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Brian87gt
newbie
Reged: 02/25/09
Posts: 3
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Was looking to get a barlow lens. Should I get the 2x the 3x or the one that offers both. It's a etx 125. Thanks
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Joe Lalumia
Post Laureate
Reged: 01/24/07
Posts: 3600
Loc: Rockwall, Texas, USA
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Just get a good "ED" "premium" type barlow--- like this one-- or a 3 element design-- either would be just fine
http://www.optcorp.com/product.aspx?pid=30-114-115-6323&kw=&st=0
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LX90 8" LNT, SV Nighthawk & TelePOD, SV 80/9D & M4 mount, ETX 90, Orion XT10i, 20x80 binoculars, SV-BV3s-- www.texasastro.org
"Great minds discuss ideas;Average minds discuss events;Small minds discuss people." Unknown
Edited by Joe Lalumia (03/03/09 01:34 PM)
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Brian87gt
newbie
Reged: 02/25/09
Posts: 3
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I called a local camera shop that sells eyepieces. The impression I got was while a barlow would definately make the image bigger, he told me that it would not appear as bright as before. Is the barlow the best option?
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Joe Lalumia
Post Laureate
Reged: 01/24/07
Posts: 3600
Loc: Rockwall, Texas, USA
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Well a 2x will double your current eyepiece collection-- of course the image will be "dimmer" the more magnification you use. But this will also happen with a eyepiece.
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LX90 8" LNT, SV Nighthawk & TelePOD, SV 80/9D & M4 mount, ETX 90, Orion XT10i, 20x80 binoculars, SV-BV3s-- www.texasastro.org
"Great minds discuss ideas;Average minds discuss events;Small minds discuss people." Unknown
Edited by Joe Lalumia (03/03/09 02:49 PM)
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Mak2007
sage
Reged: 06/24/07
Posts: 356
Loc: MD
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Quote:
Was looking to get a barlow lens. Should I get the 2x the 3x or the one that offers both. It's a etx 125. Thanks
I also have an ETX-125, and after try for a couple of years with several eyepieces (I have a nice collection) and a 2x TV Barlow, I am wondering...Do you really need a barlow in the ETX-125?
I mean, in the ETX you rarely will use less than 8 or 7 mm FL eyepiece (Actually, my most used high power FL is a 12mm). The FL of the ETX125 is large (1900mm), and IMO you rarely use a barlow in the ETX. If you have the 26mm Plossl, then you can get a 9mm TMB Planetary series for high power views (and later, a 32mm Plossl for low power views).
If still you want a barlow, try the shorty plus from Orion or the celestron Ultima (identical). These are better, and you can couple with the 26mm Plossl, so you will get 73x with the Plossl and 161x with the Plossl + Barlow (because these are actually 2.2x).
Good luck.
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10x50 Nikon Action Ex
ETX-125PE
L. Henzl Konig 24mm, 16mm Brandon, 12/9/7/5mm TMB Supermonocentrics
For my wife: 24mm Hyperion, 12/8.5 Pentax XF
My workhorses: 32/24/16/12(Zebra) U.O. Konig II
Barlows 2x TV, 2.8x Klee
"Don't worry about what telescope you own, or its quality. Just get out under the night sky, and enjoy God's wondrous universe." Thomas M. Back
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Brian87gt
newbie
Reged: 02/25/09
Posts: 3
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Quote:
Quote:
Was looking to get a barlow lens. Should I get the 2x the 3x or the one that offers both. It's a etx 125. Thanks
I also have an ETX-125, and after try for a couple of years with several eyepieces (I have a nice collection) and a 2x TV Barlow, I am wondering...Do you really need a barlow in the ETX-125?
I mean, in the ETX you rarely will use less than 8 or 7 mm FL eyepiece (Actually, my most used high power FL is a 12mm). The FL of the ETX125 is large (1900mm), and IMO you rarely use a barlow in the ETX. If you have the 26mm Plossl, then you can get a 9mm TMB Planetary series for high power views (and later, a 32mm Plossl for low power views).
If still you want a barlow, try the shorty plus from Orion or the celestron Ultima (identical). These are better, and you can couple with the 26mm Plossl, so you will get 73x with the Plossl and 161x with the Plossl + Barlow (because these are actually 2.2x).
Good luck.
I'm such a newbie, so is it as the number gets smaller the image gets bigger? I was looking at satuen last night, and was wanting something to increase the size of the image, the barlow seemed like the best option.
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Arthur Dent
Galactic Hitch-Hiker
Reged: 10/23/08
Posts: 1177
Loc: South Yorkshire, UK
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Hi Brian
Yes, the smaller the focal length (FL) of the eyepiece (EP) the greater the magnification.
There is a practical limit beyond which increasing the magnification has no beneficial effect on the image.
A good "rule of thumb" is the largest magnification to use is twice the diameter of the aperture of the scope (measured in millimetres).
Thus, for a 6" scope (150mm) this is about x300 magnification (though in practice, you probably wouldn't go to such a high magnification).
To calculate the magnification divide the focal length of the eyepiece into the focal length of the scope.
My ETX-105 has a focal length of 1470mm. So, if I slap a 26mm eyepiece into it I get a magnification of x56.
This means that for my ETX, the maximum practical magnification is around x200 and that means an EP no smaller than 7mm. A maximum mag of about x170 is probably more realistic though.
In practice, due to less than perfect seeing conditions, EP's of longer focal langth (= lower magnification) offer better views, hence x170 rather than x200 in the example above. The images with longer focal lengths are also brighter (see below).
A Barlow lens has the effect of reducing the focal length by the stated amount (or multiplying the magnification by the same amount).
Thus a x2 Barlow with the above 26mm on my ETX would behave like a 13mm giving me a magnification of approx x113.
As regards brightness...
As you get to higher magnifications, using a Barlow or shorter eyepiece (it doesn't matter), the same number of photons are gathered by your scope (you haven't made the scope any bigger - so the same number of photons are going in as before). These photons are spread over a bigger area ('cos the image is bigger) so that makes the image dimmer.
HTH
Art
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If I like it, the wife says that we can't afford it!
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Meade ETX105 (a nice "Grab & Go" scope) & Celestron NexStar 6SE with Bob's Knobs.
Various EP's from 6mm to 26mm, Baader Hyperion 8-24mm Zoom, a 2x Barlow, 2" diagonal and 7Ah PowerTank.
MRF and Antares 8x50 RACI finder scope - both for the 6SE's OTA, whilst the ETX gets a plain RDF.
Canon EOS 400D DSLR (un-modded) and SPC900 webcam. Finally climbing the AP Learning Curve!
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rcd47
sage
Reged: 03/15/08
Posts: 259
Loc: East Texas, USA
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Here's a post that may be of interest. http://www.cloudynights.com/ubbthreads/showthreaded.php/Number/955450
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Randy
Edmund Astroscan
Meade ETX125PE
Skywatcher 102mm F5 & Alt/Az Mount
Orion ST80/Mini EQ1
Tasco Starguide 114/Celestron Hand Paddle
Pentax 10x50 Binoculars
Philips SPC 900NC
Stellacam EX
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