Magnification when imaging

[Jove]

Hi all, I have a quick question regarding imaging using a webcam/ccd camera.

When observing visually I can work out the magnification by dividing the focal length of the scope by that of the eyepiece. If I were to use a camera like one of the Imaging Source DMK cameras then, if I understand, I would not have an eyepiece - so how do you tell how big the image will be, and how do you make it bigger ? Do you have to use barlows (or something similar - is this what powermates are for?) to make something appear larger? Similarly, is it possible to do something to get a low magnification and wider field (e.g. to image moons as well as planetary disc or a few degrees around a variable star) ?

Thanks & Clear Skies

[Merlin66]

When imaging we use the effective f ratio and focal length to measure image size.

Barlows or Powermates are often used to increase the focal ratio.

CCD calc is a useful package to verify the size of various objects on the CCD chip.

The New CCD Astronomy Home Page

Ken

[michael.h.f.wilkinson]

The same really holds for (D)SLRs, they only give you a focal length. Magnification is usually given (if at all) with respect to a standard lens (50 mm in the old days, and on top-end DLSRs). Thus on my old Contax RTS-III, I did planetary photography with eyepiece projection (the alternative to barlowing) at 17,500 mm focal length or 350x magnification with respect to a 50mm.

I get the equivalent resolution on my SPC900NC at 4,000 - 6,000 mm using a 2x or 3x Meade TeleXtender (similar to TV Powermate). I have also used EP projection for this.

The reason the apparent scale is the same, is that the imaging surface of the 35mm camera is HUGE compared to that of this CCD chip.