Sunday, 27 January 2013

CaK Sun - 27th January 2013

I couldn't resist taking a picture of our star today in CaK wavelengths (393.4nm), when there aren't many large active regions about there is always something to see in these near ultra violet wavelengths.  This was my first attempt at using a ND filter-less Lunt solar wedge as an ERF in conjunction with my own CaK filter stack; this worked as expected.  Using the 70mm scope at f6 I was getting exposure times of 1/500 second.  This is pretty good given the low altitude of the sun, and I expect certainly in summer to be imaging nearer 1/1000 second, which will be great to avoid image smearing due to the poor seeing that afflicts these shorter wavelengths.  This also leaves plenty of light for imaging at longer focal lengths, again when seeing allows.  Not all is as I would like with this setup; I am certain there is an infra red leak occuring somewhere, this is manifesting itself as a 'blooming' to the raw image.  The CaK PST employs a soft coated silver ITF (induced transmission filter) in its original configuration, which certainly in visual inspection would appear to be exactly the same sort as is used with their Ha PST.  The purpose of this filter is to essentially take out wavelengths >1500nm.  The next time I get some decent imaging time I want to try and image with an ITF in the stack, and also trying in its place some Schott KG5 which has similar properties at these long wavelengths.  I am fairly confident this will improve matters.