Magnification factor in barlows and binoviewers

[Andrew*]

I am unable to get my binoviewers to come to focus with a 1.25" diagonal in either my ED80 and ED120 without use of a screw on 1.6x barlow element.

I noticed on my first session with my new binoviewers that the magnification was far greater using the binoviewers than with the eyepiece and a 2x barlow.

So I decided to make a test to work out the magnification factors using the binoviewers and barlow. I pointed the ED80 at a tape measure on a wall 30ft away, and noted the FOV in cm on the tape measure. I used a 6mm Skywatcher UltraWide eyepiece with 66° FOV.

As can be seen from the attached results, there is no magnification change using binoviewers on their own (the 1mm difference is probably down to my inaccuracy!).

The barlow element works at about 3x when placed in front of the binoviewer.

The Ultima barlow is not 2x but about 2.15x.

Hope that's of some use to someone!

Andrew

[Peter Drew]

The magnification from a given Barlow is a function of the separation of the Barlow and the eyepiece. If you use a Barlow attached to the front of a binoviewer the optical distance between the Barlow and the eyepiece is much greater than just the Barlow and the eyepiece, hence the much greater resulting magnification.

[Andrew*]

Yes, but it requires some testing to establish just how much extra magnification the barlow is giving. It's very quick and easy to do these tests, and it helps to know what magnification you are using!

In this example, I was under the impression that the screw on barlow operated at 2x magnification when behind the binoviewer, so was consequently surprised to find the magnification so great. In fact, on my first use, I was putting my ED120 under 450x, thinking it was 300x!

I think I will do some more tests to see how much extra magnification is given with the barlows behind the diagonal.

Cheers

Andrew

[F15Rules]

Hi Andrew

Thanks for this, it's a very useful thread. Can you let us know your findings with other focal length eyepieces? Also, if you use a barlow with just the short component (unscrewed from the main barrel), what difference does that make?

I had the same problem with my ED100R.

cheers

Dave

[Andrew*]

Hi Dave,

Neither the Ultima or Abbe barlows have removable screw on elements, so I couldn't say. Usually when they do have these a 2x barlow will reduce to about 1.6x magnification.

What do you want to know about "other focal length eyepieces"? I think the same magnification factors will apply. I might start doing more detailed tests to work out the TFOV and actual magnification and hence focal length in an eyepiece. Still some thinking required as to how to do this though.

Cheers

Andrew

[F15Rules]

Hi Andrew,

I was just wondering for instance about my Baader Orthos..I have for instance 9mm, 12.5mm and 18mm versions and I know roughly what magnification they will give me in normal use..so, if using a barlow with binoviewers, I should just x2 or whatever your findings said for each "normal" magnification? So, using the 18mm barlowed via binoviewer would give the same (x2) as using the 9mm in "normal" mode?

At the moment I only have pairs of 20mm and 32mm EPs for bino use...(a double set of Baaders would be pricey!!)...but it seems that there is little point in putting together pairs of less than 20mm as in a bino they will operate as 10mm or less? But to get low power wide field views with a bino will be very difficult as I understood that 32mm is about as long a fl as you should go with a 1.25" barrel..so at x2 that would effectively mean a mag equivalent to using a 16mm in normal mode?

Sorry if this sounds messy, but I hope you see what I mean?

The views through a bino, when it works, are fabulous..but it does seem as though they are quite limited in variety of fl's that can be used in practice, given the barlow being essential for them to work correctly...

I'd value your thoughts on this Andrew:)

cheers

Dave

[Andrew*]

Okay, Dave,

Please note my figures will not necessarily apply to equipment other than what's used here, but will be approximately valid.

What kind of barlow are you using, and where are you putting it in the optical train? This is the vital question.

If it is just a barlow element that screws onto a 1.25" barrel and states ~1.6x magnification, then this will change to 3x when screwed on a Revelation binoviewer barrel (see third line). That is why I have ended up with 15mm eyepieces for use in my binoviewers. They act like 5mm eyepieces when used in the binoviewers and give 180x in my ED120. Even trying with the 6mm was a waste of time: it works out as a 2mm eyepiece, giving 300x in the ED80 and 450x in the ED120!!

If you are using an ordinary 2x push-fit barlow, this will end up at even more than 3x magnification when placed between the binoviewer and diagonal. Yet to work out exactly what mag. However, if placed between the eyepiece and binoviewer, it will operate at its normal 2x.

Yes, if you rely on a barlow between the diagonal and binoviewer, it will always work at more than 3x and you are therefore greatly restricted in your choice of focal lengths. My minimum magnification, with a 32mm plossl, is 90x. I'm fine with that though, because high power is great for binoviewing.

HTH

Andrew

[F15Rules]

Thanks Andrew, that's helpful..

I use a TV x2 barlow push fit, plus I have the small screw in barlow which screws into the front of my Burgess Optical Binoviewer (probably virtually identical to yours).

I've tried also putting the TV barlow in the eyepiece holder first, then the diagonal, then the binoviewer, and I recall that that combination gave even higher magnification.

So from what you say, it sounds as though using the 1.6x small barlow supplied with the viewer, is going to give the smallest increase in magnification possible when using a barlow..

Don't get me wrong, I like the high power bino views, but sometimes it would be nice to go wider field at lower power too? Perhaps I just need to save up for a TV Ethos to get that spacewalk feeling:D

thanks again Andrew.

cheers

Dave

[Andrew*]

That's about right. The screw-in will give the lowest possible mag. You could try diagonal/binoviewer/screw-in barlow/eyepiece. The barlow really would work at 1.6x and it's possible you'll buy yourself at least some in-focus. Whether or not enough is the question. If it does, just get another identical screw in barlow and voila - you get 25% lower power.

The 2x barlow behind the diagonal That would probably make about 5x magnification! Most barlows' performance would definitely start to break down pushed from 2x to 5x I would think.

I'll do some more testing and work out what the barlow magnifies with all the combinations.

What we really need is more angled binoviewers on the market, so we don't need to use a diagonal. A Baader 60° binoviewer went for a song on AB&S recently - they're pretty rare. That would work a dream and focus in any 'frac.

Andrew

[F15Rules]

BRILLIANT! Thanks again Andrew, I will try these permutations - just as soon as we get some sky to look at.....love your "stacked barlows" idea, definitely worth a try!

cheers

Dave

[Andrew*]

Okay, further tests done. I used a longer scope-tape distance and a 15mm eyepiece instead, so it was a bit more accurate.

See the attached results.The combinations go from objective-eyepiece, with a letter signifying the order of pieces used:

O= OCA (screw-in barlow)

B= Revelation binoviewer

D= William Optics 1.25" diagonal

2= Zeiss 2x barlow

C= Celestron Ultima "2x" barlow

e.g. 2DB would mean the 15mm eyepiece in the binoviewer in the diagonal in the Zeiss barlow in the ED80 - this is the highest combination tested (didn't try to stack barlows), giving almost 5x magnification: 197x in the ED80.

From the results, a barlow on the end will magnify 5% more in the binoviewer than in the diagonal, therefore the Revelation binoviewer has a longer light path than the WO 1.25" diagonal.

As I have mentioned, diagonals, barlows and binoviewers differ from brand to brand, so the magnifications here only apply to the equipment used in this test, and not necessarily to equipment from any other brand. Still, I'm sure it gives a good indication...

Andrew

[Peter Drew]

Most current Barlows are of the "shorty" design and employ optics of high negative power. I have achieved relatively low binoviewer amplifications by using the lens assembly from one of the 1980's Vixen type .96" X2 Barlows. These had a long separation between the lens and eyepiece holder to provide x2 so used in the circumstances under discussion, they would provide a lower overall magnification than normal 1.25" units. The Vixens were quite good quality and worth a try if you can find one, you will also need to devise a "Blue Peter" adaptor to fit it.