After some considerable time searching I finally have bought another second hand CaK PST. The method behind this was simple; to double stack a CaK filter. The CaK PST filter in it's native form has an advertised bandpass of <2.2a, compared to the 2.4a of the Lunt unit. By putting two CaK PST filters in series the bandpass will tighten to an estimated <1.5a. Well what is to be gained from this exercise? Double stacking in Hydrogen alpha wavelengths has the effect of improving the signal to noise ratio by reducing continuum light leakage into the image - in simple terms the contrast should be better. Images of the CaK sun taken with instruments with a tight bandpass, eg spectroscopes or BASS2000 show the disk itself to be much darker, with plage relatively much brighter, but also with tentative hints of prominences and filaments, something I have observed before in single stack CaK on certain occasions. Well when the image above was taken conditions could not have really been any worse; the sun was at <10 degrees altitude in the sky and was imaged through the haze of high cloud. In addition our star was pretty devoid of activity, there was only one or two very small spots, no proms and the filaments that were there were only very small and hairline - not untypical of our star at solar minimum! However, given the cumulative effect of all these factors I am very pleased with the outcome, the image definitely shows more contrast to what I am used to, and the most apparent thing is that the supergranulation cells are easily visible across the whole disk. If I really stretch the original TIFF file I can start to see the filaments that were visible. Typically since getting this setup the weather has been what can only be described as typically autumunal and have not been able to explore with it as much as would hope. Still, as I write this there is high pressure upon us (and cloud!) but there are breaks forecast, add to that the sun has increased in activity and if the clouds do clear then hopefully should have some good results to share!
