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How to pick an exposure time

Picking an exposure time can be a tricky thing when using a telescope. If your exposure time is too long then you will overexpose bright objects in your field. If it is too short you may not see anything. It is important when picking an exposure time to know the brightness of the object that you are looking for.

How bright is my object?

Astronomers use a logarithmic scale to measure the brightness of objects in "magnitudes." One magnitude is roughly 2.5 times difference in brightness. It is important to remember with the magnitude scale smaller numbers are brighter objects and negative numbers are really bright objects. (At this time we would suggest that you do not take pictures of objects with negative magnitudes.) Also remember that some objects may appear brighter or fainter depending on the filter you are using because of their color and the response of the CCD. (for example red stars will be brighter using the R filter than using the U filter).

Here are the visual (how your eye sees them) magnitudes of some common objects:
Magnitudes of Common Objects
The Sun-26.72
The Full Moon-11
(2nd brightest star)
Faintest you can
see naked eye
about 5.5

To find out what the magnitude of the object you want to observe is you have to look it up in a star atlas or star catalog. This option may be made available later on this site but is presently not implemented.

How long should my exposure be?

CCD plus filter response curves It is important to remember that the light detected is a function not only of the filters but also the sensitivity of the CCD. The figure at left shows the relative amount of light detected through each filter as a function of wavelength.

Exposure times for the B, R, and I filters are roughly the same to obtain the same level of exposure as in V with this camera. Exposure times with no filter are different. There is a separate table after the table for the filtered exposures for no-filter exposures.

The following is a table of recommended exposure times in direct imaging mode for the V filter. (This is currently the only combination available for online proposals.) Choose a time for which the magnitude of your object is neither under exposed or over exposed. Interpolation between times on the table is allowed and encouraged to achieve best results. This table will be updated as we get the telescope fully calibrated.
Exposure Table
(Filter, Direct)
Maximum Exp Time
Minimum Exp Time
Exposure Table
(NO Filter, Direct)
Maximum Exp Time
Minimum Exp Time

The Magnitude Cutoff

With our FLI CCD, overexposing the chip is a very bad thing. It leaves behind residual charge that causes each subsequent image to show a ghost image of the burned in region for the rest of the night. This is terribly un-cool if your exposure of a faint galaxy is scheduled for after someone has taken a picture of the star Vega and nearly burned a hole in the chip. So as a result, there is a policy where we will be enforcing the maximum exposure times listed in the above tables for exposures taken with and without filters. This also means that we will not take filtered images of objects brighter than 4.0 magnitudes or no-filter images of objects brighter than 6.0 magnitudes. See the web page on the exposure time limits for more information.

A word of caution.

Sometimes there is a difference between magnitudes listed for point objects (like stars) and extended objects (like nebulae or galaxies). The magnitude listed for extended objects is often an integrated magnitude. This means the magnitude listed is how bright the object would be if you took the entire luminosity of the object and shrunk it to a star-like point. For example, the Orion Nebula, the brightest nebula in the sky, is listed at an integrated magnitude of about 4. The nebula itself is spread over about half a square degree with the center obviously being brighter than the edges. The brightest parts of the nebula turn out to be a magnitude of about 7 or 8. So a 7 or 8 second exposure or better is needed to see any detail in this 4th magnitude nebula.

For star clusters sometimes the magnitude listed is an integrated magnitude as well. We suggest you do your homework and find out how bright the individual stars are in the cluster if at all possible.

©2001 CWRU Astronomy Dept.
Last modified March 27, 2001
Case Western Reserve University