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  1. #1
    wideopeniris's Avatar
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    Question Lens Formula for Aerial Photo lens B&H 915mm F6.0



    Folks,

    I have a huge aerial photography lens made by Bell and Howell, probably for military aerial reconnaissance. It is 36inch focal length (915mm) F6.0 (although it says f8 on the rim - presumably beacuase it vignettes at full aperature) and its intended for 9x9 inch film.

    The optics are basically a Tessar formula with four elements and the rear two cemented - I checked this by shining a laser through it. It also seems to have very little chromatic abberation so I suspect it is APO - I tried it with a pair of equal achromats as an eyepiece, which is not a perfect combination, and at ~70x magnification, fringing was minimal - in fact image quality was good enough for me to take the plunge and make a telescope out of it.

    I would like to know if anyone knows this lens and has an optical formula for it. I would quite like to run it up in OSLO, but need to know what glasses are used and the curvatures etc.

    The lens is very heavy and mounted in the most evil blade shutter you have ever seen (fairchild designed - I can hardly turn the main spring and the blades fire with a speed that would have your fingers off if they got in the way!) There is a wide flange for mounting it, with taperred posts - I guess it shouldnt be too difficult to mount it but any advice on bringing it to a telescope mount is apperecitaed.

    I'm not a telescope person, and have little experience with telescope mechanics. This would be my first excursion into telescope building - I know its not a very traditional start, but I have this thing and its a shame not to use it.

    Many thanks for any info,

    Kevin.

  2. #2
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    Default

    Hi Kevin,

    Welcome to Astronomy Forum.
    That sounds like an interesting project that you are undertaking.

  3. #3
    rick cope's Avatar
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    Default B&H 36 inch lens

    I have identical lens cell, front and back. I think that this lens was acually produced by perkin elmer in New England. Under "Perkin Elmer" 36 inch lens, there are several articles about focus mistakes as lens heated by sun light on the camera pod. I need the flange distence, the distence between the flange shoulder of the front large lens cell, and the back smaller cell. Once I have that distence, I can get a tube machined. does your lens have an iris, and if so what are the numbers? I have been told these were used in the U 2 program. maybe you could photo the complete lens?

    There are schematics (sic) on the web sites. go google and put it Perkin Elmer, 36 inch lns, heat variance, or some such. Another study deals with two of these lenses and notes they are made with seven elements and two of the elements are asphereical, and the designer was from Harvard etc. they cost big bucks thanks so much, the measurements are appreciated thanks



    Thanks so much.

  4. #4
    wideopeniris's Avatar
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    Default B+H or Perkin Elmer?

    The lens I have is definately 4 elements. I've shone a laser through it and you can clearly see that there are two air separated elements in the front cell and a cemented doublet in the rear (air interfaces give a bright reflected spot, whereas cemented interfaces give a less bright spot). It is most likely an APO tessar type formulation.

    The iris is marked f6.0 - f16, whereas the perkin elmer lenses I see on the web are much much bigger at f4.0 (must be immense!). This marking is consistent with the diameter of the front glass. The lens itself is marked f8 on the front - I expect it vignettes on 9x18 inch film at f6.0 - not a problem for me!

    My lens is probably 1950's vintage, before aspherics but after multicoating (its MC'd). Its mounted in a fairchild shutter with iris - unfortunately I cant disassemble it (its probably cemeted in place) so I cant measure the cell spacings.

    I'll try and russle up a couple of photo's...

    My project is well underway now - all I need is a custom tube to go over the rear cell. I need to find someone who can roll a tube and weld it for me..

    Kevin.

  5. #5
    rick cope's Avatar
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    Default

    are the flange threads sized at about 5 1/4 inches outside diameter? front cell is 20 per inch and the rear cell 28 per inch, or is that cemented so you can'see?

    Can you get an overall length from the front surface of the front cell to the rear surface on the rear cell? From that I can figure the seperation distince. Also my rear cell is marked " F.D.=10.338 IN.. Is that similar to your rear lens? Is that near your backfocus at infinity? I have a metal guy that can do anything, he re bodies classic cars. let me know if he can help. Also, at the rear of the front lens group, there is a metal baffle, kind of like a black wide mouth 90mm diameter funnel, looks like the one my mom put into the jar when pouring hot fruit to jars in the pressure cooker. That would change the f stop. the rear lens group ser number is 7515. Does it sound like I got the right rear cell to match the front cell. Front cell is ser 2002. The seller was not sure if they were for the same lens or if they matched. They do focus at at infinity at about 10 inches apart, that is the space between the front rear cell and the rear front cell. thanks so much, I was ready to give up. thanks

  6. #6
    wideopeniris's Avatar
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    Default

    OK - the lens has engraving on the rear element:

    BF 18.603 (back focal length)
    FF 21.893 (flange focal length)

    The FF is from the rear face of the mounting plane, whereas the BF is from the back of the last lens element.

    It appears that the lens rear principle plane is a little to the rear of the iris, so the focal length for magnification purposes is aproximately 880mm by my estimation.

    The rear cell _is_ about 5 1/4 inches diameter. Both cells are stuck fast, so I can see the threads! Both cells and the shutter are labelled serial no. 715 (the shutter stamped as an assembly as the shutter has its own serial number from Fairchild)

    The overall length from the edge of the back cell to the front of the front ring is 284mm.

    Some pics:

    www<dot>macrocosm<dot>info/lens2front.jpg

    .../Lens1.jpg

    .../lens3plonk.jpg

    .../lens4sideon.jpg

    (sorry to mash the addresses - antispam stops me posting urls <sigh>)

    Thanks for the offer of a contact, but since I'm in the UK it may turn out to be impractical.

    Good luck with the lens - its certainly a beauty

    Kevin.

  7. #7
    rick cope's Avatar
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    Default

    Kevin, my rear cell says f.d.= 10.338 inch, that is very close to the 284mm measurment whcih is flange to flange on your lens, I think. My rear cell serial is 7515. was your scribed rear number of 715 a misprint, and is yours 7515. if it is 7515, it means that I have the right rear element for the 36 inch. Am I correct in thinking that the flange (tube) mounting the two cells with the iris in the middle, is in the area of about 10 to 11 inches? ps couldn't get the photos to come up, can I give you my wab address, not sure of the rules. thnks so much.

  8. #8
    wideopeniris's Avatar
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    Default pictures

    you'll neet to replace the <dot> with a . and then it should work - I had a go to check and it worked for me from work.

    The serial numbers really are 715 and they should be different for each lens. It probably means that you have differnt front and rear cells - this might happen if the front element was damaged and then replaced.

    I dont really have a 'tube' - the cells are in a shutter - a big pancake, and the front had a long cover that hides the thread completely, so I can't measure the mounting thickness.

    perhaps a moderator would be kind enought to let me post links (it says I must post 5 posts to be allowed to post links)

    Kevin.

  9. #9
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    Default lets try again...

    http://www.macrocosm.info/Lens1.jpg

    http://www.macrocosm.info/lens2front.jpg

    http://www.macrocosm.info/lens3plonk.jpg



    http://www.macrocosm.info/lens4sideon.jpg

    hmmmn - very odd - if I click the link it screws up - if I cut and past the text into my browser address line it works - the joys of the internet....

    Kevin.

    PS at least I can pos the links now!

  10. #10
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    Default

    Hi,

    I am a retired optical engineer and served an apprenticeship in the U.K. with A. Kershaw and Son's in Leeds. The Rank organisation bought Wray Optical, Kershaws and Taylor Hobson in the sixties and, eventually, closed the first two down whilst Taylor Hobson became the subject of a management buy-out and now trade as, I think, Cooke Taylor Hobson.

    Anyway, back in the early seventies, after Rank closed down Wray I purchased a surplus Wray 36" F1:4 aerial Reconnaissance Lens and was lucky enough to find the lens data and drawings for in the Wray archives at work. If anyone has a Wray 36" F1:4 Night Camera Lens and needs this info I can send the optical specification along with an Autocad R14 .dwg or .DXF file of that lens.

    The Wray lens was also built around a massive shutter but had no Iris as it was always used at full aperture (Night Camera). I took some measurements before separating the front and rear lens sub-assemblies from the central shutter. Once seperated I made a cylindrical spacer exactly the same length and with the same threads as the shutter casing and reassembled the front and rear lens sub-assemblies. The only thing to remember is to make sure that the two sets of threads on the new spacer are exactly co-axial and square to the lens axis.

    I see that you are having trouble unscrewing the front and rear assemblies from the shutter. It is not normal practise to cement lenses together nor use left hand threads as lenes of this nature and cost might need to be serviced or repaired. They might have a sealant on the threads so that they can be nitrogen purged and stay so. But this would not stop you from unscrewing them.

    Get yourself a big pair of vernier calipers or micrometers and measure the distance between two obvious points on the two sub-assemblies such as over the extreme end faces. Look very carefully at the two cells to ascertain exactly how they are assembled. If unsure, get some professional help from a real (perhaps retired) engineer or befriend someone of that nature before you do anything. If you are sure that they are screwed into the body of the shutter look very closely for any small grub screws that might be locking the threads and remove them. Use the best screwdrivers or allen keys or whatever is needed to remove the screws. Use a small amount of lubricant in the screw holes and let it soak for a while. I can't stress enough the tenet: that the more care and thought you use now the more likely you are to succeed.

    Once all locking screws are removed, find a strong friend and each take hold of one of the front and rear lens assemblies and try and unscrew them (R.H. threads so anticlockwise) If that doesn't work take another look at the lens construction. If you are still of the opinion that the three parts are screwed together then look for more locking mechanisms. If there are none it is time to use a little more "gentle" and "thoughtful" force.

    Find some old car seatbelt strapping and wrap it carefully and neatly around one of the lens cells and around a piece of wood of about 2"x2" section and clamp thatpiece of wood, belts and all, in a large vice so that it grips the lens cell without allowing the cell to come into contact with anything harder than the belts and the timber. If you do it correctly, and I am sure you will find out how, it will tighten as you try to uncrew the shutter body from the lens cell thus gripping it ever more so. The more you turn the tighter the grip. You can also use the same technique of fastening another belt around either the shutter body or the other lens cell and using another 2"x2" piece of wood wrapped in the belt as a lever to give you more purchase.

    Before I go any further, I have to say that I have always managed to disassemble lenses with these techniques, on my own, without resorting to any brutality. I am sure that the components will come apart fairly easily with only a little effort using these techniques.

    Once you have separated the elements from the shutter body just measure everything up and make or have some make a threaded spacer to replace the shutter body.

    Hope that helps.

    Incidentally, this is my first posting and I don't really know how what the rules are about posting images. If I am allowed to post images of my modifications to the Wray Lens or someone wants me to e-mail them and it is within the rules then I would have no trouble sending some photos. Please advise.

    John.

 

 
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