Share practical advice and tips on making your own kit
Sat Sep 19, 2009 9:56 pm
I am new to this telescope making business and am wondering if anyone has built a telescope using an ordinary magnifying shaving mirror. My local pharmacist has a large stock of various sized mirrors from about 4" to 9" diameter.. I know that focal distance would have to be measured but is the optical quality or lack of it not worth pursuing this line of thought
Sat Sep 19, 2009 11:53 pm
You might find this of interest,
I did a quick search of "telescope shaving mirror" and that was at the top of the list.
Sun Sep 20, 2009 1:50 am
[quote]is the optical quality or lack of it not worth pursuing this line of thought
My feeling is that shaving mirrors are likely to be too inacurate by a factor of about 1000.
Sun Sep 20, 2009 2:53 pm
Also, modern shaving mirrors can be made in a mould rather than polished from a flat piece of glass, which means that the surface is slightly bumpy.
The difference is that a shaving mirror can be bought for around 10 quid instead of 50, but it means that the surface is not actually spherical.
A shaving mirror polished from a flat piece of glass will generally be very close to spherical.
Focal length on shaving mirrors is very long, which is good news if you're building a telescope with a spherical primary - but you do need a spherical one, a bumpy surface gives a blob instead of a point at the focus.
I built a Newtonian telescope like this with my little boy to see how this type of telescope works. We used a 7.99 shaving mirror (sadly Woolworths have since disappeared) and used a flat cosmetic mirror for the secondary. A cardboard outer-cylinder and kitchen-roll tube focuser completed the design. The size of the dot at 'sharp' focus, was around 2 cm - so completely useless as a telescope, but does demonstrate everything about a Newtonian scope (we even added a toilet-roll tube as a finder scope).
Once you've spent on a shaving mirror that is polished from an initially flat piece of glass, you can pretty much buy a parabolic primary for the same amount (from the same companies).
I would say it's worth doing this exercise to understand how this type of optics work, but you will only be able to make a useful telescope if you are able to test a large number of these mirrors to find one that actually has a small dot focus.
Hope this helps
Sun Sep 20, 2009 5:23 pm
Thanks to you both for this interesting bit of information. Probably not worth pursuing as the results will, except in rare cases with a true mirror won't be worth the effort. An interesting experiment however.. Perhaps I should work out a simple method to determine the quality of a shaving mirror and then go to my pharmacist armed with a torch and some sort of screen/viewing device and work through his stock of mirrors for the best. Watch You-tube for the resulting ejection from the shop! Faster than a shuttle launch I bet.
Thanks again folks
Wed Mar 13, 2019 12:04 pm
In astronomy, f/ratio is simply the Focal Length the distance that the mirror comes into focus divided by the primary mirror diameter. So, if we had a mirror that came to focus 100 inches away from its front surface, and that mirror was 25" in diameter, we would say that the mirror was a f/4 mirror 100 / 25 = 4 . If the same 25" mirror was ground to come to focus 150" away from its surface it would be called a f/6 mirror 150 / 25 = 6 .
If you just wanted to view the planets and our moon, a quality 10" telescope would be a good choice. But, for Galaxies, Nebula and other "deep sky" viewing you want as much aperture as you can afford. The deep sky objects are dim, so you need a big mirror to collect a lot of light.
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