Straight Thru and RACI: A Powerful Duo

 A discussion of finderscopes on astronomical telescopes.

In a post by 10 Minute Astronomy, Matt Wedel said this: "Have I ever said how much I hate, hate, hate red dot finders? My first accessory purchase for this scope, after the external battery pack, was a 9×50 RACI"

I asked, "Why do you hate Red Dot Finders? I like them quite a lot (though mostly for small scopes where you don’t need a magnifying finder aside from a low power eyepiece in the main scope)"

The response was an entire blog post. But it can be abbreviated by Matt's original comment response: 

Gregory, that’s an excellent question. Once my answer got sufficiently long, I decided it would be better as a full post, which I am working on right now. But the short version is this:
(1) I strongly prefer not having to crouch behind the scope to use the RDF, versus the (for me) more comfortable “just look down like you are anyway” approach to a RACI finder;
(2) I very much prefer a magnifying finder, for several reasons: it gives an intermediate between naked-eye and eyepiece views, it’s better IMHO for starhopping in areas of the sky devoid of bright stars, and it serves as an observing instrument in its own right, like a mini-richfield scope bolted onto the main scope;
(3) in my experience, red-dot finders are the component of a modern scope with the shortest lifespan; in fact, they’re the only component that usually wears out _at all_. This is based on nearly 20 cumulative telescope-years of experience with the StarBlast 4.5s in the Claremont Library Telescope Program–the RDFs always fail early in the lives of the scopes, so I pull the batteries and rewrite the instruction manual to direct people to use them as peep-sights, and that has solved the problem to everyone’s satisfaction. So at a base level, I don’t trust RDFs because I expect them to eventually fail, and that’s not something I feel about any other piece of kit that I own.

After having a little more experience (my 10" Dob with a Right Angle finder hadn't arrived when I made my first comment, and it had been a while since I'd used the finder on the 8" Dobs at the observatory) with finders and a Telrad, I have some more productive thoughts.

Basically, Finderscopes and Finders serve two completely different roles and like so many things in astronomy, combining them together is a compromise rather than an improvement.

A finder (or better yet a pointer) is a device used to orient the instrument against the night sky. A finderscope is a miniature instrument in and of itself, which may be used for navigating the sky for star-hopping or for detecting easy DSOs and fine-tuning the centering/aiming.

The reason there is a clash between the two use cases is that a finderscope must be as comfortable to use as a telescope--you will be at its eyepiece for as long as it takes to find a faint object in some cases. You would never accept this discomfort of using a straight-thru refractor at a high angle without a diagonal, so you shouldn't have to accept that discomfort in a finderscope. However, a right angle diagonal on the finder spoils the ability to easily line up the finder with the sky by combining the image of the crosshair with the night sky. You're back to sighting down the tube like you have no finder at all.

Now, sighting down the tube isn't the worst thing in the world when you have wide fields of view (including those afforded by a finderscope), but frankly I'm not good at this! I found, with the 10" dob, that I was never quite sure where I was pointed, and the view through the finder doesn't always disambiguate this, making it impossible to spot things directly from the sky.

A straight-thru finderscope is seldom used, in my experience, as a navigational aid or secondary instrument, almost always as merely a pointing tool. To waste an 8x50 finder on straight-thru viewing seems pointless--a diagonal must be used.

To point the telescope against the night sky, ergonomics must be sacrificed (with one exception.) Even sighting along the tube is ergonomically just as bad as using a straight-thru finder. The difference between sighting along the tube and just a simple peep-sight is that you can trade off ergonomics for accuracy when sighting along the tube, the peep-sight almost forces you to do it correctly.

Unit power finders are the way to go for pointing. Peep sights are basically as bad as sighting along the tube, but show a bit more sky. Straight-thru finderscopes have a specific eye relief distance which constrains ergonomics. A red dot finder (or other red reflex finder like Telrad or Quikfinder) has sort of infinite eye relief. As long as you see the dot, you could be looking off center, your eye could be right up against the casing, or it could be far away, and it'd basically function the same.

There are other unit power designs, but only reflex sights and the occasional peep-sight are found in commercial telescopes as far as I can tell.

Historically there was something like the distinction I make: finderscope vs pointer was sort of analogous to guidescope vs finderscope back when all astrophotos were manually guided. But I suspect in those days they could have really used a good unit finder and a diagonal on their finderscopes. The guidescope had to have a diagonal since you'd be looking through it for an hour or so--you had better damned well be comfortable!

From my experience, I can say that having a Telrad plus a Right Angle 8x50 finderscope is a winning combination. The ergonomic problems of the straight-thru pointing don't last long--just enough to get you oriented and certain of which star you've aimed at. Then you can relax at the diagonal eyepiece of the finderscope until you've found what you're looking for.

Using a right angle finder alone is always frustrating. Using a straight-thru finderscope alone is always frustrating. Using a unit power finder alone can work wonders in small scopes like Tabletop Dobsonians, or small refractors like the TravelScope 70 and ShortTube 80, it's not enough for me (and likely for most beginners) in big dobs. (Which is why their inclusion in Orion Dobsonians is probably a mistake)

For Dobsonians, if you can only have one, I'd say probably go for a nice 8x50 RACI finderscope. For scopes with focal lengths shorter than perhaps 600mm, a red dot finder is better.

But then again people can and do get away with using Dobsonians with just Telrads. Telrads give you a little more information than RDFs since their degree bulls eye can be used on star charts for more accurate positioning, but what they do is not fundamentally different from what an RDF does.

I will say, there's nothing great about a red dot finder in particular, just that it's the cheapest and most popular unit power finder around. But I will make a bit of a defense of the RDF: Though Wedel never went into detail and it's possible he knows this, of the three times two standard RDFs of mine have failed on me, it was a case of the screw holding the switch on being too loose. Tightening the switch screw fixed one, and just applying inward pressure to the switch dial fixed the other.