The Quest For Affordable CaK Solar Imaging - An Update...

I thought it was about time to do a bit of an update on where things are going in terms of finding an affordable amateur solution to observing our star in the calcium K line as over the past week things have taken on some developments.  

My recent visit to Switzerland as seen in previous blog posts involved me visiting my good friend Swisswalter, where among other things affordable CaK solar was a hot topic.

Walter has tried various combinations of filters in the past to get a CaK result and so we had alot to discuss.  Whilst looking through his collection I spotted a filter labelled 393nm SP (short pass) 10a.  Visually this looked pretty much identical to the front filter from the CaK PST that I use with the Baader K-Line to get results indiscernible from the native CaK PST and the Lunt CaK filter and so had some optimism with this.  We decided to disassemble the Lunt CaK filter to see what it contained; interestingly the filter at the eyepeice end of the unit was also a yellow filter, that looks also identical to the one from the PST and the 393SP.  The rest of the unit contains 2 identical filters that when look through visually has the deep purple hue of the Baader K-line.  

The weather was not clear enough to test the combinations on the day, but Walter kindly did this a few days later.  There were surprises in the outcome:  First off the reference image, the Lunt B1200

Then the Baader K-line and the 393SP; rather lacking in constrast in comparison to the Lunt image, but, for the homebrew filter combinations, possibly one of the best yet...

The 393SP/K-line image quite clearly lacks the contrast of the Lunt, but there is some detail being pulled out compared to what we would expect  in a normal CaK image.  I suspect the outcome is due to a broader bandpass compared to the Lunt letting in more of the continuum light and hence the poorer constrast.  However, Walter tried the the 393SP/K-Line in conjunction with the Lunt B1200 and there was a suprising improvement in results, with more contrast visible.

So, an outcome, if not the outcome that was intended!  I'm hoping Walter may be able to test the yellow filter from the Lunt B1200 with the Baader K-Line, this may also give us a result?

In the mean time, when I got back to the UK my GG400 long pass filter had arrived and so was time to try this out.  I figured the falling transmission at wavelengths shorter than 400nm may have the effect of tightening the bandpass of the K-Line/CaK PST combination I was using, well, we had about half and hour of clearish skies and so decided to try it out.  With the GG400 in this stack the image curiouslly reverted back to essentially a whitelight image lacking in any CaK features.  Hmmmm, not what I wanted or was hoping for, but, we have to take from this what we can, and while a negative result is annoying atleast it shows us that something doesn't work.

Since then I made an interesting find; Andy Dodson in New Zealand has been modding his CaK PST in much the same way as I have http://www.pbase.com/andy_dodson/evoloution_of_my_ca_k_filter&page=2  We had both wanted to replace the bf5 in the CaK PST with a larger option, we have both tried the Omega options off EBay and had reached the conclusion that these were lacking in the optical quality needed.  Now this is where our approaches differed; I had used the Baader K-line whereas Andy had used the Baader U or 'Venus' filter.  Now, when you look at Andys excellent images it's very clear this filter is also very effective when used with the 'front end' of a CaK PST.  

Now, the fact we have two different filter combinations that work very effectively tells us something about the front element of the CaK PST assembly, as this is the one that is the common factor in both setups.  Using a simple CD spectroscope the front element from the CaK PST shows no transmission below 450nm, now, this quite clearly isn't the case as it is  very effective at 393nm.  So, what I think is going on here is that the CaK PST front filter is showing the same style of transmission as the blocking filter that is used in a Ha setup.  In other words a very narrow (2.2a) pass at the chosen wavelength 393.4nm, then a band of blocked transmission to ~450nm then above this a normal transmission.  This is what happens with the blocking filters used in Ha setups, but at obviously a different range of wavelengths.  

So, where does this leave us in the search for a suitable 'commercially available' filter to use in conjunction with either the Baader K-line or U filter.  Well i've been looking for sometime now and are not aware of any off the shelf blocking type filters centred on 393.4nm - they may well be a bespoke filter, which could start to make things expensive.  

However there is one more alternative I would like to explore with the Baader K-Line, and that is a 387/11nm bandpass filter, this has really steep sloping sides and cuts off just above 393nm.  Certainly in the UK they are available here http://www.laser2000.co.uk/semrock_filter.php?code=CFW-Ex01-Clin-21.8-D but at £199 are in the same league price wise as the K-Line.  Could be expensive if it doesn't work, fortunately the company offers a 30 day no questions money back guarantee if the filter doesn't do the job you expect it to.  What makes me more confident with this setup than others i've looked at?  Well consider the  transmission curves below.

You can see the transmission of the 387/11nm in blue with the K-line in red, and then the resultant curve in green.  We can see if we zoom in this gives us a lovely tight bandpass centred on 393nm with a transmission of ~46%.

Now, maybe this is still too broad a bandpass to do CaK imaging effectively, but is clearly a lot tighter bandpass than the Baader K-Line on its own.  Fulvio Mete's excellent website on imaging at CaK wavelengths http://www.lightfrominfinity.org/VHIRSS%20Solar%20Images/VHIRSS%20solar%20images.htm he posts this image which shows the Calcium K line is actually really quite wide (compared say to Ha).

Now it is obvious that the horizontal scale on the two plots immediately above are different, but the profile of the curves is similar, and maybe, just maybe, the 387/11 K-line combination whilst not having as tight a bandpass as the 2.2a CaK PST it might come close enough through cutting out that bit more continuum light to make CaK visible?  The problem we're now getting is the cost - potentially up to £400 for 2 filters (K-line & 387/11), throw in the fact you would need an ERF - possibly another filter, or ideally a solar wedge and the price is getting pretty close to  a Lunt B1200.  However, if you have some of these elements already then the cost isn't too bad.  Oh, and the fact this system runs at 25mm clear aperture - the Lunt B1800 & B3400 CaK modules are thousands of $$$ to buy, if it works, then despite the expense, maybe it's not that bad after all...

So, the plan now.  Well, it's just total cloud cover here in the UK, but when the clouds do go for long enough then i'm going to get one of these 387/11nm filters courtesy of the 30 day money back guarantee and will test it out with the k-line.  Fingers crossed it will work!  Till then I have to wait till the clouds go, but can be sure I will report back on the results!