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How useful actually are 2" barlows?


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Hello all,

Might sound controversial which is certainly not my intention, but how useful actually is a 2" barlow in most cases?

I own a Revelation 2" barlow and although it seems to reasonably okay quality, I rarely seem to use it. The thought had been idly occurring to me for a while, and the more I mused upon't, the more it started to feel like there are only really edge cases. Given that 2" EPs don't exist really below a certain FL because after that there's no point having the larger diameter, is it just about having the huge eye lens, and having a bit of flexibility with your kit?

What am I missing? I'm sure there must be summat.

Just musing really, but it has had me thinking.

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I sometimes use my GSO 2" ED Barlow with a TV PBI to eliminate exit pupil issues and hard vignetting for the whole night just to change things up.  It allows me to use my 2" eyepieces like the 40mm Meade 5000 SWA, 40mm Pentax XW, 30mm APM UFF, 30mm ES-82, 26mm Meade MWA, 22mm Nagler T4, and 12mm & 17mm ES-92 eyepieces at higher powers.  They provide a different experience compared to my normal 1.25" eyepieces in the 6mm to 20mm range.  Longer eye relief, wider apparent field of view, etc.  Otherwise, I rarely use any Barlow, 2" or 1.25", except to reach focus with my binoviewer.

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Posted (edited)

Next to a 2.5x powermate (1.25”) i bought a 4xpowermate (2”).  I thought it would be a good addition to the collection.  Sure it will be some day but the times my complete set, powermate, eyepiece and the diagonal, were facing the floor due to the weight made me use it less and less.  
 

 

Edited by Robindonne
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And when used in a diagonal, the long moment arm of a 2" Barlow or PM with a large eyepiece like the 17mm ES-92 can make the entire telescope turn turtle on an alt-az mount unless a large mass on a long moment arm is hung below it.

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Posted (edited)

I bought a 2” 2x tele extender a while ago and only ended up using it once in about a year so sold it on. Just didn’t like using it. Too big and heavy.

Edited by johninderby
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Yes this was essentially what I thought - a bit of variety but at the unfortunate cost of everything being perhaps unnecessarily heavy in some cases, which is a bit off putting. 

Perhaps it was a silly question really, as we continue to often look at the same patches of sky for many nights, so a different setup is sometimes welcome to, and we amateurs have our own particular preferences.  

Still, very interesting to hears other's thoughts as always. 

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Posted (edited)

Maybe ask a more experienced stargazer about the possibilities of barlowing a barlow.   The setup is getting longer but i dont think as heavy as a 2” barlow.   If you already have a 2x barlow, maybe a 3x is a good addition to barlow 2x, 3x and 6x.  I know its possible but im not sure what quality the view will be with (non tele-extender) barlows

Edited by Robindonne
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8 hours ago, Robindonne said:

Maybe ask a more experienced stargazer about the possibilities of barlowing a barlow.

I would guess as you suggest that unless you're talking about TeleVue kit you'd just introduce aberrations or other issues.

I actually bought the 2" years ago when I got my first 2" EP and was less experienced, thinking I'd need it. I had a very small 1.25" EP collection at the time, and so was regularly using the 1.25" barlow to expand the range of my EPs; it seemed logical at the time that with only one 2" EP, I'd also need a Barlow for the same reason. 

In general now, I rarely use either any barlow as I can cover most useful FLs with my main EPs. 

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A 2" Barlow might be useful for enlarging the image scale in photography, when depending on the size of the sensor, a 1.25" Barlow might result in vignetting.

John 

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Posted (edited)
On 29/05/2021 at 01:13, badhex said:

Given that 2" EPs don't exist really below a certain FL because after that there's no point having the larger diameter, 

I have a few of the ES 100 degree EP's up to 9mm.

ES does do a 5.5mm in 2" so in theory you could go 2" all the way. (some TV's below this too I think)

I have a very nice Astro-Physics 2" barlow that gets a bit of use in imaging sometimes.

Being the lazy kind, I prefer just to reach for the right focal lengths straight off the tray

But a lot depends on your telescopes, budget, style of observing, eye relief etc.

Barlowed 2" EP's can give you better eye relief I believe than their native counterparts

I only use 1.25" for 6.7, 5.5 and 4.7mm which is easy,  as these are only planetary targets for me

 

Edited by EntropyTango
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1 hour ago, johnturley said:

A 2" Barlow might be useful for enlarging the image scale in photography, when depending on the size of the sensor, a 1.25" Barlow might result in vignetting.

Haha it's funny you say this - I actually wrote a draft reply earlier wondering almost exactly the same thing, but not being into AP I wasn't sure if such large sensors exist. Also, might be useful to the DIY astronomer for converted cameras/webcams that may not fit inside a 1.25" barlow?

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11 minutes ago, badhex said:

Haha it's funny you say this - I actually wrote a draft reply earlier wondering almost exactly the same thing, but not being into AP I wasn't sure if such large sensors exist. Also, might be useful to the DIY astronomer for converted cameras/webcams that may not fit inside a 1.25" barlow?

When using a full frame SLR, using a 1.25" drawtube results in quite a lot of vignetting, even with a 42 mm T thread adaptor I still get some vignetting with my Canon EOS 6D , but this can be reduced by using a 48 mm to Canon adaptor rather than the 42 mm T thread.

John 

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The more glass you add to your light path the more the image degrades. Personally I don't use barlows at all, in my experience I would get as many eyepieces as I can to plug all the magnifications I think I will need.

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1 hour ago, EntropyTango said:

I have a few of the ES 100 degree EP's up to 9mm.

ES does do a 5.5mm in 2" so in theory you could go 2" all the way. (some TV's below this too I think)

I actually wasn't aware of these, thanks - I hadn't really put two and two together that for the very wide 100˚ etc EPs you'd need the 2" barrel. Slightly off topic, but not sure exactly how the crossover scales e.g. I believe it's about 24mm / 68˚, for max TFOV without vignetting in a 1.25", but what happens to the AFOV when you increase FL by say 2mm? Presumably theres a calculation one can do.

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5 minutes ago, Doc said:

The more glass you add to your light path the more the image degrades

Yes, this too. I know that many people say they don't register the presence of the various TeleVue barlows in the optical train, but that comes at a cost of course :) 

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3 hours ago, badhex said:

I actually wasn't aware of these, thanks - I hadn't really put two and two together that for the very wide 100˚ etc EPs you'd need the 2" barrel. Slightly off topic, but not sure exactly how the crossover scales e.g. I believe it's about 24mm / 68˚, for max TFOV without vignetting in a 1.25", but what happens to the AFOV when you increase FL by say 2mm? Presumably theres a calculation one can do.

Tom Dey over on CN put this graph together to show where each barrel size maxes out:

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There is some fudging allowed if you put the field stop above the insertion barrel and accept some vignetting and lots of required in-focus.  It's sort of analogous to how you can use 2" eyepieces with 127mm Maks despite their 27mm diameter rear port.

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The only "barlow" that I've used that seemed totally invisible in the optical train is the Tele Vue Powermate. As noted already though, invisibility comes at some cost.

 

 

 

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Just as John says, the Powermate seems ‘invisible’.  Actually, I have the 2” and mostly use it ahead of the binoviewer.  I don’t generally ‘Barlow up’ longer eyepieces to achieve shorter focal lengths, except to experiment, usually preferring to use the shorter focal length eyepiece itself. But that said, some eyepieces benefit from being Barlowed (though the Powermate isn’t exactly a Barlow), one example, quoted by Bill Paolini, is the 10mm Burgess Ultra Mono.  And while some eyepieces in some systems respond very well to Barlowing and some less well, there need be no fear, imv, that a modern, high quality Barlow or Powermate, by introducing ‘more glass’ into the train, will inevitably degrade the image.  There are more bits of glass in some of our top performing eyepieces than under the counter and behind the bar at the Badger and Candlestick.

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Another invisible one is the Meade 140 APO 2x. I've had mine for years and don't really want to replace it. I have had other Barlows, plus I still have a 2" Barlow, but none of them match the image quality of that 140.
The 2" Barlow was purchased to use photographically with my Nikon. It's fine for that.

1067161967_D72_8335_DxO1200.jpg.6022fd10885276b59a3fd17ae328f09f.jpg

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4 hours ago, Louis D said:

Tom Dey over on CN put this graph together to show where each barrel size maxes out

Very interesting - thanks Louis!

 

3 hours ago, John said:

The only "barlow" that I've used that seemed totally invisible in the optical train is the Tele Vue Powermate.

I was thinking of you as I wrote it John, from a post some years ago 🙂

 

1 hour ago, JTEC said:

there need be no fear, imv, that a modern, high quality Barlow or Powermate, by introducing ‘more glass’ into the train, will inevitably degrade the image.  There are more bits of glass in some of our top performing eyepieces than under the counter and behind the bar at the Badger and Candlestick

Agreed - the six or seven+ element EPs at the top end of the range of course are optimal, but many don't have those options so it's perhaps more of a problem with stuff at the mid or lower ranges. All of that said though, I guess some people (esp. Planet fans) prefer the purity of fewer overall glass elements. 

 

35 minutes ago, Mr Spock said:

Another invisible one is the Meade 140 APO 2x. I've had mine for years and don't really want to replace it.

Good to know. If it ain't broke...! 

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2" Barlows definately do have their place. My main objection to Barlows in general is that you're never quite certain what the scale factor really is because it depends on the distance between the Barlow lens to the eyepiece's focal plane, which is all down to the eyepiece metalwork design. A *2 barlow can be anywhere *1.5 to *2.5....

They are very useful in faster scopes for planetary/ lunar photography for matching the pixel size to the focal ratio. 

They make off-axis eyepiece aberrations less obvious in my experience. YMMV. 

They can be very useful in "extracting" the focal plane further out if your scope does not have enough in-travel on the focuser for some accessories.  The focal plane moves more than the real linear mechanical movement on the focuser as you turn the knob. The increased focal length can be either a problem or a blessing depending on context; but at least you have options. 

I have a spectroscope which is quoted as being suitable for f/5 to f/30 scopes; adding a *2 2" Barlow optimizes the optical path for a f/4 Newtonian while allowing a tight and strong mechanical connection. 

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1 hour ago, rl said:

They can be very useful in "extracting" the focal plane further out if your scope does not have enough in-travel on the focuser for some accessories.

This is a very good point that I had not thought of. I have been lucky enough so far not to experience this with any of my kit but I know it's not uncommon. 

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I don't like 2" barlows due to the size of the "stack" you end up with.. I've only used a cheaper 2" Barlow (the GSO version) and I found its' very thin tube walls could " flex" when used with a heavy 2" eyepiece such as a Meade 14mm UWA. I couldn't imagine trying to barlow my 1.4kg Axiom LX 31mm megabeast!😱😂..but as I have a superb Morpheus 17.5mm, why bother anyway?

I do however use a good quality 1.25" Barlow regularly nowadays (Baader Hyperion Zoom Barlow 2.25x). I use it successfully with high quality eyepieces such as Morpheus and Nagler T2 and like the way you can get higher powers without losing eye relief..and to my eyes the Baader Barlow doesn't add any aberration, and saves me having to buy very short eye relief eyepieces separately.

I don't see much point in using a Barlow with cheap eyepieces..you just end up magnifying aberrations! - an odd exception to this seems to be when using a binoviewer..I have had wonderful views when binoviewing with relatively cheap eyepieces with a barlow at the front of the binoviewer...better than I sometimes had with a single high end eyepiece..I've never quite understood why that is, other than the "two eyes are better than one" theory.🤔

Dave

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Posted (edited)

If you get a 2" BarAdv from A-P that fixes itself -- you can install it as a screw-on barlow that disappears into the focuser and makes the combination more focuser-friendly. you do have to rack out the focuser somewhat more, though, so you lose part of what you gained that way too (depending on how good the focuser is).

Edited by Alexis
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7 hours ago, F15Rules said:

I've only used a cheaper 2" Barlow (the GSO version) and I found its' very thin tube walls could " flex" when used with a heavy 2" eyepiece

I have the same if it's the ED one. I had thought it was quite well built for the price, although only rarely used, and only then with a reasonably lightweight GSO Superview.

 

7 hours ago, F15Rules said:

I don't see much point in using a Barlow with cheap eyepieces.

I would tend to agree, and add that there's probably no point using a really high quality eyepiece in a cheap barlow for the same reason. I recently invested in a Panoptic 41mm, and though the GSO barlow pretty good for the cost, it will never stand up to the optics of the TeleVue, so would probably not use them together. 

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