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The Minor Planet Observer
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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!

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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.

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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.

 

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This page was last updated on 01/19/11 05:14 -0700.
All contents copyright (c) 2005-2011, Brian D. Warner
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