Military ep

[markse68]

I picked up this military ep on eBay for very little money taking a chance it could be used in my scope. I think it’s from a tank or some other fighting vehicle and would have been part of a binocular pair. It came with very nice quality inverting prism assembly which I won’t use but the ep assembly just unscrewed from that. It’s quite a large chunk of glass with 32mm eye side aperture and 35mm entrance aperture though there’s no field stop as such and I’ll probably add one as it gets a bit distorted right at the edge. It has a nice wide afov as large as a Nagler but it’ll be smaller with field stop.

I have no idea what it’s focal length is though and no way to mount it yet so can’t calculate fl either so I came up with a method to estimate- does this make sense to you? I found 2 other lenses that focus approximately at the joint between body and 1.25” barrel and removed their barrels. These were a 25 and a 40mm plossl. Then I balanced each lens in turn on a ruler and looked through the ep so I could see the field stop and keeping other eye open sketched the ghost image of the width of magnified ruler on a piece of paper below the ruler. Dividing this magnified width by actual width I got magnification factors for the 3 lenses. It looks like the military ep is close to the 25mm plossl so I’m guesstimating it’s also 25mm or there abouts.

Next I rested the lenses on some graph paper illuminated from beneath by an iPad. The military lens has much lower rectilinear distortion than the plossls- better than the phone capture. If I spaced it to get perfect focus in centre of field then the edges started to defocus which isn’t great but if I moved my eye view to the edges they came back into focus. I think it might be quite a nice Widefield eyepiece.

Just need to make an adapter to 2” and get round to fitting a 2” focuser on my scope and will find out 😉

[markse68]

A block of alu arrived this morning so I’ve made a start on the 2” adapter. Decided not to add a field stop as there’s none on the prism assembly and when you position your eye right I don’t think it’ll be needed. Just need to add grub screws to lock it in place as no way was I going to succeed making a female thread for it, and paint the inside flat black. Oh and mount the focuser!

[Louis D]

What a beauty!    I can't wait to hear how it works under the stars.

[markse68]

I was getting some 3D printed parts for some other projects done so I thought I’d have a little fun with this old ep- I present my “XW” ~25-30mm 😂 It did really need an eyecup as the er is quite long and eye position quite finicky so hopefully this will help

 

Edited July 2, 2020 by markse68

[DaveL59]

looks great Mark.

Jeez what with Gina, you and others doing all these 3D parts, I more think maybe I need to go get one too. Could make up some replacement eyecups for some of the bino collection for a start. Have a feeling tho that it'll be another slippery slope financially that I don't want to be on at the moment, not to mention nowhere useful to put it here.

Edited July 2, 2020 by DaveL59

[markse68]

3 minutes ago, DaveL59 said:

looks great Mark.

Jeez what with Gina, you and others doing all these 3D parts, I more think maybe I need to go get one too. Could make up some replacement eyecups for some of the bino collection for a start. Have a feeling tho that it'll be another slippery slope financially that I don't want to be on at the moment, not to mention nowhere useful to put it here.

Thanks Dave  I got this SLS printed by a company- i don’t have my own machine. it’s nylon so really tough and they dye it black. can get quite pricey but I think quite reasonable for parts you really need or want. This was about £17 i think (ouch) but i’m pleased with it so that’s ok!

[DaveL59]

ahhh smart move. That's not too bad a price but for miniature bins that cost me <£20 ea perhaps makes it expensive, esp as it'd be 4-5 different styles. Was thinking to re-work some nylon bushes but haven't got around to so far. Where needed I can just swap eyecups between pairs if I particularly want to use one that's missing one   I guess that might make getting copies made a little easier if I ever get around to it.

[markse68]

The biggest problem is the minimum order of £40 but if you’ve got a few projects to do that’s not such a biggie. Price is based on bounding cube volume so you can stuff parts inside bigger parts and get them effectively free. I also had some eye cups and caps printed to replace the annoying rubber winged ones on my BCOs- these worked out about £1.50 each or so 👍

[Don Pensack]

Note: most of these older military eyepieces had little to no rectilinear distortion to keep straight lines straight.

As a result, they had serious angular magnification distortion, with the edges usually having a lower magnification than the center.

That may make the eyepiece tough to use for astronomy, but it would be great for daytime use in a spotting scope.

[Louis D]

Check your local library to see if they have a 3D printer available for use by card holders.  My daughter used the one at our local library to print things for fun back in high school (8 years ago).  I think there was a 4 inch cube size limit and one project per week limit.  I would imagine they have an improved printer by now.

[markse68]

On 02/07/2020 at 17:07, Don Pensack said:

Note: most of these older military eyepieces had little to no rectilinear distortion to keep straight lines straight.

As a result, they had serious angular magnification distortion, with the edges usually having a lower magnification than the center.

That may make the eyepiece tough to use for astronomy, but it would be great for daytime use in a spotting scope.

I wouldn’t have wasted my time and money on it if I didn’t find it quite nice to look through with my scope Don 😉 It’s far from perfect but there’s something  really nice about it too

[Don Pensack]

Looks like a comfortably long eye relief as well.  

[Merlin]

Quite a few military eyepieces are/where of the symmetrical  type. I had a big brass one that gave superb views of the Double Cluster with the 8-inch Newtonian.   The eyepiece was purchased  from  H. W. English the war-surplus supplier.

They were an Alladin's cave of surplus optical and other gear.

[markse68]

1 hour ago, Merlin said:

Quite a few military eyepieces are/where of the symmetrical  type. I had a big brass one that gave superb views of the Double Cluster with the 8-inch Newtonian.   The eyepiece was purchased  from  H. W. English the war-surplus supplier.

They were an Alladin's cave of surplus optical and other gear.

I think this one must be an erfle type as it has 80+deg afov. Not sure if that’s possible with a symmetrical?

[Second Time Around]

2 hours ago, Merlin said:

Quite a few military eyepieces are/where of the symmetrical  type. I had a big brass one that gave superb views of the Double Cluster with the 8-inch Newtonian.   The eyepiece was purchased  from  H. W. English the war-surplus supplier.

They were an Alladin's cave of surplus optical and other gear.

I remember them well.  I think their address was on Rayleigh Road, Hutton, Brentwood, Essex.  I regularly got their paper catalogues they advertised in Exchange and Mart.

Amongst other things I recall buying a 5/8 inch Erfle from them late 60s/early 70s.

Anyone know what happened to them?

 

[Louis D]

5 hours ago, markse68 said:

I think this one must be an erfle type as it has 80+deg afov. Not sure if that’s possible with a symmetrical?

Look at the field lens end and see if it is near to the bottom.  Also look to see if the field stop is visible below it.  Most likely it has an internal field stop and is a negative-positive design like many more modern eyepieces.  Military eyepieces were often fairly cutting edge for their time because cost was not much of an issue.

[markse68]

On 05/07/2020 at 23:21, Louis D said:

Look at the field lens end and see if it is near to the bottom.  Also look to see if the field stop is visible below it.  Most likely it has an internal field stop and is a negative-positive design like many more modern eyepieces.  Military eyepieces were often fairly cutting edge for their time because cost was not much of an issue.

Looking at this nice chart John posted in another thread I wonder if it’s a Galoc- it’s got the wide field and long er and is squat and wide like that. No i don’t think it’s got a negative lens if you mean like a barlow? I don't think it has a field stop as such either- seems to use the bottom of the housing as it’s stop. could be wrong though.