6 replies to this topic

[LazyPatioObserver]

I‘m having a Skywatcher 130PDS and would like to try a binoviewer with it. As the 130PDS is intended for DSLR photography, I‘m hoping that I‘ll reach focus with a 1.6 GPC, giving me an effective focal length of 1040mm. I’ve read that cheap BVs support about 18mm clear aperture, so 25mm Plössl eye pieces would already be slightly vignetting.

This means the wider possible magnifications, apparent and true fields would be:

25mm 50°: 42x 1,2°

20mm 50°: 52x 1.0°

 

Is this correct? Does anyone have any experience using this scope with binoviewers?

 

[betacygni]

Reflectors are generally a challenge to reach focus with binoviewers (back focus is normally short to accommodate smaller secondary mirrors), but that said I’d suggest measuring your back focus so you know for sure, it’s easy to do. Point the scope at the moon and rack focuser in fully, place a piece of paper over the empty focuser opening and pull the paper back until image of focused moon forms. Measure this distance, that’s the back focus, and will give a better idea of what amplifier power you might get away with. Odds are you’re going to need something closer to 2x amplification to reach focus.

Edited by betacygni, 12 February 2025 - 01:49 AM.

[LazyPatioObserver]

I‘ll measure it once the clouds go away. Someone here on CN said the back focus of the 130PDS is about 85mm. How would I calculate the needed GPC factor from the measured back focus?

[vkhastro1]

Easiest solution:

 

A linear binoviewer has a clear of 18.7mm and focus point is the same position as a regular eyepiece.

No OCS (optical corrector system) required.

Your 130PDS will be working at its native focal length of 650mm

 

The ES 16mm 68° eyepieces will yield FL (650mm) / 16mm = 40.6X

68° / 40.6X = 1.67°

 

I have the same Skywatcher 130mm f/5 PDS (great little scope) and have an Orion linear binoviewer.

The Orion version version is no longer available but the identical linear binoviewer is available from other vendors.

[LazyPatioObserver]

The linear BV costs twice as much as the one needing the corrector, though, even if you include the price for the corrector. As I'm not sure if I really want to use the BV for longer, I'd like to keep costs down as much as possible. (And I want to understand the magical calculations of the longer light path as well…)

[BrushPilot]

The linear BV costs twice as much as the one needing the corrector, though, even if you include the price for the corrector. As I'm not sure if I really want to use the BV for longer, I'd like to keep costs down as much as possible. (And I want to understand the magical calculations of the longer light path as well…)

I'm not in Germany but are you sure of these prices? Linear Binoviewers in North America are much cheaper than the prism ones with or without correctors.

[betacygni]

I‘ll measure it once the clouds go away. Someone here on CN said the back focus of the 130PDS is about 85mm. How would I calculate the needed GPC factor from the measured back focus?

Binoviewers depending on model take about 110-140mm of backfocus. Various amplifiers reduce this number, but unfortunately they don’t all list by exactly how much. Baader does with their glasspath correctors, the 1.25x subtracts 30mm, the 1.7x 65mm, and the 2.6x 120mm. If you’re looking at cheaper binoviewers you’d have to see if they advertise the light path compensation amount for their various compensators.

Edited by betacygni, 14 February 2025 - 12:09 AM.