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Equipment Discussions >> ATM, Optics and DIY Forum

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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Refiguring a Cassegrain new [Re: KerryR]
      #4312170 - 01/13/11 03:44 PM

I've polished secondary mirrors for DKs down to about 40mm diameter, with a little forethought and care. Fairly similar to big stuff. Biggest mindset to alter when polishing- The item is far smaller in area, so lighten up on the pressure!!!!! Grinding goes fast, and can be done with 220 or finer grit on long radii. Common sense combined with a couple regular mirrors work well here.
FWIW, M.


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gregj888
professor emeritus


Reged: 03/26/06

Loc: Oregon
Re: Refiguring a Cassegrain new [Re: Mark Harry]
      #4313211 - 01/13/11 11:25 PM

Just some ramblings…

DKs are generally f/14 or longer to get reasonable performance.

You can add 2 lenses near the focal plane and make a corrected DK. I know you can get good performance to f/8. I don’t know if a 3-4 lens corrector would get to f/6 or not. Anyway, the CDK might be worth a look. Mark may want to comment.

If you can find a PCX lens with the right ROC it can be used as a secondary.

The primary of the DK is difficult to correct for prime focus work if that’s an issue.

A Classical Cassegrain is probably the most flexible. Different secondarys can give you different final focal lengths, back foci etc. If you add a 2 element corrector it’s better than a CDK. The primary is a parabola so it’s good to go.

If the CC’s secondary is made of optical glass, it can be figured from the back to a null. See the “Advanced Telescope Making Techniques” page 57 (Leonard). The secondary can also be figured kind of directly with the Gaviola test: ATMT, page 59. Both of these should also work for an RC.

Have fun,

Greg


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4389505 - 02/16/11 02:09 AM Attachment (76 downloads)

Time for an update with this project.
After a lot of discussion with the owner it was decided to refigure the optics as a Dall-Kirkham using the larger f/6.5 secondary. Here's a pic. taken with the existing optics. The main faults look to be focus and tracking. No doubt coma will be more noticable when those are improved but still not too bad. The spikes on one side of the stars are from the TDE but the primary baffle's not aligned 100% and is vignetting the edge on one side. When I did some star testing I removed it and the spikes were visible all around. We will look at coma correction later.

Dave


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4389510 - 02/16/11 02:15 AM Attachment (62 downloads)

I managed to take a ronchi (85 lpi) through the back of the smaller (f/ 9.5) secondary as it isn't as scratched as the bigger one. Another rough figure and turned edge and I assume the larger one is similar.

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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4389514 - 02/16/11 02:21 AM Attachment (69 downloads)

I've started polishing the primary, concentrating mainly on the TDE. Still a little edge left but it's looking a lot smoother. Diffraction ring is finally starting to appear. Not showing so well in the pic. though. I'm now close enough that I'll make a couder mask and start taking measurements. The conic works out to be -.555 btw.
Foucault:-

Edited by dave brock (02/16/11 05:06 AM)


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4389523 - 02/16/11 02:27 AM Attachment (60 downloads)

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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_BBi
member


Reged: 11/27/09

Loc: South Africa
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4391904 - 02/17/11 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.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: _BBi]
      #4391971 - 02/17/11 05:13 AM Attachment (49 downloads)

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

Edited by dave brock (02/17/11 05:20 AM)


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DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4392267 - 02/17/11 10:09 AM Attachment (56 downloads)

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

Edited by DAVIDG (02/17/11 10:10 AM)


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: DAVIDG]
      #4392500 - 02/17/11 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


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4785895 - 09/03/11 06:36 AM Attachment (47 downloads)

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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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4785897 - 09/03/11 06:37 AM Attachment (42 downloads)

Final Ronchi

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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4785904 - 09/03/11 06:44 AM Attachment (43 downloads)

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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Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: Refiguring a Cassegrain new [Re: dave brock]
      #4786039 - 09/03/11 09:04 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!


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: Mike I. Jones]
      #4786922 - 09/03/11 05:16 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


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mark1234
member


Reged: 04/15/08

Re: Refiguring a Cassegrain new [Re: dave brock]
      #5656894 - 02/01/13 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


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: mark1234]
      #5657288 - 02/01/13 10:35 PM

Quote:

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


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mark1234
member


Reged: 04/15/08

Re: Refiguring a Cassegrain new [Re: dave brock]
      #5657758 - 02/02/13 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


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Refiguring a Cassegrain new [Re: mark1234]
      #5658550 - 02/02/13 05:23 PM

As advised... PM sent

Dave


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EJN
Carpal Tunnel
*****

Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Refiguring a Cassegrain new [Re: mark1234]
      #5658741 - 02/02/13 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.


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