# SNR Estimates and Guidelines

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, typical seeing is 0.8":

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< 3030
5153030454560
63045606090120
76090120160180200
890120

180

220260300
9180300300480600600
10

300

480600600900900
11600

900

1200

infinity

infinityinfinity
12

900

1200infinityinfinityinfinityinfinity
>131200infinityinfinityinfinityinfinityinfinity

DCT SNR Estimation:

Estimate your SNR with this equation:

SNR=(273*sqrt(itime*expnum)*10^(-0.2*(Kmag-1))) /(1.66*ln(seeing in ")+1.9)

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

SNR=(273*sqrt(600*4)*10^(-0.2*(10-1))) /(1.66*ln(0.6)+1.9) ~ 200

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

SNR=(273*sqrt(600*4)*10^(-0.2*(10-1))) /(1.66*ln(1.0)+1.9) ~ 110

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
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