DEEP SKY WEST ASTROPHOTOGRAPHY FORUMS

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Images from DSW-South @ El Sauce Chile...NGC 253

Cool galaxy with lots going on if you can get it right.  The Ha pops pretty well in this LRGB version.

 

-Lloyd

 

 

Uploaded files:
  • Us-and-Them.png

Sculptor, yes, one I have imaged in my backyard, deep in the muddy glow of light pollution by the horizon ... that looks awesome!

Scott S
Graham, NC

Just process NGC 253 (attached). Great detail available, but some of the stars seem to have chromatic aberration (the two just above the galaxy. That appears in your image too, Lloyd. Any idea why?

Alex

Uploaded files:
  • NGC253_Sculptor_TAO150-100.jpg

I've seen the same issue.  Red left and green right.  I've got a call into Takahashi to see what they've to say.  I don't recall seeing it during testings.  The 2 starts you mention should be blue.  This "feels" like a convergence issue, but we'll see what they tell us..

-Lloyd

Lloyd - your versions, left and right, the left shows crisp blue stars.  The right shows the same kind of noise/distortion in some of the stars when I zoom in on them that I see in my minimally processed result.  What'd you do between the right and the left?  Ohhhh the one on the left, is that the model you used for reference?

Interestingly, those big blue stars (in the left image above) are not blue at all in ours. Here's a crop of one result, so far, no drizzle and have not done anything to fix the stars, and lots of noise reduction.

I've compared a few versions - one by Robert Gendler processed from Hubble data is a good example: https://pixels.com/featured/sculptor-galaxy-ngc-253-hubble-image-robert-gendler.html. And it looks like those two stars are actually blue.

I'm still learning as I go here and various new things to learn about this rig in Chile and the results.  Maybe I lost the blue somewhere in processing - will go through it again.

Scott S

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  • RGB_DBE1_Preview06.jpg

I just looked up the two stars discussed above on SIMBAD, and it lists their spectral types as G3 and G0.  Doesn't that indicate a yellow color, not blue?

Mike K

Mike,

If you look at the data, LRGB images all have overexposed stars. I talked to Lloyd about this. Processing still pulls color out of the stars and I don't really understand how. But the two stars in question are completely blown out and I don't see how reliable color could be extracted from such data. Maybe someone can help me understand that. I have had Takahashi refractors for 20 years and have never had chromatic aberration problems. The chance of the TOA150 being at fault here is zero in my opinion.

Mike Bushell

Yes, Mike and I have had numerous discussions of exposure times, etc etc.  It is said that stars of a certain exposure intensity "prohibit" the development of color in such "over exposed" stars (those measuring white frame by frame or in the stack).

Nevertheless, there is color in properly processed data.  In fact, my reluctance to further work on NGC 253 was due to the mixing of all these ideas:  are the stars overexposed?  is this causing the "blue" stars to look otherwise?  what's up with the convergence.

At the end of it all, based on input from many, my conclusions about exposure time remains mainly unchanged:  we do not shoot for stars, we shoot for the DSO.  To the extent a "normal" exposure of 10-15 minutes is appropriate for the DSO, that's what we've done and will continue to do.  In instances of particularly HDR elements we of course shoot for that.  From time to time these "elements" are stars, but not usually.   This is usually done for galaxy cores, certain nebulae, etc.

We will not be shooting HDR for stars except in special cases.  Of corse we operate at the behest of the team, but we'll not do things that are unorthodox or otherwise unsound.

Back to the stars on 253.  Lo and behold  Mike's research shows, the "offending" stars are not blue and the APOD that I used as a reference image is inaccurate.

So, I'm going back to 253 to work on the stars again...I'm happy with the core as the processing results in similar results that were achieved on a much larger scope.

I'm going to send another analysis of this and other images where the stars may be perceived to be overexposed, but yet exhibit color.  There are processing techniques that help this along...more to come.

-Lloyd

Uploaded files:
  • 253-apod-blue-stars.jpg

Great points, guys.  Awesome.  Well, I processed the 120 second RGB and I get green fringe/ringing around those stars and some stars in the foreground of the galaxy are almost totally green.  I ran the new version of SubframeSelector on all the subframes for this target and the FWHM of the stars in the G data, especially the 120s set, is 4+ while several B subframes' FWHM are <= 3 and lower range.  The R are in the middle.  The L 600s subframes are closest to the G 120s in terms of FWHM.  The B stars are smaller and dimmer, so the result is the G data shines around the edges.

Why this is, I don't know. Focus?  Filter?

Scott S

I would stake my entire DSW rig on the problem not being TPA150 color issues. Let me throw something on the wall and see if it sticks. Almost all the stars in the set, including RGB as well as L, are pegged at 65535 in the raw data. Common sense says these stars should all render as white, at least in the center. But the software seems to be trying to fix the stars, perhaps by interpolating the saturated parts from the nearest unsaturated pixels, perhaps in the outer diffraction rings. But it may not do that for very large stars? And overexposing stars certainly changes their size and profile unpredictably. They become square, flat-topped. I can easily see how this could result in R, G, B and L versions of stars not combining properly, what Lloyd calls convergence. For badly over exposed stars FWHM measurements are probably useless. The size will depend on how badly they are overexposed and it would be different for different filters.

 

In other words, there is no reason to expect color in overexposed LRGB stars to converge.

BTW Lloyd, I think the DSW image wrt the galaxy itself is actually a little better than the comparison image (an APOD image?) from a much larger scope. It just takes longer to get the data. Tak optics are superlative. So are Chile skies.

Does this make any sense at all?

Mike