Lens discolouration - seeking suggestions

[Ant-33]

Hi,

I've recently acquired an old pair of Barr and Stroud CF43 bins which are lovely bins, but there is an issue with discolouration on the inside of one of the objective lenses. I'd really appreciate your thoughts on this:

Is this damage to the lens coating resulting from moisture?  Or if it's something else, any idea what?

I'm guessing there's nothing I can do about it - Is this true?

The discolouration has a minor effect on the view through the bins, but the effect is there. If there's some way I can fix it, I'd love to know. I'm not afraid of dismantling the objective, cleaning it and reassembling it, and I'm not afraid of having to collimate them - it's just a matter of whether there's any point in the effort - I don't want to bother if it won't help.

If the coating is damaged, clearly it can't be repaired. Would it be better or worse to polish off the damaged coating?

I know some of you will suggest taking the bins to a specialist, but I just don't have the funds to do that - and if it's fixable, I want to fix it myself. 

Many thanks.

Tony

[Peter Drew]

These objectives will be cemented doublets using Canada balsam or similar. It could be, due to the age, that the balsam is yellowing. If so, there is little that is worth doing about it.  If the discolouration is a deposit on the rear lens surface it may be possible to clean it with a suitable agent. Polishing off a coating could alter the figure of the lens.  

[Ant-33]

2 minutes ago, Peter Drew said:

These objectives will be cemented doublets using Canada balsam or similar. It could be, due to the age, that the balsam is yellowing. If so, there is little that is worth doing about it.  If the discolouration is a deposit on the rear lens surface it may be possible to clean it with a suitable agent. Polishing off a coating could alter the figure of the lens.  

Thank you for your reply. Yes, from looking closely I'm pretty sure the discolouration is on the inner surface of the doublet (i.e. the surface that's exposed to the inside of the bins). If polishing it could change the figure of the lens, I'll avoid doing that, but I'd like to try cleaning it. Do you have any thoughts what agent would work on such a stain. I have some Baader 'Optical Wonder' cleaning fluid and some Pec Pad lens tissues. I realise it needs to be done in a way that doesn't damage the coating, but I was wondering what would be the best cleaning agent for such a stain.

Many thanks

Tony

[Peter Drew]

Hello Tony.  The Baader WF would be a good way to start.  

[Ant-33]

Cool, thanks, Peter 

[Merlin]

If the effect of the discolouration is minimal, I'd personally leave them as they are to avoid further complications.

[ronin]

If the lens unscrews I would be tempted to remove it from the main body and clean the rear surface. The coatings are generally hard so ther is no realistic chance of you damaging it. The worst aspect is simply maintaining it dust/hair free - that takes the hard part.

I would not attempt to remove any of the coating, cannot see it making any difference as what it does is kind of minimal, so live with a bit of an odd looking lens if you have to.

But myself I would certainly take the lens out and clean it as it may remove whatever is there, and if it does nothing you at least know.

If it is the cement that is holding the 2 elements together I do not think there is a great deal that can be done as releasing the adhesive is not easy then it needs the optical centers aligned on one another and regluing with the right stuff. Will say there was a program once that showed one being stuck together, look very easy but they were in a clean room and had been doing lens assembly for years. Still looked so simple, would I do it, or try it?  No.

[Ant-33]

4 hours ago, Merlin said:

If the effect of the discolouration is minimal, I'd personally leave them as they are to avoid further complications.

Thank you. Yes, I tend to agree with this, but the discolouration is visible when I use the bins - even if it's minimal. The bottom line is it's enough that if there's a way to improve it, I'd prefer to give it a try. 

2 hours ago, ronin said:

If the lens unscrews I would be tempted to remove it from the main body and clean the rear surface. The coatings are generally hard so ther is no realistic chance of you damaging it. The worst aspect is simply maintaining it dust/hair free - that takes the hard part.

I would not attempt to remove any of the coating, cannot see it making any difference as what it does is kind of minimal, so live with a bit of an odd looking lens if you have to.

But myself I would certainly take the lens out and clean it as it may remove whatever is there, and if it does nothing you at least know.

If it is the cement that is holding the 2 elements together I do not think there is a great deal that can be done as releasing the adhesive is not easy then it needs the optical centers aligned on one another and regluing with the right stuff. Will say there was a program once that showed one being stuck together, look very easy but they were in a clean room and had been doing lens assembly for years. Still looked so simple, would I do it, or try it?  No.

I think that's what I'll do. It's reassuring that you say the coatings are hard - I was worried about the difficulty of doing anything without damaging it. Of course I'll still treat it as easily damaged, but I can clean it with some degree of confidence. I'll plan to do it at a time when I'm willing to re-collimate the bins, since the collimation (on that side) is lost when the lens assembly is removed.

Many thanks for your feedback, y'all.

Regards, Tony

[Ant-33]

A happy update: I've taken the lens assembly off and given it a careful but thorough clean with the Baader lens cleaning fluid - and the lens now looks as good as new! (new was about 1960). I've put it all back together and done a quick alignment, and an aeroplane at 30000ft looks like a clear single image, so my first stab at alignment seems fairly good. Once I've checked it out on the stars I'll know whether I need to collimate it the hard way or just do a bit more tweaking. 

Billion thanks to you all for your advice.

Clear skies

Tony