Q S I 5 0 0 S E R I E S U S E R G U I D E
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The image above has been manipulated to highlight the effect of dust motes on a filter or CCD cover glass. Note the
3 darker circles. Because dust will tend to stay in one place over a night of imaging, the variation in pixel values caused
by the dust can be easily eliminated by properly applying a Flat Field.
A flat field is created by taking an image of an evenly illuminated subject. There are four
common ways to create flat fields.
Lightbox flats – Using a lightbox is usually the easiest way to create good flat fields.
There are a few commercial lightbox solutions, but many
astronomers make their own. You can find plans in The New CCD
Astronomy and The Handbook of Astronomical Image Processing
as well as online. Search for “Telescope light box”.
Twilight flats – There is a brief time after the sun sets or just before it rises when
the sky is appropriate for creating flat frames. Too early and the sky
is too bright. Too late and stars will begin to show up in the image.
Dome flats – If your telescope is in an observatory, you can take dome flats. A
dome flat is created by aiming your telescope at a white card
placed somewhere on the inside of the dome.
Sky flats –
Taking sky flats requires taking dozens or hundreds of images with
ction on Sky Flats in the Handbook of Astronomical
the telescope pointed at the sky with tracking turned off. All the
images are combined into a master flat to remove the effect of any
stars moving through the field. Sky flats require more time than the
other three options so few amateurs take sky flats. We recommend
reading the se
Image Processing for additional details on taking sky flats.
e
noise and then take a series of dark
frames (called flat-darks) using the same exposure you used for your flat fields. Just as with
light frames, the flat-darks are subtracted from the flat fields to remove any contribution from
dark current. Taking 16 flat fields and 16 flat-darks will yield excellent results. Luckily
Good flat fields require an exposure time such that the pixel wells are filled to approximately
half their full capacity. With a QSI 500 Series camera you should strive to achieve averag
pixel values between 20,000 and 30,000 out of a total of roughly 65,000. You should
experiment with exposure times to yield that result. Pixel values are commonly called
“ADUs”, short for Analog to Digital Units.
You’ll need to take enough flat fields to average out the