Dear All
I have bought a skywatcher 120mm x 600mm refractor to use mainly as a guide scope. However I have been trying to image through it as a test. It certainly produces a nice purple fringe around bright objects visually but I expected this from an acromat.
I can image and produce sharp stars when I use a red or green filter but I can't seem to get the image through a blue filter to focus. I realise that the focus will be in a different position through the shorter wavelength filter but I thought that the blue filter which is relatively close to the violet should come to a pinpoint focus also.
The accompanying pics are little subframes of a bright star with a cross over the front to produce diffraction spikes. These help with focus like a hartman mask.
The red and green images produce nice diffraction spikes but the blue (the first image) won't do it. All the filters also stop IR so I think this should not be the problem.
Any suggestions?
Is this inherent with all achromats or do I have a dud?
