Re: ATM couple questions...

[StepHydro@aol.com]

Roger,

There is a saying something like this. You can make an 8" plus a 16" in less 
time than you can make a 16" as your first mirror. The original sizes were 6" 
and 12" but the principle applies. I *do* think that a meticulous, determined 
beginner can do a 16" IF...the f/ratio is not too short (there is no such 
thing as a "fast" mirror... that photographic term is based upon an 
assumption concerning f/ that does not apply to telescopes, even when used 
for astrophotography. This assertion is easily supportable, but I won't do so 
here as it isn't the topic :-) and the glass is not ultra thin.

I'd *really* recommend that you go no shorter than f/4.5, and that will be a 
real challenge to a first time mirror-maker. I'd further recommend that you 
use 1 5/8" thick pyrex, or at the very least, 1 3/8". This can be obtained 
form United for a reasonable price, and they will machine-generate the curve 
at a price that makes your labor worth less than $4 per hour. Further, yolu 
will have exactly the courve you want and little liklihood of moving it very 
far in find grinding.

Now, having gotten the blank with the curve generated, You'll need to have a 
tool to match. Make it of reinforced (chicken wire or hardware cloth) 
concrete, epoxy coated and tile covered. Please email me privately if this 
sounds like fun, and I'll give you a contact who has done all this. He will 
give you contact names and phone numbers at United Lens.

However, I would most seriously recommend that you start with an 8" mirror. 
It is almost certain that having made one mirror, you will want to make 
others. You can buy from Willman-Bell an 8" kit for about $125 wehich has 
plenty of material to make two mirrors. An 8" Dob is dead easy to make, and 
your 16"er is likely to take the better part of 6 months to complete. Why not 
have an 8" around in the meantime. I assure you that you will have no trouble 
selling the two good 8" mirrors for $150 each, uncoated if you correct them 
to 1/12th wave and better both criteria of Danjon & Couder. This is easy for 
a determined amateur to acheive. Make the first f/6 and the second f/4. 
You'll need the practice of the short FL when you get to the 16. 
<A HREF="http://www.willbell.com/">Welcome to Willmann-Bell, Inc.</A> 

Make a good foucault tester with a dial indicator. Lots of websites on this.

Buy a copy of ADMIR from W-B. It is the best tool you have to keep from 
making a major error in figuring. It also increases you chances of achieving 
a really good mirror as it helps develop a figuring strategy.

Make a good 18 point cell for the 16"er.

A machine will make for less labor, but won't make a better mirror than 
handwork. As a matter of fact, it makes some things more difficult, as it 
strokes in a regular rather than semi-random fashion, and produces zones that 
have to be "strategized" out. I'd certainly recommend that the first 8"er be 
done by hand so that you understand the process intimately.

These are methods that have helped dozens of people make successful mirrors 
(and one dog :-) but are just my opinions. Others have made successful 
mirrors using different ideas. However you choose to proceed, mainly plan to 
have fun. There isnt much greater thrill than finishing your calcs (or having 
ADMIR finish them) and realizing that your hands have produced a surface 
accurate to 1/40th a wavelength of light (1/20th on the wavefront). You *can* 
do it.

Cheers/Carron


In a message dated 99-07-16 13:07:34 EDT, mailman@localline.com writes:

> I am now looking seriously into grinding my own. The questions I
>  have are:
>  
>  1. I will probably only do one. I am rather meticulous in my practises, so
>  I am not afraid of a large mirror. And I do want the largest primary that I
>  can. I need recommendations as to the largest a dedicated "newbie" should
>  attempt. I am thinking along the lines of 16", but if too many people think
>  this is beyond capabilities, than I will look at purchasing one. What would
>  be a good focal length of such a beast? It will be primarily stationary, so
>  size isn't the major problem, I just want the best one I can build.
>  
>  2. If 16" is reasonable, does anyone have a website, etc, on building a
>  grinding machine? I have woodworking tools and skills, and understand this
>  might be the way to go...comments?
>  
>  3. Mounts - I intend open truss, but definitely want it EQ, recommendations
>  on that as well...split ring appeals to me, but have seen some piers that
>  look promising as well...plan to be computer controlled, have those 
skills...
> 
>  4. Where can I find quality sources for blanks, compunds, etc? I searched
>  the web, but haven't found any that seem to have a good page.