Alignment stars

[Dawson]

Is Polaris a good [or bad] alignment star for alt-az mounts (actually a fork mount without a wedge) when undertaking a two star alignment for GOTO?

Thanks.

James

[David Swan]

Yes. I usually use Polaris as my first alignment star as it – very obligingly – drifts only very slowly without sidereal tracking engaged. Plus, as Polaris is a double star, one can straightaway suss out the optical performance of the telescope and atmospheric conditions too before completing alignment and engaging tracking.

[Dawson]

Thanks David. I think I have got stuck on the concept that it is the worst possible star for equatorial mounts and just assumed, without thinking, that it would be equally useless for alt-az mounts. I still struggle a but, because for less accurate alignments than the ones you are conducting, the user would be essentially pointing the scope at the same place in the sky every time they align, as polaris moves so little – I still struggle to see how this cannot introduce some kind of error, but again I think I’m over thinking it.

James

[Hugh Allen]

Hi James,

I don’t think Polaris is a good alignment star for Alt-Az telescopes. I have a fork-mounted Meade LX90, and Polaris is not in the list of recommended alignment stars in the handset. I vaguely recall trying Polaris in the early days and finding the 2-star alignment was less accurate. I think the other guidance is to avoid an alignment star when it is near the zenith (the tube is parallel to the forks) or below about 10deg elevation. Otherwise, provided the two stars are well separated in the sky then the Meade 2-star alignment is highly accurate every time (I use an illuminated cross-hair eyepiece to get the alignment star dead centre every time). I do spectroscopy with my Alt-Az Meade and using GoTo the target star drops every time into the field of view of my Alpy 600 guide unit.

Cheers

Hugh

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