12 replies to this topic

[Princess Leah]

A lot of spotting scopes have a three element objective, some as many as five and a focussing element to boot.

 

One of my favourites has five, the Opticron HR66ED. This can be purchased quite cheap used, for a reason beyond my comprehension.

https://www.opticron.../hred_scope.htm

 

One of the reasons this scope is so good is it's almost F7, just like the splendid older Nikon fieldscopes ((ii), ED).

These scopes are quite big and heavy in comparison to faster F6 modern varieties - but I think it's worth it.

 

But why so many elements? Is it something to do with that internal focusing mechanism.

Also with so many elements, plus Porros it's amazing the light throughput some manage - very good coatings I assume?

[TOMDEY]

If it's a zoom that would explain it.    Tom

[Princess Leah]

These spotting scopes use zooms and fixed piece EPs.

 

How would using a zoom influence this?

Edited by Princess Leah, 09 April 2025 - 06:24 PM.

[TOMDEY]

Another big consideration makes it far more complex than an astronomical telescope --- A spotting scope must image sharply over ranges from near to far.    Tom

[Princess Leah]

Mmmm my small F6 William Optics scope seems to be able to do this when I attach a 1.25 inch correct image Porro to it.  I will need to check how close it can focus.

[maniack]

Another big consideration makes it far more complex than an astronomical telescope --- A spotting scope must image sharply over ranges from near to far.    Tom

Astronomical telescopes can do pretty well for close range objects, but spotting scopes tend to be capable of much closer focus. Spotting scopes need to keep their focusers contained in smaller space, and the better ones will have additional flattening/reducing elements in place.

 

By the way it's certainly possible to find astronomical refractors with 5 elements or more. The ZWO FF65 is similar in aperture to the HR66ED and has a 3 element primary objective along with a 2 additional elements for flattening. The Sharpstar Z4 has a 3+3 layout for a total of 6 elements. And the WO Pleiades 68 has 7 elements!

[RichA]

Another big consideration makes it far more complex than an astronomical telescope --- A spotting scope must image sharply over ranges from near to far.    Tom

  The focusing lens group is the issue, how it interacts with the objective and prism system.  If focusing were outside, like with an astro-scope, then it's just Objective-erecting prism-eyepiece, same as a terrestrially-used astro-scope. 

[SandyHouTex]

A triplet will still have field curvature so they add a lens to flatten the field.  Then like Tom said, you need a couple for the zoom function.

[Princess Leah]

From opticron:

 

'All models feature a high definition 5 element ED objective lens system,' (HR66ED)

[CrazyPanda]

A lot of spotting scopes have a three element objective, some as many as five and a focussing element to boot.

 

One of my favourites has five, the Opticron HR66ED. This can be purchased quite cheap used, for a reason beyond my comprehension.

https://www.opticron.../hred_scope.htm

 

One of the reasons this scope is so good is it's almost F7, just like the splendid older Nikon fieldscopes ((ii), ED).

These scopes are quite big and heavy in comparison to faster F6 modern varieties - but I think it's worth it.

 

But why so many elements? Is it something to do with that internal focusing mechanism.

Also with so many elements, plus Porros it's amazing the light throughput some manage - very good coatings I assume?

Any modern Petzval astrograph also has 5 (sometimes 6) elements. Two or three elements are used for field flattening and maybe some additional correction.

 

If you didn't care about a flat field, then a 3 element objective would be sufficient.

 

Considering 8 and 9 element eyepieces can still give mid-high 90s transmission, it should be no sweat for a 5 element optical instrument to give high 90s transmission with the right coatings.

 

I do get a kick out of how the sport optic advertising makes a big deal about coatings and light throughput and us astronomers are like "Yeah? That should be a given."

[Princess Leah]

It's not field fatteners that the Opticron HR uses, or other field-scopes in general for that matter.

 

I think RichA hit the nail on the head with his comment above.

[SandyHouTex]

If you already knew, why did you ask?

[Princess Leah]

I didn't. Rich A answered above?