Meade LX200 10" f/6.3 Classic
Celestron Nexstar 8SE
Celestron 90mm Mak Meade 90mm f/8.8 Refractor
Celestron CR-150-HD6 ,6" f/8 Refractor
TASCO 60MM X 900 MM Refractor (My first scope that my wife gave to me for Christmas in '91 , still have it.
Celestron CG5 Advanced Mount
Celestron Bino Viewer
Orange Zone 10'X12' Roll Off Roof Observatory Southeast, South Dakota
Quote:The exit pupil is calculated by dividing the magnification into the aperture.If you use the 20mm eyepiece in the 8" SCT, you get about 101x (assuming that you are using 1.25" diagonals because it will be higher than this if you are using 2" diagonals). This will give you right at a 2mm exit pupil.In the 254mm scope working at f/6.3 (again, assuming a 1.25" diagonal) the focal length is only 1600mm so the 20mm eyepiece gives 80x.Not only is the magnification lower, but the exit pupil is now 3.175x That is a pretty huge difference.The target is going to be smaller in the 10" f/6.3 scope, and also brighter, but because the sky brightness is also much higher, the view can appear washed out.To make a fair comparison, you would need to use the 10" scope at 127x (for a 2mm exit pupil) or the 8" scope at 64x.I suspect though that the difference you are seeing is because of the difference in sky brightness due to the exit pupil size. The brightness of the sky I think is the proportional to the area of the exit pupil A bigger exit pupil means a brighter target, but a brighter background as well.
Friends call me Duane. Compustar C14, Leo Henzl's Custom C8, 6" Refractor Adv. GT mount, 6" F5 Omni XLT Newt., LXD-75 F4 Imaging SN8, Meade 8" F6 Newtonian, EX Dynamax DX6, RV-6 ETX-90 Astro, Meade 2045 4" SCT, B&L 4000 Vixen/Celestron 80mm F11 JC Penny 60mm AZ/ALT Refractor Binos 25x100