FAQs About Telescope Mirror Making in Pakistan

© 2012 Zain Ahmed

Grinding a telescope mirror with nothing but your bare hands is not only possible, it has been done in Pakistan. Some of the local ATMs that I know are Irshad sahab and Ali sahab in Lahore, Mehmood Hassan bhai in Peshawar/Islamabad, Asad Mehmood in Okara and Jalaluddin in Karachi. The process is extremely simple. You can see detailed explanations along with the list of materials you’ll need here if you use the tile tool method and here if you use the glass tool method. Note that by far the most important ingredients for successfully completing a telescope mirror are . . . enthusiasm and patience.

Now, if you’re actually interested, visit http://stellafane.org/tm/atm/index.html and go through its pages. Come back here later.

Pyrex (borosilicate glass) or untempered plate (soda-lime) glass – that’s the same green-colored stuff as ordinary window glass. Pyrex is not available in Pakistan. In Karachi, there are shops in Bohrapir where they sell nicely cut and polished plate glass disks (untempered; tempered glass in NOT for telescope making) up to a maximum of 1″ thick. You won’t have to cut them round. In fact, there is a shop where glass disks comprise about 95% of their annual sales. Glass in Karachi, however, is more expensive than it is in Lahore. So are the grits and polish. The current lowest price of float plate 25mm thick is Rs1100 per sq. ft. in Lahore. More than enough grit for an 8″ mirror can be bought for about Rs. 700 (as of February 2010) from there.

Plate glass is much lighter than Pyrex. It also takes better polish because it is harder. Its main drawback is its high coefficient of thermal expansion, but if one is careful, it is not difficult to work around this. When finished, both plate glass and Pyrex mirrors are optically excellent performers.

Whatever substrate you use, it is A LOT easier to do a full (with a thickness-to-dia ratio of 1:6) or nearly full thickness (~1:8) mirror as a first project. When one has learned the basics, he can move onto larger/thinner mirrors.

Your first mirror can be a 6″ or an 8″. Or may be a smaller size. There’s good reason why they say not to go larger than an 8″X1″ or 8X1.25″ for a first mirror. I’m not saying bigger first mirrors cannot be made. They’ll only be more difficult and time consuming, so that the chances of running out of time and enthusiam are very high. Only if you’re very enthusiastic and very patient should you try a 10″ as a first mirror.

The reason why doing a moderate sized first mirror, say a 10″, can be difficult is not its size. Rather, it’s the short f-ratio that large mirrors are often ground to.

If you still plan to do a 10″ first mirror, make its f-ratio f/6 or longer. Use the thickest blank you can find. Don’t go with anything thinner than 1″ if that is all you can have. Make sure the blank does not have internal stresses well before you start rough grinding.

This was NOT a first or even a third mirror, but here are pictures of a 20″x1″ plate glass mirror being made without a mirror grinding machine: link.

In a nut shell:

– Rough grinding with #80 grit and the hard tool to make the blank concave.

– Continue grinding (called fine grinding) with finer grits using your hard tool to make the shape spherical and the surface smoother.

– Polishing with a compound (cerium oxide or red oxide) in the form of an extremely fine powder using your pitch lap. Your mirror should end up in a smooth spherical shape.

– Parabolizing with your polishing compound and your pitch lap. The result is a parabolic curve on your mirror.

This theoretical approximation works only for mirrors 6″ or smaller. For larger mirrors, the focal ratio for thespherical-almost-equal-to-paraboloid approximation to work increases with diameter.

Young Asad Mehmood from Okara has parabolized a 6″ f/10-ish mirror.

Not necessarily. Richard Berry claims in Build Your Own Telescope (third ed.) that a mere 72 W strokes with CeO will fully parabolize a 6″ f/8 mirror.

Parabolizing gets much easier if you have someone looking over your shoulder who has done one or more mirrors. Optical masters can parabolize small mirrors in minutes. Absolute newbies may find it somewhat difficult if they work all by themselves, but if the blank is full thickness or nearly so, has no stress, the polishing compound is not contaminated, they don’t use the wrong stroke or do the ‘right’ one incorrectly, etc. etc., they can and have parabolized in a couple of weeks at most. Of course, if someone works on a mirror very inconsistently, like only for a few minutes every five or six months, it would probably not be done in a year or even more.

If a blank or glass disk has internal stress, it won’t turn into a telescope mirror regardless of whether it is worked upon a year or a century. You check for the presence of stress in your glass blank by using a source of polarized light (a lit LCD screen, the daytime sky background etc.). You then view the source of the polarized light through your glass blank and a polarizing filter (polarizing sunglasses, for example) placed in this sequence: Polarized light source > Glass blank > Polarizing filter > Your eye. Here are examples of how the strain looks like: link 1, link 2 and link 3. The third link shows a tempered glass, which as I’ve mentioned earlier, cannot be used as a telescope mirror blank.If a blank has no stress in it (most BUT not all plate glass blanks do not have internal stress), polishing should take between 4 to 6 hours for a 6″ mirror (according to stellafane.org). About figuring, I’ve mentioned that Richard Berry claims that only 72 W strokes with CeO will fully parabolize a 6″ f/8 mirror. Larger mirrors will take exponentially longer to figure. This is the reason why a 6″ or 8″ is recommended as a first mirror.

Yes, Alhamdulillah. There are aluminization chambers at the University of Punjab. Commercial front surface aluminization service is available in Karachi too. The last time I met that coater, he had just metallized an 18″ diameter glass disk.

Mirror making is a remarkably simple process. It should be kept that way. As they say, grind more, worry less!

Good luck and happy ATMing.

Zain Ahmed