Re: [ATM] beginner's questions

[Mark Holm <mdholm@telerama.com>]

I'll answer some of the questions.

haren burke wrote:
> Several questions from a beginner. Project is to be portable 10" f/6
> newtonian for visual and imaging.
> 
> Glass - I have a stack of old windows and would like to know the
> feasility of glueing (epoxy) several pieces, cut to size, of this glass
> to build up a stack of appropriate thickness. I was going to vacuum bag
> the assembly to eliminate air bubbles in the glue. Could it then be
> annealed in a kitchen oven (first it needs a preheat out-of-doors to
> drive off volatiles)?

The short answer is this will not work.  ATMs and pro's have been trying 
to come up with some way to laminate glass for mirror blanks for a long 
time.  Epoxy hasn't got a chance, based on previous reported attempts. 
The only method that has worked reasonably well, and it isn't so easy or 
at all foolproof, is heat fusing.  For a first mirror, especially, stick 
with an ordinary Pyrex or plate glass mirror blank of 1:6 - 1:10 
thickness ratio.

> 
> Grinding supplies - Got Grit and Newport Glass are two sources; are
> there any others for continental U.S.? Can I rely on their sieving or
> do I need to do my own elutriation?

They should be fine.  No need to elutriate.  Do be paranoid about cross 
contamination while you are using them.  Do not open a finer grade until 
you have cleaned up completely from the previous grade.  Oh, abrasives 
often come packed in polyethylene bags these days.  Abrasive grains will 
cling to polyethylene by static electricity.  Best to keep the unopened 
bags of finer abrasive completely away from opened bags.  I actually 
soap and water wash the outside of each bag before I open it to remove 
any stray grains that might be clinging.  It takes a couple of minutes 
each time, but can save a lot of regrinding.

> Mirror coating - Aluminum and a protective SiO2 layer (which yield
> about 84% relectivity) I understand but are the enhanced coatings worth
> the expense (claims of 94% to 97% reflectivity)? How about silver, an
> initial chemical deposition of it followed by an electroplating
> treatment? Silver tarnishes quickly but are there protective coatings
> for it?

For a first mirror, I would strongly advise overcoated aluminum.  I 
think some outfits do protected silver.  I still think aluminum is best 
for a first mirror.

Enhanced aluminum might be worth it on the diagonal mirror.

Here is a link from Melles-Griot, a major optics firm (I don't think 
they do ATM mirrors, but their statements probably apply to most 
coaters.)  http://www.mellesgriot.com/products/optics/oc_5_1.htm

Remember, when you look at reflectance spectra, that human vision peaks 
strongly in the 500 - 550 nm range.  Below about 450nm and above about 
600 nm, your visual sensitivity is naturally down by a large amount.  A 
few percent difference from the coating isn't going to make much 
difference.  So, don't worry when curves fall off a bit outside the 450 
- 600 nm range.

> Mersenne design - There've been a few past threads on this topic but my
> understanding of it is that it produces a compressed, collimated ray
> bundle but no image plane. My efforts to simulate it in OSLO have done
> poorly so I wonder if someone skilled in optical principles could
> explain how it produces a useable image, eyepiece or ccd, and also
> provide its design prescription for OSLO edu?

The Mersenne is an interesting design.  It produces a usable image 
because there is a second telescope, usually a refractor, after the 
Mersenne secondary.  I don't know a lot about the details.  I think 
field of view may be an issue.  I do know it is not a beginner's project.


-- 
Mark Holm
mdholm@telerama.com

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