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

On May 12, 5:41 am, pau...@xxxxxxx (Paul Schlyter) wrote:
In article <1178927950.515878.321...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

<wsnel...@xxxxxxxxxxx> wrote:
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,

You could do a lot of weird things of course. But will you be able to
even find an equatorial mount, with tracking, which costs not much more
than a 50mm refractor in a department store?

A Poncet table, built in the original form (S&T Jan. 1977) should do
the trick. A cheap synch motor/tangent arm would suffice as the
drive. Even 50mm scopes run $40 to $50 or more, if a tripod/mount is
included.


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.

You said: "To obtain tracking with all but the smallest scopes often seems
to double the cost of the instrument". My argument was that to get tracking
on one of those smallest scopes will more than double the cost of it.

The equatorial StarBlast w /drive is only 45% more expensive than the
alt-az StarBlast, and only 10% more w/o drive. Look at the Orion site
for other examples. In the smaller sizes, an equatorial mount isn't
much more complicated to build than an altazimuth, and the cost of
eyepieces, finders, focusers, spiders, etc., is a larger factor in the
cost of small scopes, compared to larger scopes, as it should be.


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.

Of course your argument is valid in *some* cases! I was merely
arguing against its claimed general validity.

My arguments are valid in most cases not just some cases, and usually
valid for the sort of scope/mount combinations that most amateurs
would consider buying or building.

I could point to the
Questar which costs several thousand in its 3.5" version. This is much
more than your 12-inch Dob or 6-inch equatorial, so in that particular
case "a smaller aperture is more expensive". But that's a very
special, not a general, rule!


Everything about the Questar is more expensive and it is aimed at a
somewhat different market than the usual astro telescope. However, the
lowest priced Standard Questar seems to be only 40% more expensive
than the lowest priced Field Model and the cost to convert to convert
that Field to a Duplex adds about 80% to the cost, i.e. once again the
mount is less than half the cost in small apertures.


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.

So the equatorial tracking mount will probably be about as expensive as
the rest of the scope, and adding an equatorial mount and tracking will
probably double the cost of the scope for many common scope sizes.

But since the cost of the scope rises faster than linearly with aperture,
if you omit the eq. mount you can probably not get a scope with twice
the aperture for the same cost. Perhaps 40-50% larger aperture, but
not 100% larger. In some cases (like the ones you pointed to above) you
could probably get twice the aperture, but not in general.

In the middle range of aperture, an equatorial mount w/drive will
usually be roughly equal to the cost of the OTA, although it can be
argued that this might result in an undersized mount. In that case
you might consider spending more on a sturdier mount and/or a Mak or
SCT(compact, lighter, more expensive for given aperture) instead of a
Newtonian (long and heavy.) You will probably end up with about half
the aperture of a comparably priced Dob as a result, especially if you
are seeking a rig as smooth and stable as the Dob. The OP's 4-inch
scope tracked Mars for about the same price as an XT-8, but its OTA
was short and light; try using that mount for a 6-inch f/6!

Also, a Dob isn't merely "an equatorial with the eq. mount removed".
The Dob tries to simplify the entire scope to keep the costs down.
As a result, the Dob is most likely of lower quality than the OTA of
the equatorial scope. And I'm not just talking about the optics here.
Perhaps the tube of the Dob is less stable, or causes more thermal
turbulence during the cooldown of the scope? Perhaps the focusing of
the Dob runs less smoothly? Perhaps the finder scope of the Dob is of
lower quality, or even absent? There are many ways you could cut down
the cost of a product - you do get what you pay for.

The XT-8 has a 50mm finder, 2-inch focuser, Plossl eyepieces, a steel
tube, a spider and mirror cell that look fine. Overall the OTA looks
comparable to Orion's 8-inch f/5 equatorials.

"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.

Very large scopes must have an observatory: they're not portable but
they still need shelter against bad weather. And I thought it was
unfair to compare the cost of a very large scope with observatory with
a much smaller scope without an observatory. That's how the
observatory entered the discussion....

YOU brought up the idea of an observatory; I'm not even factoring it
in WRT cost. But if your scope is so large that you really do need to
build an observatory for it, it certainly seems fair to include that
in the costs

.

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