Focal Plane area

FOCAL PLANE MASK

Data

At WHT 1 arc sec is 0.223 mm at focal plane = 3.3 pxl at detector

At TNG 1 arc sec is 0.182 mm at focal plane = 2.7 pxl at detector

(In either case 10 mm at mask is 148 pxl - in spatial direction)

After some discussion a new design for the focal plane mask was made on Oct 27, 2000. It looks as follows:

The mask consists of a grid of circular spots, 200micron in diameter (not drawn to scale in the diagram). The spots are aligned on equally-spaced columns, 10mm apart, (so that barrel distortion can be easily measured) and the columns are staggered (by 1mm) to prevent overlapping of spectra even with a wide-band filter (10mm separation in the dispersion direction corresponds to >100A), or with a higher-resolution grating.

The pattern of holes covers the full entrance aperture of the instrument. The 200micron diameter corresponds to 0.9" at the WHT, 1.1" at TNG.

The pattern is broken in two ways:

A 10mm-long slit (width 200micron = 0.9" at WHT, 1.1" at TNG), which may be used for calibration exposures on galactic PNe, or on other targets (target acquisition is not easy though). It also enables coaddition of the arc spectra to look for fainter lines.
Three of the holes (marked as filled circles on the diagram) are larger than the others (300micron instead of 200micron). The idea is that these can be picked out easily in software (they will be twice as bright as the other ones), and allow unambiguous determination of the orientation and center of the image in each camera. Armed with this initial offset the other spots can then be searched for automatically to establish the transformations between the counterdispersed images. The long side of the L can also be used during setup to align the dispersion direction with the CCD pixels.

If the tolerance on the repeatability of the hole size is sufficiently tight, the mask can also be used for establishing the field-dependence of the instrument throughput, by means of sky exposures.

A text file containing the locations of the holes is here.

ILLUMINATION

Discussion is not completed. As pointed out elsewhere even He and Ar together are not really enough.

Illumination (as uniform as possible) with a white light source is required, as is a spectral lamp.

Koen Kuijken 27 October 2000