I've posted a similar message on CCD NEWASTRO, but now realize this is probably a better place to ask these questions ... My flats have this rather odd "hot spot" look to them and are not doing a great job of neutralizing sky background, although they are doing a good job of cancelling dust motes. The imaging path is as follows: AP160EDF > FT 4" Focuser > AP 4" to 2.7" step-down adapter > AP 2.7" threaded extension > AP ADA27-42 (2.7" thread to T-Thread) > SBIG AO-8 > SBIG CFW10 (Astrodon filters) > SBIG ST2000XM. First image shows the back end of that imaging path. The second image is the R, G, and B master flats visualized as a color image (red is the red-filtered flat, etc.), as that shows the problem better than the three respective grayscale flats. Visualization is linear but aggressive (black/white point set to +/- 5% of the median). Third image is similar to the second, but narrowband: SII, Ha, OIII flats mapped to RGB, Hubble palette style. This has the same general appearance, although, interestingly, these are doing a much better job of neutralizing the background of narrowband lights than the RGB flats are doing for color imaging. I've been reading where the "hot spot", and sort of "target" based illumination pattern (concentric rings of brighter, then dimmer, then brighter spots, radiating outwardly) can often be caused by reflections, particularly when there's a Field Flattener or Focal Reducer (or Barlow or PowerMate) "near" the camera that is reflecting light back into the camera after a bounce or two. But I'm stumped here -- I have no such element in the path ... it's basically a stock AP160 with a bunch of AP adapters to get down to T-thread, and the SBIG stuff from that point onward. Note: I saw this same thing before I obtained the AO-8 so I don't think it's the culprit. Another issue I'm seeing is occasionally I'll have a "bad read" (or bad download) such as in the fourth image attached. Also, if you look carefully at the flats, there are rather large rectangles (left, right, and top), which are easier to see in the SHO (purple-ish) flat than the RGB flat. Happy to provide more details. Let me know if these problems are all related, or not, and if the camera needs to be sent in for repair. Thanks, WB
Added a single Flat Light through the red filter (a sky flat) and the original Light of the "bad read" image.
Could be a couple of causes to the "bad read". It could be caused by a pause in the middle of the read readout, resulting in additional dark current accumulating. Given that it's an interline chip it's unlikely for it to have been improperly flushed, since the substrate pulse clears any previous charge in the sensor... though the vertical columns also do require flushing. I'd make sure your drivers are up-to-date - get the latest version of the SBIG Driver Checker and update. Also you are using an obsolete and unsupported software package to run the camera. Does the same thing happen if you use CCDOPS? As for evaluating the flat-field, I'll need a few more images: dark frame, flat-dark and/or bias frame, and a light frame.
Thanks Doug. Attached are the files you requested. The one name Flat Light (Red) is the same one as above.
It doesn't look bad... but even a tiny error can look awful when you have a bright sky background. What are you using to illuminate the aperture of the telescope? Make sure entire front aperture of the instrument is evenly illuminated, right out to the edge of any dew/light shield. Sometimes light bounces off the inside edge of the various tubes that hook the camera onto the telescope. Even if it's blackened you can get substantial reflections from grazing incident light - especially in the infrared (which you can't even see visually!). You'd be amazed what a little bit of baffling in there can do.
Thanks Doug. I use Sky Flats, just after sunset, an hour East of Zenith. If the sky itself wasn't 'flat', I'd expect to see a smoothly varying gradient in the Master Flats, but that's not what we're seeing here. I'm using AP adapters all the way from the focuser to the AO-8, and Astrodon filters in the filter wheel. Could over-aggressive cleaning of perhaps the chip cover or the camera window or even the filters have affected the AR coatings enough to cause reflections that would create this pattern? Is there any way to compute where it's coming from?
AR coatings are reasonably tough, but the filters are more fragile. There is no way to "compute" where they come from. You need to use a process of elimination. Try it without a filter. If that works well then the filters are suspect. We recommend against opening the camera - if you need it cleaned it's best to send it to Bill @ SBIG Service & Repair.
