Table of Contents:

General Advice

• You can observe 30 minutes after sunset and until 30 minutes before sunrise.

• Don't get more than 4000 counts in a single exposure on a source, or strong H-band persistence will remain for ~15 minutes.

• Try to get at least 1000 counts in a single exposure for your A0 telluric stars.

• Science exposures with less than 30 counts are not easily reduced with the pipeline.

• The default Fowler number is 16 and this should be used whenever possible.

  • We do not recommend exposure times under 30 seconds (the fowler number will be reduced and introduce non physical flux issues in reduced spectra). 

  • If you have a very bright object, that requires an exposure time below 30 seconds, the best practice is to observe it through clouds rather than reducing the exposure time.

• We recommend always observing in full quads (ABBA; e.g. multiples of 4 for exposure number), it is better to reduce your exposure time for a full quad to reach the same SNR than a longer exposure with incomplete quads.

• If the seeing is below 0.6" then there is no benefit to exposures that are shorter than those recommended below.

• Typical seeing at McDonald observatory is ~1.0".

Recommended exposure times - approximate

SNR Estimation Equation

Where itime is the integration time per exposure in seconds, expnum is the number of exposures (must >=4, 1x ABBA), Kmag is the K magnitude of the target, and seeing is in arcseconds.

For example, four 600s exposures on a K=10 target with 0.6" seeing will give you:

The same target with the same exposure time but in 1.0" seeing will give:

Empirical SNR estimates based on the peak counts in the continuum