comparable factors

I'm not joking when I say, I haven't spent more time on any one site, I almost know you all already.

I first bought an etx 125 about a year ago and can easily spend a few hours in awe. I remember the first thing I'd said when I first laid eye on saturn, that the etx paid for itself then & there.

I've since bought a 10" donson skywatcher, first night out, only a few night ago, I was in 'double awe'!!

I have no idea around the constellations & or other, I think i was looking toword or at the ursa minor. after a while, I browsed the sky, came back to it again & again. I was never so cold in all my life. Again the dobson paid for itself on the night.

So what am I to think, how do I compare the 2 scopes, while I read the optics are far better on th etx, the 10" offers twice the aperature at half the cost.

I'd apreciate your suggestions on comparable factors, so as I may actually base the next scope on something other than aesthetics.

My own favourite is a small wide field ED refractor that gives tack sharp images of the Moon and is a very handy daytime spotting scope. For dimmer wide field views I have a six inch Newt and for the planets an f/9.8 4 inch refractor. I also use a C5 Spotter as both spotting scope and camera telelens, which is something the ETX 125 would also be suited to.

I don't think that there is such a thing as a perfect scope but I'm not surprised that you're so impressed with the 10 inch because it's mirror has about pi times the surface area of the ETX's primary with which to collect light. Personally I don't get in enough observing time to justify investing in anything bigger than what I have but the fact is that having allowed for all other factors aperture wins out.

The ETX125 is an f/15 Maksutov (Mirror and corrector lens) while the SktWatcher Dob is an f/4.7 reflector. Ignoring the difference in aperture (the 10" is twice the diameter and collects about four times as much light) you should notice that the 10" f/4 gives you a much wider field of view. You will probably also notice that the sky background is not as dark - this happens with faster scopes. While the 10" f/4 will give you pin-sharp images at the centre of the field of view, you will notice that the stars become progressively more 'soft' and less sharply focused towards the edge of the field. This is not a problem if you are looking at faint fuzzies! It does become a problem if you are looking at the planets or double stars - you simply have to keep the object in the dead centre of the field of view to get the sharpest focus.

The ETX125 f/15 probably had a relatively narrow field of view and was probably excellent for looking at the planets, better contrast and a darker sky baskground. This is down to the larger focal ratio. You may also have noticed colour fringing around bright objects at the edge of the field of view at high power - this is caused by the thick front Maksutov corrector plate, which is why the system was designed at f/15... to minimise this problem!

So, horses for courses! You're not quite comparing apples with oranges when you try to judge an f/4.5 against an f/15 scope... but you're getting close!

Phil.