Observation strategy for FOCAS polarimetry

Typical observation sequences

Linear spectropolarimetry

Telescope rough pointing
Fine pointing with Wollaston prism inserted
1. Take a field image with mask (mask pattern image)
2. Take a field image without mask (field image)
3. Differential pointing
4. Repeat steps (b) and (c) until pointing is correct
Observation of target (repeat as required)
1. with half-wave plate at PA= 0.0 deg
2. with half-wave plate at PA=45.0 deg
3. with half-wave plate at PA=22.5 deg
4. with half-wave plate at PA=67.5 deg
Observation of calibration sources (if needed)

Linear imaging polarimetry

Telescope rough pointing
Fine pointing with Wollaston prism out
1. Take a field image with mask (mask pattern image)
2. Take a field image without mask (field image)
3. Differential pointing
4. Repeat steps (b) and (c) until pointing is correct
Observation of target (repeat as required)
1. with half-wave plate at PA= 0.0 deg
2. with half-wave plate at PA=45.0 deg
3. with half-wave plate at PA=22.5 deg
4. with half-wave plate at PA=67.5 deg
Observation of calibration sources (if needed)

Circular spectropolarimetry

Telescope rough pointing
Fine pointing with Wollaston prism inserted
1. Take a field image with mask (mask pattern image)
2. Take a field image without mask (field image)
3. Differential pointing
4. Repeat steps (b) and (c) until pointing is correct
Observation of target (repeat as required)
1. with quarter-wave plate at PA= 45.0 deg
2. with quarter-wave plate at PA=135.0 deg
Observation of calibration sources (if needed)

Circular imaging polarimetry

Telescope rough pointing
Fine pointing with Wollaston prism out
1. Take a field image with mask (mask pattern image)
2. Take a field image without mask (field image)
3. Differential pointing
4. Repeat steps (b) and (c) until pointing is correct
Observation of target (repeat as required)
1. with quarter-wave plate at PA= 45.0 deg
2. with quarter-wave plate at PA=135.0 deg
Observation of calibration sources (if needed)

Exposure times

The estimation of total exposure time can be done in the same way as for normal spectroscopy or imaging, except that polarimetry generally needs much higher S/N. If the observer wants a polarimetric accuracy dP = 0.1%, the error due to photon statistics should be nearly equal to or less than 0.1%, i.e. the S/N should be nearly equal to or higher than 1000. Resulting limiting magnitude of polarimetry is generally 5-7 mag brighter than the corresponding non-polarimetric observation (i.e., normal spectroscopy or imaging).

Since the incident light is devided into two beams in FOCAS polarimetry, the saturation-free exposure time per an image becomes roughly twice as long as that of non-polarimetric observation.