Flats help us to deal with gradients, which are variations in
the light level of the background of our astrophotographs
The ideal is an even background where no part is darker or lighter than
another by a very small amount, ie. flat.
Variations from a
completely flat and even background are referred to as
gradients and they are generally caused by something affecting the
field being
photographed, street lighting, your neighbour’s security lighting, the
light
dome over a town, moonlight, haze, etc., and can usually be dealt with
by
software means, like anti-gradient software.
However, some are caused by the optics of your telescope, a mismatch of
telescope optics and chip, a narrowness of the end of the telescope
restricting the path of the cone of light from the primary. This is
vignetting, where the middle circle of the chip appears to be more
brightly illuminated than the corners, and this is actually the case.
The easiest way to deal with it is with flat frames.
A flat frame is a photograph of a white evenly lit area, and what you
should get of course is an evenly lit white frame, but instead, you get
a frame that shows the vignetting your system suffers from, ie. It also
has the central circle well lit and the corners darker. It will also
have the shadows of any dust particles on the chip or its window. These
show up as darker discs or circles, doughnuts. As both the light frame
and the flat frame are digital pictures you can do some mathematics and
dividing the light frame by the flat cancels out the variation in the
background.
 M31 taken
with an M25C and 80ED complete with vignetting, dust shadows and haze |
 This
is a flat taken of a white painted square on the wall of the dome |
The final one is the light divided by the flat (in AstroArt4). Vignetting and dust shadows gone, but not the haze unfortunately. |
For this to work you must obey some basic rules.
1.
The flats must be taken with the telescope and camera at exactly the
same
settings as for the light frames, don’t change the focus, even a
little, so
your best plan is to take your flats immediately after your lights.
Temperature isn’t important if the exposure is under 15sec or so.
2. The exposure must be long enough to register on the chip, but not
long
enough to saturate the pixels. Between 25 and 60% of saturation would
be suitable.
3. The flat frame must be taken of a white evenly lit area, anything
that
will
ensure the light going into the end of the telescope is white and even,
to
enable the chip to picks up the variations in the light caused by the
optical
system only.
The
even white area in my case is a white painted square on the inside of
the
dome, lit indirectly by the dome’s interior light and for good measure
I put a
white T-shirt over the end of the telescope, stretched and held there
with a
rubber band. A friend uses a light box as used to study 35mm
transparencies.
Many make their own light boxes, some use morning or evening twilight,
though
this is more difficult as the light level changes quickly then.
Some details
How much exposure is enough and not too much?
Check the camera maker’s website for details. You need the Full Well
Capacity
and the Gain.
In my camera an SX M25C, the Full Well
depth is 25000 e and Gain is 0.4 e/ADU.
Divide the 25000 by the 0.4 to get 62500 ADU.
25% of this is 15625 ADU and 60% is 37500 ADU.
You’ll
have to
practice a bit to get a
exposure that gets that amount with your kit. But don’t go for long exposures or you’ll have to
consider the noise and then the temperature….
ADU? Analogue to Digital Units, a measure to indicate a level of
brightness. Your acquiring software will
tell you what the level is at each pixel as the mouse runs over it or
as an average.
In my case AstroArt shows the number at the bottom left of the screen, use the second number at the bottom left, the Max number.
You
should take flats through every filter you use and in processing use
the
red flats with the red lights etc.
With my M25C, I aim for 16000 to 20000 ADU, and I can get this, actually about 17000 ADU, with exposures of 3sec., of a matt white painted square on the dome wall, lit by two dome lamps equally spaced each side of it, with a white T-shirt stretched smoothly over the telescope end. I take 25 frames. Then at home in processing, I use the AstroArt preprocessing page and drag the flats to the lights box and average stack them using "none" as the alignment method, nothing else checked. Then de-bayer the result and save it as a .fits master flat. Mine are usually named Target-Telescope-Date-MASTERFLAT.fit. Then, back in the preprocessing page, I drag the master flat into the flats box, and the master bias into the F.Darks box and the Darks box too. Then the lights into the lights box and set the other parameters, including the colour systhesis, to suit the occasion, and run it.
The master bias? That's an average, made like the flats, but of a series of 50 or more frames of .1sec each, with the cover over the telescope. I make a new master bias every 6 months or so and use the same one every time I do any processing on the pre-processing page.
The result is a nicely flattened and coloured frame which I then save as a
tiff file, (a tiff data file), and load it into Photoshop for the rest
of the processing.