6.2 Planning FLITECAM Observations

Two basic observing modes are available during Cycle 5, imaging and grism spectroscopy. However, commissioning of FLITECAM's longest wavelength modes are still ongoing. For this reason, it is important for GIs to verify that they are using the most recent version of this document as it will be updated regularly as new information becomes available.

6.2.1 Planning Imaging Observations

FLITECAM Imaging observations can be obtained in two different modes: Stare (with or without dithers), and Nod-Off-Array.  The choice depends on the target and astronomical background.  Chopping is not supported with FLITECAM.

Stare mode observations involve a single telescope pointing centered on the source with dither pattern relative to it to facilitate background subtraction and image calibration. Dithering can be performed by selecting from pre-programmed dither patterns, or by defining a custom pattern.  Observational experience has shown that stare observations without dithers are particularly challenging with FLITECAM due to thermal background issues.  Therefore, we recommend using dithers with at least 9 points, parcuarly longward of 2.2 microns.

If the science target is extended or if it is located in a crowded region or region of extended emission, dithering alone may not be able to produce a suitable sky background frame. In this case the GI may elect to observe using Nod-Off-Array mode which will complete a dither on-source and then nod the telescope to a defined "sky" position and complete a dither there as well. Selection of the nod direction and amplitude depends on the field of view around the target and will need to be chosen carefully to access a clean "sky" position and allow proper background subtraction.

The observing efficiency for FLITECAM Stare mode observations with dithering is expected to be on the order of 70%. Observations in Nod-Off-Array mode are expected to result in a decrease in observing efficiency to around 35%. However, these efficiencies are built-in to the SOFIA Proposal Tool (SPT) and do not need to be specified by the GI.

6.2.2 Planning Spectroscopic Observations

Spectroscopic observations with the FLITECAM grisms may be obtained with either an ''ABBA'' or an ''AB'' nod pattern that is either on-slit or off-slit.  The final choice of nod pattern may be determined by the observation's flight plan (Line-of-Sight rate) and will be communicated by the FLITECAM Instrument Scientist before flight.

Guiding during spectroscopic observations will be conducted with either the telescope guide cameras or with HIPO, if FLITECAM and HIPO are co-mounted in the FLIPO configuration (see Section 11).

It is important to note that due to the fixed position of the grisms/slits in the filter/aperture wheels, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Further, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind. These limitations may be especially important to consider when proposing observations of extended objects.

6.2.3 Estimation of Exposure Times

The exposure times for FLITECAM imaging observations should be estimated using the on-line exposure time calculator, SITE. SITE can be used to calculate the signal-to-noise ratio (S/N) for a given ''total integration time'', or to calculate the total integration time required to achieve a specified S/N. The format of the S/N values output by SITE depends on the source type. For Point Sources, the reported S/N is per resolution element, but for Extended Sources, it is the S/N per pixel. The total integration time used by SITE corresponds to the time actually spent integrating on-source without overheads. These integration times are used as input for SPT and for SSPOT, both of which will automatically calculate the necessary overheads.

The exposure times for FLITECAM grism mode spectroscopic observations should be estimated using the on-line exposure time calculator. This calculator can be used to calculate the signal-to-noise ratio (S/N) for a given ''total integration time'', to calculate the total integration time required to achieve a specified S/N, or to estimate the limiting flux for a desired S/N. For both imaging and spectroscopy modes, the estimated total integration time from the respective calculators should be used as the time for an observation in the SOFIA Proposal Tool (SPT). Remember, overheads should not be included, as SPT calculates them independently.