An MSB is simply an ordered list of observations.

A set of 4 observations, with target 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 of the focal-plane, showing the size and position of the autoguider CCD is shown in Figure 1.1, “WFCAM focal plane layout”.

Because of the way the autoguider works, offsets between autoguided positions are essentially quantised to the grid of possible guide locations. See “Autoguider CCD” for details.

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 offsets.

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. Each 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 editable.

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 file.

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 Time. 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. 

Guidelines for 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 guidelines.

Microstepping

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 is used.

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 "3×3" 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”. "1×1" microstepping is essentially "no microstepping". 

Calibrations

All WFCAM observations are required to use a standard, observatory defined, calibration strategy. This ensures uniformity of data in the science archive.