SolarChat!

Hi Mark

thank you very much

Thanks Mark for this insight. It made me start thinking, so this morning I decided to do some experiments. I'm running a pair of 2.2a filters from a CaK PST in series (to in theory reduce the band pass), so I decided to do some comparisons which i present below...

First off this morning I took a full disk in CaK using my regular favourite setup, this is a 40mm objective running at f10, with a 2x barlow after my CaK filter. I'm pretty happy with the outcome, it has lots of contrast, shows me proms and I get filaments now and then.

cak-full-disk-40mm-f20 by Mark Townley, on Flickr

So next off, I tried my 100mm scope stepped down to 50mm aperture and running at a native f20 focal length. Expecting great things I was quite surprised when the outcome was a little washed in comparison with the image above.

cak-full-disk-50mm-f20 by Mark Townley, on Flickr

Surprised, I still ran the same 50mm f20 configuration but this time with a 0.5x focal reducer on the nosepiece of the CCD

cak-full-disk-50mm-f7 by Mark Townley, on Flickr

At this point I wondered if the filter itself was designed for f10 operation, as would be in the original CaK PST, so decided a full disk with the 100mm at f10, but with the 0.5x nosepiece on the camera to make the mosaic a bit more manageable...

CaK-Full-Disk-100mm-f7 by Mark Townley, on Flickr

The contrast on this last disk is back a bit compared to the previous two. I'm certain the 0.5x reducer is throwing some spherical aberration into the mix to soften things up and lower the contrast a bit, but my conclusion here is that the filter in the CaK PST is optimised for a f10 beam. I can and have replicated the results of lower contrast by tilting the CaK elements in my filter stack. I'm certainly sticking with a native f10 running into it.

*Edit*

Just to throw confusion in, here is a disk taken in hazy skies with a 100mm f5 skywatcher refractor - this goes against convention and shouldn't work that well, but this disk isn't to bad, obviously taken on a different date than the above images...

100mm-cak by Mark Townley, on Flickr

Just shows how important the scope is to caK imaging...

The PDFs inserted further up this thread for the CaK filters used at the Paris Meudon indicate these were designed for a target wavelength slightly higher than 393.37nm wavelength so as when used in a non collimated beam would bring the target response of the filter down to 393.37nm.

Now, I wonder what focal ratio the Lunt CaK wedge was designed to work at to deliver a filter centred on 393.37nm?

Hi Mark

great results from that session. #1 seems to give really the best result

Thanks Walter, I agree.

As a question aside - if a filter is designed to run at a higher target wavelength in a collimated beam so that when it used in a focused beam it is at the target wavelength, does this mean if it is used at a large f-number than it is intended (as in my 50mm f20 disk), does this mean that it would be operating at a wavelength on the red side of the target wavelength? Hope this makes sense!

Hi Mark

I'm very sorry. I don't believe that. I would guess if a filter is made for say f/10 it should even better work at f/15, but I have seen your results and I have to accept them as beeing true. However. I'll make my own measurement and comparison on the sun to confirm your findings. Hope it happens

Mark,

Your experiment is worth to read as an example of the enthusiasm of amateur astronomer who is trying to extract every bit of performance out of his available equipment. But from the scientific point of view this experiment is not perfectly objective.

See, some possible explanetions of the effect you observed:

1. 40mm F/10 objective + Barlow 2x incidentally deliver a perfect (or at least acceptable) SA correction at 393nm.
2. In the same time a 50mm F/10 (stopped 100mm F/10, right?) + reducer 0.5x(this is BIG reducing factor and this reducer is a simple achromat and has HORRIBLE HORRIBLE HORRIBLE level of SA at 393nm) deliver poor SA performance at 393nm and also poor field curvature (imminent with this reducer).

3. You are righ that possibly the PST CaK filter can be designed for longer wave length in a parallel beam (or quasi parallel) with the intention that at F/10 the most of aperture (edges!) will work at the CWL 393,37nm.

But we don't know and your experiment does not give us the answer. Possibly yes, possibly not. 50% on 50%.

I should note, that with my attempts to make sub nm CaK filter with steep curve I include in the calculations the CWL shift with converging beam (for F/10 aperture).

Thanks Valery. Interestingly the 40mm f10 objective is one off a Ha PST, so maybe they used the same one for the CaK PST too? I don't know, would it be possible to design an objective that was corrected for SA at 656nm and 393nm?

The softness as a result of the SA of the 0.5x reducer I expected, but the one that caught me off guard was the fact that the 50mm f20 disk was distinctly lacking in contrast. I would have expected more contrast than I got.

All good fun!

Not one comment about my image,you guys are tough. That was shot with an old Astrovid (1/4" chip) with a Snappy type frame grabber. I am sure someone remembers them. There is no stacking, no processing like they do today. It was like shooting with film. You just go through the images till you find one that was good then save that one. And I shot it with an C-8! I don't remember to many K-line images with that scope. For Ha they do work pretty good for solar. In the 90's to the mid 2000's the SCT's where the most common scope for solar.

Mark, with the 100mm you may need to tilt the filters to move it better on band or it maybe, like I said about the AR's not transmitting well at that wavelength. Either would lower the contrast.

Mark Wagner

You make me feel bad now for lack of comment on your image The process of imaging today is definitely a bit different than then!

I think i'm going to keep my filters 'still' and stick with native f10 imaging, taken me ages to work out a configuration that controls the reflections / ghosts.

well, to be fair, you did post your image in someone else's post, and in the commercial solar filters/hardware section.
I am sure it would have gotten more comments in its own thread in the main forum...
I have over 20,000 videos on youtube of the Sun through some of the nicest equipment known to man and have over 5000 images of outreach events posted on this forum. I used to get a little resentful because no one really gives a crap about outreach in this hobby, or the Sun at all in the general public , but then I decided that I was doing what I was doing for myself and the students I see. There is no way I can compete with Justin Bieber and that's that...

I get the same three comments from the same three people on every post, and I treasure them each and every time...
Keep up the good work man and don't let the number of comments affect you one bit. I can assure you that I and many others like Walter and Mark read every single post on here if it makes any difference.

Hi Mark W

Now I'm feeling also a little bit bad. You posted a pic and an information. The pic was nothing special to me, because I did not know the background of it (taken years ago). I did very well give a comment to your Information though. I suggest we stay friends and go on.

Although Mark T. is very well a head of the CaK race, I'm not giving up (please note, we know each other and there is a sort of deep friendship. We will fight about every Cent and angstrom, but with the only target that we get closer to the goal

Interestingly I was looking through the 'archives' on my website and is 3 years since i made my first CaK filter http://brierleyhillsolar.blogspot.co.uk ... april.html - a single stack version at that stage, and I was already pontificating as to the bandwidth. I was using the 70mm f6 at that point so was never going to be getting as much out of the filter as I could...

Hi Mark

yes your current work has lightways improved, however, for 2011 that was still not a bad pic, congratulations , I know 3 years late ;-)

Hi Guys, You know I was kidding, about no comments. What I was trying to show is with K-line it doesn't need to be a high end scope to get good results.
What I like about this site is, if someone gets to high and mighty there is someone in this group that can put them back in place.
Getting back to the bandwidth. The 2-3 Ang range is a good place to be. It show the plage well. But it does make a difference if you are not centered on the line. The contrast will be lower. The other thing that will lower the contrast is how blue the sky is. If only a little of blue is getting through then the contrast will be lower.
Mark Wagner

Is there anyone here know why manufactures choose to produce CaK filter rather than CaH filter?
In my SHG images, I find the H line is always a bit brighter than K line.

Hi Wah

that I do not know, but out of experience my Ca II H filter is not as contrasty as the CaK filter. So it makes more fun to Image in the K line rather than the H line

Hi swisswalter, where did you get those Ca-H/K solar filters?
Are they the same bandwidth?

The K-line is a little broader. From the book, Solar Spectrum 2935Ang to 8770Ang by Rowland's preliminary tables. They have the width (full width) of K-line at 20.2 ang. and H-line at 15.4ang.
From a manufacturing point, it does matter which one you pick. You just need the correct blocker for the wave length you want. It is not unusual to have a peak at the K-line and one on the H-line with the same etalon. The K-line is just more commonly ordered.
The professional observatory tend to do study's using the K-line. I will check into it why K over H.
Mark Wagner

Wah wrote:Hi swisswalter, where did you get those Ca-H/K solar filters?
Are they the same bandwidth?

Hi Wah

the Ca II H is from Daystar, the CaK from Lunt

I have over 20,000 videos on youtube of the Sun through some of the nicest equipment known to man and have over 5000 images of outreach events posted on this forum. I used to get a little resentful because no one really gives a crap about outreach in this hobby, or the Sun at all in the general public , but then I decided that I was doing what I was doing for myself and the students I see. There is no way I can compete with Justin Bieber and that's that...

And if Stephen feels that way, imagine how the rest of us mere mortals feel.

Hale's original spectroheliograph was used to study the CaK lines and it has become the defacto standard.

Zirin "Astrophysics of the sun", p155-168 says of the CaK line "Because the central emission core of the K line is so strong, we can identify K-line spectra with corresponding magnetic features" He then goes on to enounce the "The (6) K line rules"......

Hi Ken

thank you very much for that information

swisswalter wrote: Hi Wah

the Ca II H is from Daystar, the CaK from Lunt

Do both filters have the same bandwidth? Maybe the bandwidth affects the contrast more than the "H-K line comparison"?

Wah wrote:

swisswalter wrote: Hi Wah

the Ca II H is from Daystar, the CaK from Lunt

Do both filters have the same bandwidth? Maybe the bandwidth affects the contrast more than the "H-K line comparison"?

hi Wah

thank you very much. That is a good point. The CaK is around 2,4 A and the Ca II H around 5 A

In my preliminary SHG study of Calcium, at the same bandwidth, the H and K images looked the same. Of course, my resolution was not very high. But quantitatively, I did not see any difference. So I also wonder why the typical choice of K over H.

Most cameras have higher sensitivity on H line than K line, aren't they?

Hi Wah

I don't think the sensitivity slope is inversed between 393nm and 396nm, ergo the opposite should be the case, if there is a noticable difference at all

Walter, the sensitivity of my camera (120MM) definitely decreases slightly between H (3968) and K (3934). Not a huge amount, but it is noticeable.

Hi Douglas

yes you are right and Wah too. I thought just the other way round . But I do not think that we can make out a difference in the final pic.

I've thrown in another CaK disk in the comparison series of images further up this post, just adds confusion!

Hi Mark

a fery fine shot with the f/5 gun.


quote
Now, I wonder what focal ratio the Lunt CaK wedge was designed to work at to deliver a filter centred on 393.37nm?
endquote

that is a very good question!