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Flat Fields
A flat field is an image taken with
uniform featureless light entering the
telescope, such as a blue sky in the
early morning or after sunset. Flat fields
solve a number of issues in your astro-
images.
Vignetting
Vignetting (Figure 21.1) in a telescope
reveals edge-darkening in the astro-
image. The large CCD chip in the
Parsec can easily detect vignetting
through almost any telescope, even
specialized astrographs. Vignetting is
more apparent when the telescope’s
illuminated field is not large enough to
illuminate the full area of the CCD chip.
As a result, more light is detected in
the center of the image compared to
the edge.
Dust and Particles
Dust and particles (Figure 21.2) will
inevitably show up in your raw astro-
images. Large particles on the CCD
optical window sometimes look like
unfocused circles or doughnuts in your
images. It’s too late to clean your cam-
era if you are already imaging in the field at night. And even when the camera is
clean, dust usually finds a way to show up in your images.
Telescope Artifacts
Very large particles or other artifacts in your telescope can effect your astro
images. Insufficient telescope baffling or poor collimation can also cause unsym-
metrical field illumination in your images.
To take a flat field image:
1. Ensure that the telescope is focused and ready for astro-imaging.
2. Point the telescope at a uniform and featureless light source, like the sky
at dusk or dawn, or a blank white sheet of paper. Make sure the camera
orientation is exactly the same as it is or was for astro-imaging (Although the
telescope is pointing at a featureless surface, the focus and orientation must
be set as it normally would be for astro-images.)
3. Set the
Frame Type in the Camera Control Window’s Setup tab to Flat.
Or if you are taking a sequence of images, set the
Type in the Autosave
Setup window to Flat
.
Even though MaxIm DL will already know which of
your images are flat frames, it’s a good idea to assign a preset, such as
F to
better organize your different images.
4. Set the
Seconds to 0.1 for now and click OK, then Expose
.
You want the
Max Pixel to read somewhere around 20000. Adjust the exposure time
as needed until the
Max Pixel is close to 20000. It’s a good idea to take
several flat frames and try different exposure times until you find the correct
exposure. If you are taking your flats near dusk or dawn, the sky brightness
will changes rapidly.
The flat fields also need dark frames.
5. Add another
Slot sequence in Autosave Setup and select Dark under Type.
Select the same exposure time as your flat and assign a prefix such a
FL.
6. Click
OK then Expose. Note that you can set all of your lights, darks, flats,
and dark flats at the same time in the
Autosave Setup window to make the
sequence faster and more convenient. Once you get used to all of these
features, you will be able to set all of the frames you want in
Autosave
Setup window without having to go back to add more frame types.
light and Dark 2x2 Modes
For added convenience and versatility, the Parsec’s individual pixels (light-
detecting sites) can be “binned” into units of two-by-two pixels (i.e. four individual
pixels create one binned 2x2 pixel). This creates larger and more sensitive pixels,
but with decreased resolution. This can be useful for some types of astronomical
imaging, such as capturing faint nebulae. It can also be useful for quickly check-
ing the image centering and orientation before using the full resolution to actually
capture images.
To use in “2x2” mode, select
X Binning/Y Binning to 2x2/Same.
Note to Parsec 8300C users: By binning the images in 2x2, you will bypass the
RGB filter built into the camera; so the resulting binned images will be black and
white, without the ability to convert to color.
Figure 21.1.
Larger format CCD
cameras like the Parsec reveal vignetting
through most telescopes. Vignetting occurs
when the edge of the image plane has less
illumination than the center.
Figure 21.2.
Dust or other particles on
the camera’s optical window can show up
as distracting dark shapes in your images.