Quote:
Originally Posted by alistairsam
The initial difference in distance between one side of the CCD to the top and other was over 3mm.
I reduced this down to about 1mm after some shims.
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I have the same sort of system and it was real PITA to align - some of these scopes work fine first up, but they can get to a state where collimation is possible, but something is still obviously way out of whack. Mine turned out to be the secondary position/alignment - suggest that Steve is on the money with his diagnosis - plus camera image plane tilt. A tip here is that all of the collimation guides I have read suggest that the secondary should be positioned using a sight hole, but they miss the point that the sight hole must be in the image plane - if it isn't, the mirror will be positioned wrongly. I was winding my focuser in so that I could easily see the whole secondary and got the secondary in-out-sideways position quite wrong as a result. This doesn't matter much at f5, but it does at f4.
Not sure what you mean by "distance" in the above quote. The surface of the CCD needs to be within better than about 30 microns of parallel to the front face of the camera at f4. Mine was 300 microns out, which I thought was lot, but your 3mm seems excessive - that is if we are talking about the same thing. If we are, 1mm is still far to much.
Your star test is defocused by too much to really show what the problem might be - suggest that you redo with maybe 1/10 as much defocus, which should start to show what is going on.
I used an additional shadow test to check overall mechanical alignment and found that to be extremely helpful, but am a bit reluctant to suggest it since others have found it to be a bit difficult to interpret what it shows. anyway, if you want to try it
http://www.iceinspace.com.au/forum/s...ad.php?t=82230
Might also be a problem with alignment of the MPCC. As a first step, do some tests without it and make sure that you start out with good central stars and coma on the edges. Only add the MPCC when you have the rest of the system squared away.
I have used both Moonlite and GSO focusers on mine and found that the GSO is just strong enough to hold the QHY8 with acceptable flex - the Moonlite is rock solid. Needed to reinforce the OTA tube though, to stop that from bending in the focuser vicinity. Would suggest that your current problems are much more significant than focuser/tube flex.
Also, am assuming that you do the collimation with the scope pointing at the bit of sky you will be imaging from. These scopes move around internally - quite enough to mess things up if you have aligned at some other attitude (eg on the bench)
all the best Ray