Roscoe,
I recently completed my first imaging session (M101). Took 89 Subs, and 30ea darks, flats, and dark flats. The flats were taken at 3s (per other discussions about the 294 not liking short exposures), all at the same gain as the lights, using an iPad as the light source with several sheets of paper to control the brightness.
Problem was my flats showed the unfortunately common characteristic of red splotches (no filters except UV-IR L2). I used a trial version of APP to preprocess them and the final result was awful. I emailed ZWO and was told by TJ Connelly that this type of flat was "normal" and would calibrate out. Since I failed, I dropbox'ed my files to him and he took a crack at them...and he also failed. He suggested that using an iPad was a likely cause of the goofy flats, claiming he had similar struggles until he switched to a Spike-a-flat. In subsequent testing, I moved the iPad around, tried a T-shirt instead of paper, and adjusted brightness on the iPad itself, all gave me the same results (proving to me that uneven lighting was NOT the problem).
I would greatly appreciate if somebody could take a crack at my images and hope for better luck pre-processing them (I can share them from my dropbox)
(That said, how do other 294MC users do their flats? If I'm going to live stack on my AA+, I'll need to shoot them first which could mean after dusk, meaning an illumination source could be required)
At this point I'm trying to convince ZWO the sensor is bad and wanting to return it.
Thanks a bunch for posting the link to your files. I did not download all of them and instead grabbed 5 of each type to run some experiments. This was very, very interesting to me as I have worked with others on IMX294 red pattern issues before but have rarely had any such issues with my own copy of this camera.
I will outline my experiments below but the net is that I think your Flats are at fault in this case.
The IMX294 sensor appears to have an issue with narrow-band light at longer (red) wavelengths. This has been documented here on CloudyNights in many threads. The most obvious such cases of fixed red channel patterns are seen when using longer wavelength narrow-band filters such as Ha, L-eNhance, L-eXtreme, and Radian Triad tri-band and quad-band filters. The issue is thought to be caused via a process called "etaloning" -- an interference effect of light. It shows up as light and dark patterns where the light is interfering with itself at the surface of the sensor at some wavelengths.
There have been some reports from users who have seen the effect to some extent or other even when not using a narrow-band filter of any sort. This is the case you are seeing. I have not had the opportunity to closely examine a set of files such as you supplied. It has sort of strengthened one of my suspicions I have had about the effect but was never able to reproduce.
The seed of the idea (or suspicion) I have had is that the nature of the light source used for taking Flat frames might cause the effect to show up. My thoughts were that if the Flat light source contained either narrow-band light at longer wavelengths or possibly had some specific polarization characteristics, it might cause the etalon effect to appear in the Flats even though the Lights used no filters.
Your use of an iPad might be behind the issue you are seeing. LCD screens, in general, are very polarized light sources. (Put on a pair of polarized sunglasses and look at your iPad while turning it 90° and you will see the polarization at work.) I know lots of others use LCD screens without issues but I have always had this nagging suspicion, nonetheless.
Here is what I did as an experiment in PixInsight:
Part #1:
- I integrated 5 of your Dark frames to make a Master_Dark
- I integrated 5 of your Flat-Darks to make a Master_Flat-Dark
- I calibrated 5 of your Flat frames using the Master_Flat-Dark
- I integrated the 5 calibrated Flat frames into a Master_Flat
- I calibrated 5 Lights with the Master_Dark and Master_Flat
- I deBayered the 5 calibrated Lights
- I aligned the 5 deBayered Lights
- I integrated the 5 aligned Lights
- I ran ABE to remove a linear gradient (only) from the integrated result
- I ran DBE to remove any residual vignetting (only) from the result
- The following shows a very stretched view of the result of this processing.
It shows the same fixed Red pattern you are seeing in your processing.
Part #2:
- Pick up the Master_Dark from Step #1 of Part #1
- I calibrated 5 Lights with the Master_Dark only -- No Flats
- I deBayered the 5 calibrated NoFlat Lights
- I aligned the 5 deBayered NoFlat Lights
- I integrated the 5 aligned NoFlat Lights
- I ran ABE to remove a linear gradient (only) from the integrated result
- I ran DBE to remove the significant vignetting (only) (from not using Flats) from the result
- The following shows a very stretched view of the result of this processing.
I used a similar strength of stretch to match that done in Part #1 of the experiment.
This version without the Flats shows none of fixed Red patterns seen before.
There are still remnants of dust motes and residual vignetting that DBE did not fully remove. (And DBE cannot remove Dust Mote shadows.)
From this experiment, I would conclude that the Flats are the source of the fixed red pattern in your image. Even though this fixed pattern red issue usually comes from the use of narrow-band filters, it appears to have originated in your Flats and then been imparted onto the Lights during image calibration. (It was seen even in the single calibrated Lights when I used your Flats.)
To further investigate, I would suggest a next step where you try a new method of taking Flats with a different light source. You indicated that you had taken Wall Flats and they looked the same as your iPad Flats but I wasn't able to understand if you only compared iPad Flats to Wall Flats or if you ran through the full imaging calibration process with both and compared the end results.
As I mentioned, I have not really seen this issue with with own ASI294MC-Pro but then I always used a Flat light source which used indirect reflected LED light inside the flat field device. I have also used Wall Flats without this sort of fixed red patterning.
John