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Why is high magnification on BT generally discouraged?

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#1 Huan

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Posted 07 August 2020 - 03:00 PM

I know BT are specialized towards low power large FOV viewing, but why is high magnification generally discouraged?

 

For an example, the recommended most practical magnification for APM ED 70 mm is from 16X to around 50X, yet a same aperture ED scope could easily go up to 150X with no issue. For APM ED 100 mm the recommended range is from 23X to around 80X, yet a 4” ED scope could go all the way to 200X or even 300X. What’s the catch? I’m under an impression that an ED BT is just two ED scopes aligned together.

 

Is it about collimation?

Is it about the prisms?

Or is it about eyes placement under high magnifications?



#2 Rich V.

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Posted 07 August 2020 - 03:17 PM

The catch is primarily your first two reasons. 

 

A 45° correct image prism system has six reflective faces and a 90° has four that have to meet very high accuracy.  A simple refractor only uses one reflection from a diagonal.

 

Collimation accuracy is directly proportional to the magnification involved.  Comparative acceptable angular errors between both sides are directly proportional to mag.   At 200x, collimation has to be 10x more precise than at 20x.

 

Eye placement isn't hard in my experience as long as you get the IPD set right and eye placement at the proper exit pupil distance.

 

Rich


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#3 warpsl

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Posted 07 August 2020 - 03:20 PM

Of course the objectives are usually capable of more magnification, but miscolimation and barrel/ota alignment issues arise at high magnification. I use 2x 6 inch refractors,and nearly every time I set up I need to adjust its ota alignment.the most mag I can use is 141x.small exit pupils become difficult to align.if I had a observatory I would leave it permanently set up and would not need to adjust it all the time.

#4 ArsMachina

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Posted 07 August 2020 - 03:28 PM

For me it is the tracking.

With very most binoculars and binoscopes you need to track the objects manually by hand.

This becomes more tricky ( I would call it annoying) with increasing magnification.

 

Jochen



#5 salico

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Posted 07 August 2020 - 04:04 PM

I found in my BINOPTIC ED 120 powers until 129x (Nagler 7mm) are nice, mediocre to track; in the NOCTUTEC 12"/6 my First Light tracking 150 x is easy, 257 x okayish



#6 Faber

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Posted 07 August 2020 - 04:29 PM

Salico, could I ask you why is easiest to track with the noctutec than the binoctic? Perhaps was my idea but I supposed that a binonewton is more difficult due to position of the observer. could I have more feedback about? Sorry if it’s considered OT. Regards

#7 Pinac

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Posted 08 August 2020 - 12:13 AM

.....

.....

 

Collimation accuracy is directly proportional to the magnification involved.  Comparative acceptable angular errors between both sides are directly proportional to mag.   At 200x, collimation has to be 10x more precise than at 20x.

 

.....

.....

And this is why I never go beyond 60x to 80x with almost all of my BTs (APM, Oberwerk, Vixen and Kowa), even when they appear well collimated, with the exception of the BORG, where I can fine-tune the collimation (or should I say: the conditional alignment) right during my observation, so going beyond 100x with the BORG has been a revelation (I have gone up to 125x with 4mm eps and, based on the excellent result, have ordered 3mm DeLites that will allow me to go up to 166x).

So the collimation accuracy argument is not necessarily valid for all BTs.

 

Pinac


Edited by Pinac, 08 August 2020 - 02:39 AM.


#8 edwincjones

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Posted 08 August 2020 - 04:19 AM

for me, the binoculars are for lower mag wide FOVs,

if one wants the higher mags, then a telescope is better

or 

just use the right tool for the job

 

edj


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#9 bcarter1234

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Posted 08 August 2020 - 12:46 PM

Huan,

 

You've received some very good and accurate responses.

 

The experience most closely aligned with my own is described here by Pinac whose opinions and experience I greatly respect.

"...with the exception of the BORG, where I can fine-tune the collimation (or should I say: the conditional alignment) right during my observation, so going beyond 100x with the BORG has been a revelation (I have gone up to 125x with 4mm eps and, based on the excellent result, have ordered 3mm DeLites that will allow me to go up to 166x).

So the collimation accuracy argument is not necessarily valid for all BTs."

 

It is an understandable but unfortunate fact that most manufactured BTs must be conditionally aligned at the factory and can not easily be adjusted during observation by the end user. In designs where the individual tubes can easily and quickly be conditionally aligned during observation the regularly accepted limits on magnification can be achieved, i.e. 200X or more with 4" ED scopes.

 

When viewing objects at higher magnification you are more likely to be focusing on the image near the center of the field so if the view deteriorates even 50% out from center it is not so detrimental. 

 

Here is an excerpt from a report of a recent outing with a 127mm achromatic BT for your consideration.  

 

"Jupiter was very pleasing at 116X as was Saturn nearby. Everything looked promisingly stable so on to 170x with a pair of Barlowed 9mm eyepieces. I thought no chance but then again nothing to lose but a little sleep so the 9mm EPs were replaced with 6mm using #8 filters.

 

Now it's operating at 256x, both planets looked very good (not perfect, but I'm yet to see perfect) and Jupiter's moons looked like discs as opposed to the point of a star. No whirls for me yet but nice banding and an occasional hint of some detail. I could see banding (ring shadow?) on Saturn, a tiny bit of planet disc below the ring and Cassini division was sharp.

 

Just for fun I went back over to the Double Double at 256X which featured clean, tight stars widely split. M57 was a suitable finish to a surprisingly fine evening.

 

Considering the lenses were operating at about two and a half times what they are designed to support I was very impressed. Merging is simple. I only had to adjust it a total of three times all night typically after an eyepiece change. Each time is was the work of about 30 seconds. There was absolutely no eyestrain even at 256x. If you have "normal" visual alignment it would be even easier."

 

If more factory built BTs had the mechanisms to allow for quick and simple conditional alignment during use you would see them used at the same magnifications as their monocular brethren.

 

Two other valid points have been raised.

 

IPD is indeed critical at higher magnification but if a mechanism for quick and easy adjustment during observation is provided it is the work of a few seconds to set.

 

Tracking is challenging but only the equal of the same magnification in a monocular telescope. If you have a good mount you will be able to track at higher magnifications.

 

Oh yes, prisms were also mentioned. In an astronomical telescope? What prisms? (Just kidding, the 90 degree asymmetrical BT design employs one mirror per side.)

 

I fear I must respectfully disagree in this instance with edwincjones. A well designed binocular telescope is the right tool for the job. ;-)

 

Seeing planets and other objects at medium to high magnification with two eyes, through two objectives, is a wonderful experience.

 

Take care,

Brent


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#10 Rich V.

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Posted 08 August 2020 - 03:27 PM

I think this conversation might need to differentiate between commercial "binocular telescopes" also known as "BTs" that use a combination of roof erecting prisms and rhombs for IPD adjustment such as the current APM and Oberwerk BTs and "binoscopes".

 

"Binoscopes" are two individual telescopes mounted together in parallel and equipped with mirrors or prism backs to deviate the viewing angle and align the light paths closer together to match our eye's IPD.  They may provide correct images with two mirrors as in the Matsumoto 90° backs or L/R reversed images as in the three refection Binoptic backs or the one reflection used by diagonals in asymmetrical/staggered tube binoscopes.  Binoscopes can have more points along the optical path where misalignment can happen so perhaps a provision for "conditional alignment" may be a necessity in their case.

 

"If more factory built BTs had the mechanisms to allow for quick and simple conditional alignment during use you would see them used at the same magnifications as their monocular brethren."

 

A properly mechanically executed and precisely collimated BT using the above roof/rhomb design is not "conditionally aligned" at the factory or by the end user; the light paths of both sides remain parallel throughout the IPD range the rhomb turrets provide.  The rhombs become the "hinge axis" so to speak, but they do not cause rotational changes in alignment like moving the tubes around a central hinge in a traditional binocular.  The light paths are made parallel before they reach the rhombs, either by eccentrics or prism tilt.  A state of "conditional alignment" can only apply if the eyepieces or eyepiece holders/focusers are out of parallelism and the user must tilt or rotate an eyepiece to compensate and reach a merging of images.  That is not by design, though, but instead by suffering from poor QC.  Perhaps an "eyepiece tilt" conditional alignment convention could be added to lesser quality BTs that don't get the QC they require.  wink.gif

 

I think the multiple reflective prism surfaces and glass path in commercial BTs still will likely cause some detrimental effects to image quality at the same magnifications as their "monocular brethren" with a simpler light path, though.  frown.gif

 

Rich

 

 




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