SNR Estimates and Guidelines at McDonald
- Sokal, Kimberly R
- Erica Sawczynec
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 Flower number is 16 and this should be used whenever possible.
For exposure times under 30 seconds the fowler number will need to be reduced, but should still be the largest even number possible.
If the seeing is below 0.6" then there is no benefit to exposures that are shorter than those recommended below.
Recommended exposure times - approximate:
| 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 |
Estimate your SNR with this 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:
Typical seeing at McDonald observatory is ~1.0".
Empirical SNR estimates based on the peak counts in the continuum:
| 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 |