Re: [ATM] Newport conical blank?

["Stuart Hutchins" <stuart452@earthlink.net>]

Hi Rob,

----- Original Message ----- 
From: <polaris@gsinet.net>
To: <atm@atmlist.net>
Sent: Friday, August 20, 2004 10:35 AM
Subject: [ATM] Newport conical blank?


>I'd like to build another nice dob to go with it, 10-11" aperture.  I
currently have a 14.5" commercial
> dob which I am selling due to back problems.  Back injuries will limit me
to a
> newt of more modest size.
>   So .... I have noticed that Newport offers conical mirror blanks for
sale.
> I am considering ordering one, of maybe 10" diam.   I'm just wondering how
big
> a deal it is to do a conical mirror.

Although a parabola is a conic section, I believe "conical mirror" usually
means the primary for a Schmidt-Cass system.  The back side of the mirror is
actually a gentle cone shape.  They are thick and heavy, because the mirror
is supported by the hole in the middle.

> I'd like a mirror which is a little lighter than normal and cools quickly.
>   Would I be better served grinding a very thin mirror instead?

A "very thin mirror" (meaning thickness/diameter ratio less than 1/8) were
developed for larger apertures, 16 inches and up to save weight and cost.
Cool down times depend mostly on thickness, because heat escapes mainly
through the face and the back. A 10 inch by 1.5 inch mirror will actually
cool down faster than a 20 by 1.5, and the optical quality will likely be
better while it is cooling.  Some people have made ultralight travel scopes
with small thin mirrors like 8 by .75 inch.  Thin mirrors are more difficult
to figure and to mount because they flex.  A really good one requires mirror
making experience.

For a Newtonian telescope, 10 to 12 inch aperture, your path of least
resistance to high quality instrument is to make a "normal" flat back
mirror, 1.5 to 2 inches thick.  Save weight with a simpler cell and a truss
tube, and lighter weight rocker box..  Many websites have good information,
but presentations on websites are by necessity brief and limited.  Consult
the standard books, such as Texereau,  Berry,  Kriege and Berry, or the new
one by the Lecleires, for more complete and organized information on what's
involved and how to do it.

>   What kind of problems might I have grinding my own conical mirror?  I
have
> the focault tester and compounds already.

Many writers describe mirror making as a childishly simple process, easily
done with common low tech equipment.  This is basically true, but a
first-time 10 inch mirror is not a purely mechanical exercise, there are
things to learn, and you will not save much money for the time invested.
People who enjoy the challenge and the process of making mirrors can make a
good one and feel rewarded by the experience..

It takes about 30 - 40 hours of physical labor, and another 20 - 40
learning/experience. Grinding is not too hard. Polishing is harder
physically, and problems such as TDE and astigmatism can occur. You must
learn to judge pitch hardness and care for laps.  Parabollizing and figuring
involves learning to interpret the Foucault test and understanding where
glass needs to be removed and where it needs to be put back (Hah!). You also
learn by instruction and experience how to get the pitch lap into good
contact and how good pitch lap acts and feels.  Various strokes and
pressures remove glass in various places. Many smart people find the
behavior of pitch laps and polishing to be non-intuitive and frustrating.
You are working toward a shifting reference parabola. You can apparently
turn down the edge by rubbing the center. Pitch can be temperamental and do
unexpected things, leading to frustration.  It takes some perseverance and
experience to sort these effects out.  Knowledge is required (yours, a
teacher's, a book or the ATM list) to recognize and recover from the
figuring errors that will occur.  Therein lies the challenge, and the Zen,
and that is why we get hooked.

Clear Skies,

Stuart


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