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Spectroscopy with ACAM

ACAM provides fixed-format spectroscopy, spectral range ~ 3500 - 9400 A (~ 3.3 A/pixel). The slit widths available are 0.5, 0.75, 1.0, 1.5 , 2, 10 arcsec (or no slit at all). Observers requiring very wide slits can deploy one of two focal plane masks with widths of 27 or 40 arcsec (in the dispersion direction).

Sections below:

  1. Wavelength range, scale, orientation
  2. Spectroscopic resolution
  3. Throughput
  4. Ghosts, light leaks
  5. Flexure, repeatability
  6. Calibration
  7. Atmospheric dispersion
For a step-by-step guide to actually observing with ACAM, see the ACAM observing guide.

An example spectrum is shown below.

The outer boundary of the figure corresponds to the standard CCD window used: [1:2148,800:3300].

Wavelength increases with y on the CCD, roughly as:

wavelength (A) ~ 3.3 * ( y (unwindowed) - 230)

[i.e. wavelength (A) ~ 3.3 ( y(windowed) + 570)]

or, equivalently:

y (unwindowed) ~ (wavelength/A) / 3.3 + 230

[i.e. y (windowed) ~ (wavelength/A) / 3.3 - 570]

That is, the dispersion is ~ 3.3 +- 0.1 A/pixel, and unwindowed 1230 < y < 3050 (windowed 430 < y < 2250), the range over which light is detected, corresponds to wavelengths ~ 3300 - 9400 A.

For approximate wavelengths of prominent arc lines, see the ACAM CuAr and CuNe arc maps.

The approximate (unwindowed) y coordinates of some other useful lines are given below:

y             y  
(unwindowed)  (windowed)  Wavel   Line 
pix           pix         A

2521          1721        7594    Atmospheric absorption, A band
2305          1505        6867    Atmospheric absorption, B band
2211          1411        6568    Hα
2150          1350        6330    Laser line (used in lab tests)
2009          1209        5896    NaD sky line
1916          1116        5577    OI sky line
1553           753        4366    Hg line of dome fluorescent lamps 
1456           646        4054    Hg line of dome fluorescent lamps 
The central row of the default window, i.e. y(windowed) = 1250, corresponds to wavelength ~ 6010 A.

In the spatial direction, the spectrum runs from x = 335 - 1725, corresponding to ~ 5.8 arcmin (the length of the slits in the focal-plane mask, of which the central 5 arcmin can be illuminated by light from the calibration lamps). The top end of the slit (it's vertical when the telescope points at the zenith) images at the right-hand side of the CCD. The spatial scale is ~ 0.25 arcsec/pixel, as for imaging.

At sky PA = 0 (north to the right, in imaging mode), the slit will be oriented north-south (north to the right). (This hasn't always been the case: during the first few months after commissioning, sky PA = 0 rotated the instrument so that in imaging mode, north was up, and the slit ran east-west.)

The sky lines (or arc lines) are slightly curved relative to the CCD rows, due to the VPH (and its flanking prisms) being in an f/22 beam, rather than a collimated beam. The sky/arc lines are rotated clockwise relative to the CCD rows (as conventionally displayed) by about 0.26 deg (last measured 2018).

The spectrum is also rotated relative to the columns on the CCD, by about 2.0 deg anticlockwise (on a conventional display of the image).

On the AUXCAM CCD, the underscan and overscan strips lie at x < 50 and x > 2100 respectively.

Order-sorting filters
GG395 and GG495 order-blocking filters (ING filter numbers #705 and #706, names GG395A and GG495A) can be mounted in wheel 1 (i.e. before the VPH in wheel 2). The GG495 filter is usually present in wheel 1. The filters are 60 mm across, and 3 mm thick, with throughput ~ 0.9 over the transmitted-wavelength range, and excellent optical quality (negligible degradation of PSF expected).

The transmitted wavelength ranges, and the parts of those ranges which won't and will be contaminated by second-order light, are:

Filter   Transmitted       Uncontaminated  Contaminated by
         wavelengths                       second-order
         (A)               (A)             (A)

None     3300-9500         3300-6600       6600-9500

GG395A   3950-9500         3950-7900       7900-9500

GG495A   4950-9500         4950-9500       None                 

The 400-lines/mm transmission VPH (Volume Phase Holographic) grating is modelled to deliver the following resolution R on axis (i.e. with the target near the middle of the slit) as a function of wavelength and slit width:

Wavelength0.75 arcsec1.0 arcsec 1.25 arccec
3800 A390 290 230
5650 A 580 430 350
7500 A 770570 460

NB the actual slit widths available are 0.5, 0.75, 1.0, 1.5, 2.0 and 10.0 arcsec - there is no 1.25-arcsec slit.

For slit widths < 1 arcsec, the off-axis resolution (i.e. towards the edge of the field) will be significantly poorer than tabulated above, due to aberrations introduced by the optics.

In the lab, at a wavelength ~ 5500 A, we have measured on-axis R ~ 900 for a 0.5-arcsec slit, and R ~ 450 (4 pixels = 13 A) for a 1-arcsec slit, consistent with the above predictions. The degradation of spectroscopic resolution with radius is as predicted. E.g at radius 4 arcmin, slits of 0.5 and 1 arcsec deliver the same spectroscopic resolution R ~ 400.

The disperser is usually mounted in position 6 of filter wheel 2. It is mounted in Littrow mode (i.e. cemented to prisms which ensure that the diffracted light emerges ~ on-axis).

A 500 lines/mm disperser with better throughput in the blue has been purchased, but is not yet commissioned.

Very wide slits
Observers requiring very wide slits, e.g. to minimise differential losses between a target and a comparison star, may deploy in position 8 of the focal-plane slide one of two masks. Mask #2727 (serial number in the filter database) has an aperture of 27 arcsec (in the dispersion direction) x 6.8 arcmin. Mask #4040 has an aperture of 40 arcsec x 7.6 arcmin.

These masks are useful for e.g. exoplanet-transit observers interested in high-accuracy time-series photometry.

The VPH grating reduces the throughput relative to that achieved in imaging mode (see the ACAM-imaging page). According to the theoretical-performance data supplied by the manufacturer (Kaiser Optical Systems):

(and extrapolating), the diffraction efficiency of the VPH in first order is 0.55, 0.58, 0.80, 0.70, ~ 0.50 and ~ 0.4, in U, B, V, R, I and Z bands respectively (blaze angle = 5650 A). In addition, light is lost due to reflection at air/glass interfaces (all uncoated, in the currently-installed VPH) and within the material itself, giving a factor 0.78. The net throughput of the grating at the blaze wavelength of 5650 A is therefore 0.80 * 0.78 = 0.62.

At 7000 A, the measured zeropoint (AB mag for 1 detected photon per sec per A) is 18.51.

Throughput relative to ISIS
The ratio of the throughput of WHT + ACAM/VPH + AUXCAM to that of WHT + ISIS/R158-grating + CCD (EEV12 or REDPLUS) is given below for each wavelength band:

Band U B V R I Z
Wavelength (A) 3600 4300 5500 6500 8200 9500
ACAM/ISIS 0.6 0.9 2.1 1.7 1.3  

i.e. in the red, ACAM is more sensitive than ISIS/R158R/REDPLUS, but in the blue is slightly less sensitive than ISIS/R158B/EEV12 (the QE of EEV12 is higher in the blue than that of AUXCAM).

The numbers above are based on the measured zeropoints of the two instruments, obtained at different times. As a check, near-simultaneous wide-slit observations of a standard star were obtained with the two instruments, during ACAM commissioning. At 7000 A, the throughput of ACAM/VPH/AUXCAM at 7000 A, in photons per sec per A, was measured to be a factor 1.5 higher than that of ISIS/R158R/REDPLUS, consistent with the above.

No ghosts have been observed. In particular, a spectrum of 633-nm laser light (almost central on the CCD) shows no ghost lines with intensity > 0.01% that of the main line.

Two undispersed light leaks are visible on arc spectra at unwindowed x,y ~ (420, 2860) and (1600, 2860). They are barely visible on the spectrum reproduced at the top of this page. Since these leaks affect only areas near the ends of the slit (2.5 arcmin off axis), they are unlikely to be a problem for most oberving programmes. The cause of the leaks is unknown.

ACAM sticks out from the WHT Cassegrain cluster at right angles to the telescope axis, and at low elevation there can be significant flexure. For example, when the telescope is at elevation 15 deg, and the Cassegrain instrument cluster is rotated to arbitrary mount position angles, the y-position (dispersion direction) of arc spectral lines moves on the ACAM CCD by +- 5 pixels (+- 16 A), see the ACAM flexure-tests page for details.

During a typical half-hour exposure, at any elevation, the flexure is unlikely to cause shifts > 1 pixel in either imaging or spectroscopy mode. However, there will in general be significant shifts on slewing to new targets, so for accurate wavelength calibration, arc exposures should be taken before or after observing each science target.

The repeatability with which the VPH is positioned in the beam (by rotating the wheel) translates into shifts of the spectrum on the CCD by ~ 1 pixel rms in the x direction (spatial).

The small shifts between the centre positions of ACAM's spectroscopic slits, when nominally deployed on-axis, are given on the ACAM slit positions page.

Arc and continuum illumination for wavelength calibration and flat-fielding are provided by the standard A&G-box calibration lamps.

With the CuAr and CuNe lamps both switched on, a 1-sec exposure provides plenty of arc lines covering at least the range 4200 - 9300 A. There are few usable lines at wavelengths bluer than 4200 A. Arc maps for these lamps, when used with ACAM, can be found in La Palma Technical Note 135.

If the LIRIS instrument is mounted at Cassegrain (instead of ISIS), the CuAr lamp may not be available. The CuNe lamp alone provides usable lines in the range 5000 - 9100 A, but the lines in the blue are faint, so it's advisable to take two exposures, of e.g. 1 and 15 sec.

The fold mirror reflecting light from the calibration lamps illuminates only the central 5 arcmin of the 5.8-arcmin slit.

ACAM does not include an atmospheric-dispersion corrector, so spectroscopic observations should be carried out with the slit at the parallactic angle.

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Contact:  (ACAM Instrument Specialist)
Last modified: 29 April 2012