Help reassemble objective lens cluster

[Chris Tran]

16th March 2024

Hello all

Our 'family telescope' dates from the late 19th Century I think - a 4" brass refractor, manufactured in Glasgow as far as I know.

Due to poor storage, there were some small signs of mould between the (2) objective lenses. I foolishly unscrewed the lens cluster, cleaned the lenses carefully, but found I cannot rethread the brass threads to reassemble the objective lens. Being fully aware of how soft a metal brass is , and how fine the pitch of the thread is, I did not use any force and stopped immediately when I realised this reassembly was beyond my capabilities.

Can anyone do this reassembly for me, or recommend a workshop that could do it, please?

Thanks in advance - regards

Chris Tran    (Invergordon)

 

[davidc135]

I'd have a look at the threads and check to see if there is any damage preventing the reassembly. If so, a local machine shop would be able to repair it. Either way there must be some such businesses in Inverness who could help. 

If the threads are good and clean but difficult to engage I've found that slowly rotating one element counter clockwise against the cell until a slight click is felt sometimes does the trick, but you may have tried that.

David

Edited March 16 by davidc135

[Rusted]

Brass to brass, or aluminium to aluminium threads may want some lubrication.
Sticking is called galling. Almost like local welding. Use tiny amounts of grease. NOT oil.
You don't want anything runny migrating into the lens elements.

I'd be checking the lock-ring[s] fit back on without the objective lens in place.
Before attempting to insert the lens.

ASK YOURSELF WHY YOU ARE RISKING THE PRECIOUS LENS!
It would have to be filthy to have any effect on the view. Use a hand bulb blower first.
Cleaning the visible surfaces with a lens cloth and approved cleaning fluid.
Is far safer than a clumsy amateur trying to dismantle the lens from its cell.

Removing the objective glass elements from the metal cell is fraught with serious risk!
Most amateurs try to tip the lens[es] into and out of the cell. Which is just asking for trouble!
The glass lenses will always want to tip and jam fast in its close fitting cell!

The proper way to do it is to remove the metal objective cell from the telescope's main tube first.
You then remove the lens locking rings[s] while working safely flat on the table. Not in mid air!

Let's assume that you have removed the cell from the telescope main tube.
You have already unscrewed the locking ring[s] and placed them safely aside.

Now you pad a suitably sized, upright, drinking glass rim with a soft cloth. Clean hanky?
Choose a drinking glass diameter which will pass easily though the lens cell.

You then slowly lower the metal objective cell over the padded drinking glass.
So that the objective remains supported on the padded drinking glass rim.
Once clear, the cell is lowered to rest safely on the table around the bass of the glass.

You then deal with the fragile and naked objective lens.
Which, in a common doublet, usually consists of a flint and crown pair.
One positive, biconvex lens and one negative concave lens.
The back of the flint may be flat or slightly curved.

The convex lens usually faces outwards. Though not always.
The biconvex lens may even have two different curves!
Where the curves between the lenses match they will usually have tiny, thin spacers.
Some objective lens pairs may have a spacer ring between them.

Make a note of which way around the lens sits in its cell.
Check carefully for any pencil marks on the edges.
Which, if present, will show their rotation, relative to each other.
Or may be arrowed to point towards the sky.

Your lens is now accessible for cleaning.
I'd much prefer surgical gloves to anything else.
This avoids fingerprinting the glass with skin oils.
While usually providing a safe grip.

Refitting the clean objective lens is done in reverse order.
The drinking glass is standing inside the objective cell on the table.
The lens is carefully placed on the rim of padded glass.

The metal cell is then lifted very gently and straight upwards without tilting.
Until the objective lenses sit safely within the metal cell.

If the lens tips and sticks then very carefully lift the cell again.
Squareness between the lens and cell is vital as they meet.

Let's assume you have the lens safely in its cell but still unsecured by its ring[s].
Place the cell down on the table and remove the drinking glass to avoid accidents.
You can now proceed to fit the locking ring[s.]
With the cell and its lens safely resting flat on the table.

You may not be able to remove the cell from the main tube.
Check for locking screws around the edge of the cell.
Unlikely, as most cells are screwed to the main tube with a fine thread.
Try grippy rubber gloves to unscrew a tight cell.

Don't use tools! Other than [rubber] strap wrenches.
These are commonly used for removing vehicle oil filters by friction.
You may need two. Facing opposite ways. One on the cell. The other on the main tube.

Or, get a strong helper to grip the main tube while wearing grippy rubber gloves.
While you try to unscrew the objective cell.
Using both, gloved hands where there is enough room.
Many cells will be very tight. Once loosened unscrew the cell very carefully.

The risk of damage to the objective lens is very much higher if the metal cell can't be removed!
In amateur hands the objective lenses tend to tip slightly as they enter the close-fitting cell.
Which can cause a scallop of glass to be broken away from the lens surface!
This usually occurs on the softer [rear] flint lens. Tears will soon follow such a disaster!
Painting the scallop with black paint is considered safer than leaving it visible.
A bare scallop might distort the light passing through the lens.

A lens in its cell is expected to rattle VERY SLIGHTLY.
This avoids squeezing the lens in very cold conditions.
Which might distort the glass lenses by crushing.
Glass has a much lower coefficient of expansion than metal.

There are YT videos on telescope lens removal and cleaning but I haven't watched them!

I use pointed, screw adjusting, dividers for undoing locking threads.
Though professional tools are available. These resemble beam compasses.

Edited March 16 by Rusted
additional info + image

[Chris Tran]

Thanks both for your advice and info.

I trained as an Engineer and worked in electrical engineering, but just don't feel confident about this particular reassembly job.

Can anyone recommend someone, or a workshop, who could do this reassembly for me?   Happy to pay of course.

(The telescope is actually a 3" refractor, not 4")

Thanks in advance - Chris