Combining objective lenses from binoculars

[tombardier]

Hi,

I have two pairs of Celestron binoculars that are basically junk.  I bought some 20x80s just a week or two ago (very very cheaply), but as with my previous 25x70s, they lost collimation literally after my first session.  It seems that the prisms have been pushed too far in, and the screw adjustments won't bring them out again.  I've got cats-eye exit pupils on the 20x80s, and can't get a merged image, and on the 25x70s I can get a merged image, but I don't think it's collimated at all properly, and it's making my eyes go a bit squiffy. 

Rather than repair them, I want to use the parts to make fun things.  I thought I could make telescopes, making use of my 3d-printer and various other tools.  I could probably repair them, and that's not entirely off the cards, but I'm seeing what my options are first.

I was wondering if anyone could school me in whether there are any interesting ways of combining the 4 objectives I have?  I guess they're achromatic crown/flint?  Is it possible to increase the focal length?

Is there any software I could use to simulate the effects of different combinations? 

Any advice welcome.

 

[Astrobits]

You can't change the focal length without re-grinding the glass components but you can use one lens as a transfer lens to place the final image at a different location and size.

Take one lens ( I am using the term 'lens' to mean the doublet ) and with an object at infinity the image is placed at the focal length the other side of the lens. This image can then be used as an object for a second lens to produce a secondary image. If the second lens is placed so that it's object ( the original image) is at it's focal length, then the second image will be at infinity and infinitely large. If the distance between the second lens and it's object is twice the focal length then it's image will also be at twice the distance from the lens and will be the same size as the image produced by the first lens. Varying the position of the second lens will produce an image of varying distances and sizes.

Use the formula : 1/object distance + 1/image distance = 1/focal length ( rearrange as required) to get some spacing data. You will, of course, need to measure the focal lengths of the lenses, if only roughly, to get meaningful data. Image size is a direct ratio of the two distances.

For an example using focal lengths of 300mm ( yours will almost certainly be longer):

1st lens to image =300mm

image to 2nd lens =500mm

second lens to 2nd image =750mm ( from the formula)

Total length 1st lens to final image = 1550mm and the image will be 50% ( 500:750 =1.5x) larger than the original.

Nigel

 

 

[tombardier]

Hi @Astrobits, and thank you.  That's just the sort of thing I was after.  If I can double check what you're describing, would it look something like this arrangement?  The images is 1.5x the size, and I guess that means I'm losing light from the periphery, as shown in this diagram?  Link to the online tool I used is here - https://ricktu288.github.io/ray-optics/simulator/?en.  It's quite slow, and very frustrating to use, so any suggestions for something similar are appreciated!

[vlaiv]

8 minutes ago, tombardier said:

If I can double check what you're describing, would it look something like this arrangement? 

No, it will be something like this:

this is "relay" arrangement.

First lens takes image at its focal plane and projects it to infinity - creating collimated beam. Second lens operates as "telescope" - taking beam of light coming from "infinity" and projecting it on focal plane.

Object and image are both at focal planes of respective lens and any amplification of image will depend on ratio of focal lengths of lenses - it can act both to magnify or compress the image.

Here are some fun projects that you can make with binocular parts:

- 3d printed telescope / finder scope / wide field instrument

- virtual / electronic telescope. This is rather interesting project as such telescopes otherwise cost thousands of pounds and you'll be making "half of it" very cheaply.

Setup consists out of:

1. collimating lens

2. objective lens

3. eyepiece

4. mobile phone

Schematics is a bit like above relay lens setup, and goes like this:

Then you can display astronomical object on your phone screen and "observe" it at eyepiece like when using proper telescope.

If you hook up such device to EEVA style telescope that records things in real time - you get electronic telescope.

- you can create home cinema projector with lens and smart phone as well (works best in very dark room with small wall image).

 

[Astrobits]

If you are looking at objects far away, end of garden, stars etc then the arrangement is as I have crudely drawn here:

 

A relay lens does just what it says, it relays an image from one place to another. The top arrangement is having the focal point of the second ( relay ) lens at the focus of the first lens and it is producing an image at infinity. In the lower arrangement the second lens has been moved back so that the final image is now a positive image that can be accessed with eyepiece or camera.

Vlaiv's arrangement would be found in a spectroscopy set-up where the leftmost focal point is at an image produced by another lens ( telescope ) and a diffraction grating will be placed between the two lenses. It is also the basis for the new, high-end light microscopes which have infinity optical arrangements that allow more interesting optical effects to be used. In microscopy the object at the leftmost focus would be a small specimen, flea, fly's wing, plant cell etc.

Nigel

Edited February 15, 2022 by Astrobits

[strayring]

I have made a 2 inch wide field low mag eyepiece based on Red Henry's Double plossl design. This requires 2 pairs of bino objective lenses each with the curved side facing each other.

There is a facebook group called "A Second Look: Reusing Old Lenses for Astronomy",  you can find all the info there.

[tombardier]

Thank you all.  I haven't settled on anything yet but with I should be able to do at least a couple of fun things with two pairs of binoculars.  I'm certainly going to rig something up so I can play with the relay lens idea!

@strayring, believe it or not I joined that group yesterday. Lots of great information, and I really like what Red Henry has done by sending eyepieces to people!

I bought some more of the 20x80s, because I really liked the star field views, and they'll do me until I get around to buying some slightly more apochromatic 45 or 90 degree angled ones with swappable eyepieces.

Thanks again.