Strehl ratio for a Celestron C11

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Montana
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Strehl ratio for a Celestron C11

Post by Montana »

Does anyone have a graph of the Strehl ratio for a Celestron C11 at 393nm for full aperture and stopped down in 1 inch steps? Just interested to know when it hits 0.9

Kind regards
Alexandra


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MalVeauX
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Re: Strehl ratio for a Celestron C11

Post by MalVeauX »

I don't think it will hit 0.9 without stopping it down heavily, if ever really. There are things you can do such as make or get things that undercorrect to improve this.

More information on making an undercorrector here:

viewtopic.php?f=10&t=25125&start=25#p239889

While this doesn't cover your exact scope, this is from Christian:
C-Strelh.jpg
C-Strelh.jpg (107.65 KiB) Viewed 3587 times
Very best,


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Re: Strehl ratio for a Celestron C11

Post by marktownley »

Montana wrote: Mon Oct 14, 2019 1:50 pm Does anyone have a graph of the Strehl ratio for a Celestron C11 at 393nm for full aperture and stopped down in 1 inch steps? Just interested to know when it hits 0.9

Kind regards
Alexandra
Simple answer, it won't hit 0.9 at 393nm, ever!

You'll always get better results with the TEC.


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Re: Strehl ratio for a Celestron C11

Post by MalVeauX »

I recently contacted i-star optical about measurements like this for their large achromatic doublet lenses and the owner replied that they don't bother measuring down to that nor look at it for correction at all.

Without going with a specific well figured mirror (non-SCT though), especially a longer focal-ratio one (F5, F6, etc) the better odds of a high Strehl is really going to be found in an achromatic doublet refractor and then potentially look into increasing the air gap between the lenses, depending on the need. I actually picked up a 200mm F6 quartz mirror for this purpose and attempted to use it, but, I couldn't ever get that tube to handle the radiation/heat even with my full aperture D-ERF unlike the SCT which handles the radiation/thermal just fine. So I can't really image with the 200m F6 reflector for solar at this time. I still have it and may re-visit it, because it did focus 430nm just fine, so odds are it would do 393nm too. I just can't get it to handle any solar right now even with the D-ERF due to radiation/thermal. Nothing melts, but the image is nervous/twitching even at sharp focus, including Ha 656nm.

I have a 150mm F8 achromatic doublet that is pretty well corrected down to 393nm and 430nm, so I sort of gave up on getting my C8 Edge with my D-ERF to do 393nm and 430nm at this point because the SCT mirrors/corrector plate just were never designed to really do these wavelengths.

I have the stuff to attempt to make an undercorrector, so I will attempt that soon, and I've used a chromacor in the past too for this. The seeing requirements at these apertures and image scales is immense.

Very best,


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Re: Strehl ratio for a Celestron C11

Post by christian viladrich »

Here are some actual interferometric measurements (HASO) of the Strelh ratio of various C11 OTA (EdgeHD or not) at 473 nm :
https://www.airylab.com/contenu/mesures ... 5003-a.pdf
https://www.airylab.com/contenu/mesures ... -33001.pdf
https://www.airylab.com/contenu/mesures ... 9001-a.pdf
https://www.airylab.com/contenu/mesures ... 5002-a.pdf

Unfortunatly (but not surprisingly) they are in line with the curves given by Marty.


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Re: Strehl ratio for a Celestron C11

Post by christian viladrich »

What aperture did you use to take this image ?


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