23 replies to this topic

[Tom Duncan]

This was a surprising find, a box of 19 Pyrex mirror blanks and a tile tool (correct term?) originally sent to Celestron International, 2835 Columbia St, Torrance, CA by United Lens Company, Southbridge, MA. That Celestron address would date these to the mid-70s at the earliest. 

 

I initially thought these are blanks for the C6 but the diameter is 5-11/16". Thickness is 1". Cast-in sagitta is approximately 1.78mm/0.07in. I'll let someone better at math figure out what the cast focal length/ratio is. 

 

You can see the hole in the back that I assume is cut out once the figuring is done, so I would think these would be for an SCT, Cassegrain or maybe a MAK?

 

Any idea of the applications for these? 

 

Thanks

 

Tom Duncan

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Edited by Tom Duncan, 25 September 2024 - 10:20 AM.

[J A VOLK]

Could have been for the Celestron 5" f/6 Telephoto lens from the early days (late 70's/early 80s?).  Sagitta is in the ballpark.  Is there a date on the package?

[telesonic]

Interesting find Tom.

 

I'm not really good at math, in fact... I'm terrible at it. But, punching 5 & 11/16" into the web tells me that it is close to 5.69 inches - or 144.526 MM for our metric peoples. As for F/L and ratio.... I'm curious as to that.

 

Now I have a few thoughts of what these could have been manufactured for, and I could be way off - just spitballing here.

 

The C-5 SCT comes to mind (though I believe the outer diameter of the mirrors you have - isn't correct, unless that was later changed.) The only other thing would be the 5.5" (if I recall) Celestron Schmidt Camera, but I have no actual dimensions of those primary mirrors. Some sleuthing around about United Lens Co. also hasn't really given up much of any information, which leads me to believe that until we know what these were for, it likely was a custom job maybe?

 

Now the only thing from there would have to be optics for some secret squirrel (DOD/ ETC) project, perhaps destined to be primary mirrors for SCT or other optical scopes for range / rocketry tracking scopes, or aerial reconnaissance platforms. (Think U-2 / SR-71.) Knowing the F/L would certainly hint at the application, and would be interesting to know.

 

I hope someone here can turn up a good definitive answer for ya, but that is really cool / rare for a find. If you ever want to get rid of a blank or two, I'd be interested (just for a historical / personal curiosity thing.)

 

 

Cheers my friend,

Temp

 

[TOMDEY]

F# ≈ D/16sag which would be around F/5 --- which would be a very nice small Newtonian project! These blanks are no doubt very good fine annealed "full thickness" and the pre-cast surface is also a positive.     Tom

[davidc135]

They are much too slow, if my maths is right, for either an sct or Schmidt camera. I get around f5 although the manufacturer could deepen the sag.

 

S = r²/2R or R = r²/2S

 

R = 5221.94/(2x1.78). Therefore 1466.8/(2x144.526) = f/5.07

 

David

 

Posts crossed

Edited by davidc135, 24 September 2024 - 09:11 PM.

[CharLakeAstro]

An interesting find !!

(I counted 19 in the pics)

[telesonic]

Could have been for the Celestron 5" f/6 Telephoto lens from the early days (late 70's/early 80s?).  Sagitta is in the ballpark.  Is there a date on the package?

 

J A, Tom, David, Char.... 

Wow, maybe....!

 

 

I was also thinking that these could be leftover primary mirror blanks from the Celestron 5" F/6 SCT telephoto, I have one of these scopes in my shed, but never measured the primary to exact figures. One would still need a corrector plate, and all that madness.

 

Was the 5.5" Celestron Schmidt Film Camera (unobtanium scope) primary mirror cored like this? Never owned or seen one in person, so I wonder.

 

Temp

[davidmcgo]

The C5 and 750 f6 used an f2 primary that was a conical blank and not that thick at the edge.  The earlier blue and whites also had ribbed blanks and tapered.  These look like neither.  So a one off set for a special project or an evaluation set trying to get a supply contract seem possible.  The Comet Cather has a conventional looking blank but I haven’t pulled it out of the cell to see if it had the hollow at the back.

 

Dave

[telesonic]

The C5 and 750 f6 used an f2 primary that was a conical blank and not that thick at the edge.  The earlier blue and whites also had ribbed blanks and tapered.  These look like neither.  So a one off set for a special project or an evaluation set trying to get a supply contract seem possible.  The Comet Cather has a conventional looking blank but I haven’t pulled it out of the cell to see if it had the hollow at the back.

 

Dave

Dave,

Yes Sir ... I suspect this would have be for an SCT in some format. I have 2 Comet Catchers (SN's) and the primary mirror isn't cored / hollowed out like the pics show, and that is what I can see in Tom's mirror picture.

 

More images and insight might shed some light onto what they were for.

 

 

Cheers,

Temp

[deSitter]

This was a surprising find, a box of 18 Pyrex mirror blanks and a tile tool (correct term?) originally sent to Celestron International, 2835 Columbia St, Torrance, CA by United Lens Company, Southbridge, MA. That Celestron address would date these to the mid-70s. 

 

I initially thought these are blanks for the C6 but the diameter is 5-11/16". Thickness is 1". Cast-in sagitta is approximately 1.78mm/0.07in. I'll let someone better at math figure out what the cast focal length/ratio is. 

 

You can see the hole in the back that I assume is cut out once the figuring is done, so I would think these would be for an SCT, Cassegrain or maybe a MAK?

 

Any idea of the applications for these? 

 

Thanks

 

Tom Duncan

 

Let R be the radius of curvature, S be the sagitta, and A be the 1/2 angle subtended by the mirror at the center of curvature. Then

 

cos A = (R - S) / R = 1 - S/R = 1 - 1.78mm/R

 

But also

 

sin A = 76mm / R

 

for a 152mm mirror. Squaring both and adding we get

 

1 = (1-1.78/R)^2 + (76/R)^2

 

so

 

0 = (76^2 + 1.78^2) (1/R)^2 - 3.56 (1/R)

 

and

 

R = (76^2 + 1.78^2) / 3.56 = about 1600mm

 

The focal length is 1/2 this, so 800mm, and so these are probably f/5 mirrors.

 

If the radius of the mirror is r then in general

 

F = (r^2 + S^2) / 4S

 

Note that the usual formula is the approximation r^2 / 4S because S^2 is much smaller than r^2 for typical mirrors.

 

-drl

[Kasmos]

This was a surprising find, a box of 18 Pyrex mirror blanks and a tile tool (correct term?) originally sent to Celestron International, 2835 Columbia St, Torrance, CA by United Lens Company, Southbridge, MA. That Celestron address would date these to the mid-70s. 

 

I initially thought these are blanks for the C6 but the diameter is 5-11/16". Thickness is 1". Cast-in sagitta is approximately 1.78mm/0.07in. I'll let someone better at math figure out what the cast focal length/ratio is. 

 

You can see the hole in the back that I assume is cut out once the figuring is done, so I would think these would be for an SCT, Cassegrain or maybe a MAK?

 

Any idea of the applications for these? 

 

Thanks

 

Tom Duncan

I can't say what they were for, but the address doesn't help date them since they still occupy that address.

[ryanr256]

Do you think United would have records on that part number?

[Tom Duncan]

An interesting find !!

(I counted 19 in the pics)

Yep, 19, corrected number in my original post. 

 

I did say I wasn't strong on math, right? 

 

Tom

[Tom Duncan]

Could have been for the Celestron 5" f/6 Telephoto lens from the early days (late 70's/early 80s?).  Sagitta is in the ballpark.  Is there a date on the package?

What Celestron 5" f6 Telephoto lens are you referring to?

 

As to the size at 5-11/16" is quite a ways from 5" and to my knowledge only Meade has ever mentioned an oversize mirror from the specification. 

 

No date on the package.

 

Tom

[Tom Duncan]

Do you think United would have records on that part number?

I have written them, waiting for a response. 

 

Tom

[Tom Duncan]

Interesting find Tom.

 

I'm not really good at math, in fact... I'm terrible at it. But, punching 5 & 11/16" into the web tells me that it is close to 5.69 inches - or 144.526 MM for our metric peoples. As for F/L and ratio.... I'm curious as to that.

 

Now I have a few thoughts of what these could have been manufactured for, and I could be way off - just spitballing here.

 

The C-5 SCT comes to mind (though I believe the outer diameter of the mirrors you have - isn't correct, unless that was later changed.) The only other thing would be the 5.5" (if I recall) Celestron Schmidt Camera, but I have no actual dimensions of those primary mirrors. Some sleuthing around about United Lens Co. also hasn't really given up much of any information, which leads me to believe that until we know what these were for, it likely was a custom job maybe?

 

Now the only thing from there would have to be optics for some secret squirrel (DOD/ ETC) project, perhaps destined to be primary mirrors for SCT or other optical scopes for range / rocketry tracking scopes, or aerial reconnaissance platforms. (Think U-2 / SR-71.) Knowing the F/L would certainly hint at the application, and would be interesting to know.

 

I hope someone here can turn up a good definitive answer for ya, but that is really cool / rare for a find. If you ever want to get rid of a blank or two, I'd be interested (just for a historical / personal curiosity thing.)

 

 

Cheers my friend,

Temp

I have a C5 and though I can't get at the actual mirror to measure it it's not nearly as large as these mirrors.

 

I have a very nice 5.5" Schmidt camera (post on this coming soon, some questions about it) which came from the same estate as these mirror blanks but I'm loathe to attempt to take it apart to compare the mirrors as the scope is in amazing condition and apparently unmolested. The cast-in sagitta would appear to be right though as the corrector of the camera is indeed a lens in classic SCT fashion (comparing the two reflections of the inner and outer surface as they change relationship as you move the reflections across the surface). However the hole in the back, typically for SCT, MAK or Cassegrain which as I understand it is drilled out after the final figuring, would seem to be un-needed.

 

Anyone have a 5.5" Celestron Schmidt camera that's apart such that you can look at the back of the mirror? 

 

And the special order suggestion is a good one.

 

Tom 

Edited by Tom Duncan, 25 September 2024 - 10:03 AM.

[Tom Duncan]

Just found this CN thread which discusses early versions of the 5.5" Schmidt camera having a hole in the mirror for a focusing microscope. 

 

https://www.cloudyni...schmidt-camera/

 

So I think we've pinned this down, these are blanks made for the 5.5" Celestron Schmidt camera. 

 

Now, what to do with them...

 

Tom

[deSitter]

Just found this CN thread which discusses early versions of the 5.5" Schmidt camera having a hole in the mirror for a focusing microscope. 

 

https://www.cloudyni...schmidt-camera/

 

So I think we've pinned this down, these are blanks made for the 5.5" Celestron Schmidt camera. 

 

Now, what to do with them...

 

Tom

Gregorians and Cassegrains!

 

-drl

[apfever]

Oh this was fun.  I missed out. Algebra version without trig. The trig is straight Pythagoras oriented. I did not do this for this string. I have this in a notebook of derivations I use a lot. This is the exact Radius Of Curvature (ROC) as a function of cord length which would be primary diameter D, and sagitta. I use a half cord a lot in my stuff so I use a symbal "C" hmm...that's a capital C with a dash through it.  This is exact.

 

The focal length is taken to be 1/2 the ROC. This is not exact because there is no true focal point for a sphere like there is for a paraboloid. Our mirrors are a crazy small piece of the sphere they are based on (placed on, ...faced on?...traced sorry). Being small that way makes 1/2 the ROC close enough to focal length no matter how you look at it. 

 

f = focal length

R = ROC

D = diameter of primary

r = radius of primary, it is 1/2 of D. It's the only way I think I can do this.

s = sagitta

 

R = (s² + r²) / 2s

R = (s² + (D²/4)) / 2s

The above are exact.

f = (s² + (D²/4)) / 4s  

...oh wait a second...Desitter above .something slipped through the crrack here...I like elliminating s squared as s² << D²

 

f  = D² / 16s  This is a nice approximation.  Waite...Tom Day has this in entry #4.....but they aren't the same.....

 

Edit:  Entry #4 is off by a square. It should be D², not just D

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Edited by apfever, 25 September 2024 - 11:18 AM.

[Tom Duncan]

Surprisingly I received an email back from United Lens Company within hours of sending my inquiry, the Sales Office Manager Lauran Sylvia stated: 

 

"Unfortunately, we do not have records that date back that far.

Sadly, we often do not know what a customer’s final product or intended use is when we fabricate an optic, as we are only given the specifications of the one component.

Best of luck tracking down the information you’re looking for!"

 

No surprise there...

 

Tom

[DAVIDG]

 Most catadioptric system use an oversized mirror and the  fact that the blank is molded for perforation is a good clue that they were meant for cassegrain type system.  Also the cast in sagitta doesn't always indicated that it was going to be the one for the finished optics. The way United Lens makes their mirror blanks is they have a mold and take a square glass billet and heat it until it will flow and then press it into a mold. So they could have had a mold that was the correct diameter that Celestron wanted and sagitta was, what it was.

 

                       - Dave 

[Don W]

You never know what you might run into!

[CharLakeAstro]

Indeed!

John Graham mentions (in this post) that some of the later 5.5" cameras had the hole in the back of the mirror, but not bored through.

 

Found a picture of that also from the site https://www.cometchaser.de/

 

Just found this CN thread which discusses early versions of the 5.5" Schmidt camera having a hole in the mirror for a focusing microscope. 

 

https://www.cloudyni...schmidt-camera/

 

So I think we've pinned this down, these are blanks made for the 5.5" Celestron Schmidt camera. 

 

Now, what to do with them...

 

Tom

Edited by CharLakeAstro, 25 September 2024 - 06:17 PM.

[telesonic]

Just found this CN thread which discusses early versions of the 5.5" Schmidt camera having a hole in the mirror for a focusing microscope. 

 

https://www.cloudyni...schmidt-camera/

 

So I think we've pinned this down, these are blanks made for the 5.5" Celestron Schmidt camera. 

 

Now, what to do with them...

 

Tom

 

Hey, I remember that thread.... that was the one I was thinking of, but couldn't find.

 

One thing that I read on here, was something along the lines of the Celestron Comet Catcher was kind of using leftover stuff from the 5.5" Celestron Schmidt Camera.... I seem to remember that they re-purposed the corrector plates, if I'm not mistaken.

 

I suppose that would make sense, as the demand for the Schmidt Camera would have probably not been very high.

 

If these mirrors are for those, that makes them even cooler (more rare!)

 

 

With some work, these could be probably be used in a lensless Schmidt setup. I'm not sure how they would work for visual - my bet is not well, if at all, but with a CCD and correcting lens -

https://oasi.org.uk/...chmidt_CCD.php

as seen in that link, could be doable. I've read of one or two folks doing something like that with an SCT with a broken corrector plate.

 

I'm no optical wizard like some of you, but would something like a lensless Schmidt even be doable with a CCD in place of the secondary, with one of these primaries?

 

That would make a heck of a fun ATM project!

 

Cheers,

Temp