Quote:
Originally Posted by darkskybondi
And this brings me to the 10" SW truss. I've only just decided to write about my neurotic concerns about the thing since a friend came over a few days ago (one of about 5 so far who I've quietly introduced to the sky for IYA2009) and commented on how dusty the primary mirror looked when I shone a torch at it. Yes, yes, I know - probably paranoia, right? But I'm going to make you shudder when I tell you that I took a tissue to the secondary and cleared up a patch of it and it did look somewhat better. But I'm not sure whether it's dust on the primary or a sort of intrinsic graininess in the mirror. So is it a flaw in the mirror, or is it worth cleaning my primary (not with a tissue)? Perhaps graininess is an artefact of all mirrors? In other words, is it a flaw, or my paranoia, dust, or a mixture of all three?  |
Hi DSB,
Remember what the front cover of the "Hitchhikers Guide to the Galaxy" reads -
"DON'T PANIC".
Many a scope owner has been made paranoid by shining a torch on their primary.
Based on your description and the model of the scope, I gather this scope
is almost brand new, right?
Chances are, unless you have been out a few nights in a dust storm like this
one -
http://www.iceinspace.com.au/forum/s...75&postcount=1
that dust is unlikely to be a serious issue in this instance.
Unlike lenses on your camera, the primary mirrors don't need to always be spotless.
When you shine a torch on a mirror, things always tend to look worse
than they often really are. Though dust can deteriorate the brightness of
objects, the general wisdom is that you can often tolerate quite a bit
of dust on the primary until it impacts upon the optical performance to the
point it warrants a wash.
Part of this wisdom also comes from the fact that each time you have
reason to handle the primary mirror, the probability of damaging it by way of
an accident when handling it increases. That doesn't mean you shouldn't
ever wash the mirror, it just means you might only need to do it once
or twice a year depending upon your environment.
In your dialogue, I note that not once does the 'C' word appear.
'C' of course, stands for collimation and correct collimation tends
to improve the visual performance of newly purchased optics more than
just about any other corrective action. Collimating your scope,
particularly if you have transported it, should be simply a part of the
routine each night before you being observing.
If you want to test the optical performance of your scope, consider
performing some star tests like those outlined in the book by Suiter
See
http://www.willbell.com/TM/tm5.htm
A quick Google will also reveal plenty of independent star testing tutorials.
Keep that tissue in your pocket and use a blower brush to clean off the
dust on the secondary.
Did your scope come with any 2" eyepieces? Some good 2" eyepieces are the
best thing one can add to any new scope.
Finally, when a scope is correctly collimated and in a dark sky location and
has been given time to cool down, atmospheric transparency and 'seeing' (turbulence)
tend to be the limiting factors when observing. There are many a time when
you can arrive at a dark sky site initially excited by the prospect of a fabulous
nights observing only to be hampered by seeing and transparency.
You will quickly come to recognize those nights where conditions are
exceptional.
Techniques such as averted vision and 'integrating' some tenuous image in one's
mind's eye take practice.
So don't get paranoid about a little dust and rather than concentrating on
washing the mirror today, if you haven't done so already, learn to collimate.
Enjoy!
Best Regards
Gary Kopff
Mt. Kuring-Gai
