The ubiquitous Celestron C6-R

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MalVeauX
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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

GreatAttractor wrote: Sun Jun 06, 2021 3:41 pm
Bob Yoesle wrote: Sun Jun 06, 2021 5:02 am Can't get actual proprietary data for the lens system, but appears he can get spec from Synta for their optical designs to re-space for CaK.
Makes me wonder, couldn't one just dismantle the objective, check its 4 surfaces with a spherometer, plop the data into some optics modelling software and find the best spacing to minimize the spherical aberration?
You still have to assume the glass type. Apparently it matters. But yes, you can model it. Even just knowing the aperture & focal-length you can model it. But knowing the actual glass type and stuff apparently plays into this for accuracy. This is how Mike Jones explained it to me at least.

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Re: The ubiquitous Celestron C6-R

Post by GreatAttractor »

MalVeauX wrote: Sun Jun 06, 2021 4:03 pm You still have to assume the glass type.
Sure - but we don't need to know the full characteristics, just the index of refraction at the CaK wavelength. E.g., with a violet light source, two pinholes to create a thin beam, and measuring the resulting beam offset after refraction... or something like that.


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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

Heya,

C6R (150mm F8 Synta lens):

The spacing mod on the lens:
C6R_Spacers_08.jpg
C6R_Spacers_08.jpg (28.04 KiB) Viewed 87 times
C6R_Spacers_09.jpg
C6R_Spacers_09.jpg (47.84 KiB) Viewed 87 times
C6R_Spacers_11.jpg
C6R_Spacers_11.jpg (46.06 KiB) Viewed 87 times
Update, my previous data with the C6R respaced to 2.13mm which worked out well. I then masked it to 120mm F10 and used a 1.62x amplifier and took it to F16 with the ASI290MM camera (2.9um pixels).

AR2824 & AR2825 from May 29th:

Image

Image

Image

Image

Seeing was good with 0.7 to 0.8 arc-second peaks.
Seeing_Conditions_05292021.jpg
Seeing_Conditions_05292021.jpg (106.42 KiB) Viewed 87 times
Solarsetup_05292021.jpg
Solarsetup_05292021.jpg (57.14 KiB) Viewed 87 times
============================================================================================================

Omni XLT 120 (120mm F8.3 Synta lens):

The mod with a 1.94mm ring between the lenses:
120_8_194mm_spacing_01.jpg
120_8_194mm_spacing_01.jpg (34.24 KiB) Viewed 87 times
120_8_194mm_spacing_03.jpg
120_8_194mm_spacing_03.jpg (17.38 KiB) Viewed 87 times
120_8_194mm_spacing_05.jpg
120_8_194mm_spacing_05.jpg (47.76 KiB) Viewed 87 times
Update on the synta 120mm F8.3 lens (the common lens found in scopes like the Celestron Omni XLT 120 OTA). I respaced the air gap to 1.94mm as advised by Mike Jones. I had bad weather the past few days, lots of storms, but this morning have be a small window between clouds and good enough seeing to explore the diffraction limited image scales of the system. I masked it to 100mm F10 for the moderate resolution scales, needing around 1 arc-second (0.8~0.9 arc-second ideally) to sample its resolution best, the image scale is 0.37"/pixel with a 1.62x amplifier (F16.2) and 2.9um pixels of the IMX290 sensor. For the full disc, I masked the same refractor to 60mm F16.7 and used an IMX253 sensor's 3.45um pixels for CaK and used 395nm for a photosphere disc as well. Imaged earlier in this thread of the re-spacing of the lens.

This morning's results:


Image

Image

Image


Astrobin for more info & full resolution:


Image

Image

Image


Seeing:

It was cloudy, so my graph is bad, shows the cloud gaps, peaks were sub-arc-second, so I was within the realm of being able to assess diffraction limited of 100mm at near critical sampling.

SeeingConditions_06112021.jpg
SeeingConditions_06112021.jpg (92.84 KiB) Viewed 106 times

Equipment:

Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 100mm F10 with 1.62x amplifier (F16.2) with Lunt CaK B1200 module and ASI290MM sensor (2.9um pixels)

Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 60mm F16.7 with Lunt CaK B1200 module and IMX253 sensor (3.45um pixels)

Synta 120mm F8.3 (air gap respaced to 1.94mm) operating at 60mm F16.7 with Baader Blue CCD-IR Block, Baader ND 3.0, GSO Polarizer & Altair Astro 395nm 7nm filter and IMX253 sensor (3.45um pixels)

SolarSetup_CaK_Respaced_06112021.jpg
SolarSetup_CaK_Respaced_06112021.jpg (96.23 KiB) Viewed 106 times

Summary:

Both the C6R & Omni XLT 120 are good for this. The spacing tested so far seems to work well and is a significant improvement over stock air-gap spacing at least on these two lenses. I opted to move to the Synta 120mm F8.3 lens because I can keep my C6R for red and green wavelength imaging. The Synta 120 F8.3 allows me to operate at 120mm for high res imaging (needing 0.8~0.9 arc-seconds to really benefit, better is better of course), but also mask it down to 60mm and do a full disc with naturally long focal-ratio with a bigger sensor, so this works for me to have both moderate to high res and full disc imaging on this scope optimal for near UV wavelengths and leave my C6R for longer wavelengths. Both are inexpensive lenses and this re-spacing is such an easy to do modification and inexpensive to perform to get diffraction limited results. The 120/8.3 is of course easier for most to mount and masking it to 100mm F10 or 80mm F12 makes it a much more realistic solution for anyone in worser seeing conditions more commonly.

Very best,
Last edited by MalVeauX on Fri Jun 11, 2021 9:57 pm, edited 1 time in total.


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Looks excellent Marty !-)


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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

So the next step is to find a common barlow/amplifier that is affordable for anyone that shows best results in near UV as well. Or to figure out if we actually need one in the first place, or if the common ones are actually good enough?

Ideally, a natively long focal-ratio is ideal; no other options to cause problems. But that gets impractical in the 100mm~120mm range trying to get to F16~F18. It works fine for smaller apertures, but for larger apertures basically we are still bound to using a barlow/amplifier/extender of some kind.

For my last few images in here with the re-spaced C6R and re-spaced Omni XLT 120, I used the ultra common GSO shorty 1.25" 2x barlow, and took the lens cell off the barlow and threaded it directly to my camera's 1.25" nose. This is a $20 barlow. I'm sure it's not ideal for this. But it seems to at least be holding up here. When threaded to the nose of the camera, this particular 2x barlow (like many of these 2x shorties) acts more like a 1.5x. I measured a structure with it as a 2x and as the cell on my camera nose and I found it to be more approximately a 1.62x power barlow based on image scale difference when I pixel measured a structure with the same scope and camera and only altered the barlow itself for use. While Baader has a diffraction limited near UV telecentric that is ideal, it's quite costly. Judging from the results I've gotten so far, I think its possible to still get acceptable quality from these cheaper barlows, but maybe we could find something better for this task? Mainly interested in finding things that will work at 1.5x, 1.6x, 1.8x basically (1.8 being the most for me, as I don't need to go past F18).

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Re: The ubiquitous Celestron C6-R

Post by GreatAttractor »

Next step - a motorized cell for smooth respacing with live view verification! (if people can build Newtonians with motorized mirror cells for collimation...).


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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