Introduction: Narrowing the IPD of the Celestron Cometron 7x50 Binoculars
These binoculars are a great value, not the best, but at ~$35 they're not so expensive you get nervous about other people touching them. However, they don't fit faces with narrow set eyes; the InterPupillary Distance (IPD) doesn't get smaller than ~60mm. Fortunately, it's just a silly design decision that you can fix. Shown is the stock range of adjustment.
All these parts probably have precise technical names, but I didn't know where to find them. I contrived names I hope you find fairly descriptive.
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Step 1: Stock Adjustment Range
Here you can see the stops on the underside that prevent the IPD from narrowing. There are similar stops on the top that limit how wide the IPD can be. There's some adjustment to be had there, but not a ton. The widest they can go in stock form is ~73mm. I bet you can get an extra 2 or 3mm, maybe 5, but probably not more than that without changing major structure.
We're simply going to file the minimum IPD stops so we can get more travel. This instructable is mostly about how these binoculars disassemble and reassemble. I wrecked a pair the first time I tried this, figuring out how to take them apart.
Step 2: Pop Off the Eyepiece Focuser Locknut Cover
You can probably pry it off with your fingernail. It's just held in place by some soft glue. Loosen up the set screws if you like.
Step 3: Remove the Eyepiece Focus Locknut
Both the eyepiece focus locknut and the spindle locknut have very small set screws that take a flat blade screwdriver. They're in an unusual spot on both locknuts. I suspect they help keep the locknuts from loosening up. I haven't paid them much attention, but I keep giving my binoculars away before I have the opportunity to notice them loosen up. They might not be that important, but if you have a small flat blade screwdriver, it's not taxing to loosen them first.
The focus locknut comes off with a flat blade screwdriver. It's made of cheesy brass, so it's helpful if you have a screwdriver that fits this well too. I used a larger flat blade screwdriver.
The left eyepiece will just slide off, followed by the right eyepiece. There's a brass thrust washer between them, and one more between the right eyepiece and the boss on the focus plunger. Don't lose or bend them. Here one has stuck to the eyepiece, the other one I had to remove from the focus plunger.
Everything that moves has some nice clear grease on it. You can try to not touch it, and reassemble without regreasing, but some parts will require it.
Shown as well is the spindle locknut to the right of the eyepiece locknut.
Step 4: Remove the Focus Plunger Drive Pin
Ok, we're getting closer. We're trying to take the spindle out, but the focus plunger is in the way. The focus plunger is held in by a pin, and that pin is behind a door under the rubber focus grip.
Peel the focus grip back. You can stick a small screwdriver in to get it started. It's easier if you start at the back and roll it forward as pictured. It's a little tricky, rotating the whole focus adjuster while peeling the grip back will help.
With the grip out of the way, you can just pop off the focus drive pin door. If you look at the bottom of the binoculars and rotate the door cavity towards you, the drive pin should come into view. I've done this 4 or 5 times now with this model of binoculars, and every one of these has had the Phillips drive of the focus drive pin really mangled. This is also brass, really soft, and we don't want to ruin this. Find a screwdriver that fits pretty good, fit it up very straight, and apply enough pressure to keep from stripping it out. Good luck.
Step 5: Remove the Focus Plunger & Spindle Cover
Ok, with the drive pin out, the focus plunger slides out.
There is a cap over the spindle on the end the focus plunger came out. This will slide off after you pry a little bit.
Step 6: Remove Spindle Locknut Cover
This cover is on the front, and will unscrew, probably with just your fingers. The center of it is threaded 1/4"-20 for a tripod adapter.
Step 7: Loosen Spindle Locknut
The spindle locknut takes an unfortunate flat blade screwdriver-like object to undo. Most screwdrivers aren't going to fit this very well. I used a gasket scraper from an auto parts store. It's nice if you can find something that fits this pretty well as it's made of aluminum and will yield before the might of your fingers if you're not very careful.
We're going to use the spindle locknut to help us get the spindle out. Loosen it ~1.5 turns.
Step 8: This Isn't Very Nice
So, we're about to do something kind of mean to our optics. The right way to get the spindle out is with a press, but it you don't have one you can get it moving with some rudeness.
Since we loosened the spindle locknut ~1.5 turns, it's protruding that much. If we put this locknut on the corner of a table and put our weight on the prism housing, it'll pop the spindle loose.
How rude is this? Well, we're not plastically deforming our prism housing. We are probably messing up the collimation, but that'll probably happen anyway when we file the stops down, and they probably got bumped out of collimation on their way to us, and these are rather cheap 7x50s. Also, you can use a press.
Once the spindle is loose, pull it out.
Step 9: Try to Keep the Parts in Order
With the spindle out now, we can separate the halves. There are some washers in there that help things move smoothly without binding or rocking, so we'll try not to lose, bend, or install them in the wrong order.
In the middle is the focus barrel. We know that we rolled the rubber grip forward. On that front side is a black washer (I think it's plastic) and some of that clear grease. There is no washer on the back side.
On both sides of the focus barrel is the whole right side. The left side surrounds the right side. Between the right and left side, and both the front and the back are brass thrust washers.
Step 10: Keep It Clean
Now we have it apart, but before we go filing, we can take a couple preventative steps to help keep dirt from getting in our pivots and optics. Cleaning the grease off the pivot faces with a paper towel will keep aluminum filings from sticking to the grease. We can wrap the whole housing to keep filings out of the open housing and away from the objectve lens & cover. Stretch wrap or plastic bags with masking tape work fine. Leave the ears exposed, that's what we're filing.
Step 11: Give It the Business
Try not to squeeze too hard when holding the binocular half. It's aluminum and easy to crush.
This pair got filed down about 6mm/1/4" on each of the four stops. I used a square (flat would be great too) file and stayed away from the face that the greased washer sits on until I got close and then slowly worked my way in.
At 6mm, the stops on the eyepiece will hit first, and you can feel it flex and it's kinda gross. You can do the same filing with the eyepieces, but you might wait until you've reassembled the body to see how much looks like the right amount.
If you have aluminum filings everywhere, as long as they didn't work their way into the body, they'll clean up ok with alcohol and paper towels.
Step 12: Reassemble
Make sure there's grease on all the sliding surfaces. I had a dollop of grease on the end of the focus plunger, so I used that. It might be silicone based food safe grease. That's a great option as the lubrication requirements are not that demanding and people's hands could come into contact with the grease some. Any grease intended for lubrication would work great.
Reassemble everything in the order it came apart:
From the inside moving out:
Focus barrel with the grip rolled forward with black washer forward
Right side body
Brass washers on both outer sides of the right side body
With that in place, insert the spindle. There are two notches in it that can line up with a nub on the back face of the left side body. Orient it so the slot in the spindle for the drive pin is facing down.
Step 13: Button It Up
Put the spindle lock nut on finger tight.
Insert the focus plunger with the threaded hole for the drive pin aligned with the slot in the spindle.
Push on the focus plunger cap.
Install the focus drive pin taking care not to strip it's head or threads. Verify the focus travel seems reasonable.
Place the focus drive pin door in it's home and cover it with the rubber grip.
Tighten up the spindle lock nut and snug its setscrew.
Screw on the spindle lock nut cover.
One of the smaller brass washers goes on the focus plunger.
The right eyepiece goes on next, followed by its washer, then the left eyepiece.
This is the time to check the eyepiece stops to see if you want to file them too.
The focus locknut is next, after it's tight snug up the little setscrews.
The focus locknut cover needs to be glued back on, but you don't need any serious glue. Double sided tape can work if it's the real sticky stuff like carpet tape. Hot glue it fine too. I'd stay away from superglue or epoxy. If you need to get back in epoxy might give you some real trouble. Superglue will probably be able to be peeled off because the rubber is flexible, but it might make getting the locknut off difficult depending where is sneaks into.
Step 14: Collimate & Enjoy
It's probably worthwhile googling for "binoculars collimation" and following instructions, or knowing that you're choosing to ignore that. Here's an instructable from someone else you can try: https://www.instructables.com/id/How-to-Fix-Double-Vision-on-Binoculars/
Go look at stuff!