The Photometry tool provides highly accurate photometric measurements, and allows for the plotting of light curves.
Availability of this feature depends on Product Level.
The accuracy of the photometric tool is higher than that of the Information window display. It uses a ”median-mean” algorithm for background subtraction. It also takes into account partial pixels inside the measurement annulus.
The size of the measurement annulus is determined using the right-click context menu for the circular cursor. Right-click on an image to bring it up. Please see the description under Information Window for more details.
Photometry Image Selection and Object Tagging
The first step is to decide what information best characterizes the abscissa (X axis) information for the plot. This will depend on the source of your data and on whatever preliminary processing may have been done. Use the Time/identification field dropdown to specify how MaxIm DL should derive this information. This control should be set at the beginning of analysis since changing it resets the photometry tool. The available choices are:
Date/time from FITS. This is the most common setting. By convention, the time associated with a photometry image is that of the midpoint of the exposure. MaxIm DL obtains this by preference from the MIDPOINT key of the FITS header. If this key is not present, it computes the midpoint by adding one-half the duration of the exposure to the start time. Duration is provided by any of the keys EXPTIME, EXPOSURE, or FEXPTIME, while start time comes from the DATE-OBS key, supplemented by TIME-OBS or UT if no hr:min:sec information is present in DATE-OBS. This setting works with images captured by MaxIm DL and many other camera control packages. Images appear in the Image list in chronological order; those lacking sufficient information to derive a time are omitted from the list.
File Timestamp. Images not containing FITS information, but which have been not been modified since their acquisition, or whose timestamps have been artificially restored, can be processed using this setting. MaxIm DL takes the time from the directory entry's modification date.
Sequence Number. In the absence of any other useful information, images can simply be assigned a "sequence number", i.e. is a number starting at 1 and counting up for each subsequent image in the Image list, listed in alphabetical order. Excluded images, described below, are also allotted a sequence number. When using this option, it is recommended that you close all images except for the ones being analyzed. You will also want to record the correspondence between image names and sequence numbers.
Arbitrary FITS key. When this option is picked, the Time/identification field displays [Enter FITS key] and is enabled for input. Type the name of any FITS keyword present in your image headers which contains a value that can be interpreted as a date/time. String-valued keys containing slash- or hyphen-delimited year-month-day and colon-delimited hr:min:sec values are interpreted correctly. A time in hours, minutes and seconds without a date is arbitrarily assigned a date of 2000 Jan 01. Integer- and real-valued FITS keywords are used directly as the X axis data. Images lacking the specified keyword are omitted from the Image list.
In all cases, the "time" value associated with the currently selected file is displayed in the read-only Time of Image edit box. When Date/time from FITS is selected, (Mid-exp.) is appended to the caption to remind you that the time shown is the middle of the exposure.
Next, select the images to analyze. The Image list contains all the currently open images in date/time order (the date/time must be present in the FITS header, which will be true if the images were taken with MaxIm DL). You can exclude an image from photometry analysis by selecting it and clicking the Exclude button. In the Image list, the names of excluded images are shown in a gray strikeout font. When such an image is selected, the Exclude button changes to Include, and serves to restore the image to the data being plotted. In practice, images are often excluded from analysis only after a preliminary plot has shown that they yield noisy measurements; this can be done from the plot itself, and is described below.
When you select an image in the Image list, it is displayed in the Photometry image window. You need to tag the object(s) that are to be measured, as well as at least one reference star in each image. In addition, you may tag one or more check stars. This is accomplished by picking the type of object you want to tag from the Mouse click tags as dropdown, then clicking the mouse cursor on the object itself in the Photometry image window. Alternatively, you can right-click on the object and pick one of the Tag… commands at the top of the context menu.
Set Act on all images if you want MaxIm DL to automatically find and tag the star on each image. Auto-tagging is performed by finding the closest star in each adjacent image. If Use star matching is enabled, the auto-tagging is done by pattern matching the star patterns in each image; this allows you to use images taken with the camera shifted or rotated by a large amount between images (this may take some time on the first object, since the star patterns on each image must be matched). You can turn on Snap to centroid to ensure that the measurement annulus is accurately centered on the star in each image.
When you tag something, MaxIm DL labels it with its type code (Obj, Mov, Ref, or Chk) followed by a number. To change this to the object's actual identifier, double-click on the entry in the Tagged objects list, or select it and press F2. You can reposition the label adjacent to the tagged object by dragging it with the mouse, or move the tag itself by dragging the circular measurement target. To adjust the measurement radii, right-click on the image and use the Aperture, Gap Width and Annulus commands in the context menu.
When you tag a reference star, you need to set its known magnitude in the Ref Mag field. By definition, a reference star has the same magnitude in every image. You can perform "ensemble photometry" by tagging more than one reference star, in which case you must set the magnitude for each. You can edit the magnitude of a reference star at any time by selecting its entry in the Tagged objects list and typing the new value in Ref Mag.
You can continue to select additional stars, including New Object for objects to be measured, New Moving Object for a non-stationary object such as an asteroid, and New Check Star. Check stars are usually included in measurements to monitor data quality and ensure that the selected reference stars are not themselves variable.
Moving objects must be manually tagged on at least two different images to enable the Photometry tool to calculate a "track" giving the probable location of the object at the time associated with every image in the set. Therefore, when a New Moving Object is initially tagged, Act on all images is ignored. After changing to a different image, pick the identification assigned to the new moving object from Mouse click tags as and click on the object in the new image. For best results, you should usually tag moving objects in the first and last images of a data set.
If you want to remove a tag from a star, select it in the Tagged objects list and click Untag. Act on all images controls whether the object is deleted from every image in which it appears, or only the one named in the Image list and currently appearing in the photometry image window. Objects which are not tagged in the currently selected image do not appear in the Tagged objects list, but do appear in Mouse click tags as.
When all the objects have been tagged, click View Plot… to see the results. At a minimum, you must identify at least a reference star on every included image, though images that lack objects to be measured are somewhat pointless!
Clicking Close will finish the photometry session. If you have unsaved work, MaxIm DL asks you if you really want to discard it. Saving is done from the Photometry Plot dialog, described next.
The Photometry plot window will now appear. A graph will be displayed showing the light curves of the tagged objects. Each object will have a separate curve with different markers or colors. The window is resizable by dragging edges or corners.
The Next Image and Prev Image buttons will move the associated Photometry image buffer back and forwards along the list of images. You can also click on any measurement in the graph to display the image from which it was derived and highlight the corresponding annulus. You can view the tagged objects and make sure that everything is tagged properly. If not, click the Back << button to return to the Photometry image selection and object tagging dialog.
Right-click on any point in the graph to display a pop-up menu showing the name of the file contributing that point and the three files immediately before and after. Files which were processed are checked, while those for which Exclude was checked are not. You can change the inclusion/exclusion status of any of these files by clicking on its name in the pop-up menu. The graph is immediately drawn with or without the affected point.
Click the Load… button to import earlier photometric results and add them to the plot. Note that the original images, tags, and filenames are not displayed for imported data points. Data is matched by object name, so it is important to use identifiers consistently. Click the Discard All button in the Open dialog to remove all imported data from the data set. When Date/time from FITS is set to Sequence Number, you can use the Offset sequence numbers edit box is used to adjust the sequence numbers of imported data, placing it at the proper position overall. It has no effect in other modes.
The Save… button and its attached
menu allow you to save the photometric data to disk in either Comma
Separated Values (.CSV) or AAVSO Report (.TXT) format. CSV files can be
displayed later using the Open Photometry
File command and are also compatible with other software such as
Microsoft Excel
Precision allows you to set the numeric format for magnitudes in the output. If you are saving in CSV format and have imported earlier results, you will have the option to Include previously imported data in output. Imported data is never included when AAVSO Format is selected, but the Observer Code, Filter Band, Chart ID and Group controls must be filled in. (Filter Band will be set automatically if the FITS key FILTER exists in your images and has one of the values in the dropdown.) MaxIm DL diagnoses most incorrect or missing information, but you should consult the Official AAVSO Extended Format document at http://www.aavso.org/observing/submit/extended.shtml for more information on how to complete these fields.
The Print… button allows you to print the graph. The standard Windows print dialog box will appear.
You can adjust the appearance of the plot using the Settings… button. The Photometry Plot Settings dialog box will appear.
You can change the appearance of the lines and symbols for the various tagged items. Select the object whose curve you want to edit in the Symbology list. Select the Symbol you want from the list, and select its Color… and Size in pixels. You can also set the Line thickness and Color.
The Plot Limits determines how the axes appear. Both the Time axis and Magnitude axis can be set to Auto. The plot will be automatically adjusted so that all data will be visible. Alternatively, you can turn off Auto and enter the values manually. The Time and Magnitude axes both require a Min and Max entry.
You can specify a Period folding modulus to overlay measurements from different cycles of periodic phenomena. The period can be expressed in hours or days, or folding can be turned off (none). If the horizontal axis variable is a Sequence Number or Arbitrary FITS key, the choices for Period folding are simply units or none. Line mode is ignored when period folding is in effect.
Offset (JD) allows you to set the reference time that is subtracted from every measurement time and is shown at the bottom of the plot window. When period folding is used, Offset (JD) adjusts the phase of the display curve. It is expressed as a Julian date when the horizontal axis is time, and in the units of the abscissa variable otherwise.
You can see the effect on the plot immediately by clicking Apply. Once you are satisfied with the appearance of the plot, click the OK button to make the changes permanent.