Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

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christian viladrich
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Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by christian viladrich »

Hello,

I recently bought an Alluxa 1.0 A Ca K filter. I was not very lucky with the seeing conditions until now, because of the heat wave in France. Still I could take some pictures and make extensive tests / measurements with Sol'Ex spectrometer.

http://astrosurf.com/viladrich/astro/in ... ilter.html

The filter is a single cavity (equivalent to single stack) filter with hardcoating. Peak transmission is > 80% according to the curves provided by Alluxa. It results in a very significant reduction of the exposure time, which is excellent news for high resolution imaging.

The filter is indeed 1.5 A FWHM. The 1.0 A value is the "nominal" value and not the actual one.

With such a narrow bandpass, the tuning of the bandpass right on Ca K is very sensitive (to the f-ratio and to ambiant temperature). For example, at normal incidence, the filter is +0.3 A from the center of Ca K at f/22 and 30°C. So the filter should be tilted (by about 2°), or to be cooled, to be tuned on Ca K.

When double stacked with the alluxa 0.37 nm, the combination has an measured FWHM of 0.90 A.

More to come, when I will be able to comare it with my Barr two-cavity 2.4 A Ca K filter...


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Re: Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by marktownley »

Interesting analysis Christian!

What are you using to tilt the filters?


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Re: Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by christian viladrich »

Hello Mark,

I have different tilt systems :
- one is for 30 mm diameter filters. I use it for the Alluxa 0.37 nm filter. It can be seen here :
http://astrosurf.com/viladrich/astro/in ... onomik.jpg

- two others come from Beloptik. They are a bit like the previous one, but derived from Baader M42 rings. They accept 25 mm filter. Unfortunately, the 1 A Alluxa filter is 25.4 mm. This is the first ring seen here after the long extension tube :

http://astrosurf.com/viladrich/astro/in ... ly2022.jpg

- so I use the ZWO system used to tilt camera (to remove interference rings). This is not very convenient, but better than nothing to start with.


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Bob Yoesle
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Re: Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by Bob Yoesle »

This filter appears quite similar to the Coronado PST dichroic hard-coated CaK filters:
CaK filters PST v Alluxa.jpg
CaK filters PST v Alluxa.jpg (242.27 KiB) Viewed 1403 times

It is a bit darker overall yellow transmission, with a similar purplish tint at most angles.

CaK Alluxa package.jpg
CaK Alluxa package.jpg (24.09 KiB) Viewed 1403 times

I'm hoping to follow Christian's lead with using this filter with a double or triple stack CaK PST filter module system that also uses the Baader-Beloptik filter tilters.

CaK imaging module SM.jpg
CaK imaging module SM.jpg (483.27 KiB) Viewed 1403 times


Note: These filters were originally commissioned from Alluxa by A. Lasky, who also obtained some excellent results with them.


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Re: Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by LeoD »

Very impressive research, Christian!
It's interesting that with DS system about 1A FWHM fibrils structures are already started to be visible in Ca K, in comparison with SHG.
Can't wait for updates and 2.4A+1A analysis!

Thinking to go further to see fine structures at ~0.5A, I remember I saw once in solar observatory Ca K filter switchable from 0.6A to 1.2A FWHM and thermostatic tuning. The filter sandwich was made from island spar plates, it was a heavy cylinder 300mm long... but they don't use the filter for hires imaging :)
Bob Yoesle wrote: Wed Jul 27, 2022 3:48 pm
Note: These filters were originally commissioned from Alluxa by A. Lasky, who also obtained some excellent results with them.
Was it a special batch order or is it possible to make a query for the same part number?
regards
Leo


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Re: Test of an Alluxa 1.0 A Ca filter with Sol'Ex spectro

Post by Bob Yoesle »

Hi Leo,

I believe this was a special batch order. You could email Alluxa and see if there were any others from this run, or what it might take to get another batch processed...


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