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Bino pairs


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So I'm looking for a bit more mag for my bino-viewer. Ideally I'd like another 10mm Clavè, but hens teeth and all that. I have the 20mm wo that came with the BV and a pair of 18.3 delites but those are on the limit of my IPD and obviously the mag is very similar with both pairs. So at present with the 1.7 gpc in the train im getting around 80mag. I'd like more, for planetary only.....a 10mm should give me 140ish mag, so something around that.....

Baader genuine ortho or classic ortho (half the price of genuine but don't know what the difference is, anyone?)?

Televue plossl?

Tak ortho?


Any thoughts or recommendations welcome.


This is for my zeiss which is f13, 63/840 (63/1428 f22.6 with GPC by my reckoning)

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Hi Andy,


Probably the best option is to purchase the 2.6x GPC and see how that performs with regard to how much magnification you need.


If you're ever in Derby (grocery shopping ?) you'd be welcome to drop by and borrow mine and try it out. Let me know.

I keep it in an extra Baader prism diagonal where it permanently resides. Aside from me being lazy its to save

me from fiddling about with them in the dark, and dropping / losing them etc. It's much easier to switch diagonals than it is to switch GPCs.


Just to confuse things the 1.7x GPC is not 1.7x but reportedly closer to 1.5x.

As their name suggests : glass path compensator, they are primarily designed to give you the back focus to you need to focus and

observe with a binoviewer in the train, and also of course have that 'Barlow' effect ,extending focal length for more power from a given set of eyepieces.


The 1.25x 'buys you back' 20mm of focus travel, the 1.7x 35mm and the 2.6x gives you back 65mm.


Baader also do a 1.7x GPC for Newtonians. I once had one when i had my Dob, and it worked fine, also corrected for Coma, but it

also tied you in to a fairly high power, which was not what i really wanted for low power rich field views of star clusters etc.

But then i thought : i already have a 1.7x GPC ? Why not try it out ?

So i screwed it into the back of the Mk V and attached a 2" nosepiece. Would it focus with the Newt ? Nope. Not even close.

That experiment proved the 1.7x gpc i used in my refractor is not 1.7x as they say, but much less, and indeed closer to 1.5x as suggested.


Another thing to consider. All i've said may be academic.

With the Mk V and the Maxbright 2 both the 1.25x and 1.7x GPCs screw into the back of the bino which is threaded for them.


With the Max 1 if i remember correctly the 1.25 & 1.7x have a different optical design and as they can't thread into the bino,

they have to go into the diagonal instead. If you look online at the Baader Max 1 instruction manual it explains it in there.


ONLY the 2.6x GPC (in all designs) fits in the diagonal, rather than the bino. I'm not sure why that is ?

The point i was trying to make is that the further in mm's you mount a gpc / barlow from the bino the greater the magnification it will be.

So for eg, if you are not using a short lightpath prism, but say a bigger prism or a bigger T2 compatible 2" mirror diagonal, the GPC will

be therefore further away in mm's from the bino and thus give a bigger magnification than you thought.

So the 1.7x that is actually closer to 1.5x may actually be working closer to 1.7x or even greater.


Which means if you try the 2.6x it could be closer to 3x depending on your configuration. Millimetre's count with binoviewing.


So to conclude, i would try the 2.6x gpc first, before investing £££s in a new pair of eyepieces you may not get on with.

But please borrow mine  for a few sessions and see how it performs if you wish.


Hope that helps.

Best regards, Rob 😀🔭






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And this,


I've found high power viewing with higher power eyepieces very much a personal thing.

I've tried plenty of pairs between 9mm and 16mm. Some of them i've sort of gotten on with, other i have hated. As much as i love my 24 and 19 Panoptics

for low and mid range power : they are perfect in every way, i've never found a high power Televue pair in the 10 - 15mm range i've liked.

I personally prefer smaller eyepieces for binoviewing, and i like a nice amount of eye relief as well. I can recommend the APM Flatfield range,

they are very nice, and although discontinued, i really like my high power 12mm set. Although 'only' a 60ºFOV, i've found them to be fine in

that department, and really like the twist up eyecup design where you can dial in your preferred eye relief. They are small and fairly inexpensive as well. See image below.


Rule 1. Always use a gpc in the train, whether that be the 1.7 or 2.6 to remove the bino and prism induced sphero-chromatism from the view.

           Rather than say, just using a pair of 5mm eyepieces on their own. High power sphero-chromatism is not pretty, believe me.


Everything i've ever read from all the experts in this says don't go below 10mm eyepieces when using a high power power pair. Some say 9 or even 8mm

eyepieces are ok, but the lower you go, the more difficult merging becomes, and getting a comfortable view.


I guess its like using any really high power : every little imperfection or issue you have literally becomes magnified enough to degrade the view.

So, always use the GPC to get that focal length, and stick within that 10 - 15mm range of eyepieces to get that high power. Most of the time in the UK

our conditions rarely support a really high power, most of the time the question is whether the seeing or the scopes optics, or thermal issues, or collimation will handle it.

If the seeing is crap, or if my scope has not fully acclimatised i'm often limited to 120 - 150x magnification anyway.

I've had many a session where i've planned on doing some planetary viewing, and where my scope has cooled properly for over an hour, and been greeted

with local seeing that has been so bad, i've laughed out loud, and packed up a short while later.

Its those rare nights we get when the seeing is decent, and every thing else is ok and it all comes together, and it all becomes worthwhile, and leaves us wanting more.....


APM 12mm Flat fields. (This was when they were new. Yes, the one on the right was mucky and perhaps a return ? but the Baader Optical wonder saved the day.)



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Thanks Rob. So I've converted the bv to be t2 screwed directly on to the prism with the gpc sat in the bv end of the prism and prism screwed directly on to the ota, well quick release, and is as close as possible. I'd rather be changing eyepieces to get higher mag rather than unscrewing everything in the field to change the gpc. Its only for planetary so a narrow fov is fine. Just finding the delites just a tad too wide and want something physically smaller but still quality although at the focal lengths I'm looking at I'd get alot of leeway with quality. I think 10mm ish is about my limit with this set up. The old telementor cuts through the seeing with surprising ease. Anyone want to do some swapsies? 

Edited by tuckstar
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I have both the 8.5 mm and 12 mm Pentax XFs for use in my BV.  Wall to wall crispness on the Moon, but the seeing has to be good for the 8.5s.

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  • 2 months later...
Posted (edited)

@bryand pair of 8.5 xf in the post thanks to @Bino-viewer finding some on astro buy and sell. Been reading about some field curvature in the 12mm. I realise this would be a combination of scope and eyepiece, but have you noticed any FC with yours? I'm using a native f13.

Edited by tuckstar
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Mine have been used mainly on the Moon, and I haven't noticed any field curvature.  But then there aren't many straight lines on the Moon!

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Spending other peoples money for them again......!! I'll be getting a reputation ! 😀

Looking forward to hearing how they perform 👍🏼🔭

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