Flats issue in overcalibrating

I’m not sure where to go with this issue I am having, but I will start here since I am using the Flats Calibration Wizard. I typically run 40 flats with an ADU target of 27000. My light source is a flat panel which I have set to its lowest setting. I ran the wizard for 1x1 binning, and I got what looked to me like a good flat image set showing dust and threads in the optical path. Exposure time was 1.97 seconds.

I ran 40 dark flats for the same exposure duration and calibrated the flats taken using PixInsight.

I have put all the flat and flat dark frames into a shared Dropbox folder: https://www.dropbox.com/sh/d0mx1765m34fck7/AADPBSjTixZ9CdiFoErtBXu6a?dl=0

Here is a shot of the image after calibration and stacking

The circled area is where the obstruction is in the flat frames. If I process the same subframes without the flats, this does not show up. Camera is a ZWO ASI1600MC.

Getting flats right has been a challenge for me. I hope someone can point out what I am doing wrong.



Any chance that any of the components in the optical path were rotated between light and flats?

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I don’t believe so. I took the flats immediately after the imaging session completed without any power cycles.

I seem to have this issue whenever I image with the Radian Triad Ultra Quad-band filter.

Are any of filter slots next to that filter open? I had a similar issue with my H-alpha filter that went away when I loaded a “dark filter” or anything other filter next to it. There was a open spot while the issue was happening. I have a 9-position ATIK EFW-2 setup.

I’m not using a filter wheel in this case. I am using a filter slider. I don’t think it is a reflection because it was there before and after the imaging session, and disappeared after I cleaned the optics. So it should have been there when the light frames were taken. The issue is it seems to be adding the obstruction by using the flats during calibration rather than removing it. If I process the lights with only darks (no flats) the obstruction is not visible.

What kind of flat panel is it? I have one of those from off of Amazon that is used normally as a tracing pad. I ended up having to use two opaque plastic squares and some printer paper. That helped a lot with some over correction issues I was having with my Lum filter that looked different from your issue. Seemed the lower the brightness of the panel, the worse things got, hence the panels to diffuse it to force the brightness up. Hope that makes sense.

That makes sense. I am using one from Amazon and I was setting the illumination to the dimmest point. I tried again after last night’s imaging session with a slightly brighter illumination and it seemed to work just fine.

I also have a Spike-a-Flat, but it is sized more for my 14" RC and doesn’t fit well for my 400mm Skywatcher Esprit 80.

I ask Sam (with ZWO) what adu he suggested for the ASI10600MM C pro as I use two of these. He said around 10,000. I too use a flat panel and find that an ADU of 12,000 works great in my case.

Thanks for your response! Wow, 12000 is significantly lower than what I’ve used so maybe thats the answer. It just seems like there is no rule about how to determine what the value should be.

I totally agree about the adu settings for CMOS sensors. I was using the same settings, in the 30,000 adu neighborhood which work great with my Atik383L CCD camera. I sent Sam an email and was floored with his response.

I’m also having all matter of fun and games with my flats.
I use a Alnitak Flat Man Large and with my NII, Ha and SII filters the exposure duration for an ADU of approx 20000 on my QHY600M is upwards of 65s and they look god awful.

Some advice here would be great

I use the histogram to find the ADU value for my flats. You want to be around 1/3 to 1/2 from the left.

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I’ve been trying to get the histogram left of center but the flats seem to be overcalibrating. I’ll try to push it further left. But is there a recommended exposure time for flats?

The exposure setting will depend on the brightness of the light source. Short answer, no. If your light panel is to bright, you could try the white T shirt method to attenuate the brightness and softening light over all.


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I use a flat panel (tracing pad) on the lowest setting, white t-shirt and try to position the histogram to left of center

I have an OSC as well as a mono camera, and the problem with the OSC is that the R, G and B pixels all have completely different peaks. The R or B can be down at 10,000 while the G can be up at 45,000, (it is made worse if I have something like the IDAS D2 LPS filter in line) so I have to do a balancing act between them. I compensate (at least in my mind) by taking 50 or more flats so that I can reduce the noise in the stack for the lower signal pixels. According to SharpCap, the sensor is linear over a very wide range of values, and (maybe I’ve been been lucky but) I’ve not had overcorrection problems for ages.

Thanks Terry, very interesting. So what do you typically target your ADU when doing 50 or more flat frames? I understand that more frames improve the SNR but do you lean more toward the R and B ADU?

On my ATIK 383 I would aim for 25-30k on all filters. On my colour cameras, I aim for at least 10k on the lowest colour channel as long as my highest channel does not go beyond 50k, and that seems to work for me. With the colour sensors, you need to be careful that you don’t get fooled by seeing a peak at 30k, because that could be the red and the blue and green could be off the right hand side of the screen!