Expert: Tom Whiting - 7/17/2009

Question
We have a Jason 323 Comet Chaser Reflector Telescope 160X Power 480 MM Focal Length 3" Diameter.  It has been in the closet for years.  We got it out tonight and when you look through the lens everything is upside down.  Can you tell me what we are doing wrong?  How can we fix it?  Thanks, Stev

Answer
Hi Stev,
As a matter of fact, nothing is wrong.  You don't want to "fix it"!!
On all good astronomical telescopes, the image is always inverted.  It's supposed to be.
The reason being, to erect the image takes another mirror or lens,
which means an additional 10% light loss.  We astronomers don't
like to sacrifice light loss (that's what we're paying for, as much
collected light as possible)...so why put in an extra lens to
erect the image...stars are just pin points anyway, and as far
as stuff like Deep Sky Objects, the moon, and planets, you just get
used to viewing them inverted.  So it's not only perfectly normal,
it's the accepted way we do it, in astronomy.  You can purchase
an extra erector lens for your eyepiece if you wish, but it costs
you 10% less light coming thru the system.  But we astronomers just
accept the inverted image because we want to gather as much light
as possible.
Clear skies,
Tom Whiting
Erie, PA

FOLLOW UP:
Oh, and I'll make you a bet that your highest power eyepiece, the
3 mm, is totally unusable at 160 power...(how did I know it was
a 3 mm?  480 mm/160X = 3 mm eyepiece) as 160X exceeds the maximum
effective magnification of your optical system. Even with the best
figured and ground mirrors, say 1/10 and even 1/20 wave, your maximum effective magnification is 50 power per inch of aperture (3" mirror), or 150 power in your case.
So you're getting all empty magnification above 150 power and that's
even true IF your mirrors are very high quality figure and you are
perfectly collimated.  By empty magnification we mean that you are
magnifying all the optical defects in the optical system just as much
as the object you are trying to see, so the image just gets bigger,
but fuzzier....so your 3 mm is a worthless eyepiece for your optical
system. Better would be a 6 mm eyepiece which gives you (480/6).... 80 power.  This would be a reasonable high power for a 3" scope.

Collimation means.....
Optical axis are perfectly aligned perpendicular to the primary mirror and dead centered.
{Yes, you have to learn how to collimate a reflector telescope with
those little screws on the top of the secondary mirror - which control the tilt and rotation of the secondary mirror-- and those
wingnuts or bolts behind the primary mirror, which control the tilt
of the primary mirror.}  See the booklet that comes with the scope
and the operating instuctions.  I would bet that sitting in a closet
for 2 years, it probably is out of collimation.  (I re-collimate on
every setup). Tube-type reflectors should be checked for collimation
every 4 or 5th setup, due to bumps traveling to the observing site,
rough handling, etc.
Clear Skies,
Tom