Binoviewer eyepieces

[bosun21]

19 minutes ago, Louis D said:

None beyond what effect they have on SCTs and Maks in native focus mode without an OCA/Barlow.  That is, to reach focus with a BV in a catadioptric scope without an OCA/Barlow element requires about 100mm or more additional back focus.  To do this, the primary mirror must be shifted forward which increases the scope's focal length due to the magnification factor of the secondary mirror.  There are formulas for both types to calculate the added focal length based on added back focus.

I’m a little puzzled 🤔 When you say move the mirror forward in a SCT or Mak, are you simply referring to the mirror movement by means of the focuser? When you say “forward” does it mean towards the EP or the corrector? Surely you will have to position the mirror to the place that provides the sharpest focus. I apologize for the incessant questions.

 

[Louis D]

12 hours ago, bosun21 said:

I’m a little puzzled 🤔 When you say move the mirror forward in a SCT or Mak, are you simply referring to the mirror movement by means of the focuser? When you say “forward” does it mean towards the EP or the corrector? Surely you will have to position the mirror to the place that provides the sharpest focus. I apologize for the incessant questions.

 

Yes, focusing an SCT or Mak generally moves the primary mirror unless you've opted to add a Crayford style focuser to the rear port.  To move the focus position further from the back plate, the mirror must move forward toward the corrector.  This extends more of the light cone behind the back plate.  I would think this would also cause some vignetting because a wider portion of the light cone is hitting the secondary and passing through the rear port.

Yes, you have to position the mirror to provide best focus, and if you move the position of the eyepiece back 100mm from the rear port by adding a BV, that means best focus has also moved back 100mm from the rear port.

[Don Pensack]

For example: the 8" SCT.

Every mm of additional rear focus you add adds 3.1mm to the focal length of the scope by moving the primary mirror toward the corrector.

If you add 100mm of back focus to accommodate the binoviewer, the focal length of the scope is longer by 310mm.

The 2032mm focal length becomes 2342mm and the f/ratio is now f/11.5 and has a narrower true field of view as a result.

In a moving mirror scope, like an SCT or MCT, a binoviewer is a great thing for lunar and planetary viewing.

But for deep sky, a single eyepiece and diagonal is better.  

Plus, there is the additional light loss in the binoviewer, the additional chromatic aberration, the extra weight on the back, and clearance issue if the scope is fork-mounted.

Not to mention the large amount of spherical aberration caused by moving the mirror.  The SCT is only "diffraction limited" in a very narrow range of focus.

 

[Louis D]

9 minutes ago, Don Pensack said:

Not to mention the large amount of spherical aberration caused by moving the mirror.  The SCT is only "diffraction limited" in a very narrow range of focus.

Do Maks also get a bunch of SA when moving the mirror far away from the design point?

[Don Pensack]

See:

https://www.telescope-optics.net/maksutov_cassegrain_telescope.htm

https://www.telescope-optics.net/Mak_spherical.htm

Short answer: YES.

It can be compensated for in the design.  Is it?  Probably not.  Read the 2nd paragraph above the section "Maksutov-Cassegrain Star Test "in the 2nd link.