Maximum exposure for pin-sharp stars?

[the lemming]

It would seem that I've been reading too many articles about the maximum exposure time in relation to a lens focal length to get pin-point stars and now my brain hurts.

The most recent article I read suggested that the rule of 500 or 450 was originally designed for full-frame cameras with 35mm film was no longer accurate for modern cameras. The article stated that with the advent of dSLR sensors pushing 40-50 mega pixels and modern lenses which are better designed the old 500 and 450 rules show imperfections in image quality when pixel peeping.

Am I reading too much into all this and best to stick with the 500 or 450 rule?

[Seelive]

At the celestial equator, where the relative motion of the stars is the greatest, they move at 15 arcsec/sec. Using a lens with a focal length of F mm and a camera with a pixel size of P um, then each pixel represents approximately 200 * P / F arcsecs. Thus the rate of movement of an equatorial star on the image will be (15 * F) / (200 * P) or approximately (0.075 * F / P) pixels/sec.

So, for example, with 50 mm lens and a camera pixel size of 4 um, an equatorial star will move at around 1 pixel/sec. Depending upon how many pixels of movement you find accepable will determine the maximum exposure time.

This website has a more 'accurate' method of calculating the exposure time based upon lens the focal length, F number, camera pixel size and the target declination:

http://www.sahavre.fr/tutoriels/astrophoto/34-regle-npf-temps-de-pose-pour-eviter-le-file-d-etoiles

It's in French and I have seen an English translation previously but I can't find it at the moment so if you're not up to speed with French try using a translation tool. Using that website, for a 50 mm F2 lens with a 4 um camera pixel size and a target on the celestial equator gives a maximum exposure time of about 2 sec (which would give a movement of 2 pixels).

Hope this helps.

[jambouk]

500 rule is a starting point, but it is not an abolute guarentee. 

I would conduct your own experiements with the kit you have at difference places in the night sky (due eat, due west, due north, due south, near the NCP and at the zenith).

James