I previously posted a thread to discuss issues that I have been having with poor flat frame calibration with my ZWO ASI1600MM-C with EFW and 36mm ZWO filters. This is a link to the previous thread:
Let me just start by saying that this is going to be a long post with a lot of data and that I don’t have a fix for this problem. The issue is that with my ASI1600MM-C, ZWO EFW and 36mm ZWO filters, I have had continual issue with flat frame calibration not being effective or worse contributing artifacts to my stacked light frames that I have been unable to remove with the processing techniques that I am capable of current. There may be a way to remove them with processing, but my point is that they should not be there in the first place. I am also rather new to a mono camera with filter wheel. Most of my experience is with my Canon DSLR cameras and I am also new to pixinsight for processing.
I am also not the only user of these products that has either this exact issue or one that is very similar. I have seen threads on CN from at least four other posters that should the same issue that I am going to show. Three others have very different equipment than I do and one has very similar equipment with the exception of filter size than I do. However, the common factor is the camera, filter wheel and filters from ZWO. The effects are worse on narrow band filters but do show up to a lesser extent on the LRGB filters. The others that have reported this issue have Explore Scientific, Stellarvue and Celestron OTAs. The issue is also seen for all three ZWO filter sizes: 1.25” mounted, 31mm unmounted and 36mm unmounted.
I state emphatically that I have no agenda or bone to pick with ZWO. I really like their products and have so far had four cameras from them. I have had some of the best imaging I have produced with their equipment. I really just want to fix this issue so I can take better pictures. I have posted to the ZWO forum on this issue, but so far, have had no response. Part of the issue is that it is not easy to explain and there are many possible causes. I am making this post (or posts since it is a lot of data and some I still have to process and some I am still collecting) so that others who have this same issue can help debug it and I can direct ZWO to the post for the data.
First - my equipment: the data that I am posting is from an ORION EON 115mm APO with a Williams Optics Flattener IV so the scope is at F/5.75. The camera is a generation 3 ASI1600MM-C with the 7 position EFW and the 36mm LRGB and narrow band filters from ZWO. My mount is a ORION HDX-110 EQ-G that I run with EQMOD, CdC, and Sequence Generator Pro. I have an ORION 70mm guide scope with an ASI290MM-mini guide camera and use PHD2 for drift alignment and guiding.
Second – how I take flats: basically, I have tried every way that there is and they all show the same issue. I will show data from using a Aniltak Flatman EM panel that I diffuse further with 1-2 layer of tee shirt fabric giving a LUM filter flat exposure of 0.5s and an SII filter Flat 15.9 seconds. I also have a home made light box with LED and diffusers that I also diffuse further with 1-2 layers of tee shirt fabric giving a LUM filter flat exposure of 0.002s and an SII filter Flat 0.25 seconds. I have also done sky flat with 3-6 layers of tee shirt fabric a LUM filter flat exposure of 0.03s and an SII filter Flat 1.3 seconds. So, this is three different types of light sources with different degrees of collimation, different methods of diffusion, and different exposure times. The exposures times for all three light sources was determined the same way with SGP flat calibration wizard and the flat frame were collected with SGP to give a peak of the histogram in the middle of the response at 25K ADU using 16bit (I will show histrograms of the data). All the calibration was with dark flat frame taken immediately after the flats with the exact same exposure times. I did not use any bias frames. All flats methods basically have the same issue.
Third – exposure settings: For all exposures, light, dark, flat, dark flat, all used a gain of 76, offset of 40, USB of 60 and were all taken at -15C with my ASI1600MM-COOL (ver. 3) camera.
Fourth – image and flat processing methods: The flats were mostly processed with Pixinsight using the method from Warren Keller book but with flat dark specified and not bias frame or scaled dark frames. Light frames were also calibrated, registered and integrated with Pixinsight following Warren Keller book – basically nothing fancy here. Just as a sanity check, I also used ImagesPlus 6.0 and did a full image processing with that package with exactly the same results. I really don’t think that this is a processing issue. My method is basically standard and it is highly unlikely that multiple users would be making the exact same settings errors – not impossible, but I think highly unlikely. If, however, someone thinks this is the issue, I will post all the details of my calibration method with all the setting or screen shots if necessary.
Edited by cfosterstars, 01 December 2017 - 08:46 PM.