Iris Nebula NGC 7023 with funny rings

[EyeGuy]

Hi guys,

this is my effort on the Iris Nebula with a Tak FS-128 from a dark sky site in Ireland. I was being greedy, doing multiple targets on the same night (March 17th) so I only captured 2 hours on this one.

Takahashi FS-128 with 0.75X reducer/flattener on Vixen AXD2 mount

60 X 120s with Nikon D750

Processing in PixInsight and Photoshop

There's something funny going on with the rings in the background, which I have not been able to get rid of. This was from a truly dark site (Bortle 2) and the subs don't show any sign of these rigns, just the usual vignetting. I think the calibration is actually introducing the effect.

I took flats and all that jazz, and did all the calibration in PI so I don't know what I'm doing wrong. Has anyone seen this kind of thing before? Comments and suggestions welcome!

Barry

 

[carastro]

Blowed if I can see any rings.  Nice image BTW

Carole 

[John78]

NGC 7023?

 

[EyeGuy]

17 minutes ago, carastro said:

Blowed if I can see any rings.  Nice image BTW

Carole 

Thanks Carole,

the main one is an oval shaped ring - long axis horizontal, which extends across about 80% of the image width. It's an ugly red colour and looks for all the world like off-axis glare from light pollution. There's another one outside that with a clear gap in between. They have quite sharp borders so DBE doesn't handle it well, especially with a large subject like this.

Barry

[EyeGuy]

4 minutes ago, John78 said:

NGC 7023?

 

Yup. I don't know what kind of a fat finger makes that typo happen! Anyone know how to fix?

Well spotted - and good NGC number knowledge!

Barry

[Jkulin]

Like Carole I can't see anything untoward on the image, sure it's not a mark on your screen? :-)

Nice image btw.

[Jkulin]

This is how I am seeing it: -

[EyeGuy]

1 minute ago, Jkulin said:

Like Carole I can't see anything untoward on the image, sure it's not a mark on your screen? :-)

Nice image btw.

LOL, definitely not the screen. But its reassuring that others aren't troubled by it. And thanks!

Barry

[John78]

1 hour ago, EyeGuy said:

Yup. I don't know what kind of a fat finger makes that typo happen! Anyone know how to fix?

Well spotted - and good NGC number knowledge!

Barry

Well it’s a target I wanted to try so I put it in the fov calculator and was confused why in your shot you couldn’t see the hh or dratted Alnitak so I sleuthed it out

 

i can see a large red ovally ring ring in your picture but idk what would cause it.

[Demonperformer]

I see what you are talking about - just outside the dust clouds - but it does not detract from the image for me. Will have to check other images to see how they compare.

[EyeGuy]

1 minute ago, Demonperformer said:

I see what you are talking about - just outside the dust clouds - but it does not detract from the image for me. Will have to check other images to see how they compare.

So I'm not crazy

I feel like with the right pixel math, the flats would correct the subs better, but I'm no expert in PixInsight so I've just been using Richard Bloch's YouTube guide and also the batch preprocessing script. They both result in v similar results (though Richard Bloch's is always a little better IMO) with these damn rings visible and poor vignetting correction. I'd love to know where they're coming from.

Barry

[Ibbo!]

1 hour ago, EyeGuy said:

Yup. I don't know what kind of a fat finger makes that typo happen! Anyone know how to fix?

Well spotted - and good NGC number knowledge!

Barry

Ask a mod nicely

[EyeGuy]

2 minutes ago, Ibbo! said:

Ask a mod nicely

Thanks Steve,

now I can take off my brown paper bag.

Barry

[carastro]

Well I put in Photoshop and stretched it and still can't see anything.

It's a very dusty area, are you sure you are not just seeing areas without any dust making them appear a different colour?

This is what I get when I over stretch it.  Can't see any rings, just dust and absence of dust.  I can see a faint hint of red towards the right of the Iris. 

 

[carastro]

So I then stretched the colour and finally I could see something.

Is this what you are talking about?

If so, I have no idea what the problem could be - sorry and no experience of  Nikon camera. 

 

[almcl]

There is something there (I think) see grossly overstretched image below, but what's causing it, I have no idea. (sorry, crossed with Carole)

 

[John78]

Just now, carastro said:

So I then stretched the colour and finally I could see something.

Is this what you are talking about?

If so, I have no idea what the problem could be - sorry and no experience of  Nikon camera. 

 

Yes that’s what I can see

[EyeGuy]

3 minutes ago, carastro said:

So I then stretched the colour and finally I could see something.

Is this what you are talking about?

If so, I have no idea what the problem could be - sorry and no experience of  Nikon camera. 

 

Yup, that's it. Thanks so much for going to that effort.

I don't think it's the camera - or at least I can't imagine how - as I'm using bias , darks and flats, which really should get rid of anything major from the sensor. So that means it's either coming from the telescope or it's due to my (lack of) processing skills.

Well, the thing is I've seen this ringing with all my scopes, and they're all good quality, so I really think it must be something to do with the flats, but I'm damned if I can figure it out.

Barry

[Demonperformer]

Checkout visibledark.ca/ngc-7023-iris-nebula-city-vs-rural/

The image there shows a similar ring structure. (I think, but i'm only on my phone so resolution not brilliant)

[EyeGuy]

4 minutes ago, Demonperformer said:

Checkout visibledark.ca/ngc-7023-iris-nebula-city-vs-rural/

The image there shows a similar ring structure. (I think, but i'm only on my phone so resolution not brilliant)

That's v helpful. I think I can see something a bit similar in that image, though it's hard to tell if it's the same effect. I must dig out some others with this effect and post them to see what people make of it.

Barry

[MartinFransson]

I´ve had the same effects when using some camera lenses and I think it is due to mismatch with the flats. Were they taken at the exact same focuspoint and temperature? I think flats where the vignetting is not exactly as in the lights can cause some sort of diffraction pattern (like these rings).

[EyeGuy]

12 minutes ago, MartinFransson said:

I´ve had the same effects when using some camera lenses and I think it is due to mismatch with the flats. Were they taken at the exact same focuspoint and temperature? I think flats where the vignetting is not exactly as in the lights can cause some sort of diffraction pattern (like these rings).

That's a good idea and definitely conceivable. On this occasion the flats were definitely taken at the same focus, temperature and camera angle. The only difference was that the telescope was in the home position for the flats - which I think most people do?

The Tak focuser is good and sturdy so I doubt the change in position would be significant. What do you think?

Barry

[carastro]

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

[EyeGuy]

4 minutes ago, carastro said:

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

I think that's a good idea. I'll give it a go and post results...

[carastro]

Also take a look at a master flat stretched.

Carole