First light for a "new" CaK PST (and 1st attempt to mod it!)

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thesmiths
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First light for a "new" CaK PST (and 1st attempt to mod it!)

Post by thesmiths » Sat Jun 06, 2015 11:25 pm

I finally managed to find a CaK PST here in the UK. It is of course "used" since they don't make them any more. But I'm calling it "new" since I've never seen one myself before. When it arrived, it brought back many memories since one of my first telescopes was a H-alpha PST many years ago. I had taken that one apart long ago to extract its etalon. However, good thing I kept all the pieces since the CaK PST was missing the nylon screw that holds the eyepiece.
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PSTs
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I must have reached a high level of jadedness since the first thing I did when the CaK PST arrived was to get out the tools to disassemble it! This particular example seems to have every single screw and joint fixed with incredibly strong glue to discourage opening up anything. With some persuasion, I could remove the side cover to look inside. The pentaprism was in good shape and moved quite smoothly. With more persuasion, the eyepiece holder could be removed to check out the condition of the two small filters near the exit. They looked clean and clear and without any sign of abnormality.
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pentaprisms
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Taking off the gold tube took an unusually large amount of torque (using a rubber strap wrench). My old H-alpha PST had come apart far more easily, as I recall. There was lots of squeaking and squealing as the gold tube slowly turned and came off. Using an adjustable spanner wrench (made for cameras), I could remove the #2 filter. It had a slight cloudiness on the side facing the pentaprism in an odd concentric circle pattern. But this cloudiness came off immediately with lens cleaning tissue. I'm thinking that some component of the vast amount of grease on which the pentaprism slides has a high vapour pressure and in the heat gets baked onto the back of the #2 filter. Here is a photo of the #2 filter and also one taken through the filter. It is a very reflective gold colour but dark blue in transmission.
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blue filter
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Even with two rubber strap wrenches, I could not remove the black case on the end of the gold tube which holds the #1 filter. This is apparently the "magic" filter with the very narrow spectral notch at 393.4 nm. I could see that this lovely transparent yellow filter was in good condition but I could find no way to remove it. I could also not remove the 40mm f10 lens at the other end of the gold tube. I recall the lens on my H-alpha came off quite easily. Here is a photo of the #1 filter attached to the gold tube and also a photo through the gold tube with the #1 filter and front lens in place.
4775.jpg
yellow filter
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Rather than risk destruction before even trying out the telescope, I reassembled everything, at least confident that all the components looked in "as good as new" condition. After waiting for a sunny day, I looked through the telescope and saw -- nothing at all. Fortunately, my young assistant's 13-year-old eyes could confirm the presence of a very faint violet disk. I understood then why this telescope was no longer made. It's pretty much entirely useless for visual use. The customer service desk at Coronado must have hated getting calls from customers about this product.

Next step was to try with a camera. I used my trusty old DMK 51 and it came to focus right away (using a very low profile adapter). The image scale was just right for this sensor and the amount of light was not too bad. At a gain of 500 (about midrange) and gamma of 67, the shutter speed was 4ms (1/250 sec). This is neither extremely fast nor very slow. I think for the aperture size of the lens, it's actually quite good (especially considering the DMK series cameras are not particularly sensitive in the blue). So here's my first attempt, taken on 4 June 2015 at 13:45 UT. 25% of 300 frames stacked with AS2.3, some wavelets in Registax6, then a little editing done with Photoshop Elements (click on photo for higher resolution).
Sun_13_43_56-b.jpg
cak 4 june
All in all, I'm very impressed with this little telescope. The focuser feels a little "touchy" and can make the image jump around a bit while focusing. On the other hand, it's possible to focus quite accurately with it. By comparison, the focuser on the H-alpha PST is quite wobbly and not as smooth. The image is very homogeneous compared with the awfully uneven illumination I recall with the H-alpha PST. In general, I found the build quality of the CaK PST to be somewhat higher than on the H-alpha PST (the latter says "Manufactured in Mexico", the former has a sticker that says "Made in U.S.A.").

Since I've so far failed in my attempt to totally disembowel this telescope, I may actually keep it intact for now. My plan had been to combine the #1 yellow filter with my Lunt CaK stack filter and Baader K-line filter to attempt a CaK double stack. But this CaK PST is such a lovely stand-alone telescope and gives great full disk images. For full disk, it seems the equal of the native Lunt B1200 on a 80mm refractor. This certainly cannot be said when comparing a H-alpha PST with even a Lunt 60 H-alpha telescope. I am a very happy buyer and I understand now why the CaK PST has a bit of a cult following.

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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by swisswalter » Sun Jun 07, 2015 5:05 am

Hi Douglas

great report about your mod work. A perfect first light
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by Derek Klepp » Mon Jun 08, 2015 3:28 am

Thanks for the detailed report and it seems a great little scope for a quick disc grab.

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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by marktownley » Mon Jun 08, 2015 5:45 am

Brings back memories!

Do you have a lens spanner douglas?
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by Montana » Mon Jun 08, 2015 9:00 am

Absolutely fantastic!! I love mine to bits :bow :bow :hamster:

However with my DMK41 I have to image at about 1/30 or there abouts at prime focus, so 1/250 is incredible! maybe the 51 is more sensitive than the 41.

Alexandra

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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by thesmiths » Mon Jun 08, 2015 11:38 am

Mark: yes, I have a camera lens spanner, a crucial tool for any telescope and microscope servicing (or modding!). Were you trying to suggest this could be used to remove the #1 filter? But I would still first need separate the gold tube, as far as I can see. Below is a photo of my lens spanner (from a Chinese supplier on eBay; it seems to work for the things I need it for).

Alexandra: I was also surprised at the relatively short exposure. I don't think it has to do with the difference between the DMK 41 and 51 (which I believe use the same technology). My guess is it's the condition of the filters. As I mentioned, all the filters on this one seemed very clear, with the exception of the #2 filter which had a slight film (of grease?) on it. There was a little bit of real rust on some of the exterior screws (I guess it was stored in a damp place) but there was no "rust" on any of the filters, which I assume could decrease the light transmission.
4776a.jpg
spanner
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by marktownley » Mon Jun 08, 2015 11:55 am

Use the lens spanner to release the objective from its cell, then, with the objective removed reach down the gold tube with a 30cm steel ruler and you will be able to use this to release the cell that holds the yellow 'magic' filter, it just has a screw in retaining ring holding it.
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by marktownley » Mon Jun 08, 2015 11:57 am

Montana wrote:Absolutely fantastic!! I love mine to bits :bow :bow :hamster:

However with my DMK41 I have to image at about 1/30 or there abouts at prime focus, so 1/250 is incredible! maybe the 51 is more sensitive than the 41.

Alexandra
Some of the filters on the CaK PST are soft coated and so oxidise or 'rust' - if it gets a problem and you are finding exposure time gets longer and longer we can replace them if needed.
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by thesmiths » Wed Jun 10, 2015 10:28 pm

Mark, so you mean remove the #1 filter via the front of the gold tube? Sounds a bit like endoscopic surgery!

I may first actually try the old hair-dryer trick. I've found with stuck microscope parts, heating with a hair-dryer will partially melt any old grease and differential expansion (e.g. between brass and aluminium) will loosen things up. In this case, perhaps the "glue" holding the threads together and the plastic/gold tube expansion coefficients will succumb to some targeted heating.

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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by marktownley » Thu Jun 11, 2015 5:24 am

thesmiths wrote:Mark, so you mean remove the #1 filter via the front of the gold tube? Sounds a bit like endoscopic surgery!
Yup, that's exactly what I mean ;)
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by Moriniboy » Fri Jun 19, 2015 4:03 pm

I found that this stuff dissolves the threadlocker that they use on these scopes, takes a while though i.e. days with a daily re-application to the joint.
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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by thesmiths » Fri Jun 19, 2015 11:56 pm

Thanks for the suggestion to look up info on "Loctite". I found the following interesting info:

"What is the difference between the Red and Blue threadlocker? Red Threadlocker is the highest strength. This product cures fully in 24 hours and is available in both a liquid and as a semisolid anaerobic. The red products are so powerful that they require heat to be disassembled. Blue Threadlocker is of medium strength. Again this product cures fully in 24 hours and can be dissembled with hand tools."

I believe my H-alpha (Made in Mexico) PST was assembled with Blue threadlocker, while by CaK (Made in USA) PST was assembled with Red threadlocker. This would explain the drastic difference in ease of disassembly.

"It can be difficult removing red threadlocker, and it’s a common vehicle maintenance question. While the strength of red threadlockers is formidable, and meant to be a permanent assembly method - don't believe the myths. Red threadlocker can be disassembled with the right technique. When disassembling red threadlocker the process is a little different than for other threadlockers. The key is to apply localized heat greater than 550° F. Then, once the threaded assembly is hot, the bolt can be unthreaded. Without applying heat to the assembly, it's likely that a bolt would break before coming loose."

"Don’t let the high strength threadlocker scare you, just remember: "heat things up to loosen it up!" Just don’t use it on plastic parts, that’s a whole other story."

So I think my original of using a hairdryer might be the best place to start. I have read elsewhere that a temperature of 250 degree C is sufficient.

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Re: First light for a "new" CaK PST (and 1st attempt to mod

Post by marktownley » Sat Jun 20, 2015 8:50 am

let us know how you get on
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