Spectroscopic analysis of a Lunt CaK filter

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Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

I was inspired by Christian's excellent analysis of his Alluxa CaK filter (viewtopic.php?t=37280) using his Solex spectroscope to try the same technique with my homebrew CaK filter (viewtopic.php?t=13804). The filter consists of a Baader K-line filter, followed by the yellow filter from a Lunt B1200 module, followed by a polarisation filter. Normally I use this behind a Herschel wedge with a refractor (or without the wedge with a solar Newtonian).

In general, the filter gives acceptable results (see download/file.php?id=9028&mode=view) but it does not give as high contrast as my CaK PST (see download/file.php?id=14487&mode=view).

To do this experiment, I took our foamboard Solex spectroscope off its telescope and mounted the CaK module on the front. This was easy since the T-adapter which holds the CaK filters just took the place of the 2-inch nosepiece that normally goes there. I mounted the contraption on my Vixen Polarie so it would track the Sun.

Foamboard spectroscope with CaK filter on Vixen Polarie
Foamboard spectroscope with CaK filter on Vixen Polarie
1448.jpg (279.17 KiB) Viewed 2555 times

First an image taken with direct sunlight with no filter. This is essentially highly collimated light (approximately f107).

Calcium K (top) and H (bottom) lines
Calcium K (top) and H (bottom) lines
Calcium_lines_165610.jpg (247.78 KiB) Viewed 2555 times

Then with the CaK filter in place:

Collimated sunlight through the CaK filter
Collimated sunlight through the CaK filter
CaK_filter_165748.jpg (229.55 KiB) Viewed 2555 times

With Photoshop, I overlaid the two images (using exclusion) to get a rough idea of the how the bandwidth and position of the filtering looked with respect to the spectral line. My initial impression was the bandwidth did not look too bad but the filter line was definitely not at the centre of the CaK spectral line.

CaK filter transmission superimposed on the spectral line
CaK filter transmission superimposed on the spectral line
Calcium-lines_CaK_filter_165610.jpg (701.99 KiB) Viewed 2555 times

Using the program RSpec, I could extract the data from the images and plot using Excel.

Solar spectrum near Ca K and H and the CaK filter transmission
Solar spectrum near Ca K and H and the CaK filter transmission
Calcium lines.jpg (909.46 KiB) Viewed 2555 times

Dividing the transmission spectrum by the solar intensity gives the true filter transmission profile.

CaK transmission profile
CaK transmission profile
CaK filter.png (89.15 KiB) Viewed 2555 times

The x-axes in the two charts are in pixels, but knowing the distance between the K and H lines, it is easy to convert pixels into angstroms. My estimate is the bandwidth (FWHM) of the filter is 2.52 angstroms and the offset is 1.10 angstroms (to the red). I believe the nominal bandwidth of the Lunt filter is 2.4 angstroms, with no specification for the offset. Given some of the uncertainties of my measurement method, I would say the bandwidth is "within spec" of the manufacturer, but the offset is quite considerable and surprisingly high. I believe the nominal value of the CaK PST bandwidth is 2.2 angstroms, not much different than the Lunt CaK filter. I suspect why the CaK PST gives higher contrast images than the Lunt CaK filter is likely due to the offset. My guess is if I were able to do the same test on the CaK PST, I would find it was better tuned to the centre of the CaK spectral line.

As I have mentioned elsewhere, one of the strengths of the spectroheliograph is not only the narrower bandwidth but also the ability to image at the minimum of the spectral line (or at a chosen offset). Unlike H-alpha etalons, CaK filters are difficult (or even impossible) to tune.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by marktownley »

Interesting analysis Douglas, many thanks.

You should be able to tilt the filter by a predetermined amount to compensate for the offset and bring it on band?


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by Carbon60 »

Very interesting. Thank you, Douglas.

Stu.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by christian viladrich »

Very interesting Douglas. Thanks for sharing.

Just some thoughts. Depeding on the mechanical rigidity of the SHG body, there might be some shift in wavelength between the solar spectrum image and the image with the the Ca K filter. On my side, I have to register the two spectra by a few pixels. I use the lines visible in both spectra.
For example, have you check that the first small dent on the red side of Ca K is at the same X position on the image with the solar spectrum and on the image with the Ca K filter ?
You've probably have done this already. This is just to check.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by Montana »

Very interesting, you can easily see it is just missing the centre line, I would be very interested too if you could just tilt it slightly to get it on band.

At Solarscope, Helmut showed me the plot of halpha etalon and how they purposely have it set shifted slightly to the right, this way you can tilt the filter and manually get it just perfect on the centre line. If it the other way you couldn't tilt it back. This is why tilt tuned etalons can never be tuned into the red wing.

Alexandra


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by christian viladrich »

Another effect is the temperature drift, when the filter is heated the CWL drifts to the red side. When it is cooled, it drifts to the blue side.

For example, PST CA K temperature coefficient is 0.005 A/°C according very old spec. Not sure whether this was the actual value.This would mean 0.05 A/10°C swing, which is quite good.

Coronado SolarMax Ca K is said to be better than 0.05 A/°C, or 0.5 A/10°C.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

christian viladrich wrote: Fri Jul 29, 2022 7:02 am Just some thoughts. Depending on the mechanical rigidity of the SHG body, there might be some shift in wavelength between the solar spectrum image and the image with the the Ca K filter.
I went back and checked on this to make sure and I did not find any problem with the registration. The filter is extremely light and does not put much weight on the collimator (which itself is very stiff). I actually put a "dummy" adapter on the front for the Calcium spectrum, more just to match the lighting conditions but it also would match any mechanical load.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

marktownley wrote: Fri Jul 29, 2022 1:32 am You should be able to tilt the filter by a predetermined amount to compensate for the offset and bring it on band?
After I did this, and found the quite sizable red shift, it occurred to me that perhaps this was an intentional feature by Lunt. It might be that when they mount the filter in their housing, they introduce a specific tilt to bring it on band. I had a look at the original housing and there is not an obvious tilting mechanism, but there are some rotational marks. So perhaps there is a clever but simple system whereby they rotate the housing to tilt the deliberately off band filter.

I should be able to add a little tilt and redo this experiment. Does any have an estimate how much tilt is required?


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by christian viladrich »

thesmiths wrote: Fri Jul 29, 2022 2:30 pm
christian viladrich wrote: Fri Jul 29, 2022 7:02 am Just some thoughts. Depending on the mechanical rigidity of the SHG body, there might be some shift in wavelength between the solar spectrum image and the image with the the Ca K filter.
I went back and checked on this to make sure and I did not find any problem with the registration. The filter is extremely light and does not put much weight on the collimator (which itself is very stiff). I actually put a "dummy" adapter on the front for the Calcium spectrum, more just to match the lighting conditions but it also would match any mechanical load.
Great ! So, we have this 1.1 A offset. This is not too bad for a 2.4 A filter.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by christian viladrich »

thesmiths wrote: Fri Jul 29, 2022 2:55 pm
I should be able to add a little tilt and redo this experiment. Does any have an estimate how much tilt is required?
Here is the relation between the tilt of the filter (in radian) and the wavelength shift :

http://astrosurf.com/viladrich/astro/in ... mula-2.JPG

n is the effective index of the filter. We can assume something between 1.4 and 1.6.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

christian viladrich wrote: Fri Jul 29, 2022 5:35 pm Here is the relation between the tilt of the filter (in radian) and the wavelength shift :
Thanks, Christian. Using that formula, I get a number near 2 degrees. That's in the regime which is not too small and not too large. I will have a think about the best way to accomplish a tilt.


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

I disassembled my CaK filter stack and put it back together in a way where I could manually apply some tilt to the Lunt yellow filter. The Lunt filter is sandwiched between two pieces of thin black foam board, held together with matte black Washi masking tape. This is held by friction inside the T-adapter. The Baader K-line filter is screwed on to the front of the adapter. In normal use, the T-thread would go right into a ZWO camera and the end with the Baader filter would go into a 1.25" Lunt Herschel wedge.

Lunt filter sandwich (left), Baader K-line filter (right).
Lunt filter sandwich (left), Baader K-line filter (right).
CaK-tilt-filter.jpg (157.63 KiB) Viewed 2368 times

I took a spectrum of the Calcium lines with the SHG as above and then put the filter adapter on and took another spectrum with the CaK filter. By toggling between the two images, I could see that I had applied too much tilt and now the transmission peak was too far to the blue. I straightened out the filter a bit and repeated the process. On the third attempt, I was able to visually see that the transmission line of the filter was close to being on top of the darkest part of the CaK line. I then decided to stop and do some analysis.

CaK filter transmission superimposed on the Calcium spectral lines.
CaK filter transmission superimposed on the Calcium spectral lines.
164526_162340_a.jpg (329.96 KiB) Viewed 2368 times

The image above is basically what you can see by toggling the images on the computer screen during observations. Using RSpec and Excel, I plotted the spectral lines and the transmission spectrum. Without even trying very hard, I was able to get very close to being on band (compare to the first attempt in the earlier post above).

Calcium K and H lines (blue) and the CaK filter transmission (orange).
Calcium K and H lines (blue) and the CaK filter transmission (orange).
Calcium spectral lines.png (403.71 KiB) Viewed 2364 times

I didn't have a chance to do any imaging with the filter as it was too late in the day. I'll probably put back a little more tilt to get really on band and then see if the imaging contrast has improved.

Lastly, I did a rough estimate of the bandwidth and it seems tilting the filter increased the bandwidth from around 2.5 angstroms to around 3.0 angstroms. So roughly speaking, shifting by 1 angstrom has a cost of about 0.5 angstrom of bandwidth. I did the acquisition of this data quite quickly because of non-ideal weather, so I will try to redo this to get a more accurate bandwidth measurement (it seems a little on the wide side). It was a little cloudy so this might have led to less than ideal collimated light falling on the filter. It looks like the focus also wasn't as good as in my previous attempt, which would have broadened the lines. But I am encouraged to discover that tuning the filter is fairly straightforward.

Tilted CaK transmission spectrum
Tilted CaK transmission spectrum
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Re: Spectroscopic analysis of a Lunt CaK filter

Post by marktownley »

Excellent analysis Douglas. Looks like we were both doing similar things today.

My first image is my 'default' CaK filter - very pretty etc, blah blah.

ImageCaK-FD-60mm-f10-ZWO183mm-bw by Mark Townley, on Flickr

Next picture tilting the filter to bring onto centreline. I think it looks a lot more like one of your SHG images.

ImageCaK-FD-60mm-f10-zwo183mm-off-axis-tuned-bw by Mark Townley, on Flickr

Much to explore with all of this! Just need more sunshine!

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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

marktownley wrote: Mon Aug 01, 2022 11:11 pm Next picture tilting the filter to bring onto centreline. I think it looks a lot more like one of your SHG images.
Hi Mark, that's a dramatic difference. What's your process to bring the filter onto the centreline? Don't you actually have a double stack?


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by Bob Yoesle »

For comparison, I had Mark Wagner do an evaluation of one of my legacy PST yellow dichroic CaK blocking filters (also apparently hard coated like the Alluxa):

CaK PST Filter.png
CaK PST Filter.png (340.04 KiB) Viewed 2336 times

Tilting also brings this filter on-band at its advertised nominal 2.2 A FWHM. The transmission is not as high as Christian measured for the Alluxa filter, but I'm hoping the Alluxa-PST combination will make for a good double-stack. If the profiles are Lorentzian with 1.5 and 2.2 FWHM, the resultant FWHM would be about 1.1, with a 10% width of around 2.6 A and a 1% width of 5.4 A. Depending on the pre-filtering, adding an additional PST CaK might also be possible with good exposure times...

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Re: Spectroscopic analysis of a Lunt CaK filter

Post by marktownley »

thesmiths wrote: Tue Aug 02, 2022 12:59 am
marktownley wrote: Mon Aug 01, 2022 11:11 pm Next picture tilting the filter to bring onto centreline. I think it looks a lot more like one of your SHG images.
Hi Mark, that's a dramatic difference. What's your process to bring the filter onto the centreline? Don't you actually have a double stack?
Hi Douglas.

With the disk below, I realised by moving it around the (big) frame of the ZWO183, when it is in the corner of the frame it is off axis by a couple of degrees. You can see the disk visibly tune and change appearance as you do this.

Soooo, I need to think how I want to achieve this tilt, but on axis. My obvious solution is to use Apollo Laskys skybender, which will give me a live tuneable way of doing it, or, my other way is to shim the filters to get this. A 1mm shim should give a couple of degree tilt across the face of the filter which should put me in the ball park.

I'd originally been using a double stack CaK filter - 2 yellow CaK PST filters like in Bobs picture. Yesterday I finally got round to trying a triple stack (i've had a third CaK PST for while). A bit like etalons there is an order to the filters that works, and an order that doesn't, yesterday I finally got this sussed out. I have very little loss in light comparing triple to double stack.

Todays job is to 'reconfigure' the CaK module for the 3 filters, in true fashion it is damp and grey so testing is very unlikely, and family commitments mean it is likely the weekend before I can revisit it (assuming clear skies!)

Lots of tinkering to do!

Mark


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Re: Spectroscopic analysis of a Lunt CaK filter

Post by thesmiths »

marktownley wrote: Tue Aug 02, 2022 9:50 am I realised by moving it around the (big) frame of the ZWO183, when it is in the corner of the frame it is off axis by a couple of degrees. You can see the disk visibly tune and change appearance as you do this.
That's also an interesting demonstration why a high f-ratio light beam is important with these types of filters.

I should add that that's one nice advantage I've experienced with the SHG approach. It's not really that sensitive to the f-ratio of the telescope so you can get a very narrowband using a relatively low f-ratio (e.g. f6 or f7), enabling a full-disk image with a not very big sensor. It's on the opposite extreme to the Quark in that regard (very high f-ratio required, only a narrow field of view).


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