1. Image calibration
Geoff Smith, September 2018

2. Calibration—A Word of Many Uses
Calibrate your mount so it knows where it’s pointing
Calibrate your guider so that it knows how to move the mount to make a correction
Calibrate your light frames—Today’s topic

3. What’s in a Light Frame? 1. The signal from your object.
A Light Frame is an image downloaded from the camera.
It contains a representation of the object you are imaging.
Unfortunately, it contains other unwanted stuff as well.

4. What’s in a Light Frame? 2. Read Noise
When the camera downloads an image there may be an error in the value assigned to any given pixel.
Typically
Signal reported=True signal±10 electrons
As an aside, we can convert electrons to ADU by the formula
ADU=electrons/camera gain
The camera gain will need to be obtained from your camera specs.

5. What’s in a Light Frame? 3. Bias Signal
Most cameras add a small (constant) amount of signal to the pixel sites before readout.
One purpose is to prevent a near zero signal being reported as zero because of read noise.
If you expose with the shutter closed for the shortest possible exposure you will still get a positive readout from your camera. This is the Bias Signal and the resulting frame is a Bias Frame

6. What’s in a Light Frame? 4. Dark Current
The heat in the sensor generates electrons .
These add to the electrons from the sky signal.
These extra electrons are the Dark Current Signal
Dark current signal is proportional to the exposure time and is temperature dependent.
If we expose the sensor with the shutter closed for a time we will get the dark current signal plus the bias signal.
The resulting frame is called a Dark Frame.
As an aside: Dark current is NOT noise. It is UNWANTED SIGNAL. Signal is, within the limits of physics, repeatable, noise is not.

7. What’s in a Light Frame? 5. Optical and Camera Defects
Vignetting
Dust on the sensor, producing dark spots (aka dust donuts) on the image
Variation in pixel-to-pixel sensitivity—Typically no more than 1%
These effects can be mitigated by the use of Flat Frames, which I’ll explain in the next slide.

8. Flat Frames
We generate a Flat Frame by pointing the telescope at an evenly illuminated surface and taking an exposure.
All the defects mentioned in the previous slide will be in the resulting flat frame and we can use the flat frame to reverse engineer these defects in our light frame
Vignetting

9. Flat Frames Removing dust donuts

10. Signals and noise.
ALL signals—light signal, bias signal, dark signal, the signal in flat frames— have noise.
There will be a slight frame to frame variation.
We do the usual thing and increase the signal to noise ratio by taking many exposures and stacking.

11. Taking calibration crames 1. Bias Frames
Most capture software will have a camera setting to take bias frames. Just select and click the button.
I usually take 100 bias frames, stacked to form a Master Bias
Read the manual for your chosen software!
Take bias frames when you are not imaging. The camera does not have to be connected to the telescope.

12. Taking calibration frames 1. Bias Frames (continued)
Cameras slowly change their characteristics with time. I usually redo bias frames every 4–6 months
Keep all master bias frames with dates since you may want to reprocess old images. Select the master frames nearest the date of your picture.

13. Taking calibration frames 2. Dark Frames
Most capture software will have a camera setting to take dark frames. Just select and click the button.
I usually take 50 dark frames and stack to form a Master Dark
Read the manual for your chosen software!
Take dark frames when you are not imaging. The camera does not have to be connected to the telescope.

14. Taking calibration frames 2. Dark Frames (continued)
Cameras slowly change their characteristics with time. I usually redo dark frames every 4–6 months.
Keep all master dark frames with dates since you may want to reprocess old images. Select the master frames nearest the date of your picture.

15. Taking calibration frames 2. Dark Frames (continued)
Your dark frame exposure should ideally be the same length as your light frames.
You may use dark frames of a longer exposure than your light frames, but never shorter.

16. Taking calibration frames 3. Flat Frames
Conceptually simple—Take a picture of a uniform light source
Difficult in practice
Getting a uniform light source is a challenge
Spectral characteristics of the light source and the night sky may differ

17. Taking calibration frames 3. Flat Frames (continued)
I usually take 40 frames in each filter.
Aim for 40–60% of full well depth
Exposures are short, 5 to 10 seconds, so the time investment is not large.
Flat frames must be taken under the same configuration as the light frames—no focus change, no change in camera orientation

18. Taking calibration frames 3. Flat Frames: T-Shirt Flats
Cover the telescope with a towel and point at a twilight sky, a shaded lawn or a screen in an observatory if you have one.

19. Taking calibration frames 3. Flat Frames: Flat Panel Flats
Use an electroluminescent panel
Google EL panels for local sources
Overseas links
itak/flipflat.htm

20. Taking calibration frames 3. Flat Frames: Light Box Flats
Translucent material lit from behind by LED lights.
Usually DIY. Plenty of hits if you google.

21. What’s in a frame? (Or what you want is not what you get)
This is all we want.
We don’t want any of this.

22. One-shot colour cameras Includes DSLR
Capture images as monochrome undebayered images in fits format. The capture software should allow you to do this.
Read the manual for your chosen software!
Debayer after calibration but before alignment and stacking
Captured

23. Calibration steps Dark frames same exposure length as light frames
Subtract master dark from each light frame
Subtract master bias from each flat frame
Apply bias subtracted flat frames to each image via software
Debayer the images if using one-shot colour
Align the images
Stack the calibrated and aligned images. Do this separately for each colour filter.
Fortunately, astrophotography software will reduce the number of steps considerably.
Read the manual for your chosen software!

24. Calibration procedure
One-click calibration in PixInsight once files have been entered

25. Colour Calibration An enormous and contentious topic—just dipping our toes in the water here
What causes colour problems?
Spectral dependence of the sensor
Different atmospheric transmission
Presumably other stuff as well
First we have to distinguish between colour balance and colour bias

26. Colour Bias
This occurs when the amount of each colour may be correct, but the brightnesses may be off

27. Colour Balance The amounts of colour in each channel need adjusting
Although the colour bias has been removed, there is too much red

28. Colour balanced and no colour bias

29. Questions?