Cannot get focus with a Baader Bino Viewer

[alan potts]

I am asking this for a friend out here as his English isn't so great. He has a T/S 6 inch scope the focal length is about the 900mm mark, I am not totally sure. He has a Baader Bino viewer and cannot get focus with it. He states when he moves it backward he feels it gets worst though this is not with an extension sleeve and only holding in line above the diagonal. Any pointers to help him out?

Alan

[Stu]

1 hour ago, alan potts said:

I am asking this for a friend out here as his English isn't so great. He has a T/S 6 inch scope the focal length is about the 900mm mark, I am not totally sure. He has a Baader Bino viewer and cannot get focus with it. He states when he moves it backward he feels it gets worst though this is not with an extension sleeve and only holding in line above the diagonal. Any pointers to help him out?

Alan

Just so it’s clear, do you mean one of the 152mm f5.9 refractors Alan?

Which version of the Baaders, first or second?

[Franklin]

If he puts a Baader 1.7X GPC in there it should reach focus. Also he will need to use a short diagonal, a 2" mirror diagonal is too long, they take up too much back focus. The Baader 32mm T2 prism diagonal is ideal and has the T2 fittings for even more back focus saving.

Edited February 10, 2023 by Franklin

[alan potts]

2 hours ago, Stu said:

Just so it’s clear, do you mean one of the 152mm f5.9 refractors Alan?

Which version of the Baaders, first or second?

Yes thats the refractor but I don't know the viewer, but he has only just bought it, so I imagine it's the later one.

Alan

58 minutes ago, Franklin said:

If he puts a Baader 1.7X GPC in there it should reach focus. Also he will need to use a short diagonal, a 2" mirror diagonal is too long, they take up too much back focus. The Baader 32mm T2 prism diagonal is ideal and has the T2 fittings for even more back focus saving.

Yes he is using a TeleVue 2 inch diagonal, I will pass this on.

Alan

[Franklin]

1 hour ago, alan potts said:

Yes he is using a TeleVue 2 inch diagonal

That's the culprit, with a 1.25" prism which has a light path under 70mm instead of a 2" with a light path well over 100mm he'll be saving at least 30-40mm which will make all the difference when trying to reach focus. He may still need a 1.7x GPC or regular barlow nosepiece. 

[alan potts]

2 hours ago, Franklin said:

That's the culprit, with a 1.25" prism which has a light path under 70mm instead of a 2" with a light path well over 100mm he'll be saving at least 30-40mm which will make all the difference when trying to reach focus. He may still need a 1.7x GPC or regular barlow nosepiece. 

Thank you I will pass this on to him,

Alan

[Highburymark]

The GPCs aren’t just there to reach focus. They also correct aberrations introduced by the binoviewer. Because they go from 1.25x to 2.6x, they provide lots of power options.