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OLSO model of the C14 Edge-HD ?

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#1 ch-viladrich

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Posted 02 October 2013 - 02:15 PM

I am looking for an OSLO model of the C14 Edge-HD.

The ressourcefull book "Telescopes, eyepieces, astrographs" by Smith, Ceragioli and Berry gives a number of Zemax models in the 8" size, but none in larger sizes (11" or 14").

I would be very interested if someone have got something for the C14 Edge-HD.

The objective is to compare the performances of both versions for high resolution imaging with 2k x 2k sensors (spherochromatism, Strelh ratio over the sensor frame, variation of focus with wavelenght).

Thanks !

#2 wh48gs

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Posted 04 October 2013 - 06:55 AM

Assuming that the 8-inch on p257 is it, you can just upscale it. It does fit the corrector configuration I've seen published, but it is not the best possible. Placing negative lens first, both elements BK7, gives better of axis performance, including less lateral color. I'm sure Celestron's opticians could figure that out, so kind of wonder if the published versions were accurate.

Beside, the version in the book has neutral zone at 0.866 radius, which more than doubles spherochromatism vs. 0.707 zone.

Vla

#3 ch-viladrich

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Posted 04 October 2013 - 02:08 PM

Thanks for your answer.
By "upscaling" do you mean just a linear upscale (X 355/202 factor) ?
I would have assumed the coefficients of the Schmidt plate would have been different ? But I am not an expert.

#4 gatorengineer

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Posted 04 October 2013 - 05:19 PM

I'm sure Celestron's opticians could figure that out,


:roflmao:

And about that Edge focal reducer?

#5 wh48gs

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Posted 04 October 2013 - 06:09 PM

Yes, right clicking on the surface data table, scale lens>scale lens by constant... Aspheric parameters adjusts automatically.

Vla

#6 freestar8n

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Posted 05 October 2013 - 02:39 AM

The different EdgeHD sizes are not scaled versions, as can be seen from the layouts in the whitepaper available here.

In addition, the 14" version uses a different type of glass for one of the corrector elements.

The design described in Smith, Ceragioli, Berry is not the actual Edge8 design, but their own tweaked version of it based on the original design. I don't think the actual numbers for any of the designs is available, but you can get an approximate starting design by looking at the layout diagrams and noting the glass types used in each size.

Frank

#7 BYoesle

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Posted 05 October 2013 - 03:29 AM

I don't have the "Telescopes, Eyepieces, Astrographs" book in front of me right now, but I seem to recall that Celestron did supply the actual prescriptions for the Edge HD. Whether or not these were the numbers used in the books example I don't recall off the top of my head...

#8 freestar8n

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Posted 05 October 2013 - 03:46 AM

Yes - they explicitly say Celestron provided the actual design, but they altered it a bit for the design shown in the book. So - I don't know of a public source for the actual design.

The design itself would need to be optimized for the specific purposes of visual, planetary, and deep sky imaging, while at the same time it needs to conform to some other constraints:

It needs to maintain a mechanical backfocus of 5.25" for the Edge8 and 5.75" for the others to allow space for the imaging train.

It needs to work for Hyperstar, which means I think the Schmidt corrector and primary are unchanged from the non-Edge designs. The secondary spacing can be made slightly shorter, but not longer.

The field size does not scale and instead targets fixed detector sizes. This size is roughly 42mm diameter. The field needs to be diffraction limited in the center for planetary work, but beyond that the relevant limit is deep sky imaging with stellar fwhm's of around 1-2 arc-seconds.

With the 0.7x reducer, the backfocus should remain the same - except for Edge8 where it is shorter.

I'm not sure if the secondary is identical with the non-Edge version. If so, that is another constraint.

Frank

#9 BYoesle

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Posted 05 October 2013 - 10:40 AM

As I recall the corrector and the primary remain the same, and I believe Smith et.al. stated the secondary ROC was changed. I called Celestron a while back and they stated they could not sell the new secondary or the corrector optics separately in order to upgrade an older version.

#10 wh48gs

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Posted 05 October 2013 - 11:35 AM

Thanks for the file, Frank. That gives pretty much all the info short of prescription. Here's two prescriptions that are close to what Edge 14 is, and can be used for performance evaluation. For minimum spherochromatism, LA curves should intersect at 0.71 radius, which for the 14-inch implies about 133,000mm corrector radius w/o corrector. These two examples are for the effective 0.87 zone and 0.75 zone, the former needs -104,000mm radius to eliminate lateral color, the other 144,000mm. It implies that if they use 0.71 zone (-133,000mm radius) w/o corrector, with the corrector it would be effectively 0.78-0.79 zone. That would increase spherochromatism ~50% compared to the minimum w/0.71 neutral zone. Maybe it's because I used N-BALF4 instead of N-BALF2 cited by Celestron. But in any instance, this is about as close as it gets without actual prescription.

It's funny that the arrangement with a negative lens in front does not require radius adjustment for lateral color, needs only BK7, has plentiful 167mm backfocus at 8-inch and has 1/3 of the astigmatism of this corrector.

Vla

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#11 freestar8n

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Posted 05 October 2013 - 12:42 PM

Interesting. I'd be surprised if they overlooked a simpler approach given their constraints. That doesn't mean they didn't - but given the various different layouts and the choice of special glass for the 14" version, I thought they would have explored a wide parameter space including the combinatorics of different glass options.

Two other factors I didn't mention are the avoidance of pupil ghosts and related reflections/flares, and the associated alignment/fabrication tolerances.

I can believe that a simpler overall design is possible if you allow the corrector to be changed - but they are leaving that the same. I guess the secondary radius and spacing are changed. I'm not sure if the corrector distance is changed or not, but Hyperstar does require a special spacer when used with Edge. I'm not sure if this is due to a mechanical issue in how it attaches, or if the corrector spacing is actually different. If it did move, it moved toward the primary.

Frank

#12 ch-viladrich

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Posted 06 October 2013 - 03:09 AM

Thanks for all of these information :-)
I 'm going to try some tests with the two models you kindly gave Vla.
This is what they say in "telescopes, Eyepices, Astrographs" :

""when we requested more detailed data from Celestron, the company obliged""

#13 wh48gs

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Posted 06 October 2013 - 06:06 PM

No surprise - the omission was mine :foreheadslap: The reversed BK7 corrector was only correcting for coma, not field curvature. It could be used for the complete job but, since it employs milder curves and generates less of Petzval-correcting power, it leaves more field curvature in the system, and requires adding more astigmatism (50%+) to flatten the field than the arrangement used by Celestron. In addition to using BK7, it's positive is that it has more relaxed tolerances and less sensitivity to misalignment, but the design correction factor probably prevailed.

Hyperstar may only need spacer with c14 and c925 if it is designed for f/2 primary. Just a guess.

Vla

#14 wh48gs

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Posted 06 October 2013 - 06:34 PM

I took a closer look at the drawing in the Celestron's file, and there the primary is not f/2, but about f/2.15. Also the system is f/11, not f/10 as I went with previously. Here's two systems that replicate the design as given by Celestron, except for the location of the corrector which is somewhat closer to the primary. This is most likely due to a different glass used for the second element (Schott N-BALF5 instead of N-BALF2, which seems to be pretty elusive).

One of the systems is for 0.707 radius original corrector, and the other for 0.866 radius. The former effectively acts as ~0.75 zone corrector, and the latter is about the native 0.866 zone performance. Both has nearly identical off-axis astigmatism, but the latter has nearly double the spherochromatic error of the former. I don't know which neutral zone Celestron uses; I thought it was 0.707 since it's easier to manufacture and gives less spherochromatism, but who knows? Maybe they like the appearance of those small compact circles produced by 0.866 zone.

The 0.866 system blurs are more alike the ones presented by Celestron, only the astigmatism is about 50% larger. I kind of doubt that it can be achieved just by using BALF2 (since the curvature to correct is generated by mirrors, and corrector lenses of a given shape and similar glass do not normally differ significantly in the amount of Petzval they generate), but cannot say it wouldn't work like that. So if you trust Celestron, use 0.81 ratio to the nominal field angle to evaluate field performance at that field height. It will go with the Celestron's astigmatism, and spherochromatism is quite similar.

BTW the design in the book is not accurate.

Vla

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#15 MKV

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Posted 07 October 2013 - 10:21 AM

Good job, Vla! I think the 0.866 correction has a slightly better image, and seems to be unaffected by astigmatism up to about a 25 mm FOV diameter.

But, a simple dialyte version with all spherical surfaces and BK7 componenets would most likely outperform the C14-Edge by a loooong shot. Why bother with aspherics?

regards,
Mladen

#16 wh48gs

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Posted 07 October 2013 - 06:11 PM

Mladen, did I ever do poorly? I don't remember ;) 0.866 zone blurs do look neater, but the sherochromatism is nearly doubled. Between the neater blur and half the wavefront error I opt for the latter.

Two-mirror dialyte would have astigmatism corrected, but may not be easy to make up to high standards. I remember Roland was making prototype some time ago, and the price mentioned was quite high (much more optical-quality glass, although I'm sure his SCT too would be more expensive than these run-of-the-mill varieties). He also sounded concerned if his fabrication/assembly capabilities will be up to the task. On the other hand, SCT corrector aspheric is for all practical purposes as easy to make as a sphere. And who doesn't love good old SCT?

Vla

#17 MKV

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Posted 08 October 2013 - 12:52 AM

Vla, I don't know if you ever did poorly. If you're human, you should have! :) The question is how much of that greater spherchormatism would be visible as opposed to "cross-like" star images.

From a fabricaiton point of view, making Schmidt-type correctors is no different than making a sphere. You're spot on there, but so is producing spherical surface. A Mangin is no more of a challenge than a Maksutov meniscus corrector, so they do require more precise manufacturing, especially in larger sizes. If you want a superb scope you pay for it.

Below is an example of a 14-inch f/11.9 dialyte Cassegrain with two Mangin mirrors based on a design oriignally proposed by Mike I. Jones on CN for a 200 mm diameter configuration.

The images are without a field flattener, and over the same off-axis angle as the C14 Edge-HD. On a curved filed, or with a suitable field flattener, the FOV can be as large as 2-inches across with pinpoint images.

Note the use of N-BK10 for the secondary. Judging from the chromatic residual, the configuration is not even fully optimized at this setting. This can be easily tweaked.

All in all, a much better product than the C14 in question.


Mladen

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#18 freestar8n

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Posted 08 October 2013 - 03:05 AM

Hi-

I'm interested in these explorations and I like to see what else is possible. But if you really want a challenge, it is actually very important to keep the length of the OTA short. If you can keep the length the same or less than the Edge14 and keep the field the same at the same ease of manufacturing, then it's more interesting.

For guided imaging and general mounting considerations, the nearly f/2 primary is very important - both in usage (wind, weight, rotational torque, carrying, flexure) and in potential sales.

Also, for planetary work, the secondary can't get too big.

Frank

#19 Loren Chang

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Posted 08 October 2013 - 04:30 AM

Schott N-BALF5 instead of N-BALF2, which seems to be pretty elusive.


Did N-BALF2 ever exist? Can'nt found in OSLO. :confused:

#20 Dave O

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Posted 08 October 2013 - 05:30 AM

In their 'White Paper' on the EdgeHD telescopes, Celestron states that the 14 inch EdgeHD corrector uses N-SK2 and N-BALF2 glasses. So yes, it seems to exit.

#21 wh48gs

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Posted 08 October 2013 - 06:47 AM

Mladen, that's clever to try to eliminate positive field lens with Mangin secondary, but the system is not flat field. It still has somewhat less of defocus (curvature) error than c14e would have due to astigmatism, but not really significantly.

And I agree with Frank that we should compare systems of similar length. Not only for the length itself, but for the performance level. An SCT with slower primary will have lower aberrations, both off axis and spherochromatism. This is what an f/3.5/12.2 14-inch Edge would put out, e,F,C lines.

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#22 Loren Chang

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Posted 08 October 2013 - 07:48 AM

In their 'White Paper' on the EdgeHD telescopes, Celestron states that the 14 inch EdgeHD corrector uses N-SK2 and N-BALF2 glasses. So yes, it seems to exit.


Hello Dave,

That's the problem. 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.

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#23 MKV

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Posted 08 October 2013 - 08:50 AM

Vla, a C14 EdgeHD with an f/3.5 primary rivals a (simulated*) field-flattened dilayte counterpart. The question remains: if an ATM were to make one of these which would be easier. I think the one with all spherical surfaces wins, even if its Mangin mirrors require greater precision. It doesn't require extra tooling.

regards,
Mladen

*simulated using a curved imaged field since OSLO.edu doesn't allow more than 10 surfaces in all

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#24 Dave O

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Posted 08 October 2013 - 09:31 AM

Hello Loren,

Yes, the smaller EdgeHD scopes (8", 9.25", and 11") use N-SK2 and K10 glasses in their corrector; only the 14" uses the N-BALF2 glass per the "White Paper" put out by Celestron.

#25 Loren Chang

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Posted 08 October 2013 - 11:42 AM

Dave,

Vlad said location of the corrector is closer to primary in N-BALF5(longer BFL). I'd like to see if N-BALF2 will do the right job. SK2/K10 combination intruduces lateral color off-axis. It can be defeated by using N-BALF.






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