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Magnification with a barlow

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#1 Yourjones

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Posted 09 April 2020 - 10:56 AM

Hello

 

Can I simply put an equal mark between a "600/150 Newtonian with a 2x barlow" and "1200/150 Newtonian alone" in terms of magnification? (The 600mm focal length is made up for easier math).

 

If they equal, what are some advantages  of both systems?

 

Thank you so much!

 

Regards



#2 ButterFly

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Posted 09 April 2020 - 11:27 AM

Consider a barlow a black box - you rarely know its specs unless you measure them.  The 2x is a nominal value.  It's magnification will vary depending on how far away the eyepiece's or camera's focal plane is from the barlow.  You can measure that by comparing the true field of views with and without a barlow.

 

A barlow is glass so whatever aberrations it introduces are there.  On the other hand, it increases the effective f ratio of the scope, so it hides the scope's aberrations a little more, is easier on eyepieces, and may increase the eye relief of an eyepiece.


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#3 Barlowbill

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Posted 09 April 2020 - 11:35 AM

I like Barlows because I like playing with them.  They aren't a cure all but sometimes they work well, sometimes not so well.  


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#4 russell23

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Posted 09 April 2020 - 12:19 PM

Consider a barlow a black box - you rarely know its specs unless you measure them.  The 2x is a nominal value.  It's magnification will vary depending on how far away the eyepiece's or camera's focal plane is from the barlow.  You can measure that by comparing the true field of views with and without a barlow.

 

A barlow is glass so whatever aberrations it introduces are there.  On the other hand, it increases the effective f ratio of the scope, so it hides the scope's aberrations a little more, is easier on eyepieces, and may increase the eye relief of an eyepiece.

Those are good points.  Barlow magnification factors vary from the number printed on the barrel and can vary with the eyepiece.  The biggest differences I've found involve the difference between Televue eyepieces and non-Televue eyepieces.   Televue eyepieces give a lower magnification with a barlow than the non-TV eyepieces I've measured.

 

For example, the TV 3.0x barlow is 3.05-3.08x with TV eyepieces and 3.17-3.22x with non-TV eyepieces.  I recently measured the magnification factors of my 1.6x Nikon and 3.0x TV barlows with the 13mm DeLite and the 8.5mm Pentax XF.   What I found was:

 

1.6x Nikon:  1.58x with 13mm DeLite, 1.65x with 8.5mm XF

3.0x TV:  3.05x with 13mm DeLite, 3.17x with 8.5mm XF

 

Adding a 1" extension tube to each barlow resulted in:

 

1.6x w/1":  1.84x with 13mm DeLite, 1.92x with 8.5mm XF

3.0x w/1":  3.65x with 13mm DeLite, 3.78x with 8.5mm XF

 

The quality of the optical glass can make a big difference.  I do not find the image degraded by the Nikon or TV barlow.


Edited by russell23, 09 April 2020 - 12:20 PM.

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#5 Ernest_SPB

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Posted 10 April 2020 - 12:04 AM

Hello

 

Can I simply put an equal mark between a "600/150 Newtonian with a 2x barlow" and "1200/150 Newtonian alone" in terms of magnification? (The 600mm focal length is made up for easier math).

 

If they equal, what are some advantages  of both systems?

No they are not equal.

Newton 1200/150 FOV has alot lesser influence from coma, better attitude to eyepieces (telecentrical off-axis beams) and more tolerant to misalignments. 

Newton 600/150 tube lesser in its dimensions and weight, larger central obscuration .


Edited by Ernest_SPB, 10 April 2020 - 12:10 AM.

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#6 MartinPond

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Posted 10 April 2020 - 06:36 AM

Consider a barlow a black box - you rarely know its specs unless you measure them.  The 2x is a nominal value.  It's magnification will vary depending on how far away the eyepiece's or camera's focal plane is from the barlow.  You can measure that by comparing the true field of views with and without a barlow.

 

A barlow is glass so whatever aberrations it introduces are there.  On the other hand, it increases the effective f ratio of the scope, so it hides the scope's aberrations a little more, is easier on eyepieces, and may increase the eye relief of an eyepiece.

 

A medium eyepiece, overall,  has about +60diopters strength, a common Barlow -6diopters.

Since the aberration is approximately proportional to the strength squared,

the Barlow can contribute only 1/100th  of the aberration of the eyepiece.

By carving out a subsection of the objective's light, aberration of the objective is reduced a bit.

 

There are still significant things to watch for, though:  handling of stray light, and vignetting.


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#7 Ernest_SPB

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Posted 10 April 2020 - 07:30 AM

the Barlow can contribute only 1/100th  of the aberration of the eyepiece.

By carving out a subsection of the objective's light, aberration of the objective is reduced a bit.

Barlow changes conditions of light beams passing thru eyepieces optics. It breaks telecentricity. So balance of field aberrations in eyepiece after Barlows is changed (due to difference in ray tracing). The changes can be compensated by reducing N-number, may be not. Most tested by me eyepieces with advanced aberration correction shows more or less degradation in image quality over field after Barlow. 


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#8 Starman1

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Posted 10 April 2020 - 12:55 PM

An explanation:

 

A Barlow lens will have a different magnification factor at different distances from the lens--the farther the distance, the higher the magnification.

 

Eyepieces don't all have their focal planes at the same point within the barrel--they vary from a focal plane as low as near the bottom of the eyepiece to well above the "shoulder" the eyepiece

sits on when it is inserted.

 

As a result, every eyepiece will have a slightly different magnification when used in a Barlow, and you can even change that by simply sliding the eyepiece 1/4" out of the Barlow and tightening

it down at that point.

 

Many, if not most, Barlows have their magnifications specified to be when the focal plane of the eyepiece happens to coincide with the opening of the Barlow.

Most eyepieces don't have their focal planes exactly there, so magnifications will vary.  And magnification factors on Barlows are often rounded off: 2x claimed could easily be 2.1x actual.

 

You can easily discover the magnification factor of the Barlow without measuring anything other than to take a timing of the passage of a star from edge to edge across the center of the field of the eyepiece

both with and without the Barlow.  It doesn't even matter where in the sky the star is, though your timing will be shorter if the star is near the celestial equator, if time is a factor.

 

The relationship is this:

Timing without Barlow รท Timing with barlow = magnification of the Barlow with that eyepiece.  Do a good job on the timing, and you can get the magnification factor to 2 decimal digit accuracy.


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