Having problems with Wizard Nebula

[Jm1973]

Hi Everyone. 

I am quite new to image processing, but with the help of this forum I have managed to get a reasonable image of ANdromeda recently. So bouyed with this small success I decided to take a shot (no pun intended) at Wizard Nebula. Using AstorTortilla I managed to get it pretty much centred in my sights, and took 60 subs at 60 seconds each before the clouds arrived. I also took 25 darks, 25 bias and 30 flats. I registered and stacked them in APP. But when I stretched them there was a very prominent red ring present, that I have struggled to remove as it is a very similar colour to the nebulosity I am trying to bring out.

 

In fact in order to get the nebulosity to show at all, I have to stretch the image so much that the gradients are overpowering as are the stars. I have not had much luck clearing it up with either 'correct vignetting' or 'remove light pollution' in APP, and I've also not had much luck with removing the stars before stretching. 

 

Does anyone who is good at this sort of stuff fancy taking a look at my stacked fits image and see if anything can be done with it, or whether I should try stacking it differently? I don't want to spendanother evening messing about in PS, GIMP, APP etc. if it's a lost cause.  I know there is data there as I can see the nebulosity when thoroughly stretched, but I can't seem to get to it without ruining the image. This is after stacking, then stretching, in APP. I'm not sure what caused the purple banding.

. 

Here is a link to the FITS file: https://www.dropbox.com/s/4g46xszfuf4zlw2/Wizard_Nebula-RGB-session_1.fits?dl=0

Thanks in advance.

Edited October 17, 2020 by Jm1973

[alacant]

Hi

Go through all the light frames and remove all those that have cloud or any other imperfection. Then stack again.

Any better?

HTH

[Jm1973]

19 minutes ago, alacant said:

Hi

Go through all the light frames and remove all those that have cloud or any other imperfection. Then stack again.

Any better?

HTH

I just went through my 60 lights and removed 15 that had slight amounts of cloud in them (over and above the others). Stacking now. WIll report back later.

[Jm1973]

I don't know about better... I would say different. Instead of a reddish ring I now have this:

I am still unable to get rid of the gradient, or even understand what's going on. 

I don't really understand how removing some slightly cloudy images has caused this to happen.

Is it possible my flats are causing problems?

[Uranium235]

Inspect your flats, a normal one (albeit my example is mono) should look something like this (vignetting amount will differ between optical/camera setups), but it should measure 1/3rd of the histogram - or/and have a healthy ADU reading, over 16bit that would be reading of about 21,000.

But the previous suggestion of inspecting all your lights should also be done. If you have Ps, then you already have all the tools you need to process the image correctly (after calibratrion and stacking). Dont rely on software to correct vignetting, the only sure way is with proper calibration. If you have a gradient after applying flats, that is best dealt with using a Ps action/filter - or you can create your own gradient map in Ps (a poor mans version of the DBE found in Pixinsight).

Also, if you want to simplfy your data for now - just convert it all to mono (including your flats), see if you can get anything out of it that way - I find mono is far easier to work with if youre trying to rescue some data.

[Jm1973]

Well I just calibrated again but without the flats this time and the results look far more normal.

So I think I need to look at my process for taking flats.

[Jamgood]

1 hour ago, Jm1973 said:

So I think I need to look at my process for taking flats.

How are you taking your flats?

The way I do it, which is not the only way or possibly the best, is with a ipad/tablet placed over the scope with just a plain white jpg stretched to full screen. If using a DSLR, switch to AV Mode and shoot away.

Nearly every time I've had a stacking issue it has been due to bad flats.

Also, look at focusing. You're out of focus in that stack. Zoom in the the stars look like bubbles. Do you have a Bahtinov mask?

[Jm1973]

9 hours ago, Jamgood said:

How are you taking your flats?

The way I do it, which is not the only way or possibly the best, is with a ipad/tablet placed over the scope with just a plain white jpg stretched to full screen. If using a DSLR, switch to AV Mode and shoot away.

Nearly every time I've had a stacking issue it has been due to bad flats.

Also, look at focusing. You're out of focus in that stack. Zoom in the the stars look like bubbles. Do you have a Bahtinov mask?

Do you stretch a tshirt or anything over the telescope, or do you just put the tablet straight over the end?

Yeah they are out of focus. I'd been playing around with the settings on Astro tortilla and not having much luck, then I suddenly got it working so started shooting. It was only halfway through that I realised I hadn't got round to focussing properly. 

I haven't got a bahtinov mask yet, but my mum has bought me one for Christmas (I'm 47). I usually focus by finding a bright star and then zooming in fully using live view. When in focus the star will generally have diffraction spikes. Kind of like a poor man's bahtinov man I guess.

[Pompey Monkey]

What camera are you using?

60 (or 45) 60 second subs won't get you a lot of nebulosity at all with a DSLR.

If you have poor data to start with, then you are already facing an uphill battle.

[Jm1973]

6 hours ago, Pompey Monkey said:

What camera are you using?

60 (or 45) 60 second subs won't get you a lot of nebulosity at all with a DSLR.

If you have poor data to start with, then you are already facing an uphill battle.

I'm using a d3300. 

 

I was able to get a reasonable image of Andromeda with 30 second subs, so I thought 1 minute would be ok for something like wixard nebula. You may well be right though. I've managed to get rid of the vignetting, but by the time I have brought out any nebulosity, the stars are so bloated that the image is ruined. Obviously It doesn't help that they aren't in good focus to start with, but I even if the stars were in focus.. I wonder would I be able to do anything with this.

This is the best I have been able to get. 

 

I would like to know whether it is my data that is bad, or my processing.. or both!

 

Edited October 18, 2020 by Jm1973