Spotting scope objective design (1 Viewer)

Hello..I recently bought some inventory for the now closed Kowa distributor and technical service in Spain..I bought some interesting stuff,including body parts for the 820,that i really needed,because i have a home repaired back carcass in my 823, (and now ,at some point i will be able to replace it.).Other treasure i got is a fluorite objective cell for the 823..this is good,I can replace this valuable part if needed,but ,honestly i dont think it would happen,and i feel bad about having this nice objective wrapped in a piece of paper ..So i decided that i might try and build a telescope,..mmm..buy a tube,a focusser and use the 82 mm ,450 focal fluorite doublet to make a sweet apo refractor..I decided to ask for advice ,and i asked the guys from APM teleskopes in Germany,if they could suggest a way to build a telescope,and they answered with a sad dissapointing fact...They told me that kowa optics for the 823 are designed to work with the whole prism system and if used without the prisms (i can get those too)the optical performance of the objective would be poor,specially chromatic aberrations...
That answer surprised me ,because i thought prisms took quality away from the final image,but seems like optics design is a far more complex matter that i could ever figured...
So..It is this true for any spottign scope?..Are the objectives of a prismatic scope and an astro refractor really that different?
Thanks

Hi mayoayo,

it is quite possible that the glass path of the prisms has been used in the optical prescription.
Also it could be that the doublet objective does not have the expected focal length of 450mm as this is the focal length of the whole body and there might be a lens for the internal focusing in there...

You can experiment with the lens cell and try to build a telescope yourself to check it out... just don't take apart the lens cell... as this will loose collimation and getting it back together in a well collimated state could take a professional with an optical bench some hours...

Joachim

I wouldn't be surprised to find that the prisms are included in the design, but there are such large differences in the spherical correction of individual units that you might still come up with a perfectly acceptable scope, especially if you use a diagonal prism rather than a mirror to erect the image.

On my astronomical APO scopes prisms add spherical overcorrection and throw violet wavelengths farther from green focus while bringing red wavelengths closer to green focus.

Using a diagonal prism is beneficial overall on my Tak Sky-90, which has its spherical undercorrection just about perfectly corrected by inserting a prism in the light path, but with some increase in violet CA. The same prism introduces overcorrection when used on an Astro-physics Stowaway, which is perfectly corrected without the prism.

BTW, I'm pretty certain the Kowa 800s use a moving prism for focus, rather than a focusing lens.

Henry

Hi,

thanks to Henry for the comment on the focus method and the good idea of using a diagonal or even an amici prism for image erection instead of a mirror in order to get some glass path... although amici prisms usually are not so great for astro use...

And happy experimenting to the original poster...

Joachim

Joachim,Henry..thanks for the information..The 820 uses a moving prism for focussing..I repaired my unit,and it is quite a simple scope..I do have access to lots of parts ,besides the pieces i bought,so i might even be able to put together a full specimen after all...and i have a swarovski fit bayonet from another repair,,,,!!!...
I guess i can always call kowa for information on the optical design of the 820 ,now that is discontinued
Ill keep you updated...
Regards
Manuel