|
JD,
Ultimately with newtonians most are not concened about moderate light losses - the eye is a logarithmic sensor, not linear, and losses of 20%-30% are barely detectable visually.
The main problems created by secondary mirrors are:
A) the degradation of image quality caused by a large secondary; below 20% of the aperture of the primary mirror it's not much of a problem but when it reaches 30-35% - as on fast dobsonians - it really does cause a noticeable loss of resolution.
B) the supporting vanes, if they are straight, this causes visible diffraction spikes around every bright star or planet.
C) contrast - more the lack of it, in newtonians that are poorly baffled and allow stray light to reach the eyepiece.
For these reasons a smaller refractor can often match the resolution of a much larger reflector, as well providing superior contrast.
While a 250mm dobsonian will give a brighter image at low power on deep sky objects such as galaxies, a 150mm refractor will provide better contrast and easily match it for resolution on the planets and bright objects.
|