I'm hoping that someone familiar with this can help me out.
I acquired a Meade 8 x 50 Polar Viewfinder as part of a group of equipment.
It had been exposed to desert dust/dirt that had completely infiltrated the internal optics
so disassembly and cleaning was necessary.
Unfortunately, as can sometimes happen, some of the lens elements got flipped around as they were removed, cleaned and
reassembled and the orientation and positioning was lost. Some elements were loose and the illuminator was not attached when I got
the equipment, so I'm not even sure that everything was working properly from the start. There was too much accumulated dust and
dirt to really know.
So, If there is anyone that has one of these or is familiar with it that can check, describe or verify the lens orientation and spacing, I'd sure appreciate some help or a point in the direction where some information might be available.
Attached are a few pictures, and I'll describe what I've got and it's back together so far. I think I've got it close, but it's still got issues so obviously it's not right.
1) The front objective (50 mm) lens is a two element lens, a thinner element convex on the open surface mated to a thicker element which appears flat, or nearly so on it's open surface. It's retained in the front focusing 'hood' between a wide retainer threaded in to where the internal diameter of the hood changes on the 'outside' (objective/sky facing side) and a thin retainer on the internal side. A mark to indicate which side faces the sky was removed during cleaning (I learned a lesson). I've got it with the convex side facing 'out' (towards the sky) based on the tapered edge of the retainer ring. The small retainer ring on the internal side is not tapered. It's flat. I could be wrong though.as the taper doesn't match the convex shape and it may be tapered simply to help center the lens. In any case that's my best guess... It needs confirmation please.
2) The other end of the 8x50 finder tube consists of a threaded end/reducer that in turn threads into a mirror diagonal. Nothing special there. Mirror was disassembled and cleaned by immersion.
3) The reticle/eyepiece assembly is threaded into the other port of the diagonal and also retained by a small grub screw (1.2 mm hex).
There are two parts to this assembly, the body, which holds the reticle and has the side port for the illuminator, and the eyepiece that threads into the top of the body and has two lenses.
3A) The body has the reticle glass, a translucent spacer ring and two retaining rings, a wide one and a narrow one. The wide retainer ring also has a broader flat edge where it meets the reticle or spacer. The narrow ring does not have that broader edge. The internal body diameter is smaller where the retainers, spacer and reticle glass fit. Above that point a larger diameter thread allows the eyepiece to thread in. In the edge created by the top of the smaller diameter section, there is a translucent insert immediately above where the illuminator threaded port is on the side.
One problem I ran in to during re-assembly was uneven illumination of the reticle. The illuminator threads into the side and there is no adjustment to it (other than light intensity). It leads to the following questions that I need help with.
What is the correct order and orientation of the reticle glass and spacer?
It is etched on 1 side. Should the etched side face 'Up' towards the eyepiece or down towards the diagonal?
Where should the white spacer be placed, against the etched side or the non-etched side of the reticle glass?
I have the wider retainer ring on the 'bottom' ie nearest the diagonal with it's wider edge. I believe that is how it was when I disassembled.
How should the whole set be positioned vertically?
It seems that along with edge lighting the spacer and reticle glass, light will either leak from the illuminator down into the area of the diagonal, or it can leak up towards the eyepiece lens because when the elements are snugged up, the 'stack' isn't deep enough to cover the entire threaded opening for the illuminator. If you have one of these or have been through this before, please let me know how to set this !!
3B) The eyepiece has two lenses with a spacer in between held by a single retainer. The 'eye' lens is a two element lens that appears to be a double convex element mated with a double concave or perhaps concave/flat element.
I've got that one oriented so that the convex side points towards the objective end of the eyepiece, and the concave side is open to the eye. Please confirm if this is correct !
Against the 'eye' lens doublet is an black internal spacer. It's curved off to slightly close/reduce the aperture at one end.
I have that closed off aperture end against the 'eye' lens.
Last is the eyepiece objective lens. It's convex on one surface, flat on the other. I have the convex side facing towards the eye lens, the flat surface facing towards the reticle. I need these confirmed as well.
The eyepiece threads going into the reticle body are coated with something black. It should provide resistance and fill the gap of the threads
as it's used as a focusing mechanism to focus on the reticle. Although the coating is largely intact, the fit of the eyepiece into the body is 'loose'.
It will rock back and forth and shift focus.
Is there supposed to be grease on these threads to float the eyepiece and prevent the shift? Is something specific suggested?
Well.. that's a long post...
The way I have things now.. three issues are apparent.
1) The reticle gets illuminated, but unevenly. During the day all the elements are visible, but at night, the some of the elements are bright, and some are not illuminated at all.
2) There is almost NO eye relief. Hard to even get the cell phone to snap a picture through the eyepiece of the reticle .
3) There is uneven focus and distortion of the reticle viewed through the eyepiece if I move my eye off center slightly.
4) Contrast during daytime viewing, the contrast is low. I haven't been able to look at stars through it yet. I noticed during focusing the main objective that there was image shift as it was rotated.
I noticed some flare looking through it during the day. It seems to come off the surface where the reducer is at the end of the main tube. It's smooth black finish there is shiny, not matte. I'm wondering if I need to blacken edges of the lenses or apply something to that black machined surface to have it be less reflective. It's something to look at once all the glass is back in the right place .
Thanks in advance if you've read this far. If you can help or point me at a resource, It would be nice to get it all working correctly now that all correctly now that all the desert dirt and sand is out of it.