Binoviewer Magnification *PLEASE HELP?*

[bond19]

So is there a hard and fast rule on how to work out the magnification when using a Binoviewer with a chosen eyepiece plus Barlow / GPC combination?

I currently use my TeleVue 102 and some TV eyepieces along with a x2 PowerMate. With a simple calculation I can work out the magnification with any given eyepiece.

But how do I go about working out the magnification when using my Binoviewer please?

Am I simply multiplying the stack of barrows/OCA’s to work out a given magnification?

[Louis D]

Luckily for you, the Powermate doesn't grow significantly in power with increasing separation.  As such, you should just be able to multiply the various magnification values together.  As a double-check, setup a ruler or similar and make some field of view measurements without the BV, with the BV and OCA, and with the BV, OCA, and PM.  Eyepiece choice shouldn't matter since all you're looking to calculate is the differential magnification between each step.  These values will remain more or less constant regardless of eyepiece magnification.  This is how I determined that my Meade 140 2x Barlow nosepiece yields exactly 3x in my BV.

[MalcolmM]

I also am very confused by this and have done a fair bit of searching on-line for explanations. I have not really found a consistent answer. I feel there ought to be a relatively simple formula but I have not been able to find one. In my own experience, with a 740mm FL scope, 1.6 x GPC before the diagonal and WO binoviewers with say 20mm eyepieces, I feel I get much more magnification than 740 x 1.6 / 20. But maybe this is an illusion due to different Fields of View? And then it is complicated even further if I put another GPC between the diagonal and the binoviewers!

If there are any Physicists out there who could give a simple explanation with a ray diagram please chip in

 

[globular]

The formula for the magnification of the common barlow design is
M = 1 - X / Fl
where M = magnification, X = distance of focal plane to barlow lens group, Fl = focal length of barlow lens group.

Trouble is you probably don't know the Fl of the barlow (they are seldom published) nor the position of the barlow focal plane.

However if you do a few measurements with your particular barlows / gpcs then you can produce good estimates for these.
(e.g. see how M changes with changes in X (e.g. with and without a 20mm spacer, say) and you can estimate Fl.... and then can estimate X with published M).

And you can then recalculate M with binoviewer by adding the lightpath of the binoviewer to your estimated X and recomputing.

(Note a powermate has a different design (it's not a barlow) and this formula does not apply.  But the 2x powermate is pretty close to 2x no matter the distance. So only your GPC will be varying.)

[Don Pensack]

2 hours ago, MalcolmM said:

I also am very confused by this and have done a fair bit of searching on-line for explanations. I have not really found a consistent answer. I feel there ought to be a relatively simple formula but I have not been able to find one. In my own experience, with a 740mm FL scope, 1.6 x GPC before the diagonal and WO binoviewers with say 20mm eyepieces, I feel I get much more magnification than 740 x 1.6 / 20. But maybe this is an illusion due to different Fields of View? And then it is complicated even further if I put another GPC between the diagonal and the binoviewers!

If there are any Physicists out there who could give a simple explanation with a ray diagram please chip in

 

A Barlow only magnifies by the set amount at one position back from its lens.  Increasing that distance increases the magnification, decreasing that distance decreases the magnification.

Since focal planes in eyepieces are not all at the shoulder, the magnification of a particular barlow will vary according to which eyepiece is in it.

Complicating that even more is the fact that not all barlows have their rated power exactly at the end of the eyepiece tube.  

Eek.

 

So there is a very easy way to tell the magnification of a barlow with a particular eyepiece:

--time the passage of a star on the celestial equator across the field with the eyepiece alone.  Convert that time to digital minutes (i.e. 2min.12sec. is 2.2min.)

--time the passage of the star from edge to edge with the barlow or GPC in place.  Convert that to digital minutes.

--divide #1 by #2.  Voilà! the magnification of that Barlow or GPC with that eyepiece (or whatever else happens to be in the focuser), regardless of where the GPC is placed.

 

Some Good news: the 2" PowerMate 2X only changes magnification by 0.1x over 4 inches of travel away from the lens!

So, to all intents an purposes, just assume it is 2X (remember that timing might yield 2.02 or something like that).

The Glass Path Corrector (OCA) in the binoviewer has a set magnification when used with the binoviewer.  But it is designed to be used attached directly to the binoviewer, not in front of the star diagonal.

When used in front of the star diagonal, it magnifies by more, just like a Barlow.  You will have to do the star timing trick to discover exactly by how much more.

If you have 2 GPCs (and I would not advise it), one by the binoviewer and one a few inches farther away, then the magnification facor may be somewhat unpredictable since the internal light path of diagonals are NOT all the same in a given size of diagonal.

Whatever that magnification is, just multiply it by 2 when used in a PowerMate, whether the PowerMate is used in front of a diagonal or in front of the binoviewer.

 

The star timing trick doesn't even have to be a star on the celestial equator, but the star moves across the field fastest at that location, meaning you'll spend less time timing the passage.

[MalcolmM]

2 hours ago, Don Pensack said:

A Barlow only magnifies by the set amount at one position back from its lens.  Increasing that distance increases the magnification, decreasing that distance decreases the magnification.

Since focal planes in eyepieces are not all at the shoulder, the magnification of a particular barlow will vary according to which eyepiece is in it.

Complicating that even more is the fact that not all barlows have their rated power exactly at the end of the eyepiece tube.  

Eek.

 

So there is a very easy way to tell the magnification of a barlow with a particular eyepiece:

--time the passage of a star on the celestial equator across the field with the eyepiece alone.  Convert that time to digital minutes (i.e. 2min.12sec. is 2.2min.)

--time the passage of the star from edge to edge with the barlow or GPC in place.  Convert that to digital minutes.

--divide #1 by #2.  Voilà! the magnification of that Barlow or GPC with that eyepiece (or whatever else happens to be in the focuser), regardless of where the GPC is placed.

 

Some Good news: the 2" PowerMate 2X only changes magnification by 0.1x over 4 inches of travel away from the lens!

So, to all intents an purposes, just assume it is 2X (remember that timing might yield 2.02 or something like that).

The Glass Path Corrector (OCA) in the binoviewer has a set magnification when used with the binoviewer.  But it is designed to be used attached directly to the binoviewer, not in front of the star diagonal.

When used in front of the star diagonal, it magnifies by more, just like a Barlow.  You will have to do the star timing trick to discover exactly by how much more.

If you have 2 GPCs (and I would not advise it), one by the binoviewer and one a few inches farther away, then the magnification facor may be somewhat unpredictable since the internal light path of diagonals are NOT all the same in a given size of diagonal.

Whatever that magnification is, just multiply it by 2 when used in a PowerMate, whether the PowerMate is used in front of a diagonal or in front of the binoviewer.

 

The star timing trick doesn't even have to be a star on the celestial equator, but the star moves across the field fastest at that location, meaning you'll spend less time timing the passage.

Thanks very much for your reply, that's a lot of information and I think it is in agreement with what globular is saying. I have a couple of questions if I may:

Is the star transit timing experiment (which I will attempt when the rain and clouds disappear!) saying that the magnification is proportional to the field of view? This will be a little tricky for me as without the 1.6x in front of the diagonal I cannot achieve focus. But hopefully I'll get a close enough result. I can then compare that with globular's formula.

Second question: when you say the light paths through the diagonal are different, does this mean focus will not be consistent across the field of view? So I would be better using shorter focal length eyepieces than 2 GPC's to get higher power?

If I attach the 1.6x direct to my binoviewers I cannot achieve focus. Another member in another post suggested I could remove part of the telescope to shorten the light path but I'm not sure I want to go that far on the basis of if it ain't broke, don't fix it!

Many thanks,

Malcolm

[Louis D]

1 hour ago, MalcolmM said:

Is the star transit timing experiment (which I will attempt when the rain and clouds disappear!) saying that the magnification is proportional to the field of view? This will be a little tricky for me as without the 1.6x in front of the diagonal I cannot achieve focus. But hopefully I'll get a close enough result. I can then compare that with globular's formula.

Yes, magnification is proportional to the true field of view.  If you halve the TFOV, you must have doubled your magnification if all else is constant.

That's the beauty of the ruler method.  All that extra optical path length through the BV allows you to reach close focus easier.  It's the same as adding an extension tube of equivalent optical length.  Give it a try and see if you can reach close focus with the GV without the 1.6x.  By close, I mean somewhere between 10 and 50 meters.  You're never going to reach focus on something a foot away without an insane amount of extension. 😁

[Don Pensack]

17 hours ago, MalcolmM said:

Thanks very much for your reply, that's a lot of information and I think it is in agreement with what globular is saying. I have a couple of questions if I may:

Is the star transit timing experiment (which I will attempt when the rain and clouds disappear!) saying that the magnification is proportional to the field of view? This will be a little tricky for me as without the 1.6x in front of the diagonal I cannot achieve focus. But hopefully I'll get a close enough result. I can then compare that with globular's formula.

Second question: when you say the light paths through the diagonal are different, does this mean focus will not be consistent across the field of view? So I would be better using shorter focal length eyepieces than 2 GPC's to get higher power?

If I attach the 1.6x direct to my binoviewers I cannot achieve focus. Another member in another post suggested I could remove part of the telescope to shorten the light path but I'm not sure I want to go that far on the basis of if it ain't broke, don't fix it!

Many thanks,

Malcolm

As long as the same eyepiece is used with or without the Barlow or GPC, then yes, magnification is inversely related to true field.

 

So if you cannot focus without the GPC in front of the diagonal, it says the scope is not really compatible with binoviewers.  You needn't modify the scope but you can lower

the effective magnification by using a GPC with a lower magnification factor, like 1.0 or 1.2.  Or use a binoviewer that doesn't require such a huge amount of in travel at the focuser.

Or use a scope that has the amount of in travel necessary, like an SCT.

 

Different diagonals are built in such a way that the length of the light path through the diagonal is not the same.  Prism diagonals, for an example, have a shorter light path so automatically gain you some in focus.

This has nothing to do with the focus in the field.

 

Yes, two GPCs can have a negative effect on spherical aberration in the scope (which affects high powers most), vignetting, coma, etc.  You are better off achieving high powers with shorter focal length eyepieces.

Of course, shorter focal lengths in a binoviewer means longer focal length eyepieces than many will use for planetary observation because of the Barlow effect of the GPC.  It isn't 1.6x in front of the binoviewer.

As an experiment, try timing the eyepiece in the diagonal without the binoviewer to see what the transit time is, then time in the binoviewer with the necessary GPC.

I think you'll be surprised at the magnification factor.

[MalcolmM]

Hi @Don Pensack, @globular, @Louis D

I hope you don't mind me tagging you all.

I did a wee experiment - hopefully following your instructions.

I measured the FOV (using a ruler) for (a) diagonal + eyepiece (b) diagonal + WO Binoviewers with the WO 1.6x nosepiece barlow between the diagonal and the scope and (c) as for (b) with the addition of the WO 2x nosepiece barlow attached to the Binoviewers (accepted that that may not be advisable!). 

The same eyepiece was used throughout.

I've attached a drawing which hopefully explains better than words!

Anyway, if we call magnification in (a) as x1, then (b) is ~ x2 and (c) is ~ x4

So that would seem (I think) to tie up with what everyone is saying; the x1.6 + binoviewer is actually giving x2

As an aside, I cannot achieve focus with any other combination of the above.

Many Thanks for all your replies,

Malcolm

 

[Don Pensack]

So the GPC 1.6x became 2.0x in the forward position.

[MalcolmM]

1 hour ago, Don Pensack said:

So the GPC 1.6x became 2.0x in the forward position.

Yip, that seems to be the case. Does that seem fair enough to you? Is that what you would expect?

[Don Pensack]

I would have expected a bigger difference.  But one measurement means more than any speculation.