We also have a limited number of original Pyrex blanks made in the 1980's. These were used to manufacture astro mirrors by, David Hinds (and sold by B.C. and Fuller), Jim Hysom (A E Ltd) Orion Optics and H. Wildey to name but a few manufacturers.
Because of the way they were poured, they retain original, concentric stress patterns which show if tested using polarised light. Of the thousands of mirrors made using these blanks, there are no known cases of optical problems caused during mirror manufacture.
Jim Hysom always maintained it was best not to grind the backs of these blanks. In that way, once ground and polished both surfaces retain their original surface tension. Mirrors recently tested that were ground and polished over 30 years ago still retain perfect figures confirming the safety of using these blanks.
We now also have some low expansion Suprax discs.
If you are planning on grinding a mirror have a look at the video link at the bottom of the page. Simon Lang wrote an aricle for the
'Sky at Night' magazine on how to construct an 8 1/2 inch Dobsonian. Terry Pearce wrote that part relating to the mirror making.
Simon presents the video using Terry's script to show the process of a mirror being ground and polished.
| Size | Price (£) | p&p |
|---|---|---|
| 4" Dia (103mm) x 3/4" (20mm) thickness | not available | 6.00 |
| 4 1/2" Dia (115mm) x 1/2" (12mm) thickness, with 1" centre bore | 20.00 | 8.50 |
| 6 1/4" Dia (152-159mm) x 1" (25mm) thickness Perhaps some discs partly ground . Ask | 30.00 | 12.00 |
| 8 1/2" or 8 5/8" Dia (216-219mm ) x 1 3/8" to 1 5/8" (29-35mm) thickness Just a few available | 50.00 | 18.00 |
| 11.9" Diameter x 1 1/2" Thickness - Just a few available | 120.00 | 24.00 |
| 11.9" Diameter x 1 1/2" Thickness with 2" Centre Bore Just a few available | 140.00 | 24.00 |
| Size | Price (£) | p&p |
|---|---|---|
|
The original moulded discs, flat on one side with concentric rings on the reverse are no longer made. The discs below are cut from one inch (25 mm) sheet Suprax Low Expansion Glass.This is reasonably flat with a fine textured surface. One face is guaranteed parfect, the rear surface may have minor irregularities mainly due to cutting- |
||
| 8 inch (205mm) Diameter x 1" (25mm) - | 72.00 | 18.00 |
| 10 inch (255mm) Diameter x 1" (25mm) - | 92.00 | 20.00 |
Three grades of coarse grinding powders are listed below. The table advises the probable amount of each grade of powder needed to grind the mirror sizes listed. The powders are priced at a cost per per pound (nominal 1/2 kg) in weight. Any amount, greater or smaller than this may be purchased.
| Mirror Size | 6" | 8" | 10" | 12" | Price(£) per lb |
|---|---|---|---|---|---|
| Grade 60C | 1 lb | 1 lb | 6.00 | ||
| Grade 80C | 1 lb | 1 1/2 lb | 1 1/2 lb | 2 lb | 7.80 |
| Grade 120C | 1/2 lb | 1/2 lb | 1/2 lb | 1/2 lb | 7.80 |
Two grades of smoothing powders are listed below for removing all pits produced during rough grinding and scour marks if grinding pre-generated blanks.
The table again advises probable amounts needed to grind each mirror size and, as before, any quantity may be purchased. This information is repeated below for the other mirror making materials.
| Mirror Size | 6" | 8" | 10" | 12" | Price(£) per lb |
|---|---|---|---|---|---|
| Grade 220C | 1/4 lb | 1/4 lb | 1/2 lb | 1/2 lb | 7.80 |
| Grade 320C | 1/4 lb | 1/4 lb | 1/4 lb | 1/4 lb | 7.80 |
| Mirror Size | 6" | 8" | 10" | 12" | Price(£) per lb |
|---|---|---|---|---|---|
| 125E (equiv. 600C) | 1/16 lb (1 oz) | 1/8 lb | 1/8 lb | 1/4 lb | 18.00 |
| 50E (equiv. 1000C) | 1/16 lb (1 oz) | 1/8 lb | 1/8 lb | 1/4 lb | 20.00 |
Auerpol 1 to 4 micron powder is really ideal for most polishing needs. Used in suspension, it gives a fast polishing 4 micron grade if taken soon after stiring. Left to settle out it leaves the finer 1 to 2 micron powder for fine polishing and figuring.
The Auerpol W100 is 1 to 1.4 micron grade. This would normally be needed only for the final fine figuring of an optical surface. It is sold in small 1 oz (1/14 lb) packets at a cost of £2.00.
| Mirror Size | 6" | 8" | 10" | 12" | Price(£) per lb |
|---|---|---|---|---|---|
| Auerpol 1 to 4 micron | 1/10 lb (1 oz) | 1/10 lb | 1/8 lb | 1/8 lb | 20.00 |
| Auerpol W100 | 1 oz | 1 oz | 1 oz | 1 oz | 28.00 |
| 6" | 8" | 10" | 12" | Price(£) per lb |
|---|---|---|---|---|
| 1 lb | 1 1/2 lb | 2 lbs | 2 1/2 lbs | TBA |
| 1 lb (460 gms) | 2 lbs (920 gms) | 3 lbs (1400 gms) | 4 lbs (1850 gms) | 6 lbs (2750 gms) |
|---|---|---|---|---|
| £6 | £8.50 | £10 | £12 | £15 |
If you are near to, or live in NW London you might like to know about the Amatuer Telescope Makers of London (previously known as CATS), meeting fortnightly in Highgate. For more details click HERE.
You might also like to know about the Hampstead Scientific Society and the Hampstead Observatory who have weekly (weather permitting) free public observing nights. For details click HERE.
Finally, the 'Sky at Night' magazine's Telescope Making Video by two members of CATS may also be of interest. To see the full video click on the image below.
