Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

3.2 The Optical Design of COS

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In this section the light from HST is followed as it progresses through COS to each optical element and mechanism. This path and its alternatives are shown schematically in Figure 3.3. In this chapter only a brief overview is provided to avoid unnecessary complication; the details of the optics, mechanisms, and so on are in Chapter 13.

Figure 3.3: Schematic of the Light Flow Through COS.


 

3.2.1 External Shutter

The external shutter is located at the front of the COS enclosure in the optical path before the aperture mechanism. When closed, the shutter blocks all external light from entering the COS instrument and prevents light from the COS internal lamps from exiting the instrument. The opening and closing of the external shutter is not used to determine the duration of an exposure. The external shutter will only be opened by a command at the beginning of every external exposure and is closed at the end of every external exposure, with the possible exception of one or more phases of target acquisition. The external shutter will be closed autonomously by the COS flight software whenever any over-light condition is triggered by an external or internal source or when the HST take-data-flag goes down indicating loss of fine lock; see Section 11.5.2.

For more on the external shutter, see Section 13.2.3.

3.2.2 The Apertures and Aperture Mechanism

After passing the focal plane, the light from HST first encounters the COS entrance apertures, which are mounted on the Aperture Mechanism.

In most spectrographs, the light from the telescope is focused on a slit, and the instrument's optics then re-image the slit onto the detector. In such a design, the slit width and how the slit is illuminated determine the resolving power and line spread function (LSF). COS is different: it is essentially a slitless spectrograph with an extremely small field of view.

There are four apertures: two look at the sky for science exposures, and two are for calibration. Selecting among these apertures can involve movement of the Aperture Mechanism. The two science apertures are the Primary Science Aperture (PSA) and the Bright Object Aperture (BOA).

Primary Science Aperture

The Primary Science Aperture (PSA) is a 2.5 arcsec (700 µm) diameter field stop located behind the HST focal surface near the point of the circle of least confusion. This aperture transmits 95% of the light from a well-centered, aberrated point-source image delivered by the HST optics. The PSA is expected to be used for observing in almost all instances. The PSA is in place, ready to use, at the start of a new visit. Note that when the PSA is in place the WCA (see below) is also in place and available for use.

Note also that the BOA is open to light from the sky when the PSA is being used for science (and vice versa); therefore bright object screening for the field-of-view must include both apertures.

Bright Object Aperture

The Bright Object Aperture (BOA) is also 2.5 arcsec (700 µm) in diameter with a neutral density (ND2) filter immediately behind it. The transmission of the BOA is wavelength dependent, and is shown in Figure 3.4. The straight line fit is given by transmission = [0.99 - (Å)/4500]/100. The BOA attenuates by about a factor of 200 at 2000 Å.

The BOA material has a slight wedge shape so that the front and back surfaces differ from one another by about 15 arcmin. This wedge is sufficient to degrade the spectroscopic resolution realized when the BOA is used. In fact, a secondary peak in the image is formed; see Section 7.5.3.

The BOA must be moved into place with the Aperture Mechanism to replace the PSA for science observations. Thus, science spectra obtained through either the PSA or BOA will use the same optical path and detector region (for a given channel), and so may employ the same flat-field calibrations. At the same time, the BOA is open to light from the sky when the PSA is being used for science (and vice versa); therefore bright object screening for the field-of-view must include both apertures. Moving the BOA into place for science use precludes using the WCA for a wavelength calibration exposure, and so an additional movement of the Aperture Mechanism is needed to obtain a wavecal when the BOA is used. For this reason the BOA may not be used with TAGFLASH exposures (see Section 5.5.1).

Figure 3.4: Measured Transmission of the COS BOA as a Function of Wavelength.


 

Wavelength Calibration Aperture

The Wavelength Calibration Aperture (WCA) is offset from the PSA by 2.5 mm (about 9 arcsec) in the cross-dispersion direction, on the opposite side of the PSA from the BOA (this is illustrated in Figure 13.2). Light from external sources cannot illuminate the detector through the WCA; instead the WCA is illuminated by one of two Pt-Ne wavelength calibration lamps. The wavelength calibration spectrum can be used to assign wavelengths to the locations of detected photons for science spectra obtained through either the PSA or BOA. As noted, both the PSA and WCA are available for use at the same time and no additional motion of the Aperture Mechanism is needed.

Flat-field Calibration Aperture

The Flat-field Calibration Aperture (FCA) is used only for calibration and it is not available to observers. For more information, see Section 13.1.6. The FCA is used to obtain flat-field exposures using one of the two deuterium lamps.

3.2.3 Gratings and Mirrors: The Optics Select Mechanisms

After passing through either the PSA or BOA, light next encounters Optics Select Mechanism 1 (OSM1). OSM1 is a rotating mechanism that can bring one of four optical elements into the beam. These four optical elements are located at 90 degree intervals around OSM1. One of these, mirror NCM1, is a flat mirror that directs the beam to the NUV channel. The other three elements are gratings for the FUV channel.

FUV Channel Optical Design

The COS FUV optical path is illustrated schematically in Figure 3.5. The FUV channel of COS uses only a single optical element to image the sky onto the XDL detector (described in Chapter 4). Each of the three FUV gratings is holographically ruled to disperse the light and to focus it onto the detector. The gratings also have optical surfaces configured to remove the spherical aberration produced by the HST primary mirror. Given the location of OSM1 in the HST optical chain, and given the several requirements placed on the FUV gratings (to disperse, focus, and correct), it is not possible to do all of these completely except for a point source that is centered in the aperture. In other words, the design of the FUV channel of COS has been optimized for high throughput and good spectrum resolution on centered point sources, but performance is reduced under other circumstances, such as when the source is moved away from the aperture center. However, this degradation of resolution is low for displacements up to about 0.5 arcsec from the aperture center (see Section 7.4).

The COS FUV channel provides spectra that cover the wavelength range 1150 to 2050 Å at low- and moderate spectral resolution. The XDL detector is described fully in Chapter 4, but it is important to note that it consists of two independent segments with a small physical gap between them. This gap prevents a single continuous spectrum from being obtained in one setting, but it also enables geocoronal Lyman- to be placed there in some set-ups, thereby eliminating the local high count rates that that emission line can cause. The gap can miss 14 to 18 Å of spectrum with G130M or G160M, but the missing wavelengths can be filled, as described in Section 5.6.

The nominal wavelength range for the G140L grating is 1230 to 2050 Å, and this spectrum takes up only part of one detector segment. G140L actually directs light out to 2400 Å onto this detector segment, but the XDL sensitivity to these longer wavelengths is extremely low. On the other detector segment, the G140L grating disperses light between ~100 - 1100 Å. Again the sensitivity to these wavelengths is very low, limited in this case by the reflectance of HST's mirrors and COS optics. Calculations predict that the effective area below 1150 Å plummets rapidly but is not zero. The sensitivity at these wavelengths will be measured once COS is installed in HST.

Figure 3.5: The COS FUV Optical Path.


 

OSM2 and the NUV Channel

The COS NUV channel covers the wavelength range 1660 to 3200 Å at low- and moderate spectral resolution. If the NUV channel is to be used, first mirror NCM1 is placed into position on OSM1; that directs the beam to mirror NCM2 - which collimates the light - and then to Optics Select Mechanism 2 (OSM2). OSM2 holds five optical elements: four plane diffraction gratings plus a mirror for target acquisitions or imaging. These five optical elements are located at 72 degree intervals around OSM2.

Three of the gratings are medium-dispersion and deliver resolving powers of R = 20,000 to 24,000 (G225M and G285M) or R = 16,000 to 20,000 (G185M) over the wavelength range 1700 to 3200 Å. The dispersed light from the gratings is imaged onto a MAMA detector by three parallel camera optics (NCM3a, b, c). The spectra appear as three non-contiguous ~35-40 Å stripes on the MAMA detector, allowing ~105-120 Å wavelength coverage per exposure. The gratings can be scanned with slight rotations of OSM2 to cover the entire NUV wavelength band. The NCM3a,b,c mirrors are spaced such that several correctly chosen exposures will produce a continuous spectrum from the beginning of the short wavelength stripe in the first exposure to the end of the long wavelength stripe in the final exposure.

In addition, a low-dispersion grating, G230L, produces three stripes with ~400 Å coverage per stripe at a resolution of ~1.1 Å (R = 1550 - 2900). The first-order science spectrum from G230L over the 1700 to 3200 Å region is captured in four separate exposures.

The layout of the stripes is shown schematically in Figure 4.4.

Figure 3.6: The COS NUV Optical Path.


 

The plane mirror on OSM2 is designated as MIRRORA when used in direct specular reflection. MIRRORB refers to the arrangement in which OSM2 rotates the position of this mirror slightly so that the front surface of the order sorter filter on this mirror is used. This provides an attenuation factor of approximately 25 compared to MIRRORA. Because of the finite thickness of the order-sorting filter, MIRRORB produces an image with two peaks that may impede its use for imaging; see Section 7.5.3 for details. This doubled image is generally acceptable for acquisitions, however.

3.2.4 Detectors

The detectors in COS are described in Chapter 4.

3.2.5 On-board Calibration Lamps

Four calibration lamps are mounted on the calibration subsystem. Light is directed from the lamps to the aperture mechanism through a series of beam-splitters and fold mirrors.

Pt-Ne Wavelength Calibration Lamps

COS has two redundant Pt-Ne hollow-cathode wavelength calibration lamps on its internal calibration platform; their spectra contain emission lines suitable for determining the wavelength scale of any spectroscopic mode. Either lamp may be used for wavelength calibration exposures, but the choice is not user-selectable. We anticipate that one lamp will be used until it fails and then operations will be switched to the other.

The Pt-Ne lamps are used to obtain wavelength calibration ("wavecal") exposures, either as a separate wavecal for ACCUM exposures, or during a TIME-TAG exposure when FLASH=YES is specified ("TAGFLASH" mode). The light from the Pt-Ne lamp reaches the spectrograph through the WCA (wavelength calibration aperture). The WCA spectrum is displaced at an off-axis position relative to the PSA, projected 2.5 mm away from the PSA spectrum on the FUV detector. On the NUV detector, the corresponding WCA spectral stripe lies 9.3 mm away from the associated PSA science strip. This optical offset introduces a wavelength offset between the two sets of spectra, and this is compensated for during data reduction with calcos.

Note that the WCA and the PSA are available for use at the same time; this is what makes TAGFLASH mode possible. However, the Aperture Mechanism must be moved to bring the BOA into position and that makes it impossible to use TAGFLASH mode with the BOA; see Section 5.7.3.

The Pt-Ne lamps are also used during acquisitions to provide a reference point that will define the relationship between a known location at the aperture plane and the detector pixel coordinates in which the measurements are made.

Deuterium Flat-field Calibration Lamps

COS has two redundant deuterium hollow-cathode flat-field calibration lamps. The deuterium lamps may also be used interchangeably. Usage of these lamps for flat-field calibrations is restricted to observatory calibration programs. The light from these lamps enters the spectrograph through the FCA (flat-field calibration aperture).