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Hi Simone & All,
Yep, Fomalhaut was too close to Zenith to align on early in the evening. Using the 'scope in Alt az mode, aligning on anything close to zenith is a likely to produce pointing errors. If using an Equatorial, aligning near the SCP is similarly a bad idea for the same reasons.
All these systems use optical encoders that have a certain number of ticks per revolution. What I want you to do is imagine these ticks on the aximuth axis projected out into the world -- out to the horizon. See how far apart they are spaced? Probably about one tick per few arc-minutes.
Now imagine in your minds-eye every one of these ticks at the horizon has a line connecting it to the Zenithal point. In your minds-eye again, do you see these lines converging as their altitude approaches zenith ? Near zenith they get very, very close to each other (only arc-seconds apart) and then finally converge. If you align on an object near zenith, the 'scope can move through perhaps a three dozen ticks on the az encoders with the object only moving a few arc-minutes in the eyepiece. Trace those 3 dozen ticks back in the direction of the horizon and you can see they diverge till they are in total maybe a couple of degrees apart -- this is probably a large slice of the error you saw.
Which ever system you are using in Alt-Az mode (or Eq mode for that matter), use two stars that are as far apart in azimuth as practicable -- at least 40-odd degrees, better 80-120 if possible. At 8.30pm (15-odd metres from you on the Argo Navis), I used Canopus and Hamal (Alpha Aretis) -- 90-odd degrees apart. Both were well above the horizon without being close to zenith (air-mass refraction error is therefore negligible -- zenithal error therefore negligible) -- and they are at quite different altitudes above the horizon. This means your alignment could be described as "well-sampled" and more likely to produce better accuracy. I didn't need to re-align all night. For me, every object landed comfortably inside a 12mm TII Nagler field (27 arc-mins). If you've picked an area of the sky to concentrate on (for example say Vela, Puppis, Canis Major, Lepus Orion etc) then alignment stars one on either side of this area will work particularly well -- like Canopus and Hamal for example.
Yours will do the same I'm sure -- just follow the advice above. If someone else is reading this and has a Eq mount, just substitute Dec and RA for Alt and Az
Best,
Les D
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