92
7 The Astronomical Detector
a second box colocated with the first. Drag this box to the bottom left corner.
Repeat until you have something like Fig. 7.8.
10. From the Region menu, select Select All (or press ctrl-A). This selects all the
regions. Now again from the Region menu, select Save Regions. Enter a file
name and then select DS9 for the format and Physical for the coordinate system.
This will save your region layout for future frames.
11. Pick a region and click on it. This will bring up some of the information on
the region including its centre. Record the centre, making sure it is in physical
units, not, for example, RA and Dec. Use trigonometry to determine its distance
from the centre of the array. (Hint: you can find the array size in the header.)
With the region information box, open Select Apply and a second box with more
information on the region. Find the mean count and record it on a spreadsheet
along with the region’s distance from the centre.
12. Repeat this for the remaining four frames, although this time you can load your
saved regions in, which is quite a time saver.
13. Plot distance against mean count on a spreadsheet for each frame on the same
graph.
Analysis
Write your report up in standard laboratory style: introduction, method, results, and
conclusions. You should include all your tables and your plots as well as notes about
the camera and software used in addition to any problems you may have encountered.
Your report should include the following:
1. A description of the overall appearance of your flat field image, including the
small-scale and large-scale uniformity. Are there any unusual features in the flat
field, and if so, what do you think they are caused by?
2. Your flat fields include a bias and a dark current. Given what you know about
these two calibration frames, how do the errors they introduce compare to the flat
field?
3. There might be variation between the edge of the flat field and the centre along
each of the axes. If there is, can you explain it? What would the impact be if we
imaged a star in the centre of the field and then at an edge? Can you quantify this
effect?
4. When the observatory staff take flats, they take multiple copies and combines
them into a master flat. How do you think this is done? Are they just added?
5. Dark frames are temperature and binning sensitive. Are light frames also temperature and binning sensitive? How do you think the choice of filter affects the flat?
Do you think we need to flat field for every filter, or will just one do?
7 The Astronomical Detector
a second box colocated with the first. Drag this box to the bottom left corner.
Repeat until you have something like Fig. 7.8.
10. From the Region menu, select Select All (or press ctrl-A). This selects all the
regions. Now again from the Region menu, select Save Regions. Enter a file
name and then select DS9 for the format and Physical for the coordinate system.
This will save your region layout for future frames.
11. Pick a region and click on it. This will bring up some of the information on
the region including its centre. Record the centre, making sure it is in physical
units, not, for example, RA and Dec. Use trigonometry to determine its distance
from the centre of the array. (Hint: you can find the array size in the header.)
With the region information box, open Select Apply and a second box with more
information on the region. Find the mean count and record it on a spreadsheet
along with the region’s distance from the centre.
12. Repeat this for the remaining four frames, although this time you can load your
saved regions in, which is quite a time saver.
13. Plot distance against mean count on a spreadsheet for each frame on the same
graph.
Analysis
Write your report up in standard laboratory style: introduction, method, results, and
conclusions. You should include all your tables and your plots as well as notes about
the camera and software used in addition to any problems you may have encountered.
Your report should include the following:
1. A description of the overall appearance of your flat field image, including the
small-scale and large-scale uniformity. Are there any unusual features in the flat
field, and if so, what do you think they are caused by?
2. Your flat fields include a bias and a dark current. Given what you know about
these two calibration frames, how do the errors they introduce compare to the flat
field?
3. There might be variation between the edge of the flat field and the centre along
each of the axes. If there is, can you explain it? What would the impact be if we
imaged a star in the centre of the field and then at an edge? Can you quantify this
effect?
4. When the observatory staff take flats, they take multiple copies and combines
them into a master flat. How do you think this is done? Are they just added?
5. Dark frames are temperature and binning sensitive. Are light frames also temperature and binning sensitive? How do you think the choice of filter affects the flat?
Do you think we need to flat field for every filter, or will just one do?