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Old 19-06-2013, 07:19 PM
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MC Rubinar 10/1000 modification

Couple of days ago I decided it is a right time to do something about that, potentially very good mirror lens, as per this website: http://www.strickling.net/russentonne.htm (have a look at attachment, it is not too bad for 45 sec, 16stack, 60D).

The problem is non-adequate fitting of the primary mirror, which is partly glued to a retaining ring, which is itself glued to the main tube, to prevent the ring from loosening. This glue looks like shellac, it is brown in colour and brittle.
In this particular case, the mirror is loose too much, so the system is showing a bit of astigmatism, but not always - depending on the direction the lens is pointed to.
Focus is also unstable, sometimes it is just lost after couple of minutes.
Has anyone managed to get rid of that glue and dismantle this or similar lens?

When I was doing this to my MTO-1000A, there was no glue.. only grab screws.
Obviously, this Rubinar is simplified version of it's older brother (MTO), and it is supposed to have even better resolution - but this is not the case here (not always, that is), because of a shabby mirror fixture.

I am thinking about heating the tube from outside, until the glue softens..
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Last edited by bojan; 21-06-2013 at 05:15 PM. Reason: progress
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Old 20-06-2013, 11:17 AM
Wavytone
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Hi Bojan, some years ago I had the MTO version, identical to the one in the photos shown in the link to that german page, and I had completely dismantled and stripped mine down to have the mirrors recoated and resprayed the exterior metalwork as mine really needed it.

In the MTO version:

Dismantling the rear of the primary cell, the primary is held in place by its perimeter - a compression ring around the back side of the mirror presses it up against a shoulder in front of the primary. This compression ring is a ring of plain aluminium with some slots cut in it to allow some springiness (its less than 1 mm) in the axial direction.

Unscrewing the back of the primary mirror cell should expose a threaded ring that adjusts the compression ring.

The compression ring is critical - if its too loose, the mirror will flop around, if its too tight, the force on the primary will distort it (pinching or astigmatism will be visible in images) or worse, you could chip it. By trial and error I found the threaded retaining ring it should be only firmed up with 1 finger - any more than that will apply too much force and deform the images.

In the MTO version the long conical central baffle screwed into the back of the mirror cell - the primary mirror isn't actually supported on the baffle. The front corrector on mine also wasn't a single meniscus - it was actually two menisci making a snug fit with a spacing shim - truly massive amount of glass. The secondary mirror was aluminised on the back of the first (outer) meniscus and the second meniscus had a conical hole drilled in it too. There was also a doublet correcting lens in the baffle, the surfaces had a pale straw-yellow coating.

The focussing is via a simple helical thread cut into the two tube sections, the front part (with the menisci) screws into the rear half. When I first acquired the lens it was lubricated with nasty molybdenum grease, buttery smooth but like yours the focus would NOT stay put. After the restoration work I ended up using ordinary vaseline to lubricate it - this was smooth enough yet enough friction remained that once focussed it would stay put all night.

At the back end mine had a brass plate with a Pentax thread firmly attached (6 screws) so no need for adapters.

While resolution was excellent (good diffraction pattern) the menisci were not coated and the result was the overall transmission through the lens was only 50-60%; I hope yours is better than that !

Have fun !

Last edited by Wavytone; 20-06-2013 at 11:30 AM.
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Old 20-06-2013, 12:28 PM
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Thanks for this, mate..
My MTO is the older than yours (and heavier) model, with 1kg of glass in corrector system. It is coated, though, but the glass is still yellowish in appearance and visibly darker, so absorption is there.

http://forum.mflenses.com/userpix/20...O1000A_3_1.jpg




This one below (Rubinar) is much lighter, but also it has significantly better resolution, especially at corners, so I really want to sort it out.
The glue on retaining ring is still resisting..

http://www.dvdtechcameras.com/manual.../rub1000/1.jpg

The springy compression ring is good idea - I will have it made of plastic (PVC) so I will have 3-point mirror suspension at the edge (with thin pieces of foil for laser printer between mirror and shoulder deeper in the tube) to avoid mirror deformation.
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Last edited by bojan; 20-06-2013 at 01:25 PM.
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Old 20-06-2013, 03:07 PM
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Ouch...

OK after a bit of searching found the exact model i had http://www.ebay.com/itm/RARE-RUSSIAN...-/151064405779 - it was the 1958 version. The exterior of the barrel has a lot more lumps and bumps than yours. Also it came in a solid plywood case with 3 full aperture filters and a screw-in screwcap that added another 15 cm to it. The corrector was uncoated.

If you dismantled the corrector on yours don't be surprised if its a doublet... just be very careful not to scratch the secondary. Yep, at least 3 cm of solid glass up front, easily.

Last edited by Wavytone; 20-06-2013 at 03:20 PM.
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Old 20-06-2013, 04:16 PM
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Sill struggling..
The next step would be to apply heat...
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Last edited by bojan; 20-06-2013 at 05:24 PM.
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Old 20-06-2013, 07:44 PM
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Attached below is a comparison between Rubinar 10/1000 and MTO-1000A (200% sized crop away from centre)
More or less the same, but Rubinar (left image) has less obvious diffraction pattern at brighter stars, and visibly lower CA away from the frame centre.
All in all, the mod is definitely worth the effort.
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Last edited by bojan; 20-06-2013 at 08:01 PM.
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Old 21-06-2013, 09:47 AM
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Success!
A few drops of acetone helped to soften the glue.
Now I am in business!

EDIT:
All done.. but the results from last night were not quite what I hoped for..

So I added this "springy" bit, cut out from black material used for plastic folders - I find it very useful for variety of applications.. (like hard paper, but water resistant)
The ends of the spokes (10mm) are folded back and now they act as a gentle springs, positioned exactly opposite of mirror seats, deeper in the tube (made of bits of kapton tape), so the mirror is held in position at three points, 120 apart... hopefully with no stress that might cause the deformation.

we'll see the results of all this hassle tonight..
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Last edited by bojan; 23-06-2013 at 12:40 PM.
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Old 23-06-2013, 07:43 PM
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The result of this little project is very good..
The astigmatism is almost totally gone, the stars are nice and round, even in corners of the APS frame.
It was slightly better than my MTO-1000A, now it is much better. And if I count in the lower weight, smaller physical size.. definitely a keeper.

The focussing is a bit tricky, though.. but still doable.
Right now I am pressing the front part of the lens towards the back end.. then focussing by turning the tube to the left, effectively moving the corrector plate with secondary upwards.. this means the relatively massive front corrector can't move by itself downwards any more, so the focus is stable during the photo session.
The image is 16x30sec, darks, flats, crop, size 100%.
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Last edited by bojan; 23-06-2013 at 09:26 PM.
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Old 23-06-2013, 10:28 PM
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Great - now see if you can take a shot of the jewel box showing the colours. I was able to do that on film.

The other target I managed to capture somewhat surprisingly was NGC5128, dust lane showing well enough.

Now, if only the clouds would part ...
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Old 24-06-2013, 04:04 PM
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Quote:
Originally Posted by Wavytone View Post
Great - now see if you can take a shot of the jewel box showing the colours. I was able to do that on film...
That will require a lot of shorter exposures and some specific data processing .. maybe one day (night) ..
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Old 25-06-2013, 09:37 AM
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To prevent the effect of backlash in helix focuser, I am trying this idea.. there is another identical spring (from Bunnings, of course!) on the opposite side of the tube (in principle, three springs would be better, but I didn't think of it when designing the saddle for this lens, so.. two should be good enough).
It is easier to do it on MTO-1000A (Rubinar is more difficult, there is less space available for those additions).

Edit: this works OK it seems.. it is now much easier to get the focus right (with help of live view)
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Last edited by bojan; 25-06-2013 at 09:56 PM.
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Old 25-06-2013, 10:27 AM
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Mmmm...

It's an amusing exercise if you have the patience and unlimited time for tinkering, but I ran out of that luxury long ago... a 102mm f/7 ED refractor makes a lot more sense...
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Old 25-06-2013, 11:08 AM
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Quote:
Originally Posted by Wavytone View Post
Mmmm...

It's an amusing exercise if you have the patience and unlimited time for tinkering, but I ran out of that luxury long ago... a 102mm f/7 ED refractor makes a lot more sense...
Yes it does, in principle...
However, since I already have those lenses.. and I have all the time in the world (practically retired - still trying to find adequate job but no-one wants me and my 63 years)... and Chinese 4" f/7 ED (according to last year's quote from Gilman of NortGroup) is US$815.. it is worth the effort.

Also, those lenses are ideal for Sun eclipses - being short and lightweight.

Last edited by bojan; 25-06-2013 at 11:29 AM.
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Old 25-06-2013, 04:46 PM
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Hehe I had a Meade 4" f/10 SCT for quite a few years - if you see one of these, or the little Saxon Maks they're good for eclipses too...
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Old 25-06-2013, 04:57 PM
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Quote:
Originally Posted by Wavytone View Post
Hehe I had a Meade 4" f/10 SCT for quite a few years - if you see one of these, or the little Saxon Maks they're good for eclipses too...
I have no doubts about it !

Last edited by bojan; 25-06-2013 at 06:54 PM.
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Old 29-06-2013, 10:24 AM
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Rubinar focuser mechanism is now without backlash as well.. I used those nylon hose clamps to attach the springs..This solution of the problem is even better than what I used for MTO, it is certainly cheaper and easier to do - no need for machined parts.
Also, this design offers very interesting possibility for focus motorisation, since those clamps, when tightened around the tube could be used as worm gear...
This may well become the next phase of this little project..
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Last edited by bojan; 29-06-2013 at 10:50 AM.
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Old 11-07-2013, 10:24 AM
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After some more experimentation, I came up with this solution (no springs).
The focus is stable now (focus ring is held in place with nylon screws on additional ring), it is easy to adjust it (the OTA itself is fixed properly as well) and the focus ring doesn't move by itself - so it stays OK even from the last night session.

One more detail - I replaced balls in bearing with nylon tubing, 3.5mm in diameter.. the original balls were actually too small (3mm - probably used by mistake, or the rail was not made up to specification) so they rattled inside the bearing. Now the whole structure is rock-solid.
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Last edited by bojan; 11-07-2013 at 11:48 AM.
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Old 19-04-2021, 02:34 PM
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Adding the focus mechanism, made of 3 flexiclamp straps...
I will add motor later for remote operation.
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