Acquiring Spectroscopy Sources in Imaging Mode
Raw or sky-subtracted Movie-mode images are used to position the source on the slit or IFU (these images are not saved to disk). Note that movie-mode images will be taken with the same position angle as your spectroscopy or IFU observations.
Before you get to the telescope
- In the OT, prepare standard star and target sequences which include the Target Acquisition “Eyeball” – see the programme preparation for spectroscopy web pages for details. Remember to select the shortest possible exposure time in the acquisition eyeball for the standard (1 second), though longer for the science target (20 or 30 seconds, perhaps, made up of a few coadds with 5sec exposures). The acquisition mode will be set automatically, depending on which grism is in use (i.e. with the same spectral blocking filter, but with the grism and slit removed from the beam).
- After having taken the flat and arc, load up the standard star sequence, run the sequence, slew to the target and configure the instrument for source acquisition but DO NOT continue after the green “Break for Acquisition” line (note that only the slit and grism wheels are reset, so this process should be quick).
- Having reached the break, run Movie and use the Gaia tool “View – Pick Object” to centroid on the object and measure its precise position (magnification of 2 or 3 usually works best). The telescope operator will move this target onto the correct pixel on the array, and probably ask you to check the position a second time.
- Once correctly positioned, Movie should be stopped and dismissed. Once UIST is “idle”, check that the telescope is guiding, and then just continue on down the sequence. The sequence will configure UIST for spectroscopy and pause. If you’re happy with the setup, hit continue again and start taking your spectroscopy data.
- Repeat the above process for the fainter science targets.
Note that you can run Movie with or without Sky-Subtraction; with sky subtraction, the first movie frame will be subtracted from all subsequent frames. The operator will offset the telescope a few arcseconds before you start movie, then after the first exposure move back onto the target. Pick Object in Gaia is used in the same way to centre the source on the slit/IFU.
Imaging Acquisition with Faint Sources
In the table below we list very approximate limiting magnitudes for source acquisition in imaging mode. Sky-subtraction was used with Movie in each case. The magnitudes are of course subject to seeing and transparency. Note also that, although longer exposure times may be used at shorter wavelengths, at thermal wavelengths this time is limited by saturation on the sky. Moreover, since coadds are possible with imaging acquisition, we recommend using, e.g., 5 x 6sec rather than 1x30sec , to avoid latency issues (discussed here).
|Grism||Blocking filter||Point source mag.||Exposure time used for|
Acquisition modes that use the same blocking filters should have the same limiting magnitude (these use the same exposure time).
Orientation during imaging acquisition
Because the raw frames from UIST on the Movie display are orientated with N-left and E-up, and because non-zero position angles are often used for long-slit and IFU spectroscopy, imaging acquisition can be confusing. However, clicking on the “re-orientation button” in Gaia may help (note that the acquisition process will still work with the frame re-orientated)…
The diagrams below show the orientation for a number of different position angles, with and without the array reorientated. As a general rule of thumb, with the images “flipped”, N is up and E is left for a position angle of -90 degrees.