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Stupendous Man
2002-Jan-05, 12:52 PM
Are these things as good as I am led to believe? Apparantly I can magnify the final image 2 or 3X, is that right?
Sounds good, but if I am not mistaken, something that good MUST sacrifice something about the image to make it bigger? SO the question is, Larger image at the cost of what? Color? CLarity? Brightness?

Also, what are your views with 2" eyepieces? I apparantly need one with a wide-field view to view Andromeda properly.

Scope Info: 6" Newtonian Reflector, Dobsonian Mount.

Thanks!

GENIUS'02
2002-Jan-05, 01:00 PM
On 2002-01-05 07:52, Stupendous Man wrote:
Are these things as good as I am led to believe? Apparantly I can magnify the final image 2 or 3X, is that right?
Sounds good, but if I am not mistaken, something that good MUST sacrifice something about the image to make it bigger? SO the question is, Larger image at the cost of what? Color? CLarity? Brightness?



Yes barlow lenses do increase the magnification by a fatcor of two or three (or what ever the power of the lens.)
but this extra magnification takes away, minorly, from both clarity and vision as it gives the light an extra lens to pass through. but it is particulary useful when viewing the moon. and brighter planets, (wish i'd had one when viewing jupiter other night)be careful though not to over power your telescope because you'll be unable too see anything. (just multiply the magnification achieved without the barlow by the power of the barlow to find total magnification power.)

e.g say, for example, your focal length is 500 mm, the maximum magnification of your telescope is 50*6=300X. just using a 4mm eye piece you have magnification power of 500/4=125X, add a 3X barlow and you get 125*3=375, the at this magnification would be un recocnizable.

<font size=-1>[ This Message was edited by: GENIUS'02 on 2002-01-05 08:05 ]</font>

Kaptain K
2002-Jan-05, 05:12 PM
Are these things as good as I am led to believe? Apparantly I can magnify the final image 2 or 3X, is that right?

Yes.


Sounds good, but if I am not mistaken, something that good MUST sacrifice something about the image to make it bigger? SO the question is, Larger image at the cost of what? Color? CLarity? Brightness?

As Genius said, you do lose some clarity. You also lose brightness, due to the increased magnification. This is true whether you get your magnification from a short FL eyepiece or a longer FL eyepiece + barlow. You gain eye relief (distance from the eyepiece you need to be to see the whole image. Example: Assuming you have a 6 inch f/8, 48" (1200mm) FL, a 4mm eyepiece will give you 300x (1200/4). A 12mm + 3x barlow will give you the same power, but with the longer eye relief of the 12mm eyepiece.


Also, what are your views with 2" eyepieces? I apparantly need one with a wide-field view to view Andromeda properly.

I doubt that a 2" eyepiece would be of much help to you. To utilise it properly, you would need a 2" focuser and a larger secondary (diagonal) mirror to avoid vignetting (dimming of the edge of the field). The cost of modification + a good 2" eyepice would probably exceed the cost of the 'scope.

<font size=-1>[ This Message was edited by: Kaptain K on 2002-01-05 12:15 ]</font>

Stupendous Man
2002-Jan-05, 05:46 PM
I have the 2" focuser, and an adaptor for the eyepieces.. Maybe I'll just try one for the heck of it. If it's no good, I can always return it.

A barlow only costs about 65 bucks here, so I guess that might be worth picking up..

Thanks guys.

aurorae
2002-Jan-07, 06:06 PM
Most of the modern wide field eyepieces (especially the ones that have 20mm eye relief) have a barlow designed into them. So for those, you may already be using a Barlow without knowing it.

Russ
2002-Jan-07, 07:15 PM
Since I didn't notice anybody else addressing it, I'll tackle the Andromeda issue. There is more to seeing all of Andromeda at once than the size of your eye piece.

What determines your field of view is a combination of the "F" ratio of your scope and the "F" ratio of your eye piece. The "faster" you F ratio, generally speaking, the wider the field of view. For example; an f10 scope will have a smaller FOV than a f8.5 or f5 scope. The lower the F ratio the larger your FOV. This holds true for the the eye pieces as well. There are specially configured "wide angle" eye pieces that will increase your apparent FOV on an f10 scope but will not get you the FOV that is default on an f5 scope.

Further, a 2" eye piece can give you a bigger FOV than a 1.25" but again it depends on the configuration of the eye piece and scope.

If you want to dig into this more go to http://dir.yahoo.com/Science/Astronomy/Telescopes/Amateur/ and read it all.

Good Luck.