Iris Nebula NGC 7023 with funny rings

[EyeGuy]

40 minutes ago, carastro said:

Also take a look at a master flat stretched.

Carole 

Will do - should be able to do that tomorrow morning. Thanks Carole.

Barry

[EyeGuy]

17 hours ago, carastro said:

As an experiment try stacking without the flats.  I know this is not ideal but might help with diagnosis.  Carole

Hi Carole,

Solved (I think)

Your hunch was right on and thanks so much for the very helpful suggestions. It turns out my flats were grossly underexposed - especially in red. I'm using a Gerd Neumann Aurora flatfield, and was just using the camera exposure meter to judge exposure. Mistake. I should have "overexposed" by that metric.

You can see what the camera thought of the exposure in the first image - all channels underexposed, but especially the red. It's funny how the camera exposure meter got that so wrong.

The second is a histogram from IrfanView image viewer showing embarrassingly low exposure of red. I imagine that made it impossible to correct the red light channel.

I extracted the red channel from the flat and gave it a severe stretch and low and behold, out came the ring (image 5) - but as an underexposure, just like you'd expect.

I re-ran the image without the flats as you suggested and there's no sign of the rings (image 4) - even with a massive stretch (image 3)

 

So I guess that probably solves it. That problem has been annoying me for months. Next time I'm out I'll be sure to check the histograms before I take everything down - and I'll post results!

Thanks everyone for chipping in and helping me solve this problem (I hope).

Kind regards,

Barry

PS. for bonus points, can anyone explain why the rings are oval, and not circular???

[carastro]

Glad you've got it sorted.  

No idea why it is oval though.  

Carole 

[EyeGuy]

20 minutes ago, carastro said:

Glad you've got it sorted.  

No idea why it is oval though.  

Carole 

Well, so far just a theory. And on the plus side, I've become much more proficient at DBE in PI.

Barry