Image brighter in the centre

[Druid]

So I was having a bit of a dabble last night, trying to image M81. Got a lot of things wrong that I do understand (5 min unguided exposures on poorly aligned EQ6, forgot to switch DSLR out of JPG mode etc)

I've got an issue that I don't understand though, the centre of the frame is significantly brighter than the sides. it looks too gradual to be vignetting.

It's a sort of circular gradient right across the frame, rather than something that happens only at the edge. So it looks like a glow in the middle.

Something optical presumably? Telescope is a 4" refractor.

[Psychobilly]

Dont worry there's nothing wrong... You will need to look at taking and using flat frames in your post processing...

Peter...

[Druid]

Aha, thanks. Reading up on flat frames now

[Psychobilly]

If your using Deepskystacker then the online help files will cover the various calibration files used in the stacking process. The terminology it uses for naming the various calibration images is slightly different to that used in CCD centred apps like Maxim, Astroart etc.

Get to grips with Flats,Darks and Bias frames and the camera settings and setup needed to capture them first...

Peter...

[umadog]

But the requirement to take the flat frame to get rid of this effect is ultimately due to uneven illumination, isn't it? This is a refractor, so the uneven illumination isn't due to an undersized secondary mirror. What, then, is it due to?

[Leveye]

Use a Gradient filter as well in post something like Gradient xTerminator

[Druid]

But the requirement to take the flat frame to get rid of this effect is ultimately due to uneven illumination, isn't it? This is a refractor, so the uneven illumination isn't due to an undersized secondary mirror. What, then, is it due to?

Having done a bit of reading on this and looked at flat frames from various refractors (including some very fancy ones) online, I think it actually is vignetting, due to the chip size, camera placement etc. and exaggerated by relatively long exposures.

I didn't recognise it as such because I'm more familiar with terrestrial photography and it looked too gradual to fit my preconceived idea of what vignetting looks like.

Thanks for all the help.

[sloz1664]

Having done a bit of reading on this and looked at flat frames from various refractors (including some very fancy ones) online, I think it actually is vignetting, due to the chip size, camera placement etc. and exaggerated by relatively long exposures.

I didn't recognise it as such because I'm more familiar with terrestrial photography and it looked too gradual to fit my preconceived idea of what vignetting looks like.

Thanks for all the help.

Welcome to the world of Astrophotography. :eek:

It is a world apart from terrestrial photography. A steep learning curve.

You will find you will have to takes flat frames on any scope, as well a bias frames and dependant on your

camera dark frames as well. There are many threads on typical imaging procedures on the SGL.

cheers

Steve

[ollypenrice]

But the requirement to take the flat frame to get rid of this effect is ultimately due to uneven illumination, isn't it? This is a refractor, so the uneven illumination isn't due to an undersized secondary mirror. What, then, is it due to?

All telescopes have an image circle (or light cone if you prefer) and that image circle doesn't end with a hard edge, it fades out. Vignetting, therefore, is a characteristic of absolutely all optics, not just reflecting ones. Even the Takahashi FSQ astrographs vignette, though not by much. However, when we stretch in post processing to find the hidden faint detail we also stretch the vignetting. Flats are essential - and sometimes cussèd!

Olly

[jgs001]

As has been said it sounds like Vignetting and flats will deal with it. A 2" focusser or larger will help reduce the effect of the vignette on the image. It sounds like you probably have a 1.25" focusser on the scope (or using a full frame camera). Even with a 2" focusser, if the drawtube is very long this can impinge on the image circle for larger sensors.

[umadog]

Ah, thanks guys. I understand now.