mdwmark wrote: ↑
Tue Sep 04, 2018 2:14 am
There are at least 5 company's that I know of that can make a hard coated single cavity .1nm HW filter at K-line. But you would need to order quite a few to get the price down to $1250.
This interest in K-line surprises me. I have been offering K-line filters for 35+ years and they have never been that common. These are usually 38.1mm and 50mm etalons, so they have a higher cost. But in the 25mm range, these hard coated filters should be the way to go for imaging.
I know the sixth which can make a 0,1 - 0,15nm 1" filter much cheaper if a quantity is 100 or more in order.
This interest is as high as Lunt sells his CaK filters. Due to specific details size and their structures CaK starts to be really interesting only at a very high resolution. Minimal diameter of a telescope for such impressive CaK pictures is 150mm which is equal in resolution to a 250mm telescope in Ha. The larger the aperture, the more interesting CaK images become.
However all the major details are well seen on the pictures taken with a single PST filter (paired with K-line Baader) or single Lunt CaK. Double stacking improves the contrast very little, much less than in H-a double stacking. This is because PST and Lunt filters already have very steep sides with effectively supressed skirt.
It was my wrong expectation that at subangstom FWHM we will be able to image new details - filaments and fibrills as well as bushes of spiculaes. As leading pros astronomers explained me, this is not the case with the CaK.
Such a new for Ca details can be imaged only at 8542,1A line and only if the filter will have very steep sides, well supressed parasitic skirt and FWHM will be 0,25A maxumum (much better if 0,15A). I believe these requirements are too strict and even two 0,7A etalons being DS will show almost as much as professional filters. I have a solid factual evidences what kind of filter will be enough for a new view of the sun in a Ca. And this is infrared Ca II I line.
Again, my experiments showed clearly - I can image 95% of CaK details with a cheap PST CaK filter vs DS PST or Lunt CaK filters. Only prominences becomes a bit better seen and spiculaes at the limb.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.
Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.