Sections: Diffraction Gratings
| Slit Width and Spectral
Resolution | Spatial Resolution
| Filters Available | Slit
Lengths and Dekkers | Efficiencies
| Detector Characteristics
Previous: The RGO Spectrograph | Next: Description of the Spectrograph | CONTENTS
This chapter contains all the system parameters needed to plan and apply for an observing run using the RGO Spectrograph.
d(lambda)/dx = a cos(theta - C1) / mf (Eq. 2.2)where x is the distance along the detector, f is the focal length of the camera (25 or 82 cm) and the remainder of the symbols are as for Equation 2.1.
For example, any 1200 line/mm grating in first order, blaze to collimator, on the 25 cm camera with the EEV CCD will have ~1400 Å coverage; 600 line gratings with the same setup will have ~3000 Å coverage; and the 300B grating will cover ~5700 Å.
g(theta) = G cos(theta - C1) / cos(theta - 50) (Eq. 2.3)where C1 ~ 65° G = 1.46 for the 82 cm camera, and C1 ~ 5° G = 4.8 for the 25 cm camera. These functions are plotted in Figure 2.1.
Figure 2.1 Projection factor vs grating angle for the 25cm and 82cm cameras.
Calculation of Spectral Resolution
The spectral resolution for a setup is approximated by:
R2 = RI2 + [(S x Fg) / (P x g(theta))]2 (Eq. 2.4)
where R is the resolution in pixels, RI is the instrumental resolution, S is the slit size in µm, P is the pixel size in µm, g(theta) is the slit projection factor (Equation 2.3), and Fg is the geometric factor. The geometric factor corrects for the fact that the slit is not a circular aperture, and has been determined experimentally to be 0.66 ± 0.01. Note that this is only an approximation, and the actual resolution should be checked on arcs during setup. Slit sizes < 1 arcsec should generally be avoided - assume an average seeing of 1.5" for Siding Spring.
Table 2.2 Available Filters
|Position||Star 1||Star 2||Arc 1||Arc 2||Below Slit|
While the below-slit filters are preferable as they filter both the starlight and calibration lamps, they are not often used. There are only three positions available, one of which is clear. The use of a below-slit filter changes the focus setting significantly, so frequent changes of below-slit filter are not recommended. In addition, the below-slit filters vignette the ends of the slit (Section 2.5).
The above-slit star filters are used for starlight and for chimney calibration lamps, and the internal calibration lamps have a separate wheel of filters with a more limited selection. They do not affect the focus.
Because of the vignetting, observing two objects at once is only efficient if the objects are less than about 1 arcmin apart. In addition, since the field of the slit-view mode of the acquisition system is only 30 arcsec, acquisition of widely spaced objects is a problem. Thus separate observations are typically more efficient for such objects.
Dekkers are available for the selection of limited portions of the slit. Details of the present dekkers are shown in Appendix D. For dekkers with multiple holes or slots, the separation is specified between inside edges. These diagrams are slightly misleading in that they suggest that the width (i.e. spectral dimension) is much narrower than the length (spatial direction). Actually the width of all dekkers is larger than the widest slit width available and is often considerably greater than the length.
Dekkers still in common use are 50 (clear); 56 (narrow) to provide a synthetic star for instrument setup; and 73 (double) for spectropolarimetry. However all listed dekkers are still available.
Figure 2.2 Vignetting function along the RGO slit for observations made with and without a below-slit filter
S/N = O / SQRT(O + S + DN2) (Eq. 2.5)
where O is the number of counts per pixel for the object, S the number of counts per pixel for the sky and DN2 is the square of the readout noise times the number of spatial pixels extracted times the number of exposures to be summed.
Table 2.3 Atmospheric Transmission
Table 2.4 Efficiency of the RGO
|25 cm||82 cm|
|(v)||Two lenses in camera (4 glass surfaces)||0.9854||-|
|(vi)||Mirrors in camera||0.902||0.90|
|(viii)||Field flattening lens (2 glass surfaces)||0.9852||-|
|(ix)||Window (to seal dry gas
ahead of the detector
The 25 cm camera (only) experiences vignetting for extreme grating angles. Table 2.6 shows the calculated vignetting.
Table 2.5 Vignetting
|grating angle||transmission %||slit projectiona|
Table 2.6 Slit Throughput
Figure 2.3. Efficiency curves of the current CCDs.
The EEV has 2k × 4k 13.5 µm pixels. Usually the longside is aligned with the dispersion direction. However, readout speed is much faster if aligned in the spatial direction (and suitably windowed), so this option may be useful for time-resolved work. You have to give warning well in advance of your run if you want to orientate the CCD in this way.
The EEV has a blue-sensitive coating, and is the preferred CCD for observations blueward of 6000Å. It suffers from strong fringing in the red, which appears to subtract well using flatfields, with only a small residual. A report from the commissioning run of the RGO with the EEV CCD is available here (coming!). Table 2.7 gives the characteristics of the EEV.
Table 2.7 Characteristics of the EEV CCD
|Read Time (s)|
The MITLL2A has 2k × 4k 15 µm pixels. It is Lumogen-coated to enhance blue sensitivity. It has the broadest range of sensitivity though it is less sensitive than the EEV in the blue, and the MITLL3 in the red. The MITLL2A suffers from some pixel smearing, leading to somewhat lower resolutions than might be expected. Table 2.8 gives the characteristics of the MITLL2A.
Table 2.8 Characteristics of the MITLL2A CCD
The MITLL3 has 2k × 4k 15 µm pixels. It is not possible to bin this CCD. It has the highest red sensitivity. It provides higher resolution than the MITLL2A, but has a higher cosmic ray rate and no blue sensitivity. At the time of writing, the MITLL3 is undergoing repairs and is not available. Check the webpage for news of its current status.
This detector can be used at 170 K or 200 K. Use at 200 K results in a 10% to 20% increase in QE at expense of a much increased dark current (0.1 e-/pix/s compared with 0.55 e-/pix/2000 s at 170 K).
Table 2.9 Characteristics of the Tektronix
Observers can design their own CCD window if desired. Popular windows currently available are TEK1K_RGO, 375 columns which include the full slit; TEK1K_BX1, 150 columns covering the central half of the slit; TEK1K_BX2, the same as the previous window with spatial binning × 2.
The RGO slit covers only 20 - 30% of the CCD, so you should use a narrow CCD window. A range of CCD windows suitable for RGO observing are supplied in the default area. Table 2.10 gives the most popular unbinned windows for each CCD. The spectral direction is usually in the Y direction (for pixel sizes in Angstroms for your chosen setup use RGOANG http://www.aao.gov.au/cgi-bin/rgoang_inputs.pl) and the spatial direction along the slit is usually in the X direction (spatial scales given in Section 2.3). Typical readout times are 1 - 2 minutes in SLOW mode. More information on CCD windows is given in Appendix A.
Table 2.10 Popular CCD windows for the RGO
|Window||X Size (Pixels)||Y Size (Pixels)||Overscan (Pixels)|