Binoviewer magnification?

[doublevodka]

Hi all,

 

After a fair bit of deliberation, I've picked up a Williams Optics binoviewer with the standard 20mm eyepieces. I tried this for the first time last night on Jupiter and Saturn with my Skymax 127 and noticed that the magnification seems to be more than I'd expect with a 20mm eyepiece on its own?

I've assumed that this is down to the extended light path through the extra glass in the binoviewer, but is there a calculator out ter that can help me confirm it similiar to http://astronomy.tools/calculators/field_of_view/ ?

On a side note, love the view with two eyes! Even with the bad seeing conditions last night I was really impressed, can't wait to try it again on a good night 👍

[Space Hopper]

Difficult to precisely calculate with a moving mirror type scope, but when you extend the lightpath beyond a certain point

the scopes focal length will rise more and more. I think its over 3mm of extra focal length per mm of lightpath added.

So theres always extra magnification.

Other issues as well such as a bit of aperture loss, a bit of contrast loss, and a bit of extra SA, but they shouldn't degrade the views too much.

Good to hear you are enjoying the 2 eyed views 🙂👍🏼

[doublevodka]

1 hour ago, Space Hopper said:

Difficult to precisely calculate with a moving mirror type scope, but when you extend the lightpath beyond a certain point

the scopes focal length will rise more and more. I think its over 3mm of extra focal length per mm of lightpath added.

So theres always extra magnification.

Other issues as well such as a bit of aperture loss, a bit of contrast loss, and a bit of extra SA, but they shouldn't degrade the views too much.

Good to hear you are enjoying the 2 eyed views 🙂👍🏼

Had a feeling it would be difficult to calculate, seems similar mag to a 12.5mm plossl direct into the scope but very happy with the views, looking forward to giving it a go on the moon too!

[MalcolmM]

I use the WO Binoviewers on an f7.5ish 4" refractor. I have to use the 1.6x GPC to reach focus.

I find the magnification is at least 2x what you would expect with the WO 20mm eyepieces - though it's very hard to judge.

Regardless, as you say, the view is fantastic!

Malcolm

 

[Louis D]

I figure my 127 Mak is operating at about 1800mm instead of 1500mm with the additional path length causing the mirror to be moved so much.  Thus, the 20mm eyepieces would be operating at about 16.7mm.  With two eyes, the combined image appears larger than with a single eye, so you may be experiencing this phenomenon.

[PeterC65]

I also have the WO binoviewer and the Skymax 127. I did some research a while back and concluded that the focal length of the scope is increased by 3.75 times the increase in optical path length. The WO binoviewer without a Barlow / GPC increases the optical path length by 100mm and so it increases the focal length of the Skymax 127 by 375mm. That's enough to make a noticeable difference.

 

[doublevodka]

2 hours ago, PeterC65 said:

I also have the WO binoviewer and the Skymax 127. I did some research a while back and concluded that the focal length of the scope is increased by 3.75 times the increase in optical path length. The WO binoviewer without a Barlow / GPC increases the optical path length by 100mm and so it increases the focal length of the Skymax 127 by 375mm. That's enough to make a noticeable difference.

 

That makes a lot of sense, thanks for the info 👍 Hopefully will get some clear and stable skies soon to give it a really good test 🤞

[allworlds]

The Meade ETX manual mentions this effect in an imaging context. The EXT 105 is nominally 1470 mm focal length, but when a camera is mounted on the back the T-adapter lengthens the light path and the result is a 1640 mm focal length, or 1830 with a longer adapter. So a surprisingly large change.

[spartinix]

Some calculations for SCT's...
https://www.telescope-optics.net/SCT2.htm