ALFOSC slits

Horizontal versus vertical spectrum orientation

Horizontal long slits
Vertical long slits
Vertical offset long slits
Echelle slitlets
Polarimetry slitlets
Other aperture-wheel items

Notes:

The poor quality 1.0 and 1.8 arcsec horizontal slits were dismissed and replaced in spring 2006 .

New sets of high-quality vertical slits have been purchased (see below): to be used with VPH grism #17 or for time-resolved spectroscopy.

The default orientation for spectropolarimetry is vertical spectra (i.e. horizontal slitlets). Re-introduced April 2012 due to the faster readout with the new controller.

Horizontal long slits

Simple horizontal long-slits, covering the full spatial field of view of the instrument, with slitwidths (in arcsec):
0.4, 0.5, 0.75, 1.0, 1.3, 1.8, 2.5, 10.0, 40.0,
(click for slit transfer functions anno 2006; y-axis of plot gives effective width in CCD8 pixels: 0.19''/pix).
New slit transfer functions July 2020: 0.5, 0.75, 1.0, 1.3, 1.8 with y-axis giving effective width in CCD14 pixels (0.2138''/pix).

The horizontal slits are imaged horizontally on the detector: the corresponding long-slit spectra are oriented vertically on the detector.
The slit lengths all match the FOV of ALFOSC (6.3 arcmin), except for the 1.0arcsec slit (5.3 arcmin) and the 1.8arcsec slit (5.3 arcmin).

This table lists the effective width of the long-slits in imaging mode (no grism).

Horizontal long slits: effective width

name 0.4 0.5 0.75 1.0 1.3 1.8 2.5 10.0 40.0

actual width in arcsec 0.45 0.55 0.75 1.0 1.3 1.8 2.4 9.9 40.8

actual width in CCD8 pixels 2.4 2.9 4.0 5.1 6.6 9.5 12.4 52 215

actual width in CCD14 pixels 2.1 2.6 3.6 4.6 5.9 8.5 11.1 47 192

Vertical long slits

The vertical slits are imaged vertically on the detector: the corresponding long-slit spectra are oriented horizontally on the detector. The advantage of horizontally oriented spectra is that for small rectangular windows the CCD readout time is much shorter for 'horizontal' windows than for 'vertical' windows. See this table for readout times for windows of different dimensions and orientations.

A disadvantage is that the two traps in CCD#8, at columns 462 and 909, will affect the spectrum in the sense that the two corresponding wavelength bins are lost.

Note that for vertical slits in ALFOSC the parallactic angle is 90 degrees different from that of the standard horizontal slits. Note that grisms have to be rotated by 90 degrees to accomodate the vertical slits. The vertical slits give horizontal spectra.

Note: the med-res VPH grism #17 can only be used with vertical (offset) slits !

The available vertical slits are all 5.3 arcmin long (click for transfer functions):

Vertical long slits

width in arcsec 0.5 0.75 0.9 1.3 1.8 10.0

width in CCD8 pixels 2.5 3.9 4.8 6.8 9.5 51

width in CCD14 pixels 2.3 3.5 4.3 6.1 8.5 46

Vertical offset slits: tuning the wavelength region

Especially purchased to be used with the VPH grism, these slits can in fact can be used with any non-echelle mode grims. With these offset slits the wavelength region can be tuned: it can be shifted by 1/4 of the total region (either bluewards or redwards). See the grism page for standard wavelength regions.

Example: for the VPH grism #17 the standard wavelength range is 6350-6850 Angstrom. With the offset slits the region can be shifted by about 120 Angstrom to either 6230-6730 Angstrom or 6470-6970 Angstrom.

The available vertical offset slits are all 5.3 arcmin long (click for transfer functions):

Vertical offset long slits

width in arcsec 0.5 0.8 1.0 1.3 1.9 8.8

width in CCD8 pixels 2.9 4.4 5.1 7.0 9.8 46

width in CCD14 pixels 2.6 4.0 4.6 6.3 8.8 41

Echelle slitlets

Echelle slitlets are oriented at 90^° to the long-slits and echelle spectra are oriented horizontally on the detector. The slits are 0.1-0.4 arcsec smaller than their names indicate. Slit-transfer functions.

Vertical Echelle slits: effective width
name	0.7	0.8	1.0	1.2	1.6	1.8	2.2
actual width in arcsec	0.3	0.5	0.8	1.1	1.3	1.6	1.9
actual length in arcsec	6.5	6.7	6.8	7.2	7.4	7.6	7.8

Polarimetry slitlets

The default orientation for spectropolarimetry is again vertical spectra (i.e. horizontal slitlets). Re-introduced April 2012 due to the faster readout with the new controller.

We have 3 polarimetry slitlets, 15 arcsec long, and named according to widths (in arcsec).

Polarimetry slitlets: effective width

name Pol_1.0" Pol_1.4" Pol_1.8"

actual width in arcsec 0.7 1.2 1.5

Other items

Other aperture-wheel items are: pinholes, multi-object (MOS) masks, coronographic masks, etc.

Last modified: May 31 2023