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OLSO model of the C14 Edge-HD ?

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#26 wh48gs

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Posted 09 October 2013 - 12:14 AM

I tried to google N-BALF2 but found nothing. And here is C8 EHD prescription in book. The mirrors system is f/8.5 if you remove corrector. I wonder why using corrector to increase focal length.



I don't think they had complete prescription. Why tweak otherwise? Their lenses are noticeably different (thickness, radii) than what Celestron gives in its file. The closest I can come to it - with all the dimensions nearly identical as in Celestron's drawing - is if I go with 0.707 neutral zone radius (not 0.866 as in the book). No lateral color to speak of, and pretty close blur-size-wise.

With 0.866 zone there has to be more lateral color, or a slight longitudinal chromatism (and that with the corrector pulled a half inch, or so, out). Another possibility is that they go with 0.866 zone and somewhat more relaxed Schmidt radius, giving them ~0.87 zone and minimized lateral color. The secondary is also different than in the standard SCT, so they are not a perfect match for the standard SCT attachments for prime focus imaging anyway.

Vla

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#27 freestar8n

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Posted 09 October 2013 - 01:27 AM

They say that,

we optimized the design for the largest possible field of good correction.



That makes me think the original design was targeted more at highest performance in the center, for planetary and visual work, while reducing the Strehl farther out. This is consistent with the white paper, which makes the point that deep sky imaging is seeing limited.

I don't think they are just talking about a choice of focus, since they say "optimized the design."

Since Smith/Ceragioli/Berry compare scopes based on Strehl across the field, a design optimized more for the center would not look as good. But for the intended usage modes of this scope, it would make sense to me. Minimal sacrifice of on-axis performance, while maintaining overall performance adequate to maintain small deep sky fwhm's across the field in long exposures.

That's my interpretation anyway.

Frank

#28 wh48gs

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Posted 09 October 2013 - 12:40 PM

That makes me think the original design was targeted more at highest performance in the center, for planetary and visual work, while reducing the Strehl farther out. This is consistent with the white paper, which makes the point that deep sky imaging is seeing limited.



What doesn't feet in is that the paper design (according to the blurs shown) has less astigmatism and less lateral color. Thus it can only be superior to the book "optimized" variant, not the other way around (center field is not discernibly different). The book variant would have been slightly better overall if some 5mm closer to the secondary.

When compared based on their design performance, tweaked and original corrector configurations do not show significant differences in any respect, except when the original configuration is matched with 0.707 neutral zone corrector (bottom). The blur shapes then are also quite close to Celestron's. It suggests that the standard C8s are made with 0.707 neutral zone, and that the actual C8 Edge corrector is similar to this variant.

Vla

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#29 BYoesle

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Posted 12 October 2013 - 01:23 PM

It seems activity on this thread has subsided, so at the risk of hi-jacking it, I’d like to know if anyone has pursued similar improved optical performance from a standard C8 and C14?

"Telescopes, Eyepieces, Astrographs" shows significantly improved performance from the standard SCT optics via relocation of the corrector plate -- close to the Edge HD levels. But this seems it would necessitate significant mechanical modifications including some sort of spider assembly to maintain the position and alignment of the secondary.

Has anyone explored design of a lens corrector system similar tho the Edge HD for the standard Celestron SCT designs which could be located in the OEM primary baffle tube? Could these be modeled in the OSLO edu version?

#30 ch-viladrich

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Posted 13 October 2013 - 09:43 AM

Hi all,

Thanks a lot for this very interesting discussion.

I have been playing with the two OSLO models Vla derived for the C14 Edge-HD:
http://www.astrosurf...ensitivity-a...

Remember my point of view here is from high resolution imaging on large size sensors ;-)
The C14 Edge-HD seems to give lower Strelh ratio than the classic C14 over 650 nm, which is not good news for us. Maybe this is due to the corrector ?

On the blue side, they seems to be equivalent.

http://www.astrosurf...SC14-Strelh.jpg

The version with a neutral zone at 0.866 gives less change of focus with wavelenght which is nice when changing filters in LRGB imaging :
http://www.astrosurf.../SC14-focus.jpg

The major drawback of these large size SC for planetary / lunar imaging is the fast degration of optical quality in blue and UV.
The CDK seems a promising alternative. Smith/Ceragioli/Berry gave the description of a 300 mm F-3-F7 optimized for deep sky imaging with a diffraction limited field of over a 1° field from 410 to 800 nm.

Of course this design with a 46% obstruction would make no sense for high resolution imaging.

So, I was wondering if it would be possible to design a CDK 350 mm F3-12 optimised for high resolution, with a 30% obstruction, and a flat diffraction limited field of say 7 arcmn in diameter, from 350 to 800 nm ?

The all spherical surfaces would be easy to manufacture and collimate(much easier than the hyperbolic secondary of a Cassegrain). While the absence of the Schmidt corrector would gives access to blue and UV with good image quality.

#31 wh48gs

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Posted 13 October 2013 - 12:20 PM

I was wondering if it would be possible to design a CDK 350 mm F3-12 optimised for high resolution, with a 30% obstruction, and a flat diffraction limited field of say 7 arcmn in diameter, from 350 to 800 nm ?



It can be done. Since it is relatively small aperture and long focal ratio, it can't use the simplest arrangement with two plano lenses (too much higher order astigmatism), but what can be used is still within the reach of the advanced amateurs. The system pictured, 350mm f/3/11.2 is not fully optimized, but it shouldn't make appreciable difference. The 350nm diffraction limited field is about quarter of a degree, and 800nm about half a degree.

Vla

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#32 Alph

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Posted 13 October 2013 - 12:31 PM

I have been playing with the two OSLO models Vla derived for the C14 Edge-HD:


I haven't found any C14 Edge-HD OSLO files there. :question:

#33 wh48gs

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Posted 13 October 2013 - 12:42 PM

Has anyone explored design of a lens corrector system similar tho the Edge HD for the standard Celestron SCT designs which could be located in the OEM primary baffle tube? Could these be modeled in the OSLO edu version?



The problem here is that inducing coma in simple correctors always comes with some undercorrection, so if a corrector is to be used in otherwise well corrected system, that is a price to pay. In this example corrector cuts coma nearly in half, but induces some 1/10 wave of undercrrection. More coma corrected, more undercorrection. For an SCT with 1/4 wave of overcorrection this would, however, work perfectly, correcting both, coma and residual spherical (rear lens is deliberately made thicker to allow possible use of two thin plano lenses).

Vla

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#34 ch-viladrich

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Posted 13 October 2013 - 01:09 PM

Many thanks a lot Vla :-)
I'll have a look it.

#35 ch-viladrich

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Posted 13 October 2013 - 03:03 PM

Hi Vla,
You've find an amazing design :-)
- Strelh ratio on axis is 0.99 at 300 nm, then 1 from 350 to 1000 nm !!
- the focal shift from 350 nm to 1000 nm is a mere 0.01 mm !!
With a F/3 primary ratio, the optical tube will be arround 100 - 110 cm, which is still nice.
You've just designed the perfect large and still portable planet/ moon killer :-)

#36 BYoesle

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Posted 13 October 2013 - 03:08 PM

Thank you very much Vla - it will be fun to explore the possibilities...

:bow:

#37 MitchAlsup

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Posted 13 October 2013 - 03:17 PM

Vla


Looking at the spot diagrams closely, I see some similarities between the book figure and the corrector 5mm closer, and some similarities between book and 0.866 null zone.

The 5mm closer overcorrects blue <spots> wrt book and undercorrects red.
The 0.866 corrector null undercorrects the blue <spots>

What would happen it fyou split the difference and tried 0.80 null zone and 2.5mm corrector spacing?

http://www.cloudynig...6127025-edg.PNG

#38 ch-viladrich

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Posted 13 October 2013 - 03:29 PM

BTW, is there a way to display the Strelh ratio accross the field with OSLO-LT ?

#39 wh48gs

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Posted 13 October 2013 - 04:40 PM

The 5mm closer overcorrects blue <spots> wrt book and undercorrects red.
The 0.866 corrector null undercorrects the blue <spots>



It only appears so. They are all overcorrected in the blue. Look at the lateral color graphs at right. It would take a zone below 0.7 to get them together. In effect, it is inducing some longitudinal color error in order to minimize lateral - can't have both with this glass combination.

Vla

#40 wh48gs

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Posted 13 October 2013 - 04:46 PM

Hi -

is there a way to display the Strelh ratio accross the field with OSLO-LT ?



Not that I know. But it should have the RMS OPD under Spot diagram>>Spot size and OPD vs. field. Just as good for monitoring diffraction limited field.

Vla

#41 ch-viladrich

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Posted 16 October 2013 - 03:29 PM

Hi,
I've done some more simulations.
The CDK 350 F3-F12 is clearly sensitive to the backfocus. This won't be a problem for deepsky imaging, but it might be more tricky with planetary imaging.
I have to check out the range of backfocus I use on my C14 depending on filter wheel, ADC, focal length of Barlow lens, etc.

#42 ch-viladrich

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Posted 16 October 2013 - 03:30 PM

... the simulation is at the end of this page :
http://www.astrosurf...ensitivity-a...






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