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Mounting a Clark Refractor

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#1 lplybon

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Posted 03 June 2021 - 11:00 PM

Howdy,

 

I am working with a local university to restore and resume the use of a pair of old refractors. One is clearly marked as a Clark 1882, and the other has no certain markings but is of similar build. It may be a Clark as well, with remodeling from some time between the 1800s and today. 

 

Restoration of the scopes aside (that is another fun project!) I need a way to mount the scopes to a modern mount. The mounting brackets are simply two threaded bolts on the side of the scope, normal to the tube and spaced about 20 cm apart. You can see them at the top of the scope, with a pair of pretty brass knobs. I doubt there is an off the shelf method for mounting these scopes, and in all likelihood I will need to machine some custom bracket/dovetail to hold them, but before I go sketching up a solution I knew a place to ask first smile.gif

 

If there are no solutions here, then I will make sure to update the thread with whatever solution I come across. 

 

Thanks,

Liam

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#2 jkmccarthy

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Posted 04 June 2021 - 12:50 AM

Liam --

 

That refractor is gorgeous! 

 

As to your question, my first thought was to wonder about the wall thickness of the brass tube.  Apparently(?) it is thick enough for there to be tapped holes for the two knobs you pointed out (although I do see two smaller screw heads [?] protruding from the tube near those knobs ... perhaps those are to secure some sort of curved plate [??] or thicker cylinder(s) inside the tube, into which the holes for the knobs are actually tapped ?).  Anyway, before planning your dovetail / saddle attachment around those two knobs, personally I would want to confirm that the tube in their local vicinity is strong enough to carry the weight of the tube via those two attachment points.  But clearly this must have been the original design intent, so it's probably the preferred way to go!   Here's a photo I found on-line of a vintage 4-inch f/15 Clark refractor, which shows knobs similar to yours ---

 

1059822_view%2003_03.jpg

(ref:   the 3rd photo at the auction site here:  https://www.skinneri.../2760M/lots/299 )

 

Alternatives to be considered (in case there is any doubt as to the tube wall strength locally at the two knob locations) would include either (1) incorporating tube rings into your mount attachment design, or else (2) fabricating a clamshell-type structure to surround the tube, possibly using the two knobs to secure one half of the clamshell to the tube, but overall deriving its strength from the clamshell structure and the fact that its gets clamped around the tube itself.

 

 

(1) As far as tube rings attached to a dovetail are concerned, yesterday I was doing some enjoyable reading about long refractors, and I was quite impressed by member Don Taylor's re-engineering of the tube rings on his Altair Starwave 102ED f/11 refractor:   see the first few pictures in his CN gallery starting here ---

 

gallery_80960_11011_445455.jpg

 

... which he upgraded to ...

 

gallery_80960_11011_465142.jpg

(for the associated discussion, see here:  https://www.cloudyni...-9#entry9237008 and later here: https://www.cloudyni...bob/?p=10276128 ... see especially post #3 of this 2nd thread for some close-up photos).

 

 

(2) Some nice examples of the vintage clamshell or "cradle" design I had in mind would be this:

 

post-43265-0-49408700-1620881789.jpg

(ref:   https://www.cloudyni...ion/?p=11097433 )

 

... or this ...

 

post-44298-0-89886900-1621557601.jpg

(ref:  https://www.cloudyni...ion/?p=11114816  from the same thread).

 

In both of these two photos, note how the lower sections of the two rings are connected by a curved "cradle" that additionally supports the tube.

 

Hope this helps,

 

        -- Jim

 


Edited by jkmccarthy, 04 June 2021 - 12:57 AM.

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#3 starman876

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Posted 04 June 2021 - 07:24 AM

Liam --

 

That refractor is gorgeous! 

 

As to your question, my first thought was to wonder about the wall thickness of the brass tube.  Apparently(?) it is thick enough for there to be tapped holes for the two knobs you pointed out (although I do see two smaller screw heads [?] protruding from the tube near those knobs ... perhaps those are to secure some sort of curved plate [??] or thicker cylinder(s) inside the tube, into which the holes for the knobs are actually tapped ?).  Anyway, before planning your dovetail / saddle attachment around those two knobs, personally I would want to confirm that the tube in their local vicinity is strong enough to carry the weight of the tube via those two attachment points.  But clearly this must have been the original design intent, so it's probably the preferred way to go!   Here's a photo I found on-line of a vintage 4-inch f/15 Clark refractor, which shows knobs similar to yours ---

 

1059822_view%2003_03.jpg

(ref:   the 3rd photo at the auction site here:  https://www.skinneri.../2760M/lots/299 )

 

Alternatives to be considered (in case there is any doubt as to the tube wall strength locally at the two knob locations) would include either (1) incorporating tube rings into your mount attachment design, or else (2) fabricating a clamshell-type structure to surround the tube, possibly using the two knobs to secure one half of the clamshell to the tube, but overall deriving its strength from the clamshell structure and the fact that its gets clamped around the tube itself.

 

 

(1) As far as tube rings attached to a dovetail are concerned, yesterday I was doing some enjoyable reading about long refractors, and I was quite impressed by member Don Taylor's re-engineering of the tube rings on his Altair Starwave 102ED f/11 refractor:   see the first few pictures in his CN gallery starting here ---

 

gallery_80960_11011_445455.jpg

 

... which he upgraded to ...

 

gallery_80960_11011_465142.jpg

(for the associated discussion, see here:  https://www.cloudyni...-9#entry9237008 and later here: https://www.cloudyni...bob/?p=10276128 ... see especially post #3 of this 2nd thread for some close-up photos).

 

 

(2) Some nice examples of the vintage clamshell or "cradle" design I had in mind would be this:

 

post-43265-0-49408700-1620881789.jpg

(ref:   https://www.cloudyni...ion/?p=11097433 )

 

... or this ...

 

post-44298-0-89886900-1621557601.jpg

(ref:  https://www.cloudyni...ion/?p=11114816  from the same thread).

 

In both of these two photos, note how the lower sections of the two rings are connected by a curved "cradle" that additionally supports the tube.

 

Hope this helps,

 

        -- Jim

That last mount looks familiar


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#4 macdonjh

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Posted 04 June 2021 - 07:24 AM

If you are considering modern mounting hardware, jkmccarthy's second thought was my first thought: tube rings and a standard dovetail.  Not particularly period-correct, but simple to implement and would result in secure mounting to any modern mount.  As jkmccarthy said, if you're confident the brass OTA is strong enough, it would be pretty simple to modify a standard dovetail (either Vixen or Losmandy) with a pair of appropriately spaced holes and attach it directly to the scope.  You could also have a dovetail made from brass at a machine shop.

 

If you can find an antique clamshell of the right size, that would look fantastic.


Edited by macdonjh, 04 June 2021 - 07:48 AM.

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#5 ccwemyss

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Posted 04 June 2021 - 08:13 AM

Since you are planning to put the scopes on a modern mount, I would agree with the suggestion to get a dovetail drilled to take the screws. You could use a Vixen to have something a bit more proportional to the scope. Perhaps machine a couple of shallow supports from brass to go between it and the tube (flat on one side, curved on the other, with a hole for the screws to pass through), to spread the pressure over a wider area than the contact point of the screw coming through the tube. 

 

It's likely that the tube was meant to be strong enough for this kind of mounting. If it needed rings, they probably wouldn't have gone to the trouble of also providing screws, which would have interfered with rings.

 

If you use rings, they will have to lift the tube far enough above the dovetail for the screws and knobs to clear it (or use a dovetail shorter than the distance between the screws). Also think about how much you'll need to slide the tube to get it balanced in Dec. Then decide whether you want to make that adjustment by sliding the dovetail, or sliding the tube in the rings. If sliding the tube in the rings, they need to be positioned so the screws won't hit them when you try to shift the tube. 

 

I recently picked up an Edmund 4" that has a similar mounting system, with long screws meant to go through holes in a saddle. I cut risers out of aluminum that are drilled to take the bolts, and be held on by nuts that go into a recess. Their bases are drilled for a D dovetail plate. One mistake I made was assuming that the bolts were parallel. When I got the risers mounted, they were several degrees offset from each other and it took some crude re-drilling to get them to mount flat to the plate. 

 

Edmund 4 - 1 (2).jpeg

 

Chip W. 


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#6 astrokeith

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Posted 04 June 2021 - 08:25 AM

If going the saddle/dovetail bar route, do make sure the saddle bar mates to the tube via some curved profiles that match  the diameter of the telescope tube. The top photo appears not to have done this which is wrong and will distort the brass tube in time if not immediately.


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#7 Bomber Bob

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Posted 04 June 2021 - 08:37 AM

For my Mogey 3, I used a 7" x 0.6" bar of aluminum as the adapter, and drilled 2 pairs of holes - 1 to fit the Mogey's tube bolts, & 1 to fit the standard holes on a Vixen 20cm dovetail:

 

Mogey 3 - Vixen Dovetail Mod (Meade SF Tall EQ) S02.jpg

 

The bar is wrapped in a couple of layers of felt, so no tube scratching.  Most of the time, the Mogey is on display on its MacKenzie AZ mount, but this mod is quick & easy for use on a modern(ish) EQ...


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#8 jkmccarthy

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Posted 04 June 2021 - 11:18 AM

[...]

If you can find an antique clamshell of the right size, that would look fantastic.

"If you can find an antique..." [or else design, and have a machine shop at the university fabricate new a] "...clamshell of the right size ..." then I agree with macdonjh !

 

Seems like it might be possible to start with a metal tube of the appropriate O.D., bore out the center on a lathe to an I.D. the matches the O.D of the Clark tube (with maybe an allowance for a thin layer of felt), and then divide it in half across its diameter and go from there.   If the metal tube is Aluminum 6061T6, this alloy can be welded, so another short section could be machined to fit onto the half-cylinder "cradle" in the T-fashion seen in the two vintage equatorial mounts pictured above (assuming the round thick-wall tube perpendicular to the cradle could then be mated to your modern equatorial mount).  I'd then be inclined to paint the cradle and rings to match the color of your mount.  My only caution relative to the T-shape idea here is to try and keep the telescope tube perpendicular to the declination axis of the mount, for best pointing accuracy if your mount uses setting circles and/or will include Go-To capability.

 

Regards,

 

       -- Jim

 
 



#9 starman876

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Posted 04 June 2021 - 12:43 PM

interesting.  I am going to see which modern scopes I can put on the Mogey mount.


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#10 jkmccarthy

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Posted 04 June 2021 - 01:14 PM

If going the saddle/dovetail bar route, do make sure the saddle bar mates to the tube via some curved profiles that match  the diameter of the telescope tube. The top photo appears not to have done this which is wrong and will distort the brass tube in time if not immediately.

I'm pretty sure "The top photo" that astrokeith makes reference to is the first one in my reply, which is a photo of a vintage 4-inch Clark refractor.  I completely agree with his comment, and should have said so earlier !

 

His suggestion to machine some curved brackets is certainly a good one.   But another (simpler?) approach you might consider, if you wish to emulate the "minimalist" appearance of the tube-to-mount interface that Clark used, would be to have the top surface of the dovetail (where it mates to the tube) machined to have raised rounded or chamfered edges (like a very shallow piece of "channel iron") such that the top surface of the dovetail makes line-contact with the telescope tube in three places:   down the center of the wide "channel" in the dovetail is (where the bolt holes are for the brass knobs) as well as along the tops of each of the raised edges along the top sides of the channel.

 

My interpretation of astrokeith's concern (although maybe I shouldn't speak for him ... so let me just say, my concern, at least ...) is that the interface shown in "the top photo" doesn't constrain the round tube from "rolling" side-to-side across the flat plate on the mount side of the interface (just imagine pointing the telescope on its equatorial mount at the southern horizon ... gravity pulling on the full weight of the telescope tube would be trying to make the tube "roll" down the flat plate on the mount).  By not constraining this rolling sideways degree-of-freedom at the tube-to-mount interface, the tube is indeed likely to get distorted.   But with raised edges on each side of the dovetail that make line-contact (down the length of the dovetail) with the outside of the telescope tube, this "rolling" degree of freedom will be taken away.

 

Design-wise, the only [mildly] tricky part is to make the wide channel in the dovetail of the right depth so it does make line contact with the tube at all three places ... so that when the knobs are tightened, one isn't distorting the tube by trying to close a "gap" between the tube and the bottom of the channel, if for example the channel were too deep.  And conversely if the channel is not deep enough, then the raised edges won't contact the tube so won't constrain it against rolling.   One approach to consider might be to intentionally cut the channel too deep, and simply fill that gap (only around where the knob bolts are) with washers custom-ground to be the exact thickness needed.
 

This would give you an interface that (at least from a distance) closely resembles the minimalist one in the vintage Clark-telescope-on-equatorial-mount photo (which BomberBob's solution also appears to do), but is actually more robust because of the line contact made with the tube by the two raised edges that run along the length of the dovetail bar.

 

If you look on-line at dovetails made to adapt popular fork-mounted telescope tubes (for example, 8-inch SCT's) to German Equatorial Mounts, you'll often see that these telescope-specific dovetails have the raised edges I've described above (i.e., I certainly am not taking any credit for this idea ...).   Here's one random example I just pulled off the web, just to illustrate (note the "top side" that mates to the telescope is underneath in the photo):

 

ADMACCESSORIES_VM8_1.jpg

 

Regards,

 

      -- Jim



#11 Terra Nova

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Posted 04 June 2021 - 01:21 PM

I like Parallax rings. Joe does a great job making them. I sold the original TV clamshell that came with my TeleVue Genesis SDF and opted for what I deemed to be a much easier and secure mounting route. 

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#12 Bowlerhat

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Posted 04 June 2021 - 08:40 PM

If you don't mind modern attachment I'd get a ring, or a slotted dovetail.

psfoto13-gp-level-prismenschiene.jpg



#13 starman876

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Posted 04 June 2021 - 10:38 PM

I like Parallax rings. Joe does a great job making them. I sold the original TV clamshell that came with my TeleVue Genesis SDF and opted for what I deemed to be a much easier and secure mounting route. 

looks really nice Terra


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#14 Bomber Bob

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Posted 05 June 2021 - 08:44 AM

interesting.  I am going to see which modern scopes I can put on the Mogey mount.

I haven't tried that:  Newer Scope on Antique AZ mount.  You got the hamsters in my noggin' running in their wheels...



#15 lplybon

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Posted 05 June 2021 - 01:51 PM

Thanks for all of the suggestions, everyone! I'll update with the solution I go with. Right now, I am thinking of getting some rings. 

 

Liam


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#16 RichA

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Posted 05 June 2021 - 05:46 PM

Howdy,

 

I am working with a local university to restore and resume the use of a pair of old refractors. One is clearly marked as a Clark 1882, and the other has no certain markings but is of similar build. It may be a Clark as well, with remodeling from some time between the 1800s and today. 

 

Restoration of the scopes aside (that is another fun project!) I need a way to mount the scopes to a modern mount. The mounting brackets are simply two threaded bolts on the side of the scope, normal to the tube and spaced about 20 cm apart. You can see them at the top of the scope, with a pair of pretty brass knobs. I doubt there is an off the shelf method for mounting these scopes, and in all likelihood I will need to machine some custom bracket/dovetail to hold them, but before I go sketching up a solution I knew a place to ask first smile.gif

 

If there are no solutions here, then I will make sure to update the thread with whatever solution I come across. 

 

Thanks,

Liam

 

 

Howdy,

 

I am working with a local university to restore and resume the use of a pair of old refractors. One is clearly marked as a Clark 1882, and the other has no certain markings but is of similar build. It may be a Clark as well, with remodeling from some time between the 1800s and today. 

 

Restoration of the scopes aside (that is another fun project!) I need a way to mount the scopes to a modern mount. The mounting brackets are simply two threaded bolts on the side of the scope, normal to the tube and spaced about 20 cm apart. You can see them at the top of the scope, with a pair of pretty brass knobs. I doubt there is an off the shelf method for mounting these scopes, and in all likelihood I will need to machine some custom bracket/dovetail to hold them, but before I go sketching up a solution I knew a place to ask first smile.gif

 

If there are no solutions here, then I will make sure to update the thread with whatever solution I come across. 

 

Thanks,

Liam

Weird they don't just sell the Clark to fund a larger, more modern telescope to do with the modern mount.  Unless  this is for primarily historical purposes and not education, public outreach.




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