Pinched optics – the cure

If you’ve been following my geeky astronomical posts lately, you’ll see that my new telescope suffers from triangular stars, caused by pinched optics. I’ve also been posting about the problem on Astronomy Forum. Just to recap, the telescope currently produces pictures like this. The effect is subtle, but if you click on the picture to see the larger version, you can clearly see the triangular stars.

M45 Pleiades cluster
M45 Pleiades cluster

For those who are unfamiliar (although there’s no excuse not to be familiar with the inner workings of telescopes if you follow my blog! :P), reflecting telescopes have a thick glass mirror. The mirror is held in a circular structure called a mirror cell, which is basically a disc of metal with three (or more) clips that hold the mirror in place. Here’s my mirror in its cell.

Mirror and cell
Mirror and cell

Now here’s the rub (literally). Although the mirror is thick glass (almost an inch) and very heavy, it is actually quite flexible. It doesn’t take much pressure on the mirror to warp it, which is what’s causing my triangular stars. The three clips on the front of the mirror were pressing down quite firmly, and warping it. The top part of the clips is rubber, but it was still enough pressure.

However, that’s not all. After I loosened the three clips by undoing the screws a bit, I noticed that the mirror was actually wedged tightly in between the metal pegs at the base of each clip. It shouldn’t be touching those at all.

Primary mirror clip
Primary mirror clip

Not only was my mirror touching the sides of the clips, it was wedged tightly down. It wouldn’t budge, and must have required significant effort to wedge it in there. I contacted the retailer, Teleskop Express, to ask what they recommended. This is their reply:

you are probably right – it is the sideways pressure of the mirror clips. This can be more pronounced if the temperatures get lower. The best way to deal with this problem is to take the mirror out of the cell and file/sand off a bit from the clips. You can even take off so much that you can fit a piece of felt or velours inbetween. (but this is an option not a must).

Reluctant to do irreversible modifications on a new an expensive telescope, I set about the task. I wore cotton gloves to avoid touching the surface of the mirror and tried to remove the mirror by hand. No luck – it was stuck too tight. Eventually I resorted to using a plastic ice-scraper. The scraper broke with the effort, but eventually I freed the mirror. It removed paint from the insides of the clips.

Freeing the mirror
Paint rub
Paint scuffs

Finally, I set to work filing back the metal extrusions. Unfortunately I only have a basic set of hand files which are not that accurate. A poor workman always blames his tools, but I’m a poor workman 😛 I scuffed the outer ring of the cell a few times, but it doesn’t matter. I was more upset about wearing a lot of the cork mat away. I tried to peel them off for safekeeping but unfortunately they started to break up.

Filed mirror clip
Filed mirror clip

Now, the mirror fits in the cell between the metal pegs with about a millimetre to spare in each direction. Nothing presses, bends, squeezes or squashes it. Unfortunately it’s cloudy tonight so I can’t test it on a bright star, but I am optimistic that this will have cured the problem. Watch this space for results!

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