8.7 Practical 6: The Messier Marathon
125
normally good for a month, as long as there have not been any major changes in
the setup of the telescope or new equipment installed in the dome.
3. Once you have taken your calibration frames, if you need them, open the telescope dome and go through the pointing and focus routine. Once this is done,
you can move on to imaging.
8.7.4 Imaging
1. For each of your targets you will need to slew to the target using the coordinates
or, if you have a GoTo facility, the target’s name.
2. Take a single, fairly short, image in clear or the nearest equivalent filter, for
example, a Johnson V. Use this to check your focusing.
3. Confirm that the target is in the centre of the field. If it has low surface brightness,
the object might not be visible without stretching, and even with stretching it
might be challenging to distinguish. If you cannot identify the target, use the stars
and your finder target to attempt to locate it.
4. Once you are happy with both the centring of your target and the focusing, turn on
and calibrate your guider and check that you are tracking. Remember that guiding
is a process requiring communication between camera and mount, while tracking
is just the sidereal movement of the telescope.
5. Pick your first filter and start imaging your target. Once the first image is displayed, check that it is not under- or overexposed. You may need to stretch the
image. If your exposure is too short, you can work out how much longer you need
by examining the pixel value of the brightest target. Pixel value should effectively scale with exposure time, although of course, the bias and pedestal do not.
However, the contribution to your pixel value of these components should be
quite small, and their nonlinear nature gives you a degree of headroom in scaling
up exposure times. For example, if you are exposing for 10 s and your target’s
maximum pixel count is 10,000, assuming that your maximum linear pixel count
is 50,000, you can safely expose for 50 s, because the bias and pedestal, which
likely make around 100 counts, remain constant with time; this being the case,
the maximum pixel count is more likely to be 49,600 for a 50 s exposure.
6. Once you have done this process for all three filters and you have saved your
images and noted your exposure times and file names in the log book, you can
move on to the next target. However, you may wish to take multiple images in
each band and stack them in software later, as this reduces noise, as we will see in
Chaps. 9 and 11. If this is the case, once you have your first set of RGB images (or
whatever filter series you are using) and you have confirmed your exposure time,
you may wish to use the automation service available in your camera control
package. This will take a series of images in multiple filters without requiring
your interaction. However, I strongly recommend you always take the first set
manually to confirm that the chosen setup is correct. Likewise, you should be
125
normally good for a month, as long as there have not been any major changes in
the setup of the telescope or new equipment installed in the dome.
3. Once you have taken your calibration frames, if you need them, open the telescope dome and go through the pointing and focus routine. Once this is done,
you can move on to imaging.
8.7.4 Imaging
1. For each of your targets you will need to slew to the target using the coordinates
or, if you have a GoTo facility, the target’s name.
2. Take a single, fairly short, image in clear or the nearest equivalent filter, for
example, a Johnson V. Use this to check your focusing.
3. Confirm that the target is in the centre of the field. If it has low surface brightness,
the object might not be visible without stretching, and even with stretching it
might be challenging to distinguish. If you cannot identify the target, use the stars
and your finder target to attempt to locate it.
4. Once you are happy with both the centring of your target and the focusing, turn on
and calibrate your guider and check that you are tracking. Remember that guiding
is a process requiring communication between camera and mount, while tracking
is just the sidereal movement of the telescope.
5. Pick your first filter and start imaging your target. Once the first image is displayed, check that it is not under- or overexposed. You may need to stretch the
image. If your exposure is too short, you can work out how much longer you need
by examining the pixel value of the brightest target. Pixel value should effectively scale with exposure time, although of course, the bias and pedestal do not.
However, the contribution to your pixel value of these components should be
quite small, and their nonlinear nature gives you a degree of headroom in scaling
up exposure times. For example, if you are exposing for 10 s and your target’s
maximum pixel count is 10,000, assuming that your maximum linear pixel count
is 50,000, you can safely expose for 50 s, because the bias and pedestal, which
likely make around 100 counts, remain constant with time; this being the case,
the maximum pixel count is more likely to be 49,600 for a 50 s exposure.
6. Once you have done this process for all three filters and you have saved your
images and noted your exposure times and file names in the log book, you can
move on to the next target. However, you may wish to take multiple images in
each band and stack them in software later, as this reduces noise, as we will see in
Chaps. 9 and 11. If this is the case, once you have your first set of RGB images (or
whatever filter series you are using) and you have confirmed your exposure time,
you may wish to use the automation service available in your camera control
package. This will take a series of images in multiple filters without requiring
your interaction. However, I strongly recommend you always take the first set
manually to confirm that the chosen setup is correct. Likewise, you should be