Again thanks for all the time and information you put in your response. I took your post and addressed each point to the extent that I can to shed more light on this issue - pun intended. You had so much in you post, that I drug it to MSWord to edit. Your input is in bold:
1) As mentioned, others have found uneven performance in bias frames if the exposure duration is below 0.2s, so use that value for your flats
Only my LUM flats were short at 0.09s. The NB flats were 2.44s for OIII, 10.1s for Ha and 13.86s for SII seconds. For my next set of data, I re-ran flats last night with the EL panel set to only 40 for the LRGM and 200 for the Ha, OIII and SII. This gave these exposure times: L - 0.5s, R - 2.26s, G - 1.37s, B - 1.8s – with EL power setting of 40, and OIII - 3.60s, Ha - 10.79s, SII - 15.46s for EL power of 200. I took 40 flats for each filter and then ran darks with exactly the same times. The gain was 76, offset 40 and USB 50 at -15c for all flats and flat darks. I have a nice data set of NGC7000 that I will be trying to process with these new flats as see where is goes with dark flats and no bias frames. The flats and flat darks are at the same gain, offset and temperature as the light frames.
2) Others have also found issues when trying to scale dark frames, so avoid doing that
With my first set of processing in Pixinsight, I only used the dark that was appropriate for the lights which was obviously much longer than the flat frame times. For the images and flats above, I used a master dark for a 10s exposure to be much closer to the flat frame exposure times. The next attempt will use flat darks at the same exposure time as the flats.
3) This means that the best approach is probably to either:
- a. Build a master bias based on 0.2s exposures; bias subtract flats; use a master dark taken at your exposure duration with the same cooling and gain settings as the lights, and do not bias subtract the darks (no need to if you aren't trying to scale them) or
- b. Don't bother with bias frames at all; use a master dark taken at your exposure duration with the same cooling and gain settings as the lights; rather than bias subtracting your flats, apply flat darks them (which will work like a bias frame, but with the added potential benefit of addressing some hot pixels from long-ish flat frame exposures.
I plan to try to use option b for my NGC7000 third attempt. I have all the data collected, but I need time to do the image processing.
4) You do have a couple spots on the sensor itself (or, more likely, on the cover glass on the outside of the CCD chamber) which appear to have been introduced when you did your cleaning. Did you clean just the filters, or did you clean the CCD chamber? Frankly, this is pretty minor and should calibrate out just fine.
There is one spot on the sensor itself. I can see it with a magnifying glass. There are also a couple of spots on the window of the sensor chamber. From the first set of data that showed the streaks, I cleaned only the sensor itself since that was where the streaks clearly were. I did not clean the filters. I would have expected that nothing in any of these flats would not calibrate out. That IS my issue – I don’t know why they are not calibrating out.
5) Maybe someone knows something I don't, but I'm not aware of filter orientation being relevant for any of the issues you are describing. Some filters behave better or worse with regard to reflections depending which side is facing the camera. Also, it is possible to orient the filter wheel itself in different positions which might be useful for balancing an uneven declination load such as that imposed by a finder scope or a focuser. But I'm unaware of there being any issues with how the filters are rotated with regard to the camera. As long as you don't change anything between your flat frames and lights you should be good to go.
The issue I am trying to explain but maybe badly, is not rotation of the filters, but which side of the filter faces the telescope and which faces the sensor. There have been other replies to this post and other that suggest that the Ha and SII filter ARE facing the wrong direction and the OIII is in the correct orientation. It is an easy issue to test and I will test it after I finish some image collection on another target. I don’t want to change the optical path just yet. I have a new adapter to directly thread my Williams Optics Flattener IV to the focuser of my EON. That way my image train will be fully threaded.
6) Focus should be at infinity for both imaging and for making flats. It doesn't need to be exact, though. Minor differences between infinity focus for different filters won't matter at all, and even setting an approximate value on your draw tube with a piece of tape or the lake is sufficient for taking flats prior to taking lights. If your draw tube is within a millimeter or so of correct focus you should be fine.
I have measured the focus position of several but not all of my filters as they have run autofocus in SGP. The positions are rather close with a small offset. Once I change my optics train to the new adapter, I was going to setup the focus points in SGP. There is just no point right now. I spent some time understanding how this actually works with SGP and it really seem to be the right track. I honestly don’t believe that the focus offset between filters is the issue anymore. I believe that it is the combination of having the Ha and SII filters facing the wrong direction and how I am doing the actually calibrations in pixinsight.
7) It is normal for different narrow band filters to generate flats with a different appearance. Often the thickness of coatings varies a bit over the surface of the filter, for example. HOWEVER, I have not seen narrow band filters generating the concentric circles you are getting, and it seems odd to me that both your luminance flat and your Ha flat seem to have very similar large, concentric circles. You may, in fact, have some sort of light leak. That would explain those hot spots on the right side of the Ha flat.
I believe that the hot spot on the Ha flat is actually an artifact from using a 180s masterdark scaled for my short flat exposures. If you look at the master dark, you can see the amp glow in the corners of the right side of the dark master. I think that this is being scaled incorrectly and creating a artifact in the calibration and stacking process for the flats. If this is correct, this should be addressed with the flat darks.
- There are a few things I would try. First, make sure you don't have any light leaks by taking dark frames with a flashlight pointed in various places to see if you can induce any strange results. I don't THINK you have a light leak, but it's worth ruling out. A dark frame taken under brightly lit conditions with the camera mounted on the telescope should let you know. Make sure you shine the flashlight around gaps in the focuser as well as the camera and filter wheels.
I have verified that I do not have any light leaks. I did this with a dark frame in daylight. No issues.
- Next, make sure your flats and darks are taken with the same offset, gain, and temperature as your lights. I assume they are, but I wanted to confirm. Make sure your bias frames or your flat dark frames (whichever you decide to use, but not both) are also taken with the same settings.
Yes, my darks and flats were and are taken at the same offset, gain, USB and temperature as the lights. The bias frames that have used previously were also taken similarly, but with a very short exposure. This may also have been an issue.
- Next, I'd start to suspect the EL panel not providing as even illumination as you thought. Try a 'T' shirt flat at dawn or dusk.
I tested the EL flatman panel on my other OTA with my DSLR and it performed flawlessly. The flats were better than sky flats and had very good uniformity. I have used tee-shirt flats many time previously and they work also just fine with my other OTA and Canon 6D. I am pretty sure the the EL panel is working correctly.
- After that, I'd want to look at your process overall in PixInsight. It's easy enough to make an error in calibration settings such that you get strange results.
I followed the method for Youtube from Richard Bloch which is identical to what is in Warren Keller’s book. I have checked several times to be sure.
- One other thing that is really odd, when I look at your calibrated and stacked Ha image, is the variation in results with alignment of the different frames. When everything is done properly, you will often see noise increasing in areas of the frame where you don't have coverage from all your subs--where the image rotated or shifted. However, it is very odd that your flat correction would be different for the different frames. Is there any chance your registered the images prior to calibrating them rather than the other way around? Calibration has to come first in the workflow.
Definitely not registering frames prior to calibration.
- Last thing I will mention is that in a lot of places the flat seems to be overcorrecting, not under correcting. I didn't measure it to be certain, but the top right corner of your Ha frame, for example, is actually lighter than areas near the middle of the frame (if you don't choose an area with nebulosity). Knowing that you have overcorrection rather than under correction might be useful. Usually this happens if you don't subtract out the bias / flat dark from the flat frame. Double check your settings. It can also happen if your flat frames are taken at an extreme ADU--something outside the linear range for your camera. This is pretty unusual, though, but with a 12 bit camera you might have a slightly narrower range of values that both swamp read noise and are nowhere near the anti-blooming protection.
I am not really sure what to say about this. I have been doing the bias and dark subtraction from the flat frame using image calibration in Pixinsight.
Jason, thanks for all the help and hints. If you have anything to add based on my input to your question/suggestion please respond to the post. I will keep following up as I get more data. I definitely want to get this issue solved and understood.