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A 5-3/4 inch diameter telescope objective

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#1 clamchip

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Posted 13 March 2018 - 05:07 PM

I recently purchased this lens and thought with the odd diameter I would  find the maker pretty

quickly but no luck. Anyone have a guess?

its 5-3/4" diameter, coated on all four surfaces. If I place the elements with the arrows pointing at

each other R2 and R3 appear to have the same radii, and this would make R1 a slightly smaller

radius. R4 is flat.

If I have both arrows pointing at the sky it feels like R2 is touching the center of R3 because of

R2's smaller radius.

Its interesting the distinctly different glass colors of the flint (F) and the crown©.

It may be a Baker or a Littrow, or a Cook, or another design. I feel fairly positive it is a commercialy

made lens, and with the odd diameter what telescope is it from, or who made it?

I measured the focal length roughly 80-1/2 inches, and that makes it exactly f/14.

If I use 5-5/8" which would probably be the mounted clear aperture, it becomes f/14.3, providing

my rough focal length measurement truly is 80-1/2".

I thought the 5.75" x 80-1/2" might be metric and that would make the lens 146mm x 2045mm

Yes the black splotches are clam chips!

Robert

 

IMG_8439.jpg

IMG_8435.jpg

IMG_8436.jpg


Edited by actionhac, 13 March 2018 - 05:24 PM.

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#2 clamchip

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Posted 13 March 2018 - 08:11 PM

Something I just thought of, I'll bet a bowl of clam chowder and a handful of saltines  R4 is optically

pretty flat and I can use it for a test flat! and it's in perfect condition, not a scratch, a few clams, but

no scratches!

 

Robert


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#3 clamchip

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Posted 11 December 2018 - 07:48 PM

I was upstairs looking for a finder and thought, WOW! this box is heavy! and it's unmarked!

Oh THAT lens! I'd forgotten about this one!

And with this revisit, anyone have a guess as to the maker?

Robert


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#4 clamchip

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Posted 11 December 2018 - 08:05 PM

I was thinking being such a odd ball, and it came with some other really strange lenses that

maybe a submarine periscope lens? no probably not, but this old periscope manual is very

interesting, take a lookcrazyeyes.gif

https://maritime.org...scope/index.htm

 

Robert


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#5 bremms

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Posted 11 December 2018 - 09:09 PM

That size and focal length screams telescope objective. A Littrow design has R1 R2 and R3 all the same radius and R4 flat. It has a little residual spherical and some coma. Easier to make than other designs, but it has slightly poorer performance. 

 

The Fraunhoffer, Steinheil, Baker and Clark designs are all pretty much coma free and fully corrected for spherical aberration.


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#6 clamchip

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Posted 30 June 2020 - 12:50 PM

I DPAC this lens today.

The test answered some questions I had about this lens.

It is indeed 'arrows point at each other' with the edge arrows.

This makes R2 and R3 the same radii, you can feel the woosh and suction

when placing the elements together. R1 is a smaller radius, and R4 is flat.

Also it is a edge contact lens. And I'm wondering about this.

If R2 and R3 are the same I wonder if any thickness spacer will work.

Clearly by the test results it's fine edge contact, and I suppose this prevents

any ghost.

I measured the focal length from the sky side of the objective to the grating at

81-1/4" from the eye side of the lens to grating 80". I'm not sure which is

the actual focal length.

I used my Meade field tripod for the test because the fl is longer than my bench.

It was very difficult to get everything on axis. I think its good enough I'm going to

build a telescope for it.

Everything about the lens points to Europe to me, in particular British. Speculation

on my part, given the design, and the unusual diameter.

Robert 

 

IMG_9866.jpg

IMG_9865.jpg

IMG_9863.jpg


Edited by clamchip, 30 June 2020 - 01:00 PM.

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#7 ccwemyss

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Posted 30 June 2020 - 01:54 PM

Any chance it is meant to be oil spaced?

 

Chip W. 



#8 clamchip

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Posted 30 June 2020 - 04:11 PM

Any chance it is meant to be oil spaced?

 

Chip W. 

It would be a good candidate for oil except all 4 surfaces are coated so I suspect it was

not meant to be oil spaced.

I'm not too thrilled about R2 and R3 in complete contact like they are, if any spec of dirt

got in there it could do some damage grinding away. I'll be sure and keep the retaining

ring tight to keep foreign objects out. 

 

I wish I knew more about this lens.

I've been researching and have not found the design.

I guess it's not a Littrow. Littrow R1=R2=R3, R4 flat.  

Not Baker or Clark as far as I can tell.

 

Robert



#9 davidmcgo

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Posted 30 June 2020 - 05:03 PM

The fringes look curved enough it would be worth playing with oiling and if that doesn’t help then spacing.  Oil or cement have a different refractive index than air and if the curves are equal it was probably designed to cement or otherwise make a non air gapped stack.

Dave



#10 clamchip

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Posted 30 June 2020 - 06:20 PM

My photo is inside focus lines bowing outward.

And outside focus the lines are bowing inward.

But its so close I think I'll get the lens mounted in a tube and re-check.

These are all inside focus, but I don't think they are in perfect alignment

for tilt of the lens and the flat and on axis with the tester.

At 80 inches separation between the tester and lens I was having

difficulties, in fact I almost threw in the towel and then I remembered

my grandpa mom's side a London taxi driver for 50 years saying

"keep a stiff upper lip and get the job done"

 

IMG_9862.JPG

IMG_9863.JPG

IMG_9864.JPG


Edited by clamchip, 30 June 2020 - 06:22 PM.

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#11 clamchip

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Posted 30 June 2020 - 07:53 PM

I think I will play with spacing the elements.

It doesn't make sense the maker would coat the air space unless there was indeed a

air space.

And the fact that the lines do bow indicates it may be off.

I will mount the lens on a tube first, its far easier to align everything.

 

Robert



#12 DAVIDG

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Posted 30 June 2020 - 08:10 PM

 I would start with 0.003" spacer and  see what the correction looks like, if still over double it to 0.006" and see if the correction is getting better or worse. That will tell you which way to go with the thickness. If the figure is off then the spacer thickness won't fix the correction  problem. Good luck.

 

             - Dave 


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#13 clamchip

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Posted 30 June 2020 - 08:20 PM

 I would start with 0.003" spacer and  see what the correction looks like, if still over double it to 0.006" and see if the correction is getting better or worse. That will tell you which way to go with the thickness. If the figure is off then the spacer thickness won't fix the correction  problem. Good luck.

 

             - Dave 

Thanks Dave, that's what I'll try.

I have a feeling this lens is a winner.

 

Robert



#14 RichA

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Posted 30 June 2020 - 10:13 PM

I recently purchased this lens and thought with the odd diameter I would  find the maker pretty

quickly but no luck. Anyone have a guess?

its 5-3/4" diameter, coated on all four surfaces. If I place the elements with the arrows pointing at

each other R2 and R3 appear to have the same radii, and this would make R1 a slightly smaller

radius. R4 is flat.

If I have both arrows pointing at the sky it feels like R2 is touching the center of R3 because of

R2's smaller radius.

Its interesting the distinctly different glass colors of the flint (F) and the crown©.

It may be a Baker or a Littrow, or a Cook, or another design. I feel fairly positive it is a commercialy

made lens, and with the odd diameter what telescope is it from, or who made it?

I measured the focal length roughly 80-1/2 inches, and that makes it exactly f/14.

If I use 5-5/8" which would probably be the mounted clear aperture, it becomes f/14.3, providing

my rough focal length measurement truly is 80-1/2".

I thought the 5.75" x 80-1/2" might be metric and that would make the lens 146mm x 2045mm

Yes the black splotches are clam chips!

Robert

 

attachicon.gifIMG_8439.jpg

attachicon.gifIMG_8435.jpg

attachicon.gifIMG_8436.jpg

Might make more sense to mask down the whole outer aperture, if you decide to do anything with the lens.



#15 clamchip

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Posted 30 June 2020 - 10:41 PM

The lens came with a mask reducing its aperture to about 4 inches.

I'll give the lens a chance at it's native aperture and compare the views

with my 4 inch Edmund.

No point hauling around a 6 inch telescope that is no better than a 4 inch

so off with it's head.

 

Robert


Edited by clamchip, 30 June 2020 - 10:42 PM.

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#16 DAVIDG

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Posted 01 July 2020 - 08:57 AM

 Robert,

     You said that R2 and R3 are equal. How did you determine that ? If  you place them in contact and shine monochrome or semi-monochrome light on them you should see interference fringes. If they are truly equal you won't get rings but straight fringes. My bet is that the design is the standard fraunhofer design were R2 and R3 are close in radius but not equal. If so you  will see rings. If that is the case the spacer thickness will between 0.003" to 0.005 to 6. If they are equal then it is most likely a Baker design and the spacer need to be much thicker as in 3/16" or so..

 

                Dave 


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#17 DAVIDG

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Posted 01 July 2020 - 09:08 AM

Robert,

   I just saw the picture of your tester and noticed the green filter looks to be positioned so it between the grating the optics being tested. If so any optical error in the filter is adding into the total test results and being added twice.  The filter should be placed  after the grating and behind the light source   So your only looking through the filter and only the light coming from the optics being tested pass through it once. In  that position it doesn't add any additions errors to the results. So the optical layout should be the light source, then the grating , light out to the optics being tested, reflected off the optical flat, back through the grating and finally through the green filter.

 

 

                        - Dave 


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#18 clamchip

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Posted 01 July 2020 - 10:02 AM

R2 and R3 will almost stick together like there isn't even room for air.

You can feel the suction when pulling straight apart.

I don't dare slide or twist apart for fear of any grit between the two.

I like your suggestion Dave, wonderful idea, test with light and check for

lines or rings.

And I will fix my tester so I'm not testing the filter, thanks.

 

Robert


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#19 DAVIDG

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Posted 01 July 2020 - 10:29 AM

 In  the  Fraunhofer design the two inner radius are different but that difference can by only few inches in the radius of each curve  That translates into a few thousands in the differences in the  sagitta of curves so they can feel like they are the same.  With unknown optics  what you usually do is measure the radii with a spherometer, the thickness and weight of each element. From there you can get a good idea of the glass type and knowing the radii and thickness put those number into optical program like OSLO and see what it looks like. 

 The fact that you tested the  lens in DPAC and it show that it forms a pretty good images shows that it was meant to be used with source at infinity and not some other purpose. 

    I'm 99% sure it was meant to be air spaced. While cemented lenses are usually made with the two inner radius being equal, then it would have come cemented and as you said that  the surfaces are AR coated. You coat surfaces that are meant since is a waste of money.

 

                - Dave


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#20 DAVIDG

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Posted 01 July 2020 - 10:39 AM

 I have the  Baker designed coded up in OSLO so all I need to do is scale it for different focal lengths and it calculates all the parameters for the lens. So for a 80" focal length lens. Air space is 0.248" and the two inner radii are +/- 28.679"

So you can directly Foucault/Ronchi test the concave surface of the flint element and measure the radius to see what it comes out to.

 

 

                   - Dave 


Edited by DAVIDG, 01 July 2020 - 11:23 AM.


#21 tim53

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Posted 01 July 2020 - 12:38 PM

 In  the  Fraunhofer design the two inner radius are different but that difference can by only few inches in the radius of each curve  That translates into a few thousands in the differences in the  sagitta of curves so they can feel like they are the same.  With unknown optics  what you usually do is measure the radii with a spherometer, the thickness and weight of each element. From there you can get a good idea of the glass type and knowing the radii and thickness put those number into optical program like OSLO and see what it looks like. 

 The fact that you tested the  lens in DPAC and it show that it forms a pretty good images shows that it was meant to be used with source at infinity and not some other purpose. 

    I'm 99% sure it was meant to be air spaced. While cemented lenses are usually made with the two inner radius being equal, then it would have come cemented and as you said that  the surfaces are AR coated. You coat surfaces that are meant since is a waste of money.

 

                - Dave

One really nifty way to determine density of the glass is to hang the lens on a string from an accurate balance and weigh it, first in air, then in water.  The water will buoy up the lens such that you can determine the density of the glass by taking the weight in air and dividing it by the weight in air minus the weight in water.

 

Measuring-the-specific-gravity-of-quartz


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#22 Terra Nova

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Posted 01 July 2020 - 04:06 PM

One really nifty way to determine density of the glass is to hang the lens on a string from an accurate balance and weigh it, first in air, then in water.  The water will buoy up the lens such that you can determine the density of the glass by taking the weight in air and dividing it by the weight in air minus the weight in water.

 

Measuring-the-specific-gravity-of-quartz

And density measured or defined in that way was classically referred to a SPECIFIC GRAVITY, which technically speaking is the ratio the density of a particular substance to that of water. Thus the average density of granite is 2.7 grams per cubic centimeter or 2700 Kilograms per cubic Meter, and its average specific gravity is 2.7!

 

PS, The story goes that Archimedes was first think up this method while lying in his bathtub trying to figure out how to accurately determine the amount of gold in the king's crown without damaging it, under pain of death, mind you! wink.gif


Edited by Terra Nova, 01 July 2020 - 04:10 PM.

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#23 clamchip

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Posted 01 July 2020 - 06:01 PM

R2 and R3 radii are not the same, I see Newton's rings with the edge contact

and probably with .003" spacers I'd see them farther out towards the lens edge.

Very faint, too faint to photograph with my overhead florescent shop light.  

The rings are obvious in the central 1/3rd.

I measured the R3 Radius of Curvature at 35-1/2"

R1 is a smaller radius then R2, if I try and mate R1 with R3 they touch in 

the middle.

R4 is flat. I placed R4 on my flat and I have straight fringes.

It's looking like a Fraunhofer? except for the flat R4 and the R1 stronger than

R2.

 

Robert


Edited by clamchip, 01 July 2020 - 07:41 PM.

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#24 clamchip

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Posted 01 July 2020 - 08:41 PM

I'm going to barrow my Brandt tube for this lens, they share the same focal length, 80 inches.

I'm not sure what I have but I feel lucky.

It could be all the clamchips, my best lenses are pretty chipped up.

Robert

 

post-50896-0-29187500-1451784417_thumb.jpg


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#25 clamchip

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Posted 02 July 2020 - 01:39 PM

I went upstairs this morning (nice and cool) armed with a variety of spacer.

I started out edge contact, inside focus line bow out, outside focus lines bow in

as shown in previous photos.

Increasing spacer thickness at first seemed to increase bowing, I went out as 

far as .006".

But then I spent more time getting everything aligned more precisely, and I

decided to dedicate my whole morning to this important project.

Again I went up and down with spacer thickness until I arrived at .0035" as

best. Straight lines, Newton's rings look right, looks fine.

Actually it's not that fussy about the spacer thickness, .003" thru .004" seemed

to yield the same results, but it's extremely fussy about everything being

square and on axis otherwise you get false results.

This lens acts like a Fraunhofer, like any other Frounhofer I've worked with.

Except the flat R4 and R1 being a stronger radius when usually R1 is weaker.

R2 and R3 as Dave mentioned are very very close to the same radii as they

should be for a Fraunhofer.

The color of the glass strikes me as unusual, and I'm wondering if the lens

recipe was adjusted to accommodate something other than the usual glass for

a Fraunhofer.

 

IMG_9868.jpg

IMG_9871.jpg

  

 


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