turning an old spy lense into a telescope?

[The Admiral]

A few of things have come out of this thread so far that have caught my eye.

1. The effective focal length is stated as 16.3", and you say the length of the lens is 16¾", from which I take that the image should form around the back of the barrel.
2. There are clearly mounting brackets within the baffle tube. What were they for? Camera, other optics?
3. A lens appears to be visible within the baffle tube. This reminds of a corrected Schmidt-Cassegrain (SCT) type optic.

It is worth having a look at the Celestron Edge white paper (https://s3.amazonaws.com/celestron-site-support-files/support_files/edgehd_whitepaper_final.pdf) and compare what you have with that instrument. The beauty of the Edge is that it has the simplest of all mirrors, i.e. spherical, and given the era I guess production of other mirror shapes at that time would have been extremely difficult(?), so it lends itself to this use. In order to achieve a flat field they incorporate correction lenses within the baffle tube. I imagine that the relatively large central obstruction is necessary in order to achieve a large enough field of view. Just my two pennyworth.

Edit. A couple of other things have since come to mind. Reading a little on the internet suggests that the focal length is perhaps a bit short for high flying reconnaissance aircraft. So perhaps it was used in low flying aircraft for mapping purposes rather than espionage. For that purpose the assembly might well have tracked the ground during exposure to prevent motion blur, or, alternatively, the film may have advanced at the correct rate. Who knows?

Ian

[The Admiral]

Looking at Wiki, I came across this:

"During the 1960s - 1980s the company designed and built a variety of optical and lens systems for military use including weapons sighting systems, gun cameras, night reconnaissance gear, and camera systems. They provided lenses for the camera systems used in U-2 and SR-71 Blackbird spy planes including huge 36 inch f/4.0 lenses.^[2] Strange Perkin-Elmer lenses from this era are frequently found from military surplus vendors and online auction websites."

So I do think it might be worth contacting the company.

Ian

[Ricochet]

I may be corrected by someone more knowledgeable but looking at your pictures I think we can take the following:

In this first picture from just behind the rear of the telescope we can see an internal chamber with a baffle hole/lens at the end through which we can see some reflections of the mirrors inside the telescope, but mostly the reflection back through that baffle. This chamber must be behind the primary mirror and where the original camera sensor/film was located and so to use the instrument you will have to fit a camera into this chamber. I don't think you will be able to easily convert this for visual use. You would have to shorten the tube so that the baffle we can see inside the chamber is the rear of the telescope.

To get this picture it looks like you have put one camera inside the chamber seen in the previous photo and then taken a photo of that camera's vewfinder with a second camera. It looks like you can now see out of the entirety of the front corrector plate, so the camera is at roughly the right position. However, you have put a camera with a lens on into the chamber so you are seeing the inside of the telescope, not collecting the light to see through it. If you put a camera at roughly this position, but with no lens attached to the camera, you may find that things start coming into focus.

[DaveS]

I think Riccochet is right. I couldn't make out what I was seeing at first, thinking it must be the secondary, but if it's actually additional correcting optics, then it makes sense.

What you've got here is one heck of an imaging lens. Forget about visual, and use it as it was intended.

[RT65CB-SWL]

6 hours ago, DaveS said:

I think Riccochet is right. I couldn't make out what I was seeing at first, thinking it must be the secondary, but if it's actually additional correcting optics, then it makes sense.

What you've got here is one heck of an imaging lens. Forget about visual, and use it as it was intended.

And 'we' were all thinking it was a catadioptric telescope... ...doh!

[DaveS]

Well yes, it is obviously a catadioptric design, the question being its exact optical configuration.

[Rick_It]

From telescope optics (Rutten, Van Venrooij):

Flat field SC are optimized for wide-field astrophotography on flat film.The flat focal surface is achieved, as it is in Cassegrains, by making the radii of curvature of the primary and of secondarymirrors almost equal. As a result of this measure, the diameter of the secondary mirror becomes rather large -45% to 60% of the diameter of the primary- when the focal surface lies behind the primary mirror. These photographic  systems are poor for visual use and high magnification.

Then in the book there is an example (with all parameters to build the telescope) of an 8" f4 system, which has a central obstruction of 55%.

Now, the book is describing a STANDARD SC. From the picture above, the telescope has clearly a built-in corrector (like Celestron EDGE). I think this system is a circa f3-f4 SC plus an integrated reducer to go down to this amazing f1.2.

If you make it work, you will take amazing widefield pictures of the sky using a large sensor. According to the book, the focus should not be internal to the telescope, but the reducer may push the focus inside (who knows..). In this case, a camera with a cylindric form factor should work.

[winters solace]

On 4/9/2018 at 06:10, Ricochet said:

I may be corrected by someone more knowledgeable but looking at your pictures I think we can take the following:

In this first picture from just behind the rear of the telescope we can see an internal chamber with a baffle hole/lens at the end through which we can see some reflections of the mirrors inside the telescope, but mostly the reflection back through that baffle. This chamber must be behind the primary mirror and where the original camera sensor/film was located and so to use the instrument you will have to fit a camera into this chamber. I don't think you will be able to easily convert this for visual use. You would have to shorten the tube so that the baffle we can see inside the chamber is the rear of the telescope.

To get this picture it looks like you have put one camera inside the chamber seen in the previous photo and then taken a photo of that camera's vewfinder with a second camera. It looks like you can now see out of the entirety of the front corrector plate, so the camera is at roughly the right position. However, you have put a camera with a lens on into the chamber so you are seeing the inside of the telescope, not collecting the light to see through it. If you put a camera at roughly this position, but with no lens attached to the camera, you may find that things start coming into focus.

if i were to look through  that light with the naked eye i can make it come into focus but it has no magnification at all. thats just light from the front correcter plate light you said. that light never hit the mirror at the bottom of the telescope.

 

 

i added a diagram of whats going on inside as best as i can tell. hopefully it helps. i really have no idea what that "central blockage" is there for.

 

 

[wimvb]

Looks almost like the LSST optical design:

https://www.lsst.org/about/tel-site/optical_design

But hardly practical for a "small" telescope, it seems.

[Apharov]

17 hours ago, winters solace said:

if i were to look through  that light with the naked eye i can make it come into focus but it has no magnification at all. thats just light from the front correcter plate light you said. that light never hit the mirror at the bottom of the telescope.

 

 

i added a diagram of whats going on inside as best as i can tell. hopefully it helps. i really have no idea what that "central blockage" is there for.

 

 

Any chance the central block could in fact be an IR pass filter?

[winters solace]

hi guys this project isnt forgotten but i really dont know where to go from here. it doesnt appear that central blockage is any kind of IR filter. ill look through it again tomorrow when i find the time but it appears is just a chunk os solid plastic or metal. but as i put in my illustration there is a mirror in from of it and i seriously think its some kind of cassegrain setup just with an extra p

[Ricochet]

I see no evidence of a "central blockage" as you have drawn on your diagram in any of the photos. All I see are the shadows of the secondary mirror and the baffle through the primary mirror. I am confident that any SCT owner taking a photo through the visual back would produce a similar image. 

As before, you need to place a camera with no lens attached into the camera chamber and move it back and forth to find the focal point. At this point you will not see the shadows in the central area.

[dannybgoode]

Interesting bit of kit. Love old stuff like this whether or not it can be made to work. 

As a slight aside the mirror in Hubble was in fact brilliantly made - it was the test equipment that was out and caused the wrong curvature. 

In fact it is because it was so precisely ground (albeit to the wrong specifications) that meant a fix could be engineered so nothing inherently wrong with the manufacturing process. 

[henry b]

Interesting !   i am  waiting  to see  it  produce  an  image . It  has  been  some  time,  the  correct  test  procedure  has  already  been  proposed  by  another  contributer .      henry b.

[winters solace]

UPDATE GUYS! i took the back of the lenses off and i can now look inside. there is no central blockage. i was wrong about that. but i took pictures for you guys to see.  it definitely is a cassegrain design but with an extra lenses above the primary mirror. 

does this give anyone any ideas on how to use it?

[winters solace]

i'm thinking the lenses is taking the light from the secondary mirror and "blowing it up" my guess is for the original camera that was in there. this appears to be the kind of lens in it.

https://www.google.com/imgres?imgurl=https%3A%2F%2Fmicro.magnet.fsu.edu%2Foptics%2Flightandcolor%2Fimages%2Flensesfigure6.jpg&imgrefurl=https%3A%2F%2Fmicro.magnet.fsu.edu%2Foptics%2Flightandcolor%2Flenses.html&docid=zKp40Za_AZciUM&tbnid=wSb6Mz2YGnId_M%3A&vet=10ahUKEwjwp9_XgKzbAhUeoYMKHekQA6AQMwjbASgiMCI..i&w=179&h=239&bih=949&biw=1920&q=convex concave lenses&ved=0ahUKEwjwp9_XgKzbAhUeoYMKHekQA6AQMwjbASgiMCI&iact=mrc&uact=8

i may be wrong though. it may be two lenses i see here not just one.

[henry b]

I have watched with interest, you did have a very fast wide field imaging system that now will now require re collimating!  reasemble and put a ccd senser behind the primary mirror and corrector lens . The distance i suspect could be short and the senser size required might be full 35mm.     henry b.

[winters solace]

the problem is i dont have anything like that. i also dont know how to collimate a cassegrain? in one of the pics you can see 3 adjustment screws to move the mirrors angel around and a central one which is used to keep its position. i assume that part works just like a dobsonian

i took my 25mm plossl i have and moved it about within the chamber. the closer it got. the more of that central plastic/metal piece in the middle of the secondary mirror it saw. at no points did it even try to come into focus

ive been looking around at different types of lenses to see what they do. and as i linked above i think that lens (lenses?) purpose was to "blow the image up" for the camera. im guess this was done to spread the image across a VERY large piece of film so i could take the light that was brought through the lens and put it on a large piece of film that (with a large surface area) wouldnt need to be exposed as long to take the picture.

i speculate that if those lenses were removed it would be a nice straight line of light that could go to an eyepiece like most other cassegrains

 

 

 

[henry b]

Thanks for your reply, i have sent you a PM. Please reply i can explain and help possibly. henry b.

[winters solace]

i took it apart again. the secondary mirror is completely flat. and the lens at the back appears to to convex. i also managed to take those lenses out and my hypothesis was incorrect. the light doesnt come to a focal point anywhere with out it. 

[winters solace]

sooooo i have a REALLY BAD good idea?

could i convert this into a dobsonian? i know the f ratio is garbage and this lens. which is calculated by focal length divided by aperture size. which gives it its crappy 1.21 f ratio. 

now if i were to take the primary mirror out get rid of everything else. im left with a mirror with its giant gapping whole in the middle of it. at first i thought this was s problem but i took my crappy bushnell telescope and hot glued a lens cap in the middle of its primary mirror put it back together and i could still see perfectly through it, albiet a bit darker. 

with that out of the way i calculated the surface area of the mirror from the lens.

(quick break down of the math these are rough calculations)

surface area of gap

7/2=3.5^2=12.25Xpi=38.485

13.5/2=6.75^2=~45Xpi=141.371-38.485-102 square inches.

my current 8" dobsonian

8/2=4^2=16 X pi =50.265

so it has roughly twice the surface area.

so then i got the idea. i know i cant change the primary mirrors "focal point" and make that longer to achieve a high f ratio. but lets suppose i went ahead and turned it into a dobsonian. can i just use barlows to fix that problem? i mean 1.2 with a 2x barlow is 2.4 then 3.6 with a 3x barlow etc. could i use this to get a more tame f ratio? or would that straight up not work? i know barlows change the focal length of a telescope so i figured it may be a possibility

 

[henry b]

What did you want the lens to work as,  a dob! or sct!  it wont do either, in fact it probably has an exchange value with someone who wants a fast imaging lens like me!  henry b.

[winters solace]

im not talking about the whole lens. but recycling its primary mirror into a dob  

im wondering if its possible.

[vlaiv]

1 minute ago, winters solace said:

im not talking about the whole lens. but recycling its primary mirror into a dob  

im wondering if its possible.

Probably not, it is very unlikely that it is parabolic mirror, and it will probably be extremely fast mirror like F/2 or something like that. If it is spherical mirror - you will need corrective optics on it to make it usable - otherwise it will be really blurry. If it is parabolic - coma will be extreme.

[winters solace]

well im pretty sure its parabolic. it looks exactly the same as my dobs mirror snd that tiny one minus the gapping hole. why would it make lots of coma. becuase of the hole? also its 4am here ill be on tomrrow