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Given the complexity of GeMS/GSAOI, careful planning of the observations is essential for the success of any science program. In this page we provide important information to help the users to define the observing strategies to follow with GeMS/GSAOI. The page also provides some guidelines/tips to maximize observing efficiency and avoid some common errors. This information might be also useful to prepare the GeMS/GSAOI proposal submission (Phase I).
The users may want to take into account the following factors when designed observing sequences:
- Guide stars (NGS)
- Recommended maximum exposures
- Read mode selection, co-adds and associated overheads
- Dithering and offset sizes
- Observing sky frames
Guide stars (NGS)
As it is mentioned in Guide Options page, the best and uniform corrections are provided when NGSs are positioned as close as possible to an equilateral triangle. The best asterisms are the ones that cover most of the field, or the more distant the stars are, the lower the plate scale error will be. In some cases the selection of such asterisms is not possible. When the asterism is not the optimal or less than 3 NGS (2 or 1 NGS) are used, the users might expect larger variation in FHWM across the GSAOI FoV compared to an optimal asterism selection, affecting the PSF uniformity in the field. By using only one NGS, the user can expect corrections slightly better compared to a single conjugate adaptive optics system (e.g. Altair). This is because the distribution of the 5 laser on the sky (1' x 1') can better compensate the variation across the GSAOI FoV compared to a classical AO system with 1 laser.
- The users have to pay special attention with guide stars. Some time, the available optical catalogs show the wrong R magnitude. If you have doubt about the magnitudes of the stars, always use the brighter stars available inside the Canopus patrol field.
- Remember that
only the Canopus CWFS NGSs can be used for fast tip/tilt correction. ODGW for fast tip/tilt correction are not allowed (not implemented yet). ODGW (On-Detector Guide Window) star is used only for flexure correction.
- Remember that the
brightest guide star inside the Canopus patrol field area has to be assigned to CWFS3.
Recommended maximum exposures
For broad band filters (Z J H K Kp Ks), the maximum recommended exposure time is 120 sec. Longer exposures are not recommended because the sky changes significantly between exposures, in particular in H, Kp, Ks and K bands, making sky subtraction difficult.
For narrow band filters, the maximum recommended exposure time is about 360 seconds. Make sure the exposures are long enough to be background limited. The users have to use the Faint Object or Very Faint Object read modes (see below) to observe with narrow band filters (see below).
Science programs that need separate sky frames have to alternate the science observations with the sky frame observations. It is strongly recommended to observe a set of sky frames every 10 minutes maximum (less is better), in particular for H and K filters (see below for details about sky frame observations).
Read mode selection, co-adds and associated overheads
The selection of the read mode and the number of co-adds used in any GeMS/GSAOI observation can impact directly the overheads. It is important to balance between the read mode and the co-adds needed for your observation to minimize the overhead associated to the read out time and optimize the observations an image. The overheads associated to each read mode are listed in this table. Note that in the GSAOI component, the read mode is automatically set based on the filter used and the length of the exposure time. You can overwrite the read mode using the GSAOI sequence iterator.
- If the observations are photon dominated, the Bright objects read mode may be used. If the objects in the field are too bright, then you can reduce the exposure time and add some co-adds to avoid saturation. However, the penalty will be the increase of the overheads due to the co-adds (see this table for details).
- If the observations are background-limited, the Faint or Very faint object read modes may be used. The discrimination between the two read modes depends on the filters used and the length of the exposure time. Note that the recommended exposure time per filters is automatically displayed in the GSAOI component. For broad filters filters, it is recommended to use Faint Object read mode. For narrow band filters, it is recommended to use the Faint or Very Faint Object modes, depending on the length of the exposure time. The readout noise can be reduced by optimizing the readout method. For example, you can be tempted to observe with broad band filters and using the read mode for Very Faint objects to reduce the read noise. In this example, the gain in the reduction of read noise is ~3 e-. However, the penalty in the read noise reduction is the increase in the read out time by a factor of two, from 26.2 sec to 47.7 sec. Short exposure times (< 30 sec) with Faint Object read mode is not a very good combination. Therefore, cares have to be taken in the choice of the read mode.
Dithering and offset size
The GSAOI detector is formed by four HAWAII-2RG 2k x 2k arrays mounted in a 2 x 2 mosaic. The gaps between arrays have ~2.5 mm, corresponding to an average of 2.5" on the sky. Most observations will require to dither in order to prevent any lost of information due to gaps. The arrays are not perfectly parallel to each other and the size of the gaps varies between 2.5" and 3.5" on the sky. Then, it is recommended that the dithering pattern is constructed with a
When an observing sequence is constructed, the user has to keep in mind the following:
- The CWFS NGSs must remain inside the patrol field area of Canopus in all steps defined in the GSAOI offset iterator. Note that in the case that one or more CWFS NGS remain outside the Canopus patrol field area during dithering, an error message will be appear in the OT. A visual inspection of the offsets can be done using the OT Position Editor (see Visualization of Selected Asterism page for details).
- The star used for flexure correction (ODGW) must remain inside the area defined by the array where the star was defined. In the case that the ODGW remains outside the area allowed, an error message will be appear in the OT. The area covered by the ODGW is visualized in cyan (dashed lines) in the Position Editor and can be activated using the ODGW button in the GSAOI Position Editor.
- Remember, the overheads associated to dither, i.e. the time needed to open all loops, move the telescope and close all loops again, is 30 sec.
Note about precise astrometry with GeMS: Some programs require a good astrometric accuracy (see " GeMS: first on-sky results " by F. Rigaut et al., SPIE, 2012, Vol. 8447, pp. 84470I-84470I-15A for a detailed analysis of the astrometric performance obtained with GeMS). In this case, we don't recommend to dither. The large field distortion introduced by GeMS makes difficult to obtain a good astrometric performance when offsets are used. If the users needs to dither, then we suggest to use offsets not larger than 0.1- 0.2 arcsec (5 - 10 pixels).
Observing sky frames
GeMS/GSAOI observations require background subtraction. In the case of crowded fields, fields embedded in a large nebulosity or fields containing an extended object, sky frames have to be acquired using a blank region near the science target. In the case of spare fields, the same science images can be used to create a master sky frame to subtract the background.
A. General notes
Dithers on the sky should be large enough to remove point sources when making sky images (5"-10" is more than enough since the observations are unguided). Be sure that the same exposures used for your science observations are used to observe the sky. Exposures shorter (or larger) than your science observations can make the sky subtraction (and your life) more difficult. Avoid saturating objects in your sky frames when possible. Saturated objects are very difficult to remove and can affect the background subtraction. Observe sky frames every 10 minutes maximum (less is better). The sky changes significantly
B. Spare fields
In the case of spare fields, the same science images can be used to construct a master sky frame to subtract the background. The offsets should be large enough to remove point sources and avoid the gaps when making sky images. If the field contains sources that are more extended than point sources, it is recommend to construct a dither sequence with offsets slightly larger than the average size of these objects.
C. Crowded fields and fields with extended objects
For crowded fields, fields embedded in a large nebulosity or fields containing an extended object, sky frames have to be acquired in a blank region near the science target. Some time this is not possible, and larger offsets are required to reach a blank field to observe the sky frame. In the later, the laser has to be shuttered and the science field has to be re-acquired when the telescope is back from the sky, adding additional overheads to the program. Depending on the location of the blank field, the observing strategy to follow is different.
C.1. Sky field located < 5 arcmin from the science target
- The sky observations can be mixed with science observation and included in the same observing sequence. Note that the sequence cannot be larger than 2.5 hours. A sequence larger than 2.5, requires additional acquisition.
- If the science sequence includes more than one filter, the exposures are short and the observations of all filters are not larger than 10 minutes (including overheads), then it is recommended to observe science target with all filters first and then move to the blank field and observe the sky frames in all filters. e.g. science (J) + science (H) + science (K) + sky (K) + sky (H) + sky (J). This strategy optimize the observations and minimize the overheads.
- If the science sequence includes more than one filter and the exposures are large, then it is recommended to observe one filter at time, e.g. science (J) + sky (J) + sky(H) + science (H) + science (K) + sky (K), using the same sequence.
C.2. Sky field located > 5 arcmin from the science target
In this case the laser propagation must be stopped due to Laser Clearing House restrictions, the laser has to be re-acquired and the CWFS NGS acquisition re-checked when the telescope moves back to the base position.
- For short exposures and multiple filter observations, the best strategy is to observe science field all filters and then move to the sky position. The sky observation has to be included in a separate sequence.
- For large exposure and multiple filters observations, the best strategy is to observe science field for each filter separately. In this case, one science sequence and one sky frame sequence has to be created separately for each filter.