Barlow and focal reducer primer (very simplified)

[Kaptain Klevtsov]

A Barlow lens makes a telescope act as though it had a longer focal length. This increases the magnification with either a given eyepiece, or a given camera.

A focal reducer does the reverse, making the telescope act as though it had a shorter focal length.

In some instances it is useful to work out the action of the Barlow lens or focal reducer with regard to the eyepiece focal length, but the effect is the same. This is because adding a 2X Barlow to a telescope / eyepiece combination will double the magnification as if a) The telescope focal length had doubled, or The eyepiece focal length has halved. Either way is valid as a telescope and Barlow combination without a camera or eyepiece doesn't do anything, and neither does an eyepiece plus Barlow lens. Only putting it all together makes any sense, so you can do the sums either way.

Function

The focal length of a simple refracting teelscope varies with the strength of the objective lens (the big one at the front). A more powerful lens bends the light more strongly than a less powerful one, so that the parallel rays of light froma star (for example) come together closer to the lens with a short focal length as the rays are bent more. A long focal length telescope bends the rays less strongly so that they converge further from the lens.

Plus focal reducer

A focal reducer is a converging lens like a simple magnifying glass. This goes in the cone of light as it converges after the main telescope lens and adds to the convergence making the rays come together closer to the main lens than is the case without the focal reducer. Many Newtonian teelscopes lack the in travel of the focuser necessary for this to work.

Plus Barlow lens

A Barlow lens is a biconvex diverging lens which undoes part of the action of the main teelscope lens. This causes the rays to be bent less strongly so that focus occurs further from the main lens. To acheive focus it is often necessary to extend the focus tube by adding an extension tube. This is simply a male / female hollow tube.

Using either, it makes a difference how far from the original focus point the additional lens is added. Further away from the original focus point increases the action of the additional lens, so using a Barlow lens before the diagonal mirror increases the effect of the Barlow lens and increases the magnification. Taking the lens off a Barlow lens and screwing that into the bottom of an eyepiece reduces the effect s this moves the additional lens further away from the original focaus point.

As I said, very simplified, as many Barlow lenses and focal reducers are complex designs to avoid chromatic and other abberations.

HTH

Kaptain Klevtsov

[FireForEffect]

Very simplified is exactly what I need! Thanks for that - ideal for a beginners in imaging like myself info like this may feel standard for imagers but if your new to it all, simply priceless! Thankyou KK.

[RichardBasse]

Can I mention that I have had several not simplified explanations and while they have helped me understand that I know little about my new hobby, I still did not know what my expensive focal defuser was for - just that I needed one!

Thanks for your explanation, as a result of finding it and this forum I just registered, have started reading lots of things that I should have known before I went shopping and will be hoping for many more very simplified explanations for newbies like me.

[Probablynot]

Thanks for all that, KK (or HTH).

I had been about to buy a focal reducer, on the grounds that it was supposed to be useful for widening the FOV of a webcam. But then I saw your post.

You say that "Many Newtonian telescopes lack the in-travel of the focuser necessary for this to work".

So would I be likely to be wasting my money?