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Regular Barlow vs. Shorty Barlow

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#1 mistercrisp

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Posted 16 September 2020 - 10:56 PM

I am curious about the differences between regular 1.25 barlows and shorty barlows.  Can anyone offer their experiences?

 

thanks,

 

ken

 

 



#2 Hugh Peck

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Posted 16 September 2020 - 11:20 PM

Had a couple 3 element "shorties" and they were okay in the f/10s but not in the f/5s. Not good at wide fields, losing it around the edges. Can't say about "regular" ones as only ever had two, one for newtonians and one .965 that came with an old used orabge tube C-90. Never really used either. 

 

"Orabge" is too a word. I just coined it. grin.gif


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#3 TOMDEY

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Posted 17 September 2020 - 12:48 AM

There's a fundamental principle in lens design... to avoid unnecessarily powerful ~ray-bending~ because that's where aberrations come from... related to violation of the very fundamental sine condition and other arcane ways we optics guys think of how much work we are imposing on the lenses and mirrors. I learned that personification of lenses from Kingslake and Sinclair (Rudolf and Doug) very early on.

 

The Shorty Barlow pays the price of its compactness by... having to bend (refract) the light more severely. The price is poorer performance. It's like it got to the ray-bending too late, so has to put the brakes on the too-fast feed harder, too close to the image. All sorts of things get worse than had a more gentle negative achromatic relay intercepted the light a little earlier in the game. Really no free lunch in optics. The more you try to compact a scope, the harder and harder it is to achieve good performance. And that's why a plain vanilla slowish Newtonian... is the conservatively-best telescope. One single gentle ray-bender (the PM) and maybe, just maybe a rather benign refractive field corrector. Compare that with a Schmidt Cass... which most all are out of alignment and have choked field with giant central obstruction. Note that the convex SM mirror in Cass (and its derivatives) is nothing more nor less than a camouflaged reflective Barlow --- and therefore inefficient, right out of the blocks!    Tom


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#4 213Cobra

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Posted 17 September 2020 - 01:55 AM

I am curious about the differences between regular 1.25 barlows and shorty barlows.  Can anyone offer their experiences?

 

thanks,

 

ken

We have a ShortyBarlow here: https://www.cloudyni...-shorty-barlow/.

 

Maybe he weighs, say, 160 lbs. or more. I'm pretty sure most regular 1.25" barlows weigh much less. So, you might consider that.

 

Phil


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#5 MartinPond

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Posted 17 September 2020 - 08:05 AM

After testing a number of Barlow Shorties  (1.25" context):

 

--The 2-element achromatic cell in most brand-name 2X  (Meaded, Celestron, etc)

   Barlows does quite well, chromatically.  In fact, the optical cell from a Meaded #126 can be

   swapped onto a long Orion Tri-Mag, and matches  the 3X power and lack of chromatism.

   There is a bit less cntrast....the Tri-Mag front cell holder has a bit better light baffling..

 

---Many older shorties had contrast issues.  They have an open inner tubing and

   less threaded or  stepped walls.   A internal restriction fixed that, but now

   the more recent shorties have vignetting (field reduction) for longer-fl eyepieces.

 

---3X shorties have more severe vignetting, and some astigmatism (fuzzing) before that.

   ....the Celestron 3X X-Cel LX seems to dodge that bullet well.

 

---To avoid any vignetting or eye-relief pushing, Telecentrics like the 

    ES focal extender or the Televue Powermate can be used.

    Most handy for long-focal-length(FL)  eyepieces.

 

-----> There is a very cheap SVBONY Barlow.  Some say it's great, and has a real

         achromatic cell.  Mine arrived coated one side only with a 1-element cell.

           Not great as a Barlow,  but made a great 1.25 extension plus iris, and the cell,

         after throwing the glass out, makes a great contrast-enhancing iris for 

         short-fl eyepieces.  Fun deal for parts.....not a good Barlow...for me.

 

I have 3 Meade 126's, and with a 1" eyepiece extension tube, they have

   gain closer to 2x, much less vignetting, and improved contrast.

  Wonderful, when customized.

 

A basic Televue 2X Barlow costs more, but does things just right.

Its future value is multiplied across all your eyepieces.

The 3X X-Cel XL costs more too, but has a lot of satisfied takers.

The long Orion Tri-Mag is an excellent deal....you just have to accept the length.

 

Long 2X Barlows are almost non-existent. 

 The Televue 2X is a "midi", middle length.

Smart design choice. that's what I made....but mine has

a bunch of electrician's tape on it.


Edited by MartinPond, 17 September 2020 - 08:09 AM.

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#6 mistercrisp

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Posted 17 September 2020 - 10:18 AM

Wow, thank you all for your valuable comments and observations!  I appreciate the "why" of the problem of the short barlows, in terms of how optics work.  Thank you all SO much!

 

Ken



#7 MitchAlsup

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Posted 17 September 2020 - 11:27 AM

There's a fundamental principle in lens design... to avoid unnecessarily powerful ~ray-bending~ because that's where aberrations come from... related to violation of the very fundamental sine condition and other arcane ways we optics guys think of how much work we are imposing on the lenses and mirrors. I learned that personification of lenses from Kingslake and Sinclair (Rudolf and Doug) very early on.

 

The Shorty Barlow pays the price of its compactness by... having to bend (refract) the light more severely. The price is poorer performance. It's like it got to the ray-bending too late, so has to put the brakes on the too-fast feed harder, too close to the image. All sorts of things get worse than had a more gentle negative achromatic relay intercepted the light a little earlier in the game. Really no free lunch in optics. The more you try to compact a scope, the harder and harder it is to achieve good performance. And that's why a plain vanilla slowish Newtonian... is the conservatively-best telescope. One single gentle ray-bender (the PM) and maybe, just maybe a rather benign refractive field corrector. Compare that with a Schmidt Cass... which most all are out of alignment and have choked field with giant central obstruction. Note that the convex SM mirror in Cass (and its derivatives) is nothing more nor less than a camouflaged reflective Barlow --- and therefore inefficient, right out of the blocks!    Tom

Well said.



#8 Jon Isaacs

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Posted 17 September 2020 - 02:09 PM

Wow, thank you all for your valuable comments and observations!  I appreciate the "why" of the problem of the short barlows, in terms of how optics work.  Thank you all SO much!

 

Ken

 

Ken:

 

I regularly use both standard GSO Shorty's and a 2x 1.25 inch TeleVue midl length Barlow.

 

My scopes are generally around F/5 and I have a variety of eyepieces by my main set is all TeleVue.  

 

In terms of use in the field and not theory, the Shorty Barlows do very well with most eyepieces. There can be vignetting with long focal length eyepieces and some long focal length, wide fields show odd off axis reflections.

 

But with mid focal length length and short focal length eye pieces I find the Shorty's to be good performers. The main issue is not sharpness but scattered light that reduces contrast slightly when viewing the planet's.

 

Regarding Tom's comments:

 

Eyepieces often use severely curved optical elements, comparatively Shorty Barlows are mild compared to many eyepieces.

 

https://www.chuckhaw...ece_designs.htm

 

Any shorter focal length eyepiece with acceptable relief will include a Barlow like doublet that's like more than 2x and shorter than a shorty Barlow.

 

Jon


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#9 SteveG

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Posted 18 September 2020 - 01:41 PM

 

 

Long 2X Barlows are almost non-existent. 

 The Televue 2X is a "midi", middle length.

Smart design choice. that's what I made....but mine has

a bunch of electrician's tape on it.

How does the mid-length TV 2x hold up with a 24 mm widefield? Does it vignette?



#10 MartinPond

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Posted 18 September 2020 - 03:13 PM

Jon might be able to check that out for you..

 

 

"-----How does the mid-length TV 2x hold up with a 24 mm widefield? Does it vignette?---"

I need to look up the widest 1.25//25mm...

Looks like Agena lists a 25mm // 58degrees

Others are similar..

For more afov at 1.25 you may need to step down in FL.

 

 

 

I have the "Po'boy" version, the stretched Meade #126.

I project   55 degrees (down from 57) at a 23mm FL.

So...there is a little restriction in the body still.

 

If I do the whole distance in 1.25 OD barrels taped, I get all 57 degrees.

And...all 70deg  of a 20mm/70deg.   Very dark inside, too.

Long flat-blackened threading has been a staple for many Zeiss

   widgets for over 100 yrs.  

 

 

Trimming a 1.25 lavatory (bathroom sink) extension tfor 2X and lining it with 

  1.5 wraps of pressed-outward  drywall sanding mesh (the best blacking!)

https://www.homedepo...790BG/205153709

 ...gives up all 57 degrees as well.

 

Not as sleek as the TV 2X, of course, and the TV claims high-index glass.

 

It does have that dandy LPDE compression ring EP holder, though.

Something I wish they built into scope accessories  ... no lame/bent copper rings.


Edited by MartinPond, 18 September 2020 - 03:22 PM.


#11 Thomas_M44

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Posted 20 September 2020 - 05:58 PM

I have only one suggestion:

 

Unless it's physically/mechanically impossible to use a longer Barlow with your telescope and associated optical accessories, I suggest always going with a higher-quality standard-length or long Barlow.

 

"Shorty" Barlows may be practically necessary some times, but are almost always somewhat inferior to regular/longer Barlows --assuming similar optical (lenses and coatings) quality, of course.


Edited by Thomas_M44, 20 September 2020 - 06:00 PM.


#12 Peter Besenbruch

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Posted 20 September 2020 - 08:36 PM

Shorty Barlows tend to work better in star diagonals. 3 element shorties do better than 2 element optically. The Celestron X-Cel LX Barlows use a longer top portion (where you insert an eyepiece than the 2x. Vignetting is minimal with either the 2x or 3x. The opposite extreme is the Baader Q Barlow, which vignettes most of my eyepieces. The X-Cels have some of the best light control I have seen. Explore Scientific (not a shortie) and Meade have some of the worst.


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#13 Jon Isaacs

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Posted 21 September 2020 - 06:28 AM

"Shorty" Barlows may be practically necessary some times, but are almost always somewhat inferior to regular/longer Barlows --assuming similar optical (lenses and coatings) quality, of course.

 

 

That has not been my experience.  For me, the main difference is the coatings, more scatter.  Typically though I only use Barlows with medium and short focal length eyepieces.  Generally Barlows improve the view of eyepieces that are not will corrected off-axis.  

 

Jon



#14 Ernest_SPB

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Posted 21 September 2020 - 11:23 AM

In other aspects equal short focal Barlow performs better on axis and introduces more off axial aberrations and wide field artifacts (like vignetting and FOV cut).    



#15 DRodrigues

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Posted 22 September 2020 - 07:22 PM

I like the use of barlow cells on my day.light use because of their compactness, specially important on spotting scopes.

The Baader Hyperion zoom barlow cell results well as a compact 2.25x barlow - e.g http://www.pt-ducks....3_ESBA_92_5.JPG

See text about the adapters almost at the end of the text at http://www.pt-ducks....f 5mm eyepieces


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#16 Jon Isaacs

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Posted 22 September 2020 - 11:50 PM

I like the use of barlow cells on my day.light use because of their compactness, specially important on spotting scopes.

The Baader Hyperion zoom barlow cell results well as a compact 2.25x barlow - e.g http://www.pt-ducks....3_ESBA_92_5.JPG

See text about the adapters almost at the end of the text at http://www.pt-ducks....f 5mm eyepieces

 

If you're thinking of the Baader zoom Barlow:

 

Be aware that the Baader Hyperion Zoom Barlow has a clear aperture of 13.2 mm. 

 

https://agenaastro.c...ow-2454827.html

 

Jon


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#17 DRodrigues

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Posted 24 September 2020 - 06:14 PM

If you're thinking of the Baader zoom Barlow:

 

Be aware that the Baader Hyperion Zoom Barlow has a clear aperture of 13.2 mm. 

 

https://agenaastro.c...ow-2454827.html

 

Jon

Yes, but 13.2*2.25 is equivalent to 29,7mm - no 1,25" ep will vignette and several 2" also will not.

Unlike other 2 to 2.5 barlows that reduce the effective power increase by using only the barlow cell alone, on this you get always >2.25x increase
 



#18 Jon Isaacs

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Posted 25 September 2020 - 05:24 AM

Yes, but 13.2*2.25 is equivalent to 29,7mm - no 1,25" ep will vignette and several 2" also will not.

Unlike other 2 to 2.5 barlows that reduce the effective power increase by using only the barlow cell alone, on this you get always >2.25x increase
 

I don't think it is quite that simple. Other Barlows with more than 20mm clear aperture vignette with eyepieces with larger field stops.  What you have calculated is the effective field stop diameter with a 2.25x Barlow in place. 

 

That is valid at the focal plane but does not address how the focal plane is illuminated by the converging rays from the Barlow.  

 

Have you tried eyepieces with a 29.7mm field stop with the Baader zoom Barlow?

 

Jon 



#19 MartinPond

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Posted 25 September 2020 - 01:26 PM

Under the heading of:   "not necessarily":

 

There is a peculiar side-effect of at least one telecentric multiplier.

The ES 3X  Focal Extender has lenses that are a full 1.25" in diameter

(not diminished by the usual barrel thickness) and somehow produces

a 57 degree afov   with a  few 30mm//53-degree eyepieces I have.

A handy  feature, this opposite-of-vignetting.

This is in a fat 80x400 scope, if that figures into it somehow.


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