Why do we need calibration frames?
Calibration frames deal with or remove electronic and optical
defects that all cameras and optical systems suffer from and that
plague our light frames
•
Thermal noise - amp-glow / hot & cold pixels
•
Dust bunnies, Vignetting and Light gradients
•
Manufacturing noise
The three main calibration frames are darks / flats / bias
These in full (or in part) are integrated during the stacking process and
remove or limit these electronic and optical impurities from our final
image
What are dark calibration frames?
Camera sensors produce thermal noise. This is
caused by the camera’s electronics heating up
during long exposures and are affected by:
•
Ambient camera temperature
•
Exposure time
•
ISO/gain Setting
Darks remove thermal noise including amp-glow
Hot / cold and dead pixels
How to take dark frames
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Take darks at the start and/or the end of a session
•
On the Dwarf II fully cover the front of the telescope so that no light can
enter by installing the supplied solar filters to the filter holder. (Do not put the
device in its bag, this will raise the temperature making the dark frames useless)
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The same exposure length as your light frames
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Same ISO/gain setting as your light frames
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The same temperature as your light frames
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Take between 20 and 30 exposures
•
Can be reused if the above parameters are met
How to control thermal noise
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Thermal noise on a DSLR is controlled by using dark frames
Dark calibration frames at the same temperature, exposure length and ISO setting will
remove thermal noise during the stacking process
•
Dither
This is performed via your guiding software and deals with hot / cold & dead pixels
•
Use a dedicated cooled astro camera
The temperature of the sensor can be controlled and can be significantly lowered to
minus temperatures, thus reducing or entirely eliminating thermal noise, but you may
still need to take dark frames
Why do we need flat calibration frames?
•
Arguably the most important of all calibration frames
They remove dust motes that will change through focusing or move/appear/disappear
from session to session so they cannot be reused
•
Flat frames look at the different light gradients across the picture and calculates
the correction factor. It then applies this correction to each individual pixel to give
a smooth, evenly illuminated image
•
Flats remove: Dust bunnies & vignetting
What do flat calibration frames remove?
•
Dust bunnies - small specks of dust that are
already on or will land on your cameras sensor,
telescope lens/mirror and any filters, reducers
etc.
•
Vignetting - Light fallout to the edges caused by
most optical systems, lens’ or mirror design or
using mismatched accessories in the optical
train.
How to take flat frames
•
Take after each target imaged
•
Place pure white cloth on the end of your telescope and use an evenly
illuminated light panel or white light app.
The sky just before sunrise is also good (without cloth)
•
Put your camera in Av mode if using a DSLR
The camera will select the best exposure setting automatically
•
If manual - take test exposures and ensure the histogram is roughly in
the middle. You will see dust spots and vignetting if it is present
•
Same ISO/gain setting as your light frames
•
Take between 20 and 30 exposures
How to control dust & vignetting
•
Dust bunnies
Everyone will suffer from dust during a session. The easiest way to remove them is by
taking flat frames
Do not change focus
Do not change camera orientation
Leave filters in place until you have finished taking the flat frames
•
Vignetting
Do not chance the camera orientation
Do not change focus
Use 2 inch accessories where possible.
Always use flat frames
Why do we need bias calibration frames?
•
Bias frames remove dark fixed-pattern noise that is a result of the
manufacturing process
•
All camera’s will suffer from this pattern noise to differing degrees and it shows
up when stretched in post processing
•
They work in conjunction with dark frames by removing this fixed pattern
noise
How to take bias frames
•
Install the cover or cap
•
Same ISO/gain as light frames
•
Set to the fastest exposure your camera allows
•
Some argue that the temperature should be the same but I doubt this is
the case due to the length of the exposure involved
•
Take upwards of 50 exposures (wont take long)
•
Can be reused forever as this pattern will never change
