Flat frames all have a band on edge of frame

[Andrew INT]

I have begun capturing dark and flat frames for my wide-field DSO imaging. (Equipment WO 73mm Zenithstar APO, cannon 250D DSLR, software, DSS). Whenever I take flat frames (either using an LED panel or the blue sky), I have a distinct band at the top of the frame. Is this due to the camera shutter or the mirror getting in the way? 

Any advice welcome!

Andrew

IMG_0494.CR3

Edited October 16, 2022 by Cornelius Varley
image added

[Cornelius Varley]

Can you upload a version of the file in either jpg or png format so that it can be viewed within the browser.

[Andrew INT]

1 hour ago, Cornelius Varley said:

Can you upload a version of the file in either jpg or png format so that it can be viewed within the browser.

Hi Peter - I don't appear to be able to upload the file in either png or jpeg - I just jet a 'contact us' message. Will try again later.

[Andrew INT]

I can't upload any files at the moment for some reason. Please see the screenshot below.

Many thanks,

Andrew

[Cornelius Varley]

Added for you

[ollypenrice]

You have a top to bottom gradient, dark at the top. This is fine if it is also there in your astrophotos, but is it? An easy way to find out is to open a linear image in Photoshop and go to Image, Equalize. This will give an extreme stretch in which the gradient will show, assuming you've used an image which has more stars than nebulosity. Other programs may offer the same thing.

However, I'd be very suspicious of that as a flat. It's too flat, no vignetting, no bunnies. That can't really be right.

Olly

[ONIKKINEN]

This looks like a bias frame taken with a flat panel, something is wrong with the method of taking the flat. Looks like you took this with a 1/2000s exposure and aperture priority, so your flat panel must be obscenely bright for the camera to go down to 1/2000s in aperture priority mode where the camera takes the wheel and exposes properly. But also the flat just looks plain wrong, with a straight linear gradient top to bottom and no signs of vignetting. I suppose this could be some kind of rolling shutter thing with the extremely fast exposure time? Whatever the cause, this is not a flatframe that can be used.

Below are your flat with the 250D and my old flat taken with a 550D, see the difference. They are rendered in false colour rainbow mode in Siril to make the brightness differences pop out visually. Reds are bright, purples are dim (relatively).

Yours:

My old 550D flat:

Looks nothing alike, so something has gone wrong.

[Andrew INT]

Thanks Olly and Onikkinen,

I had taken the original flats the following morning using an LED panel against the aperture - with no white tee-shirt in-between. As you have pointed out, it was a very fast exposure of 1/2000s. I tried again, this time lowering the LED as much as possible and using two layers of tee-shirt. This increased the exposure time (using AV) up to 1/100s and looks to be a more even frame. Does this look more usable now? I'm guessing the dark banding in the first attempt was due to the shutter?

Many thanks once again!