[ATM] Re: 40" mirror

["Mike Conran" <mike@guidesys.com>]

Hi Jerry,

We just completed a 30 inch F/3.8 this summer and in conjunction with a 
Parcor found the views to be great. 


Here is a website on the scope: 

 

http://www.mbm30.org/ 

 

We haven't had sufficiently good steady viewing condintions and
installation of fans, edc to really test it hard but our current assesment 
tells us we like what we see!  The key issue with fast mirrors are you need 
to have a pretty darned high degree of accuracy in order to get performance. 
Good figuring is definitely doable with the use of multiple sized 
subdiameter tools and soft pitch. 

One real advantage with a lower F value on these large guys is viewing 
heights and overall manageability in the field.  In our case an 8 foot step 
ladder does the trick quite easily.  Let's say you have a 40 inch F/6. 
That's a 20 foot focal length.  At zenith you can find yourself 20 feet 
above the ground in the dark!  Something to think about. This is assuming 
you are making a standard Newtonian. 

An F/4 takes that length down to a more manageable 13.3 feet. 

Now a 40 inch scope is much larger that a 30 so everything is scaled up.  
Weights will be subsantially more.  Our 30 weighs abut 300 pounds and two 
people can manage the movement and assembly of the scope.  Takes about 15 
minutes to setup and is trasported in a minivan, no problem. You will need 
to assess how you can transport the scope. I'm sure you will figure that one 
out. 

When you are making the scope, you will find that you will need to develope 
systems devoted to proper handling of the mirror during its production and 
testing. It is for your own safety and the mirror's.  In other words, over 
engineer things like test stands, grinding machines (if you are doing it on 
a machine) and be patient because these large scopes will have their own 
inertia that can dictate the speed of its completion. You definitely will 
learn a lot about a lot things related to its creation. 

Have fun! 

Mike Conron 

 

 


Jerry B. Hillman writes: 

> Hello everyone, 
> 
> Since I am going to invest a good deal of money and a considerable amount of
> time in the venture of melting the glass and casting a 40" mirror, I would
> like to hear anyones input about very large mirrors.
> I am interested in hearing about F ratios compared to useability and
> difficulty in manufacture.  I don't want to invest a lot of time making an
> F2 only to find out that uncorrectable coma is the inevitable result.
> Don't misunderstand me.  I have no idea if an F2 would have uncorrectable
> coma.  I just pulled an example out of my hat to help explain what I need.
> The first piece of glass I am going to cast is for the secondary.  I need to
> know what the F ratio of the telescope is going to be before I take that
> step so that the glass will be large enough.  I know that starting with the
> secondary is sort of putting the cart before the horse, but casting and
> annealing the secondary will tell me if my methods are feasible for a larger
> mirror.
> Clear skies, Jerry 
> 
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Mike Conron 

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