dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Refiguring a Cassegrain
#4308721 - 01/12/11 02:28 AM Attachment (117 downloads)
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I've been given a 16" Cass to evaluate and possibly refigure. I've done a very quick test of the primary and it has only about 30% correction toward a parabola. It's also got a rough surface and a TDE. Pics to follow. The owner believed it to be a Classical Cass. but it is evidently closer to a Dall Kirkham. I assume the secondaries (there are actually two of them, giving different focal ratios) are of similar quality. Startest actually shows the overall correction is reasonable but in focus star images are poor due to the TDE and surface roughness. It also shows a lot of coma.(Dall Kirkham?)
I've very little experience with polishing cass. optics and don't even have a spherometer so I have a question for the experts.
It seems the easiest option is to repolish the primary to fix the TDE and smoothen the surface but keep the overall correction as is. Recoat it and then repolish the secondaries using the startest to test or possibly test through the back. Use a comacorrector.
The other option is to refigure the primary to a parabola and refigure the secondaries to a hyperbola using startest. Is this feasible using the existing radii of curvature?
Here's the foucault.
Dave
Edited by dave brock (01/12/11 03:01 AM)
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4308727 - 01/12/11 02:34 AM Attachment (92 downloads)
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Ronchi 133 lpi
Edited by dave brock (01/13/11 12:19 AM)
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4308737 - 01/12/11 02:47 AM Attachment (94 downloads)
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Star at focus
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4308861 - 01/12/11 07:05 AM
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What's the focal ratio of the primary? And do you know what surface is on each of the secondary mirrors? I have heard that Dalls can be quite good, though the correction you seem to have doesn't match what I'm accustomed to seeing. Generally the correction of the Dall primary has to be in the area of 60-70% with a spherical secondary mirror.
Mark
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Mark Harry]
#4308878 - 01/12/11 07:23 AM
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The primary is 1230mm focal length so just over f/3. The 60-70% correction you mention is what I've been told is usual as well. I've read that the D K needs to be kept with a longer focal ratio to keep coma at bay. It's main advantage is it's easier to make. I have no idea what surface is on the secondaries at this stage. I'm confident I can improve the primary as there's really nothing to beat and I can take it to say 65% then polish the secondaries spherical. The owner wants to use it for imaging though (deep sky) and prefers the secondary giving the smaller f/ratio hence the bad coma.
Dave
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Loc: Fort Worth TX
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4309022 - 01/12/11 09:16 AM
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Nasty, zoney figure on the primary for sure. Put up all the Cass parameters for this system (amplification, radii, separation, back working distance, etc.) so we can determine the right amount of primary correction.
Of the RC, classical and DK mirror shapes, the DK has the most coma, but the flattest field and least sensitivity to misalignment. The RC has zero coma, but has the most curved field and strongest hyperboloidal secondary shape, and thus has very tight alignment tolerances. The paraboloid/hyperboloid classical Cass might be the best of both worlds for you. But let's play with some numbers here and see what's what.
Mike
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Refiguring a Cassegrain
[Re: Mike I. Jones]
#4309879 - 01/12/11 04:54 PM
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I'm sure Dave will fix that primary!!! Don't think he'd ever let one like that get by......
Mark
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Mike I. Jones]
#4310161 - 01/12/11 06:35 PM
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Ok, things are getting interesting.
Just did some measuring and have :-
primary 400mm dia. r/c 2460mm f/r 3.075
secondary 162mm dia r/c unknown at this moment.
secondary is 490mm inside primary focus
primary-secondary spacing 740mm
secondary to final focus 1035mm
Am I right that this means an amplification factor of 1035 ÷ 490 = 2.112 which makes the final f/r 6.49!!
I measured the primary central baffle and it is 80mm dia and extends 250mm in front of the primary surface. Looks like it might vignette to me.
I haven't swapped the secondaries at this stage but the other one is 140mm dia. and looks to have a spacing of 860 to the primary.
I will have access to a spherometer shortly so can measure the secondary r/c's in the next day or two.
Btw the 3 mirrors are plate glass and are currently silvered.
Dave
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Mark Harry]
#4310187 - 01/12/11 06:46 PM
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Quote:
I'm sure Dave will fix that primary!!! Don't think he'd ever let one like that get by......
Mark
Yes Mark, I'm confident I can improve the primary, although I've never worked a mirror with a central hole before. Nothing to beat there. It's more the secondaries and the system as a whole. I'm sorting through to see if I want to take this on as it's new ground for me. I'll be doing this for basically nothing so don't want to regrind anything or make test plates for the secondaries.
Dave
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4310192 - 01/12/11 06:48 PM
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Forgot to mention, the secondaries are very rough/scratched on the back so testing through the back will not be possible.
Dave
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4310226 - 01/12/11 07:05 PM
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It's been something of note before; testing thru the back. But I notice with PCX lenses, there is a convergence of a reflection when looking thru the convex side; seeing a larger image of a lightbulb for example. I'm sure Mikey could figure a way to test it from either side.
Mark
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Dick Parker
professor emeritus
Reged: 08/17/07
Loc: Tolland, CT and Chiefland, FL
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4310459 - 01/12/11 09:02 PM
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Dave -
If you are going to consider a Classical Cassegrain
Convex aspheres have one of their conjugates within the reflection, i.e. virtual. It is difficult to have access to it without auxiliary optics.
Without auxillary optics, your best bet might be a star test, or star test with artificial star at a known distance. Before 1859 (Foucault test) that is the way many, if not all, telescopes were made.
There is an article in Telescope Making , issue 31, pg 24 where the author describes a method where he tested the secondary by adjusting the spacing between the primary and secondary of the telescope and established conjugates in front and behind the telescope where the focuser is. The light source was at the conjugate in front and the knife edge was at the back conjugate. This system resulted in known spherical aberration which could be tested by measuring zones. Some ray trace analysis was required. The knife edge was replaced by a wire where the wire was a human hair.
Testing through the back might require that the back be quite flat, or some other curve (for best results). That seems to me like making an auxiliary optic.
Some other auxiliary optic choices are:
1) Spherical concave test plate, test per Texereau. ROCs have to be an exact match. Be careful of the shape of the intereference fringes.
2) Hyperbloidal concave test plate, i.e. the silvertooth method.
3) A Hindle sphere. You would only need a 12 inch or so Hindle sphere, but the curve is deep.
4) A 16 inch flat. You could figure the primary, then have it aluminized, then figure the secondary by testing the whole telescope as a system using autocollimation.
Don't know if that helps, but a good 16 inch Cass is worth going after.
Dick Parker
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wh48gs
Pooh-Bah
Reged: 03/02/07
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4310663 - 01/12/11 10:29 PM
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Quote:
primary 400mm dia. r/c 2460mm f/r 3.075
secondary 162mm dia r/c unknown at this moment.
secondary is 490mm inside primary focus
primary-secondary spacing 740mm
secondary to final focus 1035mm
Am I right that this means an amplification factor of 1035 ÷ 490 = 2.112 which makes the final f/r 6.49!!
Yes, this is a very fast system. For the secondary radius, you can use a simple two-mirror system relation: R2=mkR1/(m-1), where "m" is the secondary magnification, "k" is the relative size of the axial cone at the secondary (in units of aperture diameter), and R1 is the primary r.o.c. With
m~2.1 and k~0.4, that gives R2~0.76R1~1880mm.
In the classical Cassegrain, secondary conic would have been close to -8; in the Dall-Kirkham, the primary would have been about -0.54 (with lots of coma: 1.2mm, or little over 1/40 degree diffraction-limited field radius).
Vla
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Dick Parker]
#4310836 - 01/13/11 12:04 AM
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Thanks Dick.
As I've mentioned, it looks like any work done on the secondaries is going to be tested on a star. Perhaps initially using a ronchi to get in the ball park then startest to finish. I'm picking that since the easier optic (primary) is so poor, the secondaries are likely worse.
Dave
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: wh48gs]
#4310860 - 01/13/11 12:16 AM
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Thanks Vla.
I'm wondering what thought process was in the mind of the original maker. I have met him and I believe he does use the startest when figuring. The obvious thing is to give him a call but I'm hesitant to do that as he's very hard to talk to and would likely take offence to any questions.
Dave
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4311126 - 01/13/11 06:51 AM
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An addition to Dick's fine post, do -NOT- test the secondary by looking thru the concave testplate. The curve would be strong enough to mess up the bands to give an unreliable indication of actual fit.
Mark
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Loc: Fort Worth TX
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4311184 - 01/13/11 07:56 AM
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Quote:
I'm wondering what thought process was in the mind of the original maker. I have met him and I believe he does use the startest when figuring. The obvious thing is to give him a call but I'm hesitant to do that as he's very hard to talk to and would likely take offence to any questions.
If he's so uppity and hard to talk to, show him the focogram you made of his "wonderfully" made, way undercorrected, ultra not-smooth mirror 
Mike
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wh48gs
Pooh-Bah
Reged: 03/02/07
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4311379 - 01/13/11 09:51 AM
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If the primary is about 30% corrected, it takes a strong oblatre ellipsoid to compensate for the undercorrection (in this case, secondary conic should be about 4, optimally around 4.4). No wonder the optics is rough. Seems as if the maker exclusively relied on the star test while figuring - that can take you anywhere. Accidentally or not, this arrangement has zero astigmatism, but the coma is even a bit stronger than in the DK.
Vla
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Mike Lockwood
Vendor, Lockwood Custom Optics
Reged: 10/01/07
Loc: Usually in my optical shop
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Re: Refiguring a Cassegrain
[Re: wh48gs]
#4311670 - 01/13/11 12:28 PM
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I've made several classical Cassegrains.
I'd recommend refiguring the primary to a parabola with your standard techniques.
Testing the primary and secondary in combination will be very difficult unless you have a good flat. Star testing will take a long time and will probably be frustrating.
Don't feel constrained by the secondary that came with it - if a different system is desired/required, then you may need to use a different diameter or regrind to a different radius.
I'd recommend a simple test for the secondary. A concave test plate can be figured to the proper hyperbolic shape using Foucault testing, and then you can figure the secondary to match by straightening the interference fringes between them. This method had a learning curve, but it works. Alternatively you can make a Hindle sphere and create a null test for the secondary.
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KerryR
Carpal Tunnel
Reged: 12/05/07
Loc: SW Michigan
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Re: Refiguring a Cassegrain
[Re: Mike Lockwood]
#4312090 - 01/13/11 03:20 PM
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How do you grind and figure a small surface, such as the secondary of a (smaller) DK? Is it necessary to use a spindle polisher, or can such small(ish) surfaces be done by hand using 'normal' atm methods?
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Refiguring a Cassegrain
[Re: KerryR]
#4312170 - 01/13/11 03:44 PM
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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
sage
Reged: 03/26/06
Loc: Oregon
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Re: Refiguring a Cassegrain
[Re: Mark Harry]
#4313211 - 01/13/11 11:25 PM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4389505 - 02/16/11 02:09 AM Attachment (73 downloads)
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4389510 - 02/16/11 02:15 AM Attachment (62 downloads)
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4389514 - 02/16/11 02:21 AM Attachment (65 downloads)
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4389523 - 02/16/11 02:27 AM Attachment (59 downloads)
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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
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4391904 - 02/17/11 02:44 AM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: _BBi]
#4391971 - 02/17/11 05:13 AM Attachment (46 downloads)
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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
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4392267 - 02/17/11 10:09 AM Attachment (55 downloads)
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: DAVIDG]
#4392500 - 02/17/11 12:25 PM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4785895 - 09/03/11 06:36 AM Attachment (43 downloads)
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4785897 - 09/03/11 06:37 AM Attachment (40 downloads)
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Final Ronchi
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4785904 - 09/03/11 06:44 AM Attachment (37 downloads)
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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
Carpal Tunnel
Reged: 07/02/06
Loc: Fort Worth TX
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Re: Refiguring a Cassegrain
[Re: dave brock]
#4786039 - 09/03/11 09:04 AM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Mike I. Jones]
#4786922 - 09/03/11 05:16 PM
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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
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Re: Refiguring a Cassegrain
[Re: dave brock]
#5656894 - 02/01/13 06:11 PM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: mark1234]
#5657288 - 02/01/13 10:35 PM
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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
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Re: Refiguring a Cassegrain
[Re: dave brock]
#5657758 - 02/02/13 09:23 AM
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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
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: mark1234]
#5658550 - 02/02/13 05:23 PM
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As advised... PM sent
Dave
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EJN
Pooh-Bah
Reged: 11/01/05
Loc: Highway 61
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Re: Refiguring a Cassegrain
[Re: mark1234]
#5658741 - 02/02/13 07:09 PM
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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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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Refiguring a Cassegrain
[Re: EJN]
#5659141 - 02/02/13 11:45 PM
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Dave luv seeing the shadow grams again, so smooth 
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dave brock
sage
Reged: 06/06/08
Loc: Hamilton, New Zealand
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Re: Refiguring a Cassegrain
[Re: Pinbout]
#5659157 - 02/03/13 12:01 AM
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Thanks Danny.
All in all, the work I'm most proud of. Particularly the secondary.
Dave
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jasonharris
sage
Reged: 09/16/06
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Re: Refiguring a Cassegrain
[Re: mark1234]
#5661021 - 02/04/13 01:08 AM
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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.
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ausastronomer
Carpal Tunnel
Reged: 06/30/03
Loc: Kiama NSW (Australia)
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Re: Refiguring a Cassegrain
[Re: jasonharris]
#5661102 - 02/04/13 03:32 AM
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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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