Not quite clear on barlows and magnification?

[vindictive666 (John)]

hello everyone

"quote
Of course you would still need the barlow to effectively double the number of eyepieces
Another point is to not get any double ups. In the above selection the 30mm will become a 15mm, the 20mm will become a 10mm and the 12mm will become a 6mm. That way you are not spending money on "eyepieces" you will not use."



(borrowed the quote from another post)

i realise that when you have a 2x barlow it doubles the magnification, but what i dont really get is the 30mm will become a 15mm, 20mm a 10mm ect ect

can someone point me in the right direction of this (i dont really understand the halving the eps ? why that happens ?)

[iceman (Mike)]

Hi John.

Magnification is determined by the focal length of your scope, divided by the focal length of your eyepiece..

eg: your 12" dob is 1500mm FL I think.. So if you have a 30mm eyepiece, the magnification is 1500 / 30 = 50x

A 20mm eyepiece gives you 1500 / 20 = 75x

A 2x barlow lens effectively doubles the focal length of your telescope, so instead of 1500mm FL, it's 3000mm FL. So the magnification of your eyepieces is also doubled.

eg: Your 30mm eyepiece now becomes 3000/30 = 100x
and your 20mm eyepieces now becomes 3000/20 = 150x

However most people talk about it in terms of eyepiece focal length because it's easier to remember that way.

So a 30mm eyepiece with a barlow, effectively becomes a 15mm eyepiece. The 20mm eyepiece with a 2x barlow, effectively becomes a 10mm eyepeice.

So that's why people say, "don't double up". That is, if you're getting a 2x barlow, don't get a 30mm and a 15mm eyepiece, because it'll mean you're doubling up on the 15mm focal length.

Choose focal lengths that give you a broad range of magnifications, with and without the barlow.

Does that help?

[iceman (Mike)]

John I also edited your thread title to make it a bit clearer.

[[1ponders] (Paul)]

I'll have a crack Vin, but remember I've been wrong before

The way I understand it is the when you add a 2X barlow to a scope you are effectively doubling its focal length, not halving the eyepice Focal length. Therefore a 1000mm scope becomes 2000mm. So in the first a 25mm eyepiece will gove 50X magnification and in the second it will give 100X magnification. But keeping the same eyerelief and exit pupil (?) as the original 25mm. The downside is increase optical elements in the optical train.

However for ease of explaination its often expressed as halving the eyepiece focal length, rather than doubleing the OTA FL, simply because we are used to expressing increases in magnifiation as a function of eyepiece focal length.

[vindictive666 (John)]

thanks people that made a it clearer

[ving (David)]

um... glad to be of service

(not that i did anything)

[acropolite (Phil)]

Can anyone please explain how the magnification of the barlow is affected when it is placed on the EP side of a diagonal. I have read somewhere that the magnification of a 2x barlow becomes 3.5x when placed after the diagonal. I have visually observed that extra magnification does occur in this situation.

[[1ponders] (Paul)]

I've heard something similar Phil, so I'll have a guess.

If your scope has FL 1000mm and you place in a 2X barlow then your FL becomes 2000mm (see above ) If however you place it after the diagnal then you have to add the extral length of the diagnal, lets say 100mm (not actual measurement) then your initial focal length becomes 1100mm and your barlowed length becomes 2200mm. I know this doesn't even come close to the 2X a 3.5X . Don't know if this the complete story or even completely accurate, but it make sense to me. I'm sure someone else here is more an optical genius than me and will give a better explaination.

[acropolite (Phil)]

Coundn't find the original mention of 3.5x but I did a search and found a good explanation. It seems that barlow magnification is increased with any extension before the barlow. http://www.astunit.com/tutorials/barlow.htm