extension tube on focusser

[chemtom24]

so from what ive read you can use an extension tube ( literally a hollow plastic tube?) on a focusser to get a bigger image of an object when using a webcam.

wouldnt this effect apply to eyepieces too?

but then how come if i place an ep too close or too far out of the focusser, focus cannot be achieved?

OR, does the extension tube contain a piece of glass which directs the light to a focus point further out than it would have originally been?

[steppenwolf]

OR, does the extension tube contain a piece of glass which directs the light to a focus point further out than it would have originally been?

That's the answer - it's called a Barlow Lens.

[chemtom24]

lol really? dear me, so everytime ive seen someone mention an extension tube, they are really talking about a barlow lens? for crying out loud!!! i already have a couple

[steppenwolf]

You will also see people using an true extension tube (i.e. a tube with no inbuilt lens) and this is often required in place of a 'diagonal' - the extension tube makes up the 'missing' light length lost by the removal of the diagonal, however, this has no effect on magnification.

[chemtom24]

thanks steppenwolf

[michael8554]

A barlow is a "concave" lens, but a "convex" lens inside an extension tube will produce an image that gets bigger and dimmer the further away the camera is placed. This is known as "eyepiece projection" because most commercially available units require an eyepiece to be inserted inside the tube. Home made systems often use photocopier lenses.

Cheers

Michael

[chemtom24]

A barlow is a "concave" lens, but a "convex" lens inside an extension tube will produce an image that gets bigger and dimmer the further away the camera is placed. This is known as "eyepiece projection" because most commercially available units require an eyepiece to be inserted inside the tube. Home made systems often use photocopier lenses.

Cheers

Michael

hi michael, so there is such a thing as i described in my first post, but with a convex lens as opposed to a concave lens?

how easy are they to make yourself? is it literally a case of finding a convex piece of glass small enough to fit inside an empty barlow tube?

[JKB]

Umm So when I use an extension what have I done wrong?

I use a Barlow then Extension tube (hollow plastic pipe). Longer tube = greater magnification (after Barlow that is!).

I feel I have missed something in this thread?

[chemtom24]

Umm So when I use an extension what have I done wrong?

I use a Barlow then Extension tube (hollow plastic pipe). Longer tube = greater magnification (after Barlow that is!).

I feel I have missed something in this thread?

this is exactly what i was talking about except without a barlow. but even with a barlow.. surely just adding an extension tube will move the focus point out of reach of the ep or camera?

arggh so confusing! il have to wait til i can test it out myself but it just doesnt seem right

[chemtom24]

ive drawn a pic to illustrate my point.

in picture 1, the focusser has been moved to get the ep in to focus. in picture 2, the extension tube is too long for the ep to be bale to reach focus. the focus point is too deep in the extension tube.

this was what i was trying to say. how can adding a tube just suddenly make something bigger, when you wont be able to focus it!

[Cornelius Varley]

An extension tube can serve two purposes.

1. If you remove the diagonal from the image path the tescopes might then have insufficient outwards focus travel to allow a camera or eyepiece to reach focus. The extension tube replaces the lost distance of the diagonal.

2. In imaging an extension tube placed between the barlow lens and the camera will increase the effective magnification of the barlow lens.

Peter

[steppenwolf]

how can adding a tube just suddenly make something bigger, when you wont be able to focus it!

It can't unless it is used in combination with a Barlow lens.

1. An extension tube can be used in certain situations to allow you to achieve focus but on its own it cannot magnify an image. A typical use for an extension tube is to make up for the loss of light path length when you remove a diagonal from a refractor.

2. You appear to have a reflector, based on the two diagrams that you have made. It is unlikely that you would ever require an extension tube with a reflector. In fact quite the opposite - on some reflectors it is all but impossible to achieve focus using a camera as you cannot wind the focuser IN far enough.

What are you actually trying to achieve? If you are trying to increase the magnification achieved using your webcam, there are two ways of doing this:-

1. Use a Barlow lens. A 2 x Barlow lens will increase your magnification twofold AND as a by-product, help with inwards travel issues on a reflector

2. Use the eyepiece projection method. Here, you use an eyepiece in the focus tube with your webcam attached to the end of the eyepiece either directly or via a small tube to project the magnified light from the eyepiece into the webcam.

Note the mention of a small tube. Is this the tube you are talking about perhaps? If you increase the length of the tube between the eyepiece and the camera, you will increase the magnification. However, note that it only has an effect when used with an eyepiece to project the light from, again it does NOT work on its own.