"Analogue setting circles" on an alt-az tripod

Just finished reading Rob Nabholz' article on making and using "analogue setting circles" on a dob and the thread recently posted. I was heartened to learn how well it worked.

In the spring, I tried the same concept with an alt az tripod using Smart Tool and a Brunton compass acurrate to one degree. Since I have only a porthole size of sky to observe, I was hoping to get in the ball park with the compass and digital angle reader and then fine tune with a super wide angle eyepiece.

With The Sky Pocket Edition on a PDA set for my locality and time, the compass set for the current declination adjustment and the compass arrow pointed at Polaris, I visually located Jupiter and then its position relative to the compass. The azimuth position of the planet did not even come close to the degree it should have been per the planetarium's az calculation. I flipped the declination adjustment from West to East but that did not help. I moved from the back yard to the front yard to see if there was some magnetic interference but that did not help either. I double check the PDA's az reading with Starry Night and found them similar.

Theoretically, Jupiter should have been close to the az reading given by The Sky and Starry Night. But my findings proved otherwise. What am I missing? Can anyone help here?

Thanks and Clear Skies,

Darlene

Are you taking into account that the compass may not point due north? Does your compass use GPS or is it manual? I believe GPS units automatically make the adjustment from magnetic north to true north, but old fashioned manual ones don't.

Is that what you mean by, "With The Sky Pocket Edition on a PDA set for my locality and time, the compass set for the current declination adjustment and the compass arrow pointed at Polaris,..." ??

Darlene
The compass arrow never points to Polaris, it points to the magnetic north and your heading line would point to true north. Another possible problem is in your adjustment between magnetic north and true north may have been to the wrong side of magnetic north...easy mistake to do.
Hope this helps
Buck

Darlene
On a second reading I see you tried the East/west flip. Are you sure you have the correct degree of declination of change, you may want to check here:

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

I know Brunton makes a great compass, but have you compared it to another to see if it does in fact point to magnetic north. Occasionally a compass can go out of adjustment and the arrow does not point to where the magnet is pointing and some compass' can be adjusted for this. I had a friend set his compass by this method to account for the dec adjustment which made his arrow point to true north, which was all well and good while he was using it. However, he sent a another hunting friend off with a map and told him to walk for an hour at this heading to get to some rutting marks and forgot to tell him of the change. You guessed it, our friend got lost and didn't get back until late and dark. Needless to say he was p*#$ed.
Buck

Darlene,

I have played around with this very thing with my PDA. Here is what I think it takes to get enough accuracy to actually find fainter objects. Find a medium to bright star within 10-15° of the object you are searching and center it. Get the alt az readings from your PDA for that particular time and set the alt az circles to the correct reading. Now, while watching the PDA for the object you want (alt az will be continually updating), move the scope to the desired readings. Expect to require recalibration of the circles each time you move the scope more than about 30-40°.

Forget the compass and don't try to align the circles on one part of the sky and make a great swing to another part of the sky. Even excellent RA/DEC circles will accumulate error as you try this. With some patience, you will get good results.

At least this is my experience.

Randy

HI Darlene,

I think Randy's advice is good, forget the compass, and simply use a known object and set your azimuth circle and SmartTool to the match the computer's bearing computation.

As Randy said it is also a good idea is to choose an known object in the same area of the sky, this will only improve the accuracy.

The SmartTool really helps make this kind of set up very accurate.

Good Luck.

Guys,

Thanks for the interesting reads and some great suggestions. I don't think the owner of the property would take kindly to paint on nor steel stakes in the concrete.

While there are no setting circles on the alt-az mount, I can use the compass to take a reading of a near- by bright object relative to the faint fuzzy I'm trying to locate. Thank you Randy and Rod.

But I was still curious as to why my compass was outrageously off the mark. The noaa web site said the declination for Oakland is 14 degrees 46" East so I adjusted the compass accordingly. Mars was still way off.

Using Buck's suggestion, I said to heck with what noaa says, pointed the compass at Polaris and adjusted the declination to whatever would bring its alignment marks together (Brunton Eclipse). Then I adjusted the bearing to the reading Starry Night gave me and found Mars to be where the arrow was pointing! Hooray!

Curiously, the compass shows the declination is about 17 degrees East. This variance could be due to some magnetic interference in the area.

By the way, starry night's companion book say the azimuth reading follows the land based horizon co-ordinate system.

Clear Skies!

Darlene