Acquistion Images

Contents


An Overview

It is usually the case that one wants to locate the target, in both imaging and spectroscopy, on a specific pixel of the CCD. This is achieved by executing an Acquisition Template. One acquisition template (and only one) is an essential part of a science observing block - it is always the first template on a science OB and may be followed by one or more science templates.

More details on acquisition images may be found for each science observing mode on the corresponding web page (Imaging and Spectroscopy).

Actions executed by an acquisition template can be divided into 3 well-defined parts.

  • If the Preset flag is set to True
    • stop the telescope from tracking the previous target
    • rotate the adaptor to the required angle
    • go to the current target and start tracking
    It may take anything up to 5 minutes to complete this procedure  depending on far far the telescope has to move, although typically the preset will take around 1 minute.
     
  • If the Focus flag is set to True
    • Take a 20 second exposure with a focus wedge in place
    • estimate the true focus position and apply it
    • repeat the above 2 steps if required until the observer is satisfied
    • Use the last focus image (taken with the focus wedge) as the first image for the next step
    Each focus iteration requires 1-2 minutes. At the NTT this focus procedure is generally not neccesary.
    The focus image is recorded as a FITS image named EFOSC_FocWedge.#.fits
     
  • Position the target
    • Take an image with user-defined exposure time (or the last focus image if it exists)
    • Ask the user to identify the target on the image
    • calculate the telescope shift required to move the target to the user-defined pixel and apply it.
    • Repeat the previous steps until the observer is satisfied
    Each acquisition iteration takes 1-3 minutes and depends on the faintness of the object.  Each acquisition images is stored as a FITS image and the name depends on the template used.
It is possible that one is already on target - one may want to refocus during a long template or one may want to observe some more at the end of a template). But as mentioned before every science OB has to have an acquisition template. In such cases :
1. one can ask the telescope operator to skip the acquisition template - in  this case the science templates are executed immediately, or
2. one can set the Preset flag to False  - in this case the telescope skips to the Focus part in the above sequence. Important - if you want the adaptor angle changed the Preset flag must be set to True even if you are on target
Telescope Focus   The telescope must be focussed every time it is moved by 15-20 degrees on the sky  - either because the target was changed or because the telescope moved substantially while tracking a target. Of course any change in temperature will necessitate a refocus. It is not necessary to refocus from one filter to another.

Acquisition Exposure Time   It takes between 1 and 3 iterations to get the positioning of the target exactly right - and it is a strong function of how many photons the object has on the image. Always err on the side of caution by specifying a larger acquition exposure time than you think is necessary - it takes less time to keep the shutter open for another 20-40 seconds than to go through another iteration! The default 20 second exposure is sufficient for objects up to 19-20 mag (in R). Scale up the exposure correspondingly for fainter objects.

The minimum exposure time allowed by the template is 1 second. However  ultra bright stars (e.g. spectrophotometric standards) which will saturate the CCD even with this exposure and make positioning on the slit difficult. One solution would be to request the telescope operator to make the change (down to 0.1 second) directly on BOB.

Using Acquisition Images for Photometry  The acquisition images are always read out in the fast mode to save time. This means that such frames have a different gain, bias and readout noise than frames taken in the normal readout mode. We recommend that observers not use these images for science unless they have calibration frames (bias, flat fields etc) for the fast mode. Also note that the fast mode is read using 2 different amplifiers which means that the two halfs of the CCD have different biases and gains.

Sometimes observers specify long acquisition exposure times with the intention of doing photometry on the fainter objects in the field. Remember that if you go through several iterations (for whatever reason) to position your target you will be wasting time because every one of those exposures will use that long exposure time. It is better to take short (but sufficient) acquisition and then add a regular science observing template to it.

Imaging Acquistion Procedures

Simple Preset

This is the simplest acquisition template - it merely points the telescope on to the target, rotates the adaptor to the required value and starts tracking. Note The telescope pointing accuracy is a few arcseconds and there are bad pixels close to the centre; if you want to be sure that your target is not on a bad pixel you should use the Move_To_Pixel template. This template is useful when one wants to do a focus sequence on a field ..... or if the target is extended and one is not bothered by a few bad pixels.

jP2PP panel with typical parameters

The above example shows a simple preset template which makes the telescope track the target with the y-axis of the CCD aligned 25 degrees west of north.

Move To Pixel

This is the standard template for imaging programmes including simple imaging, jittered imaging and coronographic imaging. This can be used to make sure that
1.  widely scattered objects (eg. a large cluster, standard star cluster) are all are positioned on the CCD
2.  essential objects do not fall on bad pixels
The object positioning is done as follows :
  • The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the object by clicking on the screen
  • The procedure calculates the telescope offset required to move the object to the pixel defined in the template and then offers the following options
    • offset and exit from the acquisition procedure
    • offset but take one more acquisition image to make sure the shift worked fine
    • do not offset but exit from the acquisition procedure (if the offset was very small, for example)
    • abort the entire OB
One should allow for 1-3 minutes for this procedure

jP2PP panel with typical parameters

The above example shows an acquisition template which does a Preset (points the telescope to the target and starts tracking) followed by a Focus procedure and finally  moves the desired object to pixel  (1100, 1024)

The FITS image is named  EFOSC_AcqPix.#.fits

Polarimetric Imaging Acquisition

This is similar to the Move_To_Pixel template. The only differences are :
  • The Half Wave Plate should be introduced into the optical path
  • The x- and y-pixel values are to be provided not in the template but in a panel which pops up when the procedure is run. This pixel value should be determined by following the procedure described under Identifying the Acquisition reference pixel
One should allow for 3-5 minutes for this procedure

jP2PP panel with typical parameters

The above example shows an acquisition template which does a Preset (points the telescope to the target and starts tracking)  and without doing a focus  goes on to moving the desired object to a pixel which has to be defined when the procedure is run. The acquisition is done with the Half Wave Plate in position.

The FITS image is named  EFOSC_AcqPol.#.fits

Spectroscopy (long slit and spectroploarimetry)

Move to Slit

This is used to position a target on the slit in long slit spectroscopy and spectropolarimetric modes.

Note that even though the acquisition frame is an image, taken without a slit, one still has to specify the slit intended for use. This is because the  y-pixel location of different slits are different (the y location is recorded in a database when a slit is mounted in the instrument). Additionally, one also has to specify the x-pixel along the slit where the target will be positioned. One should chose a column which has no bad pixels as the x-pixel value.

The x-pixel value for spectropolarimetry should be determined by following the procedure described under Identifying the Acquisition reference pixel

One can either orient the slit in a particular direction (along the major axis of a particular source) or let the procedure align it along the parallactic angle. One selects for the parallactic angle by choosing -9999 for the rotator offset angle in the template. The other option is to input an rotator offset angle (ROA) which has to be between -180 and +180 degrees. The position angle of the slit is given by  PA = 90 + ROA. Important: it is advisable to sellect the parallactic angle when the object is low in the sky, especially when conducting spectroscopy with a large wavelength range, so to avoid losing flux due to atmospheric refraction. The importance of atmospheric differential refaction in spectrophotometry is discussed by Filippenko (1982).

The object positioning is done as follows :

  • The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the object by clicking on the screen
  • The procedure calculates the telescope offset required to move the object to the x-pixel defined in the template and on the y-column where the slit is located. It then offers the following options:
    • offset and exit from the acquisition procedure  (when the shift is less than 1-3 arcsec depending on the size of the slit)
    • offset but take one more acquisition image to make sure the shift worked fine
    • do not offset but exit from the acquisition procedure (if the offset was very small,  say <0.1 arcsec, for example)
    • abort the entire OB
One should allow for about 5 minutes for this procedure

jP2PP panel with typical parameters

The above example shows an acquisition template which does a Preset (points the telescope to the target with the adaptor angle = parallactic angle and starts tracking) followed by a Focus procedure and finally  moves the desired object to pixel  (1100, y-pixel of slit#1.5)

The FITS image is named  EFOSC_AcqSlit.#.fits

Important   Since every slit has a different y-pixel position one cannot mix observations with different slits in the same OB. One can however mix different grisms but with the same slit

Rotate to Slit

This is used to position two targets on a slit in long slit spectroscopy and spectropolarimetric modes.

Note that even though the acquisition frame is an image, taken without a slit, one still has to specify the slit intended for use. This is because the  y-pixel location of different slits are different (the y location is recorded in a database when a slit is mounted in the instrument).

The procedure itself calculates the appropriate position angle of the slit that includes both the required objects (see the procedure described below). So the easiest option is to define the rotator offset angle (ROA) = 0 in the template. However observer can save a minute of acquisition time if he/she specifies an approximate rotator offset angle such that the objects are parallel to the slit, especially if the rotator needs to be moved a long way from its default position.

Additionally, one also has to specify the x-pixel along the slit where the one of the targets will be positioned. One should chose a column which has no bad pixels as the x-pixel value. Also make sure that with this x-pixel value the second target will not fall on a column with bad values.

The x-pixel value for spectropolarimetry should be determined by following the procedure described under Identifying the Acquisition reference pixel

The object positioning is done as follows :

Rotation to align the slit with the objects:

  • The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the two objects by clicking on the screen.
  • The procedure calculates the rotator offset angle required to align the two objects along the slit. It the offers then offers the following options
    • rotate and move on to the next step in the acquisition procedure  (rotation < 1 degree)
    • rotate  but take one more acquisition image to make sure the rotation worked fine
    • do not rotate but move on to the next step in the acquisition procedure (if the offset < 0.1 degree)
    • abort the entire OB
Translation to move the objects into the slit:
  • The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the object by clicking on the screen
  • The procedure calculates the telescope offset required to move the object to the x-pixel defined in the template and on the y-column where the slit is located. It then offers the following options
    • offset and exit from the acquisition procedure  (when the shift is less than 1-3 arcsec depending on the size of the slit)
    • offset but take one more acquisition image to make sure the shift worked fine
    • do not offset but exit from the acquisition procedure (if the offset was very small,  say <0.1 arcsec, for example)
    • abort the entire OB
One should allow for about 5-15 minutes for this procedure

jP2PP panel with typical parameters

The above example shows an acquisition template which does a Preset (points the telescope to the target and starts tracking) followed by a Focus procedure and finally  rotates the adaptor to align the slit parallel to the 2 objects and then moves the selected object to pixel  (1450, y-pixel of slit#2.0)

Important: Since every slit has a different y-pixel position one cannot mix observations with different slits in the same OB. One can however mix different grisms but with the same slit.

The FITS image is named  EFOSC_AcqRotSlit.#.fits

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Glossary

Given below is a list of acquisition template parameters which should/may be modified:

  • Exposure time   should be 20 seconds for acquisition frames for targets up to 19 mag. Exposures for fainter objects should be scaled up. In both cases the focus frames are always taken with a 20 second exposure. There is a minimum value of 1 second which is still too bright for ultra bright stars (polarimetric standards for example). One can request the telescope operator to reduce the exposure below 1 second directly on the BOB.
  • Filter   R band is the default; others may be used if the observer believes that the target will be more prominent
  • Focus flag   should be set to True if focussing on the target field is required or to False otherwise
  • Insert HWP   only in Polarimetric imaging template - should be set to True
  • Preset flag    should be set to True if moving to a new field or if the adaptor needs to be rotated.
  • Rotator Offset Angle   This is the angular offset in degrees from the default position. In the default position the slit is oriented east-west and the CCD y-axis is along the north-south direction. Positive offsets will increase the position angle of the slit (PA = 90 + rotator_offset_angle)  and the y-axis of the CCD will be aligned with PA = rotator_offset_angle. The allowed range is -180 to +180 degrees. Set this to -9999 for the parallactic angle.
  • Slit for reference   This has to be defined for spectroscopic acquisition templates. The  y co-ordinate of each slit is different and is stored in a database. This identity of the slit is essential for positioning the target on the slit.
  • X,Y  pixel co-ordinate   These specify the pixel to which the target has to be moved during the acquisition procedure. In spectroscopic templates only the X-pixel co-ordinate has to be defined since the identity of the slit fixes the y position.
Do not change the values of the following parameters:
  • CCD readout speed   :  fast
  • CCD X binning   : 2
  • CCD Y binning   : 2
  • Perform combined offset   : T
 
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