OTA as finder scope

[dcwales2012]

Hi,

I have read the topics on using an ST80 as a finder scope, but is there a reason why one couldn't use a Skymax 90 instead?

Setup:

SW 150p, EQ5 Goto

Regards

David

[dcwales2012]

'Pics on u'

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[michael.h.f.wilkinson]

'Pics on u'

came from nowhere

Removed

A Skymax 90 has a very long focal length. That means the part of the sky that fits into the maximum possible field of view is much smaller than with the ST80. I have an ST80, but us a 70mm F/5 as finder (much lighter), with a 25mm Antares SWA (70^o). That gives 14x magnification and theoretically 5^o true FOV. In practice I get 4.46^o true FOV (this is due to the optimism of manufacturers when it comes to FOV. The ST80 would give me 3.9^o true FOV, which is quite usable. The Skymax at 1250mm focal length gets only 1.25^o true FOV, which means that the finder itself will need a finder.

[dcwales2012]

Many thanks for your reply, it makes it all perfectly clear now.