Is it possible to build a triplet apochromatic refractor?

[Fishie]

Hi all,

 

As the title says, is it possible to make my own triplet apochromatic refractor? I've made my own Newtonian reflector before, and I've heard of people making their achromatic refractor, but what are the difficulties of making a triplet apochromatic refractor?

 

Thanks!

 

Fishie

[ronin]

!"designing" oen is likely easy, suspect there are packages out there where you give the glass data and specify cemented or seperated and the focal length and they churn out the  radaii and seperations you would have to construct.

It will be grinding each face to the correct radaii that is the difficult area. Could also be a bit of effort in getting the glass types required, although you have more flexibility then you would think as you do not specifically need FPL-51 or FPL-53.

Designed one many years ago by hand, or rather data and calculator for a optical test item we needed.

[John]

Some of the glass required is pretty expensive and then there are 6 optical surfaces to grind, polish and multi-coat. The 3 lens elements need to be very carefully matched, spaced and mounted. The lens cell on triplets is a vital element and quite complex to design and machine.

I'm sure it has been done but whether it can be done in a way that is viable financially and in time terms now, given the availability of reasonably priced triplet apochromats from China is doubtful I feel

 

[mikeDnight]

You'll need spherometers to measure the radius and sag of the lens surface accurately and some top class engineering to create a suitable lens cell. seperation of the elements is critical in air spaced designs, or you could design an oil spaced lens. The beauty of figuring refractors though, as opposed to mirrors, is that they are easier to figure accurately, as you can work on any or all surfaces to achieve a perfect end result. You might try your hand at making an ED doublet to begin with, just so you get a feel for the science and art of lens making.  It could also fund your triplet project. There are numerous books available to help guide you. I used Advanced Telescope Making by Mackintosh but there are many more sources of information available. Google is amazing!

 

Mike

[Chris]

Wow that's a wonderful ambition you have there Fishie, I can't confess to having the answers to this one other than it would be flipping difficult. As a fellow mirror grinder you'll know that there is literally tons of material out there, but I've not seen anything on grinding, polishing (and possibly figuring) a triplet. Then again I haven't looked that deeply into triplets so maybe there is stuff out there?

I'm personally tempted to try a long focus Crown and flint achromat as my next project, I've seen a book on the subject and figure if I read it enough times I might just stand a chance! The achromat would be relatively easy compared to the triplet I should think, 4 shallow spherical faces for a long achromat, and 6 potentially deeper curves for the triplet (unless you want a slow triplet which might defeat the object?), then of course there is the extra engineering involved with the cell for a triplet, as discussed above.

Have you thought about cutting your teeth on a 'relatively' easy long focus achromat doublet? It might be a good stepping stone to your goal   

https://www.firstlightoptics.com/books/making-a-refractor-telescope-how-to-design-grind-polish-test-correct-and-mount-a-doublet-lens-book.html

[Peter Drew]

I've not made triplet objectives but I've made triplet refractors from scratch using "bare" supplied optics. The largest was a 300mm F15 triplet for a large Camera Obscura which produced a 2.5 metre diameter image with better than 1mm edge resolution on a flat surface. The lens itself cost £25K!  

[rl]

I've got a 6" f/10 oil spaced triplet made by John Owen which works very well..so, yes it can be done. I've been writing a review on it for months..one of those things that never gets finished. It's virtually colour-free in focus at *250.

Making refractors is a different ball game..you have to worry about the homogeneity of the glass and its exact refractive index,how to avoid "wedge", how to stop the oil leaking...There's a reason they cost so much..I wish you luck!

RL

[Chris]

24 minutes ago, rl said:

I've got a 6" f/10 oil spaced triplet made by John Owen which works very well..so, yes it can be done. I've been writing a review on it for months..one of those things that never gets finished. It's virtually colour-free in focus at *250.

Sorry to the OP for slightly digressing, but they had one of those scopes on ENS not so long back..I very nearly bought it and put it on the plastic!

[Peter Drew]

3 minutes ago, Lockie said:

Sorry to the OP for slightly digressing, but they had one of those scopes on ENS not so long back..I very nearly bought it and put it on the plastic!

I nearly did as well, it was a steal at the price. Jon Owen's triplets are as good as anyones. 

[rl]

That's the same one...

I ummed and ahhed about it and of course it went before I made up my mind. . A year later it turned up on ABS with a moonlite focuser mod so it was obviously meant to be....i'm getting motivated now to finish that review....

RL

[Peter Drew]

On 5/19/2017 at 18:52, Lockie said:

Wow that's a wonderful ambition you have there Fishie, I can't confess to having the answers to this one other than it would be flipping difficult. As a fellow mirror grinder you'll know that there is literally tons of material out there, but I've not seen anything on grinding, polishing (and possibly figuring) a triplet. Then again I haven't looked that deeply into triplets so maybe there is stuff out there?

I'm personally tempted to try a long focus Crown and flint achromat as my next project, I've seen a book on the subject and figure if I read it enough times I might just stand a chance! The achromat would be relatively easy compared to the triplet I should think, 4 shallow spherical faces for a long achromat, and 6 potentially deeper curves for the triplet (unless you want a slow triplet which might defeat the object?), then of course there is the extra engineering involved with the cell for a triplet, as discussed above.

Have you thought about cutting your teeth on a 'relatively' easy long focus achromat doublet? It might be a good stepping stone to your goal   

https://www.firstlightoptics.com/books/making-a-refractor-telescope-how-to-design-grind-polish-test-correct-and-mount-a-doublet-lens-book.html

If you are seriously considering having a go at making an achromatic doublet I have a pregenerated crown and flint pair for a 5" aperture Littrow design objective you could have. The Littrow design would be a good starter as r1, r2 and r3 curves are identical spheres with r4 being flat. With your mirror making experience it would be worth making a spherical mirror of the correct radius to use as a test plate for r1 and r2 on the crown and then test the flint r3 on the crown. 

[Chris]

3 minutes ago, Peter Drew said:

If you are seriously considering having a go at making an achromatic doublet I have a pregenerated crown and flint pair for a 5" aperture Littrow design objective you could have. The Littrow design would be a good starter as r1, r2 and r3 curves are identical spheres with r4 being flat. With your mirror making experience it would be worth making a spherical mirror of the correct radius to use as a test plate for r1 and r2 on the crown and then test the flint r3 on the crown. 

Sounds wonderful, Peter  I certainly have the inclination to do so, only it might be some time before I've completed my mirror, plus a scope to go with the mirror. A very kind offer, but it might be a bit early days? Having said that, let me have a think? I could always buy the blanks and shelve them until I'm ready  

[Peter Drew]

11 minutes ago, Lockie said:

Sounds wonderful, Peter  I certainly have the inclination to do so, only it might be some time before I've completed my mirror, plus a scope to go with the mirror. A very kind offer, but it might be a bit early days? Having said that, let me have a think? I could always buy the blanks and shelve them until I'm ready  

When I said you could have them I meant FOC. I'm not going to anything with them and they would only get lost or broken in due course.  

[Chris]

7 minutes ago, Peter Drew said:

When I said you could have them I meant FOC. I'm not going to anything with them and they would only get lost or broken in due course.  

Wow in that case yes please, Peter! I'd be very happy to look after them until I can turn them into hopefully working optics  I'll PM you after work tonight to sort out postage, cheers  

[Chriske]

I'd go for a Fraunhofer first.

[Chris]

16 minutes ago, Chriske said:

I'd go for a Fraunhofer first.

Hi Chris, is there a reason you would go for a Fraunhofer first?

EDIT: just had a quick Google and seen that the Littrow can produce multiple ghost images...hmm?

[Chriske]

Less surfaces to grind and polish.

Lets hope Fishie knows what he's starting....

[Chris]

5 minutes ago, Chriske said:

Less surfaces to grind and polish.

Lets hope Fishie knows what he's starting....

I wouldn't do a triplet first, even if he has mirror grinding experience I still think he should go for some kind of doublet. I wonder what he will do?

[Peter Drew]

Size for size and similar focal ratios the Fraunhofer is a better design. It is slightly more achromatic and field coverage is better if AP is a consideration. The Littrow is "easier" to make, needs little specialist equipment for testing and is less sensitive to cell mounting issues. I've used a number of uncoated Littrow objectives and can't recall ghosting issues, modern coatings, if applied should overcome any.

[Chris]

PM sent, Peter  

[Chriske]

These lenses must be coated. That process should be done by pro's.
Question is who is willing to do this job. A vacuum bell-jarr for smaller diameter lenses is arranged to allow certain diameters. These machines will allow spectacles lenses and to apply single coatings only. Although some brands do apply multi coatings. So question is : what if the diameter of this selfmade lenses do not fit in the machine's holders..? Are they prepared to adjust the machine's setup for two lenses with a different diameters. (Don't think so)
And a major problem is : will you find a lens manufacturer wiling to do the job.. in UK or Europe..?

[Peter Drew]

I wouldn't worry too much about coatings at this stage, none of the early great astronomers did.  

[Chriske]

9 hours ago, Peter Drew said:

I wouldn't worry too much about coatings at this stage, none of the early great astronomers did.  

Woow...! Al these surfaces without coatings.....'Some' light is going to bounce back and forth in that cell imho.  You'll loose contrast, a lot of it.
These early astronomers had no coatings because the technique wasn't available yet. Otherwise they surely would have applied.

[Peter Drew]

28 minutes ago, Chriske said:

Woow...! Al these surfaces without coatings.....'Some' light is going to bounce back and forth in that cell imho.  You'll loose contrast, a lot of it.
These early astronomers had no coatings because the technique wasn't available yet. Otherwise they surely would have applied.

Very true, but the Littrow can be oiled as three of the surfaces are identical so not such a "bad" scenario as it seems. 

[Chris]

3 hours ago, Chriske said:

These lenses must be coated. That process should be done by pro's.
Question is who is willing to do this job. A vacuum bell-jarr for smaller diameter lenses is arranged to allow certain diameters. These machines will allow spectacles lenses and to apply single coatings only. Although some brands do apply multi coatings. So question is : what if the diameter of this selfmade lenses do not fit in the machine's holders..? Are they prepared to adjust the machine's setup for two lenses with a different diameters. (Don't think so)
And a major problem is : will you find a lens manufacturer wiling to do the job.. in UK or Europe..?

I'm going to try not to worry about coating any lenses I get round to grinding and polishing. I'll be too busy worrying about getting a reasonable figure and having the skill to do so! Yes non coated lenses will have less contrast, but if you want the highest contrast and highest Strehl refractor, my feeling is to save up and buy one from a company that has spent many years applying the art and money in kit to do so. 

for me grinding and polishing my own optics is more for the personal achievement and satisfaction than anything else. If any I make turn out reasonable i.e. 1/4 wave 0.8 Strehl and above, I'll be very happy indeed, maybe even if the contrast isn't top notch.