Anyone who has been following my blog, or the CGEM yahoo group, might know that I have had a really tough time comprehending polar alignment – both the full principles, and the actual technique.
I mean – it’s not hard to comprehend the part about the mount needing to be aligned with the north celestial pole in order for the scope to be able to track in compensation for the earth’s rotation. But what this means, fully, for a german equatorial mount is another thing, and understanding the various techniques and their different functions and applications can be really, truly hard for a newcomer.
I think I’ve got it, though, and the key is summed up in this: The entire purpose of Polar Alignment is to get the altitude and azimuth axes into position. Thus, the ‘home position’ for RA/DEC (i.e. those lined up arrows) becomes relevant for tracking only relative to accurately positioned alt/az axes.
The Polar Axis Finder instructions are fine – if read and re-read thoroughly. But it leaves out the most important step/concept in being able to move to the Hand Controller (HC) and run through the 2-star alignment procedure (with however many calibration stars), or the “All-Star” procedure (p37 of the manual) once Polar Alignment has been achieved using the polar finder scope: Return the RA/DEC axes to the ‘home position’ i.e. with all the arrows aligned.
The alt-az axes need to be in position for the home position to be useful.
So the step by step routine (assuming that the Optical Axis of the Polar Scope has been aligned already):
- Rough align the tripod north, and level.
- Many recommend against assembling the OTA fully at this point: for both weight and range of motion. Some suggest a full assembly to avoid messing up polar alignment, since the full weight can cause bowing in the tripod legs. I’ll have to see which works for me.
- The Latitude Scale should be set to your latitude.
- Rotate DEC so that you can see through the bore hole.
- – rotate the RA axis until the big dipper (northern hemisphere) is in the same orientation within the polar axis finder scope reticule as it is within the sky. This is a nice video.
– Another method is to control the rotation of RA according to the RA setting circle and polar finder scope metal collar with hour angles on it. My scope does not have an RA setting circle, so the collar is useless. Here is a good link that Gary A. emailed that describes it, along with the rest of the routine.
– Yet another method is the Kochab’s Clock method, described here. With this method, line up Polaris and Kochab along the unloaded counterweight bar.
- Adjust the alt/azimuth axes until Polaris is within the small circle meant for it. Then lock them down.
- Return the RA and DEC to home (in bold because this is the part that kept tripping me up). Power up the scope, and run through the 2-star alignment, and then, if preferred, the “All Star” procedures.
- (Kevin adds something about re-calibration at this stage, and ‘display align’, but I’m not there yet in my learning curve!)
That is it. Of course, I’ll post again on the topic when the snow melts and the temperature is above freezing.