Science Operations

Exposure Time Calculator

A web-based Exposure Time Calculator (ETC) is provided to help to determine the exposure times needed for your science to reach your desired SNR. The ETC knows the zero points and sensitivity of both LUCIs in the different available bands and modes.

The ETC does not currently directly support AO imaging or spectroscopic observations, but for imaging approximations can be made to estimate the magnitude limit for AO observations from the seeing-limited mode. Use the seeing-limited mode on the ETC and enter a seeing of 0.5″, 0.375″, or 0.25″ for K, H, or J bands, corresponding to 4, 3, and 2 pixels on the N3.75 camera, respectively. This size in pixels corresponds to the approximate pixel size of the Airy disk on the N30 camera in those bands. Assume to first order the core of the Airy disk contains a fraction of the total flux corresponding to the Strehl ratio (at your target!), and for background-limited observations the sky noise is reduced by the square root of the ratio of the aperture areas. Since we have set this example up assuming equal FWHMs in pixels, this is equal to the ratio of the N3.75/N30 pixel sizes in arcseconds. So the magnitude limit for AO observations will be those from this seeing-limited estimate scaled by:

mAO ≅ mSL + 2.5log(Strehl%/100) + 2.5log(0.12/0.015)

For 50% Strehl, this is about 1.5 magnitudes or a factor of 4 in flux over otherwise excellent seeing-limited conditions.
If you plan to take AO observations, it is best to discuss in detail with colleagues familiar with adaptive optics observations, or contact the LBTO Science Operations group.