M15: Artifacts vs. Real Objects; How to Prevent or Remove if Artifacts

Michael Lehv

Hi Adam,

If you look at the full res view of https://www.astrobin.com/8opil7/F/ there are a good many extremely faint background objects (particularly at the bottom of the image) that might pass for stars or galaxies and are slightly more visible here: 

The processing was done using modifications of your 08-26-22 OSC Globular Cluster video series. However, at the end of the initial processing, these objects were quite blotchy and spoiled the image as shown here: 

It was very difficult to remove or minimize these objects. I tried unsuccessfully removing them with NXT and/or isolating them with a mask using e.g., MLT, Binarize or STF. Eventually, I generated star and starless images with STX and then adjusted and re-blended them using a "Screen" instance of the Image-Blend script. The resulting image which I used for Astrobin wasn't perfect but significantly improved. 

I have several questions. First, are these simply processing artifacts or highly blurred real structures? 

Second, if they are artifacts, how can they be avoided or more fully removed than I did?

Last, what is your critical assessment of my final Astrobin image?

Thank you, 

Michael

 

BlockHead

I have a guess... you didn't give me this information though.

A globular cluster has stars that range in brightness from saturated all the way to the faintest detected signal in your image. My guess is that you used BXT. Do you see the issue? There is going to be a threshold you will find that that AI determines is too noisy and will not deconvolve. This is perfectly fine in the case of nebulae where the transition between bright and faint signal (noisy stuff) is smooth. But for stars- you will probably get the result you see here. 

I am working with incomplete information on your image...so this is only a guess and I could be 100% wrong (usually also a good guess). 

Regarding the image Astrobin... 

1. It appears you might have a cable in the optical path near a secondary vane...or you have TDE... you can probably work on this to get cleaner bright stars.

2. Yes a Globular star cluster needs sharp stars... but the stars need color as well. This two things fight each other. The sharper the stars are...the brighter their centers..and the less saturated they are. This is NORMAL. However.. you can communicate color in this case by *not removing the halos* of stars. Indeed, one consideration is to use the halo adjustment of BXT. Another is to deconvolve the image differentially. This is exactly the kind of topic I would like to address in the new course I would like to put together (Deep Sky Academy) that takes into account these kinds of considerations. 

3. The center of the cluster has two issues. First there is a general color "glow" of blue. We are looking for colors of stars...but not a bias of color in the core. Second... it appears the core is "softer" than the rest of the cluster. So that might need to be handled with a little more care. HDRMT (done carefully) can generally help here.

So I do not think they are artifacts if my guess is correct.

And the rest of this note is my commentary. 

-the Blockhead

Michael Lehv

Thanks Adam,

Yes, you hit the nail on the head - I did use BXT. Early on I applied it with Correct only and then for Sharpening and I applied it to both the RGB and Superlum images. After the final STX and Screen re-blending, I applied it again which this time did sharpen the highly blurred objects. 

Regarding a possible cable near a secondary vane, no - they are completely clean. However, if you are referring to the diffuse radiations showing between the main spikes, I think this may be something inherent in images from the SW 250 10". I've seen some discussions of this and their appearance in other images using this OTA. Some people have swapped out the original vanes. I'm not sure what you mean by "TDE" unless it's Tidal Disruptive Event? At any rate I'll delve deeper into a possible vane upgrade. 

I will definitely make use of BXT's star halo adjustment and also see if I can improve the color depth to counteract BXT. 

The blue glow at the center of the cluster should be easy to correct. 

As for the core "softness," you again hit the nail solidly. I didn't use either PI's HDRMT or Russel Croman's Color Corrected HDRMT because both would only work properly on a central Preview and I didn't think Substitute With Preview would seamlessly blend into the rest of the image. However, since your eagle eyes spotted the omission, I'll go back and give it a try. 

I really appreciate your expert analyses and the time you took in your post. Many, many thanks.

Michael

 

BlockHead

Yes The substitute script (the one *I* helped make) in combination with a mask (GAME) will blend seamlessly.

TDE is turned down edge (a common malady of reflectors).

-the BLockhead

Michael Lehv

Hi Adam, 

I revised the Astrobin image so it's now: https://www.astrobin.com/8opil7/J/. ;

I do owe you an apology for not giving you due credit regarding the preview substitution methodology. I had viewed your video on how HDRMT's output is dependent on what part of an image it's using and also your suggestion for the use of a preview. 

Your critique was most helpful in giving me a better appreciation of the level of attention to detail necessary in image processing. I am a bit embarrassed because long ago in one of my previous lives I was a plastic surgeon where "Undo/Redo" was not available.   

Thanks, 

Michael