8.7 Practical 6: The Messier Marathon
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for photometry and spectrography you should stack only via addition, although
you may make a mean (but not a median) stack if all your exposures are of the
same length. Details of how to align and stack images are presented in Chap. 9.
4. Once you have your three separate frames, four if you intend to do an LRGB,
you will need to tidy them up and stretch them before combining them into your
colour image. Hot pixels and cosmic rays will appear as unusually coloured pixels
when you create the colour image and are easier to deal with at this point using a
cloning tool. If your astronomical imaging software has a hot pixel removal tool,
it is worth applying it at this point, although I find that often they fail to get them
all, especially cosmic rays.
5. Combine the images using your favourite software into an RGB or LRGB image
and then colour balance them as discussed in Chap. 9. Once you are happy with
your image, export it in TIFF or JPEG format. You may then use standard image
manipulation tools such as Photoshop or The Gimp to remove artifacts and crop
the image.
8.7.7 Analysis
When you have finished, you should write up your work and include your images,
finder charts, and exposure times. During the writeup, pay particular attention to any
problems you encountered. Describe what you did to the images, why you did what
you did, and how it influenced them. In particular, look at how the signal-to-noise
ratio improves when you add calibration frames and science frame stacking.
8.8 Planetary Imaging
It is often thought by students that the easiest objects to image are the planets, because
they are so bright, but that is not true for most teaching observatories. In fact, planetary
imaging is extremely challenging for the very reason that planets are so bright. Most
astronomical cameras cannot cope with the number of photons being received from
the brighter planets—Jupiter, Mars, Venus, and Saturn—and they overexpose even
with their shortest exposure setting and with a photometric filter in the pathway.
Another problem is that the telescope is designed to track not solar system objects
but stars. Autoguiding is not of as much help here, as it would need to guide on
the planet, which, not being a point source, causes problems with guiding software.
Hence, even if we wished to take long exposures, they would suffer from trailing.
An obvious solution to this problem is to reduce the amount of light entering
the camera. This could be done either by stopping down the telescope, i.e., putting
a cover on the front to reduce the aperture size or putting either a twin polarising
filter or neutral density filter in the optical pathway. Another challenge is that to
see detail on the planet, high magnifications are needed. At high magnifications,
127
for photometry and spectrography you should stack only via addition, although
you may make a mean (but not a median) stack if all your exposures are of the
same length. Details of how to align and stack images are presented in Chap. 9.
4. Once you have your three separate frames, four if you intend to do an LRGB,
you will need to tidy them up and stretch them before combining them into your
colour image. Hot pixels and cosmic rays will appear as unusually coloured pixels
when you create the colour image and are easier to deal with at this point using a
cloning tool. If your astronomical imaging software has a hot pixel removal tool,
it is worth applying it at this point, although I find that often they fail to get them
all, especially cosmic rays.
5. Combine the images using your favourite software into an RGB or LRGB image
and then colour balance them as discussed in Chap. 9. Once you are happy with
your image, export it in TIFF or JPEG format. You may then use standard image
manipulation tools such as Photoshop or The Gimp to remove artifacts and crop
the image.
8.7.7 Analysis
When you have finished, you should write up your work and include your images,
finder charts, and exposure times. During the writeup, pay particular attention to any
problems you encountered. Describe what you did to the images, why you did what
you did, and how it influenced them. In particular, look at how the signal-to-noise
ratio improves when you add calibration frames and science frame stacking.
8.8 Planetary Imaging
It is often thought by students that the easiest objects to image are the planets, because
they are so bright, but that is not true for most teaching observatories. In fact, planetary
imaging is extremely challenging for the very reason that planets are so bright. Most
astronomical cameras cannot cope with the number of photons being received from
the brighter planets—Jupiter, Mars, Venus, and Saturn—and they overexpose even
with their shortest exposure setting and with a photometric filter in the pathway.
Another problem is that the telescope is designed to track not solar system objects
but stars. Autoguiding is not of as much help here, as it would need to guide on
the planet, which, not being a point source, causes problems with guiding software.
Hence, even if we wished to take long exposures, they would suffer from trailing.
An obvious solution to this problem is to reduce the amount of light entering
the camera. This could be done either by stopping down the telescope, i.e., putting
a cover on the front to reduce the aperture size or putting either a twin polarising
filter or neutral density filter in the optical pathway. Another challenge is that to
see detail on the planet, high magnifications are needed. At high magnifications,