4mm eyepiece came apart in my scope....looking for advice

[quesne]

Hello All

Having an early peek at the moon about 30 minutes ago, I slid a 4mm EP into my Meade ETX 70.  I heard a small clatter in the diagonal, the same EXACT sound as Santa coming down the chimney, but if Santa were the size of a mouse ;-)

When I shook out the contents, low and behold there were two small lenses, a threaded retaining collar and a tiny spacing ring.  The EP came with the scope when I bought it used a while back.  It is marked SR4mm.

Looking for opinions / a few questions:

1.  Did I get all the parts back, or am I missing something?

2.  It's a cheap EP, so I do not mind experimenting with it.  Is it ready for the trash bin?

3.  If I choose to put it back together, how much chance do I have of getting it right?

3.  Any good advice on getting it back together properly (i.e. with lenses aligned and so it does not come apart again)?

Thanks for your feedback!

[John]

I think you have all the bits. The SR eyepiece optical design is on the left of the diagram below. There may be some slight variations from yours but it should provide enough information to put the lenses back in the right way round. I think the spacing ring goes between the 2 lenses separating them by a set distance:

[baggywrinkle]

It is a Symetric Ramsden ep, pretty low cost and often not of the best quality.

It sound slike you have everything back, there are two flat/convex lenses in a Ramsed. The curved sides should face each other inside the barrel.

[quesne]

Thanks very much for the diagram, John.  

I does look to make sense, though I will say that my spacer is incredibly thin, about 1/32".  The belly on the lenses appears to be about 1/64" or perhaps even more, which would leave the convex surfaces touching (or nearly).  That is quite different than what is depicted in the image.

Anyway, we'll see how it goes.  I'm not too worried about damaging it.  It's a very cheap EP.  I will give it a try later on when I have time to set up a "clean" space to play :-)

[John]

With a 4mm focal length eyepiece the lenses will be very close to touching. The one depicted in the diagram above is a long focal length Ramsden where the lenses are spaced much further apart.

Good luck with the rebuild !

[quesne]

Well, it took a while and several attempts, but I finally got everything back together and working (seemingly) as good as new.  

I'm sure there must be more than a few threads on the topic of rebuilding an EP, but I will post my experience here for the benefit of someone reading in the future.  Not saying it SHOULD be done this way, just saying that my attempts appear to have been successful.

I cleaned all of the parts with 70% rubbing alcohol and a Q-Tip and let them air dry.  I studied each of the lenses carefully with tweezers under a loupe.  It appeared pretty clear to me that one of the lenses was flat-convex, and the other one was convex-convex.  This was a little different from John's diagram -- and I would not swear to it on the stand -- but I am pretty sure that is the case.

Based on what John and Baggywrinkle suggested, I put the flat lens surface against the eye-side, leaving two convex surfaces to meet in the middle.  The field side of the EP now appears to me to be convex rather than flat.

There were a few notable frustrating aspects.  The first was getting that tiny lens to seat properly with the correct surface down.  No matter how careful I aimed -- dropping it gently with a tweezer -- the lens had a tendency to tumble it's way down into the recess.  Once it was down there, it was impossible to see clearly which surface it had landed on.  I realized pretty quickly that I needed to "seat" it under controlled conditions.

I created a 1 1/2" hollow rubber "post" out of some narrow electrical tubing, and stood it upright in a small hand-vise.  I slipped the empty EP upside-down over the rubber post, balanced the lens flat side down on the top of the post, and then lifted the EP slowly until the lens was seated at the bottom of the recess.  I dropped the spacer down, and then followed it up with the convex-convex (I assumed the second lens was symmetrical) and the retaining collar.

Even after figuring out the gymnastics of getting the lens down the hole, I had to repeat the process three or four times because there were persistent hairs of cotton left over in the finished assembly.  I ended up rinsing everything again with the alcohol, and then followed the Q-Tip bath with a light rub on a sterile pad (far fewer hairs).

The EP appears to work fine.  It focused fairly well on Jupiter an hour later (or, at least, as well as can be expected at 300x).  I'm not absolutely sure how to judge success since I rarely use that EP, and certainly never in the 8" Dobs.  My guess is that if the lens orientation was wrong, the image would be all be indiscernible.  Am I right about that?

Anyway, thanks for all the advice.  This was a more-than-satisfactory ending :-)  Happy New Year!

[baggywrinkle]

Well done. Sounds like you got it all back together.

It is amazing what a few pieces of lint from a cotton bud can obstruct.

I used to maintain pnuematic dead weight testers (piston gauges across in the US) for pressure calibration and the air gap between piston and cylinder could be blocked by a finger print. Cotton lint was a barn door compared to the thickness of a finger print.

I used lint free tissues as part of the final cleaning process. These would be the same as lens tissues, they can be wound around a cotton bud to avoid lint being deposited.

[happy-kat]

But if you don't get it back together do not throw the lens barrel away, it might come in handy for any future diy project.

[John]

I'm glad you got it sorted

At least your eyepiece was a relatively straightforward one.