Calvin Observatory Equipment
The sensitivity of our raw digital images to sky brightness varies
slightly across the field due to vignetting in the optics, dust grains
on the optical surfaces, and the properties of the CCD chip itself.
All of these nonuniformities may be calibrated out of our final image
in a single step by dividing the image (pixel by pixel) by an image
taken of a uniform brightness source, a flat field.
Towards this end Prof. Van Baak constructed a flat-field light box that
can be clamped to
the end of the telescope (seen here stored on a hook inside the dome).
It produces light that is uniform in intensity with respect
to direction and position.
For many images, the small variations in sensitivity are not great enough to be noticeable. However, there are two situations in which flat field calibration is essential: 1) high accuracy photometry (e.g., tracking the tiny variations in a star's brightness caused by a planet passing in front of it); 2) removing bright sky background from low brightness objects (like spiral galaxies). The ability to do the latter is often important in the light polluted skies of Grand Rapids.
A light box is one of three common approaches to measuring sensitivity to a flat field. A second approach is to observe the zenith at twilight, when the sky is bright enough to swamp any stars, but not so bright as to saturate the camera. This has the disadvantage of needing to set the telescope up early (although the astronomical target of interest may not be up until much later). It has the further disadvantage of offering a narrow time window, during which flats must be made using all configurations (various filters, resolutions, etc.).
A third approach is to view light reflected off a white spot painted
on the interior of the dome.
While this may be done at any time, it can be difficult to
get highly uniform reflectance properties.
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The fluorescent light is a triphosphor lamp that uses 22 W and produces
1200 lumens (equivalent to a 75 W incandescent bulb).
It is brighter than the incandescent at all wavelengths
except the near infrared, and is useful to calibrate configurations with
relatively low signal: ultraviolet, blue, and narrow band filters.
The incandescent light is a 7.5 W bulb connected to a dimmer switch.
It is useful for configurations with high signal: clear, visual, red,
and infrared filters, and when the focal reducer is in place.
The lower intensity keeps the image from saturating on even the shortest
exposures.
The box is not heavy, and so can be attached to the telescope regardless
of the direction towards which the scope is pointing.
It is held snugly in place with four spring-loaded clamps.
The power cables for the two lamps plug into outlets mounted on the
telescope fork, so there is nothing for an observer to trip over in
the dark.