• Cameras

  • ASI294MC Pro Flat issues - overcorrection - white corners

I have an issue with my ASI294MC Pro, plus "overcorrecting" in the corners, which end up turning white after calibration. I calibrate with all the recommended steps rigourously. Here is an example (right after pre-processing and stacking)

L-Pro, or L-Enhance or even no filter whatsoever... All my tests have yielded these results. For long, I thought this was because I was unexperienced and was doing something wrong.
I have tried calibrated flats from 10K ADU to 30K, by 2.5K increments. All of them over 4 seconds exposure. I use a spike-a-flat. They all give me these white corners.

I use PI for the whole processing.

I have seen the posts where people claim solving their issues, but I must admit the solutions (which bring nothing new to the table, to me at least) are less than satisfying. Could it be a defect?

BR/

Alex

    Afilion01 "overcorrecting"

    I wonder if this is due to the darks being taken at a different temperature and exposure as your flats. Try taking some "dark flat frames" (i.e., darks that are taken at the same sensor conditions as the flats, and with the same exposure time as your flats).

    Most post processors have a way to include dark flats; they will use them just to compute the master flat, and not used for the light frames. Since they hardly take any time to capture and post process into master dark flats, it is not a bad idea to do it even when you don't see a problem.

    If you don't take dark flats, the post-processor is making some assumption about the dark current (increases linearly with exposure time), and that assumption may not hold well for your camera model.

    (They probably should be called "flat darks," but it has been defacto called "dark flats ." Just keep in mind that the "dark flats" needs same cooling as the flats and same exposure time as the flat frames -- but with no light entering the camera; this is where light leak needs to be suppressed completely.)

    So, try experimenting with using dark flats and check for light leaks in your dark flats while you are at it.

    Chen

        w7ay They probably should be called "flat darks," but it has been defacto called "dark flats"

        There has been a LOT of discussion in many forums about what the proper name is. I guess any opinion is as valid as any other but let’s not discuss that here … again 🤣

        Good advice as always, Chen!

          dark flats/flat darks (!) have been applied. same temperature/duration as flats... which are same temperature/gain as Lights.
          Pre-processing done with wbpp in PI.
          Someone hypothesized that my ADU are too high because of gain. I will shoot between 5-8 ADU tonight and share the result.

          • w7ay replied to this.

            Flats are generally advised to have the peak around the middle of a linear histogram and I disagree with that. Further right is better because that way the dynamic range of the sensor gets better used AS LONG AS THERE ARE NO SATURATED PIXELS. This is independent of gain.

            Having said that, it is possible to shoot flats at a lower gain if you want but that requires dark flats of the same gain, offset, temperature and exposure time as the flats.

              Afilion01 Someone hypothesized that my ADU are too high because of gain. I will shoot between 5-8 ADU tonight and share the result.

              I assume that your dew shield is extended to the identical position while taking lights and while taking flats, right?

              If none of the fixes work, try placing the synthetic flat a little away (5 or 10cm) from the front of the dew shield, and cover the dew shield with some diffuse material (T-shirt held by rubber bands will do for now) to see if the problem becomes better or gets worse. That might help diagnose the problem. I.e., the miscalibration could be caused by from light from the synthetic flat reflecting off of the interior of the dew shields.

              As another sanity check, you can take two sets of flats (together with two sets of dark flats). Use one set to derive the master flat, and use the other set (at much lower exposure to simulate a dark sky) as "light" frames. If the sensor is stable, you should see an evenly illuminated frame.

              For what its worth, I use a Gerd Neumann panel, but on top of that, I also add a Daystar FlatCap diffuser to my objectives when I take flats. But I have never used an ASI294, and your problem may actually be specific to that camera model (unstable sensor) and not to the manner that you are taking the flats. If so, after finding the cause, you may be able to mitigate the problem, but not completely eliminate it.

              Chen

                  I use a good blue sky or an evenly lit wall in my house. Point the scope at it, rubber band a white t shirt over the end of your scope, and just shoot for about half of the available brightness. Your problem looks like a good old fashioned light leak. Your device doesn’t seal all the light off from around the edges where the panel sits on the tube.
                  I use a Newtonian, a 294MC Pro, and ASIAIR. The ASIAIR automatically calculates the flat exposure time.
                  This is a simple cure. Try it, you’ll like it.
                  Use the same exposure time for the dark flats, just cover the objective.
                  I put my whole rig in a lightproof closet when doing any darks.

                    w7ay I read about a gradiant issue with the ASI294 sensor, but usually it seems to calibrate out.
                    It's really the whitish corners that are causing me grief. Thanks for the advice. It's great to know there are people willing to help and share knowledge. I will pay it forward!

                      Horsehead I wrapped the image train in foil last night to try to remove any infiltration. It does make sense. Thanks a lot for your help. Deeply appreciated.
                      I am new to the hobby and I had to learn on my own (basically from the ground up), with covid closing my local club for a while...

                        If you have any more questions just put it in this thread. I may not know much, but what I do know, I learned the hard way, and don’t mind passing it on. I started out imaging with a Nikon F3, Celestron 9.25” SCT, a Celestron CI700 mount(made by Losmandy), Kodak Techpan 2415 film, and my own darkroom.

                        Issue solved!!!
                        It was both light leaks and saturated pixels due to gain...
                        Dont look at the processing... Its lacking data... lots of noise and things I cut corners to validate what you guys suggested.
                        But!!! no white corners... (forget the gradient...)
                        Thanks!

                        3 years later

                        This fucking camera sucks fucking balls. It should never have left the factory without this crap being fixed. how are you supposed to use flats then? Honestly I will never buy another ZWO cam, and therefor no need for the asi anymore. lol since it locks you to zwo crap anyway. Why is this not addressed by zwo? do they give a shit?

                          navyguy
                          Do you have any problem with your ZWO cameras? please feel free to tell me, we will try our best to solve the problem for you. thanks.

                            Write a Reply...