The J.F.B. Observatory

finderscope mods

A Sunday afternoon project : Illuminated crosshairs on an 8x50 Finder


Some modifications to my GSO 8x50 Right Angle Correct View Finder. Some forum posts on IIS mentioned focus issues especially the

cross hairs being out of focus. I pulled it to bits to see if I could tweak the focus of the crosshair ring and also the main focus.

Mine was slightly out on both.


Here is what I did:



When you unscrew the eyepiece barrel and flip it upside down to look into it, the first element is the crosshair ring. This doubles as

the locking ring for the entire optical assembly of the eyepiece barrel.

Gently trying to unscrew it with a screwdriver in one of the grooves, I could move it and undo it.

With a few turns and a re-check, the crosshairs were now in perfect focus, but because I loosened the lock ring/ crosshair ring, the lens elements

were all loose as well. So I had to think of a way to put in a spacer ring to make up for the gap I had introduced by loosening the crosshair ring.

Again I unscrewed the crosshair ring but this time all the way to the end of the thread, so it was ready to come out.

I flipped the whole barrel over carefully on to a piece of A4 paper and lifted off the barrel outer case.



The elements all sat on top of each other in order on the A4 and I took a moment to sketch what went where especially the direction of the

two glass elements: a doublet and a single element (does that make it a singlet?) :)


When you flip over the crosshair ring and look at it from underneath, I saw 4 grooves like the 2 screwdriver grooves on the other side of it.

These are the grooves that the hairs lay in. In this case it looks like they used very fine copper wire and held them with epoxy or superglue.

I then added a spacer between the crosshair ring (1) and the plastic spacer barrel (2). I made this from a sliver of 20mm PVC conduit and blackened it

with a black felt pen.

The height of this new spacer ring was the estimated amount I had backed the crosshair ring out to get focus. It was about 2mm.

Everyone's will probably be different. Mine is focused for MY eyes, and looks slightly blurry to my wife.


This is when I decided to do something completely rash and install a red LED to illuminate the crosshairs.


Adding the Red LED:


I re-assembled the eyepiece again and looked down into it again from the crosshairs end. Mounting a LED through the side wall would mean

drilling through both the outer barrel case and the plastic spacer (2).

This would illuminate the crosshairs from above so they should look well lit from also above as you look into the first lens (the eye lens?)

I had a choice of a few different LEDs in my junk collection and settled for a diffused lens 3mm type. I added some wires to it, added some

heatshrink to the joints and bent the legs over. I also blacked out the upward facing side of the LED to stop it shining up at me through the eyepiece.

Once again, un-assembled , I drilled the outer case where the yellow X is, and once again, re-assembled the eyepiece and carefully drilled through

the inner spacer (2). That hole ended up where the yellow X is on the pic.

I pulled it apart once again to get rid of the plastic debris from drilling and this is where I broke one hair on the crosshair!!!!!!!!!!!!!!!!!!


Replacing the crosshairs:


Fortunately, I have done this repair job before on a riflescope I use for a Finderscope, and also on a homemade finder I built once.

For both I used fibre-optic glass which is perfect because it glows under illumination and sits dead straight. No need to tension the hair

while glueing.

I do fibre optic work for a living so I had a bit lying around, stripped it back and cleaned it with Isopropyl alcohol , and glued it down with

some wood glue. An hour in the sun and it was ready to assemble. The LED illuminates beautifully on the fibre and works best when it

shines down on an equally spaced gap between both hairs. Picture hairs at 12 o'clock and 3 o' need to rotate the

crosshair ring so the LED is at 1:30.

(A 9v battery powers the LED for now via a 1 K potentiometer and 220 ohm in series for brighness adjustment.)

The 220 is for when the pot is one end and prevents a short and this doubles as the max brightness value.

Eventually I will plumb this circuit in to my general purpose 12v feed that I have on the GEM mount for all the other gadgets.

You can never have too many gadgets

Main focus was easier than all this! I just turned the plastic where the RACV moulding screws into the main white aluminium tube. It is a fine plastic thread,

firm enough to stay where you leave it.

I screwed it out about 2 turns and the finder was focused. Crosshairs, naturally still focused too.


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