OLSO model of the C14 Edge-HD ?
Posted 02 October 2013 - 02:15 PM
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).
Posted 04 October 2013 - 06:55 AM
Beside, the version in the book has neutral zone at 0.866 radius, which more than doubles spherochromatism vs. 0.707 zone.
Posted 04 October 2013 - 02:08 PM
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.
Posted 04 October 2013 - 05:19 PM
I'm sure Celestron's opticians could figure that out,
And about that Edge focal reducer?
Posted 04 October 2013 - 06:09 PM
Posted 05 October 2013 - 02:39 AM
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.
Posted 05 October 2013 - 03:29 AM
Posted 05 October 2013 - 03:46 AM
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.
Posted 05 October 2013 - 10:40 AM
Posted 05 October 2013 - 11:35 AM
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.
Posted 05 October 2013 - 12:42 PM
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.
Posted 06 October 2013 - 03:09 AM
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""
Posted 06 October 2013 - 06:06 PM
Hyperstar may only need spacer with c14 and c925 if it is designed for f/2 primary. Just a guess.
Posted 06 October 2013 - 06:34 PM
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.
Posted 07 October 2013 - 10:21 AM
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?
Posted 07 October 2013 - 06:11 PM
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?
Posted 08 October 2013 - 12:52 AM
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.
Posted 08 October 2013 - 03:05 AM
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.
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.
Posted 08 October 2013 - 05:30 AM
Posted 08 October 2013 - 06:47 AM
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.
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.
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.
Posted 08 October 2013 - 08:50 AM
*simulated using a curved imaged field since OSLO.edu doesn't allow more than 10 surfaces in all
Posted 08 October 2013 - 09:31 AM
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.
Posted 08 October 2013 - 11:42 AM
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.