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3.7 ISOPHOT Modes of Operation

In this section summaries of the logic behind the ISOPHOT AOTs are given. Only those operations that may be relevant for data processing are mentioned. Details on the parameters to be entered by the observer are described in the ISOPHOT Observer's Manual [10]. For details on the the filter wheel settings see section A.1.

3.7.1 PHT-P: PHT03, PHT04, PHT05, PHT17/18/19

3.7.1.1 PHT03

PHT03 includes the following operations:

1.

Pointing procedure, pointing:

on source in case of single pointing observations except when rectangular chopped mode is requested;
in between the source position and the chopper position in case of rectangular chopper mode;
on the first raster point in case of raster maps (see below).

Repeat points 2 - 5 for all detector assemblies selected:

2.

In case of detectors P2 and P3, switch on heating and wait for heating stabilization. Activate detector.

Repeat points 3 - 4 for each filter using the activated detector

3.

Rotate the filter and aperture wheels to the commanded configuration, optimised to instrument requirements.

To minimize memory effects of the detectors the measurements with the different filter/aperture combinations are performed according to increasing power on the detector. PGA should have warned the observer if the ratio between the highest and the lowest flux on one detector for the selected filter/aperture combinations is higher than 1000. Such a flux combination within one AOT could lead to poor calibration. In order to minimize heating and cooling stabilization times in multi-detector mode, the detectors are used in the order:

P3 -> P1 ->P2

4.

Integrate on the source for the specified or calculated exposure time. A measurement is performed in staring mode or one of the three following chopped modes:

(i) RECTANGULAR
(ii) SAWTOOTH
(iii) TRIANGULAR

In staring mode the observer has to provide an appropriate reference field measurement for background subtraction, which is not part of this AOT.

5.

The internal calibration is performed with the last filter/aperture combination used with the current detector. This means that there will be one FCS calibration per detector used. If the observation is in staring mode a staring measurement on FCS1 is performed. If the sky is measured in any chopper mode, both FCS's are measured in rectangular chopped mode; FCS1 is tuned close to the [source+background] power, FCS2 close to the background power as given by the user.

After calibration, the FCS's are switched off.

6.

After the last integration, the electronics are set to standby and the instrument is reconfigured to the default state which is the C200 serendipity mode configuration.

PHT03 in raster mode:

For raster mode measurements, only one detector can be chosen per AOT. If more than 1 filter is selected the measurements are sorted according to increasing power on detector. While PHT is measuring, the spacecraft independently performs a raster map giving no feedback to PHT on the pointing progress in the raster. To synchronize the spacecraft rastering and the instrument data collection, the AOT logic already calculated the total time necessary to perform a complete raster including the microslews. After the elapse of that time the integration is stopped by a ``pulse command'' (a high priority interrupt to the PHT microprocessor) which also switches off all electronics including the detector heaters. After the pulse command a reinitialisation of the instrument is necessary, this is reflected in the AOT operations.

1.: In case of a heated detector, switch on heating and wait for heating stabilization. Activate detector.

Repeat points 2 - 6 for each filter:
2.: Rotate the filter and aperture wheels to the commanded configuration.
3.: An internal calibration on FCS1 is performed in staring mode. After the calibration FCS1 is switched off.
4.: Set the instrument integration time such that it covers the time needed to perform a full raster map with the specified raster point integrations requested by the user. The spacecraft is commanded to perform a raster according to the requested parameters. PHT is collecting data during the raster slews. The raster point ID and the on-target flag are recorded to reconstruct where the spacecraft was pointing.
5.: After the raster has been completed the integration is halted by a pulse command. This pulse command also stops the electronics. The detector is activated again and in case of a heated detector, a stabilisation heating time is included.
6.: An internal calibration on FCS1 is performed in staring mode. After the calibration FCS1 is switched off.
7.: After the last integration, the electronics are set to standby and the instrument is reconfigured to the default state which is the serendipity mode configuration.

3.7.1.2 PHT04

PHT04 includes the following operations:

1.

Pointing procedure, pointing:

on source in case of single pointing observations except when rectangular chopped mode is requested;
in between the source position and the chopper position in case of rectangular chopper mode;

2.

In case of a heated detector, switch on heating and wait for heating stabilization. Activate detector.

3.

Rotate filter wheel to the position of the required spectral bandpass

Repeat points 4 - 5 for each aperture

4.

Rotate the aperture wheel to the respective aperture configuration. Measurements will be ordered with increasing power on the detector, i.e. with increasing APERTURE size.

5.

Integrate on the source for the specified (or calculated) integration time. Measurement is performed in staring mode or one of the three following chopped modes:

(i) RECTANGULAR
(ii) SAWTOOTH
(iii) TRIANGULAR

6.

The internal calibration is performed with the last aperture used with the current detector. This means that there will be one FCS calibration per detector used. If the observation is in staring mode a staring measurement on FCS1 is performed. If the sky is measured in any chopper mode, both FCS's are measured in rectangular chopped mode; FCS1 is tuned close to the [source+background] power, FCS2 close to the background power as given by the user.

After calibration, the FCS's are switched off.

7.

After the last integration, the electronics are set to non-prime- state and the instrument is reconfigured to the default state.

3.7.1.3 PHT05

For the absolute photometry AOT PHT05 only one filter and one aperture can be selected. Chopped and/or raster mode is not possible.

1.

Pointing on target

2.

In case of heated detector, switch on heating and wait for heating stabilization. Activate detector.

3.

If a dark measurement is requested then

rotate the filter and aperture wheels to the dark instrument configuration for the requested subsystem (see Table 3.5)
perform a dark measurement of 256 s

4.

Rotate the filter and aperture wheels to the commanded configuration

5.

If a cold FCS measurement is requested then

perform an internal calibration measurement of 256 s in staring mode of FCS1 with zero electrical power applied

6.

Perform the sky measurement

7.

Perform the calibration measurement in staring mode on the heated FCS1 with an measurement time equal to the sky measurement. After the calibration the FCS1 is switched off.

8.

The electronics are set to standby and the instrument is reconfigured to default mode.

Note that the dark instrument configuration is different for the different P detectors in order to ensure that no (stray-)light can reach the detector, see Table 3.5 for the selected filterwheel settings. The mnemonics given in the table are explained in section A.1.

**Table 3.5:** Filterwheel settings for dark measurements in AOTs
Detector	Wheel I	Wheel II	Wheel III
	mnemonic	mnemonic	mnemonic
P1	PHT_S	5_SEC	P3_100_UM
P2	PHT_S	5_SEC	P1_3P29_UM
P3	PHT_C_NO_POL	5_SEC	P1_11P5_UM
C100	PHT_S	C200_200_UM	P3_100_UM
C200	PHT_S	C100_50_UM	P3_100_UM
SS/SL	PHT_P_NO_POL	180_SEC	P1_7P3_UM

3.7.1.4 PHT17/18/19

PHT17 initiates the following operations:

1.: Pointing on requested position

Repeat points 2 - 3 for each filter using the selected detector
2.: Rotate the filter and aperture wheels to the commanded configuration, optimised according to instrument requirements. To minimize memory effects of the detectors the measurements are sorted according to increasing power on the detector. The measurements with the different filters are performed in the same order. The selected filters must belong to the same detector.
3.: Integrate on the source for the specified (or calculated) integration time. Measurement is performed in staring mode.
4.: The internal calibration is performed in staring mode on FCS1 with the last filter of the selected detector. After the calibration, FCS1 is switched off.

PHT18 initiates the following operations:

1.: Pointing on requested position

Repeat points 2-3 for each filter using the activated detector
2.: Rotate the filter (and aperture) wheel(s) to the commanded configuration, optimised according to instrument requirements. The measurements are sorted according to increasing power on the detector.
3.: Integrate on the source for the specified (or calculated) integration time.

PHT19 initiates the following operations:

1.: Pointing on requested position

Repeat points 2-3 for each filter using the activated detector
2.: Rotate the filter (and aperture) wheel(s) to the commanded configuration, optimised according to instrument requirements.
3.: Integrate on the source for the specified (or calculated) integration time.
4.: At the end of the filter sequence a calibration measurement is performed in the last filter in staring mode on FCS1.
5.: The electronics are switched to standby and the instrument is reset to the default configuration

All measurements are performed in staring mode, thus the observer has to provide an appropriate reference field measurement for background subtraction, which may be one or more measurements in the sequence.

Prior to PHT18, a PHT17 must have been performed. After the last PHT18 has been done, a PHT19 is performed. Prior to a PHT19, a PHT17 must have been performed. During the whole sequence the detector remains activated, but the measurements are only performed on the target positions.

3.7.2 PHT-C: PHT22, PHT25, PHT32, PHT37/38/39

3.7.2.1 PHT22

PHT22 initiates the following operations:

1.

Pointing procedure, pointing:

on source in case of single pointing observations except when rectangular chopped mode is requested;
in between the source position and the chopper position in case of rectangular chopper mode;

Repeat points 2 - 5 for all detector assemblies selected:

2.

Switch on heating and wait for heating stabilization in case C100 is used. Activate detector.

Repeat points 3 - 4 for each filter using the activated detector

3.

Rotate the filter and aperture wheels to the commanded configuration, optimised according to instrument requirements. If filters of both C100 and C200 are requested, detectors will be operated in the following order:

C200 -> C100

To minimize memory effects of the detectors the measurements per detector are performed according to increasing power on the detector.

4.

Integrate on the source for the specified (or calculated) integration time. The measurement is performed in staring mode or chopped mode. If only filters of the C100 filter set are selected one of the three following chopped modes is possible:

(i) RECTANGULAR
(ii) SAWTOOTH
(iii) TRIANGULAR

If at least one filter of the C200 filter set is selected, the chopper mode is automatically restricted to RECTANGULAR chopping with 180'' throw. In staring mode the observer has to provide an appropriate reference field measurement for background subtraction which is not part of this AOT.

5.

The internal calibration is performed with the last filter used with the current detector. This means that there is one FCS calibration per detector used. If the observation is in staring mode a staring measurement on FCS1 is performed. If the sky is measured in any chopper mode, both FCS's are measured in rectangular chopped mode; FCS1 is tuned close to the [source+background] power, FCS2 close to the background power as given by the user.

After calibration, the FCS's are switched off.

6.

After the last integration, the electronics are set to non-prime state and the instrument is reconfigured to the default state.

PHT22 in raster mode (restricted to filter sets belonging to either C100 or C200):

Procedure is identical to the raster logic presented for PHT03.

3.7.2.2 PHT25

See description of PHT05. The available detectors are C100 and C200 for which C100 needs heating. The cold measurement time in the case of C200 is 128 s instead of 256 s.

3.7.2.3 PHT32

PHT32 initiates the following operations:

1.: Pointing on the first raster point
2.: Activate either PHT C100 or C200 depending on selection of filters and wait for stabilization of the detector in case C100 has been selected.

Repeat steps 3-7 for all filters:
3.: Rotate the filter wheel to the commanded filter position.
4.: An FCS1 calibration measurement is performed in staring mode.
5.: Set the instrument integration time such that it covers the time needed to perform a full raster map with the specified raster point integrations requested by the user. The spacecraft is commanded to perform a raster according to the requested parameters. On each raster position a number of chopper sweeps are performed. PHT is collecting data during the raster slews.
6.: After the raster has been completed the integration is halted by a pulse command (see PHT03 raster description). This pulse command also stops the electronics. The detector is activated again and in case of a heated detector, a stabilisation heating time is included.
7.: The map is concluded by a repetition of an FCS1 calibration measurement in the last commanded filter at the last raster position.
8.: After the last filter, the electronics are set to standby and the instrument is reconfigured to the default state.

3.7.2.4 PHT37/38/39

The PHT37 AOT is the first AOT in the cocatenated sequence PHT37, [PHT38], PHT39 which form the sparse map. It initiates the following operations:

1.: Pointing on requested position
2.: For C100 switch on heating and wait for heating stabilization. Activate detector.

Repeat points 3 - 4 for each filter using the activated detector. To minimize memory effects of the detectors the measurements are sorted to increasing power on the detector.
3.: Rotate the filter wheel to the commanded configuration, optimised according to instrument requirements.
4.: Integrate on source for the specified (or calculated) integration time.
5.: Perform calibration in staring mode on FCS1.

PHT38 will initiate the following operations:

1.: Pointing on requested position

Repeat points 2-3 for each filter using the activated detector To minimize memory effects of the detectors the measurements are sorted to increasing power on the detector.
2.: Rotate the filter wheel to the commanded configuration, optimised according to instrument requirements.
3.: Integrate on the source for the specified (or calculated) integration time.

PHT39 is the last AOT in the concatenated sequence PHT37, [PHT38], PHT39 which form the sparse map. It will initiate the following operations:

1.: Pointing on requested position

Repeat points 2-3 for each filter using the activated detector To minimize memory effects of the detectors the measurements are sorted to increasing power on the detector.
2.: Rotate the filter wheel to the commanded configuration, optimised according to instrument requirements.
3.: Integrate on source for the specified (or calculated) integration time. Measurement is performed in the staring mode, thus the observer has to provide an appropriate reference field measurement for background subtraction, which may be one or more positions in the sequence.
4.: Perform calibration measurement in staring mode on FCS1.
5.: Switch electronics to standby and reset instrument to default configuration.

The minimum number of sparse maps is two: one PHT37 followed by one PHT39.

3.7.3 PHT-S: PHT40

PHT40 will initiate the following operations:

1.

Pointing procedure, pointing:

on source in case of single pointing observations except when rectangular chopped mode is requested;
in between the source position and the chopper position in case of rectangular chopper mode;
on the first raster point in case of raster mapping.

2.

Activate detectors

3.

Rotate wheels to configure DARK configuration for PHT-S (see Table 3.5).

4.

Integrate in dark position for the specified dark current measurement time of 32 s. Measurement is performed in staring mode.

5.

Rotate wheels to configure beam path to PHT-S.

6.

Integrate on the source for the specified (or calculated) integration time. Measurement is performed in staring mode or one of the three following chopped modes:

(i) RECTANGULAR
(ii) SAWTOOTH
(iii) TRIANGULAR

7.

After the integration the electronics are set to standby and the instrument is reconfigured to the default state.

PHT40 in raster mode:

Start with points 2- 6 at first raster position. Set the integration time such that it is longer than the time to perform the requested raster map. Perform point 6 in staring mode. After the raster has been completed the integration is halted by a pulse command (see PHT03 raster description). The instrument is measuring during micro-slews. Proceed with point 7.

3.7.4 Polarisation Observations: PHT50, PHT51

3.7.4.1 PHT50

For PHT50 only the 25 ${\mu}$ m filter can be used with the 79'' aperture. The logic rounds the requested integration time per polariser to the nearest higher multiple of 128 s with a minimum of 256 s. A description of CHW1 (change wheel I) is given in section A.1.4. PHT50 initiates the following operations:

1.: Pointing on target
2.: Switch on heating, wait for heating stabilisation, and activate P2
3.: Rotate the filter to P_25 and aperture wheel to 79''
repeat steps 4-6 to accumulate the requested integration time per polariser
4.: Rotate CHW1 to PHT-P polariser 1 ( $0^{\circ}$ ) and perform measurement of 128 s
5.: Rotate CHW1 to PHT-P polariser 2 ( $120^{\circ}$ ) and perform measurement of 128 s
6.: Rotate CHW1 to PHT-P polariser 3 ( $240^{\circ}$ ) and perform measurement of 128 s
7.: Rotate CHW1 to PHT-P polariser 1 ( $0^{\circ}$ ) and perform measurement of 128 s
8.: Rotate CHW1 to PHT-P `no polariser' position and perform a sky measurement of 128 s
9.: Perform the calibration measurement in staring mode on the heated FCS1 with an measurement time of 128 s. After the calibration the FCS1 is switched off.
10.: The electronics are set to standby and the instrument is reconfigured to default mode.

The additional measurement in the $0^{\circ}$ polariser at the end of the polariser cycle is introduced to correct for long term drifts of the P2 detector.

3.7.4.2 PHT51

For PHT51 only the C200 detector array with the 170 ${\mu}$ m filter can be used. The logic rounds the requested integration time per polariser to the nearest higher multiple of 128 s with a minimum of 256 s. A description of the first filterwheel (CHWI) is given in section A.1.4. PHT51 initiates the following operations:

1.: Pointing on target
2.: Activate the C200 detector
3.: Rotate the filterwheel 2 to C_160
repeat steps 4-6 to accumulate the requested integration time per polariser
4.: Rotate CHW1 to PHT-C polariser 1 ( $0^{\circ}$ ) and perform measurement of 128 s
5.: Rotate CHW1 to PHT-C polariser 2 ( $120^{\circ}$ ) and perform measurement of 128 s
6.: Rotate CHW1 to PHT-C polariser 3 ( $240^{\circ}$ ) and perform measurement of 128 s
7.: Rotate CHW1 to PHT-C `no polariser' position. and perform a sky measurement of 128 s
8.: Perform the calibration measurement in staring mode on the heated FCS1 with an measurement time of 128 s. After the calibration the FCS1 is switched off.
9.: The electronics are set to standby and the instrument is reconfigured to default mode.

In many cases the full sequence is repeated in a $2{\times}2$ raster map with 1 pixel displacement and the source centred on each pixel. This yields a 4-fold redundancy for the central position of the resulting $3{\times}3$ raster map. This method gives the highest source to background contrast and provides a consistency check from the results of the 4 individual pixels.

Next: 4 Instrumental Characteristics Up: 3 Instrument and AOT Previous: 3.6 Focal Plane Chopper

ISOPHOT Data Users Manual, Version 4.1, SAI/95-220/Dc