CCD Windowing

The following window coordinates are appropriate to limit the CCD readout window to the region illuminated by the polarimeter.

Mode X Y W H
Spectropolarimetry 0 760 2048 640
Imaging Polarimetry 580 640 330 640

Note that these are ``figdisp'' coordinates. You can set this window by issuing one of the following commands from a shell window on punaluu:

spectral mode: m window=0,0,760,2048,640
imaging mode: m window=0,580,640,330,640

Focusing the Spectrograph

The polarimeter may be focused in spectroscopic mode. Please see Focusing the Polarimeter for the procedure for focusing in spectroscopic mode. The polarimeter by happenstance has a focus very close to the long slit focus. So if the long slit focus is known, you may use this focus value for the polarimeter.

Focusing the Telescope in Imaging Mode

The malacq script now accomodates polarimetry mode. Run malacq as usual, with the standard value of pmfm. Check to make sure that the star lies wholly within the imaging beam when the primary is spread; the smaller field size of the polarimeter makes this more critical than with normal imaging.

Calibrations

Optimal exposure times for imaging flat fields and spectroscopic wavelength calibrations may differ between normal LRIS observing and polarimetry mode. The following tables provide estimated exposure times for the RED side.

Estimated exposure times for spectroscopic-mode calibrations

Lamp Exposure Time
using 1.0" slit using 1.5" slit
Hg+Kr* 3 sec 2 sec
Neon 3 sec 2 sec
Argon 4 sec 3 sec
Internal Halogen lamp 18 sec 12 sec

*be sure to let this lamp warm up before taking the exposure!

Estimated exposure times for imaging-mode calibrations using dome lamps

Filter Exposure time
B 80 sec
V 20 sec
R 10 sec
I ?? sec

Calibration Stars

There are lists of polarized and unpolarized stars available in /kroot/starlists/0000_polarized and /kroot/starlists/0000_unpolarized, respectively. In these lists the HST standards are given first, sorted by R.A., followed by other standards used by previous polarimetry experts. Look at the text files for the polarization magnitude and angle (for polarized standards) or polarization and uncertainty (for unpolarized standards). The text files also contain the V magnitudes and spectral types.

These lists are derived from the following papers, which can be consulted for further details:

Another source of useful polarized standards can be found at:

You will also possibly need to take spectrophotometric flux standards and/or imaging photometric standards

Notes

Note on calibrations with polarizing elements in (the HNP'B UV/optical Polaroid and the IR800 IR polarizer); in some wave plate positions all of the light will be in one beam, so don't let yourself be fooled into doubling your exposure times and saturating the images at those angles!

See also: