#10 Mirror Size

     I mentioned somewhere in an earlier post that if I had easy access to a truly dark sky, I would not require anything larger than an 8" telescope.  To any readers fortunate enough to have that dark sky on demand, along with an 8" scope, you have an ideal situation.  Or not.  It depends on what you are searching for, and how much you hope to see.  You won't be resolving any galaxies into millions of stars with it (nor with any other instrument).  A 6" scope would be quite nice in a great sky if an observer was after only the best and brightest of deep sky objects.  An 8" would be a big bonus for such a project.  For someone enamoured with the whole NGC list, however, a 12" scope in that beautiful dark sky would be darn near perfect.  A 14" objective would be a bonus. 

     I've owned a 3" reflector, a 4.5" reflector, an 8" reflector, and my current 12" Dob reflector.  Going from a 3" reflector to a 12" is not a bad upgrade.  One might think that going from a 3" to a 6" would double the light, and then double it again from a 6" to a 12".  Thus 4x the light gathering power belongs to the owner of a 12" compared to his neighbour with a 3" (refractor or reflector, makes no difference when merely gathering light).  However, this is not the case.

     In actual fact, even though my math is not exact to seventeen decimal points, upgrading to a 6" mirror from a 3" is already an increase of 4x the light-gathering power, not 2x.  7.1" sq. versus 28.3" sq. is 4x the light-gathering power.  Going from a 6" to a 12" increases it again by 4x, from 28.3" sq. to 113" sq. (pardon my non-metric calculations).  In my lifetime, I have increased my light gathering power by a whopping 16x.   I'm still not going to resolve M31 into individual stars with it, but the galaxy sure looks a lot brighter (and I can now see a few globular clusters in it!).

     When I observed with my 8" reflector, I had pre-cut cardboard circles on hand so that I could cover the top of the tube to see what certain objects looked like from a 4" objective and a 6" one.  With the holes cut away, it was also easy to actually see the difference between areas of different sized objectives.  I recently cut a new cardboard aperture to fit over the 12" tube, being 8" (off-centre).  That 8" hole now seems awfully small, with the huge amount of cardboard left over showing the full 12" circle.  Since upgrading from an 8" mirror to a 12", I have increased my light gathering power from roughly 50" sq. to 113" sq., or by about 125%.  It had not really struck me how much bigger that 12" mirror was until the cardboard cover was finished.

 A very rough 8" diameter hole cut from a 12" piece

of thick cardboard.  Black tape hides the bad cutting,
and keeps the circumference roughly even.  Not only
can I now see how objects compare at 8" and 12"

(roughly speaking), but I can really see how much

larger the 12" mirror is (more than 2x, or about 125%).

     We could push this discussion some more, discovering that a 16" mirror has about 201" sq., and a 22" one has around 383" sq.  These scope sizes and larger ones now regularly appear in amateur astronomers' inventories.  How long until we get one big enough to visually resolve Andromeda galaxy at a local star party?  Hmm, that requires around a 200" mirror.  Or maybe several 25" ones spread out and linked by computer on-site.  Maybe that day isn't really too far off after all.

Mapman Mike