Binoviewer magnification - the truth is out there

[Highburymark]

I have a WO binoviewer that I'm going to use for ha solar in my Lunt LS60 (bino has 1.6x Barlow piece that screws into nose). Until now I've had no problem reaching focus with the WO in my other scopes, but I need to make some changes to get it working in the Lunt. So a 2x Barlow piece (again, screwed into the nose) is on the way.

But which EPs to use? At the moment the only EP pairs I have are 15mm TV plossls, and the 20mm WO EPs which came with the bino. So need a couple of less powerful pairs to get full disc views with the 2x Barlow fitted. But I can't find a definitive formula for working out EP magnifications with binoviewers. Some say the Barlow factor should be used directly to establish magnifications (i.e. the 2x barlow will double the power of eps in the binoviewer - a pleasingly simple calculation which appeals to my dimwitted mind), while others maintain that the extra light path between the Barlow and eps in a binoviewer will increase the magnification significantly to something like 2.8x. If that were the case, the 15mms would be too much for my solar set up. Any answers? 

[Louis D]

It depends on the focal length of the negative element of the barlow.  Check this CN thread for some leads figuring all this out.  I would suggest figuring out how long your binoviewer's light path is first.  Second, measure the distance from the barlow element's threads to the shoulder of its original tube.  I would then apply a simple ratio calculation since I think barlows grow power at a linear rate.  If the binoviewer's path is 50% longer than the barlow's original tube, figure it will yield 50% more power in the binoviewer.

For low power in my Arcturus binos, I really like the cheap 23mm 62 degree aspheric eyepieces sold on ebay.  They're light, tiny, have very good performance, and are cheap enough not to worry about damaging them.  Just don't wipe the eye lens because it's been reported the coatings wipe right off of the acrylic aspheric lens.

[tooth_dr]

59 minutes ago, Highburymark said:

I have a WO binoviewer that I'm going to use for ha solar in my Lunt LS60 (bino has 1.6x Barlow piece that screws into nose). Until now I've had no problem reaching focus with the WO in my other scopes, but I need to make some changes to get it working in the Lunt. So a 2x Barlow piece (again, screwed into the nose) is on the way.

But which EPs to use? At the moment the only EP pairs I have are 15mm TV plossls, and the 20mm WO EPs which came with the bino. So need a couple of less powerful pairs to get full disc views with the 2x Barlow fitted. But I can't find a definitive formula for working out EP magnifications with binoviewers. Some say the Barlow factor should be used directly to establish magnifications (i.e. the 2x barlow will double the power of eps in the binoviewer - a pleasingly simple calculation which appeals to my dimwitted mind), while others maintain that the extra light path between the Barlow and eps in a binoviewer will increase the magnification significantly to something like 2.8x. If that were the case, the 15mms would be too much for my solar set up. Any answers? 

If it helps, with a 2x Barlow and a pair of 15mm EPs I don't see a full disc in my 60mm Lunt

[Floater]

As you may have guessed, Mark, I have a 2x nosepiece Barlow arriving (soon? PostNord Sweden?), too. I hope your query can get it nailed. But it ain't easy! 

As I think you wrote in another thread, the easy part is acquiring the telescope. But, as I wrote in another thread, some of the fun is in struggling with these things ... and, then hopefully getting it sorted out. ?

And we will!

 

[Highburymark]

11 minutes ago, Floater said:

As you may have guessed, Mark, I have a 2x nosepiece Barlow arriving (soon? PostNord Sweden?), too. I hope your query can get it nailed. But it ain't easy! 

As I think you wrote in another thread, the easy part is acquiring the telescope. But, as I wrote in another thread, some of the fun is in struggling with these things ... and, then hopefully getting it sorted out. ?

And we will!

 

Definitely agree Floater - I've had a remarkable few days so far with the new scope - will expand more when I've done a bit more observing - but learning is part of the deal with ha solar

[Highburymark]

2 hours ago, Louis D said:

It depends on the focal length of the negative element of the barlow.  Check this CN thread for some leads figuring all this out.  I would suggest figuring out how long your binoviewer's light path is first.  Second, measure the distance from the barlow element's threads to the shoulder of its original tube.  I would then apply a simple ratio calculation since I think barlows grow power at a linear rate.  If the binoviewer's path is 50% longer than the barlow's original tube, figure it will yield 50% more power in the binoviewer.

For low power in my Arcturus binos, I really like the cheap 23mm 62 degree aspheric eyepieces sold on ebay.  They're light, tiny, have very good performance, and are cheap enough not to worry about damaging them.  Just don't wipe the eye lens because it's been reported the coatings wipe right off of the acrylic aspheric lens.

Thanks Louis - all roads seem to lead to Eddgie whenever there's light path measurement to be debated ?

[YKSE]

16 hours ago, Highburymark said:

I have a WO binoviewer that I'm going to use for ha solar in my Lunt LS60 (bino has 1.6x Barlow piece that screws into nose). Until now I've had no problem reaching focus with the WO in my other scopes, but I need to make some changes to get it working in the Lunt. So a 2x Barlow piece (again, screwed into the nose) is on the way.

But which EPs to use? At the moment the only EP pairs I have are 15mm TV plossls, and the 20mm WO EPs which came with the bino. So need a couple of less powerful pairs to get full disc views with the 2x Barlow fitted. But I can't find a definitive formula for working out EP magnifications with binoviewers. Some say the Barlow factor should be used directly to establish magnifications (i.e. the 2x barlow will double the power of eps in the binoviewer - a pleasingly simple calculation which appeals to my dimwitted mind), while others maintain that the extra light path between the Barlow and eps in a binoviewer will increase the magnification significantly to something like 2.8x. If that were the case, the 15mms would be too much for my solar set up. Any answers? 

It'll depend on the what barlow you use (long or short), a long barlow (with tis longer focal length) will have less increased magnifcation than a short barlow.

You can easy get quite good estimation of magnification of your barlow with BW.

1. Measure the physical length of barlow lens to its EP shoulder (a couple of mm deviation wouldn't do much harm), assume it's 70mm, then the focal length of your 1.6x barlow is 70/0.6=117

2. WO binoviewer is said to have 100mm optical length, you'll get magnification

1+(100+70/0.6)/(70/0.6)=2,86x,

If you measurement in step 1 is 80mm, it'll be 2.75x, not a whole world of difference to 2.75x.

[Highburymark]

 

3 hours ago, YKSE said:

It'll depend on the what barlow you use (long or short), a long barlow (with tis longer focal length) will have less increased magnifcation than a short barlow.

You can easy get quite good estimation of magnification of your barlow with BW.

1. Measure the physical length of barlow lens to its EP shoulder (a couple of mm deviation wouldn't do much harm), assume it's 70mm, then the focal length of your 1.6x barlow is 70/0.6=117

2. WO binoviewer is said to have 100mm optical length, you'll get magnification

1+(100+70/0.6)/(70/0.6)=2,86x,

If you measurement in step 1 is 80mm, it'll be 2.75x, not a whole world of difference to 2.75x.

Thanks Yong  - just what I was looking for

[Floater]

23 hours ago, Highburymark said:

But which EPs to use? At the moment the only EP pairs I have are 15mm TV plossls, and the 20mm WO EPs which came with the bino. So need a couple of less powerful pairs ...

Might be worth repeating here that I bought a couple of "budget" 30mm plossls but didn't get on with them at all. Not sure why - eye placement may be a factor and I haven't given up yet, but it wasn't the simple fix that I had hoped it might be. ??

[Highburymark]

4 hours ago, Floater said:

Might be worth repeating here that I bought a couple of "budget" 30mm plossls but didn't get on with them at all. Not sure why - eye placement may be a factor and I haven't given up yet, but it wasn't the simple fix that I had hoped it might be. ??

Interesting - my (not very well developed) thinking was a pair of 25mm Fujiyama orthos, then 32mm and 40mm Celestron plossl pairs to keep spending down. TV plossls not cheap when you get above 20mm.

[Louis D]

17 hours ago, Highburymark said:

Interesting - my (not very well developed) thinking was a pair of 25mm Fujiyama orthos, then 32mm and 40mm Celestron plossl pairs to keep spending down. TV plossls not cheap when you get above 20mm.

I use a pair of 32mm GSO Plossls to maximize exit pupil in my Arcturus binoviewer, but they vignette significanly due to the 22m/23mm of clear aperture.  26mm Sirius Plossls work well, but are a bit short on eye relief due to ther recessed eye lens.  23mm barrel wide field microscope eyepieces work well.  I use vintage B&L 15x 31-15-74 with adapters for mid-power.  They degrade a bit to the edge, but are super comfy with eyeglasses.

[Highburymark]

7 hours ago, Louis D said:

I use a pair of 32mm GSO Plossls to maximize exit pupil in my Arcturus binoviewer, but they vignette significanly due to the 22m/23mm of clear aperture.  26mm Sirius Plossls work well, but are a bit short on eye relief due to ther recessed eye lens.  23mm barrel wide field microscope eyepieces work well.  I use vintage B&L 15x 31-15-74 with adapters for mid-power.  They degrade a bit to the edge, but are super comfy with eyeglasses.

Ok - think Arcturus BVs come from same factory as WOs so useful info - thanks Louis

[Peter Drew]

The amplification increase of a normal Barlow lens due to varying spacing between the Barlow and the eyepiece is not linear. The amplification increases rapidly as the separation increases.

Comparing an eyepiece alone in my solar telescope to a pair of the same eyepieces in a binoviewer with a 2x Barlow (lens only) screwed to the nosepiece of the binoviewer, presents an amplification of nearer 4x.  

[Louis D]

32 minutes ago, Peter Drew said:

The amplification increase of a normal Barlow lens due to varying spacing between the Barlow and the eyepiece is not linear. The amplification increases rapidly as the separation increases.

Are you sure?  According to the amplification formula from the CN thread I referenced:

M = 1 - L / fb

Where M is the magnification factor.
L is the distance from the Barlow lens to the field stop of the eyepiece, and
fb is the focal length of the Barlow lens (a negative value).

It works out to M = 1 + L / fb if you flip the sign of fb.  That's a classic linear formula of the form Y = 1 + X / fb, so it has a slope of X / fb and crosses the Y axis at 1.  That is, no magnification (x1 or unitary) if there is zero separation between the field stop and the barlow lens.  It yields 2x if the separation is fb (1 + fb/fb = 2).  It yields 3x if the separation is 2fb (1 + 2fb/fb = 3).  It yields 4x if the separation is 3fb (1 + 3fb/fb = 4).  That's a linear relationship with a Y offset of 1.  If I messed up my math somewhere, please correct me.

Actual experience bears this out.  If you pull an eyepiece up and out of a barlow, the size grows bigger at the same rate as the distance is increased.  If it was geometric or exponential, the size would grow much faster than the rate at which it is pulled out.  As another data point, my Speers-Waler 5-8mm zoom eyepiece has all of the focal lengths marked at equal intervals along the barrel.  It's basically an eyepiece with a variable barlow section.  If power increased faster than linearly, the marks would get closer together as you go from 8mm down to 5mm.  Again, personal experience bears this out that the markings are accurate when compared with fixed focal length eyepieces.

[Peter Drew]

19 minutes ago, Louis D said:

Are you sure?  According to the amplification formula from the CN thread I referenced:

M = 1 - L / fb

Where M is the magnification factor.
L is the distance from the Barlow lens to the field stop of the eyepiece, and
fb is the focal length of the Barlow lens (a negative value).

It works out to M = 1 + L / fb if you flip the sign of fb.  That's a classic linear formula of the form Y = 1 + X / fb, so it has a slope of X / fb and crosses the Y axis at 1.  That is, no magnification (x1 or unitary) if there is zero separation between the field stop and the barlow lens.  It yields 2x if the separation is fb (1 + fb/fb = 2).  It yields 3x if the separation is 2fb (1 + 2fb/fb = 3).  It yields 4x if the separation is 3fb (1 + 3fb/fb = 4).  That's a linear relationship with a Y offset of 1.  If I messed up my math somewhere, please correct me.

Actual experience bears this out.  If you pull an eyepiece up and out of a barlow, the size grows bigger at the same rate as the distance is increased.  If it was geometric or exponential, the size would grow much faster than the rate at which it is pulled out.  As another data point, my Speers-Waler 5-8mm zoom eyepiece has all of the focal lengths marked at equal intervals along the barrel.  It's basically an eyepiece with a variable barlow section.  If power increased faster than linearly, the marks would get closer together as you go from 8mm down to 5mm.  Again, personal experience bears this out that the markings are accurate when compared with fixed focal length eyepieces.

On reflection, I think I've confused magnification amplification with final focal plane position which certainly does increase rapidly to the point where the Barlow placed inside the prime focus by a factor equal to the Barlow's native focal length produces parallel light. It's a long time since I looked up the calculation but direct comparison of image scale supports the close to 4x amplification in the circumstances I mentioned.

Thanks for the correction. 

[Louis D]

19 minutes ago, Peter Drew said:

On reflection, I think I've confused magnification amplification with final focal plane position which certainly does increase rapidly to the point where the Barlow placed inside the prime focus by a factor equal to the Barlow's native focal length produces parallel light.

Yes, you are quite correct on that point.  This webpage goes into greater depth, especially on the equations related to placing the barlow inside of prime focus.  As the distance approaches the barlow's focal length, the denominator goes to 0 yielding "infinite" magnification, or parallel rays.