What power Barlow with a webcam?

I have a 6" LXD75 scope and I've just bought an SPC900NC webcam, which I'm going to modify for long exposure. Can anyone recommend what power of Barlow can I use with this combination? The maximum recommended magnification for this scope is 300x. So, since the focal length is 762mm, the smallest eyepiece will be 2.5mm.

I've read that the SPC900NC is equivalent to a 6mm eyepiece. Does this mean that a 3x Barlow would be too powerful or do different rules apply for webcams? Because of the range of eyepieces I've picked up, a 3x Barlow would be more useful for visual observing than a 2x.

Thanks for your help.

magnification is focal length of the scope divided by that of the eyepiece/camera. I believe the philips webcam is 4.5mm so 762/4.5 = 169x roughly.

a 2x barlow will literally multiply this by 2, so i think that with a 2x barlow you will get the max ( slightly above it) from your scope with this camera.

please someone correct me if im wrong in this.

Thanks Fintan. It looks like I'll have to buy both a 2x and a 3x then.

Noel,

The current fad is to image using 0.1 to 0.2 arc-seconds per pixel for solar system work and around 1 arc-sec per pixel for DSO work.

I suggest you compute your image-scale as it is right now with the webcam at the prime focus of your telescope and choose then a barlow accordingly so as to be in the neighbourhood of the above image-scales.

Noel,

Here is a handy calculator for working out arc-secs per pixel etc.

www.aozc64.dsl.pipex.com/software/formulae.htm

BTW the pixel size in the SPC900NC is 5.6 microns. - so:

Arcsec per pixel = (5.6/Focal Length of scope) x 206.3

I typically use a 3x barlow, 2x if the seeing is bad.