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Aperture Masks

The multiaperture masks consist of a number of slits (typically 300m wide = 1.5 arcsec) cut in a brass disk. The masks may be made at the telescope using a specialized manufacturing facility which provides masks of very high quality which can be made close to the time of observation. If arranged with technical staff, masks can be made in `real-time' from coordinates supplied from an analysis of a direct image taken with LDSS-2. To allow accurate sky subtraction for each object, it is best to use slits at least 5-10'' long so that there is plenty of sky either side of the object spectrum. In principle this means that there is space to fit 60-100 spectra on the detector, but the distribution of objects on the sky means that this maximum is not achieved in practice. A typical field will have space for about than 30-40 spectra.

The masks must be positioned flat in the focal plane of the telescope so that images are in focus at all positions over the mask. This means that the masks can not be tilted to allow any form of slit viewing. The acquisition of each field uses fiducial holes in the mask as described in Section 5.3.

In some cases, you may wish to use LDSS-2 with a longslit. Normally a traditional longslit spectrograph (e.g. ISIS/FOS-2) would be preferred for single object observations since these offer adjustable slit-jaws that are inclined to permit slit-viewing. However there may be occasions where you need to mix multiaperture and longslit work, especially for flux calibration where you might observe a spectrophotometric standard through a longslit. Here you have two choices:

• Treat your longslit observation as a multiaperture observation where there happens to be only one slit. In this case, you would design and manufacture individual masks for each field with fiducial holes to allow the telescope to be precisely aligned with the target field.

• Use a longslit mask that has already been manufactured (see below). Since this will not have fiducial holes, you will need to use a variant of the usual target acquisition technique.

In addition to the multiaperture masks that are custom-made for each target field, there are masks already manufactured that have special functions.

Matrix mask: A regularly spaced square grid of holes used for positional calibration and distortion mapping. The pitch is 10 arcsec. See Section 4.8 for further information.

Focus mask: This consists of three well-spaced parallel slits, each of dimensions mm ( arcsec). This is required for the Hartmann tests to check focus, rotation and tilt. See Section 4.6 for further information.

Standard longslit mask: A single, centrally placed longslit for traditional longslit spectroscopy. Dimensions are mm ( arcsec). Note that this (and all the other masks) is not inclined so cannot be used with the slit-viewing TV system.

Wide longslit mask: For spectrophotometry so that all the light from the object will go down the slit. Dimensions are mm ( arcsec).

Very wide longslit mask: Even wider than the wide longslit for really bad seeing or for easy acquisition. Dimensions are approximately arcsec. Note that the edge quality is poor.