What is telescope "Barlow" lens for?

[RobertN]

My son got an inexpensive telescope from his grandpa. With it is a 2x Barlow lense and a 1.5x erector lense.

What are the purpose of those lenses? Both appear to be 6-8" long.

 

[DieselPower]

From Wikipedia:

Barlow lens - Wikipedia, the free encyclopedia

The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, effectively increases the focal ratio of an optical system as perceived by all components after it in the system.

I couldn't find anything on the "erector" lens. Does it have another name by chance?

 

[CurlyDave]

The Barlow Lens is an eyepiece. You put it in the telescope in order to look through the telescope and actually see something. It changes the magnification of the telescope. It may be labeled 2x or 3x to indicate the magnification change.

Most astronomical telescopes turn the image upside down, because it is easier to build them that way. It doesn't make much differrence when looking at the stars, but this can be annoying when looking at a scene on the earth. The erector lens turns the image over a second time, allowing it to appear upright.

 

[dmartin88]

A Barlow typically is used with another eyepiece (insert the barlow first, then the regular eyepiece in to the barlow) and increases the magnification of the image. As others said, they're usually labled 2x or 3x to indicate how much they will magnify the image. In cheaper telescopes, the barlow lens is usually pretty cheap as well and you can see real degradation in the image when using it so won't be surprised if it's not a lot of help for anything other than terrestrial viewing, maybe when looking at the moon too?

Good luck with the new telescope, Saturn is amazing the first time you see it and can recognize the rings- sure it's not like the pictures you see but I still remember the first time I saw Saturn in a telescope, also an inexpensive model!

 

[RobS]

The Barlow Lens is an eyepiece. You put it in the telescope in order to look through the telescope and actually see something. It changes the magnification of the telescope. It may be labeled 2x or 3x to indicate the magnification change.

Most astronomical telescopes turn the image upside down, because it is easier to build them that way. It doesn't make much differrence when looking at the stars, but this can be annoying when looking at a scene on the earth. The erector lens turns the image over a second time, allowing it to appear upright.

Excellent, concise answer Dave! I've learned something for the day so I can go home now, right? A telescope is on my long term list, someday, somehow, sometime. Seems like an interesting hobby. A photo in Popular Science this month shows the space shuttle undocking from the space station, all shot directly in front of the sun. Cool pic!

 

[HomeBrew2]

Just to add a bit to Dave's good answer: Physics dictates that an image will appear inverted when viewed thru a lens .... it's the law
The more elements in a lens system, the less usable light you will have and the less definition of the viewed image, simply because you're adding more pieces of [imperfect] glass between the object and the image of it.
Purists and scientists, etc don't need the image "erected" at the cost of less light and less resolution.
Not sure what's used today, (probably GPS) but in the past, geodetic triangulation was done with very precise european theodolites (optical transits) with inverted image ... pretty weird surveying with everything upside down.

 

[mwechtal]

My son got an inexpensive telescope from his grandpa. With it is a 2x Barlow lense and a 1.5x erector lense.

What are the purpose of those lenses? Both appear to be 6-8" long.

To add to what others have said:
The lenses that come with inexpensive telescopes are a bit of a mixed bag. Usually the lowest power eyepiece is decent, and the others are there just so they can print an impressive magnification on the box. Since the designers know they can't be used, they have very bad plastic lenses in the eyepiece.

So, I would recommend that he only use the lowest power eyepiece (longest focal length), and forget the barlow for now. Once he's used it a bit, try the barlow. It probably will degrade the image, but you never know. The higher power eyepieces will likely be extremely difficult to focus, and mostly unusable. They might be OK on the moon.

Two safety notes. Please be sure he KNOWS that he can't look at the sun with his 'scope.

Check to see if there is a solar filter that screws on to an eyepiece. I think they're illegal now, but please check. These are dangerous. I recommend dropping them on concrete and stomping on them, then burning them, then burying them. The ones that screw into an eyepiece have the light from the sun focused on a small area. This almost always causes them to break sooner or later. We have a guy in our Astronomy club that has an image of the sun burned into his retina from when this happened. The correct type of filter covers the front of the telescope so that the power behind the sun's light is decreased to a safe level before it even gets to the telescope. Colored filters, and moon filters (gray) that screw into eyepieces are OK. I use them often.

Mike

 

[Alan L.]

A Barlow typically is used with another eyepiece (insert the barlow first, then the regular eyepiece in to the barlow) and increases the magnification of the image. As others said, they're usually labled 2x or 3x to indicate how much they will magnify the image. In cheaper telescopes, the barlow lens is usually pretty cheap as well and you can see real degradation in the image when using it so won't be surprised if it's not a lot of help for anything other than terrestrial viewing, maybe when looking at the moon too?

Good luck with the new telescope, Saturn is amazing the first time you see it and can recognize the rings- sure it's not like the pictures you see but I still remember the first time I saw Saturn in a telescope, also an inexpensive model!

The cheaper barlows are worthless. The barlow lens I use does not degrade the image that I can tell. I prefer it to using the higher power eyepieces because of the confort of the larger diameter on the low power lenses. And it produces just as clear an image and with just as much light as far as I can tell.

My frustration with the whole astronomy thing is sheer lack of patience in finding and identifying deep space objects. I have a manually operated Dobson mount 8" reflector, but the computerized mount would be nice to help fund stuff. I have never been able to see any galaxies, and looking at Jupiter, Saturn, Mars, Venus, the moon, the Orion Nebula, gets fairly old especially when you factor in that seeing conditions are marginal the majority of the time even on clear, dark nights.