[Wavytone]

Well it's cleaned and re-assembled now. In case anyone else needs to do this you will need:

- a lens spanner (see below),
- a packet of Johnson cotton pads (the things girls use to remove makeup)
- industrial acetone from Bunnings ($10 buys half a litre, chemists charge a fortune),
- a wooden clothespeg,
- a pencil,
- a drinking cup,
- a couple of very well-washed linen teatowels for the work area.


I made a lens spanner to open the cell using 2 screwdrivers and a block of wood. I found a pair that happened to have straight blades and shafts which were a tight press fit in a 5mm hole (see photo). I used a pair of calipers to measure the cell and get the spacing of the screwdrivers exactly right.

- Cover a work area with the tea towels to provide a soft clean surface such that the elements can sit on it and not pick up any dust.

- Using the lens spanner, loosen the retaining ring and remove it. The lenses will stay in the cell. I would not recommend trying to hold the cell with one hand and the spanner in the other, for two reasons - one slip and the blades will scratch the lens; secondly if you drop it this will be the end of your lens.

Instead I clamped the lens cell to the bench and used both hands to gently place the spanner in position and rotate it using both hands.

- Using an inverted cup and a couple of pads to support the lens by the centre well off the bench top, unscrew the cell, then gently ease the cell off the lens, making sure you do not rotate it, and keeping both elements of the lens together.
- The lens elements are airspaced, with a metal spacer ring between.
- There are a series of 4 pairs of holes around the periphery of the lens cell. Orion forced some sort of waxy gunk into these which keeps the lens elements from shifting laterally.
- Once the cell has slid off the elements be careful not to rotate it as I am fairly sure the gunk above is important to keep the lenses precisely centred with respect to each other.

- Using a cotton swab dipped in the acetone, swab a small patch on the side of both lenses to clean off the black stuff they have coated the edges with, without disturbing the alignment of the lenses and the cell.

- Using the pencil, place a set of lines on the edge of the lenses and the cell, so that you can get these marks aligned again when you reassemble it. I use a group of 3 lines so that if one rubs off during the cleaning there are still two more.

Make sure you also put a "V" mark on the side of each lens so that you can tell which way round the elements belong. The convex element is nearly symmetric so it is quite hard to tell which side is supposed to face the focal plane.

- Now you can lift the top element off. Using a wooden clothespegs instead of your fingers to grab a cotton pad, dip it in acetone, then swab a lens surface. This will save your fingers from the acetone. It should come clean. Repeat for both sides, then do the bottom element, and put both elements to one side on the tea towel.

if you want to clean the edges note there is more black gunk that will come off. Just be careful not to wipe the pencil marks off.

- Clean the cell too, mine was greasy.

- Reassemble the cell, ensuring the elements are aligned and aligned in the same way they were in the cell, using the pencil marks. Using the spanner to close the retaining ring it should be firm but not tight; otherwise the lens will be overloaded and the glass may fracture in cold conditions - aluminium shrinks more than glass. Mine was, in my view, dangerously tight in the first place.

The acetone in my case dried off almost instantly leaving a clean surface.

Looking at the city lights the lens image is OK, have to wait for the skies to clear for a star test.

PS grease is a non-polar molecule and getting it off requires either a non-polar solvent or else a caustic solution that will turn it into soap, which washes off (as in a dishwasher). For non-polar solvents acetone is one obvious one but paint thinners are also likely. Turps I tried on window glass - it won't dry clean, but thinners do. Thankfully not needed.

The pale green coating on these lenses is almost certainly zinc cryolite - OK in itself, but what bothers me more is how well the coatings have adhered to the glass underneath, or not; if the solvent gets under the coating it will come off in patches.