drift Alignment and CG-5

Since the original drift alignmeny post is getting far in the field here, I
am wondering if
a drift aligned 144 tooth CG-5 cheapo mount will hold a DSO for good picture
taking for say
5-10 minutes? I never see any exact reports about this. And the ones I
have seen are poorly
written and don't even mention drift alignment. The state of reporting on
this for the CG-5
leads me to believe that they are only good for 3-5 minutes of exposure
time.

On Fri, 05 Dec 2003 19:52:14 GMT, "Toma" wrote:

The state of reporting on this for the CG-5
leads me to believe that they are only good for 3-5 minutes of exposure
time.

Good assumption. Overall tracking is just fine; i.e., if you
"integrated" the track points, it will have excellent average
positional accuracy. You'll find, however, that the poor gear train
leads to excessive periodic error for long duration imaging.

Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

"WayneH" wrote in message
...
On Fri, 05 Dec 2003 19:52:14 GMT, "Toma" wrote:

The state of reporting on this for the CG-5
leads me to believe that they are only good for 3-5 minutes of exposure
time.

Good assumption. Overall tracking is just fine; i.e., if you
"integrated" the track points, it will have excellent average
positional accuracy. You'll find, however, that the poor gear train
leads to excessive periodic error for long duration imaging.



What is the definition of periodic error? Is it just the machining error of
the gears that add up
to an overall error that shows up as offtracking of so many arcseconds? I
presume the error
of the clock itself is negligible in all cases and does not enter in.

The term itself "periodic error" gives no indication as to the real meaning
of mechanical error.
The term "Tolerances" sucks too!! The word tolerate should onlt be applied
to structural "tolerences".

On Fri, 05 Dec 2003 21:46:22 GMT, "Toma" wrote:

What is the definition of periodic error? Is it just the machining error of
the gears that add up
to an overall error that shows up as offtracking of so many arcseconds?

Something like that. It is "periodic" in that the same error repeats
each time the worm makes a full revolution. Better drive systems can
be "trained" to compensate for most of the error - a standard CG5 has
no such capability.

I presume the error
of the clock itself is negligible in all cases and does not enter in.

True in most cases. Nevertheless, just as in all things created by
man, stuff happens. The drive speed is usually determined by a
crystal-controlled electronic "clock," and crystals not only have an
initial error (usually very small, to be sure) but change their
oscillation frequency with varying temperatures (and age). Better
clocks all but negate these errors (for our uses), but the old axiom
"Ya Gets What Ya Pay For" continues to hold true.

The term itself "periodic error" gives no indication as to the real meaning
of mechanical error.
The term "Tolerances" sucks too!! The word tolerate should onlt be applied
to structural "tolerences".

Not sure I understand - is this a rant, or are you looking for an
answer? grin

Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

I
am wondering if
a drift aligned 144 tooth CG-5 cheapo mount will hold a DSO for good picture
taking for say
5-10 minutes?

Hi:

At a telescope's prime focus? Without guiding? No way.

But most amateur mounts this side of a Paramount won't give you 10 minute
unguided exposures either. The culprit is not polar alignment, but periodic
gear error, which every gear-based telescope mount has to some degree. How much
you'll get unguided out of a CG5 also depends on how well-balanced it is, how
well-meshed/adjusted the gears are, etc. With everything tuned up, "a minute or
so" is more realistic without guiding. But, again, that's the case with most
mounts.

OTOH, If you're interested solely in piggyback photography through a normal -
medium focal length lens, you _could_ probably get 5 or 10 minutes out of a
CG5.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"Rod Mollise" wrote in message
...
I
am wondering if
a drift aligned 144 tooth CG-5 cheapo mount will hold a DSO for good
picture
taking for say
5-10 minutes?

Hi:

At a telescope's prime focus? Without guiding? No way.
How about 30sec to a minute?

"Toma" wrote in message
news:Ol5Ab.11836$d35.887@edtnps84...
Since the original drift alignmeny post is getting far in the field here,
I
am wondering if
a drift aligned 144 tooth CG-5 cheapo mount will hold a DSO for good
picture
taking for say
5-10 minutes? I never see any exact reports about this. And the ones I
have seen are poorly
written and don't even mention drift alignment. The state of reporting on
this for the CG-5
leads me to believe that they are only good for 3-5 minutes of exposure
time.
It really depends massively on the focal length involved, and on the part of
the sky. If you use a normal 35mm camera, with a 50mm lens, I have taken
widefield images of the Milky way, on this type of mount, involving
exposures of more than 10 minutes. However at focal lengths like 1m, with
either film or CCD, you would be very lucky indeed to take even a two minute
exposure, without some star trailing being visible. At 500mm, I have seen
some quite reasonable unguided images taken on film using these mounts,
using perhaps ten minute exposures. I'd suspect that the overall mount
'error', is probably perhaps 20 arc seconds in this time, which would then
correspond well with the advice given in the past by Barry Gordon, that star
movement should be kept below 0.05mm on film, for the results to be
acceptable (assuming mount error in this time is 20 arc seconds, the maximum
focal length to meet this criterion, is 206*50/20 = 515mm).
The change over the sky, is slightly different with a guided mount, from an
unguided one. On an unguided mount, there is no motion at all at the
celestial pole, but with a guided mount, if there is any error in Dec, as
the scope moves in RA (roughness on the moving surfaces), this will still
show at the pole. However RA errors will decrease in effect as you approach
the pole, allowing longer exposures to be taken here.
Using a long focal length scope like an SCT with perhaps a 2m focal length,
brings the allowable angular error on the mount down to:
206*50/2000 (the formula works either way round, by just swapping the focal
length for the allowable error), giving: 5.15 arc seconds of error being the
maximum that can be accepted without degradation of the image. I'd say that
a carefully tuned CG-5, could perhaps meet this for a couple of minutes
maximum, but 'out of the box', most would have trouble managing even one
minute...
There are 'periodic' errors on each component in the gear system. However
the largest one (and the one that is normally given this name when talking
about telescopes), is the irregularities in the worm gear driving the main
wheel. Technically, PE on the main wheel, and on this worm, will both have
the same magnitude (given the mechanical finish errors involved are the
same), but the worm one repeats every ten minutes, while the main wheel one
only repeats ever 24 hours. Periodic errors earlier in the gearbox, have a
reduced effect on the output motion (depending on the reduction ratio
between this point and the final drive).

Best Wishes

How about 30sec to a minute?

Hi:

Yes. With the scope well balanced, the gear mesh adjusted if need be, etc., you
can do 30 seconds with one. I have with an ST237.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html