Re: How much dec drift, field rot, and periodic error allowable?

I take the old-fashioned approach (or the engineering/applied science approach?)
of measuring it directly. And in any case you have to anyways to get the
periodic error. No formula exists to calculate it from the mechanical nature of
the mount. So if you're going to measure that and deal with it, then why not
measure everything else.

--- Dave
--
----------------------------------------------------------------------
Pinprick holes in a colorless sky
Let inspired figures of light pass by
The Mighty Light of ten thousand suns
Challenges infinity, and is soon gone

david.nakamoto@xxxxxxxxxxx


"canopus56" <canopus56@xxxxxxxxx> wrote in message
news:1138579794.630043.297690@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Is it possible to estimate acceptable tolerance limits for declination
> drift, field rotation and periodic error, given a specific detector
> size, focal length and image scale?
>
> Apparently refering to pre-autoguiding days, Covington (1999) 276-281
> provides a method estimating field rotation and declination drift, but
> his discussion of what would be an acceptable limit in an image (at p.
> 281) is less clear.
>
> For the post-autoguiding era, Berry's HAIP provides a convenient
> periodic error estimating formula (at p. 96 - x = x_0 + v*t +
> 1/2*a*t^2) but does not really discuss a limit at which periodic error
> combined with autoguiding will result in unacceptable visually
> detectable changes in the final image.
>
> Initially, I was thinking in terms, for projection magnification and
> extended objects, simply taking the limit of human vision divided by
> the magnification, e.g. 1 arcmin = 60 arcseconds / 5x projection
> magnification or a 15 arcsec limit.
>
> For stellar point objects, I was thinking in terms that an acceptable
> drift limit would be close the Rayleigh criteria - the point at which
> two stars appear elongated.
>
> For now, I am happy to just take images, take steps to minimize drift,
> rotation and periodic error within the limits of my equipment, record
> predicated and observed declination drifts, field rotations and
> periodic errors in a log and to work towards some empirical
> rule-of-thumb.
>
> But how have the expert amateurs thought this through? Are there other
> pre-existing equations or empirical rules-of-thumb? What's the best way
> to approach this, if at all? What am I missing here?
>
> - Canopus56
>


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