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What does it mean when it's stated the Barlows add correction?

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#1 CrazyPanda

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Posted 20 May 2019 - 09:02 PM

I've often read that barlows "add correction" in the form of correction for spherical aberration. How strong is the effect, and is it any value on telescopes with spherical mirrors? That is, would it be beneficial to always use a Barlow in conjunction with a spherical mirror telescope and then choose eyepieces around it?

 

Also, do Barlows also reduce eyepiece astigmatism at all? Presumably a Barlow will shallow up the light cone of the telescope since it effectively increases its focal length. Does this have an impact at all on the level of astigmatism present in cheaper eyepieces that would otherwise not work well with fast focal ratio telescopes?


Edited by CrazyPanda, 20 May 2019 - 09:04 PM.

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#2 Ernest_SPB

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Posted 21 May 2019 - 12:15 AM

Barlow (1) reduces angle in light cone and (2) break off-axis light beams (making them not celecentrical).

The first make following eyepiece working in better conditions, reducing appearance of its aberrations. It is good for epepieces with mediocre optics. The second makes eyepieces working in conditions far from used in their design and introduce artificial field aberrations (astigmatism, coma), vignetting and even TFOV cut. It is bad for eyepieces with good correction of fields aberrations.

Then Barlow introduces own aberrations: field curvature, astigmatism and in fast optics CA and SA. Some of the aberrations can compensate defects of EP or scope, but in most cases they just added.

Using Barlow for their primary target (getting high magnification close to upper limit) you can ignore most negative properties of Barlow.  


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#3 MartinPond

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Posted 21 May 2019 - 10:49 AM

Simpler eyepieces  (4-5 elements or less) will have some issues at the

field edges, and a Barlow indirectly does "correct" those, by giving the 

eyepiece a virtual barrel it is much happier with.  

 

Imagine the eyepiece seeing an objective with twice the focal length...

....that is a ~75% reduction in spherical aberration (of the objective!)...

 

 

A Barlow cell has a power that is typically 1/10th of the power

of the eyepiece it is used with (-6D  against +60D, for example)

so at worst it would contribute 1/100th of the aberration....un-noticeable.

Sometimes the cell in a shorty can be flipped one way or the other

  to adjust things at the outer edge of the EP field....if it's placed close by..

That for just the last little bit of field..

 

 

A so-so Barlow does pass more scattered light than a good Barlow, though.

That's a problem....for contrast and clarity.

 

We still have the vignetting issue......for EP focal lengths that are long

   for the focuser (like, 28--35mm in a 1.25" focuser)


Edited by MartinPond, 21 May 2019 - 10:53 AM.


#4 CrazyPanda

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Posted 21 May 2019 - 11:37 AM

Imagine the eyepiece seeing an objective with twice the focal length...

....that is a ~75% reduction in spherical aberration (of the objective!)...

 

So if you had say, the Celestron Astromaster 130EQ (spherical mirror), would a 10mm eyepiece with 2x barlow would be significantly better than just a 5mm eyepiece?

 

According to S&T formula about SA wavefront error (e = 22 * D / F3), a 5" F/5 scope would produce 0.88 waves of SA (yikes!), but with a barlow, then it's effectively an F/10 scope, so would it only exhibit 0.11 waves of SA?

 

Is this similar to how Bird-Jones scopes correct for SA?


Edited by CrazyPanda, 21 May 2019 - 11:40 AM.


#5 MartinPond

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Posted 21 May 2019 - 12:03 PM

-----------------------------

So if you had say, the Celestron Astromaster 130EQ (spherical mirror), would a 10mm eyepiece with 2x barlow would be significantly better than just a 5mm eyepiece?

----------------------------

 

Let's see.....the scope is 130mm // F5   .....wow!

Definitely better.  There is the aperture seen, and the aberration at the edge of the eyepiece (if it's simple)..

 

 

----------------------------------------------------------

According to S&T formula about SA wavefront error (e = 22 * D / F3),

a 5" F/5 scope would produce 0.88 waves of SA (yikes!),

but with a barlow, then it's effectively an F/10 scope, so would it only exhibit 0.11 waves of SA?

------------------------------------------------------------

 

I am not 100% sure ... there may still be some spherical effect (very strong on a reflectr)

   'ground in' to the prmary image..

 

 

 

---------------------------------------

Is this similar to how Bird-Jones scopes correct for SA?

-----------------------------------

From what I can dig up on it,,,,

the lens(es) more directly compensate for the spherical aberration...

....so more like curing one aberration with another....that would do more

than a Barlow could, actually.   For best results, they assumed a certain

eyepiece as a target... because that establishes the focal plane where it all has to come together.



#6 Jon Isaacs

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Posted 22 May 2019 - 07:12 AM

So if you had say, the Celestron Astromaster 130EQ (spherical mirror), would a 10mm eyepiece with 2x barlow would be significantly better than just a 5mm eyepiece?

 

According to S&T formula about SA wavefront error (e = 22 * D / F3), a 5" F/5 scope would produce 0.88 waves of SA (yikes!), but with a barlow, then it's effectively an F/10 scope, so would it only exhibit 0.11 waves of SA?

 

Is this similar to how Bird-Jones scopes correct for SA?

 

A Barlow does not correct for spherical error in the mirror, if it did, it would add spherical error a well corrected objective/mirror. 

 

In this context, the Barlow is best thought of as part of the eyepiece and that it reduces the effective focal length of the eyepiece. 

 

The correction it adds is to the eyepiece, a Erfle at F/5 will show a great deal of off-axis astigmatism. Add the 2X Barlow, it is seeing an F/10 light cone and will show much less astigmatism.  

 

In theory, I think Jones-Bird scopes use a Doublet but it is not a Barlow per se.  In practice, very few Jones-Bird scopes exist with even half decent optics. 

 

https://www.cloudyni...s/#entry8628699

 

jon



#7 CrazyPanda

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Posted 22 May 2019 - 11:53 AM

A Barlow does not correct for spherical error in the mirror, if it did, it would add spherical error a well corrected objective/mirror. 

 

In this context, the Barlow is best thought of as part of the eyepiece and that it reduces the effective focal length of the eyepiece. 

 

The correction it adds is to the eyepiece, a Erfle at F/5 will show a great deal of off-axis astigmatism. Add the 2X Barlow, it is seeing an F/10 light cone and will show much less astigmatism.  

 

In theory, I think Jones-Bird scopes use a Doublet but it is not a Barlow per se.  In practice, very few Jones-Bird scopes exist with even half decent optics. 

 

https://www.cloudyni...s/#entry8628699

 

jon

Yeah that's a good point. Ok, so sounds like the correction added by a barlow is mostly to do with re-shaping the light cone before it hits the eyepiece, so poorly edge corrected eyepieces see some benefit from a barlow.



#8 dmgriff

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Posted 22 May 2019 - 05:12 PM

Barlows do nothing to correct aberrations in the primary or lens or the optical train of the ota. A 2x barlow in no way makes a f/5 scope into an f/10.

 

One should consider the barlow as part of the eyepiece. A 2x barlow halves the focal length of the eyepiece, a 3x by a factor of 1/3 etc.

 

Aberrations in the ep can be magnified with the barlow.

 

One may notice in descriptions of a barlow, the phrase, "a well corrected barlow for well corrected eyepieces"....

 

Good viewing,

 

Dave



#9 CrazyPanda

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Posted 22 May 2019 - 06:44 PM

A 2x barlow in no way makes a f/5 scope into an f/10.

 

I get that it won't fix coma or chromatic aberration of course, but surely it does indeed change how steep the light cone is, therefore reducing how much astigmatism a poorly corrected eyepiece would otherwise exhibit, right?



#10 Jon Isaacs

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Posted 22 May 2019 - 07:02 PM

Barlows do nothing to correct aberrations in the primary or lens or the optical train of the ota. A 2x barlow in no way makes a f/5 scope into an f/10.

 

One should consider the barlow as part of the eyepiece. A 2x barlow halves the focal length of the eyepiece, a 3x by a factor of 1/3 etc.

 

Aberrations in the ep can be magnified with the barlow.

 

One may notice in descriptions of a barlow, the phrase, "a well corrected barlow for well corrected eyepieces"....

 

Good viewing,

 

Dave

 

Barlows do not correct for the aberrations in the objective/mirror but they do make it easier on the eyepiece.  With a 2x barlow in an F/5 scope, the eyepiece is seeing an F/10 light cone.  

 

I remember when I discovered this. I had my 12.5 inch F/4.06 and an assortment of Plossls, Orthos and one Erfle, a 20mm Celestron.  A 20mm Erfle at F/4 is not a pretty sight but when I added my 2x Barlow to the optical train, suddenly it was much better.  

 

There are some combinations that do not work well, the 24mm Meade 68 degree with the GSO Barlow used as a 1.5x Barlow is quite messy off-axis but most combinations work quite well.  The 22 mm Panoptic used with a 2X Barlow is a real treat. 

 

Many well corrected eyepieces depend on telenegative/barlow-like section in the eyepiece barrel and a longer focal length positive/magnifying eyepiece in the body.  

 

Jon


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