Wed, 04/03/2013 - 22:28
Hello! I have an SBIG-402 and 8" Meade LX200 classic. I constructed a light box along the lines of Berry and Burnell in "The Handbook of Astronimcal IMage Processing." I noticed that one of the four LEDs seems to cut off before the others as I dim the unit from full brightness.
What would be the bet way to determine whether the LEDs are lit uniformly and that the flat is as flat as possible?
I'd then like to compare the quality flats produced with my light box with twilight flats.
Thank you for your help. Best regards.
Mike
Hi Mike,
There are several ways to test your uniformity; I'll give a couple here and perhaps others can chime in.
Since you have 4 LEDs, presumably in the corners, and it sounds like you might be worried that one is cutting out early, rotate the light box 90 degrees and take a second flat. Then ratio the two flats. That will tell you if there is any non-radial non-uniformity (I don't think that is a double negative...). If the flats are unform, the ratio should be constant across your sensor to less than one percent.
For radial issues, things get a bit more complex. You will of course have vignetting in many systems, but you also have scattered light and possibly nonuniformity in your flatfield. Those are three effects that you need to separate somehow. Vignetting is normal, and a good flatfield removes it (however, be aware that a 25% vignetting means the corners of the sensor will get 25% less light and therefore lower signal/noise than the center). Radial nonuniformity can be addressed in a couple of ways. I think Berry/Burnell suggest moving the light box far away from the telescope, so that it looks like a small, more uniform or almost pointlike, light source. It will be in rough focus, so will look like a spot on your image. Then move the light box in a raster pattern, taking images. Process those images with your original flat field. The spot should be the same intensity, no matter where it appears in the field of view, if the flatfielding is correct. A second approach is to image a known area of the sky, such as covered by SDSS, CAMC, APASS, etc. and see if there is a systematic trend across your field of view after flatfielding. You can also take a single star and raster it across the field of view if you have a photometric night.
Good luck! Light boxes do remarkably well, though for most photometric purposes, I prefer small incadescent lights to LEDs.
Arne
Hello! Thank you for your note. I'll rotate the light box 90 degrees and divide one master with another to check for non-radial uniformity.
If I divide my light box flats with a twilight flat, would that show if radial problems were introduced by the light box? Best regards.
Mike
Mike asked if dividing the lightbox flat with a twilight flat would show any radial nonuniformity that might exist in the lightbox flat. It depends (my usual answer!). I prefer to change as little as possible when doing tests, as you can never be sure whether several simultaneous changes might introduce more problems. For example, the twilight flats might have a full-frame gradient if they are not taken near the zenith or if the moon is nearby; there might be a radial dependence in the twilight flat, so dividing the two might look like a radial problem in the lightbox flat when it really is in the twilight flat; etc.
Nonetheless, I do recommend taking both lightbox and twilight flats and dividing them. It shows you the difference of the two techniques. I also highly, highly recommend that you always divide new flats by old flats and watch for changes. You will be surprised how often a new dust spec or some other defect shows up.
Arne