ericjmense
member
Reged: 12/08/07
Posts: 23
Loc: Joplin, MO
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It sure seems like things get dimmer with my cheap barlow. Is it just me or does this really happen? and if so is there a barlow out there that does not loose light transmission?
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Currently telescopeless 
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Lawrence Sayre
Post Laureate
Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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Loss of light is a natural function of magnification. More magnification means a dimmer image (except for stars, which can't be magnified, since they are merely "point objects"). If you suspect that some degree of this effect is derived from the Barlow, this can only be validated by comparison against a few stand alone eyepieces with the same effective focal length.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Ahh eric, good thing your started this topic as I was just about to start one on my own.
I also have a pretty cheap zhummel barlow that is 2x, and probably 3 element. I definitely notice that all images obtained through barlow, except maybe very low magnifications, all show some light loss. All the objects viewed are darker.
Common sense tells me that the more glass a light has to pass through, the dimmer it gets. There is no such thing as glass that DOESN'T change the light that goes through it. Its just the basic properties of light.
The best barlows on the market today are the:
Televue powermate 2.5x
Meade 5000 telextender 2x
They both come in several magnifications up to 5x i believe. Note though that 5x mag is a bit high and I wouldn't recommend it. The 2.5x and 2x are far more practical.
The televue powermate and meade 500 telextender are 4 element barlows, and they use some new designs that try to decrease some of the negative effects of barlows such as discoloration of the image and distortion, as well as a few other problem areas.
One thing I do not quite understand at this point...if the new barlows are 4 element as opposed to 3 element like the older ones, would this mean that they soak up even MORE light than previous models?
Also, some telescopes have the ability to turn a 2x barlow to a 3x if placed in a different location.
So far I use my 2x zhummel barlow after the diagonal, on my meade lx90 emc (SCT).
OTC - diagonal - 2x barlow - eyepiece
Now, say I put the barlow before the diagonal like so:
OTC - 2x barlow - diagonal - eyepiece
Will this setup give me 3x power from the same barlow simply because it is placed before the diagonal?
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Deep13
Pooh-Bah
Reged: 01/25/05
Posts: 1454
Loc: NE Ohio
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Greater magnification means a narrower exit pupil. Consequently, any image will be dimmer in the best Barlows than they will without them.
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Preserve the night sky. Join the Internat. Dark Sky Assn. for less than a cheap eyepiece.
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David E
Carpal Tunnel
Reged: 05/25/06
Posts: 1729
Loc: North Carolina
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Quote:
They both come in several magnifications up to 5x i believe. Note though that 5x mag is a bit high and I wouldn't recommend it. The 2.5x and 2x are far more practical.
That's a good general statement, but I use my Meade 5x Telextender mostly on double stars and sometimes for lunar. A 25mm Plossl becomes a 5mm Plossl but with a lot more eye relief and ease of use. A 5x and 8-24 zoom eyepiece is a good combination for a short tube refractor, although you'll need steady and transparent air. There is probably some light loss with any magnifier but since I use them on bright targets I really don't care.
David E
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David E
So when at times the mob is swayed
To carry praise or blame too far,
We may choose something like a star
To stay our minds on and be staid.
-Robert Frost
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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You are 100% right david, I was more so referring to telescopes that the OT poster and I have, bigger aperture, long focal length f/10.
My eyepiece collection doesn't have anything higher than a 32mm super plossl (see my sig for details), that is why for people like me who have a longer focal length might not want to use barlows at all.
Looking through a 5mm baader hyperion alone will give you 400x on a 2000mm telescope. These eyepieces along with many other modern designs, dont have the small eye-hole that the plossls do, so theres no need to barlow a larger focal length EP because of comfort reasons.
Almost all tech support people who i have spoken to at meade, celestron, and many other telescope companies have all told me that they believe using an eyepiece without a barlow provides a "more perfect" image.
Adding a barlow puts 3-4 sets of glass between the light source and your eye, so you do the math. I would never say barlows are a bad thing, its just that in certain situations and applications when you need every last drop of light that you can get for DSOs and such, barlows should be avoided. 
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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ericjmense
member
Reged: 12/08/07
Posts: 23
Loc: Joplin, MO
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So will a powermate/TeleXtenders = less light loss? or just a better picture?
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Currently telescopeless
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Quote:
So will a powermate/TeleXtenders = less light loss? or just a better picture?
I'm not fully confident about this, but i think since the powermate/telextender are 4 element barlows, compared to the standard 3 element which most standard barlows are, it would have slightly less light transmition.
The selling point of the powermate/telextender is that they try to correct all the problems that are inherent with barlows.
Heres an except from the cloudy nights review of the powermate from televue:
"Barlows will often introduce their own set of optical problems into the telescope/eyepiece mix, often resulting in something of a compromise in performance. Since the negative lens of Barlows diverges the light, they also change the light angles of the light cone, resulting in only a rough simulation of a longer f/ratio system. The lens set of the common Barlow is often located down a tube some distance away from the eyepiece, allowing at least some light from the narrowly-converging light cone to scatter off the sides and reduce the contrast unless the tube is well baffled. By contrast, the Powermate isn't really a Barlow at all. It is a set of 4 lenses which, in effect, faithfully re-creates the characteristics of the light cone from a telescope with 2.5 times its original focal length. While this might not seem to be much of a conceptual difference, in performance, this idea results in a significant improvement in the view over many Barlow lens systems."
I suggest you read the entire review to get a better idea of the differences between a conventional barlow and a telextender(meade), or powermate(televue).
http://www.cloudynights.com/item.php?item_id=86
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Deep13
Pooh-Bah
Reged: 01/25/05
Posts: 1454
Loc: NE Ohio
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Conventonal Barlows have a cemented doublet lens with two elements. The third element when there is one is a kind of field-flattener for short model Barlows. Televue Barlows (not P.M.s) are all two-element.
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Preserve the night sky. Join the Internat. Dark Sky Assn. for less than a cheap eyepiece.
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Paul G
Carpal Tunnel
Reged: 05/08/03
Posts: 1751
Loc: Freedonia
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Quote:
The best barlows on the market today are the:
Televue powermate 2.5x
Meade 5000 telextender 2x
Have you tried the Astro-Physics Barcon?
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Gus
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Nope paul i have not yet. There don't seem to be many websites doing good reviews on the newer barlows.
I did find this however, its a comparison between the meade telextender and TV powermate.
http://www.grandeye.com.hk/barlow_comparison.htm
What do you think about the astro-physics barlow?
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Rick Woods
Post Laureate
Reged: 01/27/05
Posts: 4262
Loc: Inner Solar System
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Quote:
Quote:
The best barlows on the market today are the:
Televue powermate 2.5x
Meade 5000 telextender 2x
Have you tried the Astro-Physics Barcon?
I understand the Dakin 2.4x (Vernonscope) is also excellent.
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- Rick
14" LX200GPS
8" Meade 826C
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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Quote:
Common sense tells me that the more glass a light has to pass through, the dimmer it gets. There is no such thing as glass that DOESN'T change the light that goes through it. Its just the basic properties of light.
As we know, gravity is a force of attraction of matter, so common sense says if I stand on the first floor of a skyscraper, I weigh less than I do in the middle of large open field at the same elevation. I do, but what does it amount to? I'm not sure how many zeros to the right of the decimal point we need there to represent the difference, but...you get the idea. Point is that we need to quantify things here and there to calibrate our common sense when combining it with technical information like the scientifically derived basic properties of anything.
Light loss from a well-made multi-coated achromatic doublet might be something on the order of 2% in practice or even slightly less. Even with only single-layer coatings we're probably only looking at 5% loss. Conventional wisdom is that light loss must approach 10% to be apparent to the average viewer visually in side-by-side testing (viewing both images simultaneously). To put this in perspective, Newtonian secondaries for visual use are typically sized for about 30% loss at the edge of a widest true field view, and even that much hardly bothers most people.
Increasing the magnification, on the other hand, reduces brightness by the square of the ratio of magnifications (or focal lengths, if that is more convenient), not by "losing" light but simply by spreading the same amount of light over a wider area at the focal plane. The analogy I've always liked is moving the screen farther from a projector.
Consequently, a 2x Barlow multiplies the brightness per unit area of the view by a factor of 0.25 even with 100% transmission, i.e. no light loss at all. Compare this to something like 0.95 due to light loss from the Barlow lenses being in the path. Even if we suppose the Barlow is particularly cheap, it's loss at glass and surface will be dominated by the effect of magnification. So for a 2x effect, we're looking at something like 0.985*0.25 = 0.24625 effect on brightness per unit area of extended objects from an excellent 2x Barlow, and 0.2375 or so for a cheap one.
This dominating effect of magnification on the brightness per unit area is why Lawrence points out that Barlow use must be compared to standalone eyepieces of the same focal length, and even this must be done with great care, as having exactly, say, a 2x effect from a 2x Barlow is unlikely because the magnification of a Barlow varies due to differences in the location of the eyepieces' effective focal planes.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Great post Mike H., I actually learned something there!
You said that a barlowed image should be compared to the stand alone eyepiece of equal focal length, and this is exactly what I forgot to say in my post.
With the newer eyepiece designs having large viewing lenses instead of the tiny pin hole that plossls have at small Focal length, the reason to use a barlow to increase comfort and eye relief isn't really necessary anymore.
Of course, you can also end up hurting yourself with barlows and long eye relief. If you make the effective eye relief too long, it wont be comfortable to observe at all.
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Jan Owen
sage
Reged: 02/12/06
Posts: 216
Loc: Sun City West, Arizona
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Yes, this has been a very informative, yet concise, thread, containing lots of good info...
Another thing that I'll apologize for now if I missed it somewhere in an earlier post (I read them all, but I'm getting to be an old fart, and sometimes forget... ACK!!!), but a number of the newest VERY high power eyepieces incorporate their OWN Barlow element(s)... So, in many cases, sub-4 millimeter focal length eyepieces (and a few longer than that) are, ALREADY Barlowed... This not only brings up the magnification; it often allows a little more eye relief and apparent field...
We really ARE living in the Golden Age of astronomy!!! I've been doing this for more than 40 years now, and NEVER before have there been SO MANY superior products out there, and, when you factor in currency inflation, never have they offered this much value for the money...
Jan
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Keep on looking up...
Meade 60mm f/15 achro refractor from WAY back...
Vernonscope 94mm f/7 Brandon triplet APO
Orion EON ED 120mm f/7.5
Celestron 5.5" f/3.6 Comet Catcher Schmidt-Newt
8" f/6 Newtonian w/Spooner optics on Atlas G mount
Meade 10" f/10 SCT
12" f/5 Meade LightBridge Dob/Newt
13.1" f/4.5 Coulter Odyssey early Dob/Newt
Many eyepieces and accessories
Always lusting after large MCT, but never buy one
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ericjmense
member
Reged: 12/08/07
Posts: 23
Loc: Joplin, MO
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Thanks for the help I learned alot. I might expand the eyepice colection instead.
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Currently telescopeless
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Lawrence Sayre
Post Laureate
Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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I would opt for a good Barlow instead of expanding the eyepiece collection. I believe that the transmission lost via adding a Barlow to the optical train is quite minimal. I'm certain that the light loss you are experiencing is due to the magnification factor alone, and not due to the use of a Barlow. The transmission losses experienced within your mirror based scope itself are literally factors higher than those that a decent Barlow could possibly contribute.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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