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Barlow lens: apo vs achro?

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#1 audioaficionado

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Posted 29 June 2013 - 10:16 PM

Been browsing the 1.25" Barlow lenses and noticed some were 2 element, some were 3 element and a few were ED (2 element I assumed).

I want to keep it to $50 or less since that's about what each of my five lenses are worth.

What are the advantages and disadvantages of each type?

#2 ke4kso

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Posted 30 June 2013 - 07:46 AM

I have Celestron Ultima 2x three element, and I really like it, I also have the cheap one from the Celestron eyepiece kit. Under average seeing conditions I cant tell any difference in them. Maybe under very good conditions there would be some, but I haven't seen it yet. I like barlows because I have to observe with my glasses on, so I use longer fl eyepieces with a barlow for the better eyerelief. Some folks say there is some light loss with the extra glass,

#3 amicus sidera

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Posted 30 June 2013 - 08:52 AM

Steve, I don't think you'll gain much of anything by paying more for an APO or ED Barlow lens; I have several achromatic Barlows, including a Meade #126 shorty and an Edmund 2.5x RKE, and have never noticed any objectionable chromatic abberation while using them, even at very high powers.

A triplet APO Barlow lens will of course have more glass to traverse than a doublet, resulting in slightly dimmer views, although good coatings can mitigate this somewhat; the 2-element ED Barlow lenses wouldn't have this issue, and would likely have light throughput equivalent to an achromat.

I think a more important issue than type of lens is the magnification index of the Barlow one chooses; unless one intends to pursue very high magnifications in double star work or planetary study, a 1.6x or 2.5x model would be a good choice, and would provide a wide variety of magnifications from a standard eyepiece set.

#4 Jon Isaacs

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Posted 30 June 2013 - 02:30 PM

Been browsing the 1.25" Barlow lenses and noticed some were 2 element, some were 3 element and a few were ED (2 element I assumed).

I want to keep it to $50 or less since that's about what each of my five lenses are worth.

What are the advantages and disadvantages of each type?


Roland Christen of Astro-Physics, arguably the pickiest person on this earth and the person who developed the modern apo for amateur astronomy... he says all Barlows are "apochromatic." False color is just not an issue with a Barlow.

"
Secondly, there is no such thing as an "apochromatic Barlow". Just because a Barlow has 3 or 4 or even 54 elements does not make it an apo. A 2 element Barlow can be just as "apochromatic" as a 3 element. Because of the nature of Barlows and where they reside in the optical train, they neither add nor subtract chromatic aberration in any meaningful way. They only amplify whatever color error is already present in the image, an error that is fully caused by the telescope objective lens itself. If the objective is a mirror, then no color is added, whether it is a 2 element normal Barlow or 3 element "apo" Barlow. So why use the term "apo" Barlow? "

Barlow Distortions:Roland Christen

Jon Isaacs

#5 audioaficionado

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Posted 30 June 2013 - 03:10 PM

So why use the term "apo" Barlow?


Gotta love ad copy LOL.

'Everyone knows that apo is better than achro and 3 is better than just 2', etc.

#6 amicus sidera

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Posted 30 June 2013 - 04:42 PM

Roland Christen of Astro-Physics, arguably the pickiest person on this earth and the person who developed the modern apo for amateur astronomy... he says all Barlows are "apochromatic." False color is just not an issue with a Barlow.

"
Secondly, there is no such thing as an "apochromatic Barlow". Just because a Barlow has 3 or 4 or even 54 elements does not make it an apo. A 2 element Barlow can be just as "apochromatic" as a 3 element. Because of the nature of Barlows and where they reside in the optical train, they neither add nor subtract chromatic aberration in any meaningful way. They only amplify whatever color error is already present in the image, an error that is fully caused by the telescope objective lens itself. If the objective is a mirror, then no color is added, whether it is a 2 element normal Barlow or 3 element "apo" Barlow. So why use the term "apo" Barlow? "


I've seen a Barlow lens add CA to an image formed by a reflector.

Perhaps it didn't follow the optical prescription for a true Barlow, or possibly the eyepieces used were introducing the error (not likely, as they consisted of TeleVue Plössls), but it was present nonetheless.

Fred

#7 audioaficionado

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Posted 30 June 2013 - 05:47 PM

If I'm not mistaken, many short focal length, long eye relief EPs have Barlow elements in them.

How do the 100° UWA EPs fare with Barlows?

#8 Ed Wiley

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Posted 30 June 2013 - 07:29 PM

Barlows like the Orion shorty are fine so long as you are "on-axis." I use one (Japanese-made) for high resolution double star imaging. Smith et al. ("Telescopes ...) discuss barlows in detail. From what I have read, if you have anything "faster" than F5, then long barlows are better. Apo vs acro -- never bothered with an "apo." Powermates -- I have one and like it but I bet I am paying more for optical quality (Televue quality control) than anything else and have never bothered with testing it against a regular 5x barlow.

Ed

#9 Starman1

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Posted 30 June 2013 - 07:53 PM

A barlow lens is a negative lens.
If you don't want false color, make it a doublet with a long focal length.
No color, flat field, easy to make (lens curves are relatively flat).

But, try to make it short and have a short focal length, and unless the lenses get extreme in their curves (expensive to make, and difficult), you have to add another lens to decrease the focal length. Hence, the 3-element "shorty" barlows with short focal lengths. These short barlows are appropriate for scopes with star diagonals or those with really short focuser drawtubes.

There is another kind of barlow, called telecentric, which adds a positive lens after the negative lens to return the rays to the same path angles after being diverged by the negative lens (effectively increasing the image scale of the telescope, but not changing the focal point). A colorless version of this would have two achromats and have 4 lenses. Example: TeleVue PowerMate.

If you want a very high-power barlow that magnifies more (4X, 5X), additional lenses *may* be required, and if you check out all barlows you may see some non-telecentric barlows with 4 or 5 lenses.
These are very uncommon. Typically, the manufacturer just adds deeper curves to the lenses.

There's more to the story, but that's the gist of it.

#10 Ernest_SPB

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Posted 01 July 2013 - 01:37 AM

"APO Barlow lens" (applied to 3-lens Barlow) looks like just a marketing label. It is not more apochromatic then 2-lens Barlow.

But the Barlow has some benefits in quality of aberration correction (especially over field). Its advantage could be noticed easier in fast scope.

#11 BillP

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Posted 01 July 2013 - 04:07 PM

The terminology APO vs non-APo or Doublet vs Triplet for Barlows is IME not going to tell you anything. So it does not translate like it does for telescope objectives.

I have tested triplet designs that work much better than doublet designs (GSO shorty Barlows for example). But also have premium longer length doublet Barlows than far exceed less expensive triplet Barlows. So basically you need to test to confirm which is better and you can't go by whether it is a double or triplet design. btw, while the on-axis performance can differ, I find the introduction of off-axis problems is where the Barlows more often show the weaknesses of their design (triplet or doublet).

#12 azure1961p

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Posted 01 July 2013 - 08:32 PM


Roland Christen of Astro-Physics, arguably the pickiest person on this earth and the person who developed the modern apo for amateur astronomy... he says all Barlows are "apochromatic." False color is just not an issue with a Barlow.

"
Secondly, there is no such thing as an "apochromatic Barlow". Just because a Barlow has 3 or 4 or even 54 elements does not make it an apo. A 2 element Barlow can be just as "apochromatic" as a 3 element. Because of the nature of Barlows and where they reside in the optical train, they neither add nor subtract chromatic aberration in any meaningful way. They only amplify whatever color error is already present in the image, an error that is fully caused by the telescope objective lens itself. If the objective is a mirror, then no color is added, whether it is a 2 element normal Barlow or 3 element "apo" Barlow. So why use the term "apo" Barlow? "


I've seen a Barlow lens add CA to an image formed by a reflector.

Perhaps it didn't follow the optical prescription for a true Barlow, or possibly the eyepieces used were introducing the error (not likely, as they consisted of TeleVue Plössls), but it was present nonetheless.

Fred


Any chance the CA was actually atmospheric dispersion caused by the added magnification in less than good seeing?

Pete

#13 Starman1

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Posted 02 July 2013 - 12:49 AM

Look, barlows are doublet lenses for a reason.
They can be achro or apo.
The question is whether the false color added by a doublet can be seen, and if you look at the spot diagrams of a typical doublet barlow, the color blur is inconsequential.

#14 amicus sidera

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Posted 02 July 2013 - 06:31 AM

Any chance the CA was actually atmospheric dispersion caused by the added magnification in less than good seeing?


I don't believe so, as it was a fairly steady night, and the observations were performed near the zenith; I suspect that the Barlow lens in question had its own issues.

Fred

#15 audioaficionado

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Posted 03 July 2013 - 03:19 PM

Barlow options:

Unity: 32mm, 20mm, 14.5mm, 9mm, 5mm

1.5x: 21.3mm, 13.3mm, 9.7mm, 6mm, 3.3mm

2x: 16mm, 10mm, 7.25mm, 4.5mm, 2.5mm

2.5x: 12.8mm, 8mm, 5.8mm, 3.6mm, 2mm

3x: 10.7mm, 6.7mm, 4.8mm, 3mm, 1.7mm

Which Barlow power gives me the best magnification interleave with my EPs?

#16 amicus sidera

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Posted 03 July 2013 - 03:31 PM

Actually, the spread with the 3x looks best to me.

Fred

#17 audioaficionado

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Posted 03 July 2013 - 03:56 PM

Actually, the spread with the 3x looks best to me.

Fred

32mm, 20mm, 14.5mm, 10.7mm, 9mm, 6.7mm, 5mm, 3mm

#18 SteveG

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Posted 03 July 2013 - 03:59 PM

Be advised that the power shown on the barlow might not be what you get. I'm doing a shoot-out between (3) 3X barlows; 2 long ones and 1 shorty. The short one, when used with my Brandons, only increases magnification by about 2.5X.

Note that Achro and APO lenses utilize differing glasses, i.e. crown, flint, FPL-53, etc. I think that multiple-element barlows do not (correct me if I'm wrong).

Don't spend to much time fretting over a barlow - and just buy a budget unit. In the shoot-out I mention above, the expensive barlow is proving to be the worst.

#19 audioaficionado

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Posted 03 July 2013 - 04:09 PM

SteveG

I'm looking forward to your shoot out results. Please post the link in this thread too.

I'm just trying to get the best bang for my buck and the power spec on the various astro dealers is all that I have to go by for now. Once I figure out the best power for my EPs, I can dig deeper into each listed Barlow for other pros and cons.

#20 audioaficionado

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Posted 03 July 2013 - 11:53 PM

I think I like the 2x spread the best.

32mm, 20mm, 16mm, 14.5mm, 10mm, 9mm, 7.25mm, 5mm

Not so much use in my 102mm f10 refractor:

4.5mm, 2.5mm






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