Collimation - Alignment of Secondary

[SnakeyJ]

I've read a lot of threads and articles on newtonian collimation recently, but was really struggling on getting the secondary mirror centered using a standard cheshire/sight tube.

Step 1 centering the spider - easy.

Step 2 - check focuser alignment - no probs.

Step 3 - centre secondary mirror - (various expletives). Even after blocking out the primary mirror it was difficult to judge how accurately centered/rotated the secondary was at the bottom of a 2" plus draw tube. Having settled on a rough alignment, I had no problems using a laser collimator to centre on the primary and then, adjusting the primary to get the reflected laser light to centre on the back on the collimator. However, I was not overly convinced was struggling to see all three of the primary retainers and my star tests looked nowhere near. Have to admit I was using a cheap short tube cheshire, home made site cap and being somewhat long sighted.

Following a article at http://www.astronomyshed.co.uk/forum/viewtopic.php?f=57&t=5978 about using a webcam and on screen reticule and decided to have a look. Unfortunately non of my old webcams would work on Windows 7 x64, so I popped into PCWorld and brought a MS Lifecam HD3000 for £ 17. There were cheaper ones available, but this seems to be quite widely modded and used for PI, so I thought it probably justified a few quid more. Rather than mess it around to much, I simply opened it and removed the bracket and stuck a small piece of blue tack over the onboard LED. I then put it back together (with lens) and stuck a 1.25 nose-piece to the front using a load more blue tack. The nose-piece came from a very cheap 'n nasty celestron 2x barlow.

Anyhow with the webcam, sharpcap and mire-de collimation installed and working - I was able to mount the webcam on my focuser and get a good view down the focus tube to the secondary. Mire-de collimation adds a floating series of concentric circles which you can drag over the webcam image and resize to fit the base of the focus tube and match the secondary dimensions. I was quite shocked at how far out my rough alignment with the cheshire had been, with the secondary too far down/forward and looking rather oval rather than round due to rotation.

With this view it was a doddle to centre and rotate correctly, and once happy I simply replaced the webcam with the laser collimator and went through as normal. Low and behold it all looks right and there's almost no movement on the laser collimator when I focus in and out.

Have to wait for a gap in the clouds to confirm with a star test and certain it will need a few minor tweaks, but at least I feel I am in the right ball park and the views I get of the primary and secondary seem to match up to the guides.

Off to do my other 200p now!

[Daniel-K]

i have been doing the roughly the same got some were near just need a bit of finesse

[SnakeyJ]

Second scope done now - very easy, though this one wasn't too far adrift.

The only slight downer is we were completely clouded over when I started, I've just packed up and been out to find the cat and the sky is crystal clear and dark - had I realised I would of shelved the collimation and been outside. Too late now to setup, though I might go and stand out for 15 minutes or so...

[umadog]

If you did the axial alignment then a slightly mis-rounded secondary is not a big deal. I use a long-tube Cheshire to frame the secondary and a piece of white paper behind it (opposite the focuser) to provide sufficient contrast to see the shape of the mirror.