The theory behind Flats?

[Dave_D]

@olly

hete's the last image i took plonked in fits liberator to show the vignette

and here's what i got when i tried to make flats with APT. now, i know there's a dust bunny in the top right corner, but there's nothing in the 'flat'

something's very wrong....

[ollypenrice]

Yes, that flat is simply wrong and clearly has not succeeded in replicating conditions applying to the capture of the lights. As I say, I've had simlilar experiences. You'll need a complete rethink on how to shoot flats, starting over from scratch.

Olly

[dph1nm]

I know Olly doesn't like them, but I recommend 1-2 sec exposures of the twilight sky, especially if you are imaging at long focal lengths (where real gradients in the sky don't cause a problem).

NigelM

[ollypenrice]

I know Olly doesn't like them, but I recommend 1-2 sec exposures of the twilight sky, especially if you are imaging at long focal lengths (where real gradients in the sky don't cause a problem).

NigelM

I just find them hard to get right! I know they work a treat for most people.

You have to watch for light leaks on manual filterwheels and Newts, though.

Olly

[Earl]

Do you cover the mirror end of the scope when taking the flats to make sure there is no light leaking in down there?

edit:

any light leaking around the focuser?

shiney drawtube?

[Dave_D]

Do you cover the mirror end of the scope when taking the flats to make sure there is no light leaking in down there?

edit:

any light leaking around the focuser?

shiney drawtube?

No leaks as far as i can tell.. the ota was sat on a folded towel and the panel i use was mounted on a wooden ring that fits ont the end of the ota. I sprayed the drawtube with blackboard paint so, apart from where the bearings roll, it's internally, completely matt.

@olly, i think i may need to invest in a proper A3 EL panel.

[Earl]

download the trial of seqence generator pro and try the flats calibration wizard.

[ollypenrice]

No leaks as far as i can tell.. the ota was sat on a folded towel and the panel i use was mounted on a wooden ring that fits ont the end of the ota. I sprayed the drawtube with blackboard paint so, apart from where the bearings roll, it's internally, completely matt.

@olly, i think i may need to invest in a proper A3 EL panel.

Beware of 'completely matt.' It may not be, though I don't know. This is interesting; https://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

Personally I don't think that's the problem because I've had your kind of bad flat and sorted it without any repainting. I just changed the system completely. I don't know what I do to make them work but for me doing flats in the dark seems to be essential. I'd love to know why...

Olly

[Ibbo!]

try this if you are doing them in daylight works for me

cover the light panel with a towel or sheet of dark cardboard

Steve

[Alien 13]

I do think some of the issues with light panels is that the light is very directional i used to do a lot of precision measurement of colour and luminance related to the aerospace and defence industry some of this involved simulating ambient light and its effect on the colour of a display the results indicated that to create a true flat white field it was necessary to bounce the light source off a precision reflective plaque ensuring that there was no direct path to the spectral photometer.

The practical soulution for the amateur would be a diffuser (white T shirt or similar ovet the panel as Steve suggested) or using the panel light indirectly bounced off a painted wall or target.

Alan

[russellhq]

This might be a daft suggestion, but what happens when you take a dark with the camera attached to the scope? Do you get any light leak? This test would tell you if there is any stray light in your system.

[Dave_D]

This might be a daft suggestion, but what happens when you take a dark with the camera attached to the scope? Do you get any light leak? This test would tell you if there is any stray light in your system.

here's a 240s dark taken a few days before that 'flat'. gave it a good stretch and it looks just fine to me so i'm sure there's no leaks in the ota.