5 replies to this topic

[paulsky]

Hello,
Generally, it would be preferable when using a Bv., to use EP pairs. or GPC of different powers? I am referring to medium-high powers.
Thank you
Paul

[denis0007dl]

Hi Paul,

if you use binoviewers with all prisms system, you need some GPC to compensate some (not all) false colours, and spherical error.

Shorter focal lenghts you will using, more mentioned errors you will notice.

Bigger prisms are, more spherical error and more false colours you will have.

Kind regards,
Denis

[Eddgie]

Shorter focal lenghts you will using, more mentioned errors you will notice.

 

To elaborate on Denis's comment, the shorter the focal length of the eyepiece, the more likely that you will have merging issues if the binoviewer prisms are out of alignment or your eyepieces have any tilt in the holder.

A Barlow makes the image larger at the focal plane, so longer focal length eyepieces can be used and you don't have the issue unless once again, you go to a very short focal length eyepiece. 

 

If you are using a binoviewer with collimatable eyepiece holders, you can precisely center both images and this means that very short focal length eyepieces can be used. 

[urbanMark]

Paul,

 

I'm assuming your question was "should I buy a few GPC's of different powers (like 1.25x and 1.7x), or just more eyepieces" 

 

If so, it really depends on what binoviewer you use. The Denk's have a "power switch" that makes this easy. Changing the GPC on a MBII isn't hard, but not as easy as changing eyepieces, esp in the dark. Can you do it with your eyes closed and cold hands, wearing gloves? And will your system still reach focus with the new GPC?

[noisejammer]

My experience is that there are several interacting effects.

 

I'll limit my comments to the Baader MkV and Denk II but they will largely apply to the MBII and other large prism based binoviewers. I'll also limit comments to magnifications up to ~200x .. largely because this is what my eyes permit with my 115/805.

 

Finally, it's important to be aware that an ED doublet creates CA that is exaggerated by the other optically dispersive elements in the train.

 

Unless I was deliberately trying for the widest field possible (2.2° with my 115/805), I'd definitely use a 1.25x GPC at all magnifications. This means the effective focal ratio is f/8.75. I have a 1.7x GPC but changing GPC's is a pain in the rump. so unless I have a very specific reason, it stays in the box. (Also note that the 1.7x really delivers about 1.5x. I don't know why - it was designed by Rolando.)

 

As I increase magnification, I find the CA is negligible to ~100x. I achieve higher powers using a 1.6x Tak Extender (a 2x PowerMate would be just as good). The f/ ratio is then f/14 (f/17.5). My heuristic is to keep the exit pupil bigger than 0.65 mm which - in this case - translates to using an eyepiece of 9 mm or longer. If you were using a 2x PowerMate, the limiting eyepiece would be about 11 mm.

 

Other thoughts - the best contrast is delivered by the Baader Zeiss spec prism but the BBHS mirror is very close. My view is the BBHS mirror is significantly more versatile.

 

TL;DR - use a 1.25x and an extender then use relatively long focal length eyepieces to get the power you want.

[ABQJeff]

as mentioned, changing GPCs is not a rapid change. Usually I pick the best GPC for my conditions (ie can do over 400x use the 2.6x, can only do 300x, use the 1.7x) and stick with that changing pairs of eyepieces for different magnification.

Besides there are only a few GPC magnifications (1.25x, 1.7x, 2.6x), there are literally over 30 different magnifications with EP choices (2mm to 40mm Plossl).