We had a professional astronomer find some fault with our last photometric test procedure, where we used a reference star relatively close to the target star. He suggested that we use an artificially generated reference star as our photometric reference.

Maxim D/L has a plugin that was written by Ajai Seghal that will embed a properly formed artificial star in the top left corner of the image and we used this as the photometric reference. We then exposed a series of 8 sets of 9 images of an unsaturated star near the zenith so that the star appeared in a different place along a diagonal line across the image from top left to bottom right corner in each of the sets.

We then did a photometric analysis of the star in each frame of each set after the frames were calibrated with a master dark and a bias corrected flat made with a Flat-Man XL. The telescope was our customer Mark Manner’s 16″ RCOS with an STL-11000 and a V photometric filter. Mark also did all the data acquisition.

Here’s the results of the test:

Note that the (mean+median)/2 overall variation in flux is around 3%, which for photometers, means a photometric accuracy of 0.03 magnitudes.

In practical terms, this means that our Flat-Man XL does a great job of correcting your images for dust donuts, vignetting, and other imperfections. For those of you not interested in photometry, but who want pretty pictures to turn out good after brutal non-linear stretching, I stretched the heck out of one of the XL-flatted images and could see no sign of vignetting in the XL calibrated image.

We did another test with a CDK 24″ and an ST16803 with a clear filter and a Flat-Man XL-30 to compare the electroluminescent panel flats with twilight flats.
Here are the results:

Here you can see that the Flat-Man XL outperforms the twilight flats at each position of the star on the diagonal. I’m not sure why neither flat corrected the corner position well. It may be because of some passing high clouds.

I think this is a conclusive test that shows that EL panel generated flats are just as good or better than twilight flats and provide a reliable, repeatable calibration source.

As of July 1, 2011, Alnitak Astrosystems is wholly owned by Optec, Inc, a long-time manufacturer of accessories for the astro-imager. Pete Kalajian, co-founder of Alnitak Astrosystems will remain involved in the sales and marketing of AA’s products, but all the manufacturing and shipping will take place at Optec’s plant in Michigan.

We at AA are really excited by the sale. Jeff and Tina Dickerman are great people who have a fantastic commitment to the business and are well-known for their customer service. We are confident that the entire AA line will continue to be a boon to astroimagers everywhere.

Optec brings strong engineering and manufacturing experience to the table, and we are sure that our products will continue to meet the high standards that our customers have come to expect.

Stop by and see Pete and Jeff at AIC in California this November.

Bruce Gary of exoplanet fame asked me an interesting question the other day:

“One matter that seems to never get discussed in what focus setting to use when taking flats when using a telescope that changes length with temperature changes. This may not matter if you’re doing it in the middle of the night, but twilight flats are taken when temperatures are higher. Arne [Henden, director of the AAVSO] once posted an terse answer that you should set the focus setting to whatever is appropriate for the temperature at the time of the flats, and not the temperature that you expect to be working with at night. I haven’t tested this idea yet and I’m wondering if you have, or know of someone who has. ”

My response:

“I haven’t done any testing, but I doubt that focus is that important since dust donuts are going to be primarily on filter wheel and that distance isn’t going to change much. Ditto with vignetting. And certainly pixel to pixel variation doesn’t depend on temperature of the OTA. So my non-empirical take is that temperature changes to focus are irrelevant for flatting with the electroluminescent panels, anyway.”

Bruce, being the consummate experimenter that he is came back with the following:

“You’re right!”

“I just took a set of 4 images at a cold and warm focus setting, then ratio’d them. Wow! And for years I’ve been diligently setting the focus to whatever corresponds to the temperature when doing flats. I just assumed it mattered so I never checked it; your thought about it made me think that it’s an easy thing to test. ”

There: another myth about flat fields exposed. Thanks, Bruce!

Alnitak Astrosystems’ Pete Kalajian has an article in the March 2011 issue called “Demystifying flat fields” that goes into the (simple) math behind flatting your images and also notes how to characterize the linear range of your camera. Understanding how your camera works is essential to deciding on how choose an exposure length for your flats. Check it out. The digital issue is available online now, and the print issue hits the newsstands February 1.

I met Rogelio Bernal Andreo at this year’s AIC convention in California, and was completely blown away by his deep widefield images. He’s pretty much redefined what a good widefield image looks like, so I was very pleased to have him contribute two images to illustrate how flats are absolutely essential for those of you who would like to get APOD images.

The first image shows what a properly flatted image that has been stretched severely to bring out all the dust and nebulosity looks like (in the hands of a master processor like Rogelio). The second image shows the same field, with the same processing steps, but without the early flat calibration step. As you can see, flats are indispensable if you are contemplating non-linear stretching of your data.

Rogelio used one of our Flip-Flats on an FSQ106 with an STL1000 camera to make these images. Rogelio is quite pleased with the convenience and quality of the flats delivered by the Flip-Flat.

You can find more of Rogelio’s fantastic images, including several APODs, at deepskycolors.com. Check them out. They really are stunning and unique.

We decided to get a jump on the holiday season by dropping the price of our Flat-Man L to just $529! For those of you with 8-12.5″ objectives, this is a great way to get  high quality on-demand flats to calibrate your images. Check out the Flat-Man L page.

I got talking to Arne Henden, director of the American Association of Variable Star Observers (AAVSO), the other day at the Cambridge, Massachusetts headquarters. We discussed how best to evaluate the quality of a flat field light source. He wondered just how well our “flatted flat” technique evaluated for radial gradients in the light source, and suggested that we do another test, this time employing photometry of a star recorded on different parts of the imaging sensor.

Arne explained to me that by measuring the magnitude of a fixed-brighness star on different parts of the detector and then calibrating the images with our Flat-Man XL, we’d get a good idea of how even our flat panel’s light spatial distribution of intensity was. Poor flatting would almost certainly increase the standard deviation of photometrically derived magnitudes.

I got an opportunity to perform such a test on a particularly good night of photometric stability last week using our good friend Jacob Gerritsen’s Galaxy Quest Observatory. Galaxy Quest is located at a pretty good dark sky site in Lincolnville, Maine. There’s an SBIG ST-10 camera on a 12.5″ RCOS telescope and one of our earliest Flat-Man XL 18″ has been in residence there for some time now.

The observing program:

I picked a 10th magnitude star in a distinctive 3-star pattern near the zenith, just past the meridian as the target star. This particular grouping of stars also had an 11th magnitude star to use as the photometric reference, as well as a 13th magnitude check star.

I set up a mosaic in Maxim DL to capture the grouping on different parts of the detector by performing small slews after each set of 5 20-second luminance images. Figure 1 shows a co-added frame of one of each of the image sets. The target star, NOMAD 1345-0000396, is circled in red. As you can see, a good portion of the imaging chip is covered by the series of image sets.

I chose 20 second images so that none of the photometric sources were saturated. The ST-10 is a non-anti blooming camera that is quite linear across its entire dynamic range. Using the Maxim DL inspection tool yielded the following for one representative image:

Table 1: Target Star Statistics

Maximum	38061.512	Intensity	450883.375
Minimum	141.813	SNR	2755.662
Median	440.223
Average	2420.555	Bgd Avg	131.807
Std Dev	5681.021	Bgd Dev	11.657
FWHM	1.352″	Flatness	0.162

As you can see from Table 1, the target is not saturated, and all pixels are well within the linear range for the chip. SNR is very high. FWHM was quite good this night (Oct 14, 2010 UT) for our area. The clear sky clock showed better than average seeing and transparency throughout the observing period.

Figure 2 shows a close-up of the photometric apertures. I chose an 8 pixel aperture, one that I have used in the past for exoplanet photometry with excellent results.

Results:

I used Maxim’s photometry tool to generate a list of the magnitudes of the target and check star, setting the reference star to an arbitrary 10th magnitude for zero point.  Table 2 lists the statistics for the 50 images.

Table 2: Magnitude statistics for 50 images

Mean	9.241607843
Standard Error	0.000926337
Median	9.242
Mode	9.244
Standard Deviation	0.006615371
Sample Variance	4.37631E-05
Kurtosis	0.941365832
Skewness	-0.279024777
Range	0.034
Minimum	9.221
Maximum	9.255

As you may recall from your days at school, 99.7% of the data is within 3 standard deviations (3 sigma) from the mean. In this case then, that means that the vast majority of the data is within 0.02 Magnitudes of the mean value.

Recall that I exposed 5 images at each position. Taking the median value of the set of five is a way to average out the scintillation noise. The 3-sigma value in this case was 0.17 magnitudes. Figure 3 shows a graph of the target magnitude (with arbitrary zero-point applied) as a function of time. According to Arne, for small magnitude variations, the differences are nearly equivalent to flux fractions, so 0.17 magnitude variation is akin to a 1.7% variation in the total flux.

To put that into some kind of perspective, 99.7% of the stellar magnitude measurements are within 1.7% of the mean value. It is generally accepted that 1% precision in photometric magnitude is a great result, so we have a pretty good result. Of course, with more frames at each location, we might be able to decrease the variability somewhat, and I hope to do more testing on the next good photometric night. Stay tuned!

The great folks at the Advanced Imaging Conference will be giving away a Flat-Man donated by us. Pete will be there for the conference, so please be sure to come up and introduce yourself. We love to meet our customers, and love to make new friends. Pete is attending on a AIC/Tzec Maun scholarship this year, held over from last year because of illness. He missed last year’s conference, but is really looking forward to it this year.