Magnification

[Serotonin]

Ok. I know how to calculate the magnification when using an eyepiece. (focal length mm/mm eyepiece). How do I calculate it when using a camera (no eyepiece)? Thanks.

[jgs001]

That's a much harder thing to work out... If I've read and understood the posts on this before it's a factor of the sensor size and pixel density (or something like that). But as a rough guide... a VGA webcam, like the SPC900, is considered, for practical purposes, to have a field of view about the same as a 6.5mm ep, and this converts, roughly into 35mm as having an 8.5x crop factor (compared to 1.6x crop factor of your XT).

The crop factor is a multiple used to get an equivalent focal length in 35mm (full frame) terms. So, a 600mm scope with the webcam is the approx equivalent of a 5100mm lens in 35mm, and the 600mm on the XT is the equiv of a 960mm lens. (I'm not sure of the focal length of the 9.25" so I used the FL of my scope in the example).

[bish]

You can calculate the fov on here http://www.12dstring.me.uk/astro.htm - this should give you an idea. I think (and am probably wrong!) that my webcam is about the same as a 6mm eyepiece and camera a 20mm eyepiece. Can't even remember when/where I got this from so somebody may correct me.

Sorry - John posted as I was typing and knows more than me!

[jgs001]

... I wouldn't bet on that Bish... I did a rough guess calculation based on the crop factors and the approx mag of the webcam, and it comes out at about a 20mm ep for an SLR... Having got into photography, and this sort of stuff is normally talked about in terms of equivalent focal lengths and crop factors, I find it easier to think in those terms for the cameras.

[themos]

There's no magnification number for an image. There is image scale (arcseconds per pixel) and field-of-view (in degrees or arcminutes)

To calculate the first you need your camera sensor's pixel-pitch (how far are adjacent pixel centres) and your focal length.

To calculate the second you need your sensors dimensions and the focal length.

Here's a quick online calculator

Field Of View Calculator