What's up with my Flats?

[AstroJOE]

I decided to try adding some Flats to a batch of pre-taken Light, Darks and Biases. They made a noticeable improvement to the final stacked image that I got from DSS but one thing which struck me was that the individual Flats looked a bit odd......an area of darkness in the top right and left corners which isn't present in the bottom left and right corners. My best guess is that this may be an indication of some 'Tilt' between the SCT and DSLR sensor, although that's just a guess as I'm no expert. I've included 1 of the Flats to show you what I mean. Anyone able to help?

 

F_2016-02-21_13-04-02_0001_ISO800___16C.CR2F_2016-02-21_13-04-02_0001_ISO800___16C.CR2

 

James

[AstroJOE]

I've inserted the FLAT Frame to save having to click on and open the file. Any advice welcome.

James

[Oddsocks]

It is usually because the optical axis is not the same as the mechanical axis, with SCT's it is often caused by mirror flop which is why it is important to take the flats using the same RA and DEC coordinates that the image was taken with so that the mirror has the same tilt it had when the image was captured.

As long as this procedure is followed and the flats correct the vignetting then it is not a problem.

[michael8554]

As above, but just possibly your light source isn't "wide" enough to fully illuminate your 11" aperture?

Try one with the camera rotated 180 degrees ie inverted.

Michael

[AstroJOE]

7 hours ago, michael8554 said:

As above, but just possibly your light source isn't "wide" enough to fully illuminate your 11" aperture?

Try one with the camera rotated 180 degrees ie inverted.

Michael

I used the overcast sky as a light source as I'd read somewhere this was an option. Not sure there is anything bigger than that ? I'll try what you've suggested though and see how they come out.

Thanks

[Pompey Monkey]

Is the flat stretched too?

Stretching a flat with reveal the most minute contrast details even if the illumination is even to within a fraction of a percent.

You flat looks fine to me.

[AstroJOE]

12 minutes ago, Pompey Monkey said:

Is the flat stretched too?

Stretching a flat with reveal the most minute contrast details even if the illumination is even to within a fraction of a percent.

You flat looks fine to me.

No, this was a single RAW Flat frame that I converted to a JPEG to post. My initial thought was that my imaging path may have a fraction of tilt causing the DSLR sensor to be angled very slightly. 

[ollypenrice]

Theory and practice have an irksome habit of diverging in so far as flats are concerned. You are right to be doubtful about your flat. For reasons I have never been able to identify I sometimes find a gradient in my flats and throw them away. I then try again and find the gradient gone. This has been going on for so long that I'm past caring about the cause! In your case the gradient does not look strong enough to make me too sceptical but it's certainly there. When mine are no worse then that I believe them and they seem to work. I am entirely at a loss to know why I get this issue. It is particularly common on one instrument. It may arise from light leakage at the scope or in the observatory but so far I can't say what's behind it.

Olly

[AstroJOE]

Thanks Oli, that is reassuring to know! ? I'll have another go at taking a batch of Flats in the same manner and see if the issue is still present. As you say it's only slight and I did still see a real improvement in my final stacked and processed image. My main concern was that this may be a sign of something else, for example Tilt. Although judging by all the replies it sounds like I may have been wrong about that ?

James