preparing your observations
Terminology used in WFCAM observations can be confusing. It is
an area where different people use different words to describe the
same concepts, and different people use the same words to describe
different concepts. Thus we present here a reference set of
terminology. Please stick to these words when describing WFCAM
observations. If you believe there is a concept that needs a word,
that is not described here, please let us know.
An MSB is simply an ordered list of
An Observation is a slew to a given position on the sky and the
aquisition of a single guide star. No other guide stars will be used
during the course of the observation. The observation then proceeds
with a sequence which may include wfcam iterators, repeat iterators,
offset iterators, microstep iterators and integrations.
A set of 4 observations,
positions such that the gaps between the arrays in one observation are
filled by another observation. The result is reasonably uniform
coverage of a aquare of sky with area 0.75 square degrees.
Because of the necessary seperation between the target positions
of the 4 seperate observations, a different guide-star will need to be
used for each.
An Offset is an accurate shift in the telescope pointing. Note
that you cannot change guide-stars by doing an offset. The guide-star
is specified in the target component of the observation, and remains
fixed for the duration of the observation. This means that if the
guide-star moves off the autoguider CCD because of the offset, then
observations will be unguided untill you offset in such a way that
brings the same guide-star back onto the autoguider CCD. The geometry
focal-plane, showing the size and position of the autoguider CCD is
shown in Figure 1.1, “WFCAM focal
Because of the way the autoguider works, offsets between
autoguided positions are essentially quantised to the grid of possible
guide locations. See “Autoguider CCD”
Offsets are currently specified in arcseconds of RA and dec. The
guide-star will be pulled in to the nearest guide location that
corresponds to the end-point of the offset.
Where possible, the system will do a "fast offset". In a fast
offset, the secondary mirror of the telescope quickly tilts to move
the guide star to its new location on the guide ccd, at the same time
as the guide software moves to the new guide location on the
ccd. Essentially, this moves the image of the sky on the focal plane
by exactly the correct amount, and allows autoguiding to continue in
the new offset position, without actually moving the telescope
structure. This can happen very quickly as the heavy telescope
structure is not required to move. After guiding in the new offset
position has been confirmed and the integration started, the telescope
mount and secondary mirror will slowly move in unison to bring the
beam back to the optical axis of the telescope. This process is
transparent, and because autoguiding is maintained throughout, no
image quality degradation results.
The formal conditions that allow a fast offset are somewhat
complex, but in general, offsets less than 10" will be fast offsets,
and thus will be much quicker and more efficient than larger
Because of distortion over the WFCAM field, two integrations
that are offset more than a certain distance will not be able to be
mosaiced together without resampling. Typically the limiting case is
approximately a 10" offset.
Simply an ordered list of offsets.
offset is from the telescope base position (which is generally the
position specified in the target component of the observation), rather
than the endpoint of the previous offset.
A microstep is a special case of an offset
that results in offsets that are a particular number (N+1/2, N+1/3,
N+2/3) of pixels on the IR arrays. See “Microstepping” for details.
When a microstep pattern is done at each point in an offset
pattern, the microstep offsets are relative to the current offset
position, not the telescope base position.
There is be a pre-defined set of microstep sequence in the OT
microstep component to choose from. These have the exact
appropriate offsets specified within them, and are not user
An Integration is a sequence
of one or more
exposures. The Data Aquistion
system averages the
exposures together (using the arithmetic mean) so that an integration
results in a single image. This image is stored in the raw data
Thus, an integration on one of the arrays results in a single
image in a data file. Actually, each integration results in a single
NDF data structure inside the raw data HDS file. It should be noted at
this point that there are 4 seperate data flow channels in wfcam, one
for each array. This continues to the point where there are 4 seperate
raw data disks, each containing data from an individual array.
It is possible for there to be several integrations within the
observation, although with WFCAM, this only happens for focus
measurements. In this case, the raw data HDS container file contains
several NDF components, one for each integration.
The number of coadds is
simply the number
of exposures per integration.
To "do coadds" is an informal expression implying that ones
integrations consist of more than one exposure
An Exposure is a single
complete cycle of
the array readout
mode, generally an array reset followed by one or more array
reads over a time specified as the Exposure
Simplistically, it is the atomic unit of observation.
For details of what happens during an exposure, including why
the elapsed time for an exposure is greater than the exposure time,
see “Readout Modes”.
The data values that result from an exposure represent the
number of electrons detected by each pixel over a time period equal to
the exposure time.
The data resulting from each individual exposure is not
necessarily saved to disc.
scientific survey MSB design
These guidelines are written primarily for those preparing the
MSBs for the UKIDSS. In general, individual proposal users should
follow them too, and consult with the wfcam support scientist well in
advance if they believe they need to deviate significantly from these
The following unconfirmed assumptions are notable in the basis
of these guidelines.
Darks stable of a
>24 hours. Dark image is a function of exposure time only over this
Bad / Hot pixels can be
from darks and either daytime (dome) flats or twilight flats, and are
stable over timescales >24 hours.
Reset Anomaly predictable
either stable or can be simply modelled if
linearity measures are
and are stable over timescales of >1 week
Cross-talk matrices are
availiable and are stable over timescales of >1
Image Persistence is
and can be ignored. Although lab tests at ATC look promising, we
should bare in mind the possibility that this might not be strictly
true, and keep our options open in that respect.
Calibration MSBs are
will include darks and photometric standard fields, and will be
observed every hour.
- Maximum Duration
Each MSB should take no more than 1 hour of elapsed
time. This is to allow hourly observation of Calibration
- Minimum Duration
In general, MSBs should not take less than 10 minutes
of elapsed time.
- Number of different
positions on the sky
Each MSB should contain integrations at 16 or more
different positions on the sky in each filter used. This is to ensure
good sky estimation and as a fallback in case the processing has to
rely on self-calibrating MSBs for instrumental signature
- Offset distances
We recomend keeping offsets to <10". This enables
fast-offsetting for efficiency, and is well below the distance at
which differential distortion requires you to re-sample the data in
order to form a mosaic.
- Frequency of filter
Survey MSBs should not change filters more often than
once every 10 minutes. This allows the full set (YZJHK) of filters to
be covered in a 1 hour MSB. We recomend minimizing filter changes as
much as possible. This both increases operational efficiency, and
facilitates better sky subtraction and fringe
- Positions of
observations within an MSB
Multiple science observations within an MSB should be
grouped as closely as possible on the sky, to improve observational
efficiency and sky estimation.
- Offset sky
If an MSB requires offset sky integrations for sky
subtraction, then all necessary offset sky observations will be
included in that MSB. This ensures both that they are carried out, and
that they are carried out within 1 hour (the maximum MSB duration
given in these guidelines), which we believe to be the longest
recomendable interval for sky subtraction.
- Exposure Times and Darks
WFCAM darks are observed at the start of every night; only certain combinations of darks and coadds
are observed. The WFCAM FAQ page lists the exposure times and coadds routinely observed. Please stick to one of these combinations for your observations. If you need to use any other exposure time or coadd, please make sure to discuss with your support astronomer.
The WFCAM IR array pixel scale is approximately 0.4"/pixel (see Introduction for more
details). This allows a large survey area, though does not critically
sample the UKIRT/WFCAM point spread function under most observing
conditions. In order to obtain critical sampling of the PSF, a
technique known as microstepping
Basically, microstepping involves taking several seperate
integrations at precisely offset telescope positions, with the precise
offsets corresponding to precise fractions of a pixel on the IR
arrays along each axis. The options availiable are "2×2" and
microstepping. "2×2" microstepping involves taking 4
integrations, offset in 1/2 pixel steps, "3×3" microstepping
takes 9 integrations, offset by 1/3 pixel steps.
Because the autoguider can only allow offsets quantised to its
grid of possible guide locations (see “Autoguider
CCD”), the microstep offsets are
selected such that the offset corresponds to both an integer number of
autoguider offset stpes, and N+1/2, N+1/3 or N+2/3 pixel steps on the
IR arrays, where N is an arbitary integer.
The actuall step sizes involved are given in Table 4.1, “Microstep offsets”.
is essentially "no microstepping".
4.1. Microstep offsets
||Offset size in
||Offset size in IR
||Offset size in
All WFCAM observations are required to use a standard,
observatory defined, calibration strategy. This ensures uniformity of
data in the science archive.
A selection of dark frames are carried out before sunset at the
start of the observing night. These contain darks of standard exposure
time, coadds and readout mode combinations. A table of these
given in Table
4.2, “Exposure time / Readmode combinations of the standard dark frame
calibrations”. If your programme requires exposure times not listed
in this table, then you must
include darks in your own MSBs.
4.1. Exposure time / Readmode combinations of the standard dark
Flatfields are obtained from twilight sky. We will also be
investigating a tungsten-halogen illuminated screen in the dome.
There is no need for an individual observer or PI to prepare flatfield
observations, the details are taken care of by the observatory, though
observers may of course find that they are required to take
(pre-prepared) flat field observations on one or more of their
observing nights. The observations are done in twilight when it would
be too bright to carry out science observing, so there in no time
impact on the project in doing this.
The initial calibration plan for WFCAM involved taking a
standard star field every hour throughout the night. In light of
analysis done on the data between comissioning in late 2004 and survey
data in late 2005, it has been decided that it is sufficient to take
standard star field observations every 2 hours during photometric
conditions, and in non-photometric conditions to take them at the
start of the night and at midnight.
A set of observations of a standard field in all 5 broadband
filters takes about 5 minutes. This overhead should generally be
included in your telescope time request.
The standard procedure adopted for the calibration is to observe a
single standard star in only one of the cameras (currently camera 3),
with a 3-point jitter. This assumes that relative zero-points between
the four cameras are stable. You might want to center the standard on a
different camera if you are not using mainly camera 3.