Phil's Astronomy Amateur Page

One of the occasional jobs that befalls the owner of a reflecting telescope, is cleaning the lens when it gets dirty after years of use. Dusty atmosphere coupled with condensation (dew) can give the main mirror a thin coating of grime which can ruin image quality.

Cleaning the mirror on such a telescope isn't difficult at all (see details on my web site) - just take care not to scratch the aluminium surface (or if it's got a protective coating, don't scratch that). The trickiest bit is in reassembling it and getting the collimation right. (Collimation is the correct alignment of the mirror with the secondary and eyepiece and can be a bit of a 'black art').

If you've ever tried to collimate your optics, you'll understand what I mean. If you haven't, boy are you in for a treat!

Anyway, my little Bushnell was showing signs of mucky-mirror syndrome (it looked like brown staing on one patch of the surface) so it was time to take the bull by the horns. I removed the mirror from its cell - that took about 3 minutes - and ran it under tepid water from the tap.
With some Squezy washing-up liquid diluted in an egg-cup and a wadge of cotton wool, I carefully swabbed the surface of the mirror.

Aaaargh! What's happening? The aluminium was just washing off the glass like it was made of icing sugar! Bl***y great. Now I need a new mirror.

I don't understand to this day what happened but after the initial anguish, I decided to send the mirror off to a company called Galvoptics over in Essex county, to get re-aluminised. (Why did the Essex girl fail her driving test? Because she opened the door to let the clutch out...)

Interesting observation here: The word Aluminium is actually called Aluminum in America. Should the English expression for recoating not, therefore, be Aluminimised? Just a thought...)

Any way, back to the plot. The nice chap at Galvoptics said it could take up to 4 weeks to turn around the work, so I sent it off and waited patiently.
When it duly came back (£23 including DHL shipping) it was pristine. Yummy!

So after waiting a week or two whilst other work was completed, centred the mirror (put a dab of orange paint right slap bang in the middle to aid collimation) then returned it to its cell and reassembled it. That's another 20 minutes maximum. Then the fun bit...


I'm not a stranger to the 'black art' - indeed I also have a 10" reflector which gets collimated every time I bring it out of the shed, which isn't often these days, I regret to say. So I wasn't overly perturbed by the idea of collimating this little 'scope, even though the adjustments were horrible.
(A 'normal' reflector will normally have three spring-loaded adjusters which, with a little tweak here and there, will have you up and running in 2 minutes. This swining thing has 3 pairs of screws - one half of each pair to space the cell from the tube, the other half to lock the cell. OK once it's set because it shouldn't ever need doing again, but a pig nonetheless.)


Anyway, after 15 minutes messing about and resolving to convert the mechanism (watch this space!) asap, I had what I thought was pretty good collimation. Ready to test...

That evening was clear, like pretty much every alternate evening (until it clouded over) so far in August. Around 22:00hrs Jupiter was visible between my house and the one next door so there was my target. Sticking a Meade 12.4mm Plossl eyepiece in the focuser, (and having already set up the finderscope earlier) it was easy to get an image of the biggest planet in the Solar System, accompanied by its Galilean moons, but the image wasn't as sharp as I'd have liked.

Swinging across to Arcturus in the west I was rewarded with terrible coma. Or was it coma? It looked like a capital letter 'A' with extended crosspiece... Defocusing gave that clover-shaped blur again, instead of a disk divided by the three spider vanes.
Back to Jupiter and defocus it, looking a bit more carefully this time. There's that distinct clover leave shape emerging as I defocussed. Bizarre. So I started messing with the collimation, just in case my centre-spot on the mirror, on which the whole process rests, was slightly off. But I couldn't improve things, or indeed change them much at all. So I went to bed.

Next morning I pulled the mirror cell off. I checked the centre-spot. Yes, as near as possible to get it.
However, looking at the perimeter of the mirror, I could see what appeared to be little semicircular stress marks with the centres on the periphery. Had I perhaps mounted the mirror too tightly - I recall I'd left out some little plastic spacers because the mirror was actually loose in the cell?

So I remounted it, making sure it was snug but not tight, which included little pieces of paper around the mirror to stop lateral movement. Now it was possible to rotate the mirror in the cell without too much trouble, but it wouldn't rattle.

All back together again and collimated yet again and I was ready to test the next night.

Except the next night, Sunday, it was a bit cloudy so I watched TV. Anyway, opportunity offered itself and it was not without a little trepidation that I loaded up the 12.4mm eyepiece again and aimed at Arcturus. This time what it showed was a beautiful yellow point of light with 6 diffraction spikes. Lovely. Defocussing produced the required blurred disk. Swinging up to Vega showed its blue, again with diffraction spikes. Across to the double stars, Alcor/Mizar in the handle of the Big Dipper and they were beautifully split.

So what did I learn from this exercise?

Well, I didn't learn what caused the stripping of the aluminium from the original mirror.
However I did learn that it doesn't take nuch pressure at all to bend a 3/4"-thick piece of glass, because I believe this is what was causing the aberration after the first rebuild. It's all about trying to understand what you're doing, then if you get it wrong like I did(!), knowing how to diagnose it and provide the solution.

Next time out I'll probably hook the webcam up to it and post some images of Jupiter. Maybe even later tonight if the skies are clear! 'Til then, take care...

Labels: aluminising, collimating reflecting telescope, collimation, main mirror