Archive for the ‘Finders’ Category


Why I prefer RACI finders to straight-through finders and red-dot finders

September 26, 2020

My Orion 9×50 RACI finder mounted on my son’s XT4.5, at the Salton Sea.


A commenter on the last post asked why I hated red-dot finders. I wrote a short comment by way of explanation, but I realized that I had a lot to say on this issue, so I’m coming at it again with a full post.

Right off the bat we need to get something straight, and that is the difference between facts, on one hand, and preferences or opinions on the other. If I say that red-dot finders tend to be smaller and lighter than magnifying finders, or that straight-through and red-dot finders require users to put their heads lower than right-angle finders, those are statements of fact. I may be right or wrong about them, and if I’m wrong, I expect to be corrected. If I say that I don’t mind the extra weight and bulk of a magnifying finder, or that I hate crouching to look through a finder, those are my preferences. I can’t be wrong about them, any more than I can be wrong about liking egg salad and long solo drives. If you have different preferences, great! You can’t be wrong about them, either. It’s a big world, and there’s room for all kinds of preferences. Where we often get into trouble is when we (a) mistake preferences for facts, and (b) expect others to fall into line with our preferences. Maybe egg salad and RACI finders just aren’t your cup of tea. That’s fine, I can’t tell you that you are wrong.

So I will try to be as clear as possible in this post between the objective facts and my subjective preferences, and I expect to be called out if I get any facts wrong, or get any of those facts and preferences on the wrong side of the line.

With all that in mind, here are some issues that arise when we consider finders:

  1. Straight-through versus right-angle viewing
  2. Magnification
  3. Size and hassle
  4. Longevity

1. Straight-through versus right-angle viewing

The main difference in preference here seems to come down to just a handful of things: with a straight-through finder, whether magnifying or non-magnifying, you’re looking in the same direction as the telescope tube is pointed, which some people find more intuitive, and if you want you can use both eyes and overlap the image through the finder with the image through the other eye, whereas with a right-angle finder you’re looking in the same direction as the telescope eyepiece, which typically requires less head and body movement, and less crouching to get your head down behind the finder.

Sighting down the tube of my Apex 127 to get the finder aligned, using the top of a palm tree as a target.

One criticism that might be leveled at right-angle finders is that people using manual mounts often end up sighting down the tube anyway to get the scope in the right neighborhood, and as long as you are down there with your face against the tube, you haven’t actually saved yourself any ergonomic benefit over looking through a straight-through finder. My counter is that I’ve gotten good enough at “shooting from the hip”, just aiming the scope up at the patch of sky I want without sighting down the tube, that I can almost always put the target somewhere in the field of view of a 9×50 RACI finder. So I hardly ever sight down the tube. I realize that’s a personal experience and preference, not a universal one. (You can also get away from sighting down the tube by using a laser pointer as a finder, as discussed below.)

I prefer to sit when I observe, and I often observe things when they are high in the sky, and there’s no getting around the fact that if you do those two things, using a straight-through finder means crouching or kneeling on the ground. The only ways I know of to get around that are (1) putting the scope higher, which means either standing or using a really tall observing chair, (2) not observing near the zenith, which means giving up the darkest part of the sky, or (3) using go-to, where you only need to use the finder at all during setup, assuming the alignment is good and the pointing is accurate.

2. Magnification

Some people prefer magnifying finders, and others don’t. I prefer them, for a few reasons. First, a magnifying finder offers an intermediate step between naked-eye and ‘main telescope’ viewing, which I find especially useful for star-hopping. Particularly in dark areas of the sky, or under light-polluted conditions where big areas of the sky have all the naked-eye stars wiped out of visibility, I often need something between my eyes and the scope to help me get on target, even with the aforementioned sharp-shooting. Not everyone does, so that counts as a preference.

The Apex 127 with a little SV50 mounted as a luxo-finder and mini-RDF. The wide-field, low-power views in the little refractor complement the narrow-field, high-power views in the larger Mak.

I have also come to appreciate using a magnifying finder as an observing instrument in its own right. The finder offers a larger true field for big objects that might not fit in the FOV of the scope, and under sufficiently dark skies the finder serves as a miniature rich-field scope. From the Salton Sea, Anza-Borrego, and dark places out in the Mojave, Utah, and the Oklahoma panhandle, I’ve seen virtually all of the Messier objects in a 9×50 finder. As an enthusiastic binocular observer, I think of a 9×50 finder as a 9×50 monocular that I can park, and which I don’t have to worry about holding steady. I also find a magnifying finder useful for educating newcomers—I can have them compare the views in the finder and the main scope to learn about the tradeoffs of light gathering, magnification, and field of view.

3. Size and hassle

Here the point goes to the red-dot finder, and in fact most non-magnifying illuminated dot or ring finders. Even Telrads are voluminous but not heavy. So if size and weight are considerations, the RDFs come out ahead.

Also, dot- and ring-finders can typically be aligned with just two or three adjustment knobs. The spring-loaded finder brackets used for a lot of magnifying finders these days are equally easy to adjust, but the older two-rings-with-three-screws-each setups can be pretty tricky by comparison. They’re not exactly hard, especially once you get used to them, but they have a slightly steeper learning curve over the very simple alt-knob-plus-az-knob alignment of most red-dot finders.

I’ll say this for RDFs, they are much less hassle when you’re trying to balance a scope.

4. Longevity

Red-dot finders require batteries. So do illuminated magnifying finders, but non-illuminated magnifying finders don’t, so that’s a slight convenience win for them. More seriously, 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 (although reflectors eventually need their mirrors recoated). We use Orion StarBlast 4.5 scopes in the Claremont Library Telescope Program, which has been running so long now and with so many scopes that I have about 20 cumulative telescope-years of experience with that model. The RDFs always fail early in the lives of the scopes. After a couple of years of trying to fix or replace them, I started pulling the batteries and rewrote the instruction manuals 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 or work with.

I see you, red-dot finder, but I don’t trust you.

The third way: what about laser pointers as finders?

I have to admit, I use a green laser pointer a lot as an auxiliary finder. Several manufacturers make GLP mounts that can be precisely aligned with the main scope. I tend to just pull my GLP out of my pocket and slap it alongside any straight edge on the telescope, mount, finder base, or focuser, and that’s good enough to get the 9×50 RACI on target–or the whole scope on target, in the case of the SkyScanner, which I modified with a bespoke wooden trough to hold a GLP. On my other scopes, I can do without a mount for the GLP precisely because I use a magnifying finder, which takes over where the rough laser alignment leaves off. I can certainly understand, though, why some people prefer permanently-mounted GLPs as finders, and if I didn’t like using mine freehand so much—or if I was willing to buy a second one—I’d be awfully tempted to do the same. I wouldn’t give up a magnifying finder, though, so I’d have to dual-mount.

Cheap GLP in the wooden trough I built for my SkyScanner 100. I got the idea from fellow PVAA member Ken Crowder, who had done something similar for his 8-inch SCT!


In summary, I don’t like the ergonomics of any straight-through finders, red-dot or otherwise; I don’t trust the longevity of the cheap came-with red-dot finders, and I’m unwilling to invest in better ones because of the ergonomic issues; I strongly prefer to have a magnifying finder as a second telescope mounted alongside the first; and I don’t mind the extra size, weight, and hassle that comes along with 50mm finders in particular. In fact, if 70mm and 80mm finders were cheaper, I’d probably have them sprouting from my telescopes like mushrooms. A 9×50 RACI finder was the very first add-on I got for my first telescope back in 2007. I’ve moved it around as needed to almost every other scope I’ve owned, so I’ve been using one for so long now that if I don’t have one on a scope, it feels like I have a limb missing. (The SkyScanner 100 gets a pass here, because with a 32mm Plossl in the focuser, the scope itself serves as a 12.5×100 right-angle finder, albeit with an inverted image, and the same goes for other super-widefield scopes like the Bresser AR102S.)

Every one of those things is a preference, not a fact. Obviously many other folks have other preferences—just witness the devotion to Telrads, which IMHO are about 10 times as large as they should be to do what they do. And using my new NexStar 8SE is gradually eroding my hate for RDFs. I have to grudgingly admit that the RDF is a handy tool for the initial 2-star alignment, especially because I tend to select alignment stars near opposite horizons, so I don’t have to crouch too much to use the RDF. I still do the first round of centering using the Celestron illuminated 9×50 RACI that I bought from Doug Rennie, and I check the finder on about half the targets just to see what they look like in the smaller instrument. But the RDF is clawing for a place in my kit, and possibly when I’ve used one as much as I’ve used a 9×50 RACI, my preferences will have changed.

What do you roll with, and why? The comment field is open.