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Post by unkphysics on Jul 14, 2014 18:36:45 GMT -5
Greetings. New member here.
First, I appreciate the good people that put together this website and blog. It's been a wealth of information.
I am in the process of restoring some Cave Astrola's that the Physics Department has owned since the late 1960's. We own a 16" 'transportable' observatory scope (purchased 1966) and four 8" student scopes. (I must say, at 800lbs the 16" is not easily transportable.) The telescopes saw use in the late '60s and early 70's, and were set aside. To my knowledge, they were purchased new, everything on the scopes is original, and have not been touched since 1975.
I could use some input on the following: The mirrors on ALL of the scopes do not bear the diamond engraving that I see on the later 1970's mirrors. And I'm fairly certain that these mirrors were never replaced, but I am checking with retired professors of the department to be certain. Is it possible that Cave did not engrave the mirrors in the later 1960's? What am I missing? I find it hard to believe that all five of the scopes had their mirrors replaced. But then again, they were student telescopes. :-) One of the 8" mirrors does have a signature on the back, but it is in pencil. (and it's not Cave)
Any information would be appreciated.
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Post by Admin on Jul 15, 2014 13:58:13 GMT -5
Hello and welcome to the forums.
There is a question whether Cave marked every single mirror. This could have been an order pushed out all at once and they did not mark the mirrors. Is there a Panko paper sticker on the back of any of them?
Is the 16" a cassegrain telescope or a newtonian? Cave did not make the mirrors for the cassegrains. He farmed it out to another company in California. I believe he did that as he could not make the curved secondary that matched the primary.
Post some pictures here. I'd love to see them. Also, what college or university is this?
Tom T
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Post by unkphysics on Jul 16, 2014 14:39:13 GMT -5
Thanks for the response. I have interviewed the only living retiree from the department from the early 1970's. He was not the person that oversaw the telescopes. He *thinks* there was a refurbishment of the telescopes in 1972(ish). For a 16" mirror and several 8" mirrors, that would be quite an expenditure - especially considering that the equipment was purchased in 1966. Knowing how state university budgets work, that would require unique justification to replace the mirrors so soon. I am digging to see if there is a record of this expense somewhere. Who knows what happened. The signature on the first 8" mirror was illegible. Digging further, the 'signature' on another 8" mirror clearly states "Kearney State College". So apparently the vendor wrote our (the customer's) name on it. The only sticker I could find was on, yet another, 8" mirror. It reads: "Aluminum Silicon Monoxide By Precision Applied Products, 418 Rumsey Place, Placentia CA 92670." It looks as if the mirrors were replaced, which is unfortunate. I have attached three photos: a 'before', an image of the back of the 16" mirror, and the 95% completed telescope. |
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Post by Admin on Jul 17, 2014 14:19:46 GMT -5
Very nice 16". I too am puzzled about the mirrors. I have sent an email to the company that seemed to do the work for clarification. Maybe they still have the records. I will report back!
Admin
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Post by Admin on Jul 17, 2014 16:01:04 GMT -5
I talked to Peter nance from precision applied products and he does not have records that go back that far. He did say that early 80's and earlier, they only did coatings. Could it be that these are Cave mirrors that have had coatings removed and recoated? Can the etching be affected by the removal process? Not sure. Others could chime in here.
Jon miles of the Cave registry may know more about this.
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Post by Admin on Jul 18, 2014 7:40:42 GMT -5
I also want to verify that you are looking at the right place for the engraving. It would be on the side of the mirror and in some cases it would be very light. You may have to shine a very bright light on the edge of the mirror for it to show up. Can you verify this? Admin Attachments:
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Post by Datapanic on Jul 18, 2014 21:28:25 GMT -5
Wow that 16" is NICE! I kinda wonder why the tube weights are located at two spread-out locations around the optical axis though - that doesn't make sense. The amount of weight looks right, but even with a heavy load on the eyepiece, the tube weights would not need to be that far apart around the optical axis. I'm somewhat a stickler about the proper balance of a Newtonian and its OTA...
Anyway, just because the mirrors do not have Cave markings on them doesn't mean they're bad mirrors. Best to put them on an optical bench and test them out. And as said, sometimes Cave did not scribe the mirrors.
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Post by unkphysics on Jul 23, 2014 11:10:26 GMT -5
All, thanks for the comments.
Yes, I've seen other pictures online with Cave's engravings. These mirrors do not have that. :-( So, IF these mirrors are not original they had to be replaced in the 1970's - and the scopes have seen little use since then.
Regarding the weights: I freely admit that I am not an astronomer - my background is atomic and optical physics. I set the larger weight at the maximum distance to provide maximum torque (for a 'coarse setting'), and used the smaller weight to 'fine tune' the balance. As it is now, the scope is well balanced throughout the full range of motion (both axes) and moves smoothly. I fine tune the weights a bit when I put my camera on for PF photography... But once set, I don't need to make adjustments as I point the scope at different targets. From a static equilibrium point of view, the sum of torques would be the same if the weights are separated or together, as long as they balance the scope. Right? Or is there something I'm missing? Again, I am not an astronomer and know 'just enough to be dangerous." :-)
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Post by unkphysics on Jul 23, 2014 11:37:01 GMT -5
..... And it occurs to me that you were curious about the TUBE weights. (My applogies). There are three weight racks on the scope. They are original to the scope and I have put them back as I had found them. I suspect that the weights are distributed about the circumference to mimic a more even mass distribution which will better balance the primary mirror in all orientations.
By the way, I have reworked the weight racks some since that photo was taken. The present configuration is how i found the scope: If I assume the focuser is at 0 degrees, there are two weight racks with double masses at 90 and 270 degrees around the circumference of the tube. There is another rack with one smaller mass at 180 degrees. This smaller one is slightly further from the primary than the others and is typically what I adjust when changing cameras.
I am almost finished with the scope and would be willing to share restoration progress photos if there is interest.
Thanks again for everyone's help!
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Post by Datapanic on Jul 23, 2014 18:36:00 GMT -5
Right - the tube weights! I suppose it makes sense to have two different sets that are at different angles off the optical axis - this would effectively allow a range around the optical axis that is in between the two sets to compensate for different amounts of weight on the focuser - such as when an eyepiece or a heavier camera is used. With a 16" primary and a tube that has a diameter larger than that, the torque variance would be greater depending on what the focuser is carrying so it makes sense to do it that way and it would balance out, I think, but I'm not going to do that math! (I have the formulas).
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