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View Full Version : Home made scopes - lenses and mirrors?? Which is best?


DrStoney42
June 23rd 03, 06:48 AM
Hello,

I have heard of many who make their own telescopes, reflecting and refracting
alike.. I know enough physics to do it right but I dont know where I can buy
lenses and mirrors for this. If anyone knows good sources (esp for custom sized
ones) I'd appreciate it... Also, which do you folks find give higher quality
images? Assuming equaly stable platforms and eye pieces which looses more
light, etc?

Thanks

Roger Hamlett
June 23rd 03, 09:55 AM
"DrStoney42" > wrote in message
...
> Hello,
>
> I have heard of many who make their own telescopes, reflecting and
refracting
> alike.. I know enough physics to do it right but I dont know where I can
buy
> lenses and mirrors for this. If anyone knows good sources (esp for custom
sized
> ones) I'd appreciate it... Also, which do you folks find give higher
quality
> images? Assuming equaly stable platforms and eye pieces which looses more
> light, etc?
I would say that 75% of ATM's (amateur telescope makers), end up making
their optics as well.
If you purchase optics, you will rarely better the prices of 'off the shelf'
scopes by much. However some standard mirror sets are excellent, and will
usually result in a better scope than the equivalent priced off the shelf
unit. The cost for custom made optics will be prohibitive.
Now the 'rules' for which scope is best, remain the same, whether amateur
made, or professionally produced. There is a 'balancing act', with optics.
In small sizes (up to perhaps 4"), lenses are reasonably priced, and
reasonably easy to make. However as the diameter rises, because a telescope
primary lens has to be thicker at it's centre than it's edges, and if the
focal length is to be kept reasonably, will need to be quite steeply curved,
the volume and weight (plus cost...), of the glass rises massively.
A simple Newtonian reflector, has the 'downsides', of the central
obstruction, degrading the image slightly (very slightly indeed, with small
obstructions - perhaps 20% or lower), and will display Coma, away from the
centre of the optical field (how far away, will depend on the f ratio).
Conversely, a simple refractor, will display chromatic aberration.
The 'best' scope design, is probably a refractor, that has minimal chromatic
aberration, but unfortunately, this involves the use of expensive glasses,
and the cost of these components makes perhaps a 6" design, about the
largest that is normally practical. Now conversely, the other 'rule' on
telescopes, is that 'aperture allways wins', and at the end of the day, a 8"
Newtonian, will beat a 4" refractor, giving perhaps 3.5* the light gathering
power, lower limits on resolution, and the effects of the central
obstruction, will only reduce this advantage a small amount (the normal
'rule of thumb', is to subtract the diameter of the CO, from that of the
main scope, to get a good 'guide' to the comparable unobstructed scope).
Hence if the CO, is 20%, the Newtonian, will be comparable to a 6.4"
unobstructed scope.
The reason for talking about a 4" refractor, and an 8" Newtonian, is that
this probably represents a reasonable 'comparable price' point (in fact for
a really well corrected refractor, a 4" refractor, may well match the price
of something more like a 12" Newtonian...). On larger sizes, this ratio gets
even more massive.
The other reason for going to amateur making (as opposed to the simple
feeling of satisfaction that you made it yourself), is to produce optical
solutions that are not common in the mass production models. For instance,
the Shiefspiegler designs - folded reflectors without central obstructions,
are rarely seen in any reasonably priced models, and for imaging, designs
like the Ritchie Chretien (the Hubble, and a lot of the professional
scopes), can be built. Unconventional mountings, that allow larger mirrors,
to still be portable (variants on the truss tube designs), are a common
production.
The following sites, point to a lot of ATM resources:
http://www.crickrock.com/cgi-bin/webring/list.pl?ringid=TelescopeMaking;siteid=1162site
http://uk.dir.yahoo.com/science/astronomy/telescopes/amateur/telescope_making
http://www.iol.ie/~pkcurran/astro-09.htm
http://www.starastronomy.org/TelescopeMaking/index.html

Best Wishes

DrStoney42
June 23rd 03, 08:26 PM
>I would say that 75% of ATM's (amateur telescope makers), end up making
>their optics as well.

How exactly does one do that with out the rather expensive equipment you'd
noramly think involved?

>However some standard mirror sets are excellent, and will
>usually result in a better scope than the equivalent priced off the shelf
>unit

>Now conversely, the other 'rule' on
>telescopes, is that 'aperture allways wins', and at the end of the day, a 8"
>Newtonian, will beat a 4" refractor, giving perhaps 3.5* the light gathering
>power, lower limits on resolution

So you'd suggest my first home made being a reflector I take it? I've never
seen an 8" reflecting mirror only smaller versions, is there limits on how big
they can be short of the common sense weight/tensile strength limits?

Your response to my thread is very appreciated and has given me a lot to think
about, thanks a bunch...

D

David Knisely
June 24th 03, 05:33 AM
HI there. RE: making your own optics, you posted:

> How exactly does one do that with out the rather expensive equipment you'd
> noramly think involved?
>

Its actually fairly easy, although it is somewhat time consuming. The
grinding and polishing is done by hand with various abrasives and
polishing substances, and the tester can be made in almost any decent
home workshop (or you can buy one for not a lot of money). If you are
interested in how to do it, I might suggest the book, "HOW TO MAKE A
TELESCOPE" by J. Texereau (Interscience Publishers), as well as looking
at some of the telescope making supplies available from Willman-Bell
Publishing. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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