I'm considering a finderscope for my 70mm and 90mm refractors. Am tossing up between the GSO multi-target RDF and a 50mm optical finder.
While the 50mm is probably overkill for these scopes, my experience of unit power RDFs is limited to a Televue Starbeam a few years back, and I wasn't particularly impressed. I always felt that the position of the red dot was rather vague due to head movement/parallax, since there is no eyepiece to align your eye with. It didn't seem a whole lot more accurate than sighting along the tube of the telescope, then using a low power eyepiece. And of course the Televue is expensive.
Would anyone using a GSO RDF care to share their experiences on the issue of 'accuracy'? Since they're about the same price as an optical finder, it seems like the optical finder would be the way to go, but on my 70mm scope it may cause balance issues since I'm just using a camera tripod.
I'll be taking delivery of a RDF hopefully tomorrow morning. I'll be piggy backing it to a 76mm reflector, 300mm fl, and using it in turn as an over size finder. This monstrosity I'll be mounting onto the tail end of my 17.5" dob. The dob also has a 50mm straight through finder near the focuser.
I find that unless I'm using a 50mm finder in Sydney's eastern suburbs (home), I really struggle to pin down targets. A 30mm finder doesn't cut it here.
At a dark site, a 30mm does just fine!
The reason for the finder piggy back assembly is three fold: I don't need to climb the ladder to do the first gross find after a big move of the scope; the RDF will give me (I hope) the straight through sighting I most feel comfortable with; the 76mm reflector with cross hair EP will be used for fine aiming & quick scope nudge when showing people the sky & not need to have them come down for me to do the 'nudging' to recentre the target.
I'd suspect that if most of your viewing is from Sydney's east, a 50mm finder may be more the go. Out in the sticks, an RDF.
Here is Bojan's finder set up: a 50mm straight through he made with a RDF piggybacked onto it. It is this arrangement that inspidered me to come up with the assembly I have, , thanks Bojan.
For fast slewing or star-hopping (off a map) quickly to a point you estimate in the sky, the RDF is a real boon at 1:1 - mainly because you don't have to be looking through it to see where you are going. While you have an estimated position within in a field in your external sight, you can move the red dot into view without averting your vision and losing your objects estimated position. You can keep your eyes on the object area while you're slewing, and just move the dot into your external view to coincide with it.
I much prefer them to a Telrad which do their level best to obliterate the target with 3 concentric circles that just plain get in the way. They're ok if you use them in reference to a star chart that utilises their FOV for relative positioning, but for everything else they're a real pain - and overly large. A nice dim single dot is my fave - especially if you then have a straight-through 50mm finder as well to give you 7x mag when you get there.
I do a rough point with a red dot and then center the star with a 10X60mm stellarvue finderscope.
the two step process always seems the quickest way.
BTW, the red dot is mounted on the top of the front scope ring. I take the scope out of the rings to set up and pack up, yet the red dot never seems to need re-alignment.
, thanks Bojan.
