The filtered and clear apertures available for ultraviolet imaging are summarized in Table 5.1. Although there are only a small number of filters available, the solar-blind and solar-insensitive properties of the FUV-MAMA and NUV-MAMA detectors, respectively, coupled with their 25 x 25 arcsecond field of view, good spatial sampling, and ability to detect rapid variability give STIS ultraviolet imaging capabilities that are complementary to those of ACS.
The throughputs of the STIS MAMA imaging modes in this Handbook are mostly based on on-orbit measurements of Cycle 7 calibration observations. These throughputs are good to within 5% in the FUV and the NUV. The throughputs in the ETC, the calibration reference files, and in the STSDAS synphot package will continue to be updated as further analysis of calibration data continues.
Figure 5.11 shows an example of MAMA imaging data of a globular cluster taken as part of the Cycle 7 calibration of STIS using the quartz filter and the NUV-MAMA.
Figure 5.11: 320 Second NUV-MAMA Image of NGC6681 taken with the F25QTZ filter as part of the Cycle 7 Calibration Monitoring Program 7720. All the points are stars.
The MAMA detectors are subject to absolute bright-object limits, above which targets cannot be observed. They are particularly stringent for the MAMA imaging modes (being as faint as V=20.5 for the clear modes), and apply to all sources illuminating the field of view.
We direct MAMA observers to MAMA Bright-Object Limits. For summary tables of absolute bright-object screening magnitudes for the imaging modes, see "MAMA Imaging Bright Object Limits" on page 439.
It is the observers' responsibility to ensure that their observations do not exceed the MAMA bright-object limits.
The MAMA plate scale is ~0.0246 arcsecond pixel-1 in image mode, providing a good compromise between sampling of the PSF in the ultraviolet and field of view. Chapter 14 shows encircled energies as a function of wavelength for MAMA imaging, and provides information on the geometric distortions of the images. The MAMA detector PSFs exhibit broad wings, which are substantially higher in the NUV-MAMA than the FUV-MAMA. Figure 7.13 shows sample detector PSFs for the MAMAs.
Each MAMA can be used with the 25MAMA clear aperture to image a 25 x 25 arcsecond field of view of the sky, providing the maximum throughput and wavelength coverage in the NUV and FUV as shown in Figure 5.1. The FUV-MAMA quantum efficiency drops dramatically longward of ~2000 Å making it effectively solar blind, while the NUV-MAMA also has a reduced response toward the red, longward of ~3500 Å (see Figure 5.12). Table 5.4 and Table 5.5 give the percentages of detected photons arising in the UV versus optical for observations of different stellar types with the clear MAMA imaging modes. The red rejection of the MAMA detectors makes them well suited to UV imaging of red objects.
However, NUV-MAMA clear direct images will be slightly out of focus, because the corresponding mirror on the MSM optimally focuses for use of a filter. It is recommended that the F25SRF2 longpass filter (see next section) be used instead of 25MAMA (clear) for direct imaging with the NUV-MAMA. The same does not apply to the FUV-MAMA, which has separate MSM mirrors for clear and filtered imaging.
The sky background can be significant for unfiltered FUV-MAMA observations. The strongest contributor is the geocoronal Lyman-
line. Global count rates of several 104 counts per second over the whole detector are not unusual during daytime observations. The same applies to slitless far-UV spectroscopy. For observations of large, UV-faint targets, where background subtraction becomes critical, unfiltered imaging may introduce significant noise. In addition, the background may be variable during long exposures. Longpass filtered imaging may be profitable in this case.
The integrated system throughputs of the two UV longpass filters when used with the NUV-MAMA and FUV-MAMA are shown in Figure 5.3 and Figure 5.4 (see also Chapter 14 pages 402 and 399 for sensitivities, and signal-to-noise and saturation plots). The filter (only) throughputs of these two filters are shown in Figure 5.13. These filters image a 25 x 25 arcsecond field of view. The cutoff wavelengths of F25SRF2 and F25QTZ were chosen to exclude geocoronal Lyman- 1216 Å and O I 1302 + 1356 Å, respectively; use of these filters significantly reduces the total sky background in the ultraviolet. These filters can be used by themselves in imaging mode, or with the prism or any first-order UV grating in slitless spectroscopic observations, to reduce the background due to geocoronal emission (see Objective-Prism Spectroscopy and Slitless First-Order Spectroscopy). F25SRF2 images, combined with images taken in series with the FUV-MAMA/25MAMA clear, can also be used to obtain Lyman- images (see Lyman Alpha-F25LYA and Clear-minus-SRF2).
The filters for MAMA imaging include:
<1800 ÅF25MGII) which images the magnesium doublet at 2796-2803 Å, and a matched medium-band continuum filter (F25CN270) centered at 2700 Å.
F25CIII) which images the semi-forbidden CIII] lines at 1906-1909 Å, among the strongest nebular (low-density) lines in the UV, and a matched medium-band continuum filter (F25CN182) centered at 1800 Å.
; this filter has an unusually poor throughput, and we recommend that you consider, instead, obtaining two FUV-MAMA images, one through the 25MAMA unfiltered aperture and a second with the SRF2 longpass filter. The difference of these two images will isolate Lyman- with much higher throughput than the F25LYA filter.
The F25MGII filter images a 25 x 25 arcsecond field of view in the light of the doublet lines of Mg II (2796 and 2803 Å). Figure 5.14 shows the integrated system throughput (see also page 404 for sensitivities, and signal-to-noise and saturation plots). There is a substantial red leak in this filter starting at approximately 4200 Å and extending to at least 13,000 Å. For stellar spectral types O and B, less than 2% of the detected counts will be due to red leak. This percentage rises to 7% for an A0 star. For a K0 star, 75% of the counts will be due to red leak. The red leak for this filter is included in the passbands used by the STIS Exposure Time Calculator and synphot. Observers are encouraged to use these tools to predict source and background count rates carefully.
The 2700 Å continuum filter images a 25 x 25 arcsecond field of view and can be used to measure the continuum for Mg II emission-line images. The F25CN270 filter integrated system throughput is shown in Figure 5.14 above (see also page 407 for sensitivities, and signal-to-noise and saturation plots). There is a substantial red leak in this filter starting at approximately 4200 Å and extending to at least 12,000 Å. For a K0 star, roughly 40% of the detected counts will be due to red leak. The red leak for this filter is included in the passbands used by the STIS Exposure Time Calculator and synphot. Observers are encouraged to use these tools to predict source and background count rates carefully.
The F25CIII filter images a 25 x 25 arcsecond field of view in the light of C III] at 1906-1909 Å. The F25CIII integrated system throughput is shown in Figure 5.15 (see also page 410 for sensitivities, and signal-to-noise and saturation plots). The out-of-band suppression for this filter is fairly good.
The 1800 Å continuum filter images a 25 x 25 arcsecond field of view, and can be used to measure the continuum for C III] emission-line images. The F25CN182 filter integrated system throughput is shown in Figure 5.15 above (see also page 413 for sensitivities, and signal-to-noise and saturation plots).
The F25LYA filter images a 25 x 25 arcsecond field of view and can be used to obtain emission-line images in the light of Lyman-. The F25LYA filter integrated system throughput is shown in Figure 5.16 (see also page 432 for sensitivities, and signal-to-noise and saturation plots).
At the price of a slightly wider bandpass, and the need to take two exposures, Lyman- can be isolated by taking one image with the clear (25MAMA) aperture and a second with the longpass (F25SRF2) filter and differencing the two. The integrated system throughput for this imaging sequence is appreciably higher than for the narrowband F25LYA filter, as shown Figure 5.16.
Imaging Integrated System Throughputs
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