Picking Your Telescope
The Relativity of
by Tom Johnston
These Mirror Spec's are
Driving Me Crazy. Please Help!
the strange and mysterious world of mirror making. "Diffraction
Limited" has been used for decades as the catch phrase for minimum required
quality. Simply put, it is a measure of how well the mirror focuses the
light onto the focal plane. The minimum amount of diffraction, or error,
is considered to be about one quarter wave, depending on who you talk to.
If you want better than that, you may have to purchase a custom mirror.
There are several manufacturers, like Zambuto, who can put out a 1/8 or 1/10
wave beauty but it will cost you dearly and you may have to wait a year or so to
get it. As you know, life has its trade-offs.
Most of our 12.5" mirrors have come from
Intermountain Optics and Nova Optics and the total price of the 12.5" Next
Generation Telescope (NGT-12.5) is based on one of those mirrors. I have
seen over 100 of their 12.5" mirrors. They seem pretty consistent and
closer to 1/6 wave than 1/4 wave. I have what I believe to be a 1/6 wave
Intermountain mirror in my personal NGT-12.5 and the view is awesome. We
built an NGT-12.5 for a customer in Japan who shipped us his own mirror ($2,500
for the primary alone) made by someone in Japan and touted as 1/10 wave. I
looked through that scope and compared it with my scope and it was clear that
both were limited to the seeing conditions as the view was the same. In
most cases, only high power and excellent seeing together can show you the
visual difference between 1/6 and 1/10 wave mirrors.
In some areas of the country you might get some nights
where the difference can actually be seen. So the question is really,
"do I want to pay three times as much for a mirror that will only deliver
its full potential a few nights out of the year?" If your answer is
"yes" then get the better mirror. In my case, the answer would
probably be "no" as I do most of my observing here in Colorado where
seeing is poor. I would also add that when I did take my scope to the
incredible seeing in the Florida Keys, it did not disappoint me. Seven
Trapezium stars were clearly seen.
What About the Scope?
Everyone has the "best telescope." The
fact is, every style of telescope has pros and cons. I consider these
facts to be almost inescapable:
1) SCTs are compact (and
most are GOTO) but they have unacceptable central obstructions and when larger
than 10" the are simply not a one-man scope.
2) Refractors are clearly the best to look through (no central
obstruction) but are just too expensive for one that is big enough to find
3) Newtonians grab the most light and have the next best view (much
better than SCTs) at a good price but they too can be a bit cumbersome at
larger sizes and have to be re-collimated at regular intervals.
tried to make the NGT with these things in mind. Because of how the scope
breaks down, I can say without a doubt that the NGT-12.5 is the most portable
12.5" reflector on the planet. Easily a 10-minute one-man
setup. The split-ring mount places the center of gravity quite low and
that proves to be very stable. While it's not a GOTO scope, it does have
digital setting circles which tend to keep the observer a little more in touch
with the sky he's looking at. Some people tend to go with the "big
name" scopes due to the "everybody buys them" theory and the
overwhelming amount of advertising they can afford. You can look at it
this way—if they were truly the best, all the other companies would not exist.
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