I decided on a day CaK modding and did some experiments comparing DS and TS CaK, after several hours today i'm leaning very much to the idea that the gain in contrast is not worth the increase in exposure time and gain. So, I decided to put my attentions to maximising the results from the DS configuration. First of all stripping all the components of the filter train back, making sure all filter edges were light tight using gaskets and changing some filter spacings (quite easy as is a modular construction).Perhaps my biggest change was when I realised there was sag in the module, as this is a 1.25" tubing, that then sits in a 1.25" to 2" adapter and then in this in the scopes focuser, there was play with the module in the 1.25"-2" holder that caused sag, however changing for a clicklock adapter rectified this. The front CaK filter being perfectly orthogonal to the optical path seems to be a key factor. The second CaK filter is sat in a sky bender that allows tilt, and this tilting shows some interesting things. As it is tilted the image brightness slightly decreases and at the minimum brightness shows maximum contrast ( this could be easily seen on the values on auto gain changing, go past this minimum brightness point and contrast gets lower and image brightness increases. I believe what is happening is the CWL of the filter is passing over the K3 line.
With a 3rd CaK filter in place it is just harder to achieve this, and as a result is harder to see the spicules, and get this maximum contrast effect. It's a bit like 3 external Ha etalons - too many variables! I tried changing the order of the CaK filters, but this seems to have no noticeable effect, I had wondered if it would be like with Ha, but maybe because they are significantly wider band pass this is of much lesser effect? I have a few more things I want to explore tomorrow...
Anyway, MeteoBlue had the seeing 'improving' to around 2.5" by 3pm so I decided to give it a go with the 60mm f16. It wasn't good enough to resolve the chromospheric fringe, more of a 'fuzz' around the limb today, but it did give a half decent view, not much done to this apart from USM in ImPPG.
CaK-FD-bw by Mark Townley, on Flickr
CaK-FD-colour by Mark Townley, on FlickrMore to explore tomorrow, clear skies forecast again, and better seeing than today, I don't think i'll be getting the big guns out but experiments can continue at the smaller apertures.
Hope you like.
Mark
you are lucky to have so many CaK filters to play with