Optics-Talk-With-MikeL: Difference between revisions

Latest revision as of 11:54, 9 March 2007

Fred,

1) matching the corrector to the mirror is the expensive part ?

The quality optical glass and the labor involved to grind, polish and figure it are the expensive parts.

If the mirror is parabolic (and not spherical), then no corrector plate is needed. This is a classical cassegrain design. Just add a custom made hyperbolic secondary.

If the mirror is spherical, then the mirror can either be refigured (not too expensive) or a corrector plate can be made (expensive). If it is left spherical the corrector plate compensates for it, and that is a schmidt-Cassegrain design.

For commercial scopes, they make crappy correctors very quickly and cheaply and this is cheaper than correcting the primary. For custom telescopes, fixing the primary is far cheaper.

> The mirror is already a finished item so I thought to adapt to it.

It could still be refigured, and probably should be tested, depending on where it came from.

> 2) You're not forced to use the long effective F ratio if you accept a > larger secondary -- is this correct ? > ------ I'd be most interested in a wide field light bucket.

If you want wide field, I would not make a Cassegrain. Use the primary alone and make a Newtonian. If it is spherical I can figure it to the require parabolic figure.

> So what if one did a classical cassegrain accepting a large > secondary ? possible / practical or not ?

The minimum F/ratio you're going to get is F/10, and that's going to require a huge secondary, around 8" by my calculations, and more if you need a lot of back focus. That's still a 200" focal-length instrument.

Is this for visual use or imaging?

If you really want a wide-field Cassegrain, I'd start with a much faster primary.

Mike L.

Conclusions[edit]

For a wide field scope its best to to a Newtonian scope

@@ Line 1: / Line 1: @@
- My name is Fred Rockenberger.  I got your name from Andrew Aurigema
+Fred,
-> while talking to him about setting up a casting works.
-Sounds like a lot of fun.  Andrew has told me a little about it.
-> I am coming into possession of an ~F/5 20 inch cassegrain mirror.   It
+)  matching the corrector to the mirror is the expensive part ?
-> has no corrector plate or secondary associated with it.
-> What kind of money is involved in making the aforesaid?
-Corrector plates are expensive, and I haven't made one yet, but I plan
+The quality optical glass and the labor involved to grind, polish and
-to in the future.  I have made several classical Cassegrain secondary
+figure it are the expensive parts.
-mirrors, and I will be refiguring Andrew's in the near future.  A
-corrector plate is not necessary for a classical Cassegrain.
-Permit me to ask you a few questions about the use of the telescope.
+If the mirror is parabolic (and not spherical), then no corrector
-I have no idea what you know about telescope design, so I apologize if
+plate is needed.  This is a classical cassegrain design.  Just add a
-you have already worked through some of these issues.
+custom made hyperbolic secondary.
-What will the telescope's purpose be?  What is the desired system
+If the mirror is spherical, then the mirror can either be refigured
-focal ratio?  What are the constraints on weight and length of the
+(not too expensive) or a corrector plate can be made (expensive).  If
-tube?  Have you considered designs other than a Cassegrain?
+it is left spherical the corrector plate compensates for it, and that
+is a schmidt-Cassegrain design.
-If the primary mirror is F/5, that's a bit long for a normal
+For commercial scopes, they make crappy correctors very quickly and
-Cassegrain, especially in that size.  The final focal ratio would end
+cheaply and this is cheaper than correcting the primary.  For custom
-up at F/15 or F/20, probably, and that's a very high power system.  It
+telescopes, fixing the primary is far cheaper.
-also means the telescope's tube will be as long as a 20" F/4 telescope.
+> The mirror is already a finished item so I thought to adapt to it.
+It could still be refigured, and probably should be tested, depending
+on where it came from.
+> 2)  You're not forced to use the long effective F ratio if you accept a
+> larger secondary -- is this correct ?
+>       ------ I'd be most interested in a wide field light bucket.
+If you want wide field, I would not make a Cassegrain.  Use the
+primary alone and make a Newtonian.  If it is spherical I can figure
+it to the require parabolic figure.
+>       So what if one did a classical cassegrain accepting a large
+> secondary ?   possible / practical or not ?
+The minimum F/ratio you're going to get is F/10, and that's going to
+require a huge secondary, around 8" by my calculations, and more if
+you need a lot of back focus.  That's still a 200" focal-length
+instrument.
+Is this for visual use or imaging?
+If you really want a wide-field Cassegrain, I'd start with a much
+faster primary.
 Mike L.
+== Conclusions ==
+For a wide field scope its best to to a Newtonian scope

Optics-Talk-With-MikeL: Difference between revisions

Latest revision as of 11:54, 9 March 2007

Conclusions[edit]

Navigation menu

Search