Grinding mirror telescope tool
The eccentric arm is attached to the overarm with the same kind of swivel foot McMaster Carr. The block with the V-shaped notch is a sliding hatch that serves to keep the swivel foot from lifting up out of the hole.
Sliding friction alone keeps the hatch in place so that the eccentric can be quickly disconnected. Shown here are the bottom of the turntable and the eccentric. They both rest on 8" pulleys fastened to the shafts with set screws. Dogs transfer the torque. The groove in the turntable near the edge is a water barrier. Also shown are some T-nuts. I attach my mirrors to a plywood backing disk using pitch and then bolt the disk to the turntable through the T-nuts.
The eccentric crank is a carriage bolt outfitted with nuts, washers, and steel spacers. It screws into a 3"x3" square nut that rides in a channel. The square nut is made of soft wood with a T-nut hammered into it. The mirror in these photos is a 10" Corning molded Pyrex blank.
I hosed it off just before I took this photo so the slurry appears kind of thin. The dark spots are the pitch that fastens the mirror to the backing plate. Not long after I took the above photos, I added still another overarm eccentric. It was an interesting experiment.
Have a look at the details. The inch mirror wound up in the optical tube assembly shown here. The scope is really too big for the Celestron equatorial mount, but my son takes deep sky photos using lots of short exposures and stacking them. Use a brush and scrub the channels on your tile tool. Clean everything, including your spray bottle. One, single grain of 80 grit can cause a bad scratch once you move on to finer grit. Also, take a shower, change your clothes, and clean your fingernails before you continue with finer grits.
As we have thoroughly cleaned all the tools and working space, we can start with grit. After a wet or two, this is a good time to test the sphere on the mirror. The test is very simple. There is not really much to say about fine grinding. You will notice that the grits last longer and longer, as smaller in size. Grinding noise becomes more and more silent. Perform the grid test once per grit size. Clean thoroughly after going to finer grit. Past , the focal length will not change significantly anymore, and the glass becomes very smooth, even shiny if observed at a low angle.
It starts to look like a telescope mirror :. There are many reasons for grinding your own mirror. Either you want a unusual size, or focal length, or to satisfy your curiosity, or to have the "pride of achievement" by fabricating something as precise as a telescope mirror. You could also live in a part of the world where telescope optics are not commonly available, and this might be the only way to have a telescope.
Does it save money? Well, that depends. Mostly on the mirror size you wish to grind, and the prices of the local market.
In the US, telescope mirrors are the cheapest. In Europe, prices are usually twice, if not three times that. In some countries telescope mirrors are unavailable, so they need to be imported, at high shipping, customs, and tax cost. A 6" mm or 8" mm standard quality mirror will be cheaper if bought ready made. Until you have purchased the blank, abrasives, polish, and pitch, your expenses are very near to a ready made mirror.
The finished mirror needs to be aluminized, there are also shipping costs for this, and of course, your labor. But, as the mirror size grows, prices go sky high. No mass production at these sizes anymore..
Past the 12" mm size, grinding your own really pays off. Or, you want to make a super precise, highest quality mirror in the mm size. A good starter size is the mm 6" mirror. A mm 8" will also work. It is cheaper, it is softer and grinds faster, needing less abrasives and with these small sizes, the low expansion glass like Pyrex has no practical visible at the eyepiece advantage.
You might purchase a mirror grinding kit, and have everything you need included. If such a kit is unavailable, you need to do some scrounging. You can order a 19 mm thick disc at your local glass shop. Such glass is usually used for tabletops and window displays. A ship porthole will also do very nicely. You can use sandblasting abrasives, make your own polishing pitch. Or even silver the finished mirror if aluminizing is unavailable or too expensive. Now you need to decide about the focal length of your future mirror.
If you are making this mirror to practice your skills for a future, large mirror you want to make, a fast, F4 - F5 mirror is probably your best choice. A lot has changed since Russell Porter wrote those words - today the "average workman" can afford to buy an already made telescope and Dobsonian mountings are very popular. Many amateur astronomers still choose to fabricate their own instruments, for the pride of accomplishment, the gaining of knowledge and the insurance of quality.
Telescope making is at the heart of the Springfield Telescope Makers - after all it is two thirds of our club's name - and on these pages we hope to show you that you too can make your own telescope - and it can be an excellent performer!
A note on ATM techniques: There are almost as many ways to make mirrors and telescopes as there are telescope makers. On these pages we present one or more ways that have worked for us, but that doesn't mean there aren't many other valid approaches. In fact, many of us enjoy ATMing because we can experiment with different techniques and sometimes find better ways of making or building a telescope.
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