oops, correction to M31 comparison

[Stupendous Man <richmond@a188-l009.rit.edu>]

  My previous message compared images taken by a Mark IV and a
Meade LX200.  The limiting magnitudes _are_ similar, but I gave
an incorrect value for the LX200's aperture. 

      Mark IV aperture is        10 cm diameter
      LX200 aperture is          10 inches diameter = 25 cm

  Whoops.  Sorry about that.  

  So, in fact, the ratio of light-gathering power is not 100:1, as
I wrote earlier, but closer to 6.3:1.  The ratio of exposure times
in the two pictures was 103 minutes to 1.5 minutes, or roughly 70:1.
It would seem that the Mark IV ought to detect much fainter stars
than the LX200 ... 
 
  ... until you factor in additional considerations.  The Mark IV
had a V-band filter, while the LX200 had none; that cuts out about
60% of the light from the Mark IV.  The Mark IV pixels subtend 7.5 arcsec
on a side, while the pixels of the CCD camera I used on the LX200
subtend only about 2 arcsec.  That means that the Mark IV pixels
collect 14 times as many photons from the sky, increasing the noise
against which stellar photons must compete.  The PSF of the Mark IV
image -- a composite of many short exposures -- covers more pixels
than that of the LX200, again decreasing the signal-to-noise ratio
in the Mark IV image.

  I was hoping to avoid all these complications, but it turns out
they really are important to explain the observations.

  When I wrote my first message, I just wanted to point out that the
Mark IV image goes pretty darn deep.  I thought it would be nice
to show a picture taken by another telescope as a comparison.  
My faulty brain, confusing inches with centimeters, thought that
the ratio of apertures was a nice, round ten to one, and decided
to mention it as a bit of trivia.  Ooops.  Mea culpa. 

  Sorry for the confusion.  I realize that metric/English conversion
errors are hard to excuse, but at least mine didn't cause any big
problems (like, say, leading to the loss of a space probe).

                                                Michael Richmond