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GRACES acquisition is entirely performed using GMOS-N. Once the target is placed over a pre-determined GMOS-N CCD pixel, the GRACES cassette containing the pick-off mirror and the fiber injectors is moved into the beam. The whole acquisition sequence takes in average 10 min, inclluding the time required to move from any corrdinate in the sky to the requested position. The sequence is as follow:
- Applying an instrument offset of p = 23.6 arcsec and q = −9.5 arcsec to place the target within ±0.5 arcsec from the desired position.
- Taking an image using a 1x1 pixel binning and at 1800x1100 pixels sub-raster centered on the GMOS-N CCDs. The readout time is ~13sec when the fast read/low gain mode is used, i.e. most of the time. PIs should aim for a minimum of S/N~10. See this table to set the exposure time, or use the GMOS-N ITC.
- Measuring the centroid of the target image on the GMOS-N image.
- If the target is within 0.1 arcsec from the desired position, moving to the next step. Otherwise, applying the offset to improve the target position and going back to step 2. The offset accuracy provided by the OIWFS is ~0.1 arcsec when the offset is smaller than 10 arcsec.
- Placing the GRACES cassette into the beam, and setting the wavelength at which the system is guiding (to optimize flux at that given wavelength).
Since the GMOS filters introduce distortions to the field image, the final position on the GMOS-N CCD required to aligned the target with the GRACES fiber depends on which filter is used for the acquisition. The filter is chosen based on the target’s magnitude and color. For fainter targets (R>10mag), we use broad-band filters and for brighter target, we use narrow-band filters to avoid saturation in the acquisition image. The GMOS-N ITC can be used to decide which filter, with which exposure time and which readmode is the most appropriate. Note that the only narrow-band filters available for GRACES acquisition of bright targets are HeII, HeIIC, Hα and HαC, and the only broad-band filters available for GRACES acquisition of faint targets are g, r and i. The following table gives suggested exposure times as a function of the target magnitude:
|<=10||HeII, HeIIC, Hα or HαC||2|
|11||g, r or i||2|
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:
- Applying a 10 arcsec telescope offset in p.
- Taking an image of the field. The vignette of the GRACES cassette leaves two 14.5 arcsec2 holes through which the target is imaged.
- Correcting the position of the target within a 0.1 arcsec accuracy.
- Applying a -10 arcsec telescope offset in p.
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. On the other hand, a narrow band filter is recommended for brighter targets. 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.