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OSCA can be used in a very flexible manner. Its mount enables the observer to deploy OSCA practically instantly, and the user can choose a mask size that matches the observations and seeing from a software interface. Currently the OSCA mask wheel holds six hard edged masks with sizes 0.2, 0.6, 0.8, 1.0, 1.65 and 2.0 arcseconds. Additionally, two gaussian shaped masks for the visible domain with FWHM=0"5 and 0"6 are available, but there is currently no suitable detector for use with these masks. Interchange of masks is rapid, but they may not be centered exactly at the same position, so it will always take some time to recenter the target behind the mask. Therefore, the user might use different mask sizes during an observing night, but we do not recommend using more than s few different sizes each night, since flat field images should be obtained for each configuration used.
Be also aware that, as OSCA is not cooled, the mask substrates will contribute to the thermal background. Therefore observations in the K band should be taken with extreme care or preferably in H band (if possible). Do take a look at the OSCA features page.
Please let ING staff know well in advance if you are planning to use OSCA during INGRID observations. The AO system needs some fine tuning which has to be done during day time.
The question which mask size to use depends first on the scientific goal and second on the seeing conditions. Potential observers have to be aware that the PSF of an AO system has a diffraction limited core but a significant amount of light is also scattered in a halo with the size of the seeing disk. It is therefore recommended to use a mask corresponding to the actual seeing conditions or try to use always the maximum still suitable mask for your science case.
Which mask size to use
Subtracting a PSF reference from the science target enhances the sensitivity significantly. To do so one can either rotate the actual science observation and subtract the rotated image from the non-rotated one (this only will help if you are looking for pointsources). Or one observes a PSF reference and subtract that from the acutal science observation. In the latter case the observer should also prepare a list of possible PSF reference objects. PSF references should be selected according to the following criteria:
Be aware that PSF observations are time consuming and the observer has to account for this overhead when asking for time in his proposal. Also, keep in mind that since atmospheric conditions are always changing, PSF references may not be an accurate representation of the actual PSF during the observations of the target.
Centering an object behind a stop is currently done manually. This can be time consuming, especially for faint objects (up to 10 minutes) and this overhead should also be accounted for when applying for observing time. For bright stars, the star can be seen through the mask (see sample image on the left), which aids centering. For faint stars the star will disappear completely behind the mask unless the integration time is long enough.
Depending on the needs of the sciences, the observer should then bracket the observations of the science target with PSF reference(s) observations. Also, please have a look at the OSCA observers guide by P. Doel .
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