It’s a clear, dark, beautiful night. The seeing is off the charts and the transparency is even better than normal. And so far the scope you’re using has failed to split about half the double stars on your list.
Is it the optics? Maybe. But it could also be the focuser.
One of the most over-looked items of essential astronomy equipment — and to me one of the most critical — is a good focuser. Too much tension on the gears is an obvious problem — you can feel it every time you turn the focus knob. Every bit as frustrating — and worse because it tends to go unnoticed — is poor gearing.
If your focuser is geared improperly — as in too fast — you won’t be able to make the very minor adjustments in focus which are required to pull faint secondaries out of the glare of bright primaries, or split very close doubles. What happens is that when you turn the focus knob a slight amount, the gearing is so inadequate that the focuser moves right past the zone of critical focus. And more than likely you won’t even realize it’s happening.
I’ll give you an example. I have a C102 — a 102mm Celestron achromatic refractor — that was performing abysmally on double stars. It seemed like about seventy-five percent of the time it couldn’t split two stars if they were at opposite edges of a wide-field eyepiece. I had just about decided the optics were to blame — except that every now and then it would surprise me and split a difficult double. After a couple of nights of that, I realized the problem was in the focuser gearing. Just by pure luck, I would occasionally land on the critical focus, and it would seem like the scope was performing beyond it’s 102mm limits. But if I de-focused and then attempted to focus once more — or swapped eyepieces — it was usually impossible to find that perfect focus again. The focuser would move right past the critical point so quickly my eyes couldn’t detect it.
So how did I cure it? First, I tried changing the focuser tension by adjusting the four screws on the metal tension plate that pins the focuser to the gears. If you try that, loosen all four screws and then re-tighten them in an X pattern — top left, bottom right, top right, bottom left — just to the point where they become snug. Then rack the focuser in and out a few times and check the screws once more. One or two will probably be loose, and the others will likely be very tight. Take the pressure off the tight ones by loosening them again and then adjust them so they’re snug once more — then tighten the loose ones until they’re also snug once again. Repeat that process several times until the screws remain snug — if nothing else, you’ll at least have a focuser that turns more smoothly.
For a variety of reasons that doesn’t always work. It worked well on an Orion 90mm refractor — but I had no luck at all with it on the C102. So I took a deep breath, ordered a two-speed focuser that cost more than the scope, installed it — and found it transformed a poor scope into an excellent one. Since then, I’ve used that scope to pry more than a few faint secondaries out of the glare of much brighter primaries. Take a look at the posts on OΣ 111 in Orion and Propus in Gemini as examples.
Over the years, I’ve used several achromatic refractors, and almost every single one of them had poorly geared focusers. That list includes Meade’s AR-5 and AR-6, and Celestron’s C102 and C6R. I ended up installing two-speed focusers on all four of those scopes. Without exception, everyone of them became an absolute joy to use when the new focuser was installed.
The beauty of a two-speed focuser — or even a very well geared single speed focuser (Astro-Physics has an outstanding one) — lies in its ability to achieve a critically perfect focus. Astro-photographers are very familiar with critical focus, but I suspect a lot of visual observers don’t realize how critical “critical” is. But if you want to coax that last possible photon out of your scope, you’ll be a whole lot more successful a whole lot more often with a top quality focuser.
To literally “see” what I mean, try this on a dark, moonless night, assuming you have a two-speed focuser, or a very good single speed model. Point your telescope at a star field that is reasonably dense — not the heart of the Milky Way, and not one with a first magnitude star in the field. A good one to use is the open cluster Steph 1, which surrounds Delta (δ) Lyrae, but almost any field will show something similar to what follows.
Bring the scope to a focus on the brighter stars in the field. Now, look away from the bright stars toward the fainter parts of the field, and turn the fine focus knob very, very slowly. At some point — and it won’t take more than a few seconds, you’ll see a few very faint stars begin to appear. As you very gingerly advance the fine focus knob a bit further, you may catch some others popping into view, especially with averted vision. If you continue to nudge the focuser along, the faintest of those stars will begin to fade from view. And all that time, if you check the brighter stars in the field, you’ll find no change — or very little — in their appearance. And you’ll also find you’ve only turned the focus knob a millimeter or two, if that.
The point at which the faintest stars in the field pop into view and stay there is what I’m referring to here as critical focus. Try that same exercise using only the high speed focus knob, and unless you have an outstanding focuser, coaxing those very faint stars out of the background will be very difficult, if not impossible.
Is a two-speed focuser a necessity on a long focal length refractor? I’ve found they can be a huge help, especially when working at the optical limits of your scope. While the depth of focus is shorter — and consequently more critical — in a short focal length scope (let’s say f/7 or faster), even with the greater depth of focus provide by longer focal lengths, there is a small area of critical focus waiting to be mined.
For example — I was using a 60mm f/16.7 (focal length of 1000mm) on Polaris a few nights ago, and found I was having trouble seeing the 9.1 magnitude secondary. Normally I can pry it out of the glare without too much problem using a 60mm scope, but sometimes poor transparency or too much moisture in the air can make it difficult. I wrestled with the old single speed focuser for a good five minutes, nudging the focus knob along until finally the secondary popped into view. Just a slight nudge past that point — and it was gone again. A two-speed focuser would have saved me a lot of time. Unfortunately, I don’t know of any that are made for 60mm scopes.
So the moral of this whole story is this:
Having a focuser that will allow you to reach the critical focus I’ve described above will very often make all the difference in the world in how well you succeed in splitting difficult double stars — and as a bonus, it can turn a poor performing scope into a real treasure.
Filed under: 2. Observing Tips and Tactics |