Different slit widths between 0.25" and 2.00" can be selected. If the width is smaller than the scale of the camera used the width is set to the scale of the camera.
First of all the program calculates the number of photons reaching the slit. After that it computes the number of photons behind the slit. For a point-like source it assumes a PSF like a Gaussian or a Moffat + Airy disc. The hatched area in Figure 9 is the relevant area for transmission. The transmitted photons are split into the pixels 1 - 5 (see bottom of Figure 9). For example: if 18 photons are passing the hatched area of the slit, the light will be split up to 4.5 pixels. Pixel 1, 2, 3 and 4 will detect 4 (18/4.5=4) photons each. The fifth pixel will count 2 photons.
Top: Sketch of a slit. The hatched area is used to calculate the transmission of the slit. The parameter d depends on the observing mode. For seeing-limited mode it is the FWHM of the seeing. In diffraction-limited mode it is the diameter of the first minimum of the airy disk. Bottom: The calculated photons are split into the shaded pixels.