MPO Canopus - The Lightcurve Wizard
The general idea of lightcurve photometry is that you measure the images of the target
from a given session. If you’re using derived magnitudes, you need to set the
Magnitude/Intensity Relationship (M/IR), which relates the measured flux for a star to a
actual magnitude based on a linear equation derived by measuring the flux of a number of
stars against the catalog magnitudes. This step is not required if you’re using
instrumental magnitudes for finding the differential values of the target. When you
measure the images of the target field, you’re going to be measuring the magnitude of
at least one but preferably two to five comparison stars and the target itself.
While not difficult to do in Canopus, this means opening each image one at a time,
adding the data for each comparisons star and then adding the data for the target. Using
this process, one can measure two to three images a minute (about 120/hour). If you have
80 images to measure, that means it will take up to 40 minutes to measure all the images.
That’s not very productive.
The Lightcurve Wizard in Canopus changes all that, doing most of the work for you.
Instead of 40 minutes just to measure your images, you can go from setting the M/IR (if
required) to having an initial analysis of the period in less than 10 to 15 minutes, and
that’s if you have well more than 100 images to measure!
 
The lightcurve wizard works by setting the position of the comps on two images, one at
the start of the set of images to be measured and one at the end of the set. The X/Y
offsets of the first selected star to the other comps and target in both images are used
to compute the object’s motion, if any, and where to place the measuring apertures
for all stars as a result. The left-hand screen shot above shows the wizard after three of
the comparisons and the target have been measured on the first image. The right-hand image
shows how the wizard displays the positions of the comps on the second image so that you
don't have to guess which stars you initially selected. This feature is very handy if
working crowded fields where it's easy to get confused as to which star is which.
Once you set the two images and select all the images to measure, the Image List is
displayed. It's now that you realize the full power of Canopus for measuring images
quickly and efficiently. You start by double clicking on the first image in the list. This
loads the image and automatically places the measuring apertures about the comparisons and
target. If you like the placement, simply click the Accept button to store the complete
data set in a database table and have the next image loaded and the apertures set again
automatically. If the apertures are off a bit because the field drifted between exposures,
you click on the "anchor star" (the #1 comparison), in the shot above, the one
with darker blue apertures. This resets all apertures and you should then be able to click
the Accept button and continue on.
The above technique does require "manual" intervention but that is not
necessarily bad. As each image appears, you have the opportunity to determine if it should
be included. Maybe the tracking was poor or a cosmic ray crossed the target and/or comp
stars. Worse yet, a 747 on its way to some distant shore may have passed through the
field, its running lights obliterating almost every thing. In this case, double click on
the next image in the list without clicking the Accept button and proceed on.
Permanent Data Storage
As noted above, Canopus stores data about each image in a database table and not in
memory, only to generate a summary report later. This means you can easily work with the
data anytime without having to remeasure the images or go through a series of unnecessary
steps. The data stored for each image is extensive and so allows not only recreating the
differential values but the estimated errors as well. Furthermore, the data can be exported to be sent to other
observers or to generate custom reports.
Handling Meridian Flip During a Session
Those using German Equatorial Mounts (GEM) are familiar with the problem of the scope
tube hitting the pier at some point during a night’s run. What they (or their
automation software) do is to flip the scope to the east side of the pier around the time
the field is near the meridian. This allows the scope to follow the field down to the
western horizon. It also causes the orientation of the stars in the image to change.
If using only a single pair of images to determine the X/Y motion of the asteroid (or
offset of the fixed target from the anchor star), and you measure an entire night’s
run as a single set, then somewhere during the measuring process the X/Y offsets are no
longer going to be valid. This happens when the first image after the meridian flip
occurred is loaded. That’s because the image is rotated and the fact that the X/Y
offset of a given comp from the first star is a function of the angle between the two
stars and their distance from one another on the image.
To overcome this problem, the lightcurve wizard allows you to specify two
sets of images, one before and one after the meridian flip. In addition to storing the X/Y
values as needed, the wizard also stores the name of the first image of the second set.
It’s critical that this image be among those selected for measuring and
that the list of selected files is in time ascending order so that once the "trigger
image" is encountered, Canopus knows to switch to the second set of X/Y offset values
and calculations.
The only allowed difference between the two sets of images is the orientation. You must
use the exact same set of comparison stars and target in both sets; that includes the
order of the comps. Comp1 must be Comp1 in both sets, and so on. Also, the sets of images
cannot be from different nights or use different filters.
The only purpose for this feature is to deal with the meridian flip problem. It
should not be used to circumvent other issues or changes.
Meridian Flip - An Alternate Method
There is an alternative to measuring two sets in one step. You can measure the first
set, adding data to the appropriate session. Immediately afterwards, rerun the wizard
using the images after the meridian flip but do not create a new session. Instead keep the
same session as with the first set. The new data will be appended to the previous data. |
