Using AIP4Win's Magnitude Measurement Tool

AIP4Win 2.3.0, Magnitude Measurement Tool v1.0

by Richard Berry


The Magnitude Measuring Tool is a new AIP4Win feature that is still in active development. Its purpose is to make doing CCD photometry fast, accurate, and efficient. The MMT supports multicolor photometry and the AAVSO's new Extended format.

To see how easy it is to use the MMT, view the 10-minute video by clicking the link below. The video gives you a quick run-through of the MMT in operation. A second video shows you how to set up the MMT the first time you use it.

Further down this page you'll find a brief description of the theory behind the new MMT. Understanding the theory behind the tool makes it much easier to set up and use the MMT.

 

Introduction to the MMT: Streaming video... or MP4 download (29MB)

First time using the MMT: Streaming video... or MP4 download (14MB)

 

Notes for Users, MMT v1.0

1. The MMT is a complex and powerful tool that is functional but still not in its final form. New features will be added in response to reasonable user requests. If you wish to request a new feature, please explain fully its science value and how you and other observers would make use the new feature.

2. While doing photometry using a guide star, the guide star must remain visible on the screen through the whole image sequence. For images too large to fit in the image window, or for images series with large tracking errors, you may need to set the default image size to 50% or 33% to achieve this. If the guide star moves outside the image window, the MMT can get get stuck in an infinite loop waiting for you to click on a guide star that you cannot see. To escape, you must press the ctrl-alt-del keys and exit the AIP4WIN process.

3. In MMT v1.0, entering star data is tedious and prone to typos. Thanks to Jari Suomela, you can get comparison star data more easily. His script accesses the photometry star listing from AAVSO web site and outputs it in VSD format. The link is http://cataclysmix.com/php/vsd/. Enter the star designation, limiting magnitude, and field size, then click the "Crank up the converter" button. Select and copy the resulting text, then paste it into Notepad. Using Notepad to edit the file, delete all but four comparison stars and save the result as a text file with the VSD extension.

 

The Concept Behind the Magnitude Measurement Tool

The Magnitude Measurement Tool is the front end of a hierarchical photometry object. As you use the tool with your images, you are really populating a data structure with photometric measurements. When all the data has been transferred to the data structure, the data structure generates a report in the format of your choice. Because the data remains in the data structure, you can generate different reports; for example, you can call for a report in the bare-bones AAVSO Extended Format to send to the AAVSO offices, and then generate a much more detailed report for your own data archive. You can also generate a "quick-look" graph of time-series data.

Data in the photometric structure is hierarchical, meaning that different types of data are "nested" inside one another like a set of Russian dolls. Each level consists of "properties" appropriate to that level. Data extend from base objects to subordinate objects contained within them. These concepts may seem a bit foreign at first, but as you think about the hierarchy of dependencies found in photometric data, you will find that it all really make sense.

Observer

The base of the data pyramid is Observer. The Observer is fundamental because all of the Observer's properties are inherited by the images, stars, and individual photometric measurements. In the MMT, the Observer's properties are entered on the first (leftmost) tab of the tool. Observer properties include:

  • Name of Observer
  • Longtitude of Observer
  • Latitude of Observer
  • Time Zone setting used
  • Telescope aperture used
  • Telescope focal length used
  • CCD camera used
  • Images() created by the Observer, and array of Image.

Image

The next level in the hierarchy is the array Images() that belongs to the Observer. In the MMT, you specify a set of images using the Images tab, then pick a single Image for selecting Stars(). Luckily for us, CCD images contain this information in the FITS header, so you only have to supply the list of the images to the MMT, and the MMT obtains the properties of each Image from the image itself. Image properties include:

  • Time that Image was made
  • Exposure length (the integration time)
  • Filter (B, V, R, I) used for Image
  • Stars(), an array of Star.

Star

Star inherit the properties of Obsever and Image that contains it. Star properties come from two sources: 1) from star data entered on the Stars tab of the MMT, and 2) from the (x,y) coordinates that you enter by clicking on star images that correpond to star data entries. Star properties include:

  • Type (variable, comp star, or field star)
  • Name (chart designation, AIUD, or catalog name)
  • Right Ascension
  • Declination
  • B standard magnitude
  • V standard magnitude
  • Rc standard magnitude
  • Ic standard magnitude
  • (x,y) coordinates in Image
  • Measurement of photometric properties.

Measurement

Measurement properties are measured from a star in an image taken by the observer, in other words, Measurement inherits the properties of Observer, Image, and Star, and includes its own properties. Measured properties include:

  • Number of pixels in the star aperture
  • Total ADUs in the star aperture
  • Number of pixels in the sky annulus
  • Total ADUs in the sky annulus
  • Maximum ADU in star image

Furthermore, there are results derived from the measurement properties described above combined with properties inherited from Observer, Image, and Star. These include:

  • Raw instrumental magnitude
  • Statistical uncertainty in the magnitude
  • JD of the measurement
  • Airmass of the measurement

With data organized this way, the magnitude (i.e., what you're looking for) can be linked to all of the magnitudes obtained for all of the stars in all of the images taken by the observer.

 

Output Generated by the Magnitude Measurement Tool

When you click the Run Photometry button of the MMT, the MMT loads each Image in Images(), photometers each Star in Stars(), and for each Star, produces a Measurement. These properties are passed into the photometry object. Now for the big question: What data do you want to get out of the MMT?

The Magnitude Measurement Tool produces a variety of different reports, and more can and will be added. Furthermore, once the data has been passed to the photometry object, you ask to get different reports from the same data. The current selection of output reports includes:

  • AAVSO Format. This format is required to report data to the AAVSO. The format consists of a header with a smattering of observer information plus one line of data for each variable star measurement. The data consists of a magnitude for a variable star adjusted to match the standard magnitude of a comparison star plus raw instrumental magnitudes for one or two comp stars.
  • CBA Report. This is the bare-bones format described on the CBA's website. It contains a truncated JD, V-C1, and C2-C1. That's all! The advantage is that it transmits hundreds of differential magnitude measurements in a very compact text file. For this report, you would normally select a variable plus two comp stars.
  • Text Format Photometry. This option gets you a rather verbose set of measurements in an Excel-friendly text format. You can select a sparse set of data or everything but the kitchen sink, and specify the number of decimal places. If you select more than two comp stars, the report includes V-Ens. You would normally select a variable plus as many comp stars as you want.
  • Raw Instrumental Magnitudes. This option reports raw instrumental magnitudes for as many images or stars as you want. You might use this option to collect data for computing the transformation coeffcients (several dozen stars in a small number of images shot with different filters) or constructing the color-magnitude array of a cluster (several hundred stars in a small number of images), or collecting precision photometry on exoplanet transits (a few dozen stars in several hundred images) for detailed analysis in your own spreadsheet.
  • Signal and Noise. This option, originally used in testing the software, tabulates the raw instrumental magnitude and standard deviation expected from photon counting statistics.
  • All Input Values. This reports mirrors all the data collected to a text file. Use this as a permanent record of an observing session, or as a sanity check or diagnostic if the data output by another report format appears strange or confusing.
  • In addition, on the Execute tab, you can click the Light Curve for Time Series button for a "quick-look" graph of time-series data. For a permanent record, however, it is best to save your data in the appropriate text format. and import it into Excel or other spreadsheet.

Copyright 2008 by Richard Berry

 

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