DFOSC, Dutch and EFOSC2 Imaging Exposure Time Calculator

This form will help you plan your observations with DFOSC, EFOSC2 and the Dutch telescope by allowing you to (1) produce a graph of S/N against exposure time for a star or an extended object of given magnitude, under specified observing conditions; and (2) produce a table of S/N ratios for a range of magnitudes and exposure times. Fill in the form below (help is available for all the fields via the hypertext links) and then click on `Submit Form'.

DFOSC, Dutch and EFOSC2 Exposure Meter Version 1.4 -- Copyright©(1998) J.Brewer/European Southern Observatory

General Description

This exposure time estimator uses an HTML form to submit data to a PERL script via the Common Gateway Interface. The PERL script processes the parameters, and calls two C programs. The first C-program generates the plot in GIF and PostScript formats using SuperMongo library routines, while the second program generates the HTML page with the tabular data. To avoid problems with caching, each of the files generated has its own id number, and to avoid disk overload the (n-5)th files are deleted when the n'th files are generated (Note: not all browsers support Pragma: no cache). The PERL script generates the page returned to the user.

Instrument

The exposure time estimator is set up for the DFOSC at the Danish 1.54-m and the Dutch direct imager only. Please use the pull-down menu to select one of these two instruments.

Filter

Use the pull-down menu to select the filter which you shall observe with.

Currently, the exposure time estimator can only calculate exposure times for the B, V, R and I filters. We shall add further filters as measurements become available.

Seeing

Enter here a value for the seeing in arcseconds. As it is impossible to predict what the seeing will be on any given night, we suggest that you generate graphs for a range of seeing values. A value of 0.8 arcseconds would be `good' for the Danish and Dutch telescopes, whereas a value of 2.0 arcseconds would be `bad'.

Airmass

Enter the airmass at which you will make your observations here. This value will be used to calculate atmospheric extinction. The Extinction coefficients used (in magnitudes per airmass) by the program are:

Night Sky Brightness

You may either directly enter a value for the night sky brightness (select the `Sky Magnitude' radio button) or specify the the Moon phase (select the `Days from New Moon' radio button). If you choose to specify the sky magnitude, enter your value (units of magnitudes per square arcsecond) in the box below the radio button. You may find the below table of use. If you specify days from full moon, use the pull-down menu to select a moon age. If you use this option, the night sky magnitude will be looked up from the below table.

Note that during bright Moon, the measured values at the telescopes may be higher due to scattered light inside the dome. If you wish to find the phase of the moon on a particular date, you may use these Sun and Moon tables.

Point Source

Select this radio button if your object is a point source (such as a star).

Extended Object

Select this option if your object is an extended object such as a galaxy. If you select this option, note that: (1) the value entered for the seeing is unimportant; and (2) values for the magnitudes should be entered in terms of magnitudes per square arcsecond.

Magnitude

If you selected the `Point Source' option, enter the magnitude of the object you wish to observe in the filter which you have specified. For example, if you selected the B filter and wish to observe a star with V=20 and (B-V)=2, enter a B magnitude of 22. If you selected the `Extended Object' option, enter the surface brightness of the object in magnitudes per square arcsecond.

Minimum Exposure

Enter here the lowest exposure value that is to be plotted on the graph of S/N ratio against exposure time.

Maximum Value

Enter here the highest exposure value that is to be plotted on the graph of S/N ratio against exposure time.

Low Magnitude

Enter the lowest magnitude (i.e. brightest object) which shall appear in the S/N table. If you selected the `Point Source' option enter the magnitude, while if you selected the `Extended Object' option enter the surface brightness in terms of magnitudes per square arcsecond. The magnitude in the columns of the table are incremented in steps of 1 magnitude, so the number of columns in the table is (high_mag - lo_mag).

High Magnitude

Enter the highest magnitude (i.e. faintest object) which shall appear in the S/N table. If you selected the `Point Source' option enter the magnitude, while if you selected the `Extended Object' option enter the surface brightness in terms of magnitudes per square arcsecond. The magnitude in the columns of the table are incremented in steps of 1 magnitude, so the number of columns in the table is (high_mag - lo_mag).

Max Exposure

This is the maximum exposure time (in seconds) to be used in the table. An arithmetic progression with 50 steps is used to increase the exposure time to the maximum specified, and consequently the table will always have 51 rows.

Version Information

• Version 1.0 (April 15 1998)
  First release
• Version 1.1 (April 28 1998)
  Extended object and moon age options added.
• Version 1.2 (June 24 1998)
  Better form decoding routine added.
• Version 1.3 (Oct. 14 1998)
  EFOSC2 Added.
• Version 1.4 (Aug. 29 1999)
  HTML made HTML 4.0 Transitional