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Barlow question.


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Not sure if this is the right place to ask this question but here goes.

I read on the forum that putting a 2x barlow in front of the star diagonal significantly increases the magnification on a scope.

Why is this and how do I know by what factor it increases the mag?

Last night I managed, using a Kson 16.8mm orthoscopic with this setup on my Starmax 127, to see the gap tween Saturn and it's rings, the shadow of the planet on the rings and the rings crossing in front of the planet.

Thanks in advance for any help.

Mark

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Yes, the effective magnification is increased.

The magnification of a Barlow is achieved by placing it inside the prime focus of the telescope. Depending on the negative focal length of the Barlow ( usually about -120mm) and the distance inside focus the new focus will move further and further outwards.

The formula is:

1/s1 + 1/s2 = 1/f

where s1 is the distance of the lens from the prime focus

s2 the distance from the lens to the new focus

and f the focal length of the lens

The magnification is:

M = -s2/s1 or F/(f-s1)

When you add the Barlow infront of a diagonal you are increasing the distance s2 and hence the magnification.

Hope this helps,

Ken

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Hi Ken,

I have a terrible head for figures but in principle are you saying that the further forward (nearer to the objective lense or primary mirror) that you put the barlow, the longer it makes the focal length of the scope?

Is there an approximation that would apply. ie:. that a 2x barlow effectively becomes a 3x barlow so that I know what mag I am viewing at?

Regards,

Mark

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Yes...basically.

When the barlow is placed close to its focal length inside the prime focus, the further the final image is away and the greater the magnification.

Based on the "average" barlow and the "average" star diagonal; using it with the diagonal will turn a x2 into a x3....on "average";)

Ken

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The way to understand it is that the barlow takes the light cone and bends it outward "a bit". This means that when you place the barlow further away from the lens, you are showing the lens less of the area of the image to focus - therefore it magnifies (and gets darker as you have fewer photons hitting the lens).

Televue powermates magnify the image andf then have a correcting lens in to make the light parallel again (or very nearly). This is why they have a pretty constant magnification irrespective of distance from the sensor/eyepiece....

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