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".
Kmag | itime (sec) Seeing~0.6" | itime (sec) Seeing~0.8" | itime (sec) Seeing~1" | itime (sec) Seeing~1.2" | itime (sec) Seeing~1.6" | itime (sec) Seeing~2.0" |
|---|---|---|---|---|---|---|
4 | <10 | <15 | <20 | <25 | < 30 | 30 |
5 | 15 | 30 | 30 | 45 | 45 | 60 |
6 | 30 | 45 | 60 | 60 | 90 | 120 |
7 | 60 | 90 | 120 | 160 | 180 | 200 |
8 | 90 | 120 | 180 | 220 | 260 | 300 |
9 | 180 | 300 | 300 | 480 | 600 | 600 |
10 | 300 | 480 | 600 | 600 | 900 | 900 |
11 | 600 | 900 | 1200 | infinity | infinity | infinity |
12 | 900 | 1200 | infinity | infinity | infinity | infinity |
>13 | 1200 | infinity | infinity | infinity | infinity | infinity |
Where itime is the integration time per exposure in seconds, expnum is the number of exposures (must multiples of 4, 1x ABBA), Kmag is the K magnitude of the target, and seeing is in arcseconds.
For example, four 300s exposures on a K=6.7 target with 1.6" seeing will give you:
The same target with the same exposure time but in 1.0" seeing will give:
Peak counts in single frame continuum: | ABBA SNR (per resolution element) | ABBA(x2) |
50 | 55 | 80 |
150 | 100 | 140 |
300 | 140 | 200 |
600 | 200 | 280 |
1000 | 250 | 360 |