New CaK etalons from Solar Spectrum

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Re: New CaK etalons from Solar Spectrum

Post by christian viladrich »

Bob Yoesle wrote: Tue Jun 04, 2024 10:47 pm I spoke to Alluxa, and they confirmed the most of the filters of this type are coated on both sides: "The side 1 coating is the primary bandpass, and side 2 is the blocker, which is essentially just a wider bandpass encompassing the main bandpass (which acts as an AR), as well as all the auxiliary blocking."

Bob
Very interesting information Bob ! Yet, it is still unclear whether they add (or not) an AR coating on both sides of the filter.

AR V-coating is very effective with very low level of reflection at a specific wavelength (< 0.25 %) :
https://www.edmundoptics.com/knowledge- ... -coatings/

But maybe it is not possible (or does not make any sense) to add an AR coating on top of the "primary band" and "secondary" coatings ?

For us, reflections out of the band are really not an issue. However, we would like to minimize reflection on band. Maybe the very principle of these filters sets technical limits on this ? Or is it a design choice ?

It would be interesting to know what is the reflection coefficient calculated by Alluxa for this filter for both side.


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Re: New CaK etalons from Solar Spectrum

Post by Bob Yoesle »

Hi Christian,

I did not ask Matt at Alluxa specifically about adding an AR coating to the filter, as Mark W had already advised me that was not a good idea for a hard coated filter - and he is in regular contact with Alluxa for the hard-coated blockers he uses. I'll send you an email so you can ask Matt his thoughts on why that is not done in addition to what is stated to be acting as an AR coating quoted above.

Bob


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Re: New CaK etalons from Solar Spectrum

Post by christian viladrich »

Hi Bob,
Thanks a lot. I'll contact Matt at Alluxa.
Christian


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Re: New CaK etalons from Solar Spectrum

Post by marktownley »

Keep us updated please.


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Re: New CaK etalons from Solar Spectrum

Post by Bob Yoesle »

No K line filter as yet due to unforeseen circumstances.

So I took the opportunity to disassemble the DERF cell and paint it:
20240610_074423smcrp.jpg
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20240610_075337smcrp.jpg
20240610_075337smcrp.jpg (235.7 KiB) Viewed 971 times

I used a high quality flat white spray enamel, and clear coated it with a semi gloss clear finish to match the TOA150. This also will hopefully avoid any untoward thermal effects from the black Delrin retaining ring that otherwise might have heated up surrounding the DERF itself.

I exchanged the nylon securing bolts with stainless steel hex cap-head bolts with black plastic knobs, and added black plastic thread protectors to the tips to keep from marring the TOA paint finish:
20240610_075407 smcrp.jpg
20240610_075407 smcrp.jpg (225.51 KiB) Viewed 971 times

I will paint the four small stainless steel bolt heads with some Ultra black paint before putting it to use.

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Re: New CaK etalons from Solar Spectrum

Post by marktownley »

Looks tidy Bob!


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Re: New CaK etalons from Solar Spectrum

Post by Bob Yoesle »

Here are preliminary results from the just-arrived Solar Specturm K line filter.

I spent the morning playing with assorted adapters and other parts to get a half-way functional system assembled, but I will have to do some more McGyvering.

20240704_173415 sm.jpg
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20240704_113711 sm.jpg
20240704_113711 sm.jpg (251.06 KiB) Viewed 781 times

Caveats: Tuning the K line will take some more experimentation and evaluation. Mark stated the filter is set to be on-band at about 30 C (86 F), but today’s imaging period temperature was 95 F (35 C). So I tried a bit higher set temperature and some tilt to blue shift it downward. No matter what temp setting I tried, the K line always looked best with no tilt. Similarly, when double stacked with the Alluxa 393.37-0.1, no tilt for it always provided the best contrast. Luckily I observed no ghosting with the double stack configuration.

The imaging session was plagued with poor mid-day (local noon) seeing, mount tracking electrical problems, camera cooling failure secondary to a broken wire, and cramped conditions within my normal solar observing space with the 20 inch (0.5 m) Newtonian still under construction. With the high temps this limited my ability to spend more time getting things as well optimized as I would have liked.

Used FireCapture and tried to kept all settings but shutter speed the same, and just did a quick and dirty stacking 20% and wavelets in Registax 6, no other processing.

F/9 configuration: UV/IR Block on KG3, 394-10, SS 0.5 or DS ads the 393.37-0.1 ahead of the 0.5.
20240704_113721 sm.jpg
20240704_113721 sm.jpg (214.18 KiB) Viewed 781 times


F/27 configuration: same as above and ads the TZ-3 with 250 mm separation between it and the camera as specified by Baader, which places the telecentric in the proper position relative to the objective's focal plane.
20240704_124533 sm.jpg
20240704_124533 sm.jpg (187.23 KiB) Viewed 781 times

The results are not very good on an absolute detail scale - and there was no way to expect f/27 to work well given the poor seeing and consistent apples-to-apples camera settings I used. But I just had to try for the purpose of evaluating contrast and I’m impressed with the overall contrast improvements I observed. My goal was to see how well contrast improved at ~ 0.5 A, and if double stacking could be done. My additional hope was that this might achieve a near SHG level of contrast. With better conditions and camera settings (and using the TOA-150 when seeing allows), I'll hope for better results.

ED100/900 at f/9: Solar Spectrum 0.5. Shutter 3.32 ms, Gain 288, Gamma 647, Histogram max 169 66%
July 4 2024 SS f9 900mm 113437 sm.jpg
July 4 2024 SS f9 900mm 113437 sm.jpg (205.26 KiB) Viewed 781 times


Even at f/9 with just the Solar Spectrum I didn't detect much if any double limb while adjusting shutter speeds, but then again seeing conditions were rather poor.

ED100/900 at f/9: Alluxa 393.37-0.1, Solar Spectrum 0.5. Shutter 7.17 ms, Gain 288, Gamma 644, Histogram max 168 65%
July 4 2024 Alluxa & SS f9 900mm 120338 sm.jpg
July 4 2024 Alluxa & SS f9 900mm 120338 sm.jpg (141.78 KiB) Viewed 781 times

Note the CaK plage has about the same intensity, but overall disc brightness has diminished significantly, reflecting less photosphere continuum getting through. This indeed does appear to get close to typical SHG results.

ED100/900 at f/27: Solar Spectrum 0.5. Shutter 49.80 ms, Gain 288, Gamma 644, Histogram max 255 100%
July 4 2024 SS f27 2700 mm 122638 sm.jpg
July 4 2024 SS f27 2700 mm 122638 sm.jpg (121.2 KiB) Viewed 781 times


ED100/900 at f/27: Alluxa 393.37-0.1, Solar Spectrum 0.5. Shutter 100.00 ms, Gain 288, Gamma 644, Histogram max 255 100%
July 4 2024 Alluxa & SS 2700mm 123716 sm.jpg
July 4 2024 Alluxa & SS 2700mm 123716 sm.jpg (109.45 KiB) Viewed 781 times

Double stacking with the Alluxa filter at a minimum doubles the exposure time. It's pretty obvious that I need to use more gain and get the interface for the 0.4 Baader reducer put in place, This will require a new end plate for the etalon and some additional extension tube work...

The “full monty” configuration: BelOptik 2” UV/IR block on KG3, Edmund 394-10 in BelOptik tilter, Baader TZ-3 Telecentric, Alluxa 393.37-0.1 in BelOptik tilter, Solar Spectrum 0.5A with heated tilt module, extension tube (Barlow lens housing), PGR Chameleon.
20240704_173922 smcrp.jpg
20240704_173922 smcrp.jpg (181.71 KiB) Viewed 781 times

Bob


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Re: New CaK etalons from Solar Spectrum

Post by Dennis »

Promising results, thanks for keeping us updated Bob. I would go for a different camera (larger pixels) instead of the reducer.
Regards


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WL: Antlia 500nm/ 3nm, 393 nm/ 3nm

Ca-K: homebrew (includes 2x 1.5A filters, thanks Apollo), corrective lenses (thanks again Apollo)

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Re: New CaK etalons from Solar Spectrum

Post by KMH »

A good start! Will you need some kind of support for that imaging train?

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Re: New CaK etalons from Solar Spectrum

Post by Bob Yoesle »

Hi Dennis - yes indeed, larger pixels and more QE at 400 nm is on the list ;-)
Hi Kevin - indeed an optical rail system seems to be a necessity with these long telecentric systems. I also have to look further into the Baader ecosystem of connectors and extensions. And I thought Takahashi was bad!

This morning I had significantly improved seeing compared to yesterday at noon. Here's the f/9 single pane sample (~30 stacked frames):
085244 crp.png
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This again is the Alluxa 1.5 A & Solar Spectrum ~ 0.5 A double stack at f/9. at 15:52 UT. PGR Chameleon, 15 FPS, shutter 19.77 ms, gain 288, gamma 644, histogram max 181 70%. RS6 w wavelet sharpening.

Full disc mosaic:
050724 CaK mosaic histogram cmp.jpg
050724 CaK mosaic histogram cmp.jpg (690.42 KiB) Viewed 660 times

Bob
Last edited by Bob Yoesle on Sat Jul 06, 2024 3:09 am, edited 1 time in total.


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Re: New CaK etalons from Solar Spectrum

Post by Dennis »

Fantastic results considering the f/9 focal ratio. Looks definitive a step higher in the chromosphere then with 1A.


Triband C9.25

H-a: 2x Lunt40 rear mounted

WL: Antlia 500nm/ 3nm, 393 nm/ 3nm

Ca-K: homebrew (includes 2x 1.5A filters, thanks Apollo), corrective lenses (thanks again Apollo)

Cameras: imx432 + imx462
Barlows:
-2x Gerd Düring 2.7x
-2x TMB 1.8x
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Re: New CaK etalons from Solar Spectrum

Post by Bob Yoesle »

Thanks Dennis.

Using the graphs from Christian for mica etalons and the Alluxa bandpass filter at f/9 and assuming the curves remain Lorentzian, you get the following approximate bandwidth specifications for the system above:
Alluxa & SS DS at f9.jpg
Alluxa & SS DS at f9.jpg (28.2 KiB) Viewed 635 times

Obviously the longer the f ratio the better things will get.

While the images yesterday were pretty dim at f27, a big part of that might have been my use of the BelOptik UV/IR block on KG3. Looking at the plot for this filter - and now remembering that Oliver once told me the blue side might be shifted +/- 10 nm and therefore not too good for use at CaK, it might be causing a significant reduction at the CaK line.
KG3_UV_lin_1.jpg
KG3_UV_lin_1.jpg (105.36 KiB) Viewed 635 times

Therefore I had already planned on the contingency of McGyvering a Edmund 390-40 Fluorescence filter that blocked 250 - 1100 nm with a greater than 90% transmission, and possibly combine it with a straight Andover 3 mm thick KG3 that has a ~ 85% transmission at 393 nm. This might obviate the need for the Edmund 394-10. On the other hand, the straight KG3 with the 394-10 might be all that's needed for this particular set-up.
Andover 3 mm Heat_abs.png
Andover 3 mm Heat_abs.png (39.12 KiB) Viewed 635 times

The 50mm Andover KG3 and Edmund 390-40, and various McGyvering parts.
20240705_151944 sm.jpg
20240705_151944 sm.jpg (199.84 KiB) Viewed 635 times

Lots of ingredients that can go into making - or be removed from - a filter system stew... just need to do some more experimenting to find out.

Bob


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Re: New CaK etalons from Solar Spectrum

Post by mdwmark »

Hi group,
The idea about putting an AR on the Alluxa filter seems like a good idea till you find out that both sides of the filter is usually coated to get the blocking for the specs. The index of refraction of the coating on the back side must be close to to the grease i use because , it doesn't seem to have any problems with internal reflections .
The other problem is you need to take into consideration what F/# the filter will be used at. If the CW in on the line the F# may move the CW to far blue . Then with a hard coated filter you will never get it back on band. A little longer and a small tilt would work best.
Mark


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Re: New CaK etalons from Solar Spectrum

Post by marktownley »

Promising first results Bob! Looking forward to seeing more.

Thanks for the filter thoughts Mark.


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Re: New CaK etalons from Solar Spectrum

Post by christian viladrich »

mdwmark wrote: Sat Jul 06, 2024 5:30 pm Hi group,
The idea about putting an AR on the Alluxa filter seems like a good idea till you find out that both sides of the filter is usually coated to get the blocking for the specs. The index of refraction of the coating on the back side must be close to to the grease i use because , it doesn't seem to have any problems with internal reflections .

Mark
Very interesting Mark. Thanks for your input !

If I understand you properly, once the two surfaces of the filters are coated in order to block Ca K (large bandpass on one side, narrow bandpass on the other), there is no technical possibility to "stack" an AR layer on top on these coatings.

Christian


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