Re: 4 inch reflector can't resolve Mars at all

On May 11, 3:42 am, pau...@xxxxxxx (Paul Schlyter) wrote:
In article <1178842877.144621.36...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

<wsnel...@xxxxxxxxxxx> wrote:
To obtain tracking with all but the smallest scopes often seems to
double the cost of the instrument, conservatively.

It will probably double the cost even for the smallest scopes: try to
buy a toy scope at some department store, then buy an equatorial mounting
with a clock drive for that scope.... I don't think cost doubling is
enough in that case.

Well, yeah, I suppose you could put a 50mm refractor on an
Astrophysics mount - or a Questar OTA on an EQ-1, but in general an
equatorial mount seems to account for about half the cost of a -
typical- setup. The mount for a Dobsonian OTOH consists mostly of a
few square yards of plywood, even for the large scopes, i.e. an almost
miniscule cost in comparison to the tube assembly.

For a few examples, $500-$600 buys you a 10-inch Dob or a 5-inch
equatorial Mak; $900-$1000 buys a 12-inch Dob or a 6-inch equatorial
Mak. OTOH an 8-inch Dob is available for around $360, while an 8-inch
equatorial Newt w/drive will run well over $700.


Or for the same outlay you might get about half the aperture with a
scope that tracks.

Comparing a clock-driven 4-inch with an 8-inch Dob, by the end of the
night, you will have seen more detail in the larger scope. No matter
how long you stare at the perfectly centered image in the smaller,
clock-driven, scope, you will not, ultimately see as much as you would
through the larger Dob.

Doesn't the cost of a scope generally increase more than the aperture?
So that a scope with twice the aperture will cost more than twice as
much?

Otherwise giant scopes would be quite affordable - so that instead of
25 people each buying an 8-inch scope of their own, they could instead
join and together buy, say, the 200-inch Hale telescope. If 50 people
joined, they could buy one of the Keck telescopes. Wouldn't that be
something? ;-)

Earlier I've heard estimates that the cost of a scope is apporimately
proportional to something between the square and the cube of the aperture,
and that sounds more reasonable to me. If the cost rises as the cube
of the aperture, then some 125,000 instead of just 50 potential buyers
of personal 8-inch scopes would have to join forces to afford the Keck.

Yes, again, the cost does seem to scale up exponentially with
aperture, but the cost of an equatorial mounts rises too, as carrying
capacity increases.

"But giant scopes also have an observatory built around them!", I hear
you object. True, but people buy personal observatories around their
8-inch scopes too. And the cost of buildings will also increase
faster than their physical size.


I don't get your point WRT to observatories, but a very large scope
might no longer be portable, so, yes, you will probably need an
observatory.

.

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