[astro744]

Originally Posted by pberrett

Thanks

My telescope has an aperture of 130mm and focal length of 900mm so I presume the calculation is 500/900 = 0.55 seconds. Have I calculated this correctly?

Thanks Peter

This is the maximum exposure length before trailing is noticeable when tracking is not used. Note the 500 rule is only an approximation. Use the NPF rule (search NPF rule) for better accuracy. Note the trailing is less as the target is nearer the poles.

Which constellation is Acheron in? I have never heard of it being a star. Did you mean Achernar?