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Don't forget Exit Pupil diameter!
Hi Dave,
The exit pupil diameter should also be taken into account. This is the diameter of the beam of light leaving the e.p. Even well dark adapted eyes will not dilate more than 7mm (if you're young) maybe 5mm for the not so young.
To calculate this, take the focal length of the e.p. and divide by the f ratio. Say a 32mm at f/5 is already at 6.4mm! The same setup with a 0.5X reducer blows you out to 12.8mm. Assuming the e.p. can handle the exit pupil, you will not see eta car without having to move your head around.
Also a dob with 0.5x reducer is likely to give you bad images - I hate poor edge of fields, especially when in this case the crap stars might extend half way to the centre of the field! I don't know whether a Paracor might help? Would anyone like to comment?
I used to have a 6" f/6 rich field scope, which showed most of eta (with poor star images on the edge). This delivered some of the best visuals of eta I've seen to this day.
My advise is go for big binocs - I have a pair of 15x80s with a field of 3 degrees - fantastic!
Glenn
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