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Minimum detectable count rates


The minimum detectable count rate for the WFC depends mainly on the background count rate, and the effective size of an unresolved source. The minimum detectable count rate, tex2html_wrap_inline2388 , can be estimated from the following formula (e.g., de Korte et al., 1981, Space Science Reviews, 30, p. 495-511):


where tex2html_wrap_inline2392 is the required significance threshold (i.e., signal-to-noise), tex2html_wrap_inline2394 is the background count rate in the ``detection cell'', f is the fraction of the total source count included in the ``detection cell'', tex2html_wrap_inline2398 is the minimum number of photons required to constitute a detection and t is the integration time.

The ``detection cell'' is the region of the image used to detect the source. For the WFC, assuming an on-axis source, an appropriate cell area depends on the filter used, since the point response function is energy-dependent. For the S1, S2 and P1 filters the appropriate radius enclosing 70% of the total count (i.e., tex2html_wrap_inline3036 ) is tex2html_wrap_inline3038 , whilst for P2, tex2html_wrap_inline3040 . The corresponding cell areas, tex2html_wrap_inline3042 , are thus tex2html_wrap_inline3044  arcmin tex2html_wrap_inline1870 and tex2html_wrap_inline3048  arcmin tex2html_wrap_inline1870 respectively. The count rates in the background cell are then calculated as tex2html_wrap_inline3052 , where tex2html_wrap_inline3054  arcmin tex2html_wrap_inline1870 is the area of the whole field of view and tex2html_wrap_inline3058 it the total background count rate of the whole field.

The minimum detectable count rate tex2html_wrap_inline2388 as a function of t is shown graphically in Figure 12.1 gif for tex2html_wrap_inline3064 , f=0.7, tex2html_wrap_inline3068 . Three curves are shown, the two labelled `LOW' and `HIGH' are to be used for the S1,S2, P1 filters and are plotted for tex2html_wrap_inline3070 and tex2html_wrap_inline3072  counts s tex2html_wrap_inline1894 , i.e. for the typical range of background rates expected. The low background rates are only achieved for restricted time intervals, guest investigators should therefore assume the `HIGH' curve when proposing. The curve labelled P2 is to be used for observations with the P2 filter and assumes tex2html_wrap_inline3076  counts s tex2html_wrap_inline1894 , i.e. the night-time background rate expected in this filter (see § 7.8.4 gif).

next up previous contents
Next: Flux conversions Up: FEASIBILITY: THE WIDE FIELD Previous: Introduction

Michael Arida
Tue Jun 11 16:18:41 EDT 1996