GPC question.

[Telescope40]

Hello all. Considering a purchase of a 2nd GPC for my binoviewers. Already have a 1.7 version. Thinking of a 2.5. 
Will this help with back focus. The 1.7 I have nearly runs out of travel but works well, luckily. 
Would the 2.5 work too with no inward focus issues. To be used with my refractors.  
TIA. John. 

[Stu]

26 minutes ago, Telescope40 said:

Hello all. Considering a purchase of a 2nd GPC for my binoviewers. Already have a 1.7 version. Thinking of a 2.5. 
Will this help with back focus. The 1.7 I have nearly runs out of travel but works well, luckily. 
Would the 2.5 work too with no inward focus issues. To be used with my refractors.  
TIA. John. 

The x2.6 (I think it is, not 2.5) will actually give you more back focus than the x1.7. It will push the focus point out meaning more available travel.

[Space Hopper]

The 2.6gpc  'buys you back' 65mm of focus travel.

You may even run out of 'out' focus and have to pull out the diagonal a bit.

The 1.7x version you have is actually closer to 1.5x

[Telescope40]

Thank both @Stu @spacehopper  Confirmed my thoughts. 👍👍

[rkelley8493]

I use a 1.8x GPC for Refracting & SC Telescopes [Baader T2] with my binoviewer. There wasn't enough inward focus travel when I attached the GPC to the nose of the binoviewer, so I ended up attaching it to the T2 prism. This configuration required a ton of outward focus travel because the GPC is so far forward in the optical path. The closer the GPC is to the objective in the light path, the further [outward] distance you'll need to reach focus. The further the distance of the GPC from the light path, the more inward travel you'll need to reach focus.

This was no problem in my SVA-130T w. 3" Feather Touch, but in my FC-100DF, the draw tube length on the focuser wasn't long enough. To get around this, I ended up getting the Baader Tele-Kompendium set of extension tubes & adapters [see bottom picture]. This allowed me to attach the T2 GPC within the extension tubes for a flexure free optical train and gave me enough back focus to work with. 

Here are a few illustrations. This picture has the GPC attached to the nose of the T2 prism [requires a lot of focuser distance]. I had to pull the gpc halfway out of the focuser to reach focus and spot the weight due to flexure

 

[rkelley8493]

1 minute ago, rkelley8493 said:

I use a 1.8x GPC for Refracting & SC Telescopes [Baader T2] with my binoviewer. There wasn't enough inward focus travel when I attached the GPC to the nose of the binoviewer, so I ended up attaching it to the T2 prism. This configuration required a ton of outward focus travel because the GPC is so far forward in the optical path. The closer the GPC is to the objective in the light path, the further [outward] distance you'll need to reach focus. The further the distance of the GPC from the light path, the more inward travel you'll need to reach focus.

This was no problem in my SVA-130T w. 3" Feather Touch, but in my FC-100DF, the draw tube length on the focuser wasn't long enough. To get around this, I ended up getting the Baader Tele-Kompendium set of extension tubes & adapters [see bottom picture]. This allowed me to attach the T2 GPC within the extension tubes for a flexure free optical train and gave me enough back focus to work with. 

Here are a few illustrations. This picture has the GPC attached to the nose of the T2 prism [requires a lot of focuser distance]. I had to pull the gpc halfway out of the focuser to reach focus and spot the weight due to flexure

Solution: the Baader M68 Tele-Kompendium System with internal T2 thread