[Quark (Trevor)]

Hi Matt,

It takes about 10 minutes in a dark environment for the pupils of your eyes to fully dilate. Following on from that over about the next 15 minutes a chemical called visual purple is formed in the retina of your eye. When the maximum amount of visual purple that your eye can create has formed, then you are dark adapted. Any white light and that 20 to 30 minutes starts from scratch. Even bright red light such as a red laptop screen will effect your dark adaption.

The reason that a higher magnification eyepiece will deliver a dimmer image relates to the exit pupil, that particular eyepiece, will deliver in your particular telescope.

When the calculated exit pupil of an eyepiece reaches the diameter of the fully dilated pupil of your eye, then the resultant image will be as bright as your system can deliver for you. The diameter of this fully dilated pupil will generally be larger for younger people than for older people and may range somewhere between 5 and 7 mm.

The exit pupil for your telescope is calculated by dividing the FL of the eyepiece by the Focal Ratio of your telescope. For instance in my F 4.5 Newt a 31 mm eyepiece delivers an exit pupil of 31 / 4.5 = 7mm. The same eyepiece in an F 12 scope would be 31 / 12 = 2.5mm.

I am going on 59 and do most of my deep sky observing with a series 4000 Meade UW 14mm eyepiece. In my system the exit pupil will be 14 / 4.5 = 3mm. I find the images this produces to be relatively bright and most pleasing, but my eyes with pupils fully dilated probably don't get to 6mm.

From this you can see that with a slow Focal ratio scope, such as an F 12, the exit pupil produced by any eyepiece will be considerably smaller than with the same eyepiece in a telescope with a fast focal ratio. This is why, generally, fast focal ratio scopes are better for astro imaging as you can capture more photons quicker than with a slow focal ratio system.

Hope this helps
Regards
Trevor