Removing objective lens from Sky-Watcher 'Star Travel' 120mm refractor

[Julian0]

First post, hi all.

To be clear, I know very little about telescopes, gulp!

I need to remove the objective lens of my aforementioned telescope (no need to go into why). I see there is a threaded ring on the outside of the lens, holding it in place. It has 2 holes in it, diametrically exactly opposite each other. You can see one of them in the attached picture. I read somewhere (do not recall where) that these are used with a tool to grab and turn the ring, and that someone had done it with a piece of wood and two small nails.

Before I make a mess, might anyone have some guidance as to the best way to remove this lens? I would like it to be able to put the lens back later, if possible - so a non-destructive strategy would be best if possible!
 

[John]

Remove the lens cell containing the objective first. It unscrews from the scope tube. Careful when putting it back on that you don't cross the threading on the tube.

Once you have the objective cell with the objective in it off the scope, you can reach the inside surface of the objective which might be enough for your needs.

If you need to remove the objective from the cell, this photo illustrates the method. The lens retaining ring does need to be unscrewed to allow the objective cell to be lowered leaving the objective elements clear of it. The back surface of the lens rests on an optical cloth supported something that is a little smaller than the diameter of the objective - I think I used a sellotape roll in this instance:

It's a delicate job because you need to ensure that the elements are put back in the same alignment (hence the black marker) and the same orientation with any spacers used in exactly the same position.

Edited March 18 by John

[Nakedgun]

~

Good procedure.

 

 

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Edited March 18 by Nakedgun

[Julian0]

35 minutes ago, John said:

Remove the lens cell containing the objective first. It unscrews from the scope tube. Careful when putting it back on that you don't cross the threading on the tube.

Once you have the objective cell with the objective in it off the scope, you can reach the inside surface of the objective which might be enough for your needs.

If you need to remove the objective from the cell, this photo illustrates the method. The lens retaining ring does need to be unscrewed to allow the objective cell to be lowered leaving the objective elements clear of it. The back surface of the lens rests on an optical cloth supported something that is a little smaller than the diameter of the objective - I think I used a sellotape roll in this instance:

It's a delicate job because you need to ensure that the elements are put back in the same alignment (hence the black marker) and the same orientation with any spacers used in exactly the same position.

Thanks a lot for getting back. That seems like a sane approach. However, on my 120mm, I do not seem to have a detachable lens cell. There are no screws, and it does not seem to be turnable/unscrewable itself. I attach two photos to better explain. Please let me know if I'm missing something - not unlikely!

[mikeDnight]

The cell will unscrew anticlockwise, but you may need to grip the cell and the tube tightly. Phone a friend with a strong grip to either hold the tube stationary while you turn the cell, or visa versa. It's best not to attempt to remove the lens itself from the cell if at all possible! 

 I admit to being curious as to why you need to remove the lens, as it looks to be ok in the photo?

Edited March 18 by mikeDnight

[John]

As @mikeDnight posts above, the whole cell (the white bit) should unscrew and come off of the tube. The example photo that I posted came off a 2 piece Skywatcher cell. Yours is a 1 piece cell.

Your lens does look to be in fine condition though.

Edited March 18 by John

[Julian0]

Greatly appreciate the responses. This all makes sense. Yes the lens is in great shape. I am not removing it to replace it. As to why, that would derail this thread terribly! I will post my results when I have found another person to assist with grabbing the tube.

Thanks again!

 

[Albir phil]

1 hour ago, Julian0 said:

Greatly appreciate the responses. This all makes sense. Yes the lens is in great shape. I am not removing it to replace it. As to why, that would derail this thread terribly! I will post my results when I have found another person to assist with grabbing the tube.

Thanks again!

 

Sounds interesting 🤔

[Julian0]

I was able to remove the lens cell, thank you.