Bad Flats Calibration

@regpratt This seems to be an issue with the master flat. How do you create the flats? Are you making sure that the peak of the histogram on the flats is (more or less) in the center? Do you shoot flats every time that you shoot lights or do you reuse them? Do you calibrate the flats with dark flats or with bias (or both)? Are the gain and offset (or iso) of the flats and dark flats and/or bias frames equal? Are the flats shot with the same software and driver version as the lights, dark flats and/or bias?

@wvreeven

I use a USB drawing pad with a white t-shirt between it and the OTA. I use NINA with allows you to set target ADU and Nina exposes until that ADU is found. My target is 20-30k with the asi294mm. Flats shot at the end if session same temp, gain, and offset as the lights and are calibrated with dark flats that match the flats, as well as darks that match the lights. All calibration frames are made in NINA. 

I have noticed that this seems to only happen when using a uv-ir cut filter. I can't recall a time it's ever happened with narrowband filters. Should I start by re-shooting or re-mastering the flats? 

@regpratt I have had flat issues using an iPad in the past as well. I am afraid that the spectrum emitted by those devices is not representative for the sensor responses or filter throughput. Using natural light (essentially taking my telescope inside and pointing it at a white door that gets illuminated by light entering through windows while the telescope is covered by a white t-shirt to mock a sufficiently dispersed light source) solved my issues. I tried using a cloudy sky as well and found that it is much harder to regulate the light level. Indoors this can be done by opening and closing curtains and doors etc.

@wvreeven

That's odd because I use the same pan for all my scopes and cameras and it only seems to be an issue when doing Luminance. I would have thought the spectrum the panel gave off wouldn't matter as much since the camera is mono. Also is weird that this only happens occasionally. Most of the time Luminance lights calibrate fine. And it never seems to happen with narrowband filters. I guess I can just try sky flats. 

@regpratt Being mono just means that the camera doesn't separate colors in the pixels. Each pixel is sensitive to the entire spectrum as opposed to color cameras that have R, G and B sensitive pixels. But each pixel has a specific quantum efficiency curve that depends on the wavelength. The curve for your camera can be found here:

https://astronomy-imaging-camera.com/product/asi294mm-pro

and as you can see the sensitivity for different wavelengths varies. Is it possible that the L filter lets pass through UV or (more likely) IR radiation that falls outside the pass band plot for the filter which gets picked up by the sensor and adds to the signal? This has happened to me when I used the iPad and that for me was the single reason to stop using it for my flats. I know that that doesn't explain all the behavior you see but it may still be a contributing factor.

I tested this by pointing a remote control at the sensor with the L filter in the light path and I noticed that the sensor detected the IR radiation coming from the remote 🙂

Posted by: @wvreeven

@regpratt Being mono just means that the camera doesn't separate colors in the pixels. Each pixel is sensitive to the entire spectrum as opposed to color cameras that have R, G and B sensitive pixels. But each pixel has a specific quantum efficiency curve that depends on the wavelength. The curve for your camera can be found here:

https://astronomy-imaging-camera.com/product/asi294mm-pro

and as you can see the sensitivity for different wavelengths varies. Is it possible that the L filter lets pass through UV or (more likely) IR radiation that falls outside the pass band plot for the filter which gets picked up by the sensor and adds to the signal? This has happened to me when I used the iPad and that for me was the single reason to stop using it for my flats. I know that that doesn't explain all the behavior you see but it may still be a contributing factor.

I tested this by pointing a remote control at the sensor with the L filter in the light path and I noticed that the sensor detected the IR radiation coming from the remote 🙂

But if the panel was the issue wouldn't you think I would have this calibration issue every time instead of only sometimes? 

Posted by: @regpratt

But if the panel was the issue wouldn't you think I would have this calibration issue every time instead of only sometimes? 

I guess that depends on a lot of factors. If the panel has not been used for some time, it may have cooled down enough to reduce the IR emitted by it. Not sure if that's enough of an explanation. I'm just stating that it worked for me. Perhaps you can try and see if it works for you too?

Posted by: @wvreeven

Posted by: @regpratt

But if the panel was the issue wouldn't you think I would have this calibration issue every time instead of only sometimes? 

I guess that depends on a lot of factors. If the panel has not been used for some time, it may have cooled down enough to reduce the IR emitted by it. Not sure if that's enough of an explanation. I'm just stating that it worked for me. Perhaps you can try and see if it works for you too?

I will try taking new flats using the sky and I'll report back, thanks.