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Can this spotting scope objective lens be used on it's own?

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#1 chrisg

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Posted 18 June 2019 - 09:11 PM

Hi,

 

About to purchase an older 85mm Zeiss Diascope spotting telescope, and I was wondering if the objective could possibly be re-purposed for a small refractor, without the internal focusing optics that are used in the Diascope. 

 

Here is a diagram of the internal optics:

https://images.app.g...FJxgxVYr1bVnyJ6

 

Anyone have any idea?

 

Chris



#2 ngc7319_20

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Posted 18 June 2019 - 11:46 PM

The optics are probably optimized to all work together -- including the rear doublet focusing lens and prism assembly.   So the front triplet assembly on its own might or might not make good images.  Need to try it and see.


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#3 Jon Isaacs

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Posted 19 June 2019 - 12:06 PM

I tried making an ED doublet spotting scope objective into a simple refractor. It was quite strange, there was noticeable CA and it was sort of a blue green rather than purple.

 

Jon


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#4 chrisg

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Posted 19 June 2019 - 01:42 PM

I tried making an ED doublet spotting scope objective into a simple refractor. It was quite strange, there was noticeable CA and it was sort of a blue green rather than purple.

 

Jon

 

Hmm. Do you think possibly a field flattener/reducer/ would have helped?



#5 chrisg

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Posted 19 June 2019 - 01:45 PM

Also curious, given the optical schematic shown in the link, (and I know squat about optical design), I'm assuming the objective is a triplet, the rear, internal lens is a doublet. What purpose/effect does the rear doublet have on the whatever the triplet is projecting? The doublet appears to be attached to the focusing ring.



#6 ngc7319_20

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Posted 19 June 2019 - 01:57 PM

Also curious, given the optical schematic shown in the link, (and I know squat about optical design), I'm assuming the objective is a triplet, the rear, internal lens is a doublet. What purpose/effect does the rear doublet have on the whatever the triplet is projecting? The doublet appears to be attached to the focusing ring.

The rear doublet is effectively a Barlow lens.  It makes the focal length longer (increases magnification), and is also used to focus the scope.  Using a Barlow lens like this lets them make the scope physically shorter.  It also reduces aberrations going through the prism assembly -- slower F-ratio light cones have less spherical and chromatic aberration in prisms.



#7 chrisg

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Posted 19 June 2019 - 02:41 PM

The rear doublet is effectively a Barlow lens.  It makes the focal length longer (increases magnification), and is also used to focus the scope.  Using a Barlow lens like this lets them make the scope physically shorter.  It also reduces aberrations going through the prism assembly -- slower F-ratio light cones have less spherical and chromatic aberration in prisms.

Thanks, I wonder if any Barlow will do (I have an AP Barcon). Will give it a shot when I get a chance!



#8 Jon Isaacs

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Posted 20 June 2019 - 04:25 AM

Hmm. Do you think possibly a field flattener/reducer/ would have helped?

 

I don't, this is a chromatic aberration issue. 

 

My guess is that ngc7319_20 was correct when he said the optics were optimized to work together.  The prism assembly is complicated and prisms can affect the color correction of a fast objective.  

 

You could try it like I did, build a test setup out of plastic pipe.  Just make sure you can put it all back together as a spotting scope if it doesn't work.  I have heard and conversed with other people who have tried using spotting scopes objects to make astro-scopes with less than satisfactory results.  

 

What do you see when you use it as a spotting scope?  What does a power wire against a bright blue sky look like at high magnifications?

 

Jon



#9 starcanoe

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Posted 20 June 2019 - 07:37 AM

Just my random two cents.

 

Yes it IS entirely possible that the eyepiece and prism and what ever else in the optical train are optimized to work with that objective lens.

 

HOWEVER....there is IMO a very high probability that the objective will work fine by itself with some other random eyepieces not designed to work with that lens.

 

Think about it for a minute. You would WANT your lens to be designed to be as achromatic as possible in the first place because fixing things downstream optically so to speak would just be that much harder if it was not so.

 

Or in other words....you can certainly tweak things downstream for better performance....you why would you make a mess in the first place then tweak it later?



#10 ngc7319_20

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Posted 20 June 2019 - 03:23 PM

 

Think about it for a minute. You would WANT your lens to be designed to be as achromatic as possible in the first place because fixing things downstream optically so to speak would just be that much harder if it was not so.

 

Or in other words....you can certainly tweak things downstream for better performance....you why would you make a mess in the first place then tweak it later?

I'm not sure "fixing it later" is much of a concern.  It is likely one or two clicks in some optical design program like ZEMAX or CODEV and no more trouble than that.  One gets a big performance improvement going from one element objective to two elements.  More going to three.  So imagine going to 5... 6... 7... etc., where the whole system has been optimized together.  But that said, the OP will need to try it and see what happens.  We don't really know what the designer was doing. 

 

One of the unique properties of these spotting scopes (and binos, too) is that the designer has all the optics to work with.  It's not like e.g. a Meade SCT, with a Celestron reducer / corrector, and a TeleVue eyepiece on the back.  Some designer at Zeiss could specify each and every element, and ensure they work well together -- even complementing each other.



#11 Jon Isaacs

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Posted 21 June 2019 - 08:04 PM

Just my random two cents.

 

Yes it IS entirely possible that the eyepiece and prism and what ever else in the optical train are optimized to work with that objective lens.

 

HOWEVER....there is IMO a very high probability that the objective will work fine by itself with some other random eyepieces not designed to work with that lens.

 

Think about it for a minute. You would WANT your lens to be designed to be as achromatic as possible in the first place because fixing things downstream optically so to speak would just be that much harder if it was not so.

 

Or in other words....you can certainly tweak things downstream for better performance....you why would you make a mess in the first place then tweak it later?

 

I am not sure it works that way.  Hypothesis:

-  The prisms and the additional optics downstream are refractive and so they have some affect on the chromatic aberration.  A simple prism star diagonal is known to have an effect and these are much more complicated with a more complicated light path.  

 

-  Correcting the downstream optics for color is probably not possible, they are what they are.  However, it is possible to design the color correction of the objective to compensate for the chromatic aberration of the down stream optics.  

 

I did not design the optics in the spotting scope I experimented with but I do know that the color correction without the downstream optics was significantly poorer than it was with the downstream optics.  

 

Jon


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#12 chrisg

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Posted 21 June 2019 - 08:36 PM

Well, just thought I'd post a quick update. I received the telescope today and unscrewed the objective off the front and simply held an eyepiece behind it, looked through, and although this was a rough gander, didn't see any gross bad effects, the edges actually looked good. No CA as far as I can tell so...yeah looking forward to using this as my new objective, now I need to learn how to get the right parts and assemble a refractor!



#13 Jon Isaacs

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Posted 21 June 2019 - 09:07 PM

Well, just thought I'd post a quick update. I received the telescope today and unscrewed the objective off the front and simply held an eyepiece behind it, looked through, and although this was a rough gander, didn't see any gross bad effects, the edges actually looked good. No CA as far as I can tell so...yeah looking forward to using this as my new objective, now I need to learn how to get the right parts and assemble a refractor!

 

To see chromatic aberration, I think you need do more than hold an eyepiece up to the objective.  You need a stable configuration and a high contrast target during the day or a bright object star at night.. 

 

My suggestion would be to start out with a simple mock up so you can test color correction without going to the trouble of making a telescope. 

 

You might rig something up out of plastic drain pipe.  

 

Jon


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#14 BGRE

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Posted 21 June 2019 - 09:12 PM

All such prisms systems can be unfolded to an equivalent thick plane parallel slab of glass as far as the SA and chromatic aberrations are concerned.
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