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Looks real snazzy... kind of like the scope should have come with them as original equipment.

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Carol,

Imagine a piece of thin steel - the steel strapping used on lumber or pallets would work great. You simply attach, say, 6 inches of that to the edge of your groundboard with a couple of screws.

Then attach your pointer to a small magnet, and stick it to the center of the steel strip.

When you need to tweak your calibration you simply slide the pointer along this strip of steel strapping until it matches the computer's azimuth setting and you are good to go.

Let me know if that is clear as mud and I will try again.

Rod

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Exactly, just a few inches on both sides of zero will do the trick.

Glad to help.

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This is very informative but let me make sure I have this straight.

The pointer is on the bottom base.
The pointer is aligned with Polaris and the base leveled.
The steel band with magnet on pointer allow for adjustments so you can tweak that alignment by moving the pointer after you test on some known objects. (without having to keep moving the entire bottom base)

If the markings are on the top board and are stationary with the OTA where do you set the gradients to begin with? The zero / zero goes directly below the center of the OTA at the front of the scope? That way you always begin with the scope pointed at the Polar North.

You then match the Azm:159 and Alt:+38 as per your favorite program (my settings at the moment for Jupiter) and your on target or at least in the neighborhood.

Am I keeping up here or did I miss something?

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Or another idea would be . . . calculating the difference between a leveled platform and a maximum swing platform. Then we can use this different to a Alt/Az value from the pocket star.

What do you think about that one?

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Rob, I'm just having a little brain fart here, but I'm thinking that if you're using a tracking platform, wouldn't it be better to use the other set of coordinates (RA-DEC) and set up a different set of degree circles. I confess that I don't know what Ra-Dec coordinates are other than they are used on EQ mounts.

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I just did some calculation on delta between leveled and full swing platform. It does not work. Because the delta is not a consistant number. The object closer to Polaris for example will have a different delta from an object away from Polaris.

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In regard to your inclinometer, I assume when you mount the inclinometer on the tube - it has to be vertical in order to get an accurate reading of the altitude. But . . . when you have a platform swings all the way . . . the inclinometer is not necessarily vertical anymore.
So even using an adjustable mounting, there should be a way to compensate vertical axis.

The RA and Dec method is even more complicated because dobsonian has only two UN-adjustable plane (vertical and horizontal) With RA and Dec, you need to have a 3rd axis in order to make the 2 planes coincide with the true North (Polaris) . . . like the the equatorial mount. . . . Unless I am wrong . . .

Have fun tinkering . . . I am going to sleep on it again . . .

Ridwan

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I use Planetarium on my Palm to look for my objects. There is an option for magnetic azimuth on it, so it should solve the problem of knowing what true north is. My only concern is will the metal OTA interfere with the compass?

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Good question. I ordered the cheap wayfinder from amazon ($17) and will try it this weekend. I don't have a big tube, but do have four long aluminum poles (2" diameter), so it would probably be similar. However, it's supposed to work inside a car or truck, w/ an indicator when bothered by interference from metal, so we'll see.

pete

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Update: Received and installed the Wayfinder v7000. IMHO, it does an excellent job (accurate to 1 degree + true or magnetic North settings). With the $9 Home Depot inclinometer and the Wayfinder 7000 digital compass ($50 refurbished from Overstock.com) the total system is about $60.

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place it against a wall, read, spin it a bit and place it back against the wall, read again

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Oh, and my apologies for folks who ran out and purchased a Wayfinder 7000 and may not be thrilled about it

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Too late!

I had the smart tool for altitude, so decided to try the compass for azimuth. I was hoping for 2 or 3 degrees max, figured it was only $40 if not. I did not quite expect 8 degrees.

At any rate, given both devices, I mounted them on my scope last night and took it out to the driveway. I had more confidence w/ the inclinometer (smart tool), as others have used it successfully. W/ the wayfinder I had several concerns: 1) accuracy, 2) backlight, and 3) directionality.

For accuracy, concerns were justified. I calibrated the wayfinder and then used the magnetic declination setting to zero on polaris. I then tried to use the two to find things, with mixed success. My first attempt, M31, was off by 3 degrees of azimuth. M76 was found in the eyepiece (40mm, about 1.5 degrees). M92 two degrees off. Several stars were within a couple degrees azimuth, but two were four degrees off, and one was eight.

Interestingly, when I finally rotated all the way around, polaris was only one degree off. I'm speculating that the "map" of the surroundings the wayfinder makes during calibration is off. Perhaps this was because I didn't do it evenly enough or some such.

Another interesting point that didn't occur to me until afterwards was that I think the error was always in the same direction (of azimuth). If so, this would significantly ease the process. After all, searching in one dimension is much easier than searching in two.

As far as the smart tool, it was always within two or three tenths, at most.

My second concern was the wayfinder's backlight. Turns out this is not bright, is easily disabled, and actually somewhat useful. The wayfinder has to be mounted down on the rocker base, so it's a bit hard to see from up near the eyepiece. Situation might be different when I get to a dark site.

My last concern was directionality, by which I mean that the wayfinder is designed to be mounted on a dashboard, facing forward. Facing the compass in the directy my mount is facing would mean mounting in on the "back", i.e., I'd have to get off the chair and walk around the scope to read it. I put the wayfinder on the side of the scope, meaning that it's 90 degrees off, PLUS the magnetic declination. The wayfinder's magnetic declination can be set up to 99 degrees, which would seem to be enough. However, it didn't seem that way, I seemed to need more like 105. So I left it at 95, meaning that I have to add 10 to each azimuth I look up on the computer. No big deal.

Overall, it's good enough that I'm willing to spend another couple of evenings trying to work out the kinks. If I can't, I'll mount the wayfinder in my car and add a printed azimuth circle to the base (which will be a bit of a pain).

Cheers,
pete

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*Gosh, dang dibbity dag nabbit* - sorry Pete. That first night it was only off max 1.5 degrees.

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Beeps every 10 degrees and has soft blue backlight that can be turned off.

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Will do!
I assume this sort of gadget isn't affected if it gets a magnetic strip stuck on it so I can clip it to my scope tube? I know Rod's is held on magnetically ... I have no idea how these actually work or if anything in it can be affected by magnets. I only know there is so much metal in our building I cannot use a compass to line up so will have to go by visual sighting -hopefully using Polaris- to align everything.

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