I spent a couple of days up at Bretti Reserve this week and left the dob at home this time to give my Bresser AR102L, f9.8, 1000mm fl refractor a good go at a dark site. This refractor is identical to the Explore Scientific model with the same AR102 designation, but sold mainly in europe through Bresser (Meade EU). The scope sits on a Vixen Porta II Alt/Az mount with JMI Train-n-Track motors.
Useful magnification is around 200x given the usual refractor edict of 2 times the aperture in mm.
The two Allan's were up at Bretti as well so I had some large dob perspective on the views for comparison.
I was mainly aiming to give the planets a real go: Jupiter early on and then Mars and Saturn later in the night. The views are great for contrast and lack of coma, and of course no need for collimation at all. So in many ways more pleasing a device to use. But as with all but the longest Acrhos there is some CA to deal with, but this was ably handled by a Baader Semi-APO filter, and being a long focal length scope it was not very noticable anyway.
While I could get good views of the three planet targets I was really pushing that refractor. The relative size of the object in the EP just did not compare to that of the dobs, and Maritan detail in particular required at least 182x with the trusty 1mm Nagler. Barlowing just made the image quality worse.
Last night back at home, I got out my 16" GSO Dob and did the same comparsion again. The big dob killed the refractor on every level. It is perfectly collimated, and edge coma is never an issue with planets anyway. The sheer size of the panet in the EP and the brightness could never happen with the refractor because of the small aperture. Object detail was outstanding with the dob, picking up things the refractor could not see. Contrast was on par with the refractor on planets but for pin point stars it is hard to beat a refractor - but there is the rub: not enough aperture to make it really useful for DSO work. The reactor can resolve bright double stars (or triples like Beta Moncer..) well but it has no real reach. Even for widefield star work on clusters and Eta Carinae, the refractor can't provide the expanse of view that the dob has.
Of course it's not a fair comparison I suppose, 102mm verses 406mm gives you heaps more light to work with, and the refractor did well to stay in the ballpark with the dob albeit with a much smaller image to work with at its useful magnification limits.
Not sure where this leaves me with the refractor, it's a great grab and go setup but it leaves me wanting more aperture. I am building a 127mm refractor at the moment but I suspect it will still fall well short of the dob in pure breathtaking observation.
So why do people buy refractors (maybe they live in small apartments), or do mostly guided astrophotography on expensive mounts. But for stunning visual vistas, it's the dob hands down.
