Dobsonian Azimuth Compass

[Sconesbie (Scott)]

Hi everyone.

I've got a 10" Collapsible Dob which I have placed a 360 deg azimuth compass thingy on the bottom. I've cut a section out of the bottom board so I can see what deg it's on.

One trouble I've had is this:

I've found north in the yard using my phone compass, turned my scope around so 0 deg is pointing toward north. Following that, I've looked up an object in the sky using a Push To App I have which gives ALT and AZ settings. I've turned the scope around to the corresponding AZ deg, gone to the approx ALT required but is no where near the object I'm looking for. The AZ is way out. I hope all that makes sense.

What am I doing wrong?

[julianh72 (Julian)]

Have you allowed for the deviation between True North and Magnetic North ("Magnetic Declination") for your location? The error is about 13 to 15 degrees in Tasmania.
http://www.magnetic-declination.com/

[Sconesbie (Scott)]

Hi Julian. I haven't allowed for that. Thank you.

I just looked at the link and it says 14 deg East.

So where my phone compass says 0 deg (north), I should actually move that line 14 deg east to be at magnetic north. Then place the 0 on my telescope compass circle on that adjusted line? I hope that makes sense.

I'll give it a try anyway and see how that goes.


Regards
Scott

[byronpaul (Paul)]

Hi Scott,

If your azimuth setting circle is moveable, there is an easier and more accurate way to do this than using your compass.

Set your telescope up so your setting circle is approximately lined up. i.e. 0 pointing North, 180 pointing south, then find a known object in the sky. This should be easy with one of the more obvious reference Stars. Once you’ve done that, find the Alt/Az for that star and then spin your setting circle to align with that degree setting. You’ll then be perfectly aligned.

I have the same set-up on my 12” Lightbridge and this takes no more than 1 min from start to finish.

Regards,
Paul

[Sconesbie (Scott)]

Hi Paul.

No, it doesn't move. I've stuck it down to the base plate. Will my theory still work?


Regards
Scott

[byronpaul (Paul)]

Hi Scott,

yes most definitely your theory will work, albeit very difficult to get spot on with a compass.

Given your setting circle is fixed, go through the procedure you've described. This will get you close.

At that point you'll need to find an object like I described, check the true azimuth setting and work out your error. e.g 2 deg west, or 3 deg east.

At that point, you would try to carefully turn your base assembly to compensate for that error.

Recheck from the beginning.

This might take a couple of iterations, but you should get there in a few minutes.

Percerver with the azimuth circle and get yourself an Inclinometer for Altitude if you haven't already - you can pick these up cheap on ebay.

Once sorted, finding targets will be much easier !!!!

Paul

[Wavytone]

Scott, As Paul indicated you can make it work if the az circle is stuck to the baseplate: here's how I used to use an ancient Celestron 8 in altaz mode many years ago:

1. Choose a bright star low near the east or west horizon, and calculate the azimuth of this.

2. Turn your scope so it reads this azimuth value on the base. Now turn the whole base so the scope is pointing at the star - you can move the tube up and down (altitude) as much as you like.

3. Repeat for a star near the other west/east horizon.

You will probably find that you won't be able to eliminate the errors because this is sensitive to any lateral offset of the circle with respect to the centre of rotation in azimuth, however you may be able to get it with 2 degrees or so.

If you're good at mental arithmetic, an alternative solution is:

1. Choose a star and calculate its azimuth, low in the east/west as per above,

2. Plonk the base down on the ground with the circle roughly aligned to north, within say 30 degrees;

3. Point the scope directly at the selected star and note the azimuth as indicated on the circle. Subtract this from the calculated azimuth to get the azimuth error.

4. You can write this down, or do as I did - using some electrical tape or similar, put a marker on the circle corresponding to this error - this will effectively point to true north.

5. From then on each time you compute a star position add this offset to the azimuth.

Notes:

a) if your azimuth circle could slip around the base, at step 3 above the thing to do is to rotate the circle so that it reads the calculated azimuth of the star. But you said you glued it in place... ah well, it's a modification to carry out on a rainy day to fix this.

b) All of the above only works reasonably well if the base of the scope is level within the accuracy of the altitude circle (0.5 degree). If its tilted in some direction this will affect your calculations of the altitude position and you'll have to allow for this tilt - or level the scope.

c) In addition if the altitude circle is not concentric with the axis of rotation in altitude this will also create another source of error.

[ZeroID (Brent)]

Last piece of advice. If you use the same spot each time for viewing put down a dab of paint or something similar at the points where the feet are for the base. Next time just drop it onto them.
I did it on my driveway before the Ob was built. Solar noon to get Nth Sth line then a big T square for east west. 'X' marks the spot(s). I actually spray painted a yellow N-S line and E-W and levelled it.