Conventional narrowband filters (12nm or smaller bandwidth) do not work with the Hyperstar III system due to its very low f#. The cone angle of the rays passing through the filter are so far from normal incidence that the actual operating wavelength of the filter (at the incident light angle) is no longer at H-alpha wavelengths. One possible solution to this problem is to use a filter with a very large passband. I have just purchased a 35nm bandwidth H-alpha filter from Ian King Imaging and can’t wait to see if the Hyperstar III will work with it. My first target for testing the new combination will be CTBI, the supernova remnant in Cassiopeia that I have had trouble imaging before as it is so faint. It will be very interesting to see if the new filter allows me to get a decent image of this difficult object. If it does, I might just give Simeis 147 a go – I have always left this one well alone as it is so large and faint.
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