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cementing an achromat

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#1 Redstone2

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Posted 03 January 2014 - 11:39 PM

Hello,

How difficult is cementing a 25.0 mm diameter achromat?
Surplus Shed has a 1/4 lb. block of Canada Balsam that would be used for only two small lens... any other cement options? What I like about the lens is that is it their “precision” lens but if cementing is difficult.... might settle for something else.

Thanks for any ideas,
Tom

#2 davidmcgo

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Posted 03 January 2014 - 11:47 PM

Easiest in terms of technique is probably the Norland UV curing adhesive. Home Depot sells black light compact fluorescent bulbs that work in a mechanics light within 5 to 10 minutes to set and an hour to fully cure.

Dave

#3 PhilHerring

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Posted 04 January 2014 - 02:22 AM

Use oil instead of cement and tape the edges, maybe?

#4 GlennLeDrew

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Posted 04 January 2014 - 08:39 AM

One must be careful to observe centration, or to compensate for wedge if present in one or both elements. In practice, one way this is done is by observing the reflected spots from a collimated light source, and slipping the lenses laterally until the reflections are coincident. If not properly centered optically, lateral color and astigmatism of at least some small magnitude will result.

#5 careysub

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Posted 04 January 2014 - 09:19 AM

... this is done is by observing the reflected spots from a collimated light source...


Could you expand upon what would be a suitable light source?

#6 amicus sidera

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Posted 04 January 2014 - 09:32 AM

The refractive indices of various lens cements, including Canada balsam, vary somewhat among themselves; a lens combination designed and ground for one cement might produce aberrations (though minor) if used with another.

Glenn, I'll echo Carey's request for a more detailed description of the technique you mentioned, if you have the opportunity.

#7 GlennLeDrew

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Posted 04 January 2014 - 11:22 AM

The simplest technique I can think of for achieving proper lens centration involves a laser.

Place the negative component on a turntable. Position a laser well above the lens, and adjust table and/or lens so that when the lens is spun, the reflected spot(s) is/are directed right back into the laser's collimating lens. Lock things down. Cement the convex element, adjusting laterally so that its spot is coincident with the other(s).

I should add that the radii of curvature of one or more lens surfaces will dictate how far away you can position the laser before the spot becomes enlarged too much. But I should think that a distance at least as great as two doublet focal lengths should be far enough for sufficient precision.

The laser should have a white screen (of card) attached to the front, with the smallest hole which passes the beam. This will facilitate alignment.

#8 amicus sidera

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Posted 05 January 2014 - 11:24 AM

Many thanks for the explanation, Glenn! Have saved this information and will refer to it should the need arise to re-cement any of my lenses.

#9 BigDob Al

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Posted 05 January 2014 - 01:15 PM

The simplest technique I can think of for achieving proper lens centration involves a laser.

Place the negative component on a turntable. Position a laser well above the lens, and adjust table and/or lens so that when the lens is spun, the reflected spot(s) is/are directed right back into the laser's collimating lens. Lock things down. Cement the convex element, adjusting laterally so that its spot is coincident with the other(s).

I should add that the radii of curvature of one or more lens surfaces will dictate how far away you can position the laser before the spot becomes enlarged too much. But I should think that a distance at least as great as two doublet focal lengths should be far enough for sufficient precision.

The laser should have a white screen (of card) attached to the front, with the smallest hole which passes the beam. This will facilitate alignment.

Sounds simple in principle .
One detail I can't visualize is how would the negative element be secured to the turntable to avoid moving during rotation and subsequent gluing of the positive element.
Is there a simple/easy technique you could recommend for this?

#10 GlennLeDrew

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Posted 05 January 2014 - 02:16 PM

My previous outline was just that; it omitted much detail.

A lens can be secured to a turntable via three constraining blocks at the side, extending no higher than the lens's edge thickness. Actually, this would be done on a separate plate, which is then adjusted laterally and clamped down onto the turntable itself. After all, there must be provision for getting the lens's optical axis coincident with the turntable axis.

And the lens axis must have no tilt with respect to the turntable's axis. A convex lens surface will rock when placed on a flat surface. Such a lens requires to be supported at the edge. This means that a way of adjusting tilt to remove any tilt of the lens optical axis is likely required.

So, the bottom lens must bs adjustable both laterally and in tilt, until its optical axis is made coincident with the turntable's rotation axis. Before making up a complex support...

...one could try air-curing modeling clay or putty. Cover the lens's bottom surface with masking tape; this is quite safe for coatings. Press the lens onto a lump of clay, and adjust the lens until the laser spots reflected from it do not oscillate when the table is turning.

For his stage, the laser need not be centered, and it can be tilted. All we are looking for is rotational symmetry; a tilted, off axis laser will do this just as well.

The laser should have the smallest beam possible, so that the curved surfaces reflecting it do not make for too-large spots, they being harder to use for precise alignment. Moreover, many laser spots are elliptical, if not of even less symmetrical shape (on cheap laser pointers.) A pinhole--ideally as circular as possible--is a good way to achieve this.

#11 careysub

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Posted 06 January 2014 - 12:55 AM

Easiest in terms of technique is probably the Norland UV curing adhesive. Home Depot sells black light compact fluorescent bulbs that work in a mechanics light within 5 to 10 minutes to set and an hour to fully cure.

Dave


OTOH, for people with one-of-a-kind objectives, obtained at significant expense, who have never done this before, the ability to separate balsam-cemented lenses and re-cement them if the cementing does not go well is an attractive feature. The UV cured cement is probably irreversible.

#12 bremms

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Posted 06 January 2014 - 07:23 AM

There are room temp two part cements which work great. I recemented an old objective years ago. I borrowed the cement from someone, but its not hard to find.
http://www.optical-cement.com/
I want to cement my 5" F5 Jaegers. Makes me a little nervous. Could use oil and tape I guess.

#13 Dick Parker

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Posted 06 January 2014 - 10:15 AM

Bremms -

For a 5 inch I would suggest oil and tape. My recollection is that diameters greater than 3 or 4 inches (forget exactly) the thermal expansion properties of the two glasses in an achromat becomes a problem with cemented elements. I used oil and tape with my 4.5 inch achromat and it works great.

I used mineral oil from the pharmacy, although I am aware that there are special oils for this purpose mineral oil seems OK. You lay your concave surface face up, add a drop or oil, then nest the convex surface into it carefully working to get all the air bubbles to migrate to the perimeter and out. IMPORTANT after you do this, let them sit over night for the oil to settle before you tape up the edge and look through it. How do I know this?? When I first did my 4.5 inch I put the lens back in the telescope right after oiling and the image was horrible. I thought I had messed something up. The oil is somewhat viscous and with the very thin layer involved, it moves slowly.

I assume your lens is designed to be cemented or oiled i.e. that R2 and R3 are designed to be one surface. It is not appropriate to just take an old lens with paper shims and oil it.


Good luck
Dick Parker

#14 Chuck Hards

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Posted 06 January 2014 - 11:01 AM

Dick, Jaegers put the cut-off point at 5".

#15 Dick Parker

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Posted 06 January 2014 - 11:48 AM

Chuck -

OK. ATM Journal, issue #2, Pg 17, Editors note "many designers and opticians feel that doublets over approximately 4 inches in diameter should not be cemented, owing to the possibility of strain..."

Obviously there is no exact cut off agreed to by everybody. My point was that in larger sizes, depending on the glass type and intended use (environment) of the telescope cementing can be an issue. Oiling does not have that issue and my experience is that oiling works very well and lasts a long time.

Dick Parker

#16 Chuck Hards

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Posted 06 January 2014 - 11:57 AM

I understand, no big deal Dr. Parker. I have experience with some decades-old 5" Jaegers, and the cemented joints look as good today as they did in the 70s. 4 inches allows some peace-of-mind and a bit of insurance.

#17 hwhall

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Posted 15 January 2014 - 09:33 PM

If you'd like to try balsam but don't like buying the SS block for such a small lense (probably only need a drop or two of balsam to do the job) email me and I'll send you a bit of the SS balsam block I bought.

Wayne






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