SolarMax 70 CaK Telescopes and Beyond

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SolarMax 70 CaK Telescopes and Beyond

Post by Bob Yoesle »

Over the past few months I've been able to acquire some early 2000's era Coronado SolarMax telescopes to compliment my existing DS Ha and CaK filter systems, including a SM90 H alpha telescope. These pre-Meade etalons are superb performers. For this particular SM90 scope I'm anticipating adding a Feathertouch focuser, as my other recent acquisition is a pair of SolarMax 70 mm CaK telescopes, one of which is also Feathertouch equipped and a definite improvement over the standard helical focuser:

SolarMax scopes.jpg
SolarMax scopes.jpg (270.39 KiB) Viewed 2654 times
918411-3.jpg
918411-3.jpg (356.26 KiB) Viewed 2638 times
LA CaK 70.jpg
LA CaK 70.jpg (363.99 KiB) Viewed 2638 times

The SM70 CaK telescopes are wonderful instruments. Like their H alpha counterparts, the original ITF's are shot, but these can be replaced for imaging by the Baader K line filter (a KG3 and additional brightness filtering would possibly make it "safe" for visual use). And since the Baader K line is double stacked, an option could be to use a single filter for even better transmission about a 15% improvement, which I hope to try out relatively soon.

msg-11233-0-92201800-1621635534.jpg
msg-11233-0-92201800-1621635534.jpg (68.37 KiB) Viewed 2638 times
K-line-Viladrich etal.jpg
K-line-Viladrich etal.jpg (122.66 KiB) Viewed 2640 times
schott-kg1-kg2-kg3-kg4-kg5-transmission.png
schott-kg1-kg2-kg3-kg4-kg5-transmission.png (17.97 KiB) Viewed 2640 times


But even with the stock DS K line filter, image brightness is tremendous. The single stacked CaK image seen below required a 0.135 ms exposure with my cooled PGR Chameleon:

160521 CaK SS adj.jpg
160521 CaK SS adj.jpg (432.69 KiB) Viewed 2654 times

These CaK telescopes employ a larger version (~ 32 mm) of the 25 mm PST CaK yellow "No. 1" blocking filter set between a collimator and refocusing lens.

Front end of all elements.jpg
Front end of all elements.jpg (109.56 KiB) Viewed 2638 times

As can be seen above, this collimated light path results in excellent performance - and to me appears to give them the performance of my DIY CaK PST DS filter system at f9. So this begged the question of what would it look like if I doubled-stacked this telescope with one of my smaller PST CaK blocking filters attached to the camera nosepiece...

Here's the 0.265 ms exposure result:

160621 CaK 70 DS adj.jpg
160621 CaK 70 DS adj.jpg (162.37 KiB) Viewed 2654 times

The resulting theoretical bandpass would be 1.42 A, but given the second filter is not in the collimated position, it would likely be somewhere around 1.6+ A. This scope, double or triple stacked and used with a camera with smaller pixels, would seem to make an ideal full-disc imaging system.

In discussing these CaK filters, Mark Wagner at Solar Spectrum asked me to send him one of my PST CaK blocking filters to measure on his spectrometer, and he informs me he suspects the filter is hard-coated, has the FWHM measurement of the specified 2.2 A, and appears to have a CaK peak similar to a single cavity filter with a typical symmetric profile at Tmax of about 40% or so. If so, this would likely show a Lorentzian profile like shown here:

Ca-K v3.3.jpg
Ca-K v3.3.jpg (118.25 KiB) Viewed 2640 times

I hopefully will be able to get a graphical print-out and will post it if and when possible. The hard-coated nature of the filter may be possibly supported by Brian Stephens at Lunt telling me these 32 mm filters would cost over $1600 each when bought in lots of 100. He also told me this high cost and relatively low demand is one of the reasons Lunt no longer offers a B3400 CaK filter...

Given their high transmission, I hope to be doing additional experimenting with some multiple PST CaK blocking filters in tilters to see what triple and perhaps quad stacking might achieve to hopefully get to the Holy Grail FWHM below 1.0 A, and where hopefully disc filaments become readily apparent. My immediate plan is to re-space a Celestron C6-R to 3.2 mm for CaK and use one of the 70 mm ERF's internally with the collimator based 32 mm CaK blocking filter for high-resolution work (SSM needed here), and have the ability to double or triple stack this if it works out. One of my ED100/900's will also be re-spaced to 1.6 mm for CaK for when the seeing doesn't allow for the C6-R to be used.

The final frontier belongs to real etalons, which I'm trying to talk Mark into making at less than a 1.0 A FWHM. Brian stated at one time Coronado made a SMn quartz based CaK etalon at 0.7 A FWHM.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Merlin66 »

It would be interesting to see how close the CWL is for these filters to the CaK centre.
Tilt tuning to centreline???? Probably not.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by marktownley »

Very interesting Bob! I always suspected those magic CaK filters were hard coated.

I have a 2.5" Feathertouch on my SM90, I can whole heartedly recommend it...

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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Montana »

Wow!! good luck Bob :hamster:

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Re: SolarMax 70 CaK Telescopes and Beyond

Post by David Fox »

Beautiful just beautiful Never seen a pre Meade in the flesh.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by christian viladrich »

Hello Bob,
All of this is quite exciting !
In case you've missed it, I measured the FWHM of my Barr and Alluxa Ca K filters with Sol'Ex spectro :
http://astrosurf.com/viladrich/astro/in ... lters.html

The Sol'Ex is easy to built, and I can only urge you to make one. This is really a precious tool to evaluate Ca K and Ha filters.

For sur, 1-1.5 A is the next frontier for Ca K filters :-)


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Merlin66 »

Christian,
Why do the curves for the ALLUXA look so different?
The ALLUXA CWL infers it could be tilt tuned closer to CaK????


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by christian viladrich »

Merlin66 wrote: Mon Jun 21, 2021 8:50 am Christian,
Why do the curves for the ALLUXA look so different?
The ALLUXA CWL infers it could be tilt tuned closer to CaK????
Hello ,
For Alluxa, the first figure (with blue light) is the fiter profile, the second figure (with two lines) is the solar spectrum (blue line) and the solar spectrum filtered by the Alluxa filte (red line).
In other words, the transmission of the filter is given by the ratio solar spectrum filtered by Alluxa/ solar spectrum.
The curves there are for normal incidence. When the filter is tilted the profile is moved is to the blue (on Ca K line). I'll post the additional curve when I have some time. I am currently busy with the tests of the Ha filters ...


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Bob Yoesle »

It would be interesting to see how close the CWL is for these filters to the CaK centre.
Tilt tuning to centreline???? Probably not.
Hi Ken, I don't have the final results, but I believe Mark stated the filters are pretty close to the CaK center line. However, being rather broad, they can tolerate a bit of tilt for reflection suppression without going significantly off-band.

I also sent him the ChromaTech 393.4 0.1 filter and it definitely was a bit high and needed some tilt to be on-band, which I had previously discovered.
I always suspected those magic CaK filters were hard coated... I have a 2.5" Feathertouch on my SM90, I can whole heartedly recommend it...
Yeah, the placement of the filter ahead of the ITF was highly suspicious. Thanks for the Feathertouch feedback Mark!

Hi Christian, thanks for the Sol'Ex recommendation and filter profile info!
For Alluxa, the first figure (with blue light) is the fiter profile, the second figure (with two lines) is the solar spectrum (blue line) and the solar spectrum filtered by the Alluxa filte (red line).
It looks like neither filter CWL is on-band, but that the Alluxa can be tilted to achieve this. It would be great to put the filters in series to see to how the filter transmission curves convolute "in real life" when double stacked (hint hint ;-)


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by christian viladrich »

Bob Yoesle wrote: Mon Jun 21, 2021 2:51 pm It would be great to put the filters in series to see to how the filter transmission curves convolute "in real life" when double stacked (hint hint ;-)
Yes, I've done this with the Ca K filters and also the Ha filters. But ...there is so much data to process before sharing "official" information on this.
Sol'Ex is quite a good tool for this.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by marktownley »

Bob Yoesle wrote: Mon Jun 21, 2021 2:51 pm
I always suspected those magic CaK filters were hard coated...
Yeah, the placement of the filter ahead of the ITF was highly suspicious.
I do wonder though if the Lunt filters are soft coated as these are behind the ERF/ITF...

Rusty times ahead for all those using Lunt CaKs in f6 light cones???? :shock:


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Bob Yoesle »

Hi Mark,

I think it's hard to say based on the limited information we have available. The Lunt filter appears different in that it has a sandwich construction unlike the Tucson Coronado version:
Lunt CaK 3.jpg
Lunt CaK 3.jpg (86.44 KiB) Viewed 2477 times

This might be a way of protecting the soft-coated layers, but I'm not knowledgeable enough to make a firm conclusion.


The Lunt CaK blocker has the profile of a rather typical dichroic filter:
Lunt CaK  blocking filter.PNG
Lunt CaK blocking filter.PNG (42.79 KiB) Viewed 2477 times

From what I understand the Coronado CaK blocker may appear similar, but I will have to wait for Mark Wagner to provide a transmission profile to come to any additional understanding.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by DeepSolar64 »

Bob,
I got into Ha solar wayyy too late for pre-Meade Coronado unfortunately and I could not have afforded one at that time anyway and I bet now they are pricey hard to get collectors items. Still, I would love to look through one! With their beautiful anodized brass tubes they were certainly attractive scopes. And I wonder why in the heck Meade stopped making the CaK model, especially now that the SMIII could have a module installed on it. Meade Coronado has yet to make a CaK module for it!

Hey, Bob, is that an Edmund RKE 28mm on that SM90? I have two of those!

James


P.S. Mark, is yours a SMI or SMII? It's got the shiny brass tube. It's an early Meade model, right?


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by Bob Yoesle »

Hi James,

I think for most of the OEM's CaK is not as high a demand filter system and therefore they don't have the economy of scale production and therefore a narrower profit margin. It unfortunately seems as the narrow-band solar filter market has shifted from a niche specialty for geeks willing to pay a premium for quality to a wider distribution for more affordable "consumer grade" realities, we see less willingness to provide specialized filter systems of outstanding performance. We generally now have more widely available, less expensive, higher profit margin and generally less consistently performing products.

No - that's not an Edmund RKE eyepiece, although I have a couple and find them to be quite nice.

Mark Wagner got back to me with some spectro-scans of the Coronado PST CaK dichroic filter, and the ChomaTech "0.1" nm CaK bandpass filter:

Coronado PST CaK MWagner Plot.jpg
Coronado PST CaK MWagner Plot.jpg (487.1 KiB) Viewed 2355 times

ChromaTech CaK  MWagner Plot.jpg
ChromaTech CaK MWagner Plot.jpg (465.79 KiB) Viewed 2355 times

Note the diagrams have differing horizontal and vertical scaling. For all intents and purposes, the hard-coated bandpass filter performed very similarly to the dichroic. The bandpass filter had a bit higher CWL requiring more tilt to come on-band. Mark noted the bandpass filter didn't quite have the FWHM uniformity across the filter as the dichroic. They both had Lorentzian transmission profiles, with the bandpass having a 5% higher transmission peak and a nearly identical FWHM when appropriately tilted on-band.

These confirm what was observed with imaging.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by marktownley »

Useful info, thanks Bob and Mark.


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by christian viladrich »

Thanks Bob and Mark for the transmission curves !!


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by DeepSolar64 »

Bob,
With two 28mm RKE eyepieces, you are ready for a binoviewer! :)

James


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by marktownley »

DeepSolar64 wrote: Sat Jul 10, 2021 1:32 am P.S. Mark, is yours a SMI or SMII? It's got the shiny brass tube. It's an early Meade model, right?
It's a prototype model for the SMII ;)


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Re: SolarMax 70 CaK Telescopes and Beyond

Post by DeepSolar64 »

marktownley wrote: Mon Jul 19, 2021 4:41 pm
DeepSolar64 wrote: Sat Jul 10, 2021 1:32 am P.S. Mark, is yours a SMI or SMII? It's got the shiny brass tube. It's an early Meade model, right?
It's a prototype model for the SMII ;)

I wish they would have kept the shiny anodized tube. The matte gold tube is pretty but it doesn't equal the anodized brass finish of yours. The SMIII series has it though.


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