Barlows

A barlow simply halves the focal length and doubles the magnification of an eyepiece ,yes? If so, in astrophotography, is it just used to magnify the image comming to the camera?

Hi,
It doubles the focal lenght, that is why it also doubles the magnification, they work very well with webcams and the like, don't know about cameras with wider FOVs like DSLRs or true CCD cameras.

Yep, they're used to increase the image size. They're excellent when used for planetary and lunar imaging but cause a number of issues for long exposure imaging. e.g. say you have an f5 scope and take a shot of, say m51, with a 2 minute exposure.

If you now put a barlow in the path and the scope is effectively f10. In this case you are going to have to quadruple the exposure to get the same level of light in the photo as f10 is 4 times as slow as f5. The issue here being is that it is extremely difficult to get such long exposures without guiding or an extremely expensive mount.

Trev