Table of Pointing Corrections

Note: RA measured in azimuth arcminutes, DEC measured in arcminutes

Acronyms used in Table :

CDCC : Commanded DC Correction

MDCC : Measured DC Correction

DeltaDCC : Delta DC Correction = Measured DC Correction - Commanded DC Correction

DeltaTC : Delta Tilt Correction = Measured Tilt Correction - Commanded Tilt Correction

The TC phase is defined as follows: it is the UT time (in days) when the telescope is inferred to be pointing towards the highest point of the tilted base

Source
Date
Notes
RA DeltaDCC
RA CDCC
RA MDCC
RA DeltaTC amp
RA DeltaTC phase (Days)
Dec DeltaDCC
Dec CDCC
Dec MDCC
Dec DeltaTC amp
Dec DeltaTC phase (Days)
rcw108
2/25~28
-
-1.35
207.00
205.65
0.96
26.93
0.74
-76.50
-75.76
1.01
27.88
rcw108
3/4~6
-
-1.26
207.00
205.74
0.97
32.92
1.44
-76.50
-75.06
0.94
33.89
rcw108
3/6~10
-
-0.74
207.00
206.26
0.96
37.92
1.34
-76.50
-75.16
0.97
34.92
rcw108
3/12~13
tilty added
-0.61
207.00
206.39
1.06
36.88
1.10 -76.50
-75.40
0.35
42.48
rcw108
3/16~17
"pointing event"
1.11
202.88
203.99
0.91
41.93
0.04
-69.60
-69.56
0.19
42.77
rcw108
3/17~18
-
0.99
202.88
203.87
0.85
43.86
-0.78
-69.60
-70.38
0.40
48.85
rcw108
3/18~19
tiltx added
1.11
202.88
203.99
0.13
49.09
-1.00
-69.60
-70.60
0.25
42.40
rcw57
3/27~28
-
0.63
208.26
208.89
0.65
53.72
-0.05
-71.52
-71.57
0.51
63.51
rcw57
3/31
link
1.11
208.26
209.37
0.46
59.56
0.22
-71.52
-71.30
0.55
89.78
rcw57
4/1
-
0.95
208.26
209.21
-
-
-0.21
-71.52
-71.73
-
-
rcw57
4/3
-
0.17
208.26
208.43
0.14(std)
-
-0.03
-71.52
-71.55
0.08(std)
-
rcw57
4/3~4
-
0.26
208.26
208.52
0.44(std)
-
-0.13
-71.52
-71.65
0.19(std)
-
rcw57
4/5~7
-
0.32
208.26
208.58
0.34(std)
-
-0.10
-71.52
-71.62
0.17(std)
-
rcw108
4/16~9
-
-0.34
203.46
203.12
0.48(std)
-
-0.33
-70.44
-70.77
0.36(std)
-
rcw57
4/27~5/2
absolute fit--MTC in DeltaTC column


~209.2
2.58
88.53


-71.45
1.37
89.53
rcw57
5/26~6/2
absolute fit--MTC in DeltaTC column
0.2
208.8
209.0
3.22
118.41
1.06
-71.46
-70.40
1.72
117.47

Explanations:

Basics:

The tracking model used by SPARO in 2003 is very simple and incorporates only four parameters: The R.A. and Dec encoder offsets ("DC corrections") and the amplitude and phase of the telescope base tilt. When you point SPARO at a source of known R.A. and Dec, you will need to add these corrections to the actual coordinates or else you will not be pointed at the source. These corrections are referred to as the "DC correction" (DCC) and the "Tilt Correction" (TC).

Actual Source Coordinates + DC Correction + Tilt Correction = Corrected Source Coordinates

or

ASC + DCC + TC = CSC

Adding the DCC to the ASC is trivial. These are just constant offsets in RA and Dec. But adding the effects of tilt is more subtle. First of all, the TC correction varies sinusoidally with a period of one sidereal day. Second, the phase of this variation depends on the RA of the source. Third, the amplitude of this variation is constant for the Dec, but depends on declination for the case of the RA. This is all discussed in Dave Chuss' pointing memo.

The DCC and TC are determined by observing a point source for 24 hours and fitting both the RA pointing and the Dec pointing for the effects of tilt and for DC offsets. We have found that tilt amplitude is increasing very slowly with time. It has doubled during the time period Feb-July. The DCC do not appear to vary with time for a given source, at least not by much. (The exception occurred in mid-March when we had a sudden change in the DCC.)

We find that the DCC is different for each source that we observe. This is expected, because of various effects such as gravitational sag of the telescope and any lateral misalignment of SPARO in the focal plane of the telescope. We do not expect that the TC varies from source to source, and we have no evidence that it does.

The control computer keeps track of the CDC and TC parameters. The user supplies the ASC. The specific names of the tcl parameters that the control computer uses are as follows: For the DCC they are "pmodel::xoffset" and "pmodel::yoffset". For the TC they are "pmodel::tiltamp" and "pmodel::tiltphase".


Effects of the chopping flat mirror:

Recall that each pixel of SPARO looks at two points on the sky nearly simultaneously because of Viper's chopping flat mirror ("left beam" and "right beam"). What then does it mean for SPARO to be pointed at a source that has a certain R.A. and Dec? Does it mean that the left beam is pointed at that source? Or the right beam? Or something else? The convention used by the tcl scripts that run on control is that SPARO is pointed at a position that would put the source half-way between these two beams. So the SPARO pointing coordinates and parameters as defined above are referenced to the "center position" of the chop. So if you want to put the source in your left beam then you have to move the telescope by half-a-chopper throw towards positive R.A. from these "center-position" pointings.

CSC + 0.5*(chopper throw) = Corrected left-beam Source Coordinates

Same for the right beam but with a minus sign instead of a plus sign.

Note that this chopper throw correction is handled by the tcl scripts using the parameter "sparo::throw".

Starting with the ASC, the DCC and TC of "pmodel", and the "sparo::throw", the tcl scripts calculate the Corrected left-beam Source Coordinates and Corrected right-beam Source Coordinates and pass them to comsoft and to the Macintosh data computer. The Macintosh data computer stores the Corrected left-beam Source Coordinates in the data file.


Miscellanous notes on SPARO pointing:

More conventions: Unless otherwise stated, (1) all ASC's, DCC's, and TC's are precessed to the 2000 Epoch, (2) all DCC's and CSC's refer to pixel 5, and (3) all R.A. DCC's and CSC's are measured in "azimuth angle", not "real angle" ("real angle" = "cross-elevation angle"). However, "sparo::throw" is measured in "real angle".

Finally, note that there could be two reasons for non-zero DCC for a give source: (1) the Viper pointing is not perfect, and (2) the source coordinates are wrong. Almost always most of the DCC will be due to the first effect, but for a few sources the coordinates are not precisely known.


Details of the implementation - analysis procedure:

During the whole of winter 2003, SPARO spends some time looking at "pointing sources" that are relatively "peaky" sources of relatively well-known ASC. Then, we find the Corrected left-beam Source Coordinates of the peak of the measured flux, via gaussian fitting. Then, we see how these coordinates vary with time over the course of a day or more - specifically, we fit them to a sine curve of period one sidereal day. We then use the results of this fit to determine what the DCC and TC should have been for that observation. We refer to these post-facto pointing corrections as the "Measured DC Correction" (MDCC) and the "Measured Tilt Correction" (MTC). Then these parameters are passed to Paolo, and he inputs them into the tcl scripts for use during the next observation. At that point they are referred to as the Commanded DCC (CDCC) and Commanded TC (CTC) parameters. The iteration process continues indefinitely, in order to improve the accuracy of the pointing parameters and keep track of their changes over time.

A slightly outdated table of pointing targets with RA and dec coordinates can be found here.

 

Last modified by Giles on 07/12/03