Repairing a CaK PST

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Re: Repairing a CaK PST

Post by Montana » Sun Jan 03, 2021 2:44 pm

Sunshine :hamster: so had a good testing session this afternoon.

The pentaprisms are coated differently and I had the same sneaky suspicion as Apollo that this may not work.
ImagePST Pentaprisms by Alexandra Hart, on Flickr

and correct, it does not work to swap bodies. I had absolutely nothing come through to the camera, black black and black.

So I swapped back to the CaK body and removed Apollo's filter in the gold tube. I placed the camera in and could immediately see the Sun, it was very very bright. After knocking the exposure right down I could see that I had a double disc like a ghost and out of focus. This looked like very bad UV/IR out of focus bleed through. So to answer the unanswered questions above, the filter in the gold tube seems to block stay UV/IR light and is absolutely necessary. So I tried the Baader U filter on the camera nose and total blackness followed. It completely blocks everything. I had a suspicion this would also be the case as it blocks light above 380nm. I have no idea how Eric uses it for Calcium K?

So I placed my dodgy Baader K line filter in and wow! perfect! I took 2000 frames and checked stacking and focusing and it was great. I even tried 1.6x and that too was great.

So at 1x my old filter exposure was at 86.8ms
Apollo's filter at 23.12ms (but sadly suffered astigmatism)
Baader K line as replacement 7.975ms

I am very happy indeed, at 1.6x the exposure was 21.62ms so this is a huge shift in usability although I see now I get a reflection with this Barlow. I will try with a 2x Barlow on the next outing (just like the olden days :hamster: ).

These images are the best Calcium K images I have taken for a couple of years I think, so I am a very happy bunny today.

However, mystery eyepiece filter? no idea what that does and I won't disturb that for a few years until I am suffering again. At the moment I am very pleased with the imaging capability. I am also happy I finally have a use for my Baader K line. At low magnification it works very well, maybe it is just bad at high res with the TEC140 and C11. It now has a happy home with the PST.

ImageSun_122203_03_01_2021 by Alexandra Hart, on Flickr

ImageSun_122203_03_01_2021_f colour by Alexandra Hart, on Flickr

ImageSun_123010_03_01_2021_a by Alexandra Hart, on Flickr

Thanks everyone for all your kindness and help, I am very happy indeed and glad I have learned all this from you :)

Alexandra



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Re: Repairing a CaK PST

Post by marktownley » Sun Jan 03, 2021 3:29 pm

Whoop Whoop! I knew between us all it would sort out. Good work team!!!

You know what my suggestion is going to be... Remove the mystery filter in the EP holder and see what effect it has, might even shorten exposure time? Worth a go one of those days you have time to pass.

Even made a use of that badly behaved K-line filter of yours.

All good!


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Re: Repairing a CaK PST

Post by MapleRidge » Sun Jan 03, 2021 11:23 pm

Great new Alexandra, good to know you are back in business with with the K-Line filter !!!

What is the cause of the bright circular spot in the third pic...is this one of the ghost images or a reflection maybe?

Brian


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Re: Repairing a CaK PST

Post by Montana » Mon Jan 04, 2021 8:25 am

Brian it looks identical to a reflection I get with the TeleVue 2.5x Powermate when using Calcium K. So I guess it is the same thing, the Baader K line has a mirror surface so I think it reflects light back and hits an element in the barlow lens and then back again. This was solved in the powermate by moving the distance of the camera 4cm higher up. However, this does not change the powermate magnification, but this would probably change a Barlow and also the distance would probably render it not able to reach focus with a PST. So I am going to try my 2x Barlow instead and have my fingers crossed :)

Alexandra



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Re: Repairing a CaK PST

Post by MapleRidge » Mon Jan 04, 2021 11:49 pm

Alexandra...that reflection reminded me of that problem, but wasn't sure if you had enough focus travel to reach focus when the spacing was increased.

Nice to see it back to life ;)

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Re: Repairing a CaK PST

Post by Bob Yoesle » Tue Jan 05, 2021 3:32 pm

Hi Alexandra,

Looks like you're back in business.

I haven't yet had a chance to to try it, but the Baader K line filter has two elements; one tilted and the other not. You might try to disassemble the filter and use only the tilted element. From the little I have seen of this it will not significantly affect the filters out-of-band blocking ability, but may help remove the ghost reflection. Due to becoming a single stack filter, the filter transmission will increase - and therefore image brightness - resulting in shorter exposure times and better freezing of seeing effects.


Baader K line tranmission.jpg
Baader K line tranmission.jpg (150.99 KiB) Viewed 348 times
Christian Viladrich et.al.

When my weather improves I'm interested in trying this myself...


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Re: Repairing a CaK PST

Post by justapictureposter » Tue Jan 05, 2021 5:25 pm

The two filters in the eyepiece, come out very easily.

The one closest to the prism unthreads from the bottom.

The one in the eyepiece, is glued in and will pop out with a slight tap; using a 1.25" barrel to push it out the bottom.

Neither of the 5mm filters are required and when you remove these your 1.25" baader U-filter should work in the eyepiece.

If you look at the box of your U-filter , the calcium line is visible in the spectrum photo . Transmission is low but it does work.


block1.jpg
block1.jpg (12.98 KiB) Viewed 332 times
block2.jpg
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block3.jpg
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<---this one in the eyepiece is glued in and "pushes out" the bottom.



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Re: Repairing a CaK PST

Post by Montana » Wed Jan 06, 2021 8:01 am

Very interesting, thanks for the great info Apollo :hamster:

Alexandra



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Re: Repairing a CaK PST

Post by Montana » Sat Jan 09, 2021 5:00 pm

Today I have had another play around with the filters (very unlike me) to see what the nose piece filters do.

I unscrewed the nosepiece filter off the bottom and had a good clean with ethanol and lens wipe. By eyeball it looked good when I looked at the Sun through it. The second filter further up looked just like clear glass to me so I cleaned it and left it where it was (nervous about permanent changes).

So I tested an image with various combinations of the Baader K-line, a Baader U, Baader blue CCD (at the front of the scope as an ERF) and the nose piece filter.
Baader K-line is 390-398nm
Baader U Venus filter 320-380nm
Baader Blue CCD ?-?
CaK PST nosepiece filter ?-?

Image2021-01-09 CaK Nosepiece filter test by Alexandra Hart, on Flickr

Probably better to click through to Flickr. The result is I am sticking with the CaK nosepiece filter and Baader K-line combo :)

The 1.6x Barlow reflection is almost gone when I unscrew it from the camera by about 5mm. I can still achieve focus and it isn't too bad

ImageSun_134025_09_01_2021 1-6x a by Alexandra Hart, on Flickr

Seeing was pretty terrible today with a milky white haze and passing clouds so I am pleased I got this far.
Alexandra



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Re: Repairing a CaK PST

Post by marktownley » Sun Jan 10, 2021 6:26 am

The CaK PST #1 filter and the K-line is a killer combo.

My CaK project this year is finding the best double stack combo. I bought another CaK PST off ebay in the autumn and plan is to strip the #1 filter from it, and then from my existing CaK DS filter find which combination of filters works best. However that requires sunshine and time, so probably spring :lol:


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Re: Repairing a CaK PST

Post by justapictureposter » Sun Jan 10, 2021 7:40 am

OK! This is rather unusual; as I have had a spectacular result with my Baader U-filter in the past. :)

When did you purchase your U-filter and where? mine was purchased sometime in 2012 from highpoint; Baader must have changed the filter in recent times.

My first ever cak scope mod was with the original cak pst, using just the nose filter in the gold tube and the baader U-filter in the eyepiece; i still have my first light image in fact (and video)! at the time I also double stacked it with a lunt 3400mm cak with enough imaging transmission left. What a strange world.
Image

even the website i purchased it from still advertises it for use to image the calcium k-line
https://www.highpointscientific.com/baa ... 25in-fuv-1

Quite remarkable to see how all these filters have changed over the years.
Posted 02 October 2014 - 07:27 PM
Image



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Re: Repairing a CaK PST

Post by Montana » Sun Jan 10, 2021 11:39 am

Mark,

I already thought you had 2 CaK PST's? From my experience the filter in the eyepiece is quite crucial, the one in the gold tube not so much.
When I went to visit Luca in Italy he had a CaK modded device from a PST and I always thought the bandpass was less than my Lunt. More like the image with the Baader K line alone with the CaK gold tube filter together. I think the eyepiece filter seems quite crucial for getting a tighter bandpass and I expect his mod did not include it.

Apollo, yes I am surprised about the Baader U filter (unless I bought the wrong one?). Certainly mine is not pink, it is green. I questioned Eric about how old was his filter but he never replied. I was suspicious from the point of ordering but everyone was saying it was fine.

It was bought for me as a Christmas present so it is a 2020 edition of the filter and it quite clearly states that it blocks above 380nm. When using it with the CaK eyepiece filter it is black, no light. When the eyepiece filter is removed it just passes pure out of focus UV and nothing you can do to get it in focus. This is why I think this CaK eyepiece filter is quite crucial to reaching correct narrow bandpass. However, it must leak a little UV as adding the Baader K line with an 8nm wide filter tightens it up.
DSC00050 a.jpg
DSC00050 a.jpg (793.75 KiB) Viewed 250 times
It seems to be no longer for sale on the 365 Astronomy website so I cannot show you the link.

All very interesting, but my useless Baader K line filter has gone into service and now I have a useless Baader U filter. I guess I need to image Venus with it :lol:

Alexandra



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Re: Repairing a CaK PST

Post by Carbon60 » Sun Jan 10, 2021 12:00 pm

At least you've found a combination that works, Alexandra. I'm pretty sure the first filter (pointing at the Sun) blocks most wavelengths except for those close to 393 nm (this one appears blue) and the eyepiece filter passes all wavelengths except for those below about 393 nm (this one appears yellow as it is absorbing blue and passing green/red light. The cutoff wavelength of each filter is very close so as to allow the transmission of a narrow band of light at the required 393.4 nm when the filters work together. That's how I understand my Lunt module works, so I'd expect it to be the same with the PST. Apologies if this has already been discussed in this long thread.

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Re: Repairing a CaK PST

Post by justapictureposter » Sun Jan 10, 2021 2:55 pm

I did a little sleuthing on the -u- filter from baader planetarium.

It was changed in fact. (the u-filter i have is gold, and was discontinued in 2009.)
Image



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Re: Repairing a CaK PST

Post by Bob Yoesle » Sun Jan 10, 2021 6:21 pm

Hi Stu,
I'm pretty sure the first filter (pointing at the Sun) blocks most wavelengths except for those close to 393 nm (this one appears blue) and the eyepiece filter passes all wavelengths except for those below about 393 nm (this one appears yellow as it is absorbing blue and passing green/red light. The cutoff wavelength of each filter is very close so as to allow the transmission of a narrow band of light at the required 393.4 nm when the filters work together. That's how I understand my Lunt module works, so I'd expect it to be the same with the PST.
Depending on the viewing angle, the first filter can appear blue or a reflective gold in my CaK PST(s). It is the clear yellow in transmission (purplish in reflection) dichroic filter which is the essential filter of the PST at the 394 nm region. Brian Stephens (Tucson Coronado / LUNT) refers to this filter as the CaK "blocking filter." He states this PST filter would now days be very expensive to secure - $1500.00 - $2000.00 USD. So LUNT has now placed smaller versions of this filter in the eyepiece barrel of the LUNT CaK diagonals, and they are no longer even making the largest version due to the expense. LUNT eventually may find these CaK units to be too expensive to make considering the demand, and has hinted at their potential demise... so I would get one while one knows they can.

The transmission of this dichroic filter looks like this:

Lunt CaK Etalon_wide blocking filter.PNG
Lunt CaK Etalon_wide blocking filter.PNG (42.79 KiB) Viewed 231 times

Note this dichroic filter IS NOT an etalon as whoever made this spectrographic measurement labeled it.

Christian Viladrich uses instead a very narrow Barr bandpass filter for the same purpose, and he and Apollo have explored other hard-coated filter options from Chroma Tech and Alluxa, but these are also expensive and cost a minimum of about $2500.00 USD to produce, and production appears fraught with peak transmission CWL consistency difficulties.

The remaining filters in the PST are apparently a combination of cut-off, bandpass, and ITF's to render the image relatively bright enough and safe for eyeball imaging. Unfortunately they appear to be soft-coated and can or will deteriorate within a few years time. Replacing these with the right hard-coated filters apparently would make all the difference.

For me this has meant the move to a do-it-yourself CaK module solution (allowing a greater aperture to be used), but there's no reason these can't be incorporated into the PST - especially if it is intended for imaging only.

With my soon-to-be revised DIY CaK module, I'll be double stacking two PST "blocking filters," and using a "stew" of other mostly hard-coated filters to isolate the 393.4 nm transmission.

An Edmund fused silica 1/4 lambda hard-coated 390 45 florescence bandpass filter, followed by either an Andover KG3 or tilted BelOptik UV/IR KG3 for long IR, will replace the Baader Blue CCD filter as an "ERF":

390 nm Bandpass 45 FWHM.jpg
390 nm Bandpass 45 FWHM.jpg (128.67 KiB) Viewed 231 times

After these, and based on as yet to be made observations, I will have to decide if I will use Apollo's hard-coated Chroma Tech 393.37 0.1 (requires tilting to be on-band), a Baader K line (single stacked vs. the normal DS as posted above), or an Edmund fused silica hard-coated 400 nm short pass filter to further isolate the blocking filter's transmission while maintaining the highest transmission possible for achieving the shortest exposure times and best contrast.

400 nm SP OD 4 curve.jpg
400 nm SP OD 4 curve.jpg (132.68 KiB) Viewed 231 times

I'm not sure what the cut-on point of the slightly tilted BelOptik UV/IR K3 will be, but this Edmund short-pass and UV/IR KG might be the ideal pre-filer for the PST blocking filters:

UV IR KG3 w 400 SP v K Line.jpg
UV IR KG3 w 400 SP v K Line.jpg (203.54 KiB) Viewed 188 times

The Edmund 400 nm SP combined with the BelOptik UV/IR KG3 filter appears to offer the possibility of significant improvement in transmission to the K line as a pre-filter, especially if the UV/IR KG3 filter slightly blue-shifts when tilted. The Chroma Tech filter requires significant tilting to come on-band, and I'm not sure of its transmission percentage at that point. Indeed, most of these filters will need to be slightly tilted to minimize retro-reflections and optimize their their performance, so I've had to source some Baader-based filter-tilters from Bart and BelOptik in addition to Apollo's generous provision of SkyBenders to hopefully accomplish this task. Putting all these together in an acceptable mechanical arrangement will be a challenge, and will likely require additional machining work.

Filter tilters.jpg
Filter tilters.jpg (259.47 KiB) Viewed 231 times
Last edited by Bob Yoesle on Tue Jan 12, 2021 3:23 pm, edited 3 times in total.


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Re: Repairing a CaK PST

Post by Montana » Mon Jan 11, 2021 9:11 am

I've just heard back from Eric, he got his Baader U many years ago, so I guess pre 2009.

Alexandra



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