The ubiquitous Celestron C6-R

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The ubiquitous Celestron C6-R

Post by Bob Yoesle »

As some of you may know, the traditional achromatic refractor often performs as well or better than an apochromatic objective for narrow band imaging. This seems to be true for the well regarded Celestron 150/1200 f8 Synta sourced refractor.

C6-R.jpg
C6-R.jpg (35.04 KiB) Viewed 7337 times
For example, I use this objective for my Coronado SM140/90 double stacked H alpha telescope with good results:

viewtopic.php?p=207576#p207576

Even expensive APO's often perform more poorly at shorter wavelengths from excessive spherochromaticism due to their more highly curved optical surfaces needed to obtain shorter focal ratios for astrophotography.

https://www.telescope-optics.net/refractor.htm

I recently spoke to Mark Wagner (Solar Spectrum), and he let me know he had heard that re-spacing the C6-R objective to approximately 2.3 mm made an excellent CaK line telescope. I wanted to verify this, so I posted an inquiry to the ATM, Optics & DIY Forum on Cloudy Nights, and sure enough Mike Jones, a retired NASA optical engineer, ATM, and optical design consultant, came up with a "best guess" optical prescription for the C6-R, and when respaced it does very well at the calcium wavelength. Here's a link to Mike's post where you can download the OSLO files for both the visual (normally spaced) and Ca re-spaced "best guess" versions:


https://www.cloudynights.com/topic/6827 ... ry11094146


Looking at the visual version, you can see the objective is diffraction limited at 656 nm:

CR-6 H alpha spot Mike Jones.jpg
CR-6 H alpha spot Mike Jones.jpg (284.73 KiB) Viewed 7337 times

When respaced from 0.127 mm to 2.13 mm for Ca, the performance is even better:

CR-6 CaK spot Mike Jones.jpg
CR-6 CaK spot Mike Jones.jpg (344.46 KiB) Viewed 7337 times

Of course, stopping the aperture down to 120 mm to achieve an f10 focal ratio (ideal for the PST type of CaK dichroic filters) improves the spot diagrams even further:

CR-6 CaK 120 spot Mike Jones.jpg
CR-6 CaK 120 spot Mike Jones.jpg (393.76 KiB) Viewed 7337 times

Given the caveat that this is only a "best guess" optical analysis - even if slightly off from the real optics - you probably don't need a $13,000 TOA 150 or TEC 160 to do large aperture narrow band imaging. A pre-owned C6-R goes for about $400 + State-side, and makes for an excellent solar telescope under most circumstances. With a little bit of tweaking, it would appear to make an excellent CaK scope.
Last edited by Bob Yoesle on Wed May 12, 2021 7:31 pm, edited 2 times in total.


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Re: The ubiquitous Celestron C6-R

Post by Merlin66 »

Bob,
Do you have a set of CaK spot data for the "unmodded" C6 - for comparison before/ after???


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Hi Ken,

Here is the Mike Jones C6-R objective "best guess" un-respaced spot diagram at 394 nm:

CR-6 CaK spot visible CaK Mike Jones.jpg
CR-6 CaK spot visible CaK Mike Jones.jpg (287.52 KiB) Viewed 7313 times

Pretty not very good...


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Ooooo! This is interesting Bob, thanks for pulling all this info together from different sources. 3mm spacing would be easy to achieve as a mod. Lots of food for thought!


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Hi Mark,

I had a chance to discuss these simulations with Mark Wagner earlier today, and he stated it was actually a 2.3 mm respace he had heard about. So the 2.13 mm from Mike Jones is similar and likely within whatever tolerance is required.

And as another example, I compared a "best guess" design for the Orion/Vixen/Skywatcher (Synta) ED100/900 I have found to be very good at CaK...

ED100-900 CaK .jpg
ED100-900 CaK .jpg (497.46 KiB) Viewed 7261 times

... and a respacing from 0.15 mm to 0.42 mm also gives a significant optical improvement at 394 nm - although not a much as with the f8 achro C6-R:

100f9 CaK respace compare.jpg
100f9 CaK respace compare.jpg (201.68 KiB) Viewed 7261 times

So I'm not sure how much real-world improvement this would accomplish, but it would seem to be worth a try...


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Just a simple case of cutting a gasket to fit as the spacer :)


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Re: The ubiquitous Celestron C6-R

Post by RodAstro »

Hi Bob

Just one question: how much do you think the AR coatings on these lenses will affect the CaK lines?
From observations (I have had several of these scopes) and from what I have heard the coatings on these achromats have a minus violet filter in them and can loose light in the CaK range.
This was first seen on the original Meade 150mm and 120mm LXD55 achromats where it was extreme, almost a light yellow filter, then toned down in later models as most people did not like the yellow cast.
From what I hear this technique is used on many ED scopes making them appear more apochromatic than they are.

Mgf or no coating is supposedly better for CaK

Rod


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Hi Rod,

I really don't know how much any of the coatings would affect CaK brightness with this achro scope. I know Martin Wise uses one stopped down to f10.

https://www.astrobin.com/vi9xq7/0/

Interesting point about having no AR coatings if used for CaK...


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

RodAstro wrote: Fri May 14, 2021 8:35 pm Hi Bob

Just one question: how much do you think the AR coatings on these lenses will affect the CaK lines?
From observations (I have had several of these scopes) and from what I have heard the coatings on these achromats have a minus violet filter in them and can loose light in the CaK range.
This was first seen on the original Meade 150mm and 120mm LXD55 achromats where it was extreme, almost a light yellow filter, then toned down in later models as most people did not like the yellow cast.
From what I hear this technique is used on many ED scopes making them appear more apochromatic than they are.

Mgf or no coating is supposedly better for CaK

Rod
Totally agree Rod, I have the 5" LX55 and know exactly what you mean. I respaced the objective on this a few years ago but never really used it as it sucks up light in CaK. The Bresser 127/1200 I have now is much, much better...


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Heya Bob. Reading the links to the CN article, you mention the collimating assembly from a Solarmax 70, can you post some details about this, and how the filters are used in conjunction with it.

Thanks

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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Hi Mark,

Back in 2004 Coronado Tucson introduced the SolarMax 70 CaK, noting it was based on the H-alpha SolarMax 70:
SolarMax 70 CaK Telescope

Product Description

In order to complement our line of Hydrogen Alpha filters we are, for the first time, offering the Sun in a new light! This model telescope is based on our popular SolarMax 70 telescope design and its mechanical functions remain largely the same. However, we have developed a filter system for this unique instrument that allows for the standard production of a Coronado Calcium K telescope.

This instrument focuses on the transmission line of CaK centered at 393.4 nanometers. The thermally stable interference filters allow you to view the chromospheric network of ionized calcium emissions that are created when supergranualtion cells sweep through magnetic fields where they collect and strengthen. This emission line will not be visible by eye to all observers and the primary use of the SolarMax 70 CaK telescope is for imaging. What you will capture with this new instrument are plage, active regions, and magnetic storms in a layer of ionized calcium gas that appears in a violet/blue hue. This layer of gas is a cooler and lower level of the Chromosphere than one observes thru our H-Alpha filters and provides new information on solar activity and weather.

As with all Coronado products you can be assured of safety, quality, value and superior performance. This new telescope has the exact dimensions and mechanical performance as our SolarMax 70 telescope in H-Alpha. Binoviewers are unlikely to work with the telescope at this time and the SolarMax 70 CaK has been designed with imaging as its primary purpose. CCD, DSLR, and most digital formats will be compatible with the SolarMax 70 CaK. Some will require adaptors that Coronado does not provide but are the same as ones used for night time imaging.

Dimensions
Length – 15” to 17.5”
Weight – 5lbs
Includes: Hard travel case with die cut foam, clamshell mounting ring, Sol Ranger Sun finder.

The final product's appearance may vary from the one shown but all performance and dimesional specifications will remain the same. Price is U.S. dollars - $2,999.00. Be aware that prices outside of the U.S. may differ based on shipping and import fees.



Product Specifications
Aperture: 70mm
F/L: 400mm
F/Ratio: 5.7
Bandwidth: Max 2.2Å
Thermal Stability: 0.005 Å/C
Blocking: Full blocking >10-5 from EUV to far IR
These CaK scopes came with a front ERF, then the objective, then the internal collimator lens assembly - and the CaK collimator and refocusing lens cell even has the holes where the silicone was used to "pot" the H alpha etalon (seen around the perimeter):
Coronado 70mm CaK Filter B optics.jpg
Coronado 70mm CaK Filter B optics.jpg (90.75 KiB) Viewed 7146 times
After this came the CaK ITF, and then the trim filter.

Tim M. from a previous set of posts viewtopic.php?p=287013#p287013 decided he didn't wish to keep the CaK 70 he recently acquired from Larry Alvarez. It had a rusted ITF and missing trim filter. So I bought it for the $650 USD he paid. I removed the rusted ITF and used a Baader K-line for the trim filter as we all advised. My first impression is that this filter used single stacked in a collimator/refocusing system offers a greater amount of contrast than I am generally accustomed to for a single-stacked CaK:

CaK 70 mm imaging SM.jpg
CaK 70 mm imaging SM.jpg (313.59 KiB) Viewed 7146 times
155617 corrected orientation.jpg
155617 corrected orientation.jpg (468.24 KiB) Viewed 7146 times
With a FL of only 400 mm I could squeeze the entire disc onto the PGR Chameleon's sensor, and had really short exposures:

FireCapture v2.5 Settings
------------------------------------
Camera=Chameleon CMLN-13S2M
Filter=L
Profile=Sun
Filename=155617.avi
Date=160521
Start=155602.566
Mid=155617.566
End=155632.566
Start(UT)=225602.566
Mid(UT)=225617.566
End(UT)=225632.566
Duration=30.000s
Date_format=ddMMyy
Time_format=HHmmss
LT=UT -8h
Frames captured=547
File type=AVI
Extended AVI mode=true
Compressed AVI=false
Binning=no
ROI=1296x964
ROI(Offset)=0x0
FPS (avg.)=18
Shutter=0.151ms
Gain=160 (6%)
FPS=18.32 (off)
Gamma=512
AutoExposure=off
Brightness=0
AutoHisto=75 (off)
Exposure=1
SoftwareGain=10 (off)
Histogramm(min)=0
Histogramm(max)=255
Histogramm=100%
Noise(avg.deviation)=n/a
Limit=30 Seconds
Sensor temperature=50.6 °C


The really fortuitous findings are that the ERF is in great shape, and the main yellow dichroic filter is over 30 mm in diameter inside the collimator assembly!

aaaaaaaaa.jpg
aaaaaaaaa.jpg (64.43 KiB) Viewed 7146 times

Brian Stephens at Lunt stated the cost of these filters is now so prohibitive that the large B3400 CaK filters are no longer available even from Lunt - the largest is the B1800 CaK. So my current thought is that I will employ the re-spaced C6-R (possibly stopped down to f10 @ 120 mm), use the 70 mm objective ERF as a sub-diameter ERF, and see if the collimator can be used for high res imaging (full disc imaging will remain the purview of the ED100/900). The really short exposure time indicates that I could likely also employ additional PST yellow dichroic interference filters downstream for multiple stacking purposes...


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Very interesting indeed Bob, thanks for all this information. I guess the collimator is there as the native f5.7 of the scope isn't going to get the filter performing optimally. May have to explore mounting a PST CaK filter in the collimating assembly from a Ha PST... Be interesting to see the results...


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Indeed Mark; f5.7 would not be ideal for just any narrow band filter implementation. But with the collimator the filter performance appears to be highly optimized.

The H alpha PST collimator should be about perfect, as the I believe the original CaK PST introduced in 2006 used the same collimator optics. The other thing is that with a 2.2 A FWHM, Jacquinot spot issues from field angle magnification by the CaK collimator lens system used with longer FL objectives will be less evident than they are with H alpha filters with a 0.7 A FWHM - all the while hopefully having improved contrast performance. I'll be very interested in seeing your results. :)


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Re: The ubiquitous Celestron C6-R

Post by krakatoa1883 »

Bob Yoesle wrote: Tue May 11, 2021 11:58 pmhe let me know he had heard that re-spacing the C6-R objective to approximately 2.3 mm made an excellent CaK line telescope.
very interesting info, thank you. Having re-spaced several refractors I would just like to add that those under f/10 require spacers to be of equal thickness within 0.05 mm otherwise there will be coma at the center of the field. Best is to work during daytime, checking thickness with a Palmer-type micrometer and then the result on an artificial star.


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Re: The ubiquitous Celestron C6-R

Post by minhlead »

Bob Yoesle wrote: Tue May 11, 2021 11:58 pm As some of you may know, the traditional achromatic refractor often performs as well or better than an apochromatic objective for narrow band imaging. This seems to be true for the well regarded Celestron 150/1200 f8 Synta sourced refractor.


C6-R.jpg

For example, I use this objective for my Coronado SM140/90 double stacked H alpha telescope with good results:

viewtopic.php?p=207576#p207576

Even expensive APO's often perform more poorly at shorter wavelengths from excessive spherochromaticism due to their more highly curved optical surfaces needed to obtain shorter focal ratios for astrophotography.

https://www.telescope-optics.net/refractor.htm

I recently spoke to Mark Wagner (Solar Spectrum), and he let me know he had heard that re-spacing the C6-R objective to approximately 2.3 mm made an excellent CaK line telescope. I wanted to verify this, so I posted an inquiry to the ATM, Optics & DIY Forum on Cloudy Nights, and sure enough Mike Jones, a retired NASA optical engineer, ATM, and optical design consultant, came up with a "best guess" optical prescription for the C6-R, and when respaced it does very well at the calcium wavelength. Here's a link to Mike's post where you can download the OSLO files for both the visual (normally spaced) and Ca re-spaced "best guess" versions:


https://www.cloudynights.com/topic/6827 ... ry11094146


Looking at the visual version, you can see the objective is diffraction limited at 656 nm:


CR-6 H alpha spot Mike Jones.jpg


When respaced from 0.127 mm to 2.13 mm for Ca, the performance is even better:


CR-6 CaK spot Mike Jones.jpg


Of course, stopping the aperture down to 120 mm to achieve an f10 focal ratio (ideal for the PST type of CaK dichroic filters) improves the spot diagrams even further:


CR-6 CaK 120 spot Mike Jones.jpg


Given the caveat that this is only a "best guess" optical analysis - even if slightly off from the real optics - you probably don't need a $13,000 TOA 150 or TEC 160 to do large aperture narrow band imaging. A pre-owned C6-R goes for about $400 + State-side, and makes for an excellent solar telescope under most circumstances. With a little bit of tweaking, it would appear to make an excellent CaK scope.
Thanks Bob.
I am looking to quench my aperture thirst and there are 2 viable option with my budget
1. Get a C6R and a 150mm DERF. I am from VN and shipping the C6R to Vn can be very expensive. So I think I am looking at 2000$ total for this option.
2. Use my existing C8SE 8 inches SCT with a 200mm ERF. This option also gonna set me back about 2000$.
So which route should I choose to ensure the best image quality? I am using a Quark Chromosphere.


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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Hi Minh,

You need to be sure about how good your seeing is before jumping in at these large apertures, else, IMHO you could be wasting a whole load of £££ and time.

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Re: The ubiquitous Celestron C6-R

Post by minhlead »

marktownley wrote: Wed May 26, 2021 5:24 am Hi Minh,

You need to be sure about how good your seeing is before jumping in at these large apertures, else, IMHO you could be wasting a whole load of £££ and time.

Mark
Thanks Mark. Great advice. I'll DIY a solar scintilator and see if I got okay seeing for at least 3 months before jump the gun


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Re: The ubiquitous Celestron C6-R

Post by minhlead »

marktownley wrote: Wed May 26, 2021 5:24 am Hi Minh,

You need to be sure about how good your seeing is before jumping in at these large apertures, else, IMHO you could be wasting a whole load of £££ and time.

Mark
Also can one determine the seeing from AS3's frame quality graph? Ie: if I got x percent of frames above x quality then I got good seeing?
For example, I got this kind of graph on what I'd consider a 7/10 seeing (fair but I had much better) on my site.
Untitled.jpg
Untitled.jpg (263.62 KiB) Viewed 7001 times


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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

minhlead wrote: Wed May 26, 2021 6:28 am
Also can one determine the seeing from AS3's frame quality graph? Ie: if I got x percent of frames above x quality then I got good seeing?
For example, I got this kind of graph on what I'd consider a 7/10 seeing (fair but I had much better) on my site.
No,

You can tell more about your seeing from the live view in your screen than from the graph. The graph simply represents quality weighting of frames, it doesn't tell you the seeing. If you have highly variable seeing conditions the graph will have a lot of high quality and low quality frames. It's based on contrast essentially. But you can easily have bad seeing, but it's consistent, and so your graph may look nearly homogenous and above the 50% quality line and so it would suggest it was good seeing in that way, but in reality, its bad seeing, just consistently bad. Variation in seeing is better simply because it will allow the quality weighting (of contrast) to have better comparisons. You can have bad seeing with high contrast with HA (plage and filament in the FOV will do it easy where you have something that math will see as contrast, but its blurry so its poor seeing, yet it will show up as high quality if its consistently bad seeing but present).

Even the SSM is not perfect in how it reports seeing, it has limits, there are still ranges that it will not account for and so it could say you have good seeing, like 1 arc-second, yet it's all blurry and messy due to something out of its range way up high (or local seeing).

Ultimately its easier to get an idea of seeing simply by looking at your screen live view in your imaging software. If you can see pencil like drawings in your preview for brief moments, then seeing is good enough for your image scale and resolution potential of your aperture and sampling. If it's blurry, then it's not good of course. Hard to put a number on that, other than good or bad relative to your scale and sampling setup. And its relative too. Your 5/5 seeing might only be someone else's 3/5 seeing. That's the human side of it without reliable metrics.

Very best,


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Re: The ubiquitous Celestron C6-R

Post by minhlead »

MalVeauX wrote: Fri May 28, 2021 10:47 pm
minhlead wrote: Wed May 26, 2021 6:28 am
Also can one determine the seeing from AS3's frame quality graph? Ie: if I got x percent of frames above x quality then I got good seeing?
For example, I got this kind of graph on what I'd consider a 7/10 seeing (fair but I had much better) on my site.
No,

You can tell more about your seeing from the live view in your screen than from the graph. The graph simply represents quality weighting of frames, it doesn't tell you the seeing. If you have highly variable seeing conditions the graph will have a lot of high quality and low quality frames. It's based on contrast essentially. But you can easily have bad seeing, but it's consistent, and so your graph may look nearly homogenous and above the 50% quality line and so it would suggest it was good seeing in that way, but in reality, its bad seeing, just consistently bad. Variation in seeing is better simply because it will allow the quality weighting (of contrast) to have better comparisons. You can have bad seeing with high contrast with HA (plage and filament in the FOV will do it easy where you have something that math will see as contrast, but its blurry so its poor seeing, yet it will show up as high quality if its consistently bad seeing but present).

Even the SSM is not perfect in how it reports seeing, it has limits, there are still ranges that it will not account for and so it could say you have good seeing, like 1 arc-second, yet it's all blurry and messy due to something out of its range way up high (or local seeing).

Ultimately its easier to get an idea of seeing simply by looking at your screen live view in your imaging software. If you can see pencil like drawings in your preview for brief moments, then seeing is good enough for your image scale and resolution potential of your aperture and sampling. If it's blurry, then it's not good of course. Hard to put a number on that, other than good or bad relative to your scale and sampling setup. And its relative too. Your 5/5 seeing might only be someone else's 3/5 seeing. That's the human side of it without reliable metrics.

Very best,
Thanks Marty,
I guess experience will come with more time on the scope. :D


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

FYI - I have just learned from SkyWatcher USA that the correct CaK respace for the C6-R is 3.2 mm.

I'm also hoping to hear back what the correct ED100/900 objective respace would be.


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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

Bob Yoesle wrote: Sat May 29, 2021 1:52 pm FYI - I have just learned from SkyWatcher USA that the correct CaK respace for the C6-R is 3.2 mm.

I'm also hoping to hear back what the correct ED100/900 objective respace would be.
Would love to know if they have a value like that for their 120mm F8.3 achromatic doublet (120mm, F8.3, 1000mm focal length) for this purpose? :beanie:

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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Bob Yoesle wrote: Sat May 29, 2021 1:52 pm FYI - I have just learned from SkyWatcher USA that the correct CaK respace for the C6-R is 3.2 mm.

I'm also hoping to hear back what the correct ED100/900 objective respace would be.
Very interesting Bob. Are you able to share anymore detail about your discussions with Skywatcher?

Your man on the atm/optics forum should be able to back engineer the numbers and work out what glass is used in this scope.

I have the 127/1200, smaller equivalent of this scope. I wonder if the 3mm air gap would be the same with this...


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Hi Mark,

The SkyWatcher USA technical rep is Kevin LeGore, who posts as skyward_eyes on Cloudy Nights. Good, guy, very helpful and knowledgeable, understood immediately what I was interested in doing. 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.


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Re: The ubiquitous Celestron C6-R

Post by GreatAttractor »

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?


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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 3928 times
C6R_Spacers_09.jpg
C6R_Spacers_09.jpg (47.84 KiB) Viewed 3928 times
C6R_Spacers_11.jpg
C6R_Spacers_11.jpg (46.06 KiB) Viewed 3928 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 3928 times
Solarsetup_05292021.jpg
Solarsetup_05292021.jpg (57.14 KiB) Viewed 3928 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 3928 times
120_8_194mm_spacing_03.jpg
120_8_194mm_spacing_03.jpg (17.38 KiB) Viewed 3928 times
120_8_194mm_spacing_05.jpg
120_8_194mm_spacing_05.jpg (47.76 KiB) Viewed 3928 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 3947 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 3947 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).

Very best,


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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...).


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Re: The ubiquitous Celestron C6-R

Post by MalVeauX »

Update,

Just to show more results, this is with the Synta 120mm F8.3 (Celestron Omni XLT 120) achromatic doublet, re-spaced air gap to 1.94mm per above posts, of AR2835 two days ago in fair seeing conditions. I masked the doublet to 100mm F10 and used a super common GSO 2x shorty barlow lens threaded directly to the camera nose (took off the barlow body) resulting in 1.62x magnification (I pixel measured to establish this previously) resulting in F16 approximately on the 2.9um pixels of the IMX290 sensor.

100mm F16, on IMX290:

Image

100mm F10, on IMX253:

Image

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Re: The ubiquitous Celestron C6-R

Post by marktownley »

Yup, that works.

My 127/1200 is my holiday project in a few weeks time...


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

Those objective re-spaces really demonstrate the benefits for CaK imaging - both are incredible improvements to my eye.


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Re: The ubiquitous Celestron C6-R

Post by Bastelhannes »

Very very interesting.

But... I have questions:
1. which software is used for the spot diagrams?
2. has anyone calculated the spacing for h-alpha?

I am thinking about modifying a good ol' achromat laying around. Even thinking about creating a adjustable cell for the lenses to switch between CaK and H-alpha... if it would be ever useful...


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Re: The ubiquitous Celestron C6-R

Post by Bob Yoesle »

The biggest problem is not having the actual lens prescriptions and glass types used. I initially used Mike Jones "best guess" for the C6R design in OSLO free version. I think this is a bit off, as the OEM came up with a significantly different CaK respace distance. I'm assuming they use ZEMAX or the other expensive professional level programs.

But as can be seen, a larger space for CaK only helps - even if it might be a little off - in getting much better CaK performance.

With H alpha, there's not a lot of difference in the spot diagrams and the optimized respaced versions, so at least for the C6R I'd leave it alone.

And remember daytime seeing is a very limiting factor, no matter how good your spot diagram may look ;-)


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Re: The ubiquitous Celestron C6-R

Post by DeepSolar64 »

I plan in time to purchase a C6-R. They are quite affordable for their aperture. I don't think it's been mentioned here but they would do very well in white light with a wedge and 540nm filter. Good seeing, of course.

I have the 4" F10 version currently.

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