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Old 04-03-2018, 08:30 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,918
Hi Dunk. lots of good advice here. FWIW, maybe a little more?

1. If the mirror assembly is like the 250f4, these scopes cannot be collimated while horizontal. The mirror is free-floating on cork pads and can also slide very slightly sideways - it can flop around alarmingly if the OTA is horizontal. It can hold collimation OK, but only if you initially do it with the mirror held back against the pads by gravity - suggest that you collimate on the mount with the scope pointing above the horizon by say 30+ degrees.

2. Collimation through the CC seems to work best - the CC can have enough wedge to throw off the collimation, so leave it in.

3. a simple jig to collimate your laser can be made with a bit of wood and 4 nails (image). Project on the wall and adjust the offset screws to fix any misalignment when you rotate the tube. A well aligned laser is plenty accurate enough to get these scopes working properly - if the centre spot is right and you clamp the laser tightly in the drawtube adapter.

3. your weird stars suggest that the light column is being truncated by the OTA, because the secondary is misaligned in-out. This might just be due to collimation varying when you put the scope up on the mount, but it could also be a more fundamental problem with the secondary alignment. It is nigh on impossible to get the secondary in the right place by looking down the focuser - your eye needs to be in the focal plane, which means that you cannot see the edges of the mirror (the drawtube gets in the way) and you must guess how well it is aligned. If you find that the stars still look crook after you have fixed the collimation problem, the back-projection method woks OK to get the secondary tidied up. Will put a summary in another thread, rather than clog this one up. Cheers Ray
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Last edited by Shiraz; 05-03-2018 at 09:48 AM.
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