Optics-Talk-With-MikeL: Difference between revisions

Revision as of 11:53, 9 March 2007

Fred,

www.embeddedrf.com wrote: > 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.

@@ 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.
+www.embeddedrf.com wrote:
+> 1)  matching the corrector to the mirror is the expensive part ?
-> I am coming into possession of an ~F/5 20 inch cassegrain mirror.   It
+The quality optical glass and the labor involved to grind, polish and
-> has no corrector plate or secondary associated with it.
+figure it are the expensive parts.
-> What kind of money is involved in making the aforesaid?
-Corrector plates are expensive, and I haven't made one yet, but I plan
+If the mirror is parabolic (and not spherical), then no corrector
-to in the future.  I have made several classical Cassegrain secondary
+plate is needed.  This is a classical cassegrain design.  Just add a
-mirrors, and I will be refiguring Andrew's in the near future.  A
+custom made hyperbolic secondary.
-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 spherical, then the mirror can either be refigured
-I have no idea what you know about telescope design, so I apologize if
+(not too expensive) or a corrector plate can be made (expensive).  If
-you have already worked through some of these issues.
+it is left spherical the corrector plate compensates for it, and that
+is a schmidt-Cassegrain design.
-What will the telescope's purpose be?  What is the desired system
+For commercial scopes, they make crappy correctors very quickly and
-focal ratio?  What are the constraints on weight and length of the
+cheaply and this is cheaper than correcting the primary.  For custom
-tube?  Have you considered designs other than a Cassegrain?
+telescopes, fixing the primary is far cheaper.
-If the primary mirror is F/5, that's a bit long for a normal
+> The mirror is already a finished item so I thought to adapt to it.
-Cassegrain, especially in that size.  The final focal ratio would end
-up at F/15 or F/20, probably, and that's a very high power system.  It
+It could still be refigured, and probably should be tested, depending
-also means the telescope's tube will be as long as a 20" F/4 telescope.
+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.

Optics-Talk-With-MikeL: Difference between revisions

Revision as of 11:53, 9 March 2007

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