Good People,
While breaking in my new mirror machine, I overshot the sag on a standard 6" pyrex blank. Actually, I was going for f/4.25 and reached f/2.7 much more quickly than I expected. (I wasn't paying the attention I should have been.)
Anyway, instead of TOT to bring it back for a short-focus Newt., I'm considering a Gregorian scope with a Nasmyth focus. I've never built one before.
I would appreciate the wisdom of this group.
1. How difficult is it to figure a small concave secondary? Are there any special considerations?
2. Are there any issues regarding stray light in a Nasmyth focus, especially when using a concave secondary that doesn't mask incoming light as well as a convex secondary (being farther away)?
3. If the figure of the primary is a standard parabola, would the Gregorian secondary also be a parabola? What design would use a spherical secondary? Seems that would be easiest since it is relatively small.
4. It would seem fairly straight-forward. A curved spider for the secondary and a simple single-vane stalk for the tertiary flat. Any thoughts?
5. Would you have any online sources to recommend for design and parameters? I have Texereau, but my intuitive/subjective mind has a little trouble completely understanding him. Maybe something is lost in the translation. 8O
I hope I'm not just playing with a paper telescope here, but that would of course be my first step. I'm hoping some of you have attempted this before.
Many thanks!
Steve House
While breaking in my new mirror machine, I overshot the sag on a standard 6" pyrex blank. Actually, I was going for f/4.25 and reached f/2.7 much more quickly than I expected. (I wasn't paying the attention I should have been.)
Anyway, instead of TOT to bring it back for a short-focus Newt., I'm considering a Gregorian scope with a Nasmyth focus. I've never built one before.
I would appreciate the wisdom of this group.
1. How difficult is it to figure a small concave secondary? Are there any special considerations?
2. Are there any issues regarding stray light in a Nasmyth focus, especially when using a concave secondary that doesn't mask incoming light as well as a convex secondary (being farther away)?
3. If the figure of the primary is a standard parabola, would the Gregorian secondary also be a parabola? What design would use a spherical secondary? Seems that would be easiest since it is relatively small.
4. It would seem fairly straight-forward. A curved spider for the secondary and a simple single-vane stalk for the tertiary flat. Any thoughts?
5. Would you have any online sources to recommend for design and parameters? I have Texereau, but my intuitive/subjective mind has a little trouble completely understanding him. Maybe something is lost in the translation. 8O
I hope I'm not just playing with a paper telescope here, but that would of course be my first step. I'm hoping some of you have attempted this before.
Many thanks!
Steve House
