Another makeshift oven that heats/cools very evenly/slowly is a large cast iron skillet filled with clean sand, and another smaller heavy pan that is put upside down (into the sand) as a cover... This is put on a stove and heated for a long time until the smell comes out...
correction; I mentioned 140deg F before, but I might be confusing it with the melting point of gelatin...
Steve K
It has been a couple years since I've separated one in the oven. I remember it was north of 200 degrees. I want to say they really started sliding most of the way off at 230. So maybe boiling water would work. The skillet technique sounds a lot like what the late Mr. Grimes described to me over the phone.
Hello!
I´ve been experimenting with some cheap zoom elements and some really busted lenses from the 30´s (german and japanese lenses). Boiling in water never separated any lens, nothing really happened apart from diluting the black paint around some lenses!
a one week dip of acetone separated most of the older ones.
a two to four week dip of diclorometane separated the rest of the lenses.
UV optical adhesive can be bought cheap on bay bay! and if the one you bought does not work unglue it and repeat with a different one.
centering... well you will have no problems centering that lens: put the concave element down, pour some glue, just a tiny in excess and put the convex element over it, if
you press a little the glue will act like a lubricant and self center the lens, or you can adjust the borders the best you can... i never had a problem with centering.
i have done 3 or 4 lenses, mostly not mine but i glued an angulon 165 that i use regulary, and it works really well.
I did however had tons of problems with dust, dandruff or whatever, be cautious!
Now this is good advice! Mirrors my experiences completely.
You can soften new balsam with heat, but old balsam is often very dry and gets even more solid when you heat. Problems with allignment is just an old wives' tale - even with four lenses.
I don't recall if someone mentioned this or not, but one usually scribes a mark on the outside edge of the two elements before separating, so that on reassembly they can be realigned the same way.
Charley
Nope!Originally Posted by c.d.ewen
Everything is symmetrical and spherical. Marking is probably a good idea with complex cells, spacers and lens groups.
The typical thickness of the double convex in an achromat is less than 1mm. I have never seen allignment marks in any achromat which has obviously been redone earliar.
Thanks all for the "savants" comments. It is really a interesting subject, optics....!
BTW, any info about refractive index differences between different glues (synthetic, ecologic, balsams...)?
Warm regards,
MAC
Personally I would not use Canada balsam, especially if you think UV adhesive is being used now. There are thermal and optical performance reasons for this.
Norland NOA 61 or NOA 68 is what we call out for bonding of doublets depending on the temperature requirements. Those adhesives have been used throughout the industry for years and years. I've never had any issues with that adhesive under temperature testing much more extreme than you'll likely ever see. Part of it also is when designing optics I'm careful to ensure the glasses that make up the doublets have thermal expansion coefficients within 2 ppm / C of each other. These doublets with failing bond joints likely don't have glass types as closely matched thermally. Even so, NOA 61 should hold up over the life of the lens if used correctly.
If you use a UV adhesive, purchase a dentist's UV cure wand. When you go to set the adhesive, give it a 3-4 second cure from about 1-2" away. Then verify centering. Once you're satisfied, UV cure exactly as the manufacturer describes in the instructions. Note: Taking longer to cure is better, assuming you can maintain constant temperature.
*Highly recommend* measure the runout before you debond the doublet and use that measurement as maximum acceptable runout when you rebond. Use a radial bearing and a test indicator setup or some other suitable method to measure. Note: A V-block won't really work.
If you already debonded the lens, ensure runout is 0.003" or less when you put them back together. That's a pretty typical centering tolerance for lenses in these types of applications.
If you have any confusion about this advice or feel you're not up to it, then send it out to be repaired professionally. Otherwise you will degrade the optical performance compared to what you had before. Of course in the end it's your lens to do what you will with.
Newly made large format dry plates available! Look:
https://www.pictoriographica.com
If you have a lens where an element is much wider than what you attach it to (sitting on top, like a toadstool) then you won't think centering issues are old wives' tales. Assuming old wives were into this sort of thing. Some of the Wolly wide angles are like that. Some protars are like that.
You can eyeball it with great care and sliding something around the edges to check for uniform spacing. Some people will gently install it in the mount, tack it with a little UV exposure, then take it out and check it. If you find the lens (before separation) to be less than perfectly aligned, you've got a quandry. Did somebody in 1924 screw it up at manufacture? Or does physical centering give you optical misalignment? Without fancier equipment like a laser table that Mr. Grimes used, I don't know how you answer that.
On dust, I use a little 12 volt computer fan to blow across the lens as I work so nothing settles on it. Put a piece of poly fluff on the intake side for filtering.
The good news that is stopping down to typical LF apertures eliminates or masks a bit of imperfection. Your work doesn't have to be perfect for the lens to be saved and perfectly useful. If you can afford it, send it to Focal Point they do outstanding work.
Both points well taken, Steve. I guess I lucked out separating my lens, and was intimidated by persons discussing smaller lenses.
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