Last night ODI took several images of the comet 168P/Hergenrother. The image shown below is a single exposure in the r' band and covers an area of about 1x1 arcminutes (one OTA cell). 168P is a special case of a comet, as a few days ago a fragment broke off the main body, as can be clearly seen as the fainter spot below the comet. 168P is continues to break apart, and is actively being monitored by various research groups. See, e.g., this JPL press release.

To the right of 168P a streak of a star is visible. Since the comet moves at a different apparent velocity on the sky than the stars, we had to modify the telescope guider to actually follow the comet instead of the stars. Thus the star in the image is trailing. This different apparent movement on sky as compared to the distant background stars is common to all solar system objects (albeit to varying extent), and is a combination of the proper motion of earth itself and the orbit the solar system object.  

We achieved following the comet (called non-sidereal tracking) by modifying the ODI guide module: This module uses the video signal of a bright star to send corrections to the telscope tracking system should a star wander of its ideal position.  For the non-sidereal guiding we added a drift rate to the "ideal" star position, thus constantly pushing the telescope tracking system to follow the comet's motion.

Last night's observations of comet 168P nicely demonstrate that ODI is now fully capable of supporting solar system observations with its non-sidereal guide mode. 


Note added Nov 1st: In this image, west is left, north is up.

Comet 168P, Evening of October 30th MST

Thursday, October 25, 2012

This is gonna be technical...

... but I promise there will be a pretty picture at the end. And a pony.

We continue to test the Mosaic filter adaptors to use narrow band  (and other) filters, where in the center we place the filter of interest, and above the outside detectors there are cut outs to allow guiding the telescope. The cutouts are covered with clear glass to ensure they are parfocal with the science target area.

The dilemma is now that the clear glass lets so much more light pass than a narrow band filter, and scattered light can make its way to the central detector array. The image below shows a flat field simultaneously using the small H-alpha and the large ODI z'-band filter. Why two filters? By combining a z' and an H alpha filter, the central area under the narrow band filter should not receive any direct light. The outer detectors with clear glass in the filter module should see the equivalent of a flat field taken in the z' band.

Flat field simultaneously using a large ODI z' band filter and a small Mosaic H alpha filter.
One can clearly see the three well illuminated outer devices (a level of about 40000 electrons).  Zooming into the central 3x3 area below, we can see that some scattered light is making its way there, while under ideal circumstances we should only read noise. The level in the central array is about 0.5% to 1% of the level in the outer detectors. This scattered light will ultimately constrain the flat field quality for narrow band imaging with ODI, and we might need to opt for neutral density filters, or roughly color-matched glass inserts to cover the outer detectors instead of using simple clear glass. We expected to see such stray light, but for a first feasibility test, clear glass was cheap enough and works well for now.

Zoom into the central area of the flat field.
The next image shows a false color image of a genuine, H alpha-only flat field of the central 3x3 detector array. The scaling is such that red is near 100% illumination, green is around 50% illumination, and dark blue indicates near zero throughput. Thus, a substantial fraction of the central imaging array will be useful with narrow band filters.

Color coded H-alpha filter flat field.

Which finally brings us to the pretty picture that I have promised earlier: A 300 second exposure in H alpha of the horse head nebula, bias, dark, and flat field corrected. See? A pony!


Horse head nebula, 300s in H alpha

Wednesday, October 24, 2012

Narrow band filters

This week of commissioning started with Charles and Brent spending an entire day mounting reshuffling the large ODI filters and installing five new filters into the ODI filter mechanism. With the addition of four narrow band filters, and one SDSS u' band filter, all nine slots in ODI are now occupied. The new filters are 5 3/4 inch sized filters loaned from the KPNO and CTIO Mosaic cameras, so we do not get the full field of view. The canonical choice for the narrow band filters included an H alpha and O[III] filter – expect some nice images to show up over the new few weeks.

Monday night Ralf Kotulla (UW Milwaukee) took a first demonstration picture of M33 in H alpha, which nicely complements last run's U band image taken of the same galaxy. In the picture below we show only the unvignetted 2x2 detectors array. The total field coverage is slightly larger, though.  The exposure time of the single image was 450 seconds, but we observed the frame as part of a larger dither pattern. I cannot wait to get a full color image of M33.


M 33 – 450 sec in H alpha

Thursday, October 11, 2012

How are u?

This is night 3 of science commissioning, and I decided to come up and sit with Tod Lauer and Steven Janowiecki for part of the night.  We have had some filter inserts made (by the University of Wisconsin machine shops) to hold the 5.75 inch square CCD Mosaic filters, and they just came in yesterday.  They need to be black anodized before we really use them, but Daniel and I couldn't resist borrowing one and asking Charles to put the QUOTA u filter in.  These inserts hold the filter over the center of the 3 X 3 science field (and give you approximately 20 arcminutes square unvignetted), and they also have holes over the outer OTA fields and the focus sensors.  Here is a drawing:

We have to put optical glass in the holes over the outer OTAs so we have in-focus stars to guide on.  We elected to start with clear glass for these, but we may want to switch to red or blue glass to limit the bandpass and improve the image quality later on.  We also had aluminum "filters" made in case we want to block off any of these holes, and we decided that for this experiment we would put one of these into the hole closest to the science field to eliminate scattered light.

So, here is the first report about the u filter.  First, it is a Johnson U - central wavelength 3640 A and width 800 A - redder and wider than the SDSS u band.  Since all the other filters are SDSS bands, we won't be able to do real photometry with color term corrections.  However, the very simple things we can do are instructive.  First, focus.  It's quite windy tonight, and the seeing is poor.  But, to the extent that we can tell, the focus in u is close to the same as the focus in g. We do have a SDSS u filter, and we will put that in soon and redo some of these tests.

Second, the sensitivity through this filter.  We took a frame in Stripe 82 and analyzed a single star in terms of its SDSS u magnitude.  We can calculate a zero point, 24.7,  which is the magnitude that produces 1 count per second at airmass = 1.0  The sky has about 1.7 counts per second per pixel corrected to the same airmass of 1.0  Putting all these numbers together, we can calculate that a u=23.4 magnitude star will give a 10 sigma measurement in 300 seconds.

Finally, a picture.  Tod and Steven got some exposures of the central part of M33 in u.  Here's a quickly (and crudely) reduced 300 second exposure.  You can see we have some flat-field illumination issues, but other than that, it appears to process pretty well.  More to come.


Tuesday, October 2, 2012

Getting ready for science commissioning

After a two weeks of an ODI hiatus, we are back at the telescope. We allowed ODI to warm up for some maintenance work, and today we spend most time on pumping the dewar and going through the cool down procedure.The heater to maint a stable temperature of 170ºK just kicked in an hour ago.

Andrey is spending the time to apply some software updates to further improve the capturing of telemetry data and to enable automated data transfer to the ODI Pipeline, Portal, and Archive (PPA). We will test the new updates before the end of the week to ensure we really made things ..... "better". However, we are somewhat limited in testing since we can operate only during the daytime. By 2 to 3pm we have to yield to the regular observers at WIYN.

Next week we will have the first regular scientific commissioning run for ODI, which is a change in our operating mode: Now observers from the WIYN community, but outside the ODI team,  take over and help with the performance characterization of ODI. This will also be an exciting time for all of us.

I finally managed to get the full ODI installation time lapse video online; Pete Marenfeld edited this final version, and it looks really good. It is hard to imagine that more than two month have passed since that movie was recorded.


ODI-installation15FPS from Daniel Harbeck on Vimeo.

Wednesday, September 19, 2012

One Billion Pixels

The last night of engineering verification I was thinking about what the full focal plane version of ODI would be like.  We now know that even in non-OT mode, the instrument can produce images better than 0.4 arcseconds over its full field.  We know how to make detectors that will work fine in  static and coherent modes.  And so I produced this image of M33.  It's a five minute exposure in the r band.  It is 63 arcminutes wide by 65 arcminutes high.  The corner chips have been used for guiding, allowing us to remove telescope shake and some fraction of the ground layer seeing.  The screenshot doesn't do it justice.  Even the 30-inch display doesn't do it justice. 


Monday, September 17, 2012

pODI is verified

This is the last night of engineering verification, and it has been an enjoyable, if tiring activity.  For me, it has been great to learn about the WIYN telescope and its performance.  I've had seeing about 0.5 arcsec all night the last two nights, and I could easily get used to this.  Over the past weeks, we have demonstrated that pODI can do what it was designed to do.  We have yet to test OT shifting, but we'll get there.  Even without OT shifting, it makes superb images over the full field of view.  The flaws in the detectors are quite manageable, and we are making good progress with the instrument control and data systems.  Commissioning starts October 8, and I'm sure there will be a lot of interesting blog posts when we get into that.  Daniel and I will be writing up a report on engineering verification before then, and we will be taking (separate) vacations during the gap between now and when things gear up again.  I'll probably put another post up in the next few days, but I'll leave you with one more pretty picture. 


The bubble nebula - 5 minutes in r

Sunday, September 16, 2012

ODI, WIYN, and image quality

Daniel was right - the monsoon is on its way out.  It was clear and the seeing was good last night - around 0.5 arcsec plus or minus all night.  Among other things, I did some experiments that Pieter van Dokkum had suggested to me - to begin to understand observationally the potential gains of OT shifting with ODI.  The idea is as follows.  When you are doing OT shifting, you are taking each very short exposure of a source, shifting it according to some measure of its "center", and adding it to the accumulated image.  We are expecting to be able to do OT shifting between about 10 and 30 Hz, so 50 milliseconds is a good starting point for each individual exposure.  Pieter's suggestion was to look at an exposure of 50 milliseconds and measure the image quality.  If it is much better than a longer exposure, you know that there is a lot to be gained from OT shifting; if it isn't, then there isn't much potential gain.  One can take this one step further and use a sequence of short exposures to compare the differences between coherent and local guiding by comparing the shifts that you derive from objects over the whole field.

So, what I did was find a field near the galactic plane with lots of bright stars (to have something to measure in a 50 ms exposure).  I took a 10 second exposure, a 5 minute (guided, but not OT shifted) exposure, then 55 50 ms exposures, and then another 10 second exposure.  I haven't got all the way through the analysis, but the initial results are interesting.

The two 10-second exposures had average PSF FWHMs of 0.430 and 0.512 arcseconds, which themselves average to 0.47 arcseconds.  The 5-minute exposure has an average PSF FWHM of 0.452 arcseconds, pretty close to the 0.430 of the 10 second exposure taken immediately before it.

The 55 short exposures have the following distribution of FWHM values (I have only measured one star so far):

The average is 0.425 arcseconds, only a little better than the 0.47 that one might get from 10 second exposures over this same time period, but the spread is quite large.  So what is going on?  Two things.  First, when the PSF is spread out, its central peak is not as tall.  When you combine two PSFs, the resulting PSF will not have the average FWHM, it will be smaller than that.  Second, the PSFs that have large FWHMs tend to have multiple speckles visible and the FWHM is pushed to larger values.  In the image here, the left frame shows a typical large FWHM image, while the right frame shows a typical small FWHM image. (Note also the CTE problem evident in the faint tails going up from each image.  These will not appear in well exposed images - even those that are OT shifted.)

As a quantitative test of this, I took four frames, two with small FWHM (0.245 and 0.248) and two with large FWHM (0.675 and 0.567) values.  The average of these is 0.434, close to the average of the entire set of 55.  I then shifted these frames so that their brightest pixels were aligned.  I averaged them and measured the PSF: 0.26 arcseconds.  This is the ultimate promise of local guiding.  How much of it can be gotten from coherent guiding is still to be determined. 


UPDATE - I found a simple way to do this shift and add for the whole set of 55 frames.  The resulting PSF FWHM is 0.33 arcseconds.

UPDATE 2 - Daniel correctly points out that, in real OT shifting, you are using the image from 50 ms ago to predict the shift now.  I can't model that in my experiment, since each image took 30 seconds to read out. So, my experiment is a best case, and there may be an additional contribution to the degradation of the PSF.

Friday, September 14, 2012

Clear sky, donuts, and a marble

Almost overnight the Monsoon season came to an end. Northwestern weather systems are pressing the moist air out of Arizona, and the sky is clearing up rapidly. While Wednesday high humidity prevented us from opening the dome despite a beautiful clear sky, last night we finally saw star light again. 

During the first half of the night most images were out of focus. Intentionally, that is. By combining defocussed images where the focus has been moved inside and outside of the optimum location, one can reconstruct detailed information about the characteristics of the optical system (or more technically: one can reconstruct the wave front errors). This is what Chuck Claver from LSST and his team will do with the data we took, and provide us with hints how to optimally tune ODI's and the telescope's optic to get best images over the entire field of view.

Later the night with eastern winds the seeing at WIYN turned bad to more than 2". We used that time to prototype the workflow for automatic dithering of images. Just before calling it a night we also managed to capture Jupiter (which nicely fits into one OTA cell!), albeit I really look forward to repeat this exercise when the seeing is better.


Out of focus images ("donuts") used to analyze the imaging optics.

Jupiter – at a bad seeing of 2". WIYN can do better.

Thursday, September 6, 2012

A first (almost) completely reduced image

Another guest post from Frank Valdes:

Final reduced image of M15.  Note that gaps are filled in over 25 X 25 arcmin square region  
The data for this spectacular result started with a dither set of 9 exposures of M15 which Todd previously noted had excellent seeing and image quality.  The data were reduced using an IRAF package being developed to complement the pipeline; the pipeline is close behind in functionality.  The basic processing consists of the usual operations of overscan, bias, and dark subtractions and dome flat fielding.  Appropriate dark calibrations are required to remove the amplifier glows in these OTA detectors and, after these calibrations, the independent cell images are merged into simple OTA images.  A small constant illumination gain offset was needed in two of the central OTAs after dome flat fielding in this first extended exposure time (200 second), g'-band data.  The remarkable thing is, at least in this filter, that all the cells in an OTA are quite uniform after dome flat fielding and the other 7 central OTAs are uniform without need for a sky color (illumination) correction.  The next step is to apply a low order astrometric calibration correction to the world coordinate system (WCS) derived earlier.  Using this calibrated WCS the dithered exposures were remapped to a common sampling and combined into a final image. This combined image showed that the astrometric calibration, resampling, and stacking did not degrade the image quality by much, though further tweaking of the astrometric solutions can optimize this a bit more.  What's missing here?  Cross-talk and bad pixel masking are still needed, primarily in creating the input calibration data.  This final stack was created as a median of the exposures to compensate for the lack of explicit pixel masking.

Frank Valdes

Close-up of M15 stacked image

Wednesday, September 5, 2012

Clouds, rain, volcanos, and a real user

Plenty of moisture in the air is turning the desert greener and into a better place this week. At some point a trail of clouds formed over Baboquivari, making it almost look like a volcano. This is not the prime time for observers.

Bad bad sky conditions cannot stop us from making progress though. The first member of the ODI Commissioning Working Group, Jenna Ryon from UW Madison, is visiting us at WIYN this week, and she is the first user to come in contact with the instrument without any prior knowledge. While we have not opened the dome this week yet, she is testing the user interfaces and tries to make sense out of the early documentation. This process does not produce great looking deep-sky images (although Jenna is very proficient in creating dome flats by now!), but is fundamental to the transition of ODI towards regular operations. Based on the telescope operator's and Jenna's feedback I am now busy updating the procedures and manuals. I still hope we will get on sky this week.

Writing documents - that's what astronomers really do at night.


Clouds over Baboquivari

Jenna Ryon from UW Madison is the first outside user of pODI. 

Just a pretty picture

This week I am at the WIYN Science Steering Committee and Board meetings in Bloomington, IN, and Daniel is probably getting rained on.  I'm lining up a post from Frank Valdes in the next day or two on the first full reduction of a pODI dither sequence, but, to hold you over, here is a nice 5 minute r'-band exposure of the Helix Nebula. The bottom image is a close-up of a part of the central OTA.


Friday, August 31, 2012

It just gets better

As I wrote last weekend, Saturday night I had some of the best seeing I have ever had.  I had written a post a couple of weeks ago about the image quality we have been getting, based on a frame that Daniel took.  I thought it might be interesting to go back my focus frames to investigate the image quality on this exceptional night.  I took the raw frame that was closest to best focus and measured a half dozen or so stars in each OTA.  Here is the average on each OTA.
Wheras the previous measurements, which averaged about 0.6 arcseconds, showed little difference from detector to detector, these show substantially more.  Not surprising - as we get to better image quality, the focus or telescope-induced image quality differences become easier to see.  I have yet to  see whether I can detect focus differences from OTA to OTA, but I did notice that the bottom two OTAs (OTA 00 and OTA 61) had elongated images.  This suggests that we are seeing optical aberrations (either telescope or instrument) such as coma or astigmatism.   The telescope has some adjustments that we can explore, primarily with the active support system on the primary, that may improve these outlying fields.  However, we are seeing pretty good image quality throughout the central "science field", barely over 0.40 arcseconds on average.  

We are collaborating with the LSST wavefront group to determine the wavefront errors and how to minimize them.  This process involves calculating the sensitivity of the wavefront errors (in terms of Zernikes) to each adjustable parameter of the telescope or instrument. Then, we will look at out-of-focus images on each OTA and compare them with those predicted by the ideal optical system.  This will tell us what adjustments to make.  We'll iterate on this until we are satisfied. Of course, we'll have to wait for another night of exceptional seeing to really test the outcome, but the bottom line is we already are getting spectacular image quality, and we think we can do better.


Thursday, August 30, 2012

First baby steps with the ADC

After all these nice images of the last few days I decided that we are having way too much fun up here, and this night we return to the technical evaluation of pODI.

One problem one encounters when observing closer to the horizon is so called atmospheric dispersion. While it is a tiny effect, the atmosphere acts on light coming from outer space like a prism, and disperses it into its colors. The effect is about a factor of 200 too small to be seen by the naked eye and goes unnoticed in everyday life. For a high-resolution imager like pODI it can be a significant effect, though. To counter for this atmospheric dispersion, ODI has two large prisms in its optical train that can be tuned to compensate for the distortion of the atmospheric dispersion (hence ADC: Atmospheric Dispersion Compensator). So far we operated the ADC in its neutral configuration, but tonight we observed some first test data, albeit with a poor seeing of about 1.4 arc seconds. At an airmass of about two we took some in the g' band (which is the most affected band) with the ADC in both neutral and in active state. We still need to fine-tune the strength of the dispersion (during better seeing conditions!), but the image improvement by the ADC is apparent in the example below.

For the tuning of the ADC we will use a "UG 5" filter with transmission peaks at 3500 A and 7200 A, but we have to wait until the filter adaptors are ready. At an high air mass, this filter will create double peaks of a single star (corresponding to the separation of the red and blue part of the star's spectrum), and the goal will be to tweak the ADC such that the two stars will merge into one single one. Although one shouldn't cross the beams.


Image and contour plot of the same star observed in the g' band at an airmass of ~ 2. Left: ADC is neutral, right: ADC is engaged. Note the improvement of the roundness. 

Tuesday, August 28, 2012

Grasping StarGrasp

Engineering verification is split between daytime and nighttime activities, which is a good thing, considering the weather.  This week, we have two of the StarGrasp team, Peter Onaka and Greg Ching, here to help us with the tuning of the system and to do some technical training of mountain support personnel.  StarGrasp is the controller system that runs the detectors - voltages and signals - and receives and organizes the data coming back.  We had a very successful training session this morning in which Peter and Greg went through the entire system architecture and design, discussed how one diagnoses problems and what to do if the system is not working properly. The support model for StarGrasp is that we have a set of spare modules that can replace failed ones, and that the failed ones are then returned to Hawaii for repair.  The StarGrasp group has a second set of spares, and they send us their spare upon receipt of the failed one from us.  They then repair the failed one and it becomes the spare for next time.  For this to be effective, we will depend on the KPNO Electronics Maintenance staff to help diagnose which module has failed.  Daniel is still the local authority on StarGrasp, and he will coordinate this effort.  We will be putting this system into place over the next few months as we move into scientific commissioning and then shared risk observing.


Monday, August 27, 2012

Shooting the moon

It's raining again - not much chance of getting anything useful on the sky tonight.  I wanted to do a bit more with the frame of the moon that I took Saturday evening (In memory of Neil Armstrong).  When the first mosaic CCD cameras were built in the mid-90s, it became common to show their field of view by imaging the moon. To astronomers who had traded in their large photographic plates for tiny 800 X 800 CCDs, this was a welcome advance.  However, the surface brightness of the moon is higher than the daytime sky, and so one used to have to play tricks like partially closing the mirror cover, or occulting part of the telescope beam with the dome.  This was in addition to putting in a filter that gave you the part of the spectrum where your CCDs were least sensitive so you could take a 1 second exposure, which was about the limit of reliability and uniformity for the shutters we used.

In an earlier post, I noted that because of the design of the Bonn shutter we have on pODI, we can take exposures that have a very short effective exposure time but are still relatively uniformly exposed.  This is because the shutter consists of two sliding blades that are controlled independently by stepper motors. A short exposure can be obtained by effectively moving a narrow slit across the focal plane, with the two edges of the blades close together.  We first used this to take twilight sky flats even before the sun had set.

It was still an hour before sunset when I decided to take an exposure of the moon.  I put in the z' filter (where the sky is pretty dark and the CCDs are the least sensitive).  I set the telescope focus to the value I had used the previous night.  I tried a 5 millisecond exposure and I got about 3000 counts per pixel.  Good enough.  Finding the moon was not trivial.  John Thorstensen was my savior.  His Skycalc program gave me RA and Dec for the moon right now, right here.  It was very close.  Center it up, and - snap - 5 milliseconds - perfect exposure. About 30,000 counts per pixel in the moon.  The image that I posted was almost raw. It had the overscan level subtracted, and the display script tries to adjust the stretch to match up the individual OTAs, but it doesn't do so well for something so extended with such large dynamic range.

So tonight I did a very simple reduction.  I subtracted a bias and divided by a dome flat.  Here is the result.  This is just the central 3 X 3 OTA part of the focal plane.  You can see the three bad cells. You can see that the image display doesn't make the detector to detector gaps quite big enough.  But overall, pretty amazing.  The moon in the daytime with a 3.5m telescope.


Sunday, August 26, 2012

You gotta love this telescope

Last night was my first night at WIYN where I knew it was going to be good.  A few puffy clouds floating around in the afternoon, but the weather forecast was "clear".  This was the first night predicted to be clear since June.  The wind was blowing just hard enough to feel its motion through the open dome.  I always enjoy standing on the WIYN dome floor right after sunset.  You feel like you are outside and have a clear view in every direction.

I thought the seeing might be good, so I put in the i filter before I started a focus sequence.  I've been looking at a lot of out of focus images (see my vignette on vignetting) and as I stepped by 20's through the focus values, I noticed that even as the image got smaller and smaller, it kept its donut-like shape. I could see the hole in the middle, and some consistent bright spots and irregularities in the brightness. It got very small before the images turned into blobs with a central peak.  I cut my steps down to 10, and I passed through focus and came out the other side.  Then I went back and measured the FWHM values of the images.  0.42 arcseconds.  That was the best.  That was among the best seeing I've ever seen.  It stayed that good - or almost that good - all night.  I worked mostly in g (where the detectors are very sensitive), and I took frame after frame with images 0.5 to 0.6 arcseconds FWHM; a few below 0.5.  These were guided 5 minute exposures.  Guided with the telescope - not with OT shifting on the detectors. I took a bunch of frames of a field in Stripe 82. In each one, I could see little galaxies, but they were not just smudges; they had spiral arms and nuclei and, when they were interacting, I could see knots in the streamers that were flying off them.  It was almost like looking at HST images.  I've inserted images of a couple of cells, but these don't really do justice to the data.  Remember that each of these cells is about 1 arcminute across and pODI has 13 X 64 = 832 of them.

I've got more stories from this night - including one that taught me to ALWAYS check my shoes for scorpions in the morning before I put them on - but that's for another entry.


Saturday, August 25, 2012

In memory of Neil Armstrong (1930-2012)


One of the things I have been curious about in ODI is the vignetting.  While the optical design was based on a one degree field of view, the square format of ODI actually puts the corner pixels as far as 44 arcminutes from the field center.  I have played a bit with trying to compare the flat field illumination with the photon statistics in order to separate the gain from the vignetting, but I wasn't happy with that approach.  Last night, I got a good sequence of frames going through the best focus, and so I took the most out of focus frame and looked at the donuts - the pupil images.  I took the best image from each OTA and made up a little picture that shows them placed on top of the position of that OTA.  As you can see the illumination is remarkably uniform until you get to the far corners of the focal plane.  The two OTAs that are in one square from the corner have centers about 28 arcminutes from the field center, and you can see that there is really minimal vignetting at that point.  In the one OTA in the real corner, centered about 39 arcminutes from the field center, the vignetting increases rapidly and is about 60-70% at the outermost pixel. 


Thursday, August 23, 2012

On ODI's throughput

ODI at WIYN's sensitivity has been predicted based on modeling all optically relevant components. The throughput model includes: The atmospheric extinction and telluric absorption lines, the three telescope mirrors, absorption losses in the ODI optics (two lenses and two ADC prisms), anti-reflection coating performance, and the quantum efficiency of the OTA detectors. The predicted throughput has been verified on sky in the g', r', i', and z' bands within reasonable error margins, and the differences we found are now included in the throughput model as a fudge factor. The resulting as-build throughput of ODI is shown the figure below. We will keep tweaking this model as we get more on-sky data; I am in particular looking forward using a u'-band filter at some point in the future and verify the blue performance

The blue cut-off in ODI's sensitivity  is governed by special glass (O'Hara PBL6Y) in the ADC prisms. The fall-off in the red is driven by the vanishing quantum efficiency of the ODI detectors. The peak throughput of order of 55\% is the sum of all losses in the system; the most significant limit on the peak throughput is set by the losses in the three WIYN mirrors, though. 

Total system throughput of the WIYN telescope and ODI, including atmospheric absorption.

Wednesday, August 22, 2012

Watching the grass grow

Kitt Peak is very green right now.  The rain and overcast continues, and so I decided to delay the start of my two days of sitting in the control room until Friday.  The weather report for Friday night is partly cloudy, and for Saturday night it is clear, so hopefully.....

In the meantime, Daniel has been up working on adjusting OTA voltages, and Andrey and Erik Timmermann, from the NOAO Science Data Management group, have been installing and testing out the observing GUI that Erik has developed.  Downtown, I have been working on getting the filter inserts made  that will hold NOAO Mosaic filters (5.75 inches square) - no, I'm not cutting metal, but negotiating the schedule with the machine shop. Also, thanks to Di and Charles Harmer, I located a Schott UG 5 filter that we can use for adjusting the positioning of the ADC.

Next Monday we have a visit from the Stargrasp group - Peter Onaka and Greg Ching - who will help us with final tuning and hold a series of training sessions for the mountain engineering group.

I hope to have something more exciting to report in a couple of days.


Tuesday, August 21, 2012

First reduced pODI image

Today - a guest post from Frank Valdes

One of the early first-light priorities was to take a dataset for producing the initial world coordinate system (WCS) to be added to the raw data.  A set of 18 forty-five second unguided exposures, in pairs within a 9 point dither pattern, was taken Aug 7th of IC 4756 in the SDSS g filter.  Note the seeing was only around 1" while pODI will, hopefully, often produce even better image quality.

A WCS using an optical model of the radial distortion across the field and a general polynomial fit to the remaining residuals was determined.  A check of this coordinate system is to use it, after tweaking it for each exposure, for remapping and stacking the dither followed by examining the stars to see if they are well registered across the field of view.

The images below show the large and small scale results of this first stack.  The processing was done using the IRAF ODI and MSCTOOLS packages.  The pipeline is current being worked on to automate this same processing.  Note there are a many things that still need to be addressed -- use of bad pixel masks, cross-talk subtraction, bleed trail masking, photometric registration, etc.  Because these instrumental features are not yet handled, the quick and simple way to produce a clean looking image is by simple median stacking, in this case without any photometric scaling of the exposures.

Figure 1: The full field of the central 9 OTAs trimmed to the common area of the dither.  The size is 13.6K by 12.3K at 0.11"/pixel.

Figure 2: A close up of an approximately 4' region indicated in the full field.

Still here, in the clouds

The new week did not bring the weather improvement we were hoping for. The afternoon gave us rain and rainbows, and the clouds keep loitering around throughout the night. The small holes in the clouds have not permitted us to open the dome, yet.

What is to be done, then? I went though some older images and worked on the software module that will one day autonomously find guide stars from a short snapshot image. The same tool also allows to find stars in a longer exposure, and based on the star's size to then automatically to judge the image quality.  I came across  one shorter (60 seconds) r' band image that seemed to have a delivered image quality more around 0.55'' throughout the image. 

As the clouds might finally break up, it would be nice to demonstrate what ODI can do with long exposures. Let's give it another hour.


View from the WIYN parking lot.

Thursday, August 16, 2012

View from under the clouds

The weather has taken a turn for the worse, and it looks like we won't get any more data on the sky this week.  We are ready to test a new, improved version of Daniel's guiding software, and Andrey has been working on collecting all the instrument and telescope data with each exposure to put into the headers. This is not real exciting stuff, and I, for one, am eager to see the end of the monsoon.

One new item for prospective pODI proposers is the ODI web site (still under construction, but you can see the layout and some of the content at and, in particular, the page we have put together to tell you everything you need to know to write a proposal (  The call for proposals will be out around September 1, and we will update this page as we get more information.  One graphic I put together for this page is the layout of pODI with the formats, sizes, and gaps indicated.


Wednesday, August 15, 2012

First look at image quality

On Monday night, Daniel took a 200 second guided exposure in the r' band that he noted had stellar images with FWHM about 0.6 arcseconds.  This is the best we have seen so far, and so I took some time to examine the images in a bit more detail.  I measured the FWHM of a dozen or so stars in each OTA, and I found that the image quality was good and quite uniform over the entire field.  The picture shows the measured average (in arcsecs) in each OTA.  I did not notice any significant departure from roundness in any of the OTAs. 

The worst FWHM, 0.615 arcseconds corresponds to an additional contribution of 0.22 arcseconds (in quadrature) over the best FWHM.  Also, the OTA with the worst FWHM, OTA 55, failed its metrology test - that is, it is not within the specs for final height of the detector plus package.  Thus, part of the poorer image quality might be a focus effect, though it is not much worse than the OTAs around it.  Finally, I note that we have not yet begun to adjust the telescope optics to optimize image quality over the wide field; that is an activity planned for early September.

All in all, it looks like pODI (and later, ODI) will have the ability to deliver quite good images over its entire field of view.


Tuesday, August 14, 2012

Learning more about guiding & photometric zeropoints

The hailstorm (with about an inch diameter sized hailstones) yesterday afternoon curbed our hope for a clear night, so we used the afternoon for a new look at the noise characteristics, and we can confirm a similar read noise level as we did in the lab in Tucson. So far so good.

Todd took a first shot at the photometric zeropoints in the four ODI filters, and they are within 0.1 mag of the prediction we made a few years ago. As time permits, we will update the ODI exposure time calculator.

Surprisingly, the sky cleared up before 10pm, giving us the opportunity to spend more time to tune telescope guiding. A few hours of debugging and optimizing later, we ended up with a workable configuration of guide parameters that allows more stable guiding. Getting the guider fully tuned to a critically damped state will take some time, though.

In one instance we recorded a single bright star at a rate of >30Hz. While the telescope guider filters that signal to a slower rate, it also calculates the power spectrum of the image motion (the panels with the green histogram in the picture; ignore the units on the Frequency axis - they are meaningless at this time). Most of the time that power spectrum is boring, but when some wind blew into the dome, a clear spike showed up in the power spectrum, indicating that the telscope mount might have gone into its 8 Hz resonance. With still some bandwidth left in the system, this measurements is encouraging for the prospects of the coherently corrected OT modes with pODI, where this resonant image motion could be compensated in the detectors.


Friday, August 10, 2012

Guiding - a major milestone

One of the things that makes ODI unique is its detectors, orthogonal transfer arrays (OTAs), which are more complicated than the typical CCDs found in most astronomical imagers.  The OTAs in ODI are each divided into an 8 X 8 array of "cells", each of which contain 480 X 494 pixels.  The reason for the separation into cells is so that the guide function can be accomplished by reading out a part of a single cell frequently to measure the position of a guide star without affecting the other 63 cells on the detector.  Ultimately, this will allow us to assign 4 cells on each OTA to the guide function and independently guide each quadrant to remove the local motion from atmospheric turbulence and telescope shake.  At this point (the first week on the telescope), the challenge has been to read out a cell at video rate, extract the guide star information, and send it to the telescope every second or so. 

We accomplished this last night for the first time.  The first picture shown is the OTA Listener, which displays the entire pODI focal plane on the left and one of the OTAs on the right.  For this exposure, we had pointed at M57 the ring nebula in order to take some long exposures and measure the crosstalk between cells.  The OTA shown is OTA 33, the center one in the 3 X 3 science field.  If you look closely at the lower left corner cell, you can see that it is truly black.  This is because during the exposure (about a minute long), we read out a piece of that cell and used it to guide the telescope.  This is the first step towards OT operation, and it is prerequisite to taking exposures longer than a minute or two.  The other thing you will notice are the amplifier glows. In order to read out an OTA, we have to turn the amplifiers on (throughout that entire OTA), and they glow in this batch of detectors.  The amps are off until readout on the OTAs not used for guiding - one of these, OTA 44, is shown in the lower picture.  Our planned mode of operation for pODI is to use one of the outer OTAs for the guide function, leaving the 3 X 3 "science field" free of amplifier glow.  For the full-up ODI, we'll manufacture new detectors that do not have the amplifier glow.  The images shown are raw - no flat fielding applied yet, so small variations in gain among cells are apparent.

Todd and Daniel

A 60 second long exposure of M57, with guiding, shown in the OTA Listener
OTA 44 from the above exposure - note that the amplifiers are not glowing

Wednesday, August 8, 2012

A good night to write documentation

We made some progress on the video mode today, but it is extremely unlikely that we will ever open the dome tonight. But we need to write documentation and procedures anyway...

Daniel & Todd

A haboob in the valleys around Kitt Peak.

Hell is breaking loose over the WIYN 0.9m telescope.

So this is why no one else is up here observing

This night was particularly frustrating.  Clear all day, clear at sunset.  An hour later thin clouds forming above us and getting thicker.  We tried to shoot between the clouds for a couple of hours until the overcast was solid.

One OTA with a very bright star, showing the crosstalk ghosts in cells in the same row.
Even so, we accomplished a few things and learned a bit about ODI.  We discovered that the shutter is so good that we can do twilight flats (8 millisecond exposures) while the sun is still up.  This means that you can get good flats for all filters without having to race against the darkening sky.  We worked on video readout for guiding, and, while we didn't get it going, we found and fixed some of the bugs.  This is our highest priority for the rest of the week.  We measured the crosstalk ghosts that result when you observe a bright star, and we found that they are limited to the row of cells in which the bright star appears (see picture).  The ghosts are about 1/10,000 of the original image, so they only show up where the original is saturated.  Finally, we obtained a complete dither sequence so that the pipeline developers can have some realistic data to begin playing with.

Todd and Daniel

Tuesday, August 7, 2012

More stars, more rain, and many lessons learned

The control room was filled with people tonight to witness the official first light tonight (as opposed to the dress rehearsal last Friday). So far everything has gone well other than the weather. Around 9:30 pm Krissy opened the dome (as she predicted after dinner), allowing us to take even more images of star clusters (mostly chosen to make focussing easy). Unfortunately, only half an hour later we had to close the dome again due to imminent thunderstorms and rain. 

Nevertheless, we made progress on several fronts today: 
  1. The ring we saw on Friday's image was most likely caused by condensation on the dewar window (Thanks for the suggestion, George J). This was resolved by increasing the dry air flow. After some time spent wondering, this has turned out to be no issue at all. 
  2. The baffle to block a stray light path (from the tertiary mirror via the primary mirror into the instrument) was installed today. We tested the effect of the baffle, and on first look it seems to remove the predicted scattered light component.  
  3. We saw a << 1% pupil ghost in r' band flat fields, but apparently in no other filter. We expect that Frank Valdes' & Rob Swater's pipeline can readily handle the ghost. 
  4. Examining an 20 second exposure of a 2nd magnitude star, we have not seen any indication of strong crosstalk yet. However, before making a bold claim here, we will obtain more suitable observations to quantify any crosstalk. 
  5. A first experiment indicates that we have no obvious light leaks in the instrument – detailed testing will follow soon. 
Today marks the transition from instrument installation into the engineering commissioning phase. This week we will concentrate on fundamental detector operations, harvesting header information from the telescope, and enabling telescope guiding with ODI guide star videos. During this phase we might not produce as many spectacular images, but we will continue to keep you up to date on our progress.  

Daniel & Todd

More stars, nicely packaged into an open cluster (do you recognize it, Bob?). 30 seconds in r', bias and flat field corrected. Seeing was about 1"

Monday, August 6, 2012

pODI first light - a sneak preview

While we plan to have official pODI first light tonight, weather permitting, we were a bit fearful of discovering some difficult problem in front of an audience.  So - on Friday, we opened the dome for an hour to make sure that we could focus, point, etc.  Our first light preview went remarkably well, though we have done no quantitative analysis of the details yet.  We only obtained a couple of images and inspected them.  Here, for your viewing pleasure, is an image of M13, 30 seconds through the r' filter, no guiding.

Todd and Daniel

This is the central OTA (about 8 arcmin square) of the 3 X 3 central array.  All of the images here have been overscan-subtracted only.
This is the central 3 X 3 array of OTAs (about 24 arc min square).  There are 3 non-working cells.  The interesting artifact is the ring, which is centered just above the field center.  This appears in dome flats as well, and we are trying to figure out what this is and how we can remove it. 
This is the full field of pODI.

Friday, August 3, 2012

Loading the filters into ODI, detectors are alive

Over the course of several hours we loaded the four ODI filter into the instrument today. Each filter is about 40 cm x 40 cm large, and each costs the equivalent of a my dream sports car (I am thinking Audi TT here, if you would be so kind...). SA lot of time was spent practicing the procedure with aluminium blanks instead of glass. By the time we started the installation, we were very confident in the procedure.  The pictures of the installation are posted below. 

As of now we have four filters for pODI: SDSS g', r', i', and z'. This is the entire filter set for now, but yesterday Gary Muller released the drawings for filter adaptors that will allow Mosaic (about 6 inch size) filters to be used with pODI. We expect to have them by November or so. 

Andrey spend most of the day adjusting the data acquisition software to the new mountain environment.  At the end of the day, he was able to take the first bias and dark exposures to proof that the detectors are alive and well – a very good conclusion for the second week of installation. 


Gary Poczulp installs a filter into its module.

Lifting the filter module to the instrument. 
Charles verifies that the filter is properly seated.  
The z' filter is moving into the instrument. 

Thursday, August 2, 2012

Almost there

ODI is now cabled up and cold.  We spent the day watching the vacuum and temperatures carefully as the instrument cooled down.  This is the first time we will leave it cold while we are not around, so Daniel is briefing Krissy, the Observing Assistant who will be on duty tonight, how to do an emergency shutdown if necessary.  The rest of us will depart and return tomorrow for a day of biases and dome flats.  If all goes well, we will be opening the dome next week. 

Daniel and I worked out a schedule today for the engineering verification activities.  Since we are still in the monsoon, we will take advantage of the clear nights when they occur, and otherwise work on the stuff that can be done in the daytime.  All this is subject to change as we proceed, but the priorities are:

Aug 6-10: focus sequence; manual guide star acquisition; focus w/ guide star; telescope guiding; telescope telemetry

Aug 13-17: focus sensors; world coordinate system; on-axis wavefront adjustment; filter offsets

Aug 20-24: Image characterization; science GUI; data for PPA

Aug 27-31: ADC; begin photometric characterization; OT preliminary testing

Sept 3-7: photometric characterization

Sept 10-17: wide-field wavefront adjustment; image quality analysis; update exposure time calculator

It will be a busy period, but this should put us in good shape to begin scientific commissioning in early October.