How to fix this bad flat field?

[Nicola (Nicola)]

Dear All,

my imaging train is as follows:

GSO RC10Truss+Moonlite Focuser+Adapter+SX-AO-LF+QSI583wsg

I am getting a flat field like the one attached. Using this flat field during the calibration of the light frames is useless, the filed gets flat around the center but the corners has till some kind of reverse vignetting.

Mind that:

- I already put flocking paper inside baffle tubes, focuser draw tubes, painted with opaque black the adapter.

- I already tried to put a black card in front of the SX-AO tilting glass.

- I already tried sky flats

None of this, altogether, solved my issue.

I am running out of ideas, any hint*is appreciated, especially if you have more or less my same imaging train

Thanks

Nicola

[Exfso (Peter)]

Nicola, sounds like you need a light box. Have you tried the white tee shirt trick for flats? I make light boxes for people here in Australia. There are all sorts of home made versions around that should help. If you do a google search you should find suggestions. Reason I started making them was they cost an arm and a leg to purchase from a company in the USA, who I believe are no longer doing them anyway, so I decided to experiment and make one for myself. People got wind of this and wanted me to make light boxes for them. Unfortunately the freight to send overseas cost damn near as much as the light box, so it becomes uneconomical.

[Nicola (Nicola)]

Originally Posted by Exfso

Nicola, sounds like you need a light box

Hi Peter, I do have a light box, but unfortunately it doesn't solve the issue.

[glend (Glen)]

It looks dark and unevenly lit to me. There also appear to be many dust motes. How are you shooting these flats?

[multiweb (Marc)]

This could be some kind of internal reflection. Try to dim the light and lengthen the exposure see if it does the trick.

[Nicola (Nicola)]

Originally Posted by multiweb

This could be some kind of internal reflection. Try to dim the light and lengthen the exposure see if it does the trick.

I tried also with sky flats (very very dim), with same results

[Nicola (Nicola)]

Originally Posted by glend

It looks dark and unevenly lit to me. There also appear to be many dust motes. How are you shooting these flats?

don't get confused by the jpeg brightness it was only to show the pattern (the big dark ring) that I can't get rid of.

[troypiggo (Troy)]

Perhaps it would help if you posted flat and light frames (pre and post calibration)?

What does the flat histogram look like?

[lazjen (Chris)]

Do you use a flattener/reducer in your setup?

[RickS (Rick)]

Originally Posted by Nicola

my imaging train is as follows:

GSO RC10Truss+Moonlite Focuser+Adapter+SX-AO-LF+QSI583wsg

FYI, I have previously used a similar image train:

GSO RC10 (non truss) + FLI Atlas + SX-AOLF + SX H-18

My flats looked normal and worked correctly.

[Nicola (Nicola)]

Originally Posted by troypiggo

Perhaps it would help if you posted flat and light frames (pre and post calibration)?

What does the flat histogram look like?

Ok, tonight I'll post them. Thanks!

[Nicola (Nicola)]

Originally Posted by lazjen

Do you use a flattener/reducer in your setup?

I don't use a reducer/flattener so far.

[Flugel88 (Michael)]

Nicola wondering what ADU are you aiming for with your flats and what exposure time?

could also be a stray light issue like i have with my GSO RC 14.

[Nicola (Nicola)]

Last and final update. I'm glad to report that I was at last successful. The bright ring was caused by the SXV-AO-LF T-adapter. Putting flocking paper inside this small adapter apparently solved everything! I can't wait to try it under the stars now!