The Starry Night, 162

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Improving our lot in life.
The judgements of our pier.


2/19/2016. Finally, we're buying land out west. But not far out west. Not far at all. In fact, it's the lot next door, 300 feet west of where I sit. That project has consumed much the 2016, but it seems well under control now. I deeply regret pouring so much time and money into buying still more land in North Carolina when the same time and money could have bought acres of dark skies Out Yonder, but it can't be helped. We have way too much invested here to let its peace and privacy be compromised by someone trying to shoehorn a spec house into the sole potential building site on the lot adjacent our "estate." And since the owner of that lot has dropped the price by 70%, God knows someone would try. So we had to buy it. We've arranged to clear the easements on it (which only we seem to know about) so that we can use all of it for viewscaping, privacy, and woodland trail-building. It increases our lake frontage (beaver pond frontage) incrementally but our peace of mind considerably.

And now... putting the old pier from ETSU's early observatory into service to free up some Losmandy parts for travel has been a lovely diversion. I've reverse-engineered A-P's portable piers from photos for reassurance about placement of the struts. I aim to attack this steel monstrosity with a drill (or drill press) tomorrow. It needs guys to improve rigidity. I picked up the components (steel rods, turnbuckles, hooks, eyebolts) from Granite Falls Hardware after signing off on our refi.


2/20/2016. Guy rods in place! With all the fittings, the rods needed to be cut down from 36 to about 32". I ended up carting the drill press out into the yard to put holes in the pier (so steel hooks could grapple it) and holes in the three legs (so hooks could be bolted in place for the eyebolts on the turnbuckles to engage). This pier has rung like a bell every time I put a reasonable mass on top (such as the Mach1 with even a small OTA). With the 1/4-inch steel rods in place and tensioned with the turnbuckles (a 7/16" crescent wrench is the magic size if anyone's asking), it feels vastly more solid. We won't really know how effective this has been in unringing the bell until I add counterweights and a 'scope to the load and have a look under the stars, but by tapping, leaning, and shoving it with and without the Mach1 mount in place, I believe it has been improved, and not by just a little. If the pier needs to be shortened, I can lop 8-10 inches off (in a shower of sparks and a shriek of metal) without changing the attachment points of the struts.




Last night I put myself to sleep thinking about vectors and resonances. So now I am going to offload all those thoughts here. (That's fair warning to skip ahead.)

As acquired and as I've tried to use it so far, the pier is similar to the drawing at far left above (the ball up top represents whatever mount and OTA is attached at any given time, the red arrows represent forces working to bend the pier and introduce vibration). Mounting the guys to the pier at 45 degrees was my first thought (center), but given the length of the legs, which determines the maximum distance from the pier that the guys can be attached, that leaves a lot of the pier extending above the guy attachment, plenty of room for vibration and bending to develop. So what if anything seems so special about 45 degrees? Zero or 10 or 30 would be even more resistive, but impractical (the base would be immense and the undamped pier would be even longer). So why fixate on 45? How about a 30-60-90 triangle? Or just something close to that (far right). Look, you don't need all the strength of the guys to resist vibration. All you have to do is resist motion in one direction with tension in another. A small part of the total strength of the guy is sufficient. If 45-degrees puts 70% of the strength of a 1/4-inch steel rod into play, it's a gracious plenty. 60-degrees would still provide 50% which is still far more than needed. So raise the attachment point until the part of the pier above the guys is short enough that practically no bending can occur. By all means, attach the guys as far out on the legs as possible, but don't go crazy optimizing this and that.

What's the source of any bending/vibration anyway? Well, there are wind loads, but we practically never have discernable wind down here at the base of the pines. I suspect the torques imposed by guiding corrections would be the ones that matter. They're small angular motions, but they're accomplished in very short times: they represent significant rotational accelerations of appreciable mass and moment. Real forces are involved. An arc second or two of motion is commanded and accomplished in milliseconds; and the impulses are repeated every several seconds. That sort of thing can leave a mark in the form of continued ringing or vibration. You don't want the guider reacting to star motions it itself imposes --that ways lies madness, or at least hysteresis. And even if that bugaboo doesn't strike, slow settling after every correction will produce enlarged point spreads. Also focussing will be much easier if touching the instruments doesn't set up long-lasting oscillations. For all those reasons, making the pier as rigid as possible practical should pay dividends in the finished images.

2/26/2016. I outwaited rain and wind and applied a coat of Rustoleum semi-gloss black. Now the pier is again a piece of official astronomical equipment.



Except where noted, deep-sky photos are made with an SBIG ST2000XM CCD behind a 10-inch Astro-Tech Ritchey-Chretien carried on an Astro-Physics Mach1GTO. The CCD is equipped with Baader wide- and narrow-band filters. The internal guide chip of the CCD most often keeps the OTA pointed in the right direction (I'll let you know when an OAG or guidescope takes its place). Camera control and guiding are handled by Maxim DL 5.12. The stock focuser on the AT10RC has been augmented with Robofocus 3.0.9 using adapters turned on the lathe downstairs. A Canon 6D and a modded 50D find themselves mounted on an Orion 10" F4 Newtonian or carrying widefield glass on an iOptron Skytracker. Beginning in May 2013, PixInsight has taken over more and more of the heavy lifting -- alignment, stacking, gradient removal, noise-reduction, transfer function modification, color calibration, and deconvolution. Photoshop CS4 et seq and the Focus Magic plugin get their licks in, too.


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                   © 2016, David Cortner