Critical Sampling and filter choice for Celestron NexStar Evolution 8? - Forums

Topic starter

Jun 14, 2022

Hi All,

I recently picked up a used Celestron NexStar Evolution 8 which I am mainly wanting to use for high magnification imaging of the moon (and in time planets) using lucky imaging. Only dedicated astro camera I currently have is an ASI120MM, so I was going to look at using my Sony a7S III Full Frame mirrorless as it is capable of good quality 4k 120fps video and has large 8.4 micron pixels.

I've ordered a 2" E-mount adapter and am now looking at how to go about magnifying the image to maximize detail of the moons surface. Using a CCD Critical Sampling calculator it seems something like the Tele Vue 2" 4x Powermate would be a good selection. However I have read (never used) that a 685nm IR Pass filter would help cut through some off the seeing turbulence found at lower wavelengths, in this case it would seem something like the Explore Scientific 2" 3x Focal Extender would be a better choice for hitting critical sampling.

I've not been able to find any sort of historical database of seeing conditions for my area in Australia, however I believe them to be pretty decent as I am hundreds of km's away from the coast in a dry climate. I've read that this CCD critical sampling rate can be pushed a little bit if seeing conditions are good?

Would really appreciate any advice on which 2" powermate/focal extender to go with (even if it is other than what I've listed) and also what 2" 685nm IR Pass filter I should be looking at. I have looked at the Baader filter which is not widely available where I am, though a few places do seem to stock the Optolong filter. Is this any good?

Engaging

andrea tasselli

Frequent contributor

Jun 14, 2022 · Edited: Jun 14, 2022

I think this would be my choice as standard SCTs have coma same as the newtons:

https://www.apm-telescopes.net/en/apm-comacorrecting-1-14-ed-barlow-27-x

Having used a number of Optolong filters you won't be disappointed getting their IR-pass filter.

I'd venture to suggest you won't be able to get the full benefits of a FF camera (in terms of field) right off the bat, APS-C field size seems more within grasp. I suspect in this specific case less is more. Frankly, I doubt that the movie feature of the Sony camera are good enough for a hi-res capture of planetary bodies. You'd want a specialized camera with high frame rate, no-compression, high bit (10 or 12) capturing capabilities. No harm in trying though...

Addendum:
Incidentally, using the Sony with an IR-pass filter won't be any good as the IR-cut filter would remove nearly all of the signal. Even if it is modded with full-spectrum filter removal you'd need to go beyond 850 nm in order to get full monochrome response form the camera.

Dominic Ryan

Topic starter

Jun 14, 2022

Thanks @andrea tasselli

Am I able to use a flattener separately to the barlow, or is this complicating the imaging train too much?

Using the Sony definitely isn't ideal, I believe the baffle tube on the C8 is 37mm so I will get some vignetting I imagine (even with the 1.1x crop on the a7S III when shooting 120fps).

At some stage I've have to make the decision to spend a load of $$ on an EdgeHD 8 or (far more likely) get a dedicated astro camera with a smaller sensor so only the centre of the image circle is used.

andrea tasselli

Frequent contributor

Jun 15, 2022

Dominic Ryan:
Am I able to use a flattener separately to the barlow, or is this complicating the imaging train too much?

I'm not sure it is going to work for, never mind the expenses. AFAIK there is no flattener per-se, there is a reducer flattener bringing the focal ratio down to f/6.7 or so. You probably don't want to do that.

Dominic Ryan:
At some stage I've have to make the decision to spend a load of $$ on an EdgeHD 8 or (far more likely) get a dedicated astro camera with a smaller sensor so only the centre of the image circle is used.

In terms of pure planetary resolution the older models are to be preferred, IMO. And I sicerely doubt the viability of the Sony as a planetary camera, especialli in the near-IR. There are fairly decent monochrome cameras at a modest price from ASI there are far better suited to the task at hand. Even the 120MM will get you started albeit with a reduced FOV.

John Hayes

Frequent contributor

Jun 15, 2022

Good news! AIC has opened up their archives to everyone. My presentation included a lot of information about how to best select a camera taking into account seeing, focal ratio, and camera. I derived a simple rule of thumb that you can use to select a suitable camera. You can find the presentation (along with my slides) here:

https://www.advancedimagingconference.com/articles/secrets-long-focal-length-imaging-john-hayes

Good luck with your project.

John

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AstroNikko

Jun 15, 2022

Hi Dominic,

I'd actually suggest starting out with your ASI120MM. The smaller 3.75um pixel size will give you a smaller pixel scale. Smaller would be better, but this is a good start. I'd say use this time to get used to the camera settings available in your capture software of choice. It'll help inform you on what you might want in your next camera. For now, best to keep it simple and not worry about using a barlow or tele-extender.

As for the IR pass filter, check out the Astronomik ProPlanet 642 BP IR-Pass filter. It has a bandpass of 642nm to 842nm. Which I think helps produce sharper images than IR-pass filters without a cut-off since IR won't typically focus at the same distance over longer wavelengths. I've had good results with it.

Good luck! And have fun :-)

Clear Skies,

Nikko

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kuechlew

Jun 15, 2022

John Hayes:
Good news! AIC has opened up their archives to everyone. My presentation included a lot of information about how to best select a camera taking into account seeing, focal ratio, and camera. I derived a simple rule of thumb that you can use to select a suitable camera. You can find the presentation (along with my slides) here:

https://www.advancedimagingconference.com/articles/secrets-long-focal-length-imaging-john-hayes

Good luck with your project.

John

Excellent presentation John, thank's a lot!

Clear skies
Wolfgang