Polar Alignment

[davidpretorius]

Hi Guys,

Have been thinking about this scope buggy of M Pinner and eq platforms and wedges to be adapted etc etc.

One important thought to cross my mind this morning is polar alignment. If there is going to be a eq platform, then polar alignment must be done.

I can source a usb compass for about $85 that is accurate to 1 degree. Given magnetic north is not true north, and to some degree i can adjust that via software, how accurate mus the polar aligning be.

Given you guys are mostly doing it by hand, do you have a feel for how accurate you would want a scope buggy with polar alignment built in with a motor to automatically align?

[iceman (Mike)]

For visual, it doesn't have to be that accurate but if people are planning to do photography then it is very important and has to be very accurate.

When I go up to Kulnura I take a compass, find South and plonk the platform down roughly facing the right direction. Get it level, and away I go. Objects might only stay in the field of view for 30-60 seconds but that's enough for what I do at Kulnura.

When I do webcam imaging at home, I have to be much more accurate and it can sometimes take 30 minutes to adjust it until the planet stays in the field-of-view for the whole 3 minutes capture I need. The webcam imaging with a 3x barlow is at a much higher magnification/FL than I do with visual, so any errors in alignment will be evident immediately.

[davidpretorius]

Thanks Mike, any feel for "I am out 5 degrees, 1 degree, 2 degrees" Do we need .1 or even .001 of a degree accuracy?

I have also been thinking that once set up aproximately, step the scope to point at two known stars in the general vicinity, southern cross etc and then enter those co-ordinates and then the base can swing and adjust to the scp.

[netwolf]

For the longest time since my mate explained to me about polar alignment, i have wondered why it can not be done by a computer controlled method. I too saw the USB compas you speak of Dave, while reading through your links for USB motor control. Now i hear Celestron are suposed to be bringing out a device that you just point at the stars and it identifies them. Could such a thing be used to do polar alignment, i guess the problem would that there is no South Celestial Pole star. And that is the Crux of the problem. Unless you develop a system that uses the motion of some circum polar stars to precisely calculate the pole and go to it.

Ideas ideas.. what to do with ideas..

[davidpretorius]

if you can point the scope at star A with ra of ??, feed that into the laptop, and then step to star B with ra of ??, then the computer should be able to compute the scp easily.

i have seen where there is a siter on the base or platform to help with alignment. it would be easy from there.

you could even drill a hole in the top bit of the base of a dob, so that when you put on the platform or buggy, then if the base is pointing at scp, then the scope will be. thus polar aligning & calibration at high mag would be possible.

just like we do with the siters on top of out scopes. We use the 200X of the scope to alignt the buggy or platform.

If the base is stepper driven separate to the tracking side of things, then you plonk the buggy or platform down roughly south, make sure dob is mounted so that the scope is pointing same direction as buggy, calibrate through siting and entering co-ord into laptop, then the motor can swing the base around, but not the footings, so that when the eq is operating, you have scp lined up.

[acropolite (Phil)]

The problem with magnetic compasses is just that; they're magnetic, and affected by any localised variations in magnetic field as well as any (magnetic) metals of electomagnetic fields (cars, houses water pipes your dew heater and power supply cables) nearby. Magnetic variation can vary by large amounts depending on location even within your own area. Even Meades LNT setup is unable to accurately pick alignment stars despite sensing level and magnetic north.

[davidpretorius]

yes phil, the more i have thought about it today, the more i want to make it software driven and dependant on solid alignment with hardware and stars