Get the lead out - ehhhh . . . in
What weighs more - a pound of lead or a pound of sand?
Ouch - ok - I'm retreating to the first grade - sorry ;-) What I mean to say, is what weighs more, a tube of fine beach sand, or the same tube filled with lead shot?
If you said the lead shot tube, you're right - but I'm not sure by exactly how much. Feels like at least double, maybe triple, though. Problem is, my scale won't weight it. Why is this relevant? Because a Dobsonian telescope - when finely balanced - moves like the proverbial hot knife through butter. It's a pleasure to push it any direction - with the light touch of a finger or two - and have it stop and be steady as soon as you stop pushing.
But - add some weight up top and the beautiful balance is destroyed and the telescope becomes a sluggish, even clumsy, beast. And I'm adding a lot of weight up top. The Obsession 15 is dsigned to be used with a single eyepiece weighing perhaps half a pound or a bit more. I use two eyepieces in a binoviewer that includes some additional optics as well. The whole thing weighs in at nearly 3.5 pounds. The Obsession instructions suggests that would mean I needed to add an extra 15 pounds down low to restore the balance. Fortunately this isn't so. But while I'm sure I didn't add 15 pounds I don't know how much I did add because I dont have a scale. My guess is between 7 and 10 pounds.
Obsession sells a kit that includes three aluminum tubes, end caps to seal them, and three sets of brackets to mount them neatly on the housing behind the mirror. The catch is, one of the tubes can be placed so it's easy to put in or take out. Just sort of clips in place. The other two are semi-permanent. So I was hoping I could get away with one tube. That way if I shift to a single eyepiece for part of an observing session I can easily rebalance.
The advice is to fill the tubes with either sand or lead shot. I tried sand, lead shot being in short supply, and one tube didn't come close to doing it. In fact, it was so light I was afraid three tubes of sand wouldn't do it. So that sent me scurrying for lead shot and in my part of the country that is not easy to find. Walmart doesn't carry it and gun shops are few and far between here abouts. In fact, there's only one within a 45 minute drive for me, so I tried it first.
It's out to be a little shack on the main state road in Westport and when I went in past the out-dated Kerry hate signs - just cause you live in ablue state doesn't mean there aren't some "red" folks around - the old guy behind the counter told me he didn't have any shot and it was a "pain in the ass" to sell any ways. I think he said he'd order me some, but I didn't want to wait several days, so I left. A short ways up the street I decided maybe BB shot would work. Surely he would have BBs. So i returned and he did. But as I hefted a small container it didn't feel very heavy and it wasn't cheap.
How about .177 caliber pellets? I asked. They're at least lead, but an odd shape so not that many would fit in the tubes. You want something that uses up most of the space. And they were pricey, too. Then the old guy disapeered into the back of the store and later emerged with an old, dusty jar full of shot. I hefted it. Heavy! "You can have it for five bucks," he said.
"Fine," I replied."But you got any more. I don't know how much this weighs, but it doesn't seem like 15 pounds. Too bad you don't have a scale."
He vanished a second time, this time emerging with two very dirty, dusty plastic jars, each with some shot in it. I thnk this stuff had been sitting there since the shop opened half a century ago. "Ah, that should be enough." I said. He scooped them off the counter and meandered to the back of the store once more. "Got a scale," he said over his shoulder. Well it was something over 15 pounds total and it cost me $10 - not bad. I helpe him clean out his store, too. Curious how it materialized, though - maybe persistence counts ;-)
I filled up one tube outside - following Dave Krieger's wise advice. ("Shot is hard to vacuum up," he had written. ) It took the whole first jar - and it did the job. In fact, I may have to remove a little.
So - maybe I'm ready to tackle collimation. And if I have to add more weight - well, I got more shot. If not - who knows. Maybe I'll take up SCUBA diving - that apparently is the other market for lead shot. Divers put it in waterproof pouches they wear.