[APML]: GEG Report - Correction!

["Philip Perkins" <philip@astrocruise.com>]

> In the 'f/4 mode' (bearing in mind that actual f/ratio on my scope is
> f/5.8) I seem to recall that it was 84'. Originally I worked out the exact
> field coverage in each mode (based on actual star points at the extreme
> edges of the negative) but only to get the real f/ratios, which was what I
> was actually after. Having got the f/ratios, I subsequently trashed the
> field coverage figures! :-)  Anyway, we can work it backwards of course
> (W=arctan(3.2/FL) - I could work it backwards for you, except that my son
> has borrowed my scientific calculator - help! :-)  FL of course is derived 
> from the aperture of 254mm and the actual f/ratios - you will find these 
> under the Mode Report section, but in case you didn't note them ....
> f/10 = f/11.2
> f/6.5 = f/7.5
> f/5.5 = f/6.5
> f/4 = f/5.8

Ooops!  I don't know what I was trying to plug into the formula - my brain 
must have retired early (it was late). 
W=arctan(35/FL) sounds a lot better, assuming it's the long dimension of the
35mm frame you're interested in.
To really put matters to rights I have worked out the angular field coverage
in all four modes. Since my son still has my scientific calculator, this was
done on my computer (hope it's not one of those dodgy Pentiums :-) - at
least the numbers look good to me ...

'f/10  mode' = f/11.2 = 2845mm = 43' x 29'
'f/6.5 mode' = f/7.5  = 1905mm = 65' x 43'
'f/5.5 mode' = f/6.5  = 1651mm = 75' x 50'
'f/4   mode' = f/5.8  = 1473mm = 84' x 56'

N.B. - these are all for standard 35mm frame
Hence, taking the two extremes, if you go from f/11.2 down to f/5.8, then
you double the FOV and you only need one quarter of the exposure time.

Hope I got it right this time - sorry for any confusion.


Philip Perkins  --  philip@astrocruise.com
51 27'N 1 36'W  --  Wiltshire, UK
43 54'N 5 32'E  --  Luberon, France
Astrocruise     --  http://www.astrocruise.com