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Ritchey-Chretien uncorrected, but usable, image circle vs. aperture

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#1 xthestreams

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Posted 24 June 2021 - 08:22 AM

Hello internet!

 

Happy owner of a CDK 12.5 AND a 300mm (12") GSO RC (I know, chalk and cheese, but I really like my RC and love my little Planewave). The question I have relates to my recent experience adding the CDK to the observatory.

 

Bottom line, the PW is just incredible in terms of the technology AROUND the scope and stupidly easy to collimate, but it terms of "flat" image, to my poorly trained eye, they're pretty much equal - at least for APS-C sized sensors - when it comes to flatness (both are f8, both roughly 12", equiv focal lengths)

 

Which got me thinking about OTA #3 - ideally one day, I'd like to go bigger - ideally a 17" CDK (because they're so easy to work with) but perhaps a 16" GSO (suitably modified to include some of the automated heating/cooling smarts of the PW that I enjoy so much).

 

My question is "Can I/how do I compute the effective flat (or as good as) image circle for a given optical prescription and compare it to the CDK?"

 

I've read the ASA RC vs CDK white paper (aka the "we hate you Planewave" files) and it would seems that, even uncorrected, with a large enough aperture the field curvature for a 50mm diagonal sensor would be effectively equivalent to a CDK of a similar size (in so far as Joe Public wouldn't notice a difference in the final image and even the pixel peepers would need to, erm, peek).

 

Please don't misunderstand the post, I have a very real sense of the learning curve associated with trying to collimate an RC vs. the elegance of a CDK - I'm simply trying to get my head around the real-world technical differences and if I can save some short term pennies and go 16", I figure why not - I can always upgrade at a later date if money & passion permits.

 

Let the flames, begin!

 

 



#2 bips3453

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Posted 24 June 2021 - 05:15 PM

Buy both and see for yourself... ! wink.gif

 

Jokes apart, ASTAP app has CCD inspector tool and can be used to check Off-axis abberation. You might find it useful. 

 

https://www.hnsky.or...p.htm#inspector



#3 xthestreams

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Posted 24 June 2021 - 06:59 PM

Thanks! Yeah, NOT happening!

 

I've got CCD Inspector (and PixInsight) and have played around with looking at that on both scopes, as I said, not that big a difference.

 

I didn't know ASTAP had that feature though and I am no pleased to have a great solution for Mac and Raspberry Pi for a CCDI alternative (and one that's not old and clunky like the expensive CCDInspector on Windows) 

 

Great tip!



#4 freestar8n

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Posted 24 June 2021 - 08:23 PM

A big factor is how small your stars are in arc sec. if your seeing guiding and focus aren’t great then all stars will be swelled up and look about the same across the field despite curvature and astigmatism. And you can slightly defocus the center so the overall field is ok. Combine that with a small sensor and field correction may not help much.

But if your fwhm is below 2” or so then it becomes more of an issue.

Frank
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#5 xthestreams

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Posted 25 June 2021 - 12:23 AM

Thanks Frank - btw LOVE MetaGuide!

 

For my Micro 4/3rds chips, I suspect I would never see significant curvature on my RC - I certainly couldn't!

 

With a full frame sensor like a 16803 or IMX455 I am *guessing* it will be more of a problem, hence the question.

 

The plan is for the scope to be situated in Coonabarranbran, not quite the Atacama but I would hope sub 2" is possible. 

 

I guess my question is still, is there a simple math formula that gives me that number or do I need to run one of those fancy ray tracing simulations and hire a PhD to interpret it for me? (that's YOU Frank ;-)


Edited by xthestreams, 25 June 2021 - 12:25 AM.


#6 AtmosFearIC

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Posted 26 June 2021 - 07:19 AM

I’ve had some similar thoughts with the CDK and RCs. Planewave used to have a white paper that was CDK awesome and RCs suck but ASA take the opposing view with theirs so it’s an interesting comparison.

I can get very close to 1” seeing so I have wondered if a RC would be better than the 10” F/10 CDK I’ve got.
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#7 xthestreams

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Posted 26 June 2021 - 09:15 AM

I've got two f/8 12" scopes, one CDK and one RC - it's hard to spot the difference, but I'm not Diego either!

 

Based on what' sI've seen so far, a GSO 16" is worthy of consideration, even if it's as a setting stone scope on your path to the CDK 17"

 

Agree the white papers each set up the debate in favour of their respective products, I personally find the ASA one more credible based on my own experience and if spectroscopy is in your future, the RC gives you more choice - at the cost of a pretty gnarly collimation process.

 

p.s. where in Melbourne are you getting 1" skies :-)



#8 AtmosFearIC

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Posted 27 June 2021 - 09:25 PM

I've got two f/8 12" scopes, one CDK and one RC - it's hard to spot the difference, but I'm not Diego either!

Based on what' sI've seen so far, a GSO 16" is worthy of consideration, even if it's as a setting stone scope on your path to the CDK 17"

Agree the white papers each set up the debate in favour of their respective products, I personally find the ASA one more credible based on my own experience and if spectroscopy is in your future, the RC gives you more choice - at the cost of a pretty gnarly collimation process.

p.s. where in Melbourne are you getting 1" skies :-)


North of Melbourne ;)
Below is a comparison against the Chart-32 team, an 800mm RC in Chile against my 10” F/10 and a OSC.

https://cdn.astrobin...626bded3219.gif
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#9 xthestreams

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Posted 27 June 2021 - 09:41 PM

Given it's one of the southern most cities in Australia that gives you about 3000km to play with.... :-)

 

I've never REALLY understood what a huge difference the atmosphere can make (that and nearly triple the aperture) - that's such a cool comparison. (btw saw your posting on IIS re Tuc 47 - cool)

 

So - back to question - does this mean I need to learn to ray trace?




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