M31 Artefact

[gnomus]

I have been gathering data for a fresh run at M31.  I've done a preliminary process of the RGB - these are 16x5 minute subs on each channel.  I have noticed a strange artefact in my G and my B subs.  This is leading to the following:

This shows where the artefact is in relation to the whole image:

I wondered if this might be a processing artefact.  However, this appears on the completely unprocessed, un-calibrated subs.  I wondered if it might be a defective column that was being 'smeared' by dithering, but no - the artefact is roughly the same size and shape on individual subs.  It is visible on green and on blue.  I don't see it all on the red subs.  There is no hint of it on the Ha subs I shot last night.  If I look on the Lum subs - I am not sure.  If it is there, it is very faint.

I don't think there is a bright enough star in this area to cause flaring.  

Incidentally, this is a Moravian G2-8300 on a WO Star 71.  Filters are Baader.

Any thoughts?

 

[ollypenrice]

If it's colour dependent then I guess it has to be optical in origin rather than electronic. An internal reflection? Alternatively, is this chip microlensed? 

What happens if you reposition the target somewhat? I'd expect the artefact to disappear or move, in which case you could easily make a patch for it and it might give you a clue as to the precise origin.

I'd have another look at your colour channel registration on this, Steve. It probably won't help the artefact but I'd say that red has registered with an offset to the right, no? You don't have to start again. Just split and save the channels and then use Registar to register, say, green and blue to red. You could then import them into Photoshop and split the orginal into its channels before recombining them with the G and B channels replaced by the ones from Registar.

Olly

PS That is going to be a gorgeous M31. It has a really spooky, etherial look. The core is superbly done.

[gnomus]

11 minutes ago, ollypenrice said:

If it's colour dependent then I guess it has to be optical in origin rather than electronic. An internal reflection? Alternatively, is this chip microlensed? 

What happens if you reposition the target somewhat? I'd expect the artefact to disappear or move, in which case you could easily make a patch for it and it might give you a clue as to the precise origin.

I'd have another look at your colour channel registration on this, Steve. It probably won't help the artefact but I'd say that red has registered with an offset to the right, no? You don't have to start again. Just split and save the channels and then use Registar to register, say, green and blue to red. You could then import them into Photoshop and split the orginal into its channels before recombining them with the G and B channels replaced by the ones from Registar.

Olly

PS That is going to be a gorgeous M31. It has a really spooky, etherial look. The core is superbly done.

Thanks, Olly.  I'd noticed the colour offset - not really sure why I got that.  I 'StarAligned' in PixInsight, which is normally pretty reliable.  I also got issues around some individual stars.  I am sure I saw a post somewhere explaining what these were and how to correct, but I can't find it now.  This is zoomed some 300% to show the issue:

 

I am planning to do some 60 second RGB subs for the core.  That might give less of an issue and could function as a patch.  Failing that, I will try offsetting as you suggest.  Incidentally, I thought that if I shot about 30x1 minute each of RGB, then I could use an extracted luminance from that as a 'core luminance'.    Would that work?  Also, what about some long RGB (like 15 minute subs) - would there be any value in that?

[johnrt]

Was the red and blue taken either side of the meridian with a flip? I see a misalign in my data sometimes when I have shot the red and blue either side of a meridian flip.

[ollypenrice]

I found a huge benefit in going for 30 minute luminance subs in a direct comparison with 15 minute ones. The extended outer glow, which was my goal, really popped in the long subs. However, that's with very deep pixel wells and a dark site.

I didn't find short subs all that remarkable for the core, to be honest. I did take some at longer focal length. The main thing was a pretty brutal attack on core contrasts. Because the signal is very strong and the available details few, I really knocked the data about something rotten! Thuggery all the way... But I did find spiral hints quite a long way in.

I've seen PI fail on red alignment once before in an image on here.

Olly

[gnomus]

1 hour ago, johnrt said:

Was the red and blue taken either side of the meridian with a flip? I see a misalign in my data sometimes when I have shot the red and blue either side of a meridian flip.

That is almost certainly the case.  I shot the reds first and then green followed by blue.  There was a flip at some point during the greens.  I wonder why that leads to a misalignment.  And if it is that, how does one correct it?  (I will try RegiStar, but I won't be able to do this for a few days.)

[gnomus]

1 hour ago, ollypenrice said:

I found a huge benefit in going for 30 minute luminance subs in a direct comparison with 15 minute ones. The extended outer glow, which was my goal, really popped in the long subs. However, that's with very deep pixel wells and a dark site.

I didn't find short subs all that remarkable for the core, to be honest. I did take some at longer focal length. The main thing was a pretty brutal attack on core contrasts. Because the signal is very strong and the available details few, I really knocked the data about something rotten! Thuggery all the way... But I did find spiral hints quite a long way in.

I've seen PI fail on red alignment once before in an image on here.

Olly

Interesting idea on the 30 min lum subs.  I did try some 10 minutes before settling on 5 min lum subs (of which I now have 5 hours).  I was finding that I was getting a similar 'shaped' histogram with both settings.  The 10 minute subs were, of course, marched over to the right with a big (seemingly empty) gap on the left.  I assumed that I would only be taking in the black point anyway and, therefore, there was little if any benefit from the longer sub. 

[ollypenrice]

51 minutes ago, gnomus said:

Interesting idea on the 30 min lum subs.  I did try some 10 minutes before settling on 5 min lum subs (of which I now have 5 hours).  I was finding that I was getting a similar 'shaped' histogram with both settings.  The 10 minute subs were, of course, marched over to the right with a big (seemingly empty) gap on the left.  I assumed that I would only be taking in the black point anyway and, therefore, there was little if any benefit from the longer sub. 

I found I could only capture the upturned end of the outer glow, which I had seen just once in another image, by using the very long subs. Mind you, I didn't shoot the 15 minute set myself. They were Tom's, shot in Spain, so conditions may not have been fully comparable. This processing is a bit on the head banging side but the idea was to find that elusive outer structure.

Olly

 

 

[MrsGnomus]

 

40 minutes ago, ollypenrice said:

I found I could only capture the upturned end of the outer glow, which I had seen just once in another image, by using the very long subs. Mind you, I didn't shoot the 15 minute set myself. They were Tom's, shot in Spain, so conditions may not have been fully comparable. This processing is a bit on the head banging side but the idea was to find that elusive outer structure.

Olly

 

 

Thanks Olly

I'm responding on @gnomus behalf as he is busy driving us to the snooker in York.

When it comes to combining the Lum are you blending in PS (using a layer/eraser technique), or are you "averaging" the different Lums in RegiStar?

[ollypenrice]

8 hours ago, MrsGnomus said:

 

Thanks Olly

I'm responding on @gnomus behalf as he is busy driving us to the snooker in York.

When it comes to combining the Lum are you blending in PS (using a layer/eraser technique), or are you "averaging" the different Lums in RegiStar?

You mean combining the short subs for the core with the long ones for the rest? If so, I used Registar to align, crop and pad the short subs from the TEC but I blended them into the long ones in Photoshop. Before blending them, though, I tortured the short subs until they confessed to hiding a tiny bit of spiral structure. I would not have wanted any SGL members to witness this heartless cruelty!

Enjoy the snooker!

Olly

[gnomus]

Thanks again Olly.

I actually meant combining your 'outer glow' with your 'regular' view of the galaxy.  However, I think the point might be moot.  Examining your masterpiece carefully, it is clear that your 'upturned bit' would be outside my FOV.  I'll probably just shoot for a vanilla M31, and forget any other aspirations (which are borderline delusional, in any case).

[ollypenrice]

Ah, for the outer glow I did some carful Curves stretches, which also stretched the stars, so I star reduced that stretch then layered the outer glow into the original image.

Olly