ASI294MM Pro Flats from ASIAIR Plus

keramos

Hello,

Does anyone know why I would get such a circular pattern (and it is pixelated when zoomed in) when taking flats with ASI294MM Pro and ZWO filters (LRGB SHO), didn't happen before.

Side by side image, one on right is zoomed in view of the circular pattern.

Camera is cooled gradually to target temp.

w7ay

keramos I would get such a circular pattern

I recommend doing a test flat without cooling the camera, after letting the camera rest for a few hours from the last time you cooled it.

If the "warm" flats do not exhibit a problem, you could be having icing problems (not the hockey kind :-) from too much moisture in the camera; in which case, it is time to replace the desiccants.

That being said, you can take flats with any reasonably bright source that does not require you to use long exposures. In which case, there is no need to cool the camera when taking flats, eh? The dark current noise increases linearly with exposure time; so there should be no reason to cool the sensor with 1 or 2 second type of exposure.

Chen

keramos

w7ay Thanks Chen

I'll try your recommendations.

keramos

w7ay

Hi Chen, I haven’t tried your recommendation as yet. When I was taking flats from last nights session, I didn’t get any circular patterns or weird pixelation due to icing.

Though I did leave cooling on for quite a while before I took any flats.

Next, tomorrow I’ll try your suggestion. Hopefully clouds won’t roll in tonight..for now on to my latest question for the forum.

Nihal

w7ay

keramos Though I did leave cooling on for quite a while before I took any flats.

There is no reason to use cooling for flat frames. The cooler only affects the dark current noise of the sensor, and not the vignetting.

It is more important that the OTA tube and objective lens glass are at the same temperature as when you took the light frames. And even so, for most telescopes, the small temperature related changes do not cause a visible change in vignetting and size of dust spots (which is the reason you take flat frames).

You can test if your flat frames are temperature dependent by taking two sets of flat frames, with the focuser nudged by 50 µm to 100 µm in between. If there is no visible difference, you need not worry about temperature effects when taking flat frames.

Chen