ASI 1600MM-Pro darks - weird square in frame?

[SiD the Turtle]

Hi folks,

I picked up an ASI 1600MM-Pro kit, with the electronic filter wheel etc. This is attached to my William Optics ZS73 II which until now has only been used for DSLR.

Today I'm building my darks library and noted a weird artefact when stretched. I assume there's something in my image train, but I've never seen it before switching cameras (though maybe I missed it with the DSLR?), and there's nothing occluding the filter wheel as far as I can tell. Any ideas? More visible in the lum and red channels, and not perceptible (to my eyes) in blue, green or narrowband. Tried at ambient temperature and down at -20, same thing. Starts to become visible in any exposure over 15s. All pictures are auto-stretched from PixInsight.

Also apart from that do they look otherwise okay?

Luminance:

Luminance boosted stretch:

Red:

Green:

Blue:

SII:

 

[vlaiv]

Why do you have different darks for red, blue and green?

Darks don't depend on selected filter - they depend only on exposure length (and other parameters - like gain, offset and set temperature). It is customary to have same exposure length for R, G and B (and often for L as well) as it simplifies things.

If you are taking darks on your scope and using filter wheel and for some reason changing filters for each set of darks (which btw are not supposed to have any light signal so using filters is sort of moot) - then I suspect you have light leak. Most probably IR light leak as IR can pass thru materials like plastic.

[SiD the Turtle]

Yeah not intending on doing separate darks, just trying combinations to work out the cause. You're right though about IR, seeing as it's visible on the broad spectrum and red filters. I'm doing it attached to the rig, will try disconnected with a cap on and perhaps covered in something else to minimize light leak.

[vlaiv]

Just now, SiD the Turtle said:

Yeah not intending on doing separate darks, just trying combinations to work out the cause. You're right though about IR, seeing as it's visible on the broad spectrum and red filters. I'm doing it attached to the rig, will try disconnected with a cap on and perhaps covered in something else to minimize light leak.

I found that I get the cleanest darks if I take camera off, put cap on it and place it "face / sensor down" on wooden table .

[SiD the Turtle]

On 28/08/2021 at 15:35, vlaiv said:

I found that I get the cleanest darks if I take camera off, put cap on it and place it "face / sensor down" on wooden table .

Cheers vlaiv, did exactly that, no artefacts.