Re: ATM 8" refractor project

[Aplanatic@aol.com]

Hi David,

> I'm going to be particpating in the 8" refractor class at the western
> washington university telescope optics workshop.  I want to prepare a few
> things in advance: spherometer and wedge tester.

That sounds like a great project, have fun.
 
> For the spherometer, there seems to be a lot of talk back and forth about
> what is suitable for a dial indicator/micrometer.  Some people have said
> that the cheap .001" dial indicator is fine.  Others say that you need
> .0001", which would require either a micrometer spindle, dial test
> indicator, or precision dial indicator.  I have found .0001" dial
> indicators with very limited travel, .0005" with suitable travel, and of
> course the standard cheap .001", but nothing of the sort that Roger
> Ceragioli calls for in his instructions: .0001" with 1/4" travel.  Is it
> simpler to just take a hacksaw to a micrometer to salvage the spindle?

I now have a fancy digital dial indicator that reads to 1 um, but for many years I used an indicator that I picked up at a surplus shop for $30.  It reads 0.0001" and has 1/4" travel.  You might be able to find something like this if you look around.

To answer the question of accuracy and resolution, you need to know what the tolerances are on the radii.  Typically, the most sensitive surface on a refector lens has a tolerance of roughly 0.25%, but this can vary a lot with design.  If the lens has a sagitta of 10 mm, then you're indicator should have an accuracy of 25 microns (0.001") and somewhat better resolution.  To be very safe, you really want an indicator with a factor of three better accuracy and resolution than this.  I'm sure that's why Roger makes the recommendation to get one that reads to 0.0001".

Over the years I've found that good tools, materials and instruments are worth what you pay for them (or the time it takes to find good surplus ones.)  A good dial indicator is a valuable device for a lot of things.  Get a decent one.
 
> What size base should I be making for working on an 8" lens?  I'd guess
> 5-6".  I have access to a machine shop which I'll make use of for this
> project.   Should I try to make a fancy base with multiple tapped holes
> for the feet, or just keep it simple and permanently fasten the feet.
> I've seen some plans for adjustable spherometers of this sort which seem
> interesting.

Since you have acess to a machine shop, make two bases: one 3" diameter and one base 7" in diameter.  If you enjoy making lens and mirrors, these bases will be a valuable addition to your tool set.  I would not make an adjustable spherometer.
 
> For the wedge tester, I think I've got the general idea.  I'm going to
> take a sheet of melamine (laminated particle board), and put in some
> threaded inserts for holding the feet and the stops, and for the dial
> indicator post.    No need for metal or high precision in 
> the construction
> of a wedge tester, right?

Since you have access to a machine shop, why not make one really nice wedge tester?  It's very important for refractor lenses to measure and remove wedge accurately.  So, again, take the time to make a nice instrument.  You won't be sorry in the long run.

-Dave-