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Final Jupiter from 9-17-21 at f/13

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#1 dcaponeii

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Posted 17 September 2021 - 10:42 AM

So I've been having some success at controlling the effects of the astigmatism in my scope by using short stacks (1% - 5%).  This final image from last night is a four image derotation of 2% stacks (of 31000 frame stacks) 1.5x drizzle, taken at GAIN 350 and 2.4ms exposure.

 

2021-09-17-0345_4-DWC-L-Jup__M__AS_P2_lapl5_ap44_Drizzle15wdrAP.jpg

 

Maybe using too much denoise now but as usually I start out heavy handed and then get it under control for awhile.   But this one is not that bad all things considered.


Edited by dcaponeii, 17 September 2021 - 10:43 AM.

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#2 Borodog

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Posted 17 September 2021 - 04:47 PM

Don,

 

Do you have a stack of an in-focus, non-clipped star you could post that illustrates the astigmatism?



#3 Kiwi Paul

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Posted 17 September 2021 - 04:56 PM

Yes Don, that is a pretty good looking image. I was thinking about the astigmatism issue. You will recall that as you move through focus from one side to the other, the elongation of the image rotates by 90 degrees. It seems to me that one way to minimise astigmatism is to focus to that sweet spot between the two. With the aberration there may well be some scattered light in the image and this will lower the contrast in the image, but you would be able to adjust the image for this i.e. increase the contrast etc.
Cheers Paul

#4 dcaponeii

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Posted 17 September 2021 - 05:11 PM

Don,

 

Do you have a stack of an in-focus, non-clipped star you could post that illustrates the astigmatism?

 

Yes Don, that is a pretty good looking image. I was thinking about the astigmatism issue. You will recall that as you move through focus from one side to the other, the elongation of the image rotates by 90 degrees. It seems to me that one way to minimise astigmatism is to focus to that sweet spot between the two. With the aberration there may well be some scattered light in the image and this will lower the contrast in the image, but you would be able to adjust the image for this i.e. increase the contrast etc.
Cheers Paul

You are exactly correct.  The star image changes from elongated in one direction to elongated in the perpendicular direction as you move through focus.  Using the moons to focus with the post-processing in Firecapture set to average (about 10 frames) has been useful the past couple of sessions to "average" out the excursions around critical focus.  However, the worse the seeing is the larger the excursions around the "critical" focus are and I then end up with small "cross-shaped" moons or stars that you've seen before.  I did capture a short video of a star near focus the other night.  This cruciform shape is also the reason why features on the disk end up rather "blocky" in nature.

 

2021-09-11-0138_8-DWC-L-Star__M__AS_P15_lapl5_ap1wP.jpg

 

As you can clearly see, I'm not quite collimated as there is a flare on one side but the diffraction rings are visible but so it the cross formed by the 15% stack sampling the entire seeing range around critical focus. 

 

I keep hoping that I'll find something in the secondary or optical train that can be corrected but frankly I'm running out of ideas and may just have to live with it.


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#5 Borodog

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Posted 17 September 2021 - 05:22 PM

Very interesting; which scope is this again?



#6 Kiwi Paul

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Posted 17 September 2021 - 06:24 PM

Very interesting Don. Ill keep my thinking cap on. Not all is lost. It looks like you could do well on those nights of very good seeing. That’s when it matters!
Cheers Paul

#7 dcaponeii

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Posted 17 September 2021 - 06:46 PM

Very interesting; which scope is this again?

12" Meade f/8 ACF on the LX600 mount



#8 Kokatha man

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Posted 17 September 2021 - 08:52 PM

I think about this constantly also Don without making any comments in your threads - mainly because I'm at a loss to come up with any definitive comments.

 

Astigmatism, which is obviously present could well be the cause of the seemingly "laterally blurred" outcomes - I would be surprised if an SCT scope had this as a major manufacturing defect but I guess all things are possible...

 

I will dig out some old planetary images if I can find them (might be very hard though...) where I had captured Mars with an astigmatic scope - I have this strange notion that there was some sort of "smearing" of the features but it was quite a long time ago.....

 

This thread where a couple of old planetary imaging comrades discuss the issue are worth reading - particularly Glenn Jolly's (DesertRat) with the animation of Mars which you will need to click on to get it working in Post #10 here:  https://www.cloudyni...ct-on-imaging/    

 

Glenn mentions "de-centred correctors" as a possible cause but I have no idea how they are held in your scope - the old C11 used cardboard shims to centre the corrector in its flange whereas the C14 has adjustable nylon-tipped grub-screws in the flange to set the corrector alignment...no idea about Meades.

 

Kevin (astrovienna) who has been seen here again recently comments in the thread I've linked to...& he made a lot of test images & went through a lot of conniptions trying to nail the matter down way back when...perhaps PM him for some more info (Glenn rarely appears here nowadays) but for us cooling & seeing play strong roles in determining what your defocused/diffraction rings look like!

 

You can send yourself crazy googling "planetary images from an astigmatic telescope" just on CN.....SCT's very often appear to display astigmatism - we get oval diffraction rings after rapid cool-downs (remembering we use salted ice to very rapidly lower the temperature of our C14's primary) but we do have a temperature gauge to monitor the primary & always allow for a "primary relaxation period" after the rapid cooling.

 

In these situations the diffraction rings revert to the more circular pattern as the night wears on but it is important to remember that an oval appearance will often be seen in defocused stars regardless with SCT's as does the "trefoil" appearance also...but neither produces much of a problem in imaging...I imagine that you have nailed down the cooling/acclimatisation situation but thought it worth mentioning. shrug.gif

 

Further reading to send you crazy would be this recommendation lol.gif https://www.cloudyni...ormal-in-scts/ 

 

If I haven't confused you further with the preceding, it should be said that these aberrations whether seeing/temperature or actual optical in nature (or both!) can make collimation much more of an issue, so I guess it's worth stating that a night of good seeing where your scope is thoroughly acclimatised would help you no end..! wink.gif

 


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#9 dcaponeii

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Posted 17 September 2021 - 09:14 PM

I think about this constantly also Don without making any comments in your threads - mainly because I'm at a loss to come up with any definitive comments.

Astigmatism, which is obviously present could well be the cause of the seemingly "laterally blurred" outcomes - I would be surprised if an SCT scope had this as a major manufacturing defect but I guess all things are possible...

I will dig out some old planetary images if I can find them (might be very hard though...) where I had captured Mars with an astigmatic scope - I have this strange notion that there was some sort of "smearing" of the features but it was quite a long time ago.....

This thread where a couple of old planetary imaging comrades discuss the issue are worth reading - particularly Glenn Jolly's (DesertRat) with the animation of Mars which you will need to click on to get it working in Post #10 here: https://www.cloudyni...ct-on-imaging/

Glenn mentions "de-centred correctors" as a possible cause but I have no idea how they are held in your scope - the old C11 used cardboard shims to centre the corrector in its flange whereas the C14 has adjustable nylon-tipped grub-screws in the flange to set the corrector alignment...no idea about Meades.

Kevin (astrovienna) who has been seen here again recently comments in the thread I've linked to...& he made a lot of test images & went through a lot of conniptions trying to nail the matter down way back when...perhaps PM him for some more info (Glenn rarely appears here nowadays) but for us cooling & seeing play strong roles in determining what your defocused/diffraction rings look like!

You can send yourself crazy googling "planetary images from an astigmatic telescope" just on CN.....SCT's very often appear to display astigmatism - we get oval diffraction rings after rapid cool-downs (remembering we use salted ice to very rapidly lower the temperature of our C14's primary) but we do have a temperature gauge to monitor the primary & always allow for a "primary relaxation period" after the rapid cooling.

In these situations the diffraction rings revert to the more circular pattern as the night wears on but it is important to remember that an oval appearance will often be seen in defocused stars regardless with SCT's as does the "trefoil" appearance also...but neither produces much of a problem in imaging...I imagine that you have nailed down the cooling/acclimatisation situation but thought it worth mentioning. shrug.gif

Further reading to send you crazy would be this recommendation lol.gifhttps://www.cloudyni...ormal-in-scts/

If I haven't confused you further with the preceding, it should be said that these aberrations whether seeing/temperature or actual optical in nature (or both!) can make collimation much more of an issue, so I guess it's worth stating that a night of good seeing where your scope is thoroughly acclimatised would help you no end..! wink.gif


I always appreciate the time you spend crafting these detailed replies. I spent some time tonight before starting to image generally moving the secondary collimation screws tighter to see if that might improve things. These spring-loaded screws are not supposed to be tight as the spring should maintain tension on the secondary mounting plate. Seeing is pretty bad this evening so I’m not sure what I’ll learn at the scope although the details on my preview window look pretty good even though the larger scale turbulence makes it appear that I’m viewing through pond water. I’ll review those references and factor that into my adjustments. I may just be stuck with a lemon but I’ve not yet opened the OTA so I’ve not exhausted all possibilities yet. Thanks again for all your help. Images this year I’m achieving are still generally better than last year for sure.

#10 Foc

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Posted 17 September 2021 - 10:31 PM

I think after all your effort you have achieved one of your better looking Jupiter images.  Good luck Don in finding the source of your frustrating optical issue.


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#11 Borodog

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Posted 18 September 2021 - 09:35 AM

You are exactly correct.  The star image changes from elongated in one direction to elongated in the perpendicular direction as you move through focus.  Using the moons to focus with the post-processing in Firecapture set to average (about 10 frames) has been useful the past couple of sessions to "average" out the excursions around critical focus.  However, the worse the seeing is the larger the excursions around the "critical" focus are and I then end up with small "cross-shaped" moons or stars that you've seen before.  I did capture a short video of a star near focus the other night.  This cruciform shape is also the reason why features on the disk end up rather "blocky" in nature.

 

attachicon.gif2021-09-11-0138_8-DWC-L-Star__M__AS_P15_lapl5_ap1wP.jpg

 

As you can clearly see, I'm not quite collimated as there is a flare on one side but the diffraction rings are visible but so it the cross formed by the 15% stack sampling the entire seeing range around critical focus. 

 

I keep hoping that I'll find something in the secondary or optical train that can be corrected but frankly I'm running out of ideas and may just have to live with it.

The star is on-axis?



#12 Borodog

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Posted 18 September 2021 - 10:43 AM

As you can clearly see, I'm not quite collimated as there is a flare on one side but the diffraction rings are visible but so it the cross formed by the 15% stack sampling the entire seeing range around critical focus.

 

What do you mean here? The image does not appear to be at critical focus; there's no central spot. As you move away from critical focus you will lose the central spot. Is it possible that your planetary images are simply not well focused? If the scope is otherwise well collimated, looking at the in-focus Airy disks, I would not expect a radical decrease in resolution. It's a puzzler for sure.

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#13 Kiwi Paul

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Posted 18 September 2021 - 02:42 PM

Yes, the focus issue is a good point Borodog.
Paul
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#14 dcaponeii

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Posted 18 September 2021 - 05:13 PM

Yes, the focus issue is a good point Borodog.
Paul


I agree but I moved back and forth through focus and am pretty sure the capture was at focus. It’s had me stumped for months. What’s really bizarre is that there is a clearly resolved centroid in Metaguide!! Maybe I’m doing something dumb with my camera or in stacking. Ugh

#15 Borodog

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Posted 18 September 2021 - 05:55 PM

And if you clearly defocus, you get a nice concentric donut?



#16 Kiwi Paul

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Posted 18 September 2021 - 06:02 PM

And if you clearly defocus, you get a nice concentric donut?


… which should display an oval shape due to the astigmatism.

#17 dcaponeii

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Posted 18 September 2021 - 06:04 PM

Yes very much so and on both sides of focus.  Poisson point dead center as well.

 

This was the starting point the other night.

2021-09-11-0251_8-DWC-L-Star__M__AS_P15_lapl5_ap26wP.jpg

 

Here's where I left it before starting to image

2021-09-11-0256_3-DWC-L-Star__M__limit000750-001300_AS_P15_lapl5_ap33wP.jpg


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#18 Kiwi Paul

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Posted 18 September 2021 - 06:12 PM

These still look to be a long way out of focus. Could you not contract the very bright outer ring and the other fainter inner rings down to achieve even a bloated airy disk?
Paul

#19 Kiwi Paul

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Posted 18 September 2021 - 06:14 PM

Oh, I see what you mean. You left the shape as you depict after adjustment. Well they look really good in terms of being concentric.
Paul

#20 dcaponeii

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Posted 18 September 2021 - 06:14 PM

These still look to be a long way out of focus. Could you not contract the very bright outer ring and the other fainter inner rings down to achieve even a bloated airy disk?
Paul

I didn't image close to focus except for the image above which was essentially at focus.  I'm headed back out tonight I'll try and take an entire set through focus with the IR Pass filter in place.  I didn't think to grab the Metaguide screen shot I don't think.  I'll look and see and edit the image back in if I have it.  FYI - They are about the same size as Jupiter's disk at that f/number.


Edited by dcaponeii, 18 September 2021 - 06:15 PM.


#21 dcaponeii

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Posted 18 September 2021 - 06:17 PM

This was the most recent Metaguide image I had on my computer.  Wasn't quite centered yet but you can clearly see the centroid is there and the Airy Disk as well.  When I image with Firecapture I get the cross image above.

 

MG_DonCapone_12LX600ACF_210904_2.png


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#22 Kiwi Paul

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Posted 18 September 2021 - 06:17 PM

The other thing that strikes me is, if those you are getting this shape either side of focus, lm not sure you have an astigmatism problem? Where is the oval shape? Mind you that star image you posted did compare to the in focus image Borodog posted.
Paul
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#23 Kiwi Paul

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Posted 18 September 2021 - 06:25 PM

That stacked Metaguide image shows a pretty good concentricity. The airy disk in the centre has a hint of that idealised astigmatism signal Borodog posted ie a cross shape on the airy disk. The side graph having the blue line of your image above the idealised profile is indicating the amount of scattered light outside the inner airy disk area in your image. I guess that could be seeing dependant too.
Paul

#24 Kokatha man

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Posted 18 September 2021 - 07:58 PM

Don, the images in your Post #7 aren't really too bad at all, SCT's will often exhibit diffraction rings that appear somewhat warped/misshapen. Raising the levels of the first one does not seem to elicit a clear P.P. however...

 

Do you have another image of this night where you have moved nearer to focus but still showing 2 or 3 diffraction rings..?



#25 Kokatha man

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Posted 18 September 2021 - 08:00 PM

ps: otherwise, I would suspect the seeing was quite good on this night! 


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