Ah yes i understand now!
"Quaoar" wrote in message
...
pete wrote:
I hope this post isnt off topic but could could one answer my
question .

The aperture off the Etx is 125mm and of the newt 130mm the Meade
etx which im getting hopefully this week is a thousend and a bit
pounds.
My newt cost 200 pounds .
Ive been advised not to buy the Etx 125 as the results from my
reflector would be better as it has a bigger, if only slight aperture
.
So is this true ? im confused !
I also own an Etx70, now ive only had it a couple of weeks but the
results from that seem much better than from the reflector.

Please Help !

Since the SC 125mm has a central obstruction of diameter d for the
secondary mirror, there is some loss of d*d/(125*125) fraction of
incident light. d is about 37mm (AIRC) which is about 9% loss. There
is additional reflective loss at the corrector lens and the secondary
mirror which are small, but which the reflector does not have. The
captured light of the SC relative to the reflector is
0.91*125*125/(130*130) = 0.84, less the additional reflective losses in
the SC.

These losses are offset by, IMO, the additional flexibility, ease of
setup, and viewing pleasure of the SC compared to the reflector.
Flexibility includes attaching filters and cameras. The SC is
considerably more expensive, and for the price you can get a really nice
but awkward large diameter (10-12 inch) dobsian, which has really
superior optical performance, but low portability, with big cash left
over. The 125mm SC are not quite capable of resolving much more than
the polar caps on Mars; color and the red spot on Jupiter are wanting,
and this is in excellent high altitude (1800m) seeing. One needs an 8
inch SC (200mm) for these. The SC is decent for lunar and terrestrial
observations and as a so-so lens for either objective or prime focus
photography/astrophotography.

IMO, IMO. YMMV, of course!

Q