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# Barlow with extension

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### #1 daveyfitz

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Posted 01 February 2013 - 03:36 AM

I am considering how to populate my eyepiece collection, and I'd like to expand its usefulness by adding a barlow.

I have seen posts here and there that seem to indicate that you can increase the magnification of a barlow by adding an extension tube.

Is there any truth to that?

### #2 orveko

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Posted 01 February 2013 - 02:06 PM

Absolutely. The magnification factor of a barlow (M) is determined by:

M = 1+e/|b|, where 'b' is the focal length of the barlow and 'e' is the distance from the eyepiece's focal plane to the center of the barlow lens.

Let's say you have a 2x barlow and a 50mm extension tube. First you need to determine the focal length of your Barlow. For a 2x barlow, it's easy. Simply measure the distance from the center of the glass barlow lens (more accurately, it's the center of the double concave element) to the top of the barlow. Let's say this is 100mm. Technically, barlows have negative focal lengths, so the focal length of the barlow is -100mm. For simplicity, let's assume that the focal plane of your eyepiece is right where the barrel and eyepiece housing meet (where it rests in your focuser). When the eyepiece is sitting in the barlow, that means the absolute value of the focal length of the barlow and the distance from the barlow lens to the eyepiece focal plane is the same, thus:

1+100/|-100| =
1+100/100 =
1+1 =
2

That's how they determine it's a "2x" barlow. But we already knew that. Okay, so add the extension tube. Now, the distance from the barlow lens to the eyepiece focal plane has increased by 50mm. Therefore:

1+150/|-100| =
1+150/100 =
1+1.5 =
2.5

The magnification factor has increased to 2.5x. So, as you can see, the actual magnification factor of the barlow depends on how far the eyepiece is from the barlow lens.

IMPORTANT: this only works with standard two element barlows, not telecentric barlows aka focal extenders aka powermates aka whatever.

ALSO IMPORTANT: The location of the effective focal plane for the barlow / extension / eyepiece combo will be drastically different than just the eyepiece and barlow together, which is likely different than just the eyepiece. So, using crazy barlow and extension combinations do work, but you may have some difficulty bringing your monstrosity to focus in your scope. Keep this in mind before you put down money for extensions or Barlows or whathaveyou. If you can, it's always best to try before you buy.

### #3 daveyfitz

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Posted 01 February 2013 - 04:00 PM

Absolutely. The magnification factor of a barlow (M) is determined by:

M = 1+e/|b|, where 'b' is the focal length of the barlow and 'e' is the distance from the eyepiece's focal plane to the center of the barlow lens.

Thank you, that is very interesting.

I've read about folks stacking barlows to increase mag, but it would be so much cheaper to just get (or make) extension tubes to do this.

Is there some drawback you didn't mention?

Is there any light lost by moving the eyepiece away from the barlow lens, for example?

By the way, I'm considering getting the Antares 1.6x 2" barlow.
I've seen many reviews stating it's optics are excellent, and I'm thinking a short extension could bring it to 2 or 2.5x,
effectively tripling my (future) EP collection.

In fact, looking at the design of the Antares, I'm guessing one could tighten the set screws
with the EP not all the way down into the barlow, increasing the mag somewhat.

### #4 orveko

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Posted 01 February 2013 - 05:07 PM

The only real optical drawback is the same as with any barlow: not all eyepieces barlow well. Barlows will extend the eye relief of an eyepiece. That sounds like a good thing, and sometimes it is, but other times it leads to vignetting of the image or eye placement difficulty. Barlows can end up being a game of trial and error.

The Antares 1.6x barlow is great and I owned one for a while. I did experiment with adding extension tubes to achieve 2x magnification but I could never get it to come to focus in my scope. There just wasn't enough in focus. Now if there was a way to extend the barrel instead adding an extension to the top of the barlow, that wouldn't be a problem ...

That's why a 2" GSO 2x ED Barlow is probably best suited to what you're looking for. The barlow element can be unscrewed and screwed into the bottom of 2" eyepieces for ~1.5x magnification. The element is threaded for filters as well. Also, you can buy some threaded spacers to add length to the barrel for pretty much any magnification between 2-3x (any more than 3x and the barlow becomes way too long and unwieldy). You may have to pull the barlow out of the focuser a little to get it to come to focus, but that is no big deal. So, with one barlow and a few spacers, you can easily achieve 1.5-3x magnification.

I actually followed this train of thought for a while, but my 13mm Nagler didn't take too well to barlowing. The image was always vignetted. I ended up buying a telecentric barlow, which doesn't play around with the eye relief (i.e., no vignetting in my Nagler). Unfortunately, you can't really modularize (is that a word?) a telecentric barlow, so I went with a small magnification jump of 1.6x.

With this, you can survive on a minimalist set of just two or three eyepieces and the barlow. Typically, I only use my 40mm and 13mm eyepieces during an observation session. That gives me an effective eyepiece spread of 40mm, 25mm, 13mm, and 8mm. During very clear nights (which are rare), I pop in my 3-6mm Nagler zoom when I want to get a good close look at something.

### #5 Scanning4Comets

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Posted 01 February 2013 - 06:09 PM

I used to have a very "minimalist" set of two eyepieces and a barlow with extension tubes. I got by with this combo, but grew tired of "insert barlow, add extension, add eyepiece" in the dark. I am now down to one barlow: A 2" GSO ED.

Cheers,

### #6 Doug Culbertson

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Posted 01 February 2013 - 06:20 PM

I use the Antares with a 2" extension to get 1.6x (1.55x to be precise) and 2x, and it works vey well in all of the scopes and eyepieces in my sig, though it is mostly used with the Leica ASPH and the 13E.

I have never tried the aforementioned GSO, but it does seem to get high marks.

### #7 russell23

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Posted 02 February 2013 - 03:11 PM

I have never tried the aforementioned GSO, but it does seem to get high marks.

The 2" GSO ED barlow I tried was just so-so. Perhaps I simply had a sample on the low end of the quality range.

Dave

### #8 Doug Culbertson

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Posted 02 February 2013 - 03:24 PM

That's how I felt about the GSO1.25" 2x barlow; just so-so.

### #9 Tophat3

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Posted 03 February 2013 - 03:14 AM

I'm gonna have to try that.

### #10 Scanning4Comets

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Posted 03 February 2013 - 04:10 AM

I've had both the 2" Antares and 2" GSO ED barlows and the regular 2" GSO barlow and cannot see any difference between the three.

### #11 BillP

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Posted 03 February 2013 - 03:22 PM

I've read about folks stacking barlows to increase mag, but it would be so much cheaper to just get (or make) extension tubes to do this.

Is there some drawback you didn't mention?

The Barlow does not have an "unlimited" ability to handle increased or decreased distance. For instance, with a 2.7x Barlow I have reducing the distance below that needed to achieve 2.2x introduces off-axis astigmatism and lateral color. So there is an operating range when the image is "best". Go outside the range and your image starts to deteriorate.

### #12 daveyfitz

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Posted 07 February 2013 - 05:43 PM

The Barlow does not have an "unlimited" ability to handle increased or decreased distance. For instance, with a 2.7x Barlow I have reducing the distance below that needed to achieve 2.2x introduces off-axis astigmatism and lateral color. So there is an operating range when the image is "best". Go outside the range and your image starts to deteriorate.

Decreased distance? How would one do that?

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