- Imaging with COS may only be done with the NUV channel.
- It is not necessary to use imaging mode if what is desired is a confirmation of an acquisition. Use of ACQ/IMAGE mode automatically records and downlinks an image taken after the centroid is determined and HST is moved to that position.
- The COS field of view is very small, but because the optics image the sky onto the detector – not the aperture – the image records some of the light from sources out to a radius of about 2 arcsec. However, only point sources within about 0.5 arcsec of the aperture center have essentially all their light imaged, and so the photometric interpretation of a COS image can be inherently complex.
- COS is very sensitive and there is a limit on the maximum count rate per pixel (75 counts per second for the NUV). The imaging mode of COS concentrates an object’s entire NUV flux into a diffraction-limited image, and so that limiting count rate can be exceeded easily.
- MIRRORB and/or the BOA can be used to obtain images of brighter objects, but MIRRORB produces a secondary image and the BOA produces an image with coma that degrades resolution; see Chapter 6.
Configurations and Image QualityThe NUV imaging mode requires the observer to make only two optical element selections. First, either the PSA or BOA is selected as the aperture. The second selection required is MIRRORA or MIRRORB. MIRROR B provides an attenuation factor of approximately 25 compared to MIRRORA, and produces a doubled peak in the image. Ground testing shows that the peaks are easily separable for an isolated point source but may present difficulties for extended sources or crowded fields. Similarly, the BOA includes a neutral density filter and the optical properties of that filter degrade the image compared to what is possible with the PSA.
COS is very sensitive, and the imaging mode concentrates an object’s NUV flux into a diffraction-limited image rather than dispersing the light.
For information on imaging strategies with COS see chapter 6 of the COS Instrument Handbook.