How to grind your own mirror

[Will Gater]

Hi everyone,

I thought this might be of interest to someone on Stargazerslounge. It's our new video exclusive of 'How to grind your own mirror'. It's about 40mins long and is aimed at supplementing an article in May's issue of the mag.

http://www.skyatnightmagazine.com/mirror_video.asp

Hope it is of use to some of you who are perhaps thinking of grinding a mirror but haven't yet taken the jump.

All the best,

Will

[Sam]

Thanks for posting that - Seems quite an involved process - but not as bad as I thought. The shadow testing at the end with the knife edge was quite interesting.

[barkis]

Anyone seeing their own mirror under the Foucault test, would be absolutely fascinated by the shadow patterns at the different focii of their mirror. The smoothness of the mirrors surface is sight to behold in itself. Flap your arms, and watch the resultant airflow move across the surface. You would not do this with a coated mirror, but if you were to place your thumb on the mirror for a few seconds, and look at the area as the knife cuts into the light cone, you will see what looks like a large bump on your mirror.

It really is a lump on the glass caused by the heat from your thumb. It looks massive, but actually it is only a few microns high. It will gradually disappear as the area cools again.

Now you can see the reason for allowing your optics to cool to ambient temperature.

If anyone likes a challenge, have a go at making a mirror. I think maybe the ATM population has fallen in recent years, but probably still fairly high in the US.

Ron.

[Ajohn]

There is lots of info on the web on the subject. Having attempted one and then actually made one I have one or two reservations about the video but can't really comment without looking at the article.

The rough grinding shown was interesting. My 1st attempt was with a pyrex mirror blank and a full sized plate glass tool. I think that the blank shown is from the same source. Spent hours rough grinding and hardly touched the mirror blank but got a lovely curve on the tool. Reason is simple pyrex is much harder than plate glass. The method shown might just get round that problem but could prove a little tricky. Tiled and other thinged tools are much used though. I stuck mine on with plastic padding.

2nd go was entirely successful but used pre ground blanks made out of the same material. From Telescope House. Much better and much better quality blanks too. With those the rough grinding stage isn't needed not that this takes all that long. I sometimes wonder why I later ground an F3 curve in it and bored a hole in it for a cas. It's sitting around the house some where awaiting polishing. I will do it some day. It doesn't take as long as you might think to remove glass - but note the comment about the plate glass tool above.

I found 2 books very useful. Texereau's How To Make a Telescope, useful for a lot of other things too and Beginners Guide To Astronomical Telescope Making by Muirden. Texereau was a professional optician and Muirden was not such a purist. His minor changes to Texereau's methods helped me a lot. Particularly with making a lap and obtaining a perfectly spherical mirror before figuring it.

There is a little bit of info on this site too http://www.jimsastro.com/ . There is lot and lots else where on the web. The US seem to use ronchi tests almost exclusively but in my experience that isn't the best way to do it.

John

[Ajohn]

Bit disappointed having read the article. It all seems a bit rushed and not enough detail.

At least it does show people that it isn't as bad a thing to do as they might think. Rough and smooth grinding and then polishing doesn't take very long at all. The final bit may take just as long but it's interesting work and with luck can take just a couple of hours.

John