At 07:07 PM 11/2/04, Orin wrote:
>Your photo looks very sharp, Richard.
>
>I am currently trying to get familiar with polar alignment and related
>issues of mount type and focus tips. I would enjoy hearing your views. I
>have several OM-1s and OM-2s and will be trying for some meteor shots during
>the Leonid and Geminid showers this winter.
>
>One of my questions is whether I will get star trails if I put several
>cameras on my scope after polar alignment... but the cameras are covering
>different areas of sky?
No . . . provided you have done proper polar alignment. What prevents you
from getting star trails is proper polar alignment which puts the Right
Ascension axis of rotation parallel with the Earth's axis of rotation. If
you have an RA drive that matches Earth rotation, then you can turn that on
(but guiding is still strongly recommended). Where you aim things in terms
of Right Ascension or Declination after that doesn't matter for the "fixed
stars" of the celestial sphere (everything else *will* move relative to the
celestial sphere; planets, Sun, Moon, asteroids, etc.). Guiding using the
'scope at about 100X - 200X on a star allows periodic error correction . .
. very, very worm and ring gears are perfect . . . likewise with most drive
motors. The importance of this increases.
Even if you guide perfectly, you can still get field rotation in long
exposures if polar alignment is off. The longer the exposure, and the
closer you are aiming the camera toward the pole, the greater the effect of
field rotation if polar alignment is not done with some precision. The
easiest method is:
(a) Check to ensure your polar scope is parallel to the scope mount's RA
axis. This is usually done by aiming the polar scope at a star, then
loosening its lock screw, rotate it on its axis to see if the center
reticle moves off the star (ensure you're using a true "center" reticle . .
. not an offset one). Adjust the "jack screws" that allow tweaking polar
scope alignment until a star centered in it no longer drifts as it's
rotated on its axis. BTW, this can be done during the day by aiming at a
very distant terrestrial object. Once set, this is like a "finder" scope
that shouldn't need realignment unless it's disassembled from the scope
mount, bumped or otherwise messed with enough to warrant checking it.
(b) Set up field tripod so that the "north" leg is as close to true north
as possible. Allowable error for this is usually several degrees . . .
just needs to be close enough so that you don't run out of fine adjustment
tweaking scope mount aziumuth later.
(c) Level tripod base so that tweaks to azimuth of mount base don't affect
tweaks to its altitude, and vice versa. Any error to this will be taken
care of later . . . and the system will be independent of any error in this
. . . just makes life easier and alignment faster.
(d) Set inlination of RA axis for your latitude. Within a degree is
usually close enough.
(e) Do moderate polar alignment using a polar axis scope. In the northern
hemisphere (IIRC), the rotational pole is on an imaginary line connecting
the last bright star in Ursa Major's "handle", Polaris, and the last bright
star in Cassiopeia on the opposite end it from Ursa Major's handle. Along
this imaginary line, it's about 44 arc-minutes from Polaris in the
direction of Cassiopeia. Some polar scopes have this line and some reticle
marks for Ursa Major, Cassiopeia and Polaris that can be rotated inside the
scope. Tweak scope mount azimuth and altitude as needed.
(f) Do fine polar alignment using the "drift" method. This is also the
step that frees the system of any error in base leveling. Doing a proper
drift alignment takes about 20-40 minutes . . . at the end of this there
should be *zero* detectable drift in altitude or azimuth for about five
minutes at about 200X magnification. This is done using two stars. One is
at zero declination along (or very close to) your meridian to the south (in
the northern hemisphere). The other is east or west, also at zero
declination, about 20 degrees above the horizon (to prevent atmospheric
refraction effects). Look for specific instructions on the net on how to
tweak mount azimuth and altitude for the drifting of these stars in your
'scope (it's too lengthy to post here). See these for method and
explanations about how drift alignment works:
http://members.aol.com/ccdastro/drift-align.htm
http://www.mindspring.com/~jeffpo/polar.htm
http://www.starizona.com/ccd/settinguppolar.htm
This doesn't take as long as you think once you've done it a few times and
starting setup at dusk so you can do the polar scope and drift aligning
just as Polaris and a few brighter stars are visible maximizes "dark
time." It's the last step . . . drift alignment . . . that is important
for long exposures to prevent field rotation . . . and it makes guiding
corrections using the scope much easier by reducing the "nudges" to one
every few minutes for periodic RA gear errors or perhaps a drive motor that
isn't dead on in its rotational speed.
BTW, having a piggyback mount that allows shifting camera direction off of
the scope's axis some is preferred . . . it keeps the scope tube out of the
bottom of the photograph.
-- John Lind
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