If CaK is best at F10 -

[mdwmark]

Hi Group,
OK, I did a model for an 2.5Ang single cavity bandpass from F/30 to F/7. I assumed it to be hard coated and how I would design it.
So with the broader filter , anything above F/20 it fine. So F/15 HW was about 3.4Ang( which is really no bad) , F/10 was 5Ang HW , f/7 was 10 Ang Hw.
The question was asked, is a Barlow OK. I would keep away from those simple one ( neg lens doublets). Something like a PowerMate level would be fine. But I'm not sure how well they are corrected that far blue. The SunDancer 2 telecentric was design to work at K-line. Outside of that I'm not sure what else is.
Also, 1 Ang K-line filters are my average HW. So they are not uncommon.

[mdwmark]

Hi group.
You can see like all filters, the cone of light coming in at a faster F/# is the same effect that tilting does. The center wavelength is shifted towards the shorter wavelengths. So at K-line you may not be in the center of the absorption line but you will see some of the effects of the line.

[christian viladrich]

Great ! Thanks a lot for sharing Mark.

[marktownley]

Very insightful. Thanks Mark.

[marktownley]

I put the 2 graphs side by side for easier comparison:

CaK Filters by Mark Townley, on Flickr

Important thing to notice here is the difference in scale on the x-axis. For the wider 2.5a filter there is a lot more blue shifting of the filters target CWL compared to the narrower 1a filter for a given faster f number. There is also greater relative loss of tranmission with the wider 2.5a filter compared with the narrower filter when the operating f ratio decreases.

[marktownley]

Maybe these 2 images allude to the difference of bandpass at CaK; Both taken with a Chroma Technology 1 angstrom filter double stacked with a 2.2a filter from a CaK PST. Both taken with Bresser 127/1200, one at 60mm f20, the other 80mm f15. Camera was the zwo183mm using ROI

First off the f15 shot
Ar13423 CaK 80mm f15 zwo183mm by Mark Townley, on Flickr

Next the f20 shot
ar13423 CaK 60mm f20 zwo183mm by Mark Townley, on Flickr

Things maybe look a little softer and fuzzier around the plage in the active region, whereas with the f15 shot it looks a little cleaner.

Difficult to compare apples with apples!

Mark

[mdwmark]

OK ,
Christian's filter would had been designed for 1Ang . But in reality the very narrow ones, are harder to keep to what the program says it should be.
The first one is with an High index spacer. The next one is a low index spacer. In a collimated beam they are both 1 Ang HW.
1), F/30,F/20,F/15,F/10,F/7

k-line plot high index F30 to F7 1ang.gif (11.93 KiB) Viewed 394 times

2) F/70,F/30,F/20,F/15,F/10

k-line plot low index F70 to F10 1ang.gif (11.78 KiB) Viewed 394 times

This is just for general knowledge about what happens.

[marktownley]

Thanks Mark, interesting to see the modeling.

[Dennis]

I can totally confirm the f/ratio thing. I have no images, but i tried some time ca-k ds (Lunt) with only one apm 2.7x barlow and the imx462. Results were ok but as soon as i switched to the imx432 and added another apm 2.7x in front of the filters the contrast boost was incredible. Also the signal was better.

Apart of this i also tested the new Baader K-line Gen2 as blocker for the Lunt ca-k filters versus the Antlia k-line filter (images). Also here the Antlia gained such a contrast boost that i have to think it actually contributes to the filter stack. Seeing was not working but you can still see the difference. Images as they came out of Autostakkert presharpened. I did want to change to the Baader filter since it had the better transmision on the paper. In fact i got better results with the Antlia, (less gain use). This might be another indicator that the Antlia helps with the bandwith of the Lunt ds..

[marktownley]

A clear difference there Dennis.

[TareqPhoto]

So can i use Antlia CaK filter with Lunt CaK module to be like a DS one? If yes, should i use Antlia Herschel Wedge with CaK inside and mount/connect Lunt CaK module to it or just take out the filter and place it somewhere in imaging train without wedge?

[marktownley]

So can i use Antlia CaK filter with Lunt CaK module to be like a DS one?

No

Antilia filter is completely different. It will have zero effect when used with a Lunt CaK filter.

[TareqPhoto]

So can i use Antlia CaK filter with Lunt CaK module to be like a DS one?

No

Antilia filter is completely different. It will have zero effect when used with a Lunt CaK filter.

I see, good to know

So the question should be like this, which filter is good to be paired or used with Lunt CaK then?

[marktownley]

So can i use Antlia CaK filter with Lunt CaK module to be like a DS one?

No

Antilia filter is completely different. It will have zero effect when used with a Lunt CaK filter.

I see, good to know

So the question should be like this, which filter is good to be paired or used with Lunt CaK then?

so, cheapest option is to stop the aperture down. Next cheapest option is stacking a second Lunt CaK filter, next would be double stacking with a CaK PST filter, finally double stack with a bespoke CaK filter such as the 1 angstrom ones in this discussion.

[TareqPhoto]

No

Antilia filter is completely different. It will have zero effect when used with a Lunt CaK filter.

I see, good to know

So the question should be like this, which filter is good to be paired or used with Lunt CaK then?

so, cheapest option is to stop the aperture down. Next cheapest option is stacking a second Lunt CaK filter, next would be double stacking with a CaK PST filter, finally double stack with a bespoke CaK filter such as the 1 angstrom ones in this discussion.

I see, thank you very much Mark

[marktownley]

Question to Christian et al; has the filter from a CaK PST or from a Lunt been scanned with the same resolution etc as the ALluxa 1a?

[christian viladrich]

Question to Christian et al; has the filter from a CaK PST or from a Lunt been scanned with the same resolution etc as the ALluxa 1a?

There is this very old post, unfortunately with low resolution :
viewtopic.php?t=12203

[marktownley]

Ahhh I remember that thread now.

Ok, your next mission Christian, hi res scans of Lunt and CaK PST filters

[hopskipson]

These look good James. If the filter is seeing a longer focal ratio it will operate at a tighter bandpass that if it operates at a shorter focal ratio. Tighter bandpass equals more contrast.

So it shouldn’t matter if I use an F/30 scope or a F/10 and a TZ-3. As long as the resulting F ratio is 30 I should see similar results.

[marktownley]

These look good James. If the filter is seeing a longer focal ratio it will operate at a tighter bandpass that if it operates at a shorter focal ratio. Tighter bandpass equals more contrast.

So it shouldn’t matter if I use an F/30 scope or a F/10 and a TZ-3. As long as the resulting F ratio is 30 I should see similar results.

As long as the filter is seeing f30, ie it is after any TZ or barlows used.

[Bob Yoesle]

Question to Christian et al; has the filter from a CaK PST or from a Lunt been scanned with the same resolution etc as the ALluxa 1a?

Mark scanned my 2.2 A PST CaK filter and Chroma 1.0 A CaK here:

CT v PST CaK filter curves.png (983.75 KiB) Viewed 117 times

Here's the Alluxa 1.0 A CaK filter (no tilt):

Alluxa 393.37 1.0 M Wagner scans.jpg (587.69 KiB) Viewed 117 times

[marktownley]

Very interesting, thanks Bob and Mark.

So the CaK PST sits high above target CWL in it's native form, and with a 'tighter' bandpass than it's specified 2.2a. However, sat in it an f10 beam that the CAK PST is, the filters CWL should shift blue towards the actual target wavelength at the expense of a wider bandpass (more like the specified 2.2a?) and reduced transmission.

I have 3 CaK PST filters, and while they are all good, one has better performance than the other two. Guess it is system synergy and / or subtle variations in target CWL of filters when manufactured to a tight bandpass.

[Bob Yoesle]

So far the Alluxa 1.0 CaK filters appear overall to be the better of the two between the Chroma and Alluxa, with the chroma having less uniformity. And while the 2.2 PST CaK filters also appear very uniform, they have a wider overall bandpass in real world implementation. Double stacking with either or both of them appears to work well, and gets the effective FWHM down to about 1.5 and 1.0 respectively.

At the specified 1.0 FWHM, the Alluxa CaK filters all came in at about 1.5 A - collimated. It appears the filter manufacturers have reached the limits of coating technology, and for any given coating batch, there is significant individual filter variation of CWL:

Alluxa 393.37 1.0 batch spreadsheet & graph.jpg (734.15 KiB) Viewed 97 times

Alluxa production run spreadsheet. The dotted line is the target for the production run.


Given the variation of f ratios that might be used with these filters, the bottom line is that if you want to get narrower you need to do the same thing as with H alpha - go to an etalon. And given the variability of the CWL with the best available optical coated filters, perhaps some ability to shift the CWL via heating or cooling (i.e. using a TEC) would seem ideal - just as with etalons.

Bob

[christian viladrich]

Great inputs Bob !

[Dennis]

Would it be difficult to make a etalon based filter like the Suna or Sundancer 2 just for Ca-K with h-a like fwhms? Maybe they would make a run if enough people would want one..