Refractor lenses

[banjaxed]

I decided to clean the lenses in my 102mm Celestron refractor which I did very methodically and to the letter of all the advice received on this forum. As I removed the lenses I placed them in order of removal and kept them the right way up so I could replace them exactly as they were removed. All was going well until I recieved a phone call from a friend asking me for some assistance with a job he was doing. When I returned home my wife informed me that she needed the space and had moved my cleaning gear including the lenses 😲 and were now mixed up. Could anyone point me in the right direction as to which order the lenses should be replaced. There are 2 lenses, one is quite thick (about 12mm) and has one side slightly convex and the other thinner lens is about 6mm and is convex on both sides. Any help would be greatly appreciated.

If this is in the wrong place could the mods please move it.

[Star101]

Blame the wife!!!😱

I hope she does not read this 🤪

I found this info. You dont say what model scope you have so its the best I can do.

Manual

[banjaxed]

Thank you very much for your quick reply. Firstly I am not brave enough to blame my wife and will make sure she doesn't read this 😀

The model I have is the Celestron Nexstar 102 GT and the manual you linked does not cover removal and replacement of the lenses.

Hopefully someone may be able to help, thanks again.

[Guest]

Search on the Web for a diagram of an achromatic doublet refract objective. It will give you an idea of the crown and flint lenses positions. When they are positioned you can the rotate one lens until the image is at it's best. This is the sweet spot. 

[Peter Drew]

The correct sequence is Biconvex lens nearest the sky with the shallower curve facing outwards followed by the concave of the rear lens. Be sure to replace the thin shims at 120 degrees to space the two lenses.    😀

[banjaxed]

3 minutes ago, Peter Drew said:

The correct sequence is Biconvex lens nearest the sky with the shallower curve facing outwards followed by the concave of the rear lens. Be sure to replace the thin shims at 120 degrees to space the two lenses.    😀

Thank you very much Peter, you may have just saved a marriage 😀

[banjaxed]

16 minutes ago, Peter Drew said:

The correct sequence is Biconvex lens nearest the sky with the shallower curve facing outwards followed by the concave of the rear lens. Be sure to replace the thin shims at 120 degrees to space the two lenses.    😀

Thank you very much Peter, you may have just saved a marriage 😀

Just to be clear, the lenses are replaced in the front end of the scope so first would be the rear lens with the concave facing the sky then the biconvex lens with the shallower convex facing the sky.

[Rusted]

Convex facing the sky.

 

[Peter Drew]

Yes, convex facing the sky.   😀

[Gina]

Oh dear!!  I've never had the lenses out of a refractor telescope but I've heard it's tricky.  Just hope you can get it back together and working as well as it was before.  Good luck.

Edited September 8, 2019 by Gina

[banjaxed]

Thank you all for your help and advice, I have replaced the lenses and had a look through the scope and the views are crystal clear without any need for collimation 😀 how lucky am I (should have done the lottery)

[Gina]

That's great!

[Rusted]

It is not usual to have to clean between the glass elements of an objective.
So leave it assembled and clean only the front [and back] if necessary!

Blower first and then only drape a lens tissue in gentle, radial movements.
Never rub hard! No matter how disgusting you have let the precious lens become.

There is a considerable risk to any lens removal and replacement from its cell.
The problem is the lens getting out of square and jamming fast in its cell.
So just tipping the lens out of its cell is the worst possible thing to do!
This error is likely to lead to a scallop of glass being cracked away from the lens surface.
Usually from the softer and weaker flint.

There is a simple method to lens removal and replacement which helps to keep the lens square to its cell.
It involves a suitably sized short, drinking glass with a piece of cloth or several tissues placed on the rim to protect the lens.
The short glass having already been placed on a firm table at which you are sitting comfortably.
Swab please, Nurse? Now send her out of the room. You don't want any distractions!

First you unscrew the retaining ring and place it safely aside on the table.
Then you lower the cell with its objective over the drinking glass while keeping the cell horizontal.
So that the lens is left behind, lying on top of the protected glass.
With the cell now resting safely down on the table around the glass.

Now carefully check the lens for edge marks or arrows. There should be pencil lines across both elements.
The lens must be reassembled so that the marks match their original position and orientation.
To do otherwise may result in serious optical problems or damage.
Photograph the lens maker's marks if you can.
Or make a drawing and keep it safe from marauding kids or negligent wives!!
Send them on holiday first! Or take a day off work while they are at finishing school.
This will help to ensure the lenses are re-assembled in the correct order and orientation.

Reversing the procedure involves placing the lens sky face down onto the protected drinking glass.
With the objective cell already lying on the table, the same way up as before, around the glass.

You then LIFT the cell slowly and carefully around the lens until it rests safely inside the cell.
Lower the cell again at the slightest sign of jamming and be more careful with your levelling of the cell.

Once insertion is achieved, the cell and its objective are laid down on the table.
Ready for for the retaining ring to be replaced.
DO NOT PUT IT BACK DOWN ON TOP OF THE GLASS!
NEVER TIGHTEN THE RETAINING RING TOO TIGHTLY!

There should always be the slightest rattle in an assembled objective
Unless, of course, a compression ring is part of the design.
Remember where the ring went! Front or back? In between?
Any spacer rings? YOU MADE NOTES or took photographs, or both didn't you?
The sky face of the lens will always be facing downwards before re-assembly.

Those of nervous disposition may like to start with the supporting glass half full of whiskey.
[Or a spirit of your own choosing.]
Which may ONLY be consumed afterwards as a reward for success!
Or ONLY afterwards to drown your sorrows for abject failure.
Probably leading to a nasty flake being gashed from the surface...
of the priceless, historical and utterly irreplaceable, 24" Zeiss, Triplet, APO objective.

[banjaxed]

1 hour ago, Rusted said:

It is not usual to have to clean between the glass elements of an objective.
So leave it assembled and clean only the front [and back] if necessary!

Blower first and then only drape a lens tissue in gentle, radial movements.
Never rub hard! No matter how disgusting you have let the precious lens become.

There is a considerable risk to any lens removal and replacement from its cell.
The problem is the lens getting out of square and jamming fast in its cell.
So just tipping the lens out of its cell is the worst possible thing to do!
This error is likely to lead to a scallop of glass being cracked away from the lens surface.
Usually from the softer and weaker flint.

There is a simple method to lens removal and replacement which helps to keep the lens square to its cell.
It involves a suitably sized short, drinking glass with a piece of cloth or several tissues placed on the rim to protect the lens.
The short glass having already been placed on a firm table at which you are sitting comfortably.
Swab please, Nurse? Now send her out of the room. You don't want any distractions!

First you unscrew the retaining ring and place it safely aside on the table.
Then you lower the cell with its objective over the drinking glass while keeping the cell horizontal.
So that the lens is left behind, lying on top of the protected glass.
With the cell now resting safely down on the table around the glass.

Now carefully check the lens for edge marks or arrows. There should be pencil lines across both elements.
The lens must be reassembled so that the marks match their original position and orientation.
To do otherwise may result in serious optical problems or damage.
Photograph the lens maker's marks if you can.
Or make a drawing and keep it safe from marauding kids or negligent wives!!
Send them on holiday first! Or take a day off work while they are at finishing school.
This will help to ensure the lenses are re-assembled in the correct order and orientation.

Reversing the procedure involves placing the lens sky face down onto the protected drinking glass.
With the objective cell already lying on the table, the same way up as before, around the glass.

You then LIFT the cell slowly and carefully around the lens until it rests safely inside the cell.
Lower the cell again at the slightest sign of jamming and be more careful with your levelling of the cell.

Once insertion is achieved, the cell and its objective are laid down on the table.
Ready for for the retaining ring to be replaced.
DO NOT PUT IT BACK DOWN ON TOP OF THE GLASS!
NEVER TIGHTEN THE RETAINING RING TOO TIGHTLY!

There should always be the slightest rattle in an assembled objective
Unless, of course, a compression ring is part of the design.
Remember where the ring went! Front or back? In between?
Any spacer rings? YOU MADE NOTES or took photographs, or both didn't you?
The sky face of the lens will always be facing downwards before re-assembly.

Those of nervous disposition may like to start with the supporting glass half full of whiskey.
[Or a spirit of your own choosing.]
Which may ONLY be consumed afterwards as a reward for success!
Or ONLY afterwards to drown your sorrows for abject failure.
Probably leading to a nasty flake being gashed from the surface...
of the priceless, historical and utterly irreplaceable, 24" Zeiss, Triplet, APO objective.

Great instructions, wish I had them before I started with my little project. Fortunately for me things worked out fine.