When I first started out in astrophotography, there was a lot to get my head around – how to use my DSLR on manual; how to use my kit lenses, what additional gear to get; even how to focus in the dark!
My first images were stacked in Deepskystacker and when the
results emerged – dust spots, red pixels, and ‘darker’ patches were scattered
across the image. Very disappointing. Hardly surprising really. I hadn’t come
across the term ‘calibration frames’ then. Now I have and I fully realise their
importance in obtaining great astrophotography images.
So, what are these ‘calibration frames’ and what do they do
that is so important? The frames are additional sets of images taken at the
start, part way through or end of our astrophotography session. Simply put, they
help clean up our final images by removing or correcting imperfections for
known defects or inconsistencies on our camera sensor; they clean up our data
before we start stretching and post editing it.
I take three types of calibration frames. Each one has a
specific role to play in improving the quality of my final stacked image. The
types are:
Dark Frames
Biases Frames
Flat frames
But before getting into the specifics, lets remind ourselves
of the other type of frames – lights!
Light frames are the main images we take of our deep
sky or milky way sky objects. Rich in signal from our intended target, they
will often contain amounts of the dreaded ‘noise’. Then there may be
aberrations, vignetting, hot pixels, plane and satellite trails; not forgetting
sensor irregularities, read noise, uneven light gradients, dust motes and more.
Basically then, all the stuff that needs to be removed or corrected within
them. Our light frames are
straightforward enough to capture aren’t they. We select our deep space object,
align our lens towards it, set our ISO or gain and correct our focus, before
taking multiple images for stacking later.
So, why is taking calibration frames as well such an
important thing to do?
Dark Frames work on sensor noise. Heat produced by
our camera creates thermal noise and hot pixels. A dark frame photo is taken
with the lens or telescope cap on so that no light can get into the imaging
train. Same exposure time, ISO/gain and temperature as our light frames. All we
capture is the noise! Having identified this noise, we can now use the frame to
eliminate or ‘subtract’ this unwanted data from our stacked data. No more or
severely reduced hot pixels, thermal noise patterns etc.
We can build a library of darks frames – for different
exposure times – and reuse them; if you are using a dedicated cooled astro cam.
However, I mainly focus on use of DSLR on this blog and here it is trickier to
build such a library. This is because dark frames also depend on the ambient
temperature changes encountered from one imaging session to the next. Keep our
DSLR either on or off our scope when taking dark frames. I just put my
lens/telescope dust cap back on and continue shooting using the same exact
settings as my light frames! It takes longer in the field but I don’t mind
being out under the stars! One tip for taking darks with a DSLR is to ensure
that no stray light can enter the sensor area, so I block off my eyepiece
viewfinder with tape. A friend puts a small, lightweight, dark coloured fleece
blanket over her rig when taking them. Now that’s dedication!
So, how many dark frames should we take? How long is a piece
of string? Reading around, between 10 – 20 seems the norm. I’ve seen some
astronomy sites recommend between 20 – 30, arguing the more the better for
obtaining a clean image. The key bit – DO NOT wait until the following night to
take your darks. We take them on the night we are shooting. Remember same or as
close as temperature to when we shoot our light frames.
Bias frames capture Read noise; the electronic ‘pattern’
and ‘read’ noise generated when the sensor reads the data and the camera
downloads an image. We set our camera to the fastest shutter speed it can do –
in my case 1/4000”, keep the cap on the lens/telescope and keep the ISO the
same as our lights. So, we are shooting very short exposures. Take the stray
light prevention measures outlined above when doing dark frames. Stacking these
with our light images will help correct ‘fixed pattern’ noise and remove
unwanted artifacts from dark and light frames.
How many? 20 – 50 seem the consensus needed to ensure
accurate calibration.
Flat frames correct uneven illumination and vignetting
and remove dust motes on our sensor thus ensuring a smooth background in our
final images. There are a number of different methods for taking flat frames
and you can go research these to find which is best for your set up, but here
is what I do:
I cut out part of a clean white T shirt and stretch it
between a small embroidery hoop that is just a slightly larger diameter than my
Samyang 135mm lens and Zenithstar 61 ii aperture. I then place it over the
aperture of my lens/telescope and then place a small tablet over the top of
this. It has an ‘evenly lit’ white screen. Keeping my camera and telescope/lens
in the same set up as my light frames i.e. no changes to focus, ISO or imaging
train etc, I change my DSLR to AV mode and then take short exposures, avoiding
any clipping of the histogram. I am aiming to have the histogram about a third
or half way across from the left-hand side. I cannot emphasis this point enough
– we must NOT change our imaging train or shooting circumstances. So, no
changes to camera rotation, focus, ISO etc.
Adding flat frames into the stacking of our images helps
correct vignetting, dust shadows etc and they are one of the easiest ways of
boosting our final image quality and should therefore not be missed out.
How many do I take? Normally around 10 – 20 flat
frames. I take mine at the end of my imaging session after my darks and bias
frames.
Some quick clarification by the way – vignetting is the light fall off towards the
outside of your light frame. It might resemble a dark circle.
I tend to use SIRIL, Affinity Photo, Deepskystacker and
Sequator for post editing my astro images and each has a calibration phase when
you add your calibration frames into the stack with your lights. The software
then applies the various corrections before any alignment and integration.
Clever software!
Essentially what seems to happen is that bias frames are
combined to create a master average bias frame. This is then subtracted from
the dark frames to remove bias noise. Flat frames are calibrated and then
either the master bias or the master dark frame is subtracted from flat frames.
Calibrated flats form a master flat frame. The master dark is subtracted from
our lights. Everything is then aligned
and stacked. Probably an oversimplification of the process.
So, should we skip
doing any of them? You will hear, see, read that people do! All I can say is I don’t. For me they are the
difference between a clean astro image and one with those distracting artefacts
in them. Do I keep a library of them as
a DSLR user? No! Why? I’m constantly switching lenses and telescopes. There are
temperature and humidity variations from night to night. I suspect there is
dust on some of my optics. Adjustments to ISO, exposure time, focus. Camera
rotation to get best framing of my DSO target. The list is endless isn’t
it.
In conclusion – I hope this simple overview helps you
understand calibration frames better. They are foundational to our astro
imaging post processing work. Used correctly we can remove/reduce noise,
aberrations, vignetting and produce higher quality finished images. Most free
programs I use do it all for me, so I just need to ensure that I have taken my
time and care over capturing my calibration frames correctly.
Good luck in capturing yours.
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