Imaging Science Modes

Contents

• Simple Imaging
• Jittered Imaging
• Coronographic Imaging
• Polarimetric Imaging

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The 4 science imaging modes are described below. The observer is also referred to the pages on Acquisition images and Miscellaneous information.

Simple Imaging

In this mode one can obtain one or more images of a field through broad and/or narrow band filters.

The following acquisition templates may be used in this mode:

• Simple Preset
• Move To Pixel

A sample jP2PP observing block

The above example shows an OB comprising a sequence of:

• A simple telescope preset to target with Rotator offset angle = 0 degree
• 1  R-band  image  of 300 seconds with normal readout  and  2x2 binning  followed by
• 3  U-band  image  of 1200 seconds with normal readout  and  2x2 binning

The FITS file is called EFOSC_Image.#.fits

Parameter Values One does not have to (in fact, should not) change the value of any parameter other than :

• Filter:  See the Filter Set for a list of the available choices
• Exposure Time: This is a list of exposure times. Whatever appropriate in seconds - keep in mind the reflections of heavily saturated stars and cosmic ray hits.
• CCD readout speed: normal ,  fast  or  slow .
• CCD binning: 2x2  or  1x1
• Number of exposures:  whatever appropriate. If the number of elements in the exposure time list is less than the number of exposures the procedure cycles through list from the beginning. For example if the Exposure time entry was 300 600 and the number of exposures were 4 then the 4 images would be observed with exposure times of 300, 600, 300 and 600 seconds.

Jittered Imaging

This is very similar to Simple Imaging  except for the folowing difference :

• A different template is used
• The telescope is moved by a specified amount between each exposure in the template. The telescope offsets are cumulative.

A sample jP2PP observing block (the OB has been erroneously named Coronographic imaging)

The above example shows an OB comprising a sequence of

• An acquisition template including preset ,  focus and with the rotator offset angle = 0 degrees
• This template will move a selected object or location in the field to the pixel (1100, 1024)
• 4 B-band images of 900 seconds each taken with normal readout and 2x2 binning. The 4 images were taken with cumulative telescope offsets of  (RA,DEC) : (-10, -10) , (20, 0), (0, 20) and (-20, 0) -  i.e. with reference to the original pointing the locations are (-10, -10), (10, -10), (10, 10) and (-10, 10). The telescope is returned to the original pointing at the end of the sequence.

The FITS file is called  EFOSC_ImaJit.#.fits

Parameter Values   In addition to the parameters listed under Simple Imaging the following may need to be changed :

• List of Telescope RA Offsets: These are offsets in RA from one image to the next (and not from the initial pointing). Note that the first image will not be taken at the initial pointing unless the first value in the list is set to 0. In case of fewer elements in the list of RA offsets than the number of exposures then the procedure repeats the sequence of offsets.
• List of Telescope DEC offsets: similar to the previous parameter.
• Return to Origin:   If set to True the telescope returns to the original pointing - this would be helpful if one has several Jittered Imaging templates and does not want the field to fall off the edge of the CCD. If set to False the telescope will continue from the position of the last exposure of the previous template.

Coronographic Imaging

The Coronographic mode is very similar to the Simple Imaging Mode  except for the following differences :

• The Coronographic imaging templates lacks the Starplate option available for the Simple Imaging template. This is because the Coronographic procedure automatically puts in the coronographic mask into the light path.
• One has to use the Move To Pixel  acquisition template since the object has to be accurately placed behind the coronographic mask. Note that the mask is not in place during the acquisition image. Obviously, the pixel specified in the Move To Pixel template should be the centre of the coronographic mask (see Coronographic Calibrations)
• Typically Coronographic targets are extremely bright stars and the acquisition template is taken without the mask - one should use very small integration times in the acquisition template.
• The bright targets and the consequent saturation may result in small deviation from ideal positioning using the template. In such cases one may have to position the telescope using trial-and-error offsets. We recommend the following procedure  (see the example OB below) :
• Include in the OB a very short duration Coronographic Imaging template between the acquisition template and the programme Coronographic imaging templates.
• Execute the OB and terminate it after the acquisition template - this will position the star close to the centre of the coronographic mask
• Execute the OB again but skip the acquisition template and go directly to the very short exposure template.
• If the star looks properly positioned in this short exposure continue with the programme templates
• If not, terminate the OB, ask the telescope operator to apply the desired telescope offset using the telescope control panel and repeat the OB (after skipping the acquisition)

A sample jP2PP observing block

• The above example shows an OB comprising a sequence of:
• A Move To Pixel Acquisition template with a very short exposure to avoid saturating the bright coronographic target star and a target location at pixel (1150, 1050) which was determined to be the centre of the coronographic mask
• A short duration coronographic imaging template to carry out fine adjustments to the target position (see the procedure described above)
• 3  B-band coronographic images of 600 second  exposure time ,  normal readout  and  2x2 binning  followed by
• 1  R-band coronographic images of 600 second  exposure time ,  normal readout  and  2x2 binning

The FITS file is called  EFOSC_ImaCor.#.fits

Parameter Values  see under simple imaging

Polarimetric Imaging

Note that since EFOSC2 was moved to the Nasmyth focus of the NTT there is a significant instrumental polarisation that varies with telescope pointing. We are working to characterise this effect, but users should be aware that it may limit the accuracy to which polarisation can be measured. When preparing proposals for polarisation, extra time for observation of a large number of calibration stars should be included.

The acquisition template to be used is Img_Acq_Pol.

The imaging is similar to Simple Imaging except for the following additional features :

• The Starplate is not free but contains a Wollaston Mask
• The Grism wheel contains the corresponding Wollaston Grism
• The Half Wave Plate should be moved in (set to True)
• The continuous rotation of half wave plate should be
• set to True to get depolarise the radiation. Note: in this case there has to be 1 and only 1 value (any value is fine) entered against List of HWP rotator positions
• set to False to keep it fixed. In this case there has to be at least 1 value entered against List of HWP rotator positions   Usually the value entered is a list of 4 angles 0 22.5 45.0 67.5. Note: in this case the template takes one exposure at each of the 4 HWP positions for each exposure requested.
• Finally, only half the field is imaged in any exposure. A second exposure has to be taken if the remaining half also needs to be imaged.

There is also a quarter wave plate for measuring circular polarisation (as opposed to linear polarisation with the HWP). Only one wave plate can be mounted at a time, as they occupy the same position in the instrument. Observing with the QWP is the same as with the HWP, but the QW versions of the templates are used.

Refer to Polarisation optics for more details.

IMPORTANT When the half wave plate (HWP) is set to continuous rotation in a template the counter ends up with 360xN degrees. When a signal is sent to set the HWP to some small fixed value (say, 45 degrees)  it tries to undo all the N rotations and in doing so triggers a time-out error. So we have included a command at the end of every Polarimetry template to initialise the HWP if it was set into continuous rotation mode - this just takes a few seconds. However when such a template is aborted, for whatever reason, this initialisation does not take place. The only solution is to cold-start the EFOSC2 instrument which takes about 1-2 minutes. So avoid aborting a Polarimetry template - use the STOP option instead.

A sample jP2PP observing block

The above example shows an OB comprising a sequence of:

• A Polarimetric imaging acquisition template: the procedure will pop up a panel at run-time seeking the pixel value where the target has to be located. This pixel value has to be determined using the procedure described in Polarimetric calibrations.
• 2 V-band  Polarimetric images of 300 second  exposure time with 20" perpendicular Wollaston mask and prism ,  normal readout  and  2x2 binning  at each of 4 half wave plate positions (at 0, 22.5, 45 and 67.5 degrees) , i.e. 8 exposures in all ,   followed by
• 1 V-band  Polarimetric image of 300 second  exposure time with 20" perpendicular Wollaston mask and prism,  normal readout  and 2x2 binning  with the Half Wave Plate in continuous rotation mode.