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To Barlow, or not to Barlow....

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#1 Dave_L


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Posted 26 June 2019 - 12:03 AM

Howdy all. If observing with a 25mm Plossl and I want more magnification, is there much difference between switching to a 12.5 mm eyepiece vs. dropping in a 2x Barlow with the 25mm? The last time I did it, the view with the Barlow seemed better. The answer probably depends on the Barlow and the eyepiece in question, but I would be curious to hear any thoughts on this. Also, any thoughts on the Celestron 8-24mm Zoom eyepiece? Thanks!

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#2 Jeff Morgan

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Posted 26 June 2019 - 12:25 AM

A barlow effectively reduces the focal ratio of the telescope, and pretty much all eyepieces like that.


The flip side is barlows are somewhat of a nuisance to deal with. Lots of extra physical handling, especially when filters come into play. Do it mutliple times per night, it starts cutting into the flow of the night and your time at the eyepiece. Hence the love/hate relationship barlows engender.


Personally, I would not use them then to go from Low to Medium power. I prefer to use them to get from High to Very High power. But like all things, preferences vary.


On balance it's smart to have at least one in your collection, even if only for infrequent use. 

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#3 MartinPond



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Posted 26 June 2019 - 12:35 AM

That's been my experience as well, but there are a number of factors at play.


---First, if the barrel is "short" (F4 to F6), the Plossls outer field will have its aberration

    issues wiped out by the Barlow.


---Second, Barlows can improve stray light suppression, and so, contrast.

   (some might make it worse...depends on the EP and Barlow's chemistry together)


---Third, the eye relief will be pushed out a little....many would find that a gift.

    A 12mm Plossl has an ER of about 8mm, which is tricky sometimes.


----Fourth, many people have mentioned how the 25mm is somehow the 'best' of

    a product line, for sharpness....  I noticed that in the Meade 4000 line, the Sirius line,

    and my own home-made symmetrics.  My theory is that at moderate prices,

    the same lens grinding roughness will have less effect on a longer FL than on smaller

    lanses / shorter FL.   This also applies to little bits ofdust and gick on the surfaces,

    especially the eye-facing one.   Of course, when you go to 30mm in a 1.25" eyepiece,

    the field width shrinks due to the 1.25" limit nearby.



The "fourth" reason seems like the the biggest to me.   A 25mm Plossl is a real

   stand-out across many moderate brands, and a good Barlow lets you

  keep that nice view, across almost all barrel lengths.



"A barlow effectively reduces the focal ratio of the telescope, and pretty much all eyepieces like that."

---I'm pretty sure Jeff M.   meant the Barlow INCREASES the (apparent)  focal ratio of the telescope. 


I like using the right Barlow more often, but my use is similar to Jeff's:

   I really like using a 3X Barlow or extender to go from low to high power.

A lot of that has to do with the small aperture of a 3X....it acts as a strong iris to

  suppress stray light and increase contrast.  This does not bother the afov of the



Because the glare suppression of the Meade #126 (and others) has gotten better

even as the price has dropped, I keep 3 around and have a few semi-permanently bonded

to things like my 75 degree 1,2,1 Konigs.   I salvage those in pairs so it's fun to have the 

 same EP in different effective FLs.   


I keep the ES 3X extender on its own...

....it's a big block, and one has to think before unleashing it.

Edited by MartinPond, 26 June 2019 - 12:49 AM.

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#4 Taosmath


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Posted 26 June 2019 - 12:36 AM

One advantage of using a Barlow with  25mm plossl is that you get the eye relief of the 25mm rather than the eye relief of the 12.5mm which is much lower.

Possible disadvantage of the barlow is it adds glass to the lightpath, so there is the possibility of light loss or extra reflections (reducing contrast) , but, as you say. it depends upon the Barlow.


I like the Celestron 8-24mm zoom.  I enjoy its flexibility.  I can pick the magnification that gives the best image (i.e. best framed, greatest detail, etc.)  All of this is done of course with a single eyepiece , so I can just twist the barrel to change the image, rather than stop observing and switch over eyepieces.  


The biggest downside to the zoom is that at low magnification (long focal length) the field of view is narrow - probably only 42 degrees or so.  That increases at shorter focal lengths but is't still only 60 degrees at 8mm.  The quality of the image is also probably a little inferior to fixed eyepiece options of similar quality.


All of that said, I do 90% of my viewing with two eyepieces - a wide angle (30+mm, 2" barrel) and a zoom.  In some scopes I use a Celestron 8-24mm & in others I use a Baader 8-24mm (maybe 20% (?) better than the Celestron but almost 4 times the price).  The next most common eyepiece I uses is a 20 something wide angle eyepiece so that I can overcome the narrow FOV of the zoom at 24mm. 


Some of us love zooms, others hate them.  You need to find out which you are.

#5 Astro-Master



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Posted 26 June 2019 - 01:39 AM

I use a TeleVue 2" Big Barlow, and or a Astro Physics convertible barlow with my 18" Obsession f/4.5 with my 17mm Ethos. 


The TV big barlow changes the 17mm to about an 8mm with no vignetting and increases the focal  ratio, for a nice flat field.

The higher power darkens the background, and increases the contrast and detail on galaxies, adds a little more eye relief, and with the 100 degree AFOV allows the whole galaxy to be viewed.


It is the best combination I have found to observe DSO's from a dark site with a fast F ratio.  If the seeing is good I increase the power of the barlow with 2" by 1.5" extenders between the barlow and eyepiece.  One Ethos can become many.


Being able to view all of M51 at 280x from a Bortle 2 zone at 8,600ft with good seeing is like adding 5 or 6 inches of aperture to my 18" Dob.  The galaxy fills the field of view, and looks like a photograph!   


A good quality barlow will not degrade the image of a good eyepiece, if the seeing is good.

#6 buddy ny

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Posted 26 June 2019 - 08:28 AM

I'm in Jeff Morgan's camp
I don't like them,but you should have at least one to get to very high power
I'd rather use a 12mm ortho barlowed to 6mm (and get the benefits that go. With it,,,than use a 6mm ortho pin hole. But hey my eyes are old
They key is getting a really good Barlow. I've had the best and the worst
Best is better.
I use a. Celestron Ultima 2x circa- 1987, 88
If & when I use one
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#7 vdog



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Posted 26 June 2019 - 09:36 AM

Personally, I would not use them then to go from Low to Medium power. I prefer to use them to get from High to Very High power. But like all things, preferences vary.


On balance it's smart to have at least one in your collection, even if only for infrequent use. 

+1 on the above.


IMO, the Celestron zoom is a great eyepiece for a newbie using a small telescope.  That describes me a little less than a year ago when the Celestron was my main eyepiece.  It's an inexpensive and convenient way to start learning your way around the sky and what magnifications you like for what targets. 


But you may outgrow it. If it's going to be a long-term main eyepiece for use in larger scopes, you may want to invest in a premium one like the Baader.

Edited by vdog, 26 June 2019 - 11:14 AM.

#8 bobito


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Posted 26 June 2019 - 09:44 AM

It would be difficult to tell the difference between barlow and no barlow when viewing.  But I agree with Jeff above that they add a bit of extra effort beyond using dedicated EPs in most cases. 


However, for planetary use, I find the barlow to be beneficial with my 140 f/7.  Not only for eye relief, but because it gives me fractional focal lengths so I can match my magnification to the nights seeing conditions.  So my 11, 12, 13, and 14mm planetary EPs become 3.7, 4, 4.3 and 4.7mm with my 3x barlow (rounded to first decimal).  I can just run through the different focal lengths and the barlow stays in place.


So I find barlows useful for planetary viewing in scopes that require very short focal length EPs (less than 8mm) for high powers.  Otherwise I have dedicated eyepieces for various focal lengths.


On the zoom, I had the 8-24mm Celestron zoom and didn't like it at all.  It's like looking through a straw at 24mm.  However, zooms are good tools to help you find what magnification is working on different objects.  But I prefer to just try individual eyepieces over a zoom.

Edited by bobito, 26 June 2019 - 09:45 AM.

#9 Stardust Dave

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Posted 26 June 2019 - 11:48 AM

I've just had 2 barlows over the last 30 plus years.  Have a 1.25" 1.8X Televue that is outstanding but is only 1.25"

Had purchased a budget 2" 2X barlow 20 plus years ago on Telescope Warehouse , and I do /did not like the star images or fit in focuser ( too tight). Never used since.


I've had huge gaps in my magnifications for most the time I've observed with my Dobs.


Saturday on impulse (and "permission") lol.gif ordered the 2" Agena Astro GSO ED barlow.

Had resisted purchasing a costly barlow in the past - Tele-vue  ect. because of budget, and the fact that all my EP's are TV Pan and Naglers. Figured the inexpensive Agena barlow would not be good enough.  What a surprise I got!


This new Barlow gave me both duplicate and the in-between mags I had been missing out on.

It came with a really nice 1.25" adapter and my favorite feature is removing the element from the bottom of the barlow barrel and putting on the EP threads for 1.5X mag. 


The barlow has great contrast , bright pinpoint stars.  Spend most the night barlowed-up running new mags of 190X and 250X on DSO's . Planetary performance was superb on Jove,watching Io transit the disk on the NEB before tree got in the way.

Can see right away on the 13" the barlow will get used more often than not. Was a great session. smile.gif

I have yet to test this new barlow on the 20" or with the paracorr on either scope. 

Edited by Stardust Dave, 26 June 2019 - 11:59 AM.

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#10 dscarpa



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Posted 26 June 2019 - 01:04 PM

 I have all the 1.25" TV barlows-Powermates and a 1.6X Siebert barlow. I don't think a quality barlow the degrades the image. I've barlowed a   XW or T6 in my WO ZS110 to the same power with the same eyepiece alone in my C9.25 or IM715D mak for L&P. The smaller scope was nipping of the heels of the cats.  David

Edited by dscarpa, 26 June 2019 - 01:05 PM.

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#11 russell23



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Posted 26 June 2019 - 02:34 PM

A quality barlow does not degrade the image and improves edge performance.  With planning you can cover a lot of additional ground with barlows and extension tubes.   For example, I can cover ~95% of my lunar/planetary observations with the 18.2mm DeLite, 1.6x Nikon barlow, 3.0x TV barlow and a 1" extension tube with my 120ED.


18.2mm DeLite - 50x

 w/1.6x Nikon - 80x

w/1.6x Nikon and 1" Extension - 92x

w/3.0x TV - 150x

w/3.0x TV and 1" extension - 180x


Rare is the night when I go above 180x and in those cases I can use shorter FL eyepieces  with one of the barlows.

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#12 nicoledoula


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Posted 26 June 2019 - 03:53 PM

To barlow. For sure. 1.5X, 2X,and 3X. Why not? "Fumble with" EP's or fumble with barlows.   Low, medium, medium high and high magnifications are needed. Unless you WANT to collect EP's.   Carefully thought out scope purchases lessen the need for museums full of EP's. One good widefield and one good medium power EP and a couple, maybe 3 good barlows can cover all one needs. Zooms are also indispensable  OR, every EP set should contain a Zoom and 2 or 3 barlows. If fumbling is a/the problem than a Leica zoom is the answer.  1 excellent WF, the Leica zoom and the Nikon 1.6X barlow is all I'd need. 

#13 Jon Isaacs

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Posted 26 June 2019 - 09:08 PM

I find Barlows to have various advantages.  They often improve off-axis correction of an eyepiecein a fast scope, they allow for greater eye relief with Plossls, orthos and the like. Generally the only optical nevative is slight increase in scatter.


But in the field, I am like Jeff Morgan, I only use Barlows to increase the magnification for high to very high. .My shortest focal length eyepiece is the 3.5mm Nagler. For higher mags I use the 5 mm and 3.5 mm Type 6s with a 2x TV 1.25 inch Barlow.


For longer focal lengths, Barlows are too much hassle.  I swap eyepieces very frequently and use a Paracorr in my Dobs. A 2 inch 2X Barlow and eyepieces like the 31 mm Nagler and 21 mm Ethos make the optical train very long and heavy.


I prefer individual eyepieces covering the range of necessary focal lengths.. it's just so much simpler.


But currently I have a dilemma. I have the 21 mm, 13 mm and 8 mm Ethos. I'd really like to have a 10 mm, 6 mm and 4.7 mm Ethos.  But even used, that's well over $1000.  The right 2 inch 2x Barlow would fill those gaps. The 21 mm Barlowed is too massive but the 13 and the 8 would be 6.5 mm and 4 mm and cover those slots.


The 13 mm and 8 mm are 1.25 inch eyepieces and can be used with a 1.25 inch Barlow. It works. But I use them with 2 inch Barrel extenders so  Barlowing with 1.25 inch means more messing around... 




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#14 epee



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Posted 27 June 2019 - 09:35 AM

Barlows certainly have their place. As money savers each Barlow doubles the number of focal lengths you have available.


I use a x2, x3 & x5 in three roles: filling in preposterously short focal lengths that I will likely never use (but hope too), allowing me to creep closer and closer to the atmospheric limit of seeing, and to allow me to increase the magnification on my Plossls for comfort, outreach, and use on smaller scopes.

#15 dgordontx


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Posted 28 June 2019 - 10:56 AM

In agreement with the statements above with the exception that I did not care for the Zoom. I had the Celestron and used it 5-10 times but prefer fixed EPs. Last night I order the APM HDC 9mm, which is the first EP that i have even bought in the range of the zoom. Prior to that, I used my 32mm and 6mm exclusively, occasionally barlowing the 6mm for planets. My 5.5 UWA barlows way better than my 6mm ever did but there's a well deserved price difference as well.

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