is this a normal newtonian flat image?

[blinky]

im trying to gather some flats for last nights Witches Broom image Im going to process, does the flat below look normal?  Do newts always show the shadow of the secondary in the flats?

[andrew s]

No this is not normal. Is the centre over exposed?  You should not see a shadow of the secondary  but I doubt that is what you are seeing.

It should look like another flat brighter in the middle with some vignetting towards the edges.

Regards Andrew 

Edited September 28, 2019 by andrew s

[blinky]

I never have much luck with flats - either I get a nice smoot image or I get the above!  This is 25 of the subs stacked and its still there (unsurprisingly!) - the image stats show its not overexposed:

integration

K

count (%) 100.00000

count (px) 16389120

mean 28470.748

median 28769.440

avgDev 2090.349

MAD 2031.027

minimum 22828.140

maximum 33058.464

[andrew s]

How are you taking them? 

[blinky]

a white pillow case over the front of the scope and the scope pointing at the sky, using the SGPro flats wizzard

[andrew s]

Is it a closed tube? The only things I can think of are stray light or reflections.

Regards Andrew 

[Scooot]

Just a thought, probably not this but could the camera have an auto focus on which is causing it to try and focus on the secondary? 

[blinky]

Nah focuser is  not connected 

[Peakedge]

On 28/09/2019 at 12:25, andrew s said:

Is it a closed tube? The only things I can think of are stray light or reflections.

Regards Andrew 

I had a similar problem and found light was entering from around the primary. Quickly cured by covering the mirror end with a few thick bin bags. Didn't notice at night because no ambient light!!!

 

David

[blinky]

19 minutes ago, Peakedge said:

I had a similar problem and found light was entering from around the primary. Quickly cured by covering the mirror end with a few thick bin bags. Didn't notice at night because no ambient light!!!

 

David

Good thinking!  Bet thats what it was as it was a cracking sunny day (for a change!)