OM Tool for Planning Grism Observations

A web based form that will aid you in planning grism observations.

This tool is the third in a series of OM web tools to aid users in proposal submission.

Information about the Grism Tool

Below is a tool to aid in planning grism observations using the Optical Monitor. On submission the results will return:

  1. A list of observer inputs.
  2. The total calculated count rate of the source over the whole spectral range of the chosen grism.
  3. The expected signal to noise of the source (calculated using the total count rate).
  4. The calculated count rate of the source at a given wavelength.
  5. The expected signal to noise of the source (calculated using the wavelength dependant count rate).
  6. The assumed background count rate. This is calculated from the combination of zodiacal light, diffuse galactic light, and the average dark count rate of the detector; and therefore depends upon the position of the source. The background in the grisms is complex and dominated by starlight, therefore the assumed background is a lower limit.
  7. Signal to noise warning, if the calculated signal to noise for the total count rate of the source is below 8.09 or 8.75 for the Optical and UV grisms respectively. (Note: that these limits are minimum values for flux only; for accurate positioning of spectral lines, you will need a higher signal to noise value).
  8. Grism count rate warning, if the grism count rate exceeds 10 count/s (this equates to 10% coincidence loss above which is not advised because this causes corruption of the spectral information).
  9. Notes on calculations.

If you require information about the maximum and minimum magnitude limits for your observed star in lenticular filters, then please use Tool 1.

If you require information on the optical and UV grism dispersion direction with respect to the sky, and its relation to a given XMM-Newton Position Angle please use Tool 2.

The input required from the observer to complete these calculations are as follows:

  1. Right Assention of the source (units in hours mins, and secs).*
  2. Declination of the source (units in degrees, arcmins and arcsecs).*
  3. The epoch for the right assention and declination coordinates. The choices given are 2000 or 1950.
  4. Spectral type of source. The choices include B0, A0, G0, G2, K0, M0, and white dwarf stars. If your source type does not appear on the list then please use the closest source type available.
  5. Visual magnitude (mv) of source, in order to calculate the expected count rate.
  6. The grism required for the observation.
  7. The wavelength (in Å) required to calculated the count rate for a given wavelength. The visible grism range is from 3000Å to 6000Å, and the UV grism range is from 1800Å to 3600Å. If you pick a wavelength that is not within these ranges for your chosen grism, you will receive an error message.
  8. The exposure time required for the grism observation. (Note: this is different to the total observation time available for the observation required in Tool 1; Tool 1 calculates the expousre time for each lenticular filter).

On submission of this data in the Observational Details Form , a new web page will be open containing the calculated results.

*Right Ascension and Declination are required for sky maps of the diffuse galactic background (Lienert, 1998) and zodiacal light background (Levasseur-Regourd & Dumont, 1980). These maps are combined with the average dark count rate of the instrument to produce a total sky background count rate for the signal-to-noise calculation.

Please use Tool 2 to check for possible contamination of grism spectrum from other sources using the downloadable tool..

Avoiding Errors on Submitting the Form

Here is a list of possible errors that might occur, and how they can be avoided.

  1. If the exposure time is small, a "NaN" response is produced in the `Signal to Noise' count results. To solve this problem increase the exposure time.
  2. Entering a space (` ') anywhere in a text entry field, whether it is before, after, or in the middle of a number, will produce a "Bad Request" error message, so avoid using spaces.
  3. Comma's (,) within numbers eg. ten thousand written as `10,000', will result in a "NaN" response in the results, so avoid using comma's.


Leinert, C. et al., 1998, Astr. & Astrophys. Suppl., 127, 1.
Levasseur-Regourd, A. C., & Dumont, R., 1980, Astr. & Astrophys., 84, 277.

Observational Details Form

For observing with the Grism's, fainter stars with longer exposure times are recommended.

Right Ascension of source (h,m,s):
Declination of source (deg,arcmin,arcsec):
Epoch of given coordinates:
Spectral type of source:
Visual magnitude of source (mv):
Wavelength (in Å):
Exposure time of grism observation (in seconds):
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Questions about this grism tool should be addressed to email address.
Last Updated by T. S. Poole on 8th May 2006