7.5 Calibration Frames
91
this practical you will familiarise yourself with the flat field method, the operation of
the camera, and the use of SAO DS9. The analysis will give you an understanding of
both the importance of flat fielding and the impact that poor or irregular flat fielding
has on image quality.
Flat Fielding
1. Turn the dome flat field lights on. These might just be the normal white lights,
but do not use the dome red lights if the dome has them, as you want the light
going into the telescope to be as spectrally as flat as possible. Take the lens cap
off the telescope.
2. Rotate the dome so that the flat field board is facing the telescope. You may have
to move the telescope in alt to get it directly facing the flat field board. Rather
than power the telescope down, turn its tracking off. If your telescope has a flat
field diffuser, put this in place.
3. Power on the camera, connect the control software, put the cooler on, and set
it to the correct temperature. While you are waiting for the camera to get down
to operating temperature, make sure the binning is either off or set to 1 × 1 and
that the subframing and any autocalibration are off.
4. Change the filter to R or ´ r and take a short exposure. If your camera control
software has the option to select the frame type as “flat,” select it.
5. Take a very short exposure, not more than 1 s. Once it is displayed on the camera
control software, check to see the mean count near the centre. In MaximDL this
is done by bringing up the information window (ctrl-i) and selecting Aperture
from the Mode drop-down. If you are not using MaximDL, it is likely that your
software has a similar capability. Failing that, you could open the image in FITS
Liberator and use the mean value in the image statistic box. You are trying to
obtain a mean count of around 30,000–40,000 for a 16-bit camera. If you are
getting too many counts, you should reduce the exposure time until it is in the
correct range. Likewise, if the count is too low, increase the exposure time.
6. Once you are happy with your exposure times, take five flat fields and save each
one. Don’t forget to record your exposure time in your lab book.
You can perform the analysis of your flat field images in DS9. You will need
to determine how flat your field is. To achieve this, you need to measure the
average count in a number of small regions spread over the field. This cannot
be done with FITS Liberator, whence the need to use DS9.
7. Open your first flat in DS9 and set the scale. Using the Scale menu, select 99.5%,
and using the Zoom drop-down menu, select zoom to fit the frame.
8. You will now need to set up your regions. From the Region menu, select Shape
and then Box. Double click on the centre of the image, and a box will appear.
Single click on the box and drag it out so that it is square with width about an
eighth the width of the frame.
9. Move the box to the top left-hand corner of the frame. From the Edit menu,
select Copy and then paste (or press ctrl-C and then ctrl-V). This is creating
91
this practical you will familiarise yourself with the flat field method, the operation of
the camera, and the use of SAO DS9. The analysis will give you an understanding of
both the importance of flat fielding and the impact that poor or irregular flat fielding
has on image quality.
Flat Fielding
1. Turn the dome flat field lights on. These might just be the normal white lights,
but do not use the dome red lights if the dome has them, as you want the light
going into the telescope to be as spectrally as flat as possible. Take the lens cap
off the telescope.
2. Rotate the dome so that the flat field board is facing the telescope. You may have
to move the telescope in alt to get it directly facing the flat field board. Rather
than power the telescope down, turn its tracking off. If your telescope has a flat
field diffuser, put this in place.
3. Power on the camera, connect the control software, put the cooler on, and set
it to the correct temperature. While you are waiting for the camera to get down
to operating temperature, make sure the binning is either off or set to 1 × 1 and
that the subframing and any autocalibration are off.
4. Change the filter to R or ´ r and take a short exposure. If your camera control
software has the option to select the frame type as “flat,” select it.
5. Take a very short exposure, not more than 1 s. Once it is displayed on the camera
control software, check to see the mean count near the centre. In MaximDL this
is done by bringing up the information window (ctrl-i) and selecting Aperture
from the Mode drop-down. If you are not using MaximDL, it is likely that your
software has a similar capability. Failing that, you could open the image in FITS
Liberator and use the mean value in the image statistic box. You are trying to
obtain a mean count of around 30,000–40,000 for a 16-bit camera. If you are
getting too many counts, you should reduce the exposure time until it is in the
correct range. Likewise, if the count is too low, increase the exposure time.
6. Once you are happy with your exposure times, take five flat fields and save each
one. Don’t forget to record your exposure time in your lab book.
You can perform the analysis of your flat field images in DS9. You will need
to determine how flat your field is. To achieve this, you need to measure the
average count in a number of small regions spread over the field. This cannot
be done with FITS Liberator, whence the need to use DS9.
7. Open your first flat in DS9 and set the scale. Using the Scale menu, select 99.5%,
and using the Zoom drop-down menu, select zoom to fit the frame.
8. You will now need to set up your regions. From the Region menu, select Shape
and then Box. Double click on the centre of the image, and a box will appear.
Single click on the box and drag it out so that it is square with width about an
eighth the width of the frame.
9. Move the box to the top left-hand corner of the frame. From the Edit menu,
select Copy and then paste (or press ctrl-C and then ctrl-V). This is creating