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Hi Nigel,
I was confused after reading your post, because it is physically
impossible for afocal photography to fail, so I read it again and realised
that what you are doing is not afocal imaging.
Firstly, afocal imaging is exactly what you said , either manually
holding your camera up close to the eyepiece, or using a bracket to hold
it in place. The scope is visually focused and the camera is then auto or
manually focused, and the image taken. The camera MUST have it's own
lens.
Secondly, what you are attempting, is actually eyepiece projection,
and I am pretty sure that your problem is that something is wrong with
one of the dimensions within the optical train between the focusser and the camera's sensor. For example, the eyepiece may be too far from the camera. My eyepieces are around 10-15mm from the lens mounting
face. This part of your problem is actually quite hard to come to grips with as I haven't seen your set up.
Thirdly, many mass produced Dobs have similar focusser
assemblies, and if yours has a 2" with a 1.25" adapter, there is a good chance that you can achieve prime focus with standard DSLRs.
Unscrew and remove the nose piece on the focusser that the 1.25" eyepieces slide into, which should expose a male thread. The female thread on the
camera's T adapter will screw directly onto the male thread without
any tubes in between, so the camera is almost hard up against the focuser. Using a barlow, even when it does work, is rather defeating
the purpose of prime focus imaging, doubling the mag, halving the
field of view, increasing exposure times, and making guiding a bit
more critical, and lastly, degrading the images a little, unless the
barlow is optically first rate.
raymo
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