Repairing a CaK PST

[Montana]

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

[MapleRidge]

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

Brian

[Bob Yoesle]

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 (150.99 KiB) Viewed 4766 times

Christian Viladrich et.al.

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

[Montana]

Very interesting, thanks for the great info Apollo

Alexandra

[Montana]

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 ?-?

2021-01-09 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

Sun_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

[marktownley]

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

[Montana]

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 (793.75 KiB) Viewed 4668 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

Alexandra

[Carbon60]

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.

Stu.

[Bob Yoesle]

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 (42.79 KiB) Viewed 4649 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 (128.67 KiB) Viewed 4649 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 (132.68 KiB) Viewed 4649 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 (203.54 KiB) Viewed 4606 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 (259.47 KiB) Viewed 4649 times

[Montana]

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

Alexandra

[DavidG]

Like others that own a CaK PST, the filters will hazy over with age. So far the main yellow one in the OTA is still in good shape but the blocking filter in the eyepiece tube had hazied over. I took it into work and measured it. It was only passing less than 2% at the CaK wavelength . I was still able to see an image in the scope but had to cover my head with a dark cloth and let my eyes dark adapt to see the image.
I had worked with Maier Photonics on other projects and sells an ITF filter that works very well in H alpha PST so I looked on their website to see if anything might work. Maiers does offer a 393nm filter that looked very promising. I sent them the scan of the filter from my CaK PST and ask how their filter would match up. Chris Maiers sent me back the scan of his filter and sent me one to try. The Maiers filter is larger than the original and I was able to mount it in place with a couple of O-rings. The scan of the Maier's filter showed it transmits around 50% at CaK wavelengths and is block into the IR out OD4.
The image is now much brighter and I can see the image visually without covering my head with a cloth. The image is now back to the brightness I remember it when the scope was new.
https://maierphotonics.com/bandpassfilters.aspx

- Dave

CaK Maiers filter mounted.jpg (75.05 KiB) Viewed 3671 times

Maier CaK filter.jpg (59.11 KiB) Viewed 3671 times

CaK PST Blocking Filter.jpg (35.33 KiB) Viewed 3671 times

[Bob Yoesle]

Thanks for finding this Dave!

I believe the primary yellow dichroic filter is hard-coated and pretty resilient.

The only other issue for visual use would be how far out the Maier filter is blocked for IR out to about 3000+ nm. It might be necessary to employ a KG3 filter for this purpose when the other ancillary filters have deteriorated.

Bob

[Montana]

Thanks Dave, that is really interesting. Can I ask whether you have imaged through this at all? I have been using a Baader K line filter in replacement to the busted blocking filter but my surface contrast is much reduced to what I used to get with the original blocking filter. So I was wondering if this filter is giving a similar tight bandpass as the original. I would say that my PST is no longer 2.2A, but much greater (to my sadness). I would like to get back to 2.2A again somehow

Alexandra

[DavidG]

Bob,
Here is what is listed on Maier's site for his 393 filter that I used
393 Bandpass Filter - sulfur line gas chromatography
Center wavelength=393 nm +/- 2 nm
Bandwidth=13 nm +/- 2 nm
Size=12.0 mm dia. x 5.0 mm thick
Out of band blocking > optical density 4.0 (xray to 2000 nm)
Coating type= Non hydroscopic

So it is pretty well blocked into the IR. There is also another filter directly below the eyepiece in my CaK PST and it seems to be IR blockers as well.

- Dave

[DavidG]

Alexandra,
I have not imaged through my CaK with the new filter but it is on my list and will post images when I do. I can say that the image is much bright and I see it visually pretty well and even see some CaK features on the disk when I cover my head with dark cloth.

- Dave

[DavidG]

Hi All,
Happy 2023 ! I finally had both some free time and clear skies to use my repaired CaK PST that I used a Maier Photonics filter to replace the bad one in mine. Visually today I could see the CaK features and was very pleased with the images it was producing with my ZWO 120 monochrome camera.
Here is an image made from a short 10 sec video I took and my good friend Matt Considine processed for me. Matt said it is only lightly processed. The images were taken around 3:30pm so the Sun was low and the seeing wasn't great but again I very pleased at how well the CaK features are visible.

- Dave

[Montana]

Is this the one you bought David https://maierphotonics.com/bandpass-filter.aspx

Alexandra

[DavidG]

Alexandra,
Yes that is the Maier's filter I'm using.

- Dave

[marktownley]

Good fix!

[estra]

Hello Alexandra

Did you buy the Meiers filter? Did your Ca Pst recover the contrast you were looking for?

Best regards
Paulo

[Montana]

Hi Paulo,
Yes I did buy the Maier filter although it was very complicated to buy and took a few weeks to place the order because the website didn't work for overseas customers. My PST is much better than it was with the Baader Kline in but I have always had my worries that my Baader K line is faulty. It is still not as good as it was when brand new, but at least it is usable now. Here is an image taken in July
2023-07-19 CaK solar disc f colour b by Alexandra Hart, on Flickr

I need to take a photo of how I fixed the Maier in the eyepiece

Alexandra