Optical Filters

3.8 Red Leaks in UV Filters

The "red leaks" in the UV filters are shown in Figure 3.10
for F122M, F160BW (the new Wood's filter), F170W, F185W F218W F255W, F300W, and F336W. The presence of significant red leaks in the UV filters, together with the much greater sensitivity and wavelength coverage in the red part of the spectrum, makes calibration of UV observations difficult. Table 3.13 shows red leak estimates as a percentage of the total detected flux from de-reddened stellar sources, ordered by spectral type. In each column, the red leak is defined as the percentage of the detected flux longward of the cutoff wavelength in the second row. In the presence of interstellar reddening, the red leaks will be larger.

Figure 3.10: UV Filter Red Leaks. Includes the on-orbit measurement of system response.

Red Leak in UV Filters. A synthetic photometry calculation with de-reddened BPGS stellar spectra and system response from on-orbit data.

Note that the SYNPHOT synthetic photometry package can be used to estimate the counts contributed by red leak for various particular situations, and for filters other than those plotted below.

There is significant variation of the UV throughput due to build-up of molecular contaminants on the CCD windows, and decontamination procedures used to remove this contamination. See "Time Dependence of UV Response" on page 137.