Mounting a primary mirror

[sharness]

I have an 8 inch Coulter telescope. As I went to clean the mirror in June, the primary mirror fell off it's wood base. It had been glued on. I tried to re-glue the mirror back on to it's base but now it won't collimate. I need to locate some brackets that would help to attach the mirror to it's base. Any suggestions?

[Doc]

Hello and welcome to SGL.

Primary mirrors are normally held on by silicon on the bottom, approx 6 blobs and then three mirror clips 120° around the outside.

[NGC 1502]

You could try refixing the mirror with silicone. It needs to be the sort that stays

flexible when dry, so it doesn't stress the mirror. I've used one that's intended

for making aquariums, apart from fixing glass, it stated that it stays flexible.

If you cannot collimate now that you've refixed the mirror, perhaps one reason

could be if the blobs of glue are of different thicknesses, so you run out of

adjustment before you achieve collimation.

You need to put some removable spacers between the mirror & cell while the

silicon cures, nails or similar, use 3 at 120 degree spacing & projecting out so

you can remove them easily.

Some ventilation holes are a good idea too, to aid mirror cooling when you set up.

Hope this helps, Ed.

[acey]

What do you mean by "won't collimate"? Do you mean that it doesn't hold collimation, or that you can't get the primary tilted to the required angle? If the former then it's not being held securely enough so you could try re-gluing. If the latter, then check that you reassembled the mirror cell as it was previously, and haven't introduced a tilt that can't be sufficiently corrected by the collimation bolts.

How have you tested collimation? If you've done it by star-test and have got poor results, maybe it's because you introduced stress on the mirror when you re-glued so that you're now getting astigmatism. If anything needed screwing and tightening then check you haven't overtightened, and make sure nothing is stressing the wooden mirror cell.

If you want to use clips then what fits will depend on the configuration of your mirror cell. Many people prefer to do without clips altogether because of the diffraction they introduce, and with an 8" it's certainly reasonable to have it held with glue alone, as yours was. The downside is when you need to get the mirror recoated. For cleaning just leave it fixed on the cell in future.