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Slight donut and CA issue

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#1 iamfurgus

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Posted 28 February 2021 - 04:42 AM

HI experts, first run out this weekend with my new Starwave 70mm ED quadruplet.  First session I had some 'egging' of stars, but it still stacked ok and I managed to work out that is a tilt issue with my optics train.  So the below issue is with no filter, just basic scope and lens setup on the ioptron mount.

 

Second session I have attached the 'best' frame with decent round stars, but zoomed in I can see the smaller stars are donut shaped sightly and there appears to be a lot of CA to my untrained eye.  Focus was bang on using Bhatinov mask.  This is only a 68sec sub, so just asking for opinions, I am running the stacking now so will also share that result to compare.

 

Donut Issue

 

Link to Raw sub image:
 

 

Thanks!


Edited by iamfurgus, 28 February 2021 - 05:16 AM.


#2 robbieg147

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Posted 28 February 2021 - 05:52 AM

Your image is overexposed this alone can cause artifacts, always check your histogram to make sure nothing is getting blown out. Unless you are doing HDR and combining images taken at different exposures etc. 



#3 sharkmelley

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Posted 28 February 2021 - 06:08 AM

Your image is overexposed this alone can cause artifacts

I don't understand that argument.  Do photons in large numbers have different properties to a smaller number of photons?

 

Mark



#4 iamfurgus

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Posted 28 February 2021 - 06:08 AM

Your image is overexposed this alone can cause artifacts, always check your histogram to make sure nothing is getting blown out. Unless you are doing HDR and combining images taken at different exposures etc. 

 

 

Its a good point, the moon was so bright this weekend, I didn't factor that in on the exp time, I am going to try again tonight with short exp and also try with a filter.  I did check the histogram and all channels were about halfway across so I didn't think that was too bad (I know it should be about a 3rd from the left). 



#5 Tapio

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Posted 28 February 2021 - 06:26 AM

If the problem was doughnut shape stars and CA I think we should answer that instead of overexposure.

Frankly I don't see either of those but that might be my problem using tablet in sunny day.
One piece of advice though - before starting imaging run test focus with dimmer star also.

#6 jrcrilly

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Posted 28 February 2021 - 06:51 AM

Focus looks soft. That will create both doughnuts and CA, and will exacerbate coma (as also seen in the image). Remember that focus shifts with cooldown and temp changes so having it "bang on" at one point doesn't mean it is still good for later exposures. This is the primary weakness of focus masks; it is too much trouble for most folks to reinstall the mask and use it again every few minutes. Better to monitor FWHM and refocus as indicated by that.



#7 robbieg147

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Posted 28 February 2021 - 09:17 AM

I don't understand that argument.  Do photons in large numbers have different properties to a smaller number of photons?

 

Mark

Hi Mark,

 

If you think I am wrong that's ok I was just trying to help, looking at the image I see artifacts which to me are either caused by chromatic aberrations or blooming.

 

Blooming is caused by overexposure, I would agree with you if we were talking about film cameras.

 

So to me getting the exposure correct is the first course of action? If you think something else that's ok.



#8 iamfurgus

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Posted 28 February 2021 - 09:19 AM

If the problem was doughnut shape stars and CA I think we should answer that instead of overexposure.

Frankly I don't see either of those but that might be my problem using tablet in sunny day.
One piece of advice though - before starting imaging run test focus with dimmer star also.

Thanks, its marginal, and only on the faint stars, but maybe I am being picky.  It all seems to stack well so I probably shouldn't worry too much.

 

 

Focus looks soft. That will create both doughnuts and CA, and will exacerbate coma (as also seen in the image). Remember that focus shifts with cooldown and temp changes so having it "bang on" at one point doesn't mean it is still good for later exposures. This is the primary weakness of focus masks; it is too much trouble for most folks to reinstall the mask and use it again every few minutes. Better to monitor FWHM and refocus as indicated by that.

You could well be right here, as I have no way of checking the FWHM as i'm running dslr and a intervalometer, not through a laptop.  Thinking about it as well the dew shield moves easily, i wonder if that would give me a diff result.  I'll try again tonight but its relatively easy for me to keep checking between a bunch of frames tbh.  But at the very least I can check at the end of the session and see if its still 'bang on' (regretting using that phrase now lol).

 

Overall the takeout seems to be focus on the focus.....so thanks to all for your input!



#9 Kevin_A

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Posted 28 February 2021 - 01:27 PM

It just looks a bit soft on focus or short on exposure length to me. I didn’t check the data or your sky quality, but i am leaning to it being just a hair slightly out of focus. In a bortle 8 my data was similar but when i went to a bortle 4 my faint stars looked better as my subs were much longer to get to the same histogram. 


Edited by Kevin_A, 28 February 2021 - 01:30 PM.

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#10 iamfurgus

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Posted 28 February 2021 - 05:35 PM

It just looks a bit soft on focus or short on exposure length to me. I didn’t check the data or your sky quality, but i am leaning to it being just at hair slightly out of focus. In a bortle 8 my data was similar but when i went to a bortle 4 my faint stars looked better as my subs were much longer to get to the same histogram. 

Interesting, I'm bortle 5 and I would like try longer subs again but assumed if I'm struggling with 1 min, well, it'll only get worse. Done 2 more hours on same target tonight with filter as well, will look tomorrow at the results... 



#11 Kevin_A

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Posted 28 February 2021 - 10:07 PM

Interesting, I'm bortle 5 and I would like try longer subs again but assumed if I'm struggling with 1 min, well, it'll only get worse. Done 2 more hours on same target tonight with filter as well, will look tomorrow at the results... 

unfortunately the higher the light pollution the shorter time it takes to get to 1/4 to 1/3 histigram and most of the signal is from the sky and not stars. In darker skies more of the signal to get the same histogram is from the stars. My mages in bortle 8 were bright purple and faint stars were few and very dim. You cannot change this other than accepting more lower signal images to get to a higher total integration time. You can lower your iso but you cant beat dark skies. Bortle 5 skies are fairly good but your variables may vary as if you have a canon or nikon, the nikon will be better due to its lower iso noise characteristics. The most important thing is just using the camera at a good noise level that is low enough to get max 1/3rd histogram at yourr sky quality and then just get lots of those images to aquire total integration time.



#12 sharkmelley

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Posted 01 March 2021 - 02:30 AM

If you think I am wrong that's ok I was just trying to help, looking at the image I see artifacts which to me are either caused by chromatic aberrations or blooming.

 

Blooming is caused by overexposure, I would agree with you if we were talking about film cameras.

Understood.  I can see where you're coming from now.  But I don't think star saturation is the problem here.

 

Mark



#13 iamfurgus

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Posted 01 March 2021 - 10:02 AM

Hi so a short update, i've purchased a new t ring adapter as the last one wasn't flush so I couldn't discount it.  The 3rd session I used a filter and worked on the focus issues above, halfway through the session (1 hour) I checked and it was indeed out of focus again.  I think I need to get the scope out earlier (going from 20c to 5C probably takes a good hour for all the glass to chill) but Orion is quite early in the night for me atm so thats not so easy.  But at least I can monitor this issue now.  Anyway I share the 'best' sub here from the 3rd night, I used the Lmax pro filter with all the house lights on for an LP test too:

 

https://drive.google...iew?usp=sharing

 

I think the stars are pretty round and less coma that i can see suggesting the scope is fine and probably the whole optics train (i am still getting the new t ring).  Next test will be higher ISO and shorter subs to see if I can get these sharper still.

 

Also while I wait for a retest i've stacked 5 hours from 3 sessions, here is the final stack unprocessed in any way.  Blue channel will be high as I had the filter on the last session. 

 

https://drive.google...iew?usp=sharing

 

I'll keep my post processing questions on another thread smile.gif


Edited by iamfurgus, 01 March 2021 - 10:04 AM.


#14 iamfurgus

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Posted 01 March 2021 - 12:00 PM

Ok further update on the stack processing, tons of light pollution (as expected) and I have abandoned ship on the 5 hour stack.  However, the 3rd stack on its own yielded just 1 hour total integration but the filter did a great job and I managed a satisfactory result with such short overall time:

 

orion Processed

 

I will still play with the exposure and focus to improve further, thanks for everyones input, it really helps a lot!




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