Flats issue - Samyang 135mm

[Speedmaster]

Hello,

 

i'm starting to get my hands on some AP and i'm enjoying it so far. But until now i've been using only lights with no DOF. You all know that the samy 135mm has a lot of vignetting, so i figured: let's take some flats to correct that.

Problem is my flats are not very clean. See below an example, taken at 100ISO F2.8 and histo at 2/3. The square pattern is due to my xtrans sensor, it goes away after process in siril. My concern is that smokey-cloudy pattern at the center, any idea of what that could be?

 

I took the flats a couple of days after my astro session with the white t-shirt technique pointing at the sky.

 

thanks

 

 

[Jerry Barnes]

Do you mean the variation in lightness across the centre circle, a bit like clouds?  Is your teeshirt stretched uniformly and if you take another set of flats do you get the same pattern or a similar one?

What do you get if you compare a stack with and without the flats?  

I'm having trouble with flats so its an interesting topic.

[carastro]

The histo should be 1/3 to just under 1/2 way across. 
 

Are you pointing at a bit of sky with no obstructions ie no branches etc in the light path.  I normally use 2 - 3 layers of either white paper or a folded pillowcase.  Make sure any fabric is flat  

l use the same lens but a CCD camera.  
 

Carole

Edited May 7, 2022 by carastro

[Speedmaster]

Thanks both for your feedback.

 

Yes jerry i mean the variation in light throughout the circle. My t-shirt was stretched uniformly and tight. I got the same pattern the day before using the same combination. If i integrate the master flat in my images, i can see the variation in light across the final image and the vignetting is not gone, it's accentuated with brighter corners.

 

carastro, My histo looks like is ok. If you separate it into 5 parts, i would be at the start of the 4th part. No obstruction in the sky, no clouds..

 

i tried another technique with the white iluminated screen of an ipad on my lens. This time i took them right after my astro session, so every setting exactly the same. It's better but i still see this cloudy pattern, and the integration of the master flat in my stack is a disaster with a reverse vignetting effect.

 

Maybe i'm missing something in siril. But i'm following step by step every tutorials around.

 

 

Edited May 7, 2022 by Speedmaster

[900SL]

What camera? Are you using a step down ring? Do you have a filter installed? Any marks on the sensor or back of lens? Was it cloudy ( ignore that.. )

[Speedmaster]

Camera is a fuji xt20, no step down ring, and a filter is installed on the last photo (not on the 1st)

 

no marks for what i know

 

thanks

[ollypenrice]

You're not trying to take a pretty flat! The idea is that the flat capture the irregularities in the illumination of your system. What you are showing, here, is not a remarkably irregular flat but a remarkably regular one. If you keep getting similar irregularities that just means tat they are there, which is what you want your flat to deal with.

Over correction is a problem sometimes and not always easy to fix. Are you dark-subtracting your flats as you stack them? (Use darks with the same settings as used to make the flats but with no liggt getting in. Blocking all light can be very difficult. Or you could try using a master bias as a dark for your flats.

Olly

[Speedmaster]

ok, after a little research, i think i found the problem with the over-correction of my flats: i don't treat them at all, meaning that i don't substract the offset, neither from the flat or the lights.

I need to get the offset out in order for the vignetting to be corrected. Will try this and come back with the results.

 

 you were spot on ollypenrice regarding my flats treatment 😉 thank you

[Jerry Barnes]

Thank you, the point about offsets put me on the right track with my problem.