Astronomical Institute AS CR Academy of Sciences CR
Astronomical Institute,
Academy of Sciences of the Czech Republic
Stellar
Physics
Department

Signal to noise

The weather in Ondrejov is quite variable. Thus it is not easy to prepare something like a "calculator of a exposure time".

Generally, one can say that systematic decreasing temperature makes poor seeing while increasing temperature makes good seeing.

Typical seeing seems to be somewhere about 3arcsec. When the temperature is slowly systematically increasing, the seeing is much better. Opposite, if the temperature is decrasing, the seeing is poor and may even be about 6arcseconds.

Thus we think it is appropriate to take into account better spectra with the same exposed signal (i.e. exposuremeter) than that with the same exposure time.

For you to get the picture about our abilities, we present a short table with measured signal to noise. We used measurement of the star eta UMa (Benetnasch, Alcaid, HD120315) which is one of our common test star.

For the determination of s2n ratio we used spectra interval where no spectral lines are expected for this type of stars.

H alpha

The s2n ratio was measured in the interval 6620-6665 angstroms. See figure:

Exp. timeExposuremeterS2N ratio
100.6146
201.3202
302.0247
603.9335
805.3419
1207.9429
16010.4452
20013.0502
24015.7536
36023.4650
42027.4621
24015.7563
603.8311
301.9239
201.3177
100.6118

One can find that the graph exposuremeter versus s2n ratio is close-to-logarithmic:

H beta

The s2n ratio was measured in the interval 4780-4805 angstroms. See figure:

Exp. timeExposuremeterS2N ratio
1207.5201
24015.4333
48031.1433
96061.6481
3000194.3564

H gamma

The s2n ratio was measured in the interval 4490-4505 angstroms. See figure:

Exp. timeExposuremeterS2N ratio
1207.7195
1207.6186
24015.5240
24015.6308
96062.4407