8.7 Exposure Times

While the figures in this section will help you to estimate exposure times for various acquisition scenarios, the COS Exposure Time Calculator (ETC) should be used for all proposal preparation.

8.7.1 Imaging Acquisitions

Acquisition images obtained through the PSA should strive for a minimum S/N of 20. Due to the complex shape of images obtained through the BOA, a S/N > 30 is recommended. For MIRRORB acquisitions, the recommended S/N refers only to the primary image; the ETC performs this calculation appropriately. Figure 8.3 shows approximate exposure times needed to reach these S/N levels for various target fluxes. A flat source spectrum (Fλ = constant) is assumed.

Figure 8.3: Exposure Time Needed for an ACQ/IMAGE Acquisition.

Approximate exposure time needed to achieve S/N = 20 (PSA) or 30 (BOA) as a function of target flux. This calculation assumes a flat source spectrum.
Figure 8.4: Exposure Time Needed for a Dispersed-Light Acquisition with the PSA.

Approximate exposure time needed to achieve S/N of 40, assuming a flat source spectrum.

8.7.2 Dispersed-Light Acquisitions

We recommend a minimum S/N of 40 for all dispersed-light acquisition exposures. STScI calibration programs routinely use up to S/N = 100 to minimize the influence of Poisson noise and background when very precise pointing is required. Figure 8.4 is a guide to the exposure time needed for a dispersed-light acquisition with the PSA, assuming a flat source spectrum (Fλ = constant). Note that these exposure times apply to each separate dwell point of a pattern, which is the quantity entered into APT in Phase II.