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Binoviewer use question to aid scope choice


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I have spent many hours reading through some very entertaining, interesting and very informative topics throughout the forum over the past few months. This novice has learned an awful lot from this. I love the banter between some obviously good friends.

I am aiming some time soon to acquire a 4"/5" doublet apo refractor (if there are any actually left in stock) when I finally decide which to go for. One aspect that puzzles me in reading of the benefits of binoviewing is that some refractor descriptions describe them as being bino ready/friendly.

What makes some more readily bino friendly than others? Is it the size of the focuser, availability of focus extensions? or am I misunderstanding something.

Are all refractors capable of being used with binos as well as eyepieces?

I would welcome your advice

John

Edited by Rangefinder10
omission of 5" in description
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Problem with binoviewers is that they use up a lot of optical path.

Sometimes telescopes don't have enough back focus distance to accommodate binoviewers and in those cases you need to use glass path correctors with them - optical elements that add magnification but push focus plane further out enabling you to reach focus.

Binoviewer ready/friendly just means that focus position is further away than in "regular" scope and focuser has enough travel to be able to accommodate regular diagonal / eyepiece and binoviewers.

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Generally you need around 100mm of inwards focuser travel to be able to focus when using a binoviewer without adding a barlow lens element or similar to the binoviewer.

SCT's and mak-cassegrains tend to have lots of focus travel so this is more straightforward on those designs. Refractors and newtonians can struggle.

 

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7 minutes ago, Rangefinder10 said:

Thanks Vlaiv

So is it then just a matter of asking the question at the point of scope selection, are they bino friendly or not?

John

I think it will also depend on binoviewer in question.

I've never used binoviewer so do take my answers with a grain of salt (hopefully somebody that uses binoviewers will be along shortly to confirm or give alternative advice) but I do think that catadioptric telescopes don't have any issues with binoviewer use.

This is due to fact that they focus by moving primary mirror and have very large focus range. This means Maksutov and SCT telescopes (which are good planetary performers and as I understand it - most people use binoviewers for planets primarily).

Newtonian scopes are rather poor at back focus and almost none will work with binoviewer without modification - which means moving primary mirror up in tube closer to focuser.

For refractors, I believe that imaging refractors stand better chance of being binoviewer friendly as they are made to accommodate many accessories like focal reducers / field flatteners, filter wheels, OAGs and such - which means a lot of back focus. Some models are even "modular" - with extension tubes that you can remove when binoviewing.

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19 hours ago, Rangefinder10 said:

I have spent many hours reading through some very entertaining, interesting and very informative topics throughout the forum over the past few months. This novice has learned an awful lot from this. I love the banter between some obviously good friends.

I am aiming some time soon to acquire a 4"/5" doublet apo refractor (if there are any actually left in stock) when I finally decide which to go for. One aspect that puzzles me in reading of the benefits of binoviewing is that some refractor descriptions describe them as being bino ready/friendly.

What makes some more readily bino friendly than others? Is it the size of the focuser, availability of focus extensions? or am I misunderstanding something.

Are all refractors capable of being used with binos as well as eyepieces?

I would welcome your advice

John

Some very good advice already John. All I would add is that some scopes have removable sections in their OTA so as to give more inwards focus to cater for binoviewers. You often uses GPC (Glass Path Corrector) with binoviewers which helps correct for some of the inherent CA from the prisms, but also applies a barlow affect which gains back some optical path length. This obviously also gives higher magnification, which may not be what you want so it is often a compromise somewhere along the line. I use mine almost exclusively for high power observing so use a x1.7 or x2.6 GPC. 


https://www.teleskop-express.de/shop/product_info.php/info/p476_Baader-Glasspathcorrector-1-2-6-for-Baader-Binoviewers-with-T2-Connection.html

This scope for example has a section by the focuser which can be removed to give more inwards focus.

https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-Doublet-SD-APO-125mm-f-7-8---FPL-53---Lanthan-objective.html

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If you're going for an apo frac with your binoviewer, make sure you get a 1.25" diagonal rather than a 2" one (with 1.25" adapter) and preferably prism over dielectric. Prism diagonal consumes less backfocus distance compared dielectric. And 1.25" can save you somewhere between 35mm to 45mm backfocus distance compared to 2".

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5 hours ago, Stu said:

Some very good advice already John. All I would add is that some scopes have removable sections in their OTA so as to give more inwards focus to cater for binoviewers. You often uses GPC (Glass Path Corrector) with binoviewers which helps correct for some of the inherent CA from the prisms, but also applies a barlow affect which gains back some optical path length. This obviously also gives higher magnification, which may not be what you want so it is often a compromise somewhere along the line. I use mine almost exclusively for high power observing so use a x1.7 or x2.6 GPC. 


https://www.teleskop-express.de/shop/product_info.php/info/p476_Baader-Glasspathcorrector-1-2-6-for-Baader-Binoviewers-with-T2-Connection.html

This scope for example has a section by the focuser which can be removed to give more inwards focus.

https://www.teleskop-express.de/shop/product_info.php/info/p10133_TS-Optics-Doublet-SD-APO-125mm-f-7-8---FPL-53---Lanthan-objective.html

Stu thanks for your comments. That is indeed one of the scopes that I've been looking at, but I felt was perhaps a little too heavy.

Ideally I would like to try and keep the OTA weight down to no more than 5 kilos if possible.

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11 minutes ago, Rangefinder10 said:

".......and preferably prism over dialectric" for my education what is the difference?

John

A dielectric generally means a mirror diagonal ie the reflective surface is a first surface mirror and a prism is a glass prism where the reflection happens via total internal reflection off the diagonal side of the prism.

Plenty of info here:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

AD6240A5-346C-494C-A168-C42DF073A011.jpeg

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9 minutes ago, Rangefinder10 said:

Stu thanks for your comments. That is indeed one of the scopes that I've been looking at, but I felt was perhaps a little too heavy.

Ideally I would like to try and keep the OTA weight down to no more than 5 kilos if possible.

Which mount will you be using John?

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20 minutes ago, Rangefinder10 said:

".......and preferably prism over dialectric" for my education what is the difference?

John

Just to add to answer already provided by Stu, prism diagonals are better suited to slow scopes.

When used with fast scopes, due to angles involved there is possibility of introducing chromatic aberration.

For this reason, mirrors are preferred for faster scopes and prism diagonals for slower scopes. For fast scopes, there are options like this:

https://www.teleskop-express.de/shop/product_info.php/info/p9870_TS-Optics-T2-90--Star-Diagonal-Prism-with-28-mm-free-aperture-for-Observation-and-Photography.html

which you can equip with low profile adapters to shorten optical path.

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9 minutes ago, Stu said:

Which mount will you be using John?

I have a Skytee 2 and an AZ5.

My concern about weight is more to do with lifting capability.

Whilst manageable now, I'm trying to look ahead a year or two where I may not be so capable of lifting heavier weights.

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17 minutes ago, Stu said:

A dielectric generally means a mirror diagonal ie the reflective surface is a first surface mirror and a prism is a glass prism where the reflection happens via total internal reflection off the diagonal side of the prism.

Plenty of info here:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

AD6240A5-346C-494C-A168-C42DF073A011.jpeg

Very helpful, thanks, I will have a more detailed review

John

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I'm not totally hung up on the matter of binoviewers. It's just that a bought a pair out of interest from Ian King when he was shutting up shop last year, because I wanted to experience the difference in use,  but not yet been able to make them function, and was curious about the critical factors to take regard of when choosing my next scope.

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8 minutes ago, vlaiv said:

Just to add to answer already provided by Stu, prism diagonals are better suited to slow scopes.

When used with fast scopes, due to angles involved there is possibility of introducing chromatic aberration.

For this reason, mirrors are preferred for faster scopes and prism diagonals for slower scopes. For fast scopes, there are options like this:

https://www.teleskop-express.de/shop/product_info.php/info/p9870_TS-Optics-T2-90--Star-Diagonal-Prism-with-28-mm-free-aperture-for-Observation-and-Photography.html

which you can equip with low profile adapters to shorten optical path.

Thanks Vlaiv

I'm soaking these details up as fast as I can.

John

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This is the one I have:

https://www.teleskop-express.de/shop/product_info.php/info/p609_Baader---ZEISS-90--diagonal-prism---T2-thread-on-both-sides.html
 

There is a cheaper non Zeiss Baader one which would still be very  good. The T2 format is very good and has a short optical path. Depending on which binoviewer you have, you may be able to make a direct connection to the prism to keep the path length short. That’s what I did here when using my little Telementor. The short extension just allows space for a GPC so it doesn’t hit the prism; that is one thing you do have to be careful about. Some eyepieces have long barrels and can hit and damage the prism surface, as I found out once to my cost. Some GPCs screw directly into the binoviewer so the extension may not be needed.

BC588019-F958-4C91-B710-B171357F2846.jpeg

95053A98-0369-40FF-AE23-F889F474B3AB.jpeg

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25 minutes ago, Stu said:

This is the one I have:

https://www.teleskop-express.de/shop/product_info.php/info/p609_Baader---ZEISS-90--diagonal-prism---T2-thread-on-both-sides.html
 

There is a cheaper non Zeiss Baader one which would still be very  good. The T2 format is very good and has a short optical path. Depending on which binoviewer you have, you may be able to make a direct connection to the prism to keep the path length short. That’s what I did here when using my little Telementor. The short extension just allows space for a GPC so it doesn’t hit the prism; that is one thing you do have to be careful about. Some eyepieces have long barrels and can hit and damage the prism surface, as I found out once to my cost. Some GPCs screw directly into the binoviewer so the extension may not be needed.

BC588019-F958-4C91-B710-B171357F2846.jpeg

95053A98-0369-40FF-AE23-F889F474B3AB.jpeg

I have the W.O. Version ,  cost me £50, and at that price I thought “ nothing to lose”

John

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2 minutes ago, Rangefinder10 said:

I just simply assumed that it would be ok with my Megrez 72 but didn’t seem to work

Maybe post up a picture so we can see if there is any opportunity to save any path length?

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10 minutes ago, Rangefinder10 said:

I just simply assumed that it would be ok with my Megrez 72 but didn’t seem to work

From my experience with binoviewers it is not just a case of attaching them and adjusting the focus because there are a few other adjustments that you need to try depending on which ones you have. I am only familiar with my Astro Engineering binos so cannot offer any suggestions, maybe someone with the same one you have could help.

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7 hours ago, Rangefinder10 said:

I just simply assumed that it would be ok with my Megrez 72 but didn’t seem to work

The physical length of the Megrez 72 is about 300mm and its focal length is 430mm. If you also factor in the length of the 2" and 1.25" adapter on the focuser drawtube, it is highly unlikely to have enough backfocus distance for the binoviewer without any GPC.

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