I may be wrong, but here's my take: An altitude-azimuth or alt-az system merely projects what works well for ground-based coordinates to the celestial sphere. The horizon, regardless of direction is always at zero degrees altitude and the zenith is always at 90 degrees altitude. Azimuth equates to the cardinal directions, i.e. north is zero/360 degrees, east is 90 degrees, etc. Due, of course, to earth's rotation, celestial objects are always changing in both altitude and azimuth. Most planetarium software should give you both RA/dec and alt-az coordinates, but the alt-az coordinates will only be accurate for a given moment and specific location. (On my program I set the location and time of day just before I start to use it.) For most casual observing, if your program gives you alt-az coordinates and it takes you a moment to get to your scope to set the coordinates on your setting circles, the object shouldn't have moved enough for you to miss it, especially if you're also using a small finder scope for final centering in the main scopes field of view. This is how I've used my software to help locate things. Hope it helps. Kim
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy- bounces@mailman.xmission.com] On Behalf Of Troy Davidson Sent: Sunday, April 19, 2009 1:35 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Dobsonian Setting Circles?
Erik,
I have seen that site and it is what got me thinking. But, I'm still confused how the altitude scale works. On an equatorial mount, if I point the scope at the celestial equator and then move it to an object to the far west that is on the horizon, the scope is still at 0 degrees on the celestial equator.
But, on a dobsonian, if I pointed the scope at the same object on the horizon, it will say 0 degrees. Then, if I turn the scope due south and keep pointing at the celestial equator, the altitude is no longer at 0 degrees. It's at 50 degrees, if I remember correctly.
This is the dilemma I am having. The azimuth is easy to deal with, but altitude is still screwing me up on how it would work on a dobsonian.
Thanks for the help.
Troy
On Sun, Apr 19, 2009 at 10:14 AM, <erikhansen@thebluezone.net> wrote:
Troy,
Have you looked at this website? It suggests refinements to an inclinometer.
http://www.homebuiltastronomy.com/ManualDobSettingCircles.htm
I would not ask John Dobson for his opinion on setting circles for Dobsonians.
Erik
Hello all,
I'm looking at adding some setting circles to my Dobsonian. The azimuth circle will be easy to handle on the base. I have found sites that allow you to print out whatever size you need.
But the altitude scale seems to be more of a problem. I can use an inclinometer to tell me the angle of my scope. A simple math problem and I would have my Dec. But, this will only work if the scope is directly pointing north or south. When the scope is pointing to an object that isn't directly north or south, then the math gets a bit harder.
I have seen digital setting circles, but those seem to start at $500. Is there a better way to handle setting circles, especially the altitude, on a Dobsonian?
Thanks.
-- ++++++++++++++++++ Troy Davidson
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-- ++++++++++++++++++ Troy Davidson
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