Aladdin SW imaging exposure times

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

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

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

The ETC has not yet been updated for Aladdin SW imaging. To help with calculation of exposure times, a comparison of Aladdin and Hawaii zero-points is given in the following table. As a rough guide the Aladdin 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 the Aladdin have been kept the same as with the Hawaii, i.e. a waiver request should be submitted if the field contains stars brighter than 11th 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 pickup that 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 the pick-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 modes that do not use chopping are between 0.35 and 5s. NDIT should be set so that the total exposure time is between one and a few minutes. However the optimal DIT value depends sensitively on slit width and wavelength setting. See these spectra to get an idea of the background in LWS3-MR observations, or consult the ISAAC Exposure Time Calculator. Users should allow for night-to-night and seasonal variations by including a wide safety margin. A factor of 4 is not unreasonable.