Strehl ratio for a Celestron C11

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Montana
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Strehl ratio for a Celestron C11

Post by Montana » 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



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Re: Strehl ratio for a Celestron C11

Post by MalVeauX » Mon Oct 14, 2019 4:16 pm

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 2211 times
Very best,



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Re: Strehl ratio for a Celestron C11

Post by marktownley » Mon Oct 14, 2019 4:39 pm

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 » Mon Oct 14, 2019 5:08 pm

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 » Mon Oct 14, 2019 6:36 pm

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 langleif2 » Mon Oct 14, 2019 8:16 pm

While I like CaK pictures done with a refractor it's possible as mentioned to correct the Schmidt plate for spherocrhomatism in the CaK wavelength. I made a modified Barlow to do this and it worked out successfully despite I by accident flipped a lens. At 2nd of oct I took this picture of AR2749 and one may notice it's not sharp all over because of the flipped lens but the active region is within the sharp area:

video0018_g4_apa.jpg
video0018_g4_apa.jpg (188.4 KiB) Viewed 2185 times

This was in the afternoon and I can take these pictures allmost at will all day long with the Barlow. I can't measure the Strelh value but I would say it's minimum diffraction limited (safe to say :P ).



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Re: Strehl ratio for a Celestron C11

Post by christian viladrich » Mon Oct 14, 2019 9:19 pm

What aperture did you use to take this image ?


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Re: Strehl ratio for a Celestron C11

Post by langleif2 » Tue Oct 15, 2019 8:42 am

C8 with full aperture Aries D-ERF. You can also take a look at viewtopic.php?f=10&t=25125&start=25#p239889 and get the story behind the Barlow. Now I'm waiting for a nice sunspot to appear and for winter so I can try it on Venus.

In the first place I experimented with a Skywatcher 102 f5 optimized in green and turned it into a good performer in CaK with the Barlow inducing a severe undercorrection.



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