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Polar Alignment with a GoTo telescope mount can be significantly simpler than methods required when the telescope has no GoTo.
All you need is a GoTo telescope, this might be one of the following brands: Meade LX200/LX200R/LX200GPS, Vixen, Losmandy, Celestron just to name a few.
Ordinarily I would recommend a drift method of alignment. This is a fool proof way to achieve a high accuracy of polar alignment. However if a GoTo telescope is being used you can greatly expediate the process by just using a little common sense.
When you tell an equatorially mounted GoTo telescope to slew to a star, it goes where it thinks the star will be, not necessarily exactly where the star is. Ignoring mechanical factors such as incorrect drive rate, motor problems, sensor problems, backlash, etc, the most obvious reason for the telescope not pointing at the desired object is polar alignment. For it to not be pointing at the object, there must be some error in your alignment. What is that error? Well, if you've just slewed your telescope to a known object, chances are you are looking straight at the exact error in your polar alignment.
So, let's run through a simple proceedure:
Like with drift alignment, we need a star on the east horizon and a star on the meridian, slightly to the east of the meridian so you don't encounter any mechanical problems such as mirror flop. The stars don't have to be bright, you just need to be able to consistently find them. I use whatever SAO stars appear where I want them, as long as their obvious in the CCD image and I can note down their catalogue number for repeat slews then I'm happy.
1) GoTo & Sync on the star that's on the meridian.
2) Slew to the star on the east horizon.
3) Adjust the rotation of the mount (azmith) such that the star is as close to the centre of the eyepiece as you can get.
4) Using the motion controls of the telescope, centre the star & Sync.
5) Slew to the star on the meridian.
6) Adjust the Altitude such that the star is as close to the centre of the eyepiece as you can get.
7) Using the motion controls of the telescope, centre the star & Sync.
8) Repeat steps 2 through 7 until you are happy that the star is now being positioned acceptibly close to the centre of the eyepiece.
Congratulations, you have a surprisingly accurate polar alignment! Tighten up the bolts on your mount while centred on a star, to ensure there's minimal flexing in the telescope mount and that you don't alter the alignment considerably when doing that tightening.
In step 8 I say "acceptibly close". I carefully chose this terminology because there are reasons why the star will never be perfectly in the centre of the eyepiece/CCD. These are usually mechanical, such as:
- Flexing of the mount/wedge.
- Backlash in RA and/or DEC.
- Retrograde motion in RA and/or DEC.
- Lose optics causing image shift.
are just a few.
Including the time it takes me to choose 2 stars this proceedure takes me a total of about 5-10 minutes from start to end. At the end of that, I can take an unguided CCD image at 2160mm focal lenght and 1.043 arc/seconds per pixel, for 5-10 minutes with no drifting visible at all.
Amazing!
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