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Jupiter this apparition, and questions about necessity of a coma corrector

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#1 Baikalic

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Posted 29 September 2022 - 05:12 PM

This is my 2nd year of observing and my first full summer of imaging (read: virtually the only season for imaging in cloudy PNW!) and after absorbing and lurking to optimize my setup and workflow, I've now started getting images I'm satisfied with. I've got:
1) Jupiter with Io on 8-29-22 at 0742.8 UTC, with 9100 frames manually tracked on my 8" dob with 28% stacked in AS!3, and
2) Jupiter on morning post-opposition on 9-27-22 at 1003.6 UTC, using an 8-image derotation in WINJUPOS, each image comprised of 9000-15000 frames tracked with my 8" dob over 18 mins on a DIY EQ mount I constructed for the express purpose of making in time for the Jupiter opposition (troubleshooting delays notwithstanding).

 

1) 2022-08-29-0742_8.jpg

 

2) 2022-09-27-1003_6-09-27-0954_0-L-Jup Registax.jpg

 

I am nursing a Captain Ahab-level obsession with maximizing the potential of my modest-budgeted setup (signature contains details), and will be attempting to improve the following: better collimation, improve friction dynamics of gearing on my EQ mount (it only tracks for 5-10 minutes reliably with planet reasonably stable on ASI462 sensor/ROI in Firecapture), and look into coma correction.

Coma correction as relevant to the coma-free field of view in my setup is what I'm desperate to get some insight on as I am close being completely ignorant. My understanding from searches in this fantastic community is that in visual observing, the coma-free field of view in diameter is ~ 0.022*(f ratio)^3. In my case, that would be 0.022*(6)^3 = 4.752 mm. Since my sensor size in the ASI462MC is 5.6mmx3.2mm, with diagonal of 6.45mm, that would imply the planet target could have coma introduced if it leaves the central ~70% of my sensor area.
HOWEVER
if my setup has me imaging at f/14.75 (2.5x barlow), does the coma-free field of view blow up to 0.022(14.75)^3 = 70.6 mm? And therefore there is no risk, even in totally manual tracking situations, of encountering coma? I would like to completely eliminate coma as a risk to image smearing/blurring. Thanks for any insight...


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#2 RodgerDodger008

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Posted 29 September 2022 - 05:27 PM

Great work!
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#3 dcaponeii

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Posted 29 September 2022 - 09:15 PM

Coma correction completely irrelevant for planetary imaging. Just extra glass you don’t need between your chip and the object. Barlow to make f/ratio = 5x camera pixel size. ADC. and a UV/IR Cut filter. Read Andrew’s FAQ pinned to the top of the forum.
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#4 Baikalic

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Posted 30 September 2022 - 04:49 PM

Coma correction completely irrelevant for planetary imaging. Just extra glass you don’t need between your chip and the object. Barlow to make f/ratio = 5x camera pixel size. ADC. and a UV/IR Cut filter. Read Andrew’s FAQ pinned to the top of the forum.

I have read Andrew's very helpful FAQ multiple times and improved myself to this point ; many thanks to his and others' knowledge. My question about coma though (sort of ) stands, in a nonstandard case where I have imperfect tracking (pending further improvements on the DIY mount) and planet wander on sensor, inevitably toward the edges, (ROI) is a near-term issue. I suppose one way to check if coma is an issue is simply visually, perhaps with gamma on to evaluate contrast/detail loss near the edges of my max ROI on the ASI462MC sensor. 



#5 Ittaku

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Posted 30 September 2022 - 04:52 PM

The edge of the sensor is still very close to the centre of the sensor as the sensor is absolutely tiny. Now if you were using a full frame camera and the full width of the sensor, it would matter, but no one does that in planetary as it's incredibly wasteful.


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#6 RedLionNJ

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Posted 30 September 2022 - 10:17 PM

Your question is a very good one, Baikalic. I agree with your math regarding the size of the coma-free area around the optical axis at prime focus. But what happens if we extend that focal length by a factor of 2 or more?  Would the "good area" stay at the same absolute size, or would it increase (or decrease) proportionately to the additional amplification?

 

I believe the coma corrector is supposed to provide a reasonably flat plane only at the prime focus, so I am uncertain as to the effect of extending the light path through a barlow. On one hand, tt would seem reasonable to expect the "good area" to be expanded in diameter, proportional to the increase in focal length.  On the other hand, it's not as if the entire area inside the "good area" is perfectly flat - there is still some curvature, increasing toward the edges. It seems reasonable that curvature would also be enhanced by the increase in focal length.

 

Not that you can place a high-contrast grid far enough from your scope to take some test images of it, but that would seem a reasonable path to pursue to determine how much of the sensor would be usable at the higher focal length.

 

Interesting dilemma!


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#7 Baikalic

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Posted 03 October 2022 - 01:07 PM

Your question is a very good one, Baikalic. I agree with your math regarding the size of the coma-free area around the optical axis at prime focus. But what happens if we extend that focal length by a factor of 2 or more?  Would the "good area" stay at the same absolute size, or would it increase (or decrease) proportionately to the additional amplification?

 

I believe the coma corrector is supposed to provide a reasonably flat plane only at the prime focus, so I am uncertain as to the effect of extending the light path through a barlow. On one hand, tt would seem reasonable to expect the "good area" to be expanded in diameter, proportional to the increase in focal length.  On the other hand, it's not as if the entire area inside the "good area" is perfectly flat - there is still some curvature, increasing toward the edges. It seems reasonable that curvature would also be enhanced by the increase in focal length.

 

Not that you can place a high-contrast grid far enough from your scope to take some test images of it, but that would seem a reasonable path to pursue to determine how much of the sensor would be usable at the higher focal length.

 

Interesting dilemma!

RedLionNJ - thanks for the feedback. I think I will just live in a bit of ignorance for the remainder of Jupiter's opposition window and capture as best as I can whenever conditions are good (we got smoke problems here with the forest fires so coma is now downgraded to "least concern"!). I'll pursue some studies in the winter off-season, your suggestion of high-contrast grid captures to evaluate curvature is a clever one. Cheers!



#8 Baikalic

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Posted 03 October 2022 - 01:09 PM

The edge of the sensor is still very close to the centre of the sensor as the sensor is absolutely tiny. Now if you were using a full frame camera and the full width of the sensor, it would matter, but no one does that in planetary as it's incredibly wasteful.

I've partially obviated the problem by making some improvements in the plastic sheathing on the worm gear on my diy EQ mount, and it tracks reasonably well for 10 minutes now with target at center (Jupiter last night), which is plenty of time to capture my frames. Previously it only tracked well for 2-3 minutes before some slippage.



#9 dcaponeii

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Posted 03 October 2022 - 01:10 PM

There are folks on here generating outstanding planetary images with f/4.5 Dobs.  Since they don't need coma correction then you don't either I suspect.


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