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Refiguring a Cassegrain

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#26 dave brock

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Posted 16 February 2011 - 02:27 AM

Ronchi 133 LPI.
A bit edgy around the central hole but as the secondary is 162mm it'll be well covered.

Dave

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#27 _BBi

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Posted 17 February 2011 - 02:44 AM

Looking much better.

How did you attack the TDE on a Cass?
Seen that the centre is not filled in, you obvoiusly cannot use a full size lap.
 

#28 dave brock

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Posted 17 February 2011 - 05:13 AM

I've been using a 110mm (4 1/4") lap with facets tapering toward the edge. I don't scratch the lap like a lot of people here but press microfacets in. I use circular, oval, and chordal strokes. When using the chordal stroke although I do rotate often I tend to hold the lap so I'm stroking across the channels which helps to avoid zones. I actually have two laps, a hard one (shown) for use during the day and a softer one if I'm polishing at night. We're coming off summer here and it's quite hot in the garage during the day.

Dave

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#29 DAVIDG

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Posted 17 February 2011 - 10:09 AM

Dave,
If your going to figure the secondary to convex hyberbola, you might consider testing thru the back and moving the light source to a calculated distance. The result will be a null when the convex surface is correctly figured.
Here is a outline of the test. I worked this out for a hyberbolic Cass secondary that I'm making from a piece of optical quality quartz but it's easy to do with any type of glass using OSLO.
There is also another method of adding a concave spherical surface to the back of the secondary to form a menicus. Again it's easy to calculate the needed radius to result in a null when tested thru the back and you achieve the correct aspheric surface of the convex part. In this method both the light source and focus are at the same position. You can also just use a plano concave lens with the needed radius on the concave surface, then place the two "flat" surface together with a drop of oil between them. When you do this the two "flat" surface don't need to be optically flat any longer. If you want me to calculate any of these conditions for you, then please provide me with the glass type, secondary thickness, ROC and conic needed on the secondary.

All the Best,
- Dave

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#30 dave brock

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Posted 17 February 2011 - 12:25 PM

Thanks Dave.
Unfortunately the back of the secondary I'm using is so badly scratched/ground it's not possible to see through it. It's plate glass and I'm going to do a Dall-Kirkham so it'll be spherical. I'll test using a Ronchi on a star then startest. I know this is going against the advice on this thread. If I fail with this method then I'll grind and polish the back and take up your kind offer.

Dave
 

#31 dave brock

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Posted 03 September 2011 - 05:36 AM

Time for an update.
The refigure is now complete and the owner has been doing some imaging. Results so far have been encouraging.
In the end I refigured the f/6.4 secondary using only the star test. What made it easier was a program called RonchiExplorer which allows you to input a mirror profile and simulates the corresponding star test.
Here's the finished primary foucault. It looks a little stiggy but it isn't, I didn't have the knife edge properly aligned.

Dave

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#32 dave brock

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Posted 03 September 2011 - 05:37 AM

Final Ronchi

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#33 dave brock

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Posted 03 September 2011 - 05:44 AM

Here's a pic. of NGC253. It was taken using a Baader MPCC coma corrector. Collimation still needs a tweak but, to my eye at least, considering this is an f/6.4 Dall-Kirkham with an off the shelf corrector, the coma isn't too severe.

Dave

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#34 Mike I. Jones

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Posted 03 September 2011 - 08:04 AM

Dave,
Thanks loads for your update. What an improvement over what you started with! Now that is one beautifully figured, smooth primary. I see very little evident, but any residual TDE or zoning immediately around the hole will be shadowed by the secondary baffle tube anyway. The NGC253 image is definitely better. Good work! :waytogo: :thewave: :bow:
 

#35 dave brock

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Posted 03 September 2011 - 04:16 PM

Thanks Mike.
There is a TDE around the central hole that I didn't bother to spend the time on. As you say, the baffle will cover it not to mention the 40% secondary obstruction.
The owner is very happy.

Dave
 

#36 mark1234

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Posted 01 February 2013 - 06:11 PM

Interesting work.
The configuration the original maker was aiming for is a well known variation with an extreme example being a spherical primary and a severely oblate figure on the secondary. Any secondary can be figured just using the star test as you have done and I have done, but large departures from a sphere are more difficult because you are relying on the seeing to reveal subtle errors with possibly many magnitudes and sources.
The guy did his best with what tools he had at hand.
Since he isn't posting or reading the forums here, we have no way of ascertaining the validity of your remarks.
Since the specifics of the scope could identify him, doesn't that bother you?
Moderator please review or I'm off.

Mark
 

#37 dave brock

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Posted 01 February 2013 - 10:35 PM

The guy did his best with what tools he had at hand.
Since he isn't posting or reading the forums here, we have no way of ascertaining the validity of your remarks.
Since the specifics of the scope could identify him, doesn't that bother you?
Moderator please review or I'm off.

Mark


Hi Mark.
Perhaps you could highlight the remarks that I've made that you are referring to, then I can answer your question.

Dave
 

#38 mark1234

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Posted 02 February 2013 - 09:23 AM

Dave,
this is an interesting and helpful thread.
I feel obliged to explain, although it is some time ago the post was made. The internet never forgets though, which should be borne in mind.
You guys got the chance (it did sound like these optics were free or thereabouts) to show your skills and knowledge and there was an improved outome.
post 4310860 and subsequent interpretations of what 'difficult to talk to' might mean doesn't set a good example for sticking to the optics.
I was surprised to see this creep into an otherwise sensible discussion.
I've met some real characters in ATM, they all have something to contribute, even if just mute optimistic enthusiasm.

Mark
 

#39 dave brock

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Posted 02 February 2013 - 05:23 PM

As advised... PM sent

Dave
 

#40 EJN

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Posted 02 February 2013 - 07:09 PM

Let me see if I follow this correctly: You bring back a thread after 17 months
because you are upset that an unnamed person was described as 'difficult to talk to'
and you think the mirror specs could identify this unnamed person?


This is just....bizarre.
 

#41 Pinbout

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Posted 02 February 2013 - 11:45 PM

Dave luv seeing the shadow grams again, so smooth :grin:
 

#42 dave brock

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Posted 03 February 2013 - 12:01 AM

Thanks Danny.
All in all, the work I'm most proud of. Particularly the secondary.

Dave
 

#43 jasonharris

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Posted 04 February 2013 - 01:08 AM

The internet never forgets though, which should be borne in mind.

Mark.


The last post was a bout a year and a half ago. Looks like it was well on it's way to being forgotten.

It reminds me of silly stories people take to their newspapers about things that they dont like. All of a sudden something they didnt like being known is all over the headlines because they made a fuss about it.
 

#44 ausastronomer

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Posted 04 February 2013 - 03:32 AM

This thread has run its course and can only go downhill from here. Dave has done a fine job of refiguring the optics and the scope has been returned to it owner, who is happy with Dave's work. One can only question the reasons for resurfacing this thread when it was done and dusted over a year ago.

Thread now locked.

Cheers,
 






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