Aladdin SW imaging exposure times

During P69 it was necessary to commission SW (JHK) imaging in theAladdin arm. This new mode is offered for P70 Phase II.

Here is a summary of information which will be useful to observers wishingto prepare P70 OBs using this mode. Information is also provided inthe User Manual (v1.10.1).

The recommended DIT/NDIT/total_exposure_time for Aladdin SW imagingis the same as that for Hawaii SW imaging and is given in Table 23 ofthe User Manual. The minimum DIT with the Aladdin is 0.3447s.It is not necessary to submit a waiver request to use the Aladdinminimum DIT (unlike when using the Hawaii). This is because the readout time with the Aladdin is negligible. This short read time makesobservations with the Aladdin more efficient. However data reductionwith the Aladdin is more complicated, and photometry less accurate,than with the Hawaii (see section 2.2.1 in the User Manual for a moredetailed comparison of the two modes).

The ETC has not yet been updated for Aladdin SW imaging. To helpwith calculation of exposure times, a comparison of Aladdin and Hawaiizero-points is given in the following table. As a rough guide theAladdin is ~1.2 times faster than the Hawaii.

Aladdin ZP (ADU) ZP (e) Hawaii ZP (ADU) ZP (e)
J+Block 24.5 26.8 J 24.9 26.6
H 24.1 26.4 H 24.6 26.3
Ks 23.7 26.0 Ks 24.1 25.8

We are still testing the effects of saturation in the Aladdin.Therefore, for P70 Phase II, the bright star limits for SW imaging with theAladdin have been kept the same as with the Hawaii, i.e. a waiverrequest should be submitted if the field contains stars brighter than11th magnitude.

Recommended minimum DIT for SW and LW no chopping spectroscopy

SW spectroscopy

As discussed in the ISAAC user manual, the Hawaii array suffers from electronic pickupthat is, in some cases, strong enough to dominate over the read noise.The readout of the array for SW spectroscopy has been tuned to minimize this pick-up.However this only works for some DITs. We recommend that users use DITs for which thepick-up is weak, namely, 30, 60, 90, 100, 120, 150, 180, 200, 250, 300, 400, 500, 600,750 and 900s.

LW no chopping spectroscopy

As discussed in the ISAAC user manual, the optimal DIT values for the LW spectroscopy modesthat do not use chopping are between 0.35 and 5s. NDIT should be set so that the totalexposure time is between one and a few minutes. However the optimal DIT value dependssensitively on slit width and wavelength setting. See thesespectra to get an idea ofthe background in LWS3-MR observations, or consult the ISAAC Exposure Time Calculator. Usersshould allow for night-to-night and seasonal variations by including a widesafety margin. A factor of 4 is not unreasonable.