Remove objective lens

[Prazz]

I hope this post is in the correct place. This is my first post.

I have a Celestron Next star 102SLT, the objective lens has marks on the inside, they look a bit like oil, see image. How do I remove the lens to clean it.

Thanks in advanve

[Naughty Neal]

Sadly not oil but Fungus.

[Rob]

Welcome to the forum Prazz. I agree looks like fungus. A few things you can do here to try and salvage.

Step One: First is to leave the scope (cap off of course) pointed at direct sunlight for some hours, even over a few individual days for a good UV treatment. Make sure there is no diagonal, eyepiece or cap attached at the focuser. Also remove the finder scope/rdf. Repeat the UV treatment over a few bright sunny days this SHOULD kill the fungus and reduce it slightly visually. 

Step Two : Now this is not for the faint hearted so if your not happy doing this yourself, find someone (poss camera shop for example) to cover this. The doublet lens will need to be removed from the lens cell and individually cleaned/treated with either Baader wonder fluid or another clean with anti fungal additive in.

Now I have done this on a previous scope around 15 ish years ago with good results. But step two is at your own risk. Step one is a 'Nothing lost' step. So worth the try.

Good luck.

Rob

[Louis D]

Generally, there's a threaded ring holding the lenses in from the front of the scope.  Take a look down the dew shield end of things to see if you can make out a retaining ring of some sort.

[davidc135]

It isn't too difficult to disassemble and clean the objective if it's taken step by step.

Remove dewshield.

How is the cell fixed to the tube? I'm assuming that it isn't collimatable but screwed in place. Collimatable cells would be attached by three pairs of push/pull scews. Remove cell.

As Louis says check for a retaining ring which should have two indents to aid removal. Usually it would be on the outside of the glass but it's possible that it screws in from the rear. Remove ring.

To remove the objective first find a cylindrical object say 3'' diameter and of ample depth such as a small glass on which is placed a protective tissue or cloth. This supports the objective whilst the cell is lowered down carefully avoiding any jamming on the sides and possible resulting 'clamchips'.

With the objective high and dry, inspect for any side pencil marks or scratches indicating best orientation. If there are none it's still an idea to mark a V or similar with a waterproof sharpie just in case it matters. A V can also be a quick check on the order of surfaces during reassembly.

Check what they use to separate the two lenses. It could be a ring, or three foil or similar spacers. Retain as the spacer thickness is especially important in a fast doublet.

I'd give them a soak in tepid soapy water and carefully rub the surfaces with fingers. The lenses can be rinsed and drained with the last drops removed with a tissue before being allowed to thoroughly dry.

Ensure objective is reassembled in the correct order.

Hope this helps

David

[Rob]

Come to think of it David is absolutely right with marking the edge of the lenses with a V for returning to the original orientation.  I'd forgotten I did that also.. 

[Prazz]

Thank you for all your replies. I have taken it apart. Now I just have to clean the lens.