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Why does my Barlow lens have removable parts?

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#1 paddymcg

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Posted 21 July 2021 - 06:23 PM

I got this 3x Barlow recently and while I was playing around with it I found that the small black end and the middle silver piece could be independently screwed apart.

 

Screenshot_20210722-001255_Amazon Shopping.jpg

 

All the lenses of the Barlow are contained in the small black end and the rest is just a hollow tube. The black end has screw threads that can be connected directly to eyepieces.

 

Is there any practical use for this or the other silver piece?

 

Also, why is the Barlow so long if all the lenses are contained in the small black end?


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#2 Sky Muse

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Posted 21 July 2021 - 06:41 PM

The 3x-multiplier is accomplished via the long tube.  The silver(chromed) section is simply the 1.25" nose-piece that enables the barlow to be secured within the visual-back of a 1.25" focusser.  If the small, black end can be screwed into the bottom of an eyepiece, give it a try; observe with it.  Since it is much shorter than the main tube, it may end up being a 1.5x-multiplier, or nothing at all(1x).


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#3 jmillsbss

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Posted 21 July 2021 - 06:43 PM

I bet the 2x from the same company has a shorter barrel.  Screw the barlow directly into the eyepiece and have a look.  I bet it's a mess.  They need the distance to correct the magnification without having massive chromatic aberrations (although aberration isn't the correct word. The colors don't all come to focus at the same plane, whatever that word is)



#4 jmillsbss

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Posted 21 July 2021 - 06:45 PM

The 3x-multiplier is accomplished via the long tube.  The silver(chromed) section is simply the 1.25" nose-piece that enables the barlow to be secured within the visual-back of a 1.25" focusser.  If the small, black end can be screwed into the bottom of an eyepiece, give it a try; observe with it.  Since it is much shorter than the main tube, it may end up being a 1.5x-multiplier, or nothing at all(1x).

And I didn't ring in quick enough!!!  Dang you Sky Muse!!!  DANG YOU STRAIGHT TO CLOUDY SKIES AND HIGH JET STREAMS!!!!


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#5 Sky Muse

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Posted 21 July 2021 - 06:48 PM

And I didn't ring in quick enough!!!  Dang you Sky Muse!!!  DANG YOU STRAIGHT TO CLOUDY SKIES AND HIGH JET STREAMS!!!!

Alas, we're all hungering for fresh beginner fare.


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#6 barbarosa

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Posted 21 July 2021 - 06:49 PM

Any of these things can be called an aberration--

 

Optical aberration, an imperfection in image formation by an optical system
Chromatic aberration, caused by differences in refractive index for different wavelengths of light, in contrast with monochromatic aberration, which occurs for all frequencies of light
Spherical aberration, which occurs when light rays pass through a spherical lens near the edge

Defocus aberration, which occurs when a system is out of focus 



#7 jmillsbss

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Posted 21 July 2021 - 07:06 PM

Alas, we're all hungering for fresh beginner fare.

Wait for it.......and......

 

Any of these things can be called an aberration--

 

Optical aberration, an imperfection in image formation by an optical system
Chromatic aberration, caused by differences in refractive index for different wavelengths of light, in contrast with monochromatic aberration, which occurs for all frequencies of light
Spherical aberration, which occurs when light rays pass through a spherical lens near the edge

Defocus aberration, which occurs when a system is out of focus 

...BAM!!!  Maybe I should receive the credit for said low hanging fruit, seeing how I actually went deeper for the OP, explaining the root cause that "Seben" was trying to sort out, albeit with some loosely applied and vague vocabulary, I laid the foundation for a better understanding why things look and behave the way they do.  I'm just saying.....



#8 Migwan

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Posted 21 July 2021 - 08:51 PM

I got this 3x Barlow recently and while I was playing around with it I found that the small black end and the middle silver piece could be independently screwed apart.

 

attachicon.gifScreenshot_20210722-001255_Amazon Shopping.jpg

 

All the lenses of the Barlow are contained in the small black end and the rest is just a hollow tube. The black end has screw threads that can be connected directly to eyepieces.

 

Is there any practical use for this or the other silver piece?

 

Also, why is the Barlow so long if all the lenses are contained in the small black end?

Hello paddymcg.  Welcome to Cloudy Nights.   Post 2 seemed reasonable.

 

No worries about some of the rather cryptic replies you got.  
 


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#9 jmillsbss

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Posted 21 July 2021 - 09:00 PM

I got this 3x Barlow recently and while I was playing around with it I found that the small black end and the middle silver piece could be independently screwed apart.

Screenshot_20210722-001255_Amazon Shopping.jpg

All the lenses of the Barlow are contained in the small black end and the rest is just a hollow tube. The black end has screw threads that can be connected directly to eyepieces.

Is there any practical use for this or the other silver piece?

Also, why is the Barlow so long if all the lenses are contained in the small black end?


Migwan is right. This forum has a way of compounding content. You'll most often get the most right answer in the first, or second, in this case, reply. The rest of it is friendly banter meant to assuage our sore hearts at the sorry weather we have in the south. Or Mid-soutb. Or Northeast. Or anywhere else for that matter... welcome aboard!
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#10 Littlegreenman

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Posted 21 July 2021 - 09:58 PM

For what it is worth:

This Barlow just a small refractor telescope. A lens at one end, and at the other end an image comes to focus. With a telescope an eyepiece and diagonal are used at the far end.

The length of the tube contributes to magnification.

Other Barlows are shorter, using more complicated optical elements.

The design of the one you have is at least 70 years old, 1950's? maybe decades older.


Edited by Littlegreenman, 21 July 2021 - 09:59 PM.

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#11 jmillsbss

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Posted 22 July 2021 - 09:11 AM

For what it is worth:

This Barlow just a small refractor telescope. A lens at one end, and at the other end an image comes to focus. With a telescope an eyepiece and diagonal are used at the far end.

The length of the tube contributes to magnification.

Other Barlows are shorter, using more complicated optical elements.

The design of the one you have is at least 70 years old, 1950's? maybe decades older.

A succinct explanation.  We'll done!



#12 Sky Muse

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Posted 22 July 2021 - 12:43 PM

The barlow is quite old actually, 1833...

 

https://en.wikipedia...(mathematician)


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#13 Littlegreenman

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Posted 22 July 2021 - 12:52 PM

The barlow is quite old actually, 1833...

 

https://en.wikipedia...(mathematician)

I knew someone here would educate me and elucidate on Barlows. Aside: being the name of a person, Barlow, Plossl, and many other terms we use are capitalized.

I wonder when the specific design of the bar... Barlow the OP posted, with inscrutable... unscrewable parts and all came about. I can imagine Zeiss doing something like in 1920? At least.


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#14 SteveG

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Posted 22 July 2021 - 01:38 PM

The barlows with screw off lenses are often used with the lens portion threaded into the filter threads of an eyepiece. Not all of them do this. The standard 2-element barlows typically are a little better optically, due to the "less steep" light cone, however many "shorty" barlows incorporate a 3rd lens making them just as good, and some of those thread off as well. 

 

The barlow is a telenegative lens, and these are sometimes built in to eyepiece designs, such as the Televue T6 Naglers.

 

Most 1.25" barlows will vignette a low power, wide field eyepiece, just slightly.

 

A barlow is designed to be placed after the diagonal on a refractor or SCT, but if placed before the diagonal, it will have a higher amplification factor. In many modern refractors, there won't be enough "outward focuser" movement to do this without the use of an extension tube.

 

No barlow is apochromatic. That is simply marketing speak. Most modern barlows are excellent optically, and can reduce eyepiece astigmatism in fast instruments due to the increase of focal length. They cannot reduce chromatic aberration at all.


Edited by SteveG, 22 July 2021 - 03:01 PM.


#15 Sky Muse

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Posted 22 July 2021 - 01:56 PM

I knew someone here would educate me and elucidate on Barlows. Aside: being the name of a person, Barlow, Plossl, and many other terms we use are capitalized.

I wonder when the specific design of the bar... Barlow the OP posted, with inscrutable... unscrewable parts and all came about. I can imagine Zeiss doing something like in 1920? At least.

I've always liked barlows.  I have three nicer ones...

 

barlows2.jpg

 

I have them placed on a mantle, flanked with candles, and incense.




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