Refractor, removing dust from the rear of the cell.

[Rustang]

I picked up a Sky watcher Evostar 72 ED DS Pro yesterday, thanks Raul 👍 I know that dust should not matter for visual and imaging as it will focus pass the front lens anyway so dust is just one of the things you either get on with it being there or not. Most would say leave it. Dust and debris has settled on the rear of the doublet cell, Ive tried with a blower and its a little to stubborn to remove that way. So, I will probably use as is for a while anyway but wanted to ask about removing the front cell. I can see a retaining ring the same as old cameras have to remove the lens, I have the tool so that's ok. I'm guessing that once the retaining ring is unscrewed the cell will pop out? I dont want to risk taking the cell apart so can it just be carefully handled with out risk of the cell coming apart? Can then the same cleaning solution be used to tidy up the back of the cell and remove the dust? Are there any coatings to the rear to be concerned about and in regards to coatings both front and rear of the sell, are they fairly robust to take a gentle clean?

 

Thanks in advance.

 

Russ

[MarkAR]

Why not try a stick with a lens cloth padded on the end, or is that daft and dangerous.

[Rustang]

38 minutes ago, MarkAR said:

Why not try a stick with a lens cloth padded on the end, or is that daft and dangerous.

That would be my question, lol. I dont know if the back of the cell is fairly tough to take that if anyone can advise!?

[DaveL59]

I do similar to what Mark suggests on Bino's that I don't want to disturb the objectives or where the lens ring won't shift, tho if you do then use a very light touch so as not to scrape the dust across the lens.

I don't know the scope but the lens cell isn't retained by screws in the side of the OTA? If it is then the cell would come out complete and you could clean and re-fit and not affect the lens orientation or spacing. A lens ring may well be what is holding the lens in the cell and then you risk them coming out as separate lenses and have to orient and get the spacers placed when you reassemble. A pic of what you are looking at would help in terms of asking advice sp we know we're talking about the same things.

[RT65CB-SWL]

If you do decide to DIY... you need to remember the objective cell 'normally' comprises a minimum of two elements of glass, spacers, etc. 

If you don't feel confident, then it maybe best left to a specialist, i.e. someone or company that repairs optics and get a few quotes for the work. Then decide.

I don't know whether SGL's sponsor @FLO can/would be able to advise, as your scope is second-hand.

[Rustang]

1 hour ago, DaveL59 said:

I do similar to what Mark suggests on Bino's that I don't want to disturb the objectives or where the lens ring won't shift, tho if you do then use a very light touch so as not to scrape the dust across the lens.

I don't know the scope but the lens cell isn't retained by screws in the side of the OTA? If it is then the cell would come out complete and you could clean and re-fit and not affect the lens orientation or spacing. A lens ring may well be what is holding the lens in the cell and then you risk them coming out as separate lenses and have to orient and get the spacers placed when you reassemble. A pic of what you are looking at would help in terms of asking advice sp we know we're talking about the same things.

Photo attached, no obvious screws on the outsids, dew shield slides on and off the outside which has black felt around it, I don't believe that is covering any screws. 

[mdstuart]

I would try to clean it gently from the other side.

A very soft cloth. Baader fluid if you have it.

If you take it apart record the sequence carefully. It's easy to forget which way up lenses came out!

Mark

[DaveL59]

2 hours ago, Rustang said:

Photo attached, no obvious screws on the outsids, dew shield slides on and off the outside which has black felt around it, I don't believe that is covering any screws. 

Short of lifting the felt to see I can't really advise, not having one to examine. Hopefully someone else here may know more and comment. For sure I'd not disturb the lens ring until certain that's the way to remove the cell. Might be easier to remove the focuser and work carefully from that end, a long thin paint brush with lens cloth might reach and also act as a pressure control against the lens. You likely won't be able to reach your hand down the OTA as it'll have baffles I expect.

[mikeDnight]

Personally I'd wrap some lens cleaning fabric around a short stick and insert it through the draw tube, gently removing the particles on the rear face of the back element. Another way may be to attach a long tube or straw to a bulb blower and attempt to blow the particles off without touching the lens. I certainly wouldn't remove the lens from its cell but if the above fail, I may be inclined to remove the cell from the tube. You may be able to vac out the loose dust from inside the tube.

[Rustang]

Thanks guys, I think I will look to tackle it from the rear of the scope for now before removing anything. Thanks everyone.

[Starwatcher2001]

This bit looks like a keyway to unscrewing the front ring, you can see the thread behind it. However, without the right tool, it looks like a receipe for scratching the front lens if you slip. Approaching it from the rear does seem like a prudent idea.

 

[Rustang]

6 minutes ago, Starwatcher2001 said:

This bit looks like a keyway to unscrewing the front ring, you can see the thread behind it. However, without the right tool, it looks like a receipe for scratching the front lens if you slip. Approaching it from the rear does seem like a prudent idea.

 

I didnt notice that but yes your right! Cheers