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GRACES acquisition is entirely performed using GMOS-N. Once the target is placed over a pre-determined hotspot on GMOS-N's detector, the GRACES cassette containing the pick-off mirror and the fiber injectors is moved in place. The sequence is as follow:
- A first image of the field is obtained using GMOS-N detector. PIs should aim for a minimum of S/N~10. Use the GMOS-N ITC to determine the corresponding exposure time.
- The telescope is offset to position the target over the GRACES pick-off mirror hotspot.
- A series of image are taken to fine tune the target position until a 0.1 arsec accuracy is reached.
- The GRACES cassette is moved into the field, and the beam is directed into the fiber injectors.
The flections of the GRACES cassette into GMOS are expected to be smaller than 0.07 arcsec 95% of the time, with a maximum flexure of 0.12 arcsec. However, we offer the possibility to re-acquire the target during a long exposure. The sequence is as follow:
- The telescope is offset in p by 10 arcsec.
- An image of the field is taken. The vignette of the GRACES cassette leaves two 14.5 arcsec2 holes through which the target is imaged.
- The target is offset to the hotspot.
- The telescope is offset in p by -10 arcsec.
Acquisition observations must be defined by the PI in the Phase II definition. It is NOT necessary to acquire using the GMOS filter that is the closest to the central wavlength of interest. The effect of atmospheric refraction is taken into account when the GRACES cassette is moved in the beam. It is hence recommended to use the filter into which the target is the brightest, or the r filter by default. PIs should specify target brightness (and finding charts if needed) in the filter specified for acquisition, if possible. It is also useful to add notes with acquisition instructions if any special conditions apply (e.g., "center on fainter source W of bright nucleus","acquisition in Z filter to match finding chart", etc). See the GRACES OT details for more information on defining GRACES acquisition observations.