Quote:
Originally Posted by alpal
Not only that - an OAG is working at the native focal length &
so is far more accurate than a little guide scope.
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That seems intuitive, but I don't think that it is any longer the case Alan - Craig Stark explains why in
http://www.stark-labs.com/craig/reso...raig-Stark.pdf
Back in the days of film imaging and
manual guiding, there used to be a rule of
thumb that the guide scope should be at
least half the focal length if not the same
focal length as your imaging scope. ...With CCD-based
autoguiding this rule can be thrown away.
Computers are far better at spotting very
small movements and far faster and more
accurate in their reactions.
With modern guide software, motions
that are small fractions of a pixel can be
accurately estimated....
Personally, I use a 66-mm telescope with a
388-mm focal length (William Optics
Zenithstar 66 SD doublet). I know many
who use scopes of similar focal lengths and
have even seen excellent results from an
8x50 finderscope that had been converted
into a guide scope (200-mm focal length).
The SBIG eFinder accessory for their STV
guider is even shorter at only 100 mm! The
days of very long focal length guidescopes
are over
FWIW, I use a 180mm fl guidescope with a QHY5L2. At times it keeps the EQ8 within 0.4 arcsecRMS, normally about 0.7 RMS and in bad seeing it is >1arcsec RMS - and then I don't usually bother with imaging. With 8 inch wheels, the EQ8 is inherently better than the EQ6, since minor fluctuations in mesh due to surface and/or grease irregularities (dirt etc) are less noticeable. An EQ8 will handle the Meade with ease.