Drift maps in J2000 coordinates.
26feb05
Drift scan are are normally done in coordinates of
date
(current coordinates). A starting ra, dec is used with an ra rate
of 15 arc seconds per sidereal second. The telescope then remains fixed
as the sky drifts by. This is fine for small maps, but it
can
be a problem for larger surveys that extend over many years.
Since the az, za is remaining fixed, the drift is
being done in current not J2000 coordinates. If you were planning to
map
in J2000, or to repeat a strip 2 years later, this would be a problem.
To see how large this change was, a drift scan in J2000
coordinates
(15 asecs/sidereal sec epoch J2000) was computed and then the relative
motion of the az, za coordinates were plotted. For each day computed:
-
The telescope was positioned to az=180,za=[2..18] in 2 deg za steps.
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The ra, dec for 0 hours utc was computed using the az, za values.
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A rate of 15asec/sec sidereal was applied to the starting ra over
the entire day
-
The az, za was then recomputed for these new J2000 ra, decs over the
day.
These are the az, za's that would be tracked if the drift was really in
J2000 coordinates.
-
The az, za motion was then plotted over the day removing the starting
az,
za (which was at the correct J2000 position for the start of the strip).
The
plots
show the az/za relative motion (.ps) (.pdf)
for the entire day. The top plot is the azimuth encoder motion. The
middle
plot is the azimuth motion on the sky. The bottom plot is the za motion
(encoder/sky).
-
Fig 1: 01mar05 The azimuth encoder moves by less than .2
degrees
(except for low za). On the sky the az motion is < 1 arc
minute.
The za amplitude is 1.75 arcminutes
-
Fig 2: 01mar06 The za amplitude has now increased to an
amplitude
of 2.13 arcminutes.
-
Fig 3: 01mar07 The za amplitude is 2.53 arcminutes.
The amplitude is increasing yearly since the precession is moving the
pole
away from the J2000 position. The plots show that:
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The az/za encoders would move very little over a day while doing a
drift
scan in J2000 at transit (< .1 degrees).
-
If you did drift scans in current coordinates then a drift would move
away
from the j2000 dec. The maximum values by year are:
-
2005: .5 amin/hour
-
2006: .6 amin/hour
-
2007: .7 amin/hour
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The amplitude change of the sine wave is about .4 arc minute per
year.
-
A 1 hour strip done in 2005 and then repeated in 2007 over the same sky
would differ by about .2 amin at the end of the scan.
processing: usr/a2010/chkdrift.pr
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