Philostopher1 Build your own lenses

Posted by: Philostopher1 Apr 2 2018, 11:18 AM
I thought I should start a new thread about making lenses.

On the telescope side of this forum, making mirrors or even refractor lenses is within the realm of that hobby \ profession.

We need lenses too!

I am talking simple plano convex lenses for individual stars not multi-element lenses for star plates like Zeiss, GOTO and Minolta. That is way outside of the scope for a hobbyist as these are complex imaging lenses. We just want to sharpen up our pin hole stars a bit.

Depending on one's finances small lenses can be ordered cheaply from China or more expensively from a western optical company. I can't afford lenses from any American company, and my finances are tight enough I am afraid of being burned in an overseas transaction. So I have decided to make them, as I want lenses on all the stars down to about 1mm. That's hundreds!

Recently I have been in contact with Ed Jones who is a veteran in American optical production, and has been teaching me the process for simple plano convex lenses. If you know how to grind a mirror for a telescope you know how to do this AND it is less critical as the lenses we want are spherical rather than parabolic like a mirror.

It's amazingly difficult to find anything about this process online. We tend to think in terms of 'one-at-a-time' and that is a daunting proposition. To do this we use standard production techniques.

This can be done completely by hand like making a mirror, or using an overarm machine like a mirror 0-matic. I am looking into the latter due to chronic shoulder pain.

For now, I'll share what I have learned. I am still months away before I embark on this for the projector I am building.

Finding lens blanks

Ed suggested microscope slides. These can be cut into small squares using a good (Toyo) glass cutter. Squares are fine but if you want round cores\blanks a scintered diamond core bit is required and water cooling. This set up is expensive. There is a water cooled bit holder for drill presses, that requires a drain and a recirculating pump.

See video below
youtu.be/sOLW1OShnpw

The glass slides are waxed together and stacked on a piece of plate glass to drill down to like a wood backing board. The cores are drilled out cleaned and set aside.

The tools

Now we need tools with the proper radius for our lenses. It is best if these tools are smaller -around 3 inches diameter- as a larger tool will generate more sag. As we know when grinding two pieces of glass together one becomes concave and the other convex. Thus we generate our desired radius while checking with a spherometer. We can have these generated by Newport, United Lens or make them by hand. The process of grinding is continued to 9 micron grit.

The Lapping Block

Now we cast dental plaster into our concave part. The plaster lap is sealed to make it water proof. Pitch is applied over this convex lap about 1\8" thick. Depending on the quantity of lenses, one may wish to make numerous lapping blocks to support their production run.

Blocking the blanks

Returning to the concave glass tool, our clean little blanks are arranged closely together and laid into the tool filling it about 80-90%. The pitch on the lap is brushed with acetone to make it sticky and this is carefully lowered into the lens blocks, then warmed in an oven so the pitch will flow enough to grab the blanks. The tricky part is preventing the pitch from overflowing and potentially gumming up the grit while grinding. Repeat this as necessary for desired number of blocks. One block will yield over 180 -5mm diameter lenses.

Grinding

Grinding now begins with 40 micron grit (about 320 grit). First the lens block, then another one then the glass convex tool we made at the beginning. If you are making three lens blocks then 1,2,3, followed by the glass tool. During the grinding sessions and between each grit the radius is carefully checked on the concave and convex glass tools with a spherometer so it is maintained throughout the process while we proceed through the grits.

Polishing

Polishing is carried out by attaching a 3M LP66 .020 polishing pad to the concave glass tool and applying cerium oxide.

When completed the blocks are placed in a freezer and the pitch will break away, then clean up the lenses with kerosene and acetone for use.


Since this process is dealing with a basic spherical surface, and we are not trying to finely image onto film or our eye, the process is pretty forgiving. Also we are not dealing with figuring a parabola which makes the process less critical.

I have estimated the cost (using used parts to build a machine and a new Water Drill set up) at around $800. The lenses from China would cost about $1500, in the US about $3000. I have more time than money...

Here is a video of Lenses being made professionally but the process can be done by a hobbyist. It's a little confusing because they edited footage of concave lens production alongside of convex lens production. It's just a difference in tool and lap orientation.

youtu.be/sDZDsn-ir3E
Posted by: Ron Walker Apr 2 2018, 12:51 PM
WOW, you have more "grit" then I have.

I'm sure you have perused "Surplus Shed" for simple plano/convex lenses. I would need to go back and review to see what focal length you would need but their lenses appear to be priced at $4.50 each. I have followed them and know that they have 50% off sales from time to time which would bring the price down to $2.25 each. If you did all of the stars through the third magnitude that would be less then 200 stars (if memory serves me correctly) and then use pinholes for the 4th through 6th magnitudes.

Finding lenses of the correct focal length is the major problem but they must exist in surplus out there somewhere.

All I can say is, more power to you.
Posted by: Philostopher1 Apr 2 2018, 08:26 PM
QUOTE(Ron Walker @ Apr 2 2018, 11:51 AM) *
WOW, you have more "grit" then I have.

I'm sure you have perused "Surplus Shed" for simple plano/convex lenses. I would need to go back and review to see what focal length you would need but their lenses appear to be priced at $4.50 each. I have followed them and know that they have 50% off sales from time to time which would bring the price down to $2.25 each. If you did all of the stars through the third magnitude that would be less then 200 stars (if memory serves me correctly) and then use pinholes for the 4th through 6th magnitudes.

Finding lenses of the correct focal length is the major problem but they muse exist in surplus out there somewhere.

All I can sat is, more power to you.

Surplus shed has some 235mm x 11mm PCX lenses I bought 50 of them a while back. They actually work pretty well at 250mm. Then I bought another 35 experimental grade PCX lenses at 265mm x 9mm. They work well too. It seems at these focal lengths things are pretty forgiving. That's 85 but still not enough...

I have searched and searched, sent numerous inquiries and nothing small enough. There are lots of 250mm lenses at 20mm diam and up but those are obviously too large.

I posted a while back about cutting up plano convex lenses. This works too but as the cut lenses move out from the center wedge increases and creates an offest that must be compensated for in a particular sized dome at a particular star location on the ball. This really just makes more work than grinding the lenses. I want this to work in a dome up to 40 feet. Not that I'll ever have one, but if someone ever wanted to buy the projector (which I don't think I could ever sell), it would be useful in a variety of domes.

By the way, in some threads about lenses there was a discussion about using eyeglasses. It works with 2 x lenses. Again only in the middle then the angular offset gets really bad out to the edges. Could be handy for steering stars to a certain area if needed.

I discovered something else though. I need to do the math but an eyeglass lens, depending on the diopter will shrink a pinhole cluster of stars like the Pleiades, etc. focus them into brighter points in a more compact area. A regular pcx lens won't do this on a curved surface as it must be axially aligned to the desired star.

Anyway, I hope that by May, I can start building the grinding machine. Maybe by September I'll be able to start on the lenses. Slow cash flow...
Posted by: mrgare5050 Apr 3 2018, 12:26 AM
By the way, in some threads about lenses there was a discussion about using eyeglasses. It works with 2 x lenses. Again only in the middle then the angular offset gets really bad out to the edges. Could be handy for steering stars to a certain area if needed.


Yeah I glued a bunch of cheap reading glasses inside my star cylinder, the higher power the better (3x worked fine), but centering them WAS key, and I was never able to break them cleanly into small round pieces, which made it tricky in Orions Belt etc - even Steve Smith had to break lenses into pieces to get those stars accurate - maybe Rons dremel (wheres my stinger socket instructional video??) would work to cut them down. so Ive got all these huge glued lenses. Even reading glasses vary wildly in price though. None of this wouldve been necessary if Edmund hadnt jacked up the price of their 9mm x 265mm.

Slow cash flow...

I'm going to have to get out and push mine pretty soon!
Posted by: Philostopher1 Apr 3 2018, 12:44 AM
QUOTE(mrgare5050 @ Apr 2 2018, 11:26 PM) *
By the way, in some threads about lenses there was a discussion about using eyeglasses. It works with 2 x lenses. Again only in the middle then the angular offset gets really bad out to the edges. Could be handy for steering stars to a certain area if needed.
Yeah I glued a bunch of cheap reading glasses inside my star cylinder, the higher power the better (3x worked fine), but centering them WAS key, and I was never able to break them cleanly into small round pieces, which made it tricky in Orions Belt etc - even Steve Smith had to break lenses into pieces to get those stars accurate - maybe Rons dremel (wheres my stinger socket instructional video??) would work to cut them down. so Ive got all these huge glued lenses. Even reading glasses vary wildly in price though. None of this wouldve been necessary if Edmund hadnt jacked up the price of their 9mm x 265mm.

Slow cash flow...

I'm going to have to get out and push mine pretty soon!

Hey Gare!

Yeah, I think I bought the last of the 9mm from edmund. I was going to order 80 but someone beat me to them. So I only got 35.

If this project of making lenses goes well there will be extras. I am going to be makiing 4.3 mm diam (the inside diameter of the diamond scintered bit) x 250 mm fl. I don't know if these would work for you. How big is your starball? If the project does work out I'd only charge for materials. Microscope slides, plaster and grit are cheap...

To do something with a different radius for a desired focal length would require two tools for that radius.
Posted by: Ron Walker Apr 3 2018, 10:51 AM
In my mind I never thought that a microscope slide would be thick enough but then again I guess the radius of curvature isn't that much when you consider a 250 mm f.l..
Posted by: Ron Walker Apr 3 2018, 10:58 AM
QUOTE(Philostopher1 @ Apr 2 2018, 08:26 PM) *
Surplus shed has some 235mm x 11mm PCX lenses I bought 50 of them a while back. They actually work pretty well at 250mm. Then I bought another 35 experimental grade PCX lenses at 265mm x 9mm. They work well too. It seems at these focal lengths things are pretty forgiving. That's 85 but still not enough...

I have searched and searched, sent numerous inquiries and nothing small enough. There are lots of 250mm lenses at 20mm diam and up but those are obviously too large.

I posted a while back about cutting up plano convex lenses. This works too but as the cut lenses move out from the center wedge increases and creates an offest that must be compensated for in a particular sized dome at a particular star location on the ball. This really just makes more work than grinding the lenses. I want this to work in a dome up to 40 feet. Not that I'll ever have one, but if someone ever wanted to buy the projector (which I don't think I could ever sell), it would be useful in a variety of domes.

By the way, in some threads about lenses there was a discussion about using eyeglasses. It works with 2 x lenses. Again only in the middle then the angular offset gets really bad out to the edges. Could be handy for steering stars to a certain area if needed.

I discovered something else though. I need to do the math but an eyeglass lens, depending on the diopter will shrink a pinhole cluster of stars like the Pleiades, etc. focus them into brighter points in a more compact area. A regular pcx lens won't do this on a curved surface as it must be axially aligned to the desired star.

Anyway, I hope that by May, I can start building the grinding machine. Maybe by September I'll be able to start on the lenses. Slow cash flow...

If those 20mm lenses are cheap enough couldn't you just use that water cooled drill to cut smaller diameter lenses out of the center of the larger ones? That would save a lot of grinding and polishing. You will probably find a lot of places where there are no other stars close by and you could use the full diameter.

Posted by: Philostopher1 Apr 3 2018, 11:07 AM
QUOTE(Ron Walker @ Apr 3 2018, 09:51 AM) *
In my mind I never thought that a microscope slide would be thick enough but then again I guess the radius of curvature isn't that much when you consider a 250 mm f.l..

Yeah, a 1mm thick slide is enough to support that radius. As I mentioned, I have some 235mm and 265mm lenses. I can not see the curve from the edge. Only by viewing a reflection off the surface can the curved side be identified. At least without a loupe.

Ed Jones and I have been discussing this extensively. He is confident 1mm is enough. Since we start at 40 grit (about 320 sandpaper coarseness) we are not doing much more than shaving off microns of glass at a time. Very little grinding to the rough curve. Then as the grits get finer, one must carefully check the radius of the tools with the spherometer, working towards the final radius and polishing.

My biggest concern with thin slides is keeping them held in the pitch on the lens block. We talked about using 2mm Borosilicate. I am leaning in that direction but it's about $75 for a single 100x100mm sheet. Borosilicate microscope slides are much cheaper. I am still googling suppliers. There must be someone that will sell cheap...
Posted by: mrgare5050 Apr 3 2018, 01:54 PM