7 Reduction in Observed Place

Normally, the ASTROM reduction is carried out (internally) using mean places for the epoch of the plate – positions corrected for precession, but not for nutation, aberration, deflection, and refraction, the effects of which are simply absorbed into the fit. This approach keeps the input file simple, and delivers perfectly adequate results for most practical purposes. However, there are some occasions on which a more precise reduction may be worthwhile.

Although the nutation, aberration and deflection are always relatively innocuous – the nutation produces a small and harmless rotation, the aberration varies very slowly across the sky, and the deflection is tiny except close to the Sun – the effects of atmospheric refraction can be quite important. As far as ASTROM is concerned, the refraction has two aspects:

• Differential refraction causes the picture to be distorted. The distortion is in the form of a non-linear scale reduction in the vertical direction, the reduction being larger near the bottom; this cannot be fully corrected by ASTROM’s linear model.
• Atmospheric dispersion, important for detectors of wide spectral coverage, causes the images of stars of different colour to appear shifted vertically from their nominal positions.

Both these effects can be eliminated if the optional time, observatory, meteorological and colour records are included in the input file. The advantages of bothering to do this are as follows:

• A more accurate result. The improvement is likely to be modest in most instances, but may be significant for low elevations and wide fields.
• More nearly equal scales will be reported in x and y, and will be constant all over the sky. Apart from providing additional reassurance that the fit is good, accurate knowledge of the scales is clearly vital if the optical parameters of the telescope are being measured for later use in predictions for guide stars or fibre feeds etc.
• The effects of colour may be important and can at least be quantified.

The optional records all begin with an explicit identifier, of which only the first character (T, O, M or C) is significant. They must immediately follow the plate data record, but can be in any order. Here is an example:

If the time, observatory and meteorological records are all omitted, any colour records subsequently encountered will be ignored. In the absence of full information, ASTROM makes plausible guesses to make good the deficiencies. If insufficient information for the observed place predictions is available, warnings are issued and the astrometry is done using mean place. If any of the three observation data records appears twice, the new information supplants the old, and no error is reported.

The TIME record specifies the time for mid-exposure; this can be given as a UT date and time (as five numbers, year, month, day, hour, minutes), or a local sidereal time (hour and minutes) or a Julian epoch (a single float). If the time record specifies a UT, and an epoch is specified on the plate data record, the latter is ignored. If the ST option is used, the epoch on the plate data record must be specified (and should be accurate to a day or two if the annual aberration and solar deflection are to be correctly computed). In the absence of a UT, it is reasonable to guess that the exposure occurred near upper culmination, which simply requires the ST to be set equal to the plate centre $\alpha$. For example, to perform an observed place reduction on a plate of a field at $\alpha =19^{h}13^m$, the following TIME record might be used:

The OBSERVATORY record can either specify one of the observatory identifiers recognized by the SLALIB routine SLA_OBS (see SUN/67):

or the observatory position can be given explicitly as in the example given earlier. If the TIME record specifies sidereal time, the observatory longitude may optionally be omitted. The height (metres above sea level) is of limited importance unless the meteorological record is absent, in which case the height is used to estimate the pressure.

The METEOROLOGICAL record specifies the temperature and pressure at the telescope, in ${}^{\circ }$K and mB respectively. The temperature defaults to $278^{\circ }$K; the default pressure is computed from the observatory height.

COLOUR records can appear anywhere after the time, observatory and meteorological records, except between a pair of reference star records. Here is an example:

The effective wavelength specified by such a record applies to all stars from that point onwards. Should two colour records follow consecutively, the second supplants the first, and no error is reported. Prior to the first colour record, a default of 500nm is assumed. Appendix C contains rough estimates of the effective wavelength for sources of different colour temperature and detectors of different passband. For the photographic case, the following table (compiled with the help of D. Malin) suggests effective wavelengths for some common combinations of emulsion, filter and star colour; the blue and red columns refer to very blue and very red (thermal) sources respectively (the effects may, of course, be more extreme for emission-line objects and other non-blackbody sources). For a star of spectral type G0, the effective wavelength will lie about halfway between the blue and red figures.

It must again be pointed out that there may be other important colour effects, apart from atmospheric dispersion, notably where refracting optics have been used. There is no attempt in ASTROM to model such phenomena.