1. Phys 1830: Lecture 10 Test in class this Friday!
Test information is on the class website – check it out!
format
topics
Schedule website with the topic list has also been updated.
ALL NOTES COPYRIGHT JAYANNE ENGLISH AND ORIGINAL PRODUCERS OF THE WORK.
For 2011, restretch the BWs so they are brighter when projected.
Previous Class:
radio telescopes, interferometry
continuum emission, 21 cm spin flip transition
This class:
How optical images are made.
Workshop on B&W radio images  on test
Astronomy Club
Check Facebook page
(listed on class website)
Last tutorial before test is today! 3pm

2. Organization of Physics Undergraduate Students.
OPUS! Help with math.
Organization of Physics Undergraduate Students.
Tutoring Schedule:
Where the tunnel from Armes meets the tunnel going to University Centre. Face the bank machine, look left  Allen Building. Go down left hand corridor to find their office and directions to the tutoring office.

3. The Electromagnetic Spectrum
Constructing Colour Images:
summary
The Electromagnetic Spectrum
Recall column
Now we have enough background to discuss how images are made.
What we call light is radiation that we detect with our eyes. It is really just a small part of the spectrum of electromagnetic radiation. IT IS ONLY IN THIS ENERGY RANGE THAT WE PERCEIVE COLOUR.
At the high energy range we have gamma and xray and ultraviolet radiation… at the low energy range we have ir, microwave, and radio radiation.
We call the part of the spectrum which we see with our eyes the optical or visual or visible range. Note that high energy light is blue and lower energy light is red.
Think of fires – a gas burner burns blue while the embers of a dying fire are red. (If you had to stick your hand in a fire and were given the choice, which fire would you put your hand into? Blue is hotter and red is cooler. That is, blue has higher energy than red light.)

4. Constructing Colour Images:
The following process is relevant to radio data too. But let’s start with UV/Optical/near IR examples. None of the hardware of the CCD is used to store colour information. Instead filters are used to select out sections of the EM spectrum.
“FITS stands for `Flexible Image Transport System' and is the standard astronomical data format endorsed by both NASA and the International Astronomical Union. FITS is much more than an image format (such as JPG or GIF) and is primarily designed to store scientific data sets consisting of multi-dimensional arrays (1-D spectra, 2-D images or 3-D data cubes) and 2-dimensional tables containing rows and columns of data.”
Data in astronomy are acquired as black and white FITS format files.
For example, a CCD is most sensitive when all of its capacity is devoted to counting numbers of photons.
Therefore we produce greyscale data and images.

5. For radio telescope data we produce greyscale (black & white) images because:
the colour is too faint to encode
to use all the electronics to capture photons
there is no colour at radio wavelengths

6. Constructing Colour Images:
More than 3 images are used.
Note that each filter gives light from different phenomena – at the lower energy end is emission from old stars, at high energy range young stars are furiously burning.
Light passes through a filter in front of the satellite's instrument (CCD). This produces black and white images for parts of the spectrum of electromagnetic radiation (and EM radiation includes light).

7. Charge-Coupled Device for Optical Images
The number of photons per pixel are converted into electrons
the electrons are are read out and converted into numbers inserted into data files.
each pixel can hold about electrons  FITS format files.
however image files (ppm, jpeg, tiff) only have 256 levels of grey.
Willard Boyle (lived in Halifax) and George Smith recently won 1/2 of the Nobel prize for their development of the charge-coupled device (CCD), an electronic chip that converts light into a digital signal.

8. Charge-Coupled Device for Optical Images
FITS format files
1:65536
high dynamic range
image files (ppm, jpeg, tiff)
1:256
low dynamic range
eye looking at monitor
1:50 grey levels (tones) /
Dynamic range is the ratio between the minimum value and the maximum value.
Otherwise the eye has good dynamic range since it looks a dark area and the pupil opens and then a second later at a bright area and the pupil closes so your brain “sees” both areas simultaneously.

9. Colour Image-making: Stage 1
Mapping “values” to 256 tones (grey levels) is historically called “stretching”
Can be done using powers of ten for example – a logarithmic stretch.
Other functions used are linear (1-to-1), square root and arbitrary functions.

10. Colour Image-making: Stage 1
Veil Nebula
EXAMPLE IN THE OPTICAL
Stretch HST images in preparation for colourizing and combining them.
Tip: do not use pure black and white. Black loses faint structures and white saturates when images are combined together.

11. Colour Image-making: Stage 1
W4 Chimney
Data central wavelengths are
25 microns – Far IR
60 microns – Far IR
21 microns – Radio continuum
74 microns – Radio continuum
Take notes! This will be on the test.
There are 4 wavelength ranges.
I’ve mapped them from the FITS file to a selected range of counts. Now try and do your best stretch for each filter to bring out detail.
Take sufficient notes that you can make a “recipe” so that you can re-do this experiment.
Levels - look at histograms – in logarithmic form.
- do a selection on the histogram.
- Gamma is a power on the Intensity from the pixel. This can also be adjusted.
Curves – look at histogram view.
- click in image to find the level you want to adjust.
Save as jpeg (or tiff).
Remove “noise” along the edges if you want to.

12. Review of “Stretching”
Write down notes about what you did to stretch an image. (Take a few minutes.)
Discuss the following points with your team. Take notes about the discussion and answers (they won’t be on the ppt):
What is a logarithmic histogram?
What GIMP tools did you use to adjust intensities in the black and white images?
How do these tools differ from each other?
Your notes: They should be like a recipe that allows you to repeat the experiment - in this case, how to stretch images. They should also include “why” you do each step.

13. Review Question:
You are making images for the public and want to show that at least 3 chemical elements exist in an object. Images are observed through 3 different filters. Each filter should be stretched between the same data values and generate the same grey scale levels.
True
False
There may be more photons from ionized hydrogen than from ionized oxygen. If you set them the same way, then oxygen may never be visible in the image.
If you are making a scientific image, setting them the same way would give the relative amounts of each element. You could try these 2 approaches on your own and see if you can balance both goals.

14. Review Question:
All the stretches shown at the end of class looked the same. That is, the grey in a specific filament was the same for every team. That filament’s intensity level can only be displayed in an absolute way.
True
False

15. Colour Image-making: Stage 1
To make B&W stretched images for the colourizing workshop:
Download everything in the section:
Needed for the Black and White Workshop Component
Try to repeat what we did in class on your own.

16. Which wavelength range(s) do you plan to observe at? Why?
Review:
Imagine that you and your neighbours are part of an international consortium planning to build a telescope to study very powerful, energetic supernova explosions. Discuss the following details of your plans:
Which wavelength range(s) do you plan to observe at? Why?
Will it be a space-based or ground-based observatory? Why?
Will these choices give you high or low resolution? Why?
Did roster for iclickers in 2009.
Results of discussion (which is a synthesis of the material recently covered):
the wavelength regime for objects with a lot of energy: Gamma-ray, X-ray, UV.
Multi-wavelength observations are good too since Gamma-rays and X-rays are absorbed by dust that might be in between the earth and the SN. Supernova remnants (SNR) are discovered at radio wavelengths for example.
space-based telescopes are necessary for the high energy wavelengths like gamma-ray and x-ray since they are affected by the atmosphere being opaque for those wavelengths. Space-based is good for optical observations due to seeing.
ground-based for the radio telescopes since seeing doesn’t affect the longer wavelengths of IR and beyond.
it is also good to go to high energy regime since the resolution is higher with shorter wavelength.
(Note – there is no one right answer, as one can see from the multi-wavelength discussion.)

17. Doppler Shift Velocity fields Upcoming Topics
Image by J. English with support from Russ Taylor (U. Calgary) constructed for the Canadian Galactic Plane Survey. Uses Far IR and 21cm neutral hydrogen (HI) emission.
Doppler Shift
Velocity fields