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[ATM] Dob vs Eq mount
Richard,
For my second telescope, (8 inch f/7.6 Newtonian), I traveled the same
thought path as you just described.
A child of the 60's, I am biased toward equatorial mounts. GEM's annoy
with their counterweights and bad habit of running into the tripod legs, but
I like forks.
When I re-entered the hobby in 2001, I found the large truss tube
Newtonians, with the balance point near the mirror, on a Dobsonian mount a
beautiful and exciting concept. But it doesn't track the stars. And I
believe the vast popularity of big Dobs has confused fundamental observing
issues for a lot of newcomers.
I did several iterations of design adapting the truss/Dob idea into a PED
(polar-equatorial-disk) style mount. This mount concept is illustrated in
Texereau but can be improved.
The PED differs from the split ring by locating the ring at the base of the
fork. It is viable and it can be made lightweight and compact as your
intuition tells you. My 8" equatorial fork works as I thought it would.
Another member of our club, Denver Astronomical Society, has a 12.5" of
similar design. Unfortunately, my drawings are web-challenged (ie pencil).
I am just starting to build
a site to publish some of these ideas.
IMHO, here are the pros and cons for an 8' travel scope:
A. The Dob is good because:
1. Every astronomer should start with naked eye, binoculars, and star
hopping.
2. Extreme portability means no batteries or motors or computers.
3. Low mag views of big clusters sail serenely through the 1.5 degree
field of view.
4. You can build one with little knowledge, simple instructions, and few
tools.
5. You can build one with materials from a well-stocked dumpster and be
accepted at star parties, if you just wave printouts from the San Francisco
Sidewalk Astronomers website.
B. The non-computerized Dob will seem inadequate when you need to track the
sky:
1. You want time to study a planet at 120x.
2. You want to want to split a close double star at 240x.
3. You want to show a companion some object you found at 300x.
4. You want to find any bright object at 400x.
5. You believe in celestial coordinates, and you want to find a dim
object at any power.
6. You try to do almost any kind of imaging.
Many Dob owners have added tracking by building or buying a Poncet or other
equatorial platform. It works, for up to an hour, but you have added
complexity, a third axis, and sacrificed some portability.
Dob owners now have choices of several motor+computer systems, tracking, and
go-to. These will track objects in the sky or automatically slew to them.
Downside? Added cost, motors, computers and the hassle of setting them up
and troubleshooting. Just read the use groups. And still the field rotates
while imaging. And for any alt-az (read Dob) it rotates fastest crossing the
zenith where the clearest view is found. This is fixed by adding more
motors. Now, did you bring your travelscope on vacation and hike to a
secluded glen to fight with software and repair broken wires to make your
instrument work? Or did you want to kick back, look for some new objects
from a dark site, and maybe feel a connection to ancient stars whose dust
makes up your blood and bones? Either answer is OK, but it has to be your
choice.
The simplest solution to tracking and field rotation is to mount the scope
on an axis parallel to the earth's. Tracking can be performed by hand via
flexible shaft. Or, a very small, slow gear motor, a trivial timing
circuit, and a small battery can take over the job, while you ponder our
impending collision with M31.
My conclusions from all my (unwebbable) drawings of a PED mount?
1. The width (E-W) of your minimum mount will be about mirror diameter D
+ 6" vs a Dob's D+4.
2. Length (N-S) will be D+8 to D+10 vs a Dob's D+4 or D+5.
3. Weight about equal.
4. 8" mirrors are light, so the tube balance point will be a min of 30
to 40% up from mirror end.
5. The Serurrier truss tube is an easy, good idea, and cancels some
deflections.
5. Yes, for visual use a rotating cage is mandatory. This is not the
bugbear or weight that some report, but it does require extra care in
collimation.
5. Yes, the CG of the scope tube overhangs the roller bearings under
the disk, as mentioned by Tom Krajci. This is not a big deal, and actually
helps add load to the friction surface which is driving RA. The force path
for this load actually flows to ground a very short distance north of those
rollers.
6. Mount deflection in Declination can easily be held below 2 arc
seconds using a steel triangle truss for the fork arms.
7. Arm deflection in RA is larger because the E-W dimensions of the fork
arms have been minimized to keep the PED small.. But the flexures can be
made perpendicular to the polar axis and parallel to each other so they do
not change the telescope pointing. (Also true in a Dob).
8. Bronze, ball, and roller bearings are cheaper to buy and easier to
use than the half-moon+virgin Teflon recipe for a Dob. (I know, I know:
HERESY! Dobs are supposed to be cheap! So prepare the kindling and the
stake).
As TK keeps pointing out, all these words are much better understood with
drawings and pictures posted to a website. I am working on it. This post
is too long anyway. Meanwhile, Richard, perhaps you and I can quietly start
the Second Equatorial Revolution (the first started with Copernicus) and
drive all those Flat-earth Dobsonians from the field!
Go for it.
Stuart
----- Original Message -----
From: "Richard Brodie" <richardbrodie@optusnet.com.au>
To: "'Mailing List: Amateur Telescope Makers'" <ATM@atmlist.net>
Sent: Monday, March 01, 2004 2:54 PM
Subject: [ATM] Dob vs Eq mount
> I am attempting my first ATM project by (re)building a 2nd Hand 8" dob
> into something that is more compact, lighter and can track/find. My
> main criteria is compact travel as while I do have a large car, when
> traveling to my "dark site" it is usually for a weekend (at least) with
> wife and kids, 3 bikes and lots of luggage etc etc.
>
> It will be an all Alu construction truss for the OTA.
>
> I am about to start work on the design of the mount and I'm intrigued by
> what looks to me to be an obsession with dob mounts rather that
> equatorials for the miminalist alu telescopes being constructed. I've
> looked at lots of work done by ATMs on the web and those building alu
> scopes all seem to be dob mounts (maybe I just haven't looked far
> enough).
>
> Is there a particular reason(s) for this? I would have thought that an
> equatorial (eg split ring) would be just as easy to construct, just as
> compact but easier for tracking and better for photography.
>
> I'll be happy to build a low profile dob (flex rocker?) if there is good
> reason to do so, but I am currently leaning towards a split ring equ
> mount as I would like to be able to do photography, even though visual
> observing will be my most common way.
>
> If this project goes well then I will be attempting something a little
> more ambitious, as I am getting aperture hungry.
>
> Comments appreciated
>
> Regards
>
> Richard
>
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