Binoviewers, GPCs and Barlows....

[Stu]

A question.....

Does a GPC always gives its true multiplying factor regardless of how/where it is installed, or does the position and optical path length make a difference.

@YKSE I'm putting my faith in you for an answer here

For instance, I have a Baader Zeiss Mark IV binoviewer and install a x1.7 into the base of it (rather than into a 1.25" barrel in front of the prism diagonal). Does that give x1.7 or some other factor?

In addition to this, if I use an AP Barcon element with two extension tubes fitted with T2 quick changers ahead of a Baader 2" Zeiss Herschel Wedge then the binoviewer fitted with the x1.7 GPC and 25mm Zeiss orthos. As a very rough estimate I think I'm at around x200 but would love to be able to properly calculate it. I can provide photos and measurements if needed....

Thanks!

Stu

EDIT Incase not clear, I mean the Barlow and extension tubes are all after the HW for obvious reasons.

[Moonshane]

Look forward to the answers!

As someone who hopes to use BVs on my 16" & 12" f4 dobs I'd be interested in another related query if you don't mind Stu?  That is do you normally use BVs with the paracorr removed?

My understanding of GPCs is that they do act as a Barlow in terms of increasing effective focal length by the factor of the GPC.

[YKSE]

Stu,

Answer to your questions is No according to my measurements. magnification factor will be higher when installed in front of a diagonal than behind. The end of this thread is about GPC factors:

The extention(s) was needed there for the measurement when focusing on much closer day time target.

As you can see,  when a GPC installed just in front of BW, the magnification is lower than the nominal value and it increase and varies with different diagonals too when installed in front. wtih  2.6x GPC showed largest magnification increase.

[Space Hopper]

3 hours ago, YKSE said:

As you can see,  when a GPC installed just in front of BW, the magnification is lower than the nominal value and it increase and varies with different diagonals too when installed in front. wtih  2.6x GPC showed largest magnification increase.

Yes, the further 'in front' an amplifier is the greater the magnification.

With the 2.6x gpc being in the diagonal (with both the Maxbright and the Mk V) some refractors may even run out of out focus.

My 132mm refractor is pretty much fully racked out when i use the 2.6x and i actually have to pinch a few mm extra by unseating the nose a touch when using certain eyepieces.

I did have a 50mm extension tube (which was a faff) but can't use it since modifying my focuser. 

[niallk]

8 hours ago, Moonshane said:

Look forward to the answers!

As someone who hopes to use BVs on my 16" & 12" f4 dobs I'd be interested in another related query if you don't mind Stu?  That is do you normally use BVs with the paracorr removed?

My understanding of GPCs is that they do act as a Barlow in terms of increasing effective focal length by the factor of the GPC.

FWIW, I don't use a Paracorr with my binoviewer @ f/4.7 and f/4.4 (saves weight, I use 50deg afov plossls at medium-high power on planets, lunar).

I have a 2.5x GPC for my Maxbright for my 10", but couldn't reach focus in my 15". I tried my 2.5x Powermate, and it worked - a possible alternative to save money if you already have one??

From the Televue website, I think I might be getting closer to 2.2x from the PM given the distance through the binoviewer (?- could be misinterpreting):

 

[Moonshane]

Cheers Niall

I am fortunate that the paracorr free focus position on two of my dobs (6" f11 & 12" f4) is at about 75mm out. I am hopeful that removing the 50mm extension and using trial and error with gpc placement,  I should be able to get good results again with just the one pair.

[Space Hopper]

As a point of interest, i measured focusing distances last year on a solar session with the Mk V.

Basically a 1.25x gpc with give you an extra 20mm of infocus. The 1.7x gpc will give you 35mm. (this is using a Herschel wedge which swallows up a whole length of lightpath)

But this is with the gpcs screwing directly into the binoviewer though ; i'd imagine these distances to be longer with the Maxbright.

Its quite a tricky balancing act with a refractor sometimes : all these different gpcs : will they all reach focus ? Do i need an extension tube ? How long ?

Should i saw off a bit of the OTA tube ? Maybe get a new focuser ? Lots of ifs and buts with binoviewers........

[Moonshane]

I think I'd be happy to cut a bit off my 120ED as it's quite an old one and others would also benefit if I ever sold it.