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  #1  
Old 20-05-2017, 09:44 AM
Tropo-Bob (Bob)
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Barlows

3 questions here regarding barlows:

1. Using a F5 reflector, will a 2x barlow reduce coma to an amount similar to a F10 scope, or simply double the magnification of the EP, so in essence, the scope still performs as a F5 system? I favour doubling the magnification only, but am interested in the opinions or experiences of others.

2. Using a refractor, will the introduction of a barlow reduce CA, or again, is it simply doubling the magnification.

3. Does the use of a barlow allow a EP not normally suitable for fast scopes (say a H) to work effectively?

I have been in the game for a long time, but sometimes its still hard to know what is myth and what is reality!
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  #2  
Old 20-05-2017, 12:26 PM
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Merlin66 (Ken)
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I say yes to all three......
You're changing the effective focal length and focal ratio...
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  #3  
Old 20-05-2017, 01:50 PM
astro744
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The Clairaut cemented aplanatic doublet as manufactured and sold by Clave as a 2x Barlow with -113.4mm focal length is coma correcting to f5 according to Texereau in his book "How to Make a Telescope", Willmann-Bell.

This is a long Barlow with a large clear aperture (available in 1.25" as well as the slightly smaller French standard size). I have found it to be essentially invisible with my Clave Plossl eyepieces and my 15cm f5.5 Newtonian. However I didn't realise it was coma correcting until recently and never really looked for it as I mainly observed planets on axis with RA tracking.

There is some debate as to whether the Klee 2.8x Barlow is coma correcting and I don't have one but did try one many years ago with a Tele Vue 24mm Widefield eyepiece and I found it to vignette. The same eyepiece in my Clave Barlow did not vignette. I always thought it was the smaller clear aperture of the Klee but I think it had more to do with the length of the Barlow with the Clave almost twice the length of the Klee.
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  #4  
Old 20-05-2017, 02:31 PM
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Merlin66 (Ken)
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Re the Klee x2.8 "debate"
See: https://stargazerslounge.com/topic/4...lee-28-barlow/
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  #5  
Old 20-05-2017, 05:49 PM
astro744
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Quote:
Originally Posted by Merlin66 View Post
Yes the cloudynights link gets a bit heated.

I know about the Pretoria eyepiece (28mm I think) and it WAS coma correcting but can't remember what optimal focal ratio / focal length Newtonian it was designed for.
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  #6  
Old 20-05-2017, 06:00 PM
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Merlin66 (Ken)
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The Pretoria was optimized for f4
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  #7  
Old 21-05-2017, 09:02 AM
Tropo-Bob (Bob)
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I tried a combination of eyepieces with a barlow with fast refractors that have loads of CA to answer some of my own questions .

I could see no less CA or improvement in the off-axis view I compared EPs 18mm ortho, 20mm plossl and a 12mm radian with a
2x barlow to a 9mm ortho, 10mm plossl and 6mm radian,.

I used 100mm F5 & F6.5 achromatic refractors during the bright daylight, yesterday. The CA was certainly less (actually much less) with the F6.5 scope than the F5. However, the F5 did not magically start to perform like a F10, or even outperform the F6.5 scope with the insertion of a barlow.

I also tried using a lowly performing 20mm Huygenian EP. Again, the barlow caused no magical improvement in the image; just more magnification.

This result is not what I would expect when I read the literature on these matters, but is inline with my observing experiences over many years.
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  #8  
Old 21-05-2017, 10:18 AM
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Merlin66 (Ken)
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Bob,
Interesting results....
You don't mention which barlow you used - Standard long focus, Shorty, triplet etc.
Notwithstanding....

In "Telescopes, Eyepieces Astrographs" by Smith, Ceragioli & Berry, p 374-388 they analyse the design and performance of two barlows and the Powermate.
They say that the typical (good) barlow will control longitudinal and lateral colour, spherical aberration and the amplification factor, but do little for coma, astigmatism and field curvature (p376)

When the barlow is used with slower telescopes >f6 they also say that "slowing the f ratio..... reduces the angular aberrations inherent in the eyepiece" (p388)

My practical experience with barlows has been with spectroscopes (to obtain f ratios >f7) and solar imaging. My preference has been the TMB x1.8 ED barlow or the Powermates.
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  #9  
Old 21-05-2017, 10:20 AM
Barnacle (Bill)
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Agree!

Bob,

I agree with your observations, as I tested this also with barlows. A short tube frac will not magically perform like a long tube frac by sticking a barlow in between.

This link sums things up pretty well on these issues:

https://www.cloudynights.com/topic/2...low-reduce-ca/

Hope it helps.

Kind regards,

Bill
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  #10  
Old 21-05-2017, 10:44 AM
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Merlin66 (Ken)
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Bill,
I'd rather put my trust in professional optical designers like Ceragioli than the uninformed rabble who populate the CN forum.

Sorry......
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  #11  
Old 21-05-2017, 03:52 PM
Tropo-Bob (Bob)
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Quote:
Originally Posted by Barnacle View Post
Bob,

I agree with your observations, as I tested this also with barlows. A short tube frac will not magically perform like a long tube frac by sticking a barlow in between.

This link sums things up pretty well on these issues:

https://www.cloudynights.com/topic/2...low-reduce-ca/

Hope it helps.

Kind regards,

Bill
Thanks Bill,

That was a quite interesting read.
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  #12  
Old 21-05-2017, 04:09 PM
Tropo-Bob (Bob)
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Quote:
Originally Posted by Merlin66 View Post
Bob,
Interesting results....
You don't mention which barlow you used - Standard long focus, Shorty, triplet etc.
Notwithstanding....

In "Telescopes, Eyepieces Astrographs" by Smith, Ceragioli & Berry, p 374-388 they analyse the design and performance of two barlows and the Powermate.
They say that the typical (good) barlow will control longitudinal and lateral colour, spherical aberration and the amplification factor, but do little for coma, astigmatism and field curvature (p376)

When the barlow is used with slower telescopes >f6 they also say that "slowing the f ratio..... reduces the angular aberrations inherent in the eyepiece" (p388)

My practical experience with barlows has been with spectroscopes (to obtain f ratios >f7) and solar imaging. My preference has been the TMB x1.8 ED barlow or the Powermates.
Thanks Ken,

I used a Celestron, short barlow. I can't say too much about it as it was given to me some years ago and I am normally disappointed when I use it.

Yes, the literature certainly would have me believe other than what I saw for myself. In Rutten & van Venrooij's Telescope Optics, it is stated (P155): "... a Barlow lens often permits the use of relatively poorly corrected eyepieces.."

Furthermore, I have read elsewhere that wide-field EPs often have a negative lens or built in barlow and this is vital to their success.

So in essence, I am still trying to match up what I understand of theory with my own observations. Currently, it doesn't make sense!

I can only now say that a barlow increases the magnification, is useful for better ER and sometimes causes vignetting.
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  #13  
Old 21-05-2017, 05:55 PM
astro744
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Have a look at http://www.televue.com/engine/TV3b_p...d=52&Tab=_back

and then http://www.televue.com/engine/TV3b_p...d=53&Tab=_back

and a picture of the Clave Barlow at https://commons.wikimedia.org/wiki/F...arlow_lens.jpg

I would recommend getting the Tele Vue 2x as it is still in production and stay away from Shorty types. The Shorty Plus is supposed to be better than the shorty and perhaps an attempt to mimic the Klee but I have never used any 'short' Barlow other than a friends Klee briefly so cannot comment on their pros and cons. I can say the the Clave and Tele Vue 2x and 3x work very well as do the Powermates with the latter having the advantages mentioned on the Tele Vue web site.
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