8.3 In-Dome Pre-observation Preparation
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For the master dark frames, pixel-to-pixel co-add all the frames with the same
binning and exposure time. Divide every pixel in the resultant frame by the number
of co-add frames. From the result of this pixel-to-pixel mathematics, remove the bias
master that has the corresponding binning. The result is a master dark for that binning
and exposure time.
To create master flats pixel to pixel, co-add all the frames with the same binning
and filter. Divide every pixel in the resultant frame by the number of co-add frames.
From the result of this pixel-to-pixel process, remove the bias master that has the
corresponding binning and the master dark with the corresponding exposure and time
binning. Find the mean pixel value within the frame and divide every pixel by that
value in order to normalise it. The result is a master flat for that filter and binning.
Applying the masters to light frames is straightforward, and your imaging processing software should be able to do this for you. From your light frame, pixel-to-pixel
subtract the master bias with the same binning; then from this subtract the master
dark with the correct exposure time and binning. Finally, pixel-to-pixel divide the
resultant frame by the master flat with the correct binning and filter. The result is the
science frame.
8.3.2 Pointing
Once you have all your calibration frames done, open the dome (assuming that it is
dark outside). Some telescopes require that you align to two or three bright stars.
If you need to do this, do it now. Your observatory staff should be able to help you
perform a pointing alignment if you have not done one before. Even if you don’t need
to use alignment stars, it is always a good idea to check the alignment by slewing
to a bright star and taking a short exposure. Centre the star in the image, and if
the telescope has this ability, sync the telescope to the star. Syncing recalibrates the
encoders to match the position of the target star. Syncing to the wrong target can
cause problems, so make sure that the star you are syncing to is the one you think
it is. If you cannot see the star, then slew around in small steps, taking images until
either you can see the star or it is clear that your telescope is out of alignment. Note
that you might be able to see some stray light coming from the star at the edges of
the frame before you see the actual star, which can provide a hint as to the star’s
location relative to the frame.
If you are unable to find the target star within a reasonable distance, use the finder
scope. Most telescopes have a small telescope with a wide field of view co-mounted
on the main telescope. The finder scope should have cross-hairs, or if you are lucky,
an illuminated dot in the centre. Try placing your star in the centre of the finder
scope. If you can’t see the star in the finder, you have a more serious problem, but
most such difficulties are easy to resolve.
If you cannot find the target star, there are several possible reasons. You may
have an incorrect RA and Dec. The time, date, or location could be incorrect on
the telescope controller. The telescope could have been incorrectly parked or the
109
For the master dark frames, pixel-to-pixel co-add all the frames with the same
binning and exposure time. Divide every pixel in the resultant frame by the number
of co-add frames. From the result of this pixel-to-pixel mathematics, remove the bias
master that has the corresponding binning. The result is a master dark for that binning
and exposure time.
To create master flats pixel to pixel, co-add all the frames with the same binning
and filter. Divide every pixel in the resultant frame by the number of co-add frames.
From the result of this pixel-to-pixel process, remove the bias master that has the
corresponding binning and the master dark with the corresponding exposure and time
binning. Find the mean pixel value within the frame and divide every pixel by that
value in order to normalise it. The result is a master flat for that filter and binning.
Applying the masters to light frames is straightforward, and your imaging processing software should be able to do this for you. From your light frame, pixel-to-pixel
subtract the master bias with the same binning; then from this subtract the master
dark with the correct exposure time and binning. Finally, pixel-to-pixel divide the
resultant frame by the master flat with the correct binning and filter. The result is the
science frame.
8.3.2 Pointing
Once you have all your calibration frames done, open the dome (assuming that it is
dark outside). Some telescopes require that you align to two or three bright stars.
If you need to do this, do it now. Your observatory staff should be able to help you
perform a pointing alignment if you have not done one before. Even if you don’t need
to use alignment stars, it is always a good idea to check the alignment by slewing
to a bright star and taking a short exposure. Centre the star in the image, and if
the telescope has this ability, sync the telescope to the star. Syncing recalibrates the
encoders to match the position of the target star. Syncing to the wrong target can
cause problems, so make sure that the star you are syncing to is the one you think
it is. If you cannot see the star, then slew around in small steps, taking images until
either you can see the star or it is clear that your telescope is out of alignment. Note
that you might be able to see some stray light coming from the star at the edges of
the frame before you see the actual star, which can provide a hint as to the star’s
location relative to the frame.
If you are unable to find the target star within a reasonable distance, use the finder
scope. Most telescopes have a small telescope with a wide field of view co-mounted
on the main telescope. The finder scope should have cross-hairs, or if you are lucky,
an illuminated dot in the centre. Try placing your star in the centre of the finder
scope. If you can’t see the star in the finder, you have a more serious problem, but
most such difficulties are easy to resolve.
If you cannot find the target star, there are several possible reasons. You may
have an incorrect RA and Dec. The time, date, or location could be incorrect on
the telescope controller. The telescope could have been incorrectly parked or the