Barlow - How does yours work ?

[Piero]

For the reader who is wondering why this thread began, here is the link: 

http://stargazerslounge.com/topic/254027-using-a-barlow/#entry2772566

In there there is my comment too.

I apologise for all this noise, I am not an expert in optics as well as in many other things concerning astronomy and non. 

In the other thread I simply wanted to point out that features in a telescope *should not* change when a *theoretically perfect* focal extender is introduced. If this were the case, the focal extender would play a role as a telescope corrector, which is generally not the case.

Whether people prefer to think in terms of 'doubling the telescope focal length' or 'dividing the eyepiece focal length by a factor' in order to calculate a magnification is irrilevant to me, because the outcome is the same.

My 2 cents opinion. 

Piero

[Ruud]

Drawing of how a barlow magnifies:

made with Geogebra

[John]

While looking into all this I came across this quite interesting explanation of why a barlow lens cannot reduce chromatic abberation:

http://www.cityastronomy.com/barlow-lens.jpg

[Mak the Night]

TeleVue have some interesting points on Barlows:

http://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_back

[LukeSkywatcher]

I have read online that barlows technically dont magnify objects. What they do is decrease the distance between object and observers eye.

Isnt that the same thing?.

[Ruud]

I have read online that barlows technically dont magnify objects. What they do is decrease the distance between object and observers eye.

Not really, but what you read is true for a magnifying glass: it allows you to get closer and still see sharp. (An eyepiece like the Plössl is a lot like a magnifying glass)

A barlow stretches the focal plane of a telescope.