Mirror-grinding questioons

Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?

Where do I get blanks and the grinding stuff?

How do I test it?

How do I get it coated?

Thanks.
--

----
Joe S.

Joe S. wrote:

Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

I like Jean Texereau's HOW TO MAKE A TELESCOPE (available through Sky
Publishing).

Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?

Probably not, although you never know. Most people who are doing their first
mirror will start with a 6 or 8 inch and go up from there (although not very
many do mirrors larger than 12.5 inches). For a 20 inch, I would probably go
with a custom optical company rather than attempting it myself.

Where do I get blanks and the grinding stuff?

Willman-Bell has mirror kits.

How do I test it?

You build (or buy) a knife-edge tester and use the Foucault test (or one of
its variants). The techniques and mathematical analysis are covered in detail
in Texereau's book.

How do I get it coated?

There are a number of companies which will coat mirrors including Clausing
(probably the least expensive) and Spectrum (expensive enhanced coatings).
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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"Joe S." wrote in message
...
Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?


20-inch? Where did that come from -- should say "10- or 12-inch mirror."


--

----

Joe S.


Where do I get blanks and the grinding stuff?

How do I test it?

How do I get it coated?

Thanks.
--

----
Joe S.

On Sat, 20 Mar 2004 17:29:14 -0500, "Joe S."
wrote:

Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?

^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In addition to what David has said from most of reading I have done is
a 8 inch F6 mirror is the best overall first mirror. Suggest that you
get a mirror with pregenerated curve of choice. Saves about 6 hrs of
hard grinding and measureing to determine if you have the correct
curve. Also get a blank that is about 1:6 ratio of thickness to
diameter. If you make mistakes or have difficulty in polish, you have
spare thickness of glass to work with.

There have been people that have ground larger mirrors for their first
mirror. I would not reccommend above 12.5 to 15 inch though.


Where do I get blanks and the grinding stuff?

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Newport Glass sells kits also

You can also buy just the blanks from them and order the grinding
stuff from Got Grit or other places.

Reccommended sites for ATM suppliers links is

http://www.freenet.tlh.fl.us/~blombard/

In 2002 I bought an 8 inch full thickness blank from Newport Glass
with pregenerated curve for $46. A blank with no pregenerated curve
was $5 less. So you can see the extra work taken out for $5 is worth
it.


How do I test it?

How do I get it coated?

Spectrum is by far one of the better but is also one of the more
expensive coaters. He does excellent work. see the link above and you
wil find links to a few that will coat the mirror.Coating can cost
between $40 and $100 for an 8 inch mirror depending on who does it and
how it is coated.

Thanks.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For the rest as David said Jean Texreau's book on how to make a
Telescope is almost the Gospel of Telescope making. If you do not own
it and want to make your own, it should be on your bookshelf.

IF you take care in the steps of fine grinding and polishing you can
create a very nice mirror. For your first time do not get into a rush.
Also don't panic. Scratches and errors can be corrected. Probably the
biggest thing that most beginners do not do when going to the next
smaller grit is getting everything clean, hands, blank, gringing tool,
stand and clothes before the next step. Clean so that you do not
scratch the surface in the next step.

Remember if you make a mistake do not panic.

james

Joe S. wrote:

20-inch? Where did that come from -- should say "10- or 12-inch
mirror."


It would probably prove beneficial if you initially attempt something
smaller knowing that the experience gained will result in fewer "surprises"
on the larger mirror you envision.

From the various effects of different strokes used in the grinding and
polishing procedure, to interpreting foucault results and parabolizing the
smaller mirror, to just plain getting the feel for the whole process, you
can't go wrong starting small. I believe that a 150mm is an excellent place
to begin. . .even after you have finished your larger scope, you'll still
have a nice sized scope to use when you just don't have the time or energy
to drag the larger one out. I have a 12.5 inch dob. I also have a 114mm
newtonian. Believe me, the smaller scope doesn't gather dust.

Almost 40 years ago, I did a 114mm mirror before subsequently progressing to
producing a couple of other 200mm ones and believe the introductory
experience of a smaller mirror to be indispensible.

--
Martin

On Sat, 20 Mar 2004 17:29:14 -0500, Joe S. wrote:

Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

I learned from Sam Brown's "All About Telescopes" from Edmund Scientific.
That was about 35 years ago. Since then I've come to know that the book
has some technical errors in stating acceptible surface accuracy, and
perhaps a few other areas. None the less, the instructions are clear, and
very well illustrated. Today, the book is a bit out of date, since it
refers to a number of items that Edmund no longer handles, though the
principles are still valid.

There are a number of other books you may wish to consider:
1. "Amateur Telescope Making" - a three volume series from the mid-
20th century, rearranged and updated by Willman-Bell. Good reference,
but not terribly current for beginners.

2. "How to Make a Telescope" by Jean Texareau (sp?) - another 'classic'
of the genre. Translated from french, and updated by Willman-Bell.

3. Richard Berry had a good book out 20 years ago or so. I forget the
title, but it was relatively current, well illustrated and had clear
instructions.

4. "The Dobsonian Telescope" or something to that effect. This may be
the one book you'll need if you plan to work on a large aperture scope.

5. "The Standard Handbook of Telescope Making" by Howard Neale (sp?).
Out of print, but available in many well stocked libraries.

Other sources you may wish to consider:
6. Search the web for 'Amateur Telescope Making' and 'Astronomical Mirror
Grinding'.

7. Scientific American published a CD-ROM with all the articles it ever ran
on amateur astronomy. There may be some valuable info contained there.
Title "The Amateur Astronomer" or similar.




Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?

"Possible"? Yes! Probable? Not very likely.
The difficulty increases with the surface area to be figured
and the steepness of the light cone. A 6" f/8 is considered
the standard beginner's project. A 20" f/5 would probably be 20X more
difficult!




Where do I get blanks and the grinding stuff?

A few years ago, I started an 8" f/8 with a preground
rough curve in both Mirror blank and plate glass tool.
The source I chose was Newport Glass Works. One feature
of the kit from Newport is that the tool had a series of
concentric rings molded into it. This keeps the abrasive
slurry in active contact with the glass much longer than
would two pieces of uniform glass.

As David pointed out, Willman-Bell also (supposedly) deals with grinding
kits, though I have no direct experience with them. You may wish to check
with club websites which have
current links to various "Amateur Astronomy Resources".
(Do a web search for most key phrases in quotes.)




How do I test it?

The traditional instument is the Focault (Knife Edge) Tester.
I prefer a variation called the "Slit-less Focault" with
an finder scope behind the knife edge to magnify any mirror
operating at f/6 or greater.



How do I get it coated?

There are a number of Aluminizing / Coating services.
Again, check Amateur Telescope Maker resource lists (web pages).

Be sure to do a reality check before flinging yourself into the
ATM abyss. Enthusiasm is nice, but persistence, dedication and
patience will give you the discipline and commitment needed to
actually complete such a major undertaking.

Telescope making can be a fascinating and demanding hobby. There
is a great deal to learn, even for such a seemingly simple optical
system as the Dobsonian / Newtonian. Issues which seem trivial
in your daily life, may spell the difference between success and
disaster in a 10" telescope. If you really have to start with a large
aperture, you may be wise to start with 10" as a maximum
size starter project. Once you master that, then you might be
ready for a 20".

A mentor (or two) is highly recommeded.

Cheers,
Larry G.



Thanks.



--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

Cheers,
Larry G.

Hi Larry-

I hope you're well. The 8" f/8 mirror was finished last fall and the
figure checks on the bench to around 1/12th wave on the wavefront. I'm
doing the carpentry now to finish it, maybe in a few weeks or so. Thanks
again, many times. -Chris

I made my first mirror from information collected on the web. The end
result wasn't exaclty perfect since I ended up with a deep central hole and
turned down edge. But this is because I didn't follow the instructions
properly. When the web sites said polish 20 strokes a minute, I was probably
doing more like 80 strokes a minute. I used a lap that was smaller than the
mirror, which wasn't a good idea for a first mirror. My opinion (from
experience), don't try a 10 inch first time.

Good luck and clear skies

Jack
South Africa


"starburst" wrote in message
...

Cheers,
Larry G.

Hi Larry-

I hope you're well. The 8" f/8 mirror was finished last fall and the
figure checks on the bench to around 1/12th wave on the wavefront. I'm
doing the carpentry now to finish it, maybe in a few weeks or so. Thanks
again, many times. -Chris

I almost forgot. I used ordinary plate glass that was only 10mm (3/8")
thick. Now that was stupid.

Jack

"Jack Kotze" wrote in message
...
I made my first mirror from information collected on the web. The end
result wasn't exaclty perfect since I ended up with a deep central hole
and
turned down edge. But this is because I didn't follow the instructions
properly. When the web sites said polish 20 strokes a minute, I was
probably
doing more like 80 strokes a minute. I used a lap that was smaller than
the
mirror, which wasn't a good idea for a first mirror. My opinion (from
experience), don't try a 10 inch first time.

Good luck and clear skies

Jack
South Africa


"starburst" wrote in message
...

Cheers,
Larry G.

Hi Larry-

I hope you're well. The 8" f/8 mirror was finished last fall and the
figure checks on the bench to around 1/12th wave on the wavefront. I'm
doing the carpentry now to finish it, maybe in a few weeks or so. Thanks
again, many times. -Chris

Start with a 6" f/8 mirror. Your success rate will be higher than with an
8".
Im partial to "Standard Handbook of Telecope Making" by Howard Neal.
Because, chapters are laid out in step-by-step to a finished mirror. A
perfect
blend of theory and practical "how to, do this, do that" all in a logical
order.
Great large illustrations. Nothing in left out - its a complete textbook
and
well written & thought out for the average person. Howard's book assumes
no prior experience.

Engineers like Texareau.

Jerry




"Joe S." wrote:

Which book(s) is/are/am best for telling me step-by-step how to grind a
mirror?

Is it possible for someone who has never ground a mirror to make a
respectable 20-inch mirror?

Where do I get blanks and the grinding stuff?

How do I test it?

How do I get it coated?

Thanks.
--

----
Joe S.