Barlow and extension tube, estimating effective magnification

[badgers]

Hi All,

I'm about to buy a televue 3x barlow for general use and planetary imaging, but I see that televue charge a fortune (almost £40) for a t-thread adapter.

I was planning to unscrew the lens element from my skywatcher 2x barlow and use it as an extension tube, as it already has the t-thread.

I was wondering whether this might give me too much magnification, I guess over 4x in poor seeing will not be helpful.

Is there a formula somewhere for working this out ?

I guess the sw barlow as an extension tube would be adding about 40mm + whatever the distance to the CCD sensor.

Badgers

[Merlin66]

It depends on the focal length of the barlow lens.

Typically if you double the distance from the lens to the CCD you get 50% change in magnification.

How do you measure the focal length of a negative lens I hear you ask....

Measure the clear aperture of the lens, say 30mm. Draw a circle on a piece of card twice this diameter i.e. 60mm and then "project" an image of the Sun onto the card until it just fills the circle. The distance from the lens to the card is the focal length. Simples!