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Make a telescope from lens elements?
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PostPosted: Wed Feb 07, 2018 4:24 am    Post subject: Make a telescope from lens elements? Reply with quote

This might not be the correct section of the forum to post this, but I didn't know where else to put.

I have a huge Century Optics 1.6x teleconverter, the afocal type that you place on the front of a lens. It's too big and heavy to be practical with any of my camera lenses, but I would like to make use of it.

What I'd like to do is make a telescope for bird watching, obviously I would need an eyepiece, but what others optics would be needed behind the Century TC to turn it into a scope?

Can a 50mm lens be utilised as an eyepiece?


Last edited by iangreenhalgh1 on Wed Feb 07, 2018 4:53 am; edited 1 time in total


PostPosted: Wed Feb 07, 2018 4:37 am    Post subject: Reply with quote

http://www.bobatkins.com/photography/technical/Lens-scope-adapter.html

Click here to see on Ebay


PostPosted: Wed Feb 07, 2018 4:56 am    Post subject: Reply with quote

Cheers Calvin. I would need to add something between the 1.6x TC and the eyepiece to act as an objective.


PostPosted: Wed Feb 07, 2018 7:24 am    Post subject: Reply with quote

Yes, that was just what I was going to mention. You need and objective. And given that your converter is a 1.6x, you're gonna need an objective with sufficient power such that you'll have enough reach to make it worth your while. So I'd say at least 300mm. 400mm would be better. 500mm even better.

I have an idea -- you know those 400mm f/6.3 and 500mm f/8 presets that used to be advertised in the backs of photo magazines for budget prices? They were actually decent telephotos. So you could either attach your converter directly and add a telescope adapter to the rear, or do it manually with your own tube assy design.

I have a telescope adapter made for t-mount lenses by Spiratone. Works pretty well, but it's a fixed eyepiece. Ideally you'd want an adapter where you could add your own eyepiece -- preferably a 1.25" diameter. The 0.96" size ones are cheap crap, and the 2" ones are overkill.


PostPosted: Wed Feb 07, 2018 9:27 am    Post subject: Reply with quote

Cheers Michael, that's helpful.

The front glass of the 1.6x TC is 102mm, the rear glass is 62mm, so I'd need an objective suitably large enough.

Those 400s and 500s are pretty cheap and common, I've owned a few. My concern would be that they are slow - f6.3/400 or f8/500, would that be bright enough for a scope?

Maybe I should just buy a cheap scope and add the TC to the front?


PostPosted: Wed Feb 07, 2018 10:47 am    Post subject: Reply with quote

You don't need a very fast lens for the scope unless you need to use it in very low light condition(your eye will adjust the brightness). However, the lens has to be sharp enough in the center. A good 500/8.0 mirror lenses will be also a good candidate for using the scope adapter.


PostPosted: Wed Feb 07, 2018 11:33 am    Post subject: Reply with quote

Yeah, except a 500/8 mirror is gonna have a front filter size quite a bit bigger than 62mm, whereas with one of those cheap teles, 62mm is probably very close to their actual size. a step-up (or down) ring is all that will be required, assuming they would even need one.

Ian, telescopes are often slow. F/8 is not unusual. The smaller the aperture, the greater the focal length, everything else being equal. Using the formula FL = Aperture x Objective Diameter, lets use f/8 for the aperture and 100mm (about 4") as the front objective size, we get 8 x 100 or 800mm focal length. But what if this were an f/10 scope? Well then we'd have 10 x 100 = 1000mm focal length. So the speed of the optic (aperture size) is not a drawback, at least for astronomy. For terrestrial viewing, then typical lens issues apply.

Oh, one thing I didn't mention in my above message: regarding eyepieces (just in case you aren't familiar), the smaller the eyepiece is in mm the greater the magnification. I like the Plossl formula -- it gives better eye relief and a wider field of view than a more traditional design.


PostPosted: Wed Feb 07, 2018 12:22 pm    Post subject: Reply with quote

Thanks guys, you have given me some thinking to do, before I was quite clueless.


PostPosted: Wed Feb 07, 2018 12:52 pm    Post subject: Reply with quote

Yes. Front converter does not works well with catadioptric lenses they are large. A good 1.4x or 2x tele-converter will do. Wink


PostPosted: Wed Feb 07, 2018 1:05 pm    Post subject: Reply with quote

Surplus Shed sells rather cheaply large achromats which would be the lens in front of your TC.

This one here for example: https://www.surplusshed.com/pages/item/L3926.html

There is even a booklet how to build a Refractor: https://www.surplusshed.com/pages/item/M4114.html

Should work nicely for you Ian...


PostPosted: Wed Feb 07, 2018 1:31 pm    Post subject: Reply with quote

+1 what Klaus says. Surplus Shed!


PostPosted: Wed Feb 07, 2018 5:41 pm    Post subject: Reply with quote

Normal tc's are effectively a negative (= concave ) optic. However your front mounting tc would be, I think, a positive optic (if you look at eg a newspaper through it does the writing look bigger?)? In other words, your tc itself can act as your objective.
You could get n estimate of the focal length by setting it up to project an image eg of a window, onto a piece of paper. When the image is in focus that's the focal plane. (more...)
If it is a positive/negative 4 elements in 2 groups two achromat combination then the trick may be to separate the main front achromat from the secondary doublet and use that as your main objective.


PostPosted: Wed Feb 07, 2018 7:09 pm    Post subject: Reply with quote

I thought of the shed, but I was hoping I could use parts I already have in my rather large parts bin.

I have two of these TCs, identical, new and boxed so I could indeed try taking one apart as Marcus suggests, cheers.

Yes, they are indeed positive lenses, I'll see if I can work out the FL: roughly.


PostPosted: Wed Feb 07, 2018 11:27 pm    Post subject: Reply with quote

This is a topic which has often intrigued me, for the simple fact that I now own
a few long-reach lenses I might like to use as spotting scopes.

Some interesting ideas put forward so far, but not understanding much of it.
I'd like to ask if anyone has suggestions for an erect-image solution?
I understand it's easy enough to adapt to an upside-down imaging scope,
but with a 300mm, 400mm, and 500mm(+ 2X TC for 1000mm) lens on hand,
I'd like to try this out without spending a load of cash.

Oh, and I also have a Celestion telescope on the way with 1.25" objectives,
which may offer further available opportunities.

Isn't there a simple way to get this to happen other than drilling lens caps,
using PVC pipes, hose clamps, electrical tape, and paying lots for a 45-degree
erecting objective optic?


PostPosted: Thu Feb 08, 2018 12:01 am    Post subject: Reply with quote

I've been eyeing up a Tamron 02F Wide Field Tele View adapter that a dealer has for sale at the camera fair, but he wants to sell it with a 300 / 5.6 that I already have two of Rolling Eyes But, I will wear him down if he hasn't sold it....

They are very good and on the 55B 500 / 8 mirror lens they become a 25X telescope. Having optics obviously makes a difference, and this guy likes his on the Tamron 06B 350 / 5.6 together with a Tamron SP teleconverter Model 01F

http://www.gyes.eu/photo/reviews_tutorials/tam06b.htm


PostPosted: Thu Feb 08, 2018 5:39 am    Post subject: Reply with quote

+1 for the Tamron 02F Wide Field Tele View eyepiece.

This is a very good optical piece, allowing to transform any Tamron Adaptall telephoto lens into a terrestrial telescope with fine optical quality. As it features an erecting prism (roof prism), the image appears right side up, contrary to a simple telescope. If you already have Tamron Adaptall lenses, this is a very good choice. I bought mine for $25 from Keh a few years ago and I guess it should be possible nowadays to get one for less than $50 with a bit of shopping around.

With the 90mm Macro, it makes a handy portable 4.5x spotting scope / macro loupe. With the 350mm Mirror lens, it makes a compact but powerful terrestrial scope with 17.5x magnification for bird watching. With the 200-500/5.6 zoom and a sturdy tripod, it becomes a terrific widefield astro refractor scope with variable 10x-25x magnification for stargazing!

Cheers!

Abbazz


PostPosted: Thu Feb 08, 2018 9:09 am    Post subject: Reply with quote

iangreenhalgh1 wrote:
I thought of the shed, but I was hoping I could use parts I already have in my rather large parts bin.

I have two of these TCs, identical, new and boxed so I could indeed try taking one apart as Marcus suggests, cheers.

Yes, they are indeed positive lenses, I'll see if I can work out the FL: roughly.

Surly they don't have a focal length.
They are designed to have parallel rays enter & leave as parallel rays that are further apart, otherwise the focus on the lens they where mounted on would be changed drastically.


PostPosted: Thu Feb 08, 2018 2:24 pm    Post subject: Reply with quote

Lloydy wrote:
Tamron 02F Wide Field Tele View adapter
They are very good


+1 - I use mine a lot. With the 500mm mirror it works splendidly, crisp sharp fringe free images, an in-your pocket spotting scope (almost). With my 300mm f2.8 + 1.4x tc it matches the sharpness of most of the spotting scopes i compare it to. A bit more fringing with that combo. The fact is the eye is less discriminating than 16/20/24...MPx, as a few experiments digiscoping with the 02F demonstrated (similar IQ to using the 01F 2x tc).
You did very well getting one so cheaply Abbazz, no way one would go for so little on auction here in the UK. They only crop up once or twice a year, if that (quick browse - non on ebayxx update one in Germany #173143373784, one in Italy, search "tamron tele-view"), often with a lens. I paid about £110 for 02F + 55BB and considered that a good purchase.
Just one thing: if only, for general use, tamron had made this with a 45° prism rather than a 90° prism!
PS Lloydy, I happen to have one spare...


PostPosted: Thu Feb 08, 2018 2:53 pm    Post subject: Reply with quote

marcusBMG wrote:
Lloydy wrote:
Tamron 02F Wide Field Tele View adapter
They are very good



PS Lloydy, I happen to have one spare...


I'm interested Marcus. The dealer is looking to sell the one he has with the SP 670AU that it's mounted on and also a SP 19AH with it as a job lot, and I've got two copies of both those lenses.


PostPosted: Thu Feb 08, 2018 4:50 pm    Post subject: Reply with quote

Lloydy wrote:
I'm interested Marcus. The dealer is looking to sell the one he has with the SP 670AU that it's mounted on and also a SP 19AH with it as a job lot, and I've got two copies of both those lenses.


Do you mean the SP 54B (the 670Au is the adaptamatic and the 02F won't fit)? You know if his deal price is right you can sell the 54B and the 19AF, they are both very saleable on ebay, and end up with the 02F for free(or nearly)!
PM sent.


PostPosted: Thu Feb 08, 2018 5:04 pm    Post subject: Reply with quote

DConvert wrote:
iangreenhalgh1 wrote:
I thought of the shed, but I was hoping I could use parts I already have in my rather large parts bin.

I have two of these TCs, identical, new and boxed so I could indeed try taking one apart as Marcus suggests, cheers.

Yes, they are indeed positive lenses, I'll see if I can work out the FL: roughly.

Surly they don't have a focal length.
They are designed to have parallel rays enter & leave as parallel rays that are further apart, otherwise the focus on the lens they where mounted on would be changed drastically.


ehemm, sorry but this is simply not true


PostPosted: Thu Feb 08, 2018 5:06 pm    Post subject: Reply with quote

Abbazz wrote:
+1 for the Tamron 02F Wide Field Tele View eyepiece.

This is a very good optical piece, allowing to transform any Tamron Adaptall telephoto lens into a terrestrial telescope with fine optical quality. As it features an erecting prism (roof prism), the image appears right side up, contrary to a simple telescope. If you already have Tamron Adaptall lenses, this is a very good choice. I bought mine for $25 from Keh a few years ago and I guess it should be possible nowadays to get one for less than $50 with a bit of shopping around.

With the 90mm Macro, it makes a handy portable 4.5x spotting scope / macro loupe. With the 350mm Mirror lens, it makes a compact but powerful terrestrial scope with 17.5x magnification for bird watching. With the 200-500/5.6 zoom and a sturdy tripod, it becomes a terrific widefield astro refractor scope with variable 10x-25x magnification for stargazing!

Cheers!

Abbazz


I have such an adapter .... somewhere... made by and for Nikon. Should dig it out, I guess Smile


PostPosted: Thu Feb 08, 2018 5:20 pm    Post subject: Reply with quote

I had & sold a Spiratone with erecting prism inside and a Russian-made with erecting optics, both for M42 lenses. The exit pupils are tiny. Used with nifty fifty to 135mm lenses handheld. Longer require sturdy tripod & small exit pupil is harder to see through.

Iirc, the Tamron exit pupil is larger. I don't know about the Nikon.


PostPosted: Thu Feb 08, 2018 7:29 pm    Post subject: Reply with quote

Thanks for all the input guys, I didn't think there would be much interest in this hare-brained idea.

I'll do some experimenting and post my progress.

I have been playing around with stacking wide angle adapters and am finally getting somewhere, after a lot of trial and error, mostly error.



In these winter months where we have very little light and lots of rain up here in the north of England I tend to spend more time fiddling with these projects than shooting.


PostPosted: Fri Feb 09, 2018 9:08 am    Post subject: Reply with quote

kds315* wrote:
DConvert wrote:
iangreenhalgh1 wrote:
I thought of the shed, but I was hoping I could use parts I already have in my rather large parts bin.

I have two of these TCs, identical, new and boxed so I could indeed try taking one apart as Marcus suggests, cheers.

Yes, they are indeed positive lenses, I'll see if I can work out the FL: roughly.

Surly they don't have a focal length.
They are designed to have parallel rays enter & leave as parallel rays that are further apart, otherwise the focus on the lens they where mounted on would be changed drastically.


ehemm, sorry but this is simply not true


Can you put me right then Klaus - how do they work?