SALT/PFIS Observer's Guide

3.3 Fabry-Perot Imaging Spectroscopy

The PFIS Fabry-Perot system provides two-dimensional imaging spectroscopic capabilities. Three spectral resolution modes are provided, each over the wavelength range 430 - 860 nm. The system works with the camera in its imaging configuration (camera angle = zero degrees), the gratings removed, and one or two of the FP etalons inserted into the collimated beam. The full 8 arcmin field of view is imaged onto the detector, with the spectral band selected by the etalons and the appropriate order-selecting filter.

A typical observing sequence consists of taking a series of exposures of an astronomical target, changing the wavelength setting of the FP system for each exposure to cover the relevant spectral range about a spectral feature of interest. Atmospheric transparency is monitored during each exposure by the guider. Wavelength zero-point calibration exposures of a standard spectral lamp are taken before and afer the sequence. Flat-field and full wavelength calibration sequences are run during daylight hours.

3.3.1 Etalons

The FP system uses servo-controlled etalons manufactured by ICOS. Piezoelectric positioners set the parallelism and the gap of the etalon plates, and the plate positions are monitored by capacitance sensing.

The spectral resolution of an etalon is set by the size of the spacing between its plates and by their reflectivity; this resolution is fixed for a given etalon (although the lowest resolution etalons have small enough gaps that they can be tuned by their piezos through approximately a factor of two in resolving power). The system has four spectral resolution modes: tunable filter (TF), low (LR), medium (MR), and high (HR). Tunable filter and low-resolution modes use a single etalon, with an interference filter to select the desired interference order (corresponding to wavelength). The medium- and high-resolution modes use two etalons in series, with the low-resolution etalon and its filter selecting the desired order of the medium- or high-resolution etalon, respectively.

Be aware of the standard spectroscopist's rule of thumb: with reasonable signal-to-noise, you can measure a velocity accurate to about 1/20 the FWHM of the line profile. For example, in measuring galaxy rotation curves, you are probably interested in a precision of about 5 to 10 km/sec; that suggests a resolution of about 1500, or MR mode. If, on the other hand, you are investigating line profile shapes, you obviously need higher resolution. Don't over-resolve! The cost in total observing time varies as the square of the resolution

Approximately 30 interference filters (of resolving power R=50) will be required to isolate the FP orders over the entire spectral range. Only 15 will be installed in the magazine at any one time, so the operating queue will be structured to limit the number of filters needed on a given night.

The wavelength of a single FP image is not constant over the field, but varies quadratically with distance from the optical axis. The field of view at approximately constant wavelength (the so-called "bull's-eye") is 1.3' x (10450/R)^(1/2), set by the focal length of the PFIS collimator. The total wavelength variation from the optical axis to the edge of the field of view is 0.9969 x the central wavelength (2.1 nm at 656.3 nm).

3.3.2 Efficiency

For a continuum object of magnitude B=V=R=I=20, the table below lists the expected detected photon rate per second in each FP resolution bandpass, and the corresponding sky rate (photons/sec) per square arc second at new moon. Binning 2x2 or 4x4 gives 0.25" or 0.5" pixels respectively, and the read noise is about 3 e-per pixel. For emission lines, a typical HII region has a luminosity of 10^38 erg/sec, so at a distance of 10 Mpc it will give about 440 detected Ha photons/sec and subtend ~1"; the diffuse Ha background in galaxies is typically 10 - 100 x fainter.

Table 1: Detected photon fluxes (counts/second) for continuum objects
Wavelength
(nm)
TF LR MR HR
m=20 sky m=20 sky m=20 sky m=20 sky
450
28.6
2.88
11.8
1.19
2.8
0.28
0.54
0.054
550
27.2
6.84
10.0
2.52
3.0
0.69
0.56
0.14
650
21.4
10.2
8.9
4.23
2.9
1.25
0.58
0.26
800
17.4
18.9
9.5
8.17
2.9
3.23
0.60
0.65

3.3.3 Filters

There are forty narrow-band intereference filters that cover the entire spectral range of the Fabry-Perot system:

Filter Data


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