Barlow servicing tips?

[vlaiv]

I have GSO x2.5 1.25" APO barlow that needs good cleaning and maintenance.

Problems are as follows:

I suspect there is quite a bit of dirt and grease on lenses, I see quite a few specks when out of focus on bright target like Jupiter. There is also too much light scatter when observing with this barlow which I think is due to greased up lens. There is one more thing that concerns me quite a bit. Barlow introduces aberration that resembles atmospheric dispersion. Jupiter shows blue fringe on one side and red on the other. It is not atmosphere induced, because it is not there when I remove barlow and it rotates in position when I rotate barlow in focuser. I guess that lens might be tilted / not seated properly in housing?

Never done thorough cleaning of glass optics before so I was wandering what is the best way to bring this item to a good working condition?

[Physopto]

Read "  Cleaning Aperture" above.

The fringing could be caused by the grease on the barlow. I would try gentle cleaning first.

Derek

[vlaiv]

So, high purity alcohol, does it matter if it is isopropyl or ethyl and what is acceptable purity (would 92% be ok?). Can I dismantle lens cell to take out glass element? It is triplet APO but I don't know if it is cemented or airspaced. I think I will have hard time cleaning it thru barlow tube. I would also like to make sure it is seated properly.

[Physopto]

Yes the purity matters. The extra can be water or anything. Impurities can leave marks or can cause damage to the anti reflective coatings or even pitting of the glass. 92% is not good enough. If you buy isopropyl alcohol at a chemists it quite often is only something like 70% and the rest water.

Derek

 

[Ruud]

The specks might come from the eyepiece. With the barlow you get a 2.5x smaller exit pupil, which makes anything on the eye lens more visible (2.5² as much). Try turning the eyepiece only to see if the specks turn with it. If they do, clean the eyepiece.

Ideally, to troubleshoot the barlow, you should only turn the barlow itself and keep the eyepiece in a fixed position while you turn the barlow under it.

For cleaning lenses, I first blow off any particles. If some are stuck to the glass, I push them loose gently with the tips of the hairs of a lens brush. A soft make-up brush is fine too. 

Alcohol is fine. When the lens is free of dust, apply the alcohol to a cotton ball and clean the lens gently. It may leave rainbow streaks but you can remove those by fogging the glass with your breath and wiping it dry with a clean tissue. (Kleenex regular is very soft. Don't use tissues with oil, perfume or whatever added to them). I wipe the glass with with Baader optical wonder on a cotton ball or q-tip. Denatured alcohol is fine too. Isopropyl  alcohol will do as well. Any alcohol or combination of alcohols is okay, as long as the only impurity is water.

Don't apply your solvent to the lens directly. It may creep into or under things. Apply it to a cotton ball and only use a little bit.

If alcohol is not strong enough I use acetone as a last resort. Acetone dissolves plastics so it is a risky cleaning solution. My eyepieces have no plastic, but I'm still wary of using acetone. 

Do not use these: Some rubbing alcohols are just isopropy, but others have a greasy lotion added to them. You don't want a greasy film on your optics.

You have to be more careful with the barlow than with the eye lens of an eyepiece. The coatings on the eye lens are extra hard. The coatings on the barlow may be softer.

As your cleaning arsenal I suggest a blower, a brush, Baader's wonder fluid, acetone for when the wonder fluid is not strong enough, cotton (non-synthetic surgical cotton is best) and Kleenex regular.

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While you're at it, check if the lens cell of the barlow is tightened properly. If there's any movement in the lenses of the barlow or anything is cross-threaded you may have your reason for the colour dispersion you observe.

Good luck.