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Markarian's Chain Revisited

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#1 Achernar

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Posted 14 June 2024 - 09:51 AM

Last week I revisited Markarian's Chain with a better mount, better guiding and a better understanding of how to acquire and process image data with a monochrome camera than the last time I imaged it. This image is a stack of 90 two-minute subs taken through red, green and blue filters with a ZWO 533MM monochrome camera, LRGB filters and a EFW filter wheel. An 80mm Astro-Tech EDT apochromatic refractor and Skywatcher EQ6R-Pro mount were also employed to gather the image data. The skies were average in terms of seeing and transparency for Mobile Alabama during the month of June from a suburban site. The color balance is certainly better than the previous attempt.

 

Taras

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#2 JF1960

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Posted 14 June 2024 - 10:09 PM

The image appears a bit out of focus.  Do you use an auto-focuser?



#3 Sacred Heart

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Posted 15 June 2024 - 09:17 PM

Taras,

 

Understanding what you are doing is sometimes better than knowing.  For if you understand what you did, you can plan ahead to try another thing or different approach.  It is all experimentation.  I believe.  Also, to me, focus on improving one aspect at a time.  Each night is different,  seeing is different, Moon position is different, temp / humidity.  Refocus times are different, ability to collect color is different.  My thinking anyway.

 

I think you did great, you have a lot on your plate with mono set up.

 

Question, what does lawyers, doctors, astronomy and cooking all have in common??   Think about what they do, not what they are.

 

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#4 Achernar

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Posted 16 June 2024 - 06:57 AM

The image appears a bit out of focus.  Do you use an auto-focuser?

Yes, however seeing could the reason. The hot weather here often means lots of radiational cooling at night, which leads to poor seeing. That has in the past made stars larger than they otherwise would be. Longer exposures also makes the brighter stars larger, normally I don't expose RGB frames for two minutes, but galaxies generally require longer exposures than other objects. Using a ZWO EAF and ASI Air has thus far worked better for me than manually focusing with a Bhatinov mask.

 

Taras


Edited by Achernar, 16 June 2024 - 07:26 AM.


#5 rj144

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Posted 16 June 2024 - 09:33 AM

Yes, however seeing could the reason. The hot weather here often means lots of radiational cooling at night, which leads to poor seeing. That has in the past made stars larger than they otherwise would be. Longer exposures also makes the brighter stars larger, normally I don't expose RGB frames for two minutes, but galaxies generally require longer exposures than other objects. Using a ZWO EAF and ASI Air has thus far worked better for me than manually focusing with a Bhatinov mask.

 

Taras

Why do galaxies require longer exposures?



#6 Achernar

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Posted 16 June 2024 - 04:13 PM

Why do galaxies require longer exposures?

The outer regions are often much dimmer than the central regions, and depending on the galaxy in question longer exposures can improve matters. However, I have not so far seen much benefit from going past three minutes with RGB filters unless I'm at a dark site.

 

Taras


Edited by Achernar, 16 June 2024 - 04:15 PM.


#7 rj144

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Posted 16 June 2024 - 04:50 PM

The outer regions are often much dimmer than the central regions, and depending on the galaxy in question longer exposures can improve matters. However, I have not so far seen much benefit from going past three minutes with RGB filters unless I'm at a dark site.

 

Taras

The dimmer parts don't matter with individual subs though.  It's total integration that will bring that out, not the length of the sub.



#8 Spaceman 56

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Posted 17 June 2024 - 02:24 AM

The dimmer parts don't matter with individual subs though.  It's total integration that will bring that out, not the length of the sub.

this is often stated. But I wonder if it is entirely 100% true ?

 

if stacking is averaging, then the average will still reflect the SNR values of the individual subs. (because we are not adding subs)

 

would stacking and averaging a set of subs with 3x the exposure length, reveal a higher SNR ? I guess it might.

 

experts will make this complicated I am sure. smile.gif

 

as an experiment I shot a galaxy with 180 second subs, instead of my normal 60 seconds.

 

I got this. click to enlarge.

 

Centaurus A. Closer
 
it was brighter and more explosive that the same target with 60 second subs.
 
I am unsure what this proves and I am still experimenting with all this.  smile.gif
 
Spaceman

Edited by Spaceman 56, 17 June 2024 - 02:25 AM.


#9 imtl

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Posted 17 June 2024 - 03:19 AM

 

this is often stated. But I wonder if it is entirely 100% true ?

 

Yes it is as long as you are shot noise limited, or "swamping your readout noise".

 

if stacking is averaging, then the average will still reflect the SNR values of the individual subs. (because we are not adding subs)

 

No, stacking or averaging increases the SNR. It does NOT reflect the SNR of individual subs. If that was the case then we wouldn't be stacking.

The signal grows faster than noise by square root, hence SNR grows as we stack.

 

would stacking and averaging a set of subs with 3x the exposure length, reveal a higher SNR ? I guess it might.

 

Stacking is averaging. And yes it will have higher SNR, because the total integration time will also be 3 times longer.

 

experts will make this complicated I am sure. smile.gif

 

No they won't.

 

as an experiment I shot a galaxy with 180 second subs, instead of my normal 60 seconds.

 

I got this. click to enlarge.

 

 
 
it was brighter and more explosive that the same target with 60 second subs.
 
I am unsure what this proves and I am still experimenting with all this.  smile.gif
 
It proves that longer integration time yields higher SNR. Compare your single 180s light with 3 stacked 60s subs. 
 
Spaceman

 


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