Re: (SNES) Eye Pieces

["Les Lytton" <Starman@SAMnet.net>]

Barry wrote:

> I am always on the lookout for a good inexpensive ep for my Orion XT-8
(1200mm). I was wondering  what  the max. power I can use on this telescope,
and if anybody is familiar with the eyepieces of Siebert Optics? <


Good morning, Barry...

It's c-c-c-cold out there this morning!  340 and gloomy gray skies in
Wilbraham as I am writing this!

Maximum usable magnification is a very subjective thing... so many variables
enter the picture, on a use-by-use basis:

Sky transparency - how much water vapor is in the air, in the form of
visible haze, thin clouds, or even invisibly, has a drastic effect on the
maximum usable magnification.

Seeing conditions - how many temperature inversion layers the light of the
object being viewed is punching through before it enters your telescope also
has a dramatic effect on maximum usable magnification.  As you already know,
magnification magnifies everything, including the dancing and blurring of an
image created by refraction.

Local light pollution - I need not say more on THIS subject!

How high in elevation above the horizon the object is being viewed - best
observing conditions always exist at the zenith, and as a rule, viewing an
object in this area of the sky will support higher magnification than
anywhere else, because the telescope is looking through the thinnest amount
of atmosphere possible.  But...  how many times do we actually observe in
this special spot?

Resolution - the ability of the telescope to see finer and finer detail is
tied directly to the overall size of the objective lens.  The bigger the
lens, the finer the detail that can be seen clearly.  An upper limit is
reached for all telescopes where further magnification causes the image to
degrade.  The 60X rule of thumb is based on this factor.

These are the biggest determining factors for successfully applying maximum
magnification...  and all are variable, from one observing session to
another, and even from viewing one object to the next.  There are many other
things that come into play, but not to the same degree.

Several rules of thumb have been developed over the years to determine the
best usable maximum magnification, and it depends on who you listen to as to
which is the best.  The most common one states that 60X per inch of
objective diameter is the best that can be expected.  I have also seen
proponents of 30X per inch, and even one pessimist who claims that 20X is
the tops for planetary detail.  Personally, I have found that mother nature
is actually the determining factor for 80% or so of all observing sessions,
with 10% attributable to your local conditions.

Using the 60X/inch rule, the top end usable magnification for my C-14 works
out to 840X.  I own three 2" eyepieces and a 2" barlow calculated around
this value.  Here is a table for my own arrangement:

Eyepiece    Magnification    Barlow
55mm            78X                No
55mm           156X               Yes
13mm           300X               No
 9mm            450X               No
13mm           600X               Yes
 9mm           900X                Yes

I have successfully observed at 900X for longer than a minute or two exactly
ONCE in the 10 years or so that I have had the 9mm Nagler!  That was 2
weekends ago, as you read in the observation report I posted.  Almost all of
my viewing is done at 78 156, and 300.  When I attended the CT Star Party
last month, the skies beautifully supported 600X the entire weekend, but
this is by far, the exception, rather than the rule.

Now...  how does this all apply to your 8" diameter?  If you follow my
example with the 60X rule of thumb, 60 X 8 = 480X upper limit before
resolution begins to appreciably degrade for you.  Can you achieve it?
Well...  If we lived atop Kitt Peak in Tucson, AZ, things would be a far cry
different.

I'm sorry, but I can't be of help with the Seibert optics.  I have never
used one of their eyepieces, so I'm not of help here.  Personally, I feel
that the eyepieces are one of the most crucial elements in the entire
light-train of a system, and am of the firm school of thought that the best
is not good enough.  A one-time purchase for good quality ones is a lifetime
investment.

Respectfully,

Les Lytton
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