Note: my thoughts on collimation have changed since this post was written. I now only use a collimation cap, and prefer not to try to use two methods as I only end up chasing my tail trying to get them to agree. While the Baader laser worked well enough, I feel the collimation cap is more accurate, especially when combined with the tips mention in my more recent post here.
Everyone’s got their preferred method for collimation a Newtonian telescope. I used to use a Cheshire eyepiece, but never felt totally happy with it. I felt it was too long and heavy, and, when fitted to the focuser with the 1.25 eyepiece adapter, exacerbated the play in the system (I was able to wobble it around in the focuser … not ideal).
I finally sold it on and bought an Aline collimation cap. Being much lighter, this didn’t have the same weight/play/balance issues in the adapter, and I found it very easy to use. Collimation was a simple matter of popping the cap in the eyepiece adapter, checking that the secondary appeared central and circular in shape, that the all the primary mirror clips are equally visible (adjust the secondary as required), then adjusting the primary to get the centre spot on the mirror aligned with the hole in the cap. Simple!
Although generally happy with this approach, I started to feel that there was just too much judgement involved: are the primary clips all equally visible; does the secondary mirror appear completely circular; is the dot exactly dead centre on the primary? Also, it’s not possible to use the collimation cap out in the field, as it were, so although I could set up the scope in the house, would it still be accurate after a half hour drive to a dark site?
When the chance to buy a Baader laser collimator came up I jumped at the chance (35 quid from a fellow Stargazers Lounge member!).
These are very simple to use (just pop it in the eyepiece adapter, tweak the secondary until the laser is in the centre of the primary, then adjust the primary until the red dot is in the centre of the collimator screen. Very easy to use, can be used in the field, and, unlike many cheaper lasers, doesn’t need to be collimated itself (so Baader claims).
It has to be said, I do prefer the laser over the collimation cap. It’s just that bit more accurate, and can be used in the dark.
I had noticed that there was always a discrepancy when I compared the results of using the cap with the laser. I’d get the laser dots lined up perfectly, but the primary mirror clips (when viewed with the collimation cap), were out. Set the mirror clips to appear equally using the cap and the laser dots were out …
It took me a while to work out what was going on. Then it dawned on me. I’d always used the collimation cap in the 1.25in eyepiece adapter (below left), and the laser in a more snug-fitting 2in-1.25in adapter that came with the replacement focuser I’d fitted (below right).
Using the same adaptor for both removed the discrepancy … obvious, really. The 2/1.25 eyepiece adaptor I was using with the cap introduced some play into the focuser when used with the laser, which isn’t there with the shorter and more snug adaptor. So, if using two methods to confirm collimation, use the same focuser adapter, ideally as short and snug as possible to minimise play. Top tip there 🙂
The astro guy 