WFI Science Exposures
Dithered Imaging: WFI_img_obs_Dither
This is the only option that encompasses all types of observation. It offers you the following choices:
- Auto-dithering or you can define your own dithering pattern
- Combined Offsets: do you want the autoguider to follow the guidestar when you offset?
- Note: For large offsets (>?? arcsec) this should be set to no. The guidestar will fall outside of the tracker chip after the offset and a new one will have to be selected
- Return to Origin: do you want the telescope to return to the original position after each dither pattern?
- This should be set to true if you want to perform the same dither patter with more than 1 filter inside the same OB. Otherwise, the dither pattern of the 2nd filter will start at the last dither position of the first filter.
- Filter: should be the same one as you used in your aquisition to minimise overheads
- Subexposure time: How long do you want each exposure to be?
- Number of Subexposures: How many exposures do you want in the dither sequence?
- This number can be 1 if you only want 1 exposure
- If you are defining your own dither sequence, this number must match the number of offsets you have given
- CCD Binning Mode 1x1 is default
- Observation Type choose from menu to reflect the purpose of the observation
The above example shows an OB comprising a sequence of:
- An aquisition template that will execute a simple preset, turn the autoguider on, and put the V/89 filter in the beam
- Take 5 dithered exposures of 120 seconds each through the V/89 filter in a dithering pattern optimised for 5 exposures
- Autoguider follows the offsets and doesn't lose the guidestar
- Returns to the original preset position after dither sequence is finished
- Take 3 dithered exposures of 20 seconds through the B/123 filter in a user defined dither pattern.
- The 1st exposure is offset from the preset position by 560,560 arcseconds
- The 2nd exposure is offset from the 1st position by -1000,-1020 arcseconds
- the 3rd exposure is offset from the position of the 2nd exposure by 500,480 arcseconds (i.e. the 3rd exposure should be taken -60,-60 arcseconds from the preset position)
- Note: The offsets are cumulative!
- The telescope does not return to the preset position since Return to Origin = false
- Take a single exposure of 500 seconds through the I/203 filter at the position of the 3rd exposure of the above dither sequence
- This happens since we did not set Return to Origin = true in the preceding part of the OB
Not strictly a science exposure, but certainly something you need to know about! In particular, the fact that a focus sequence takes about 10 minutes to complete. You have to be patient!
Within the template, one needs to define:
- Filter: Which filter you wish to focus in?
- Sub-exposure time: How long you want each exposure to be?
- Number of sub-exposures: How many exposures do you want to define the focus sequence?
- Telescope Focus Offset: How many encoder units do you want to move M2 by between each exposure?
- Image charge shift: How many pixels do you want each image of the star separated by?
You should use the default values in all cases. The only parameter you need to change is the Filter you wish to use
Click here to download a standard Focus OB (for the V/89 filter). You can then import this directly into your P2PP, duplicate it and edit the duplicates to make Focus sequences for other filters.