Before we can delve too deeply into what we can do with Chandra data, it will help to know more about what it is.

The ACIS Detector

    The Chandra telescope only has two detectors.  The first is called ACIS for Advanced CCD Imaging Spectrometer.  This is a very similar to the CCD you would find in a digital camera or a camcorder.  The big difference is not in this detector but what they are dectecting.  X-ray photons have a million times more energy than optical photons.  (Photon energy is equal to Planck's constant times the photon's frequency.)  Your CCD has to gather millions of photons during some exposure, because only lower energy, optical photons are focused on it.  Since high energy photons are focused on ACIS, each individual photon can be detected and recorded.  During an ACIS observation, every 3.2 seconds the CCD chips are read out.  For each photon hitting ACIS, Chandra records its position, (approximate) energy and the time it arrived.

Why Chandra Data Is Great

    If you attach a CCD detector to your telescope for an observation, you end up with a picture.  If you want to measure the color of a star with this setup, you would make two exposures.  One with a blue filter and one with a red filter and from the change in brightness between these two observations, you can compute a star's color.  If you wanted to answer some other question instead of "what color is it", you would adjust the filters and duration of the exposure to create the right picture.

    But Chandra data isn't a picture.  It is a record of every individual photon that arrived during the observation.  Where detectors on optical telescope create a picture, the ACIS detector on Chandra creates a multi-dimensional data cube that contains sky position (x and y), energy and arrival time.  For any reasonably bright object observed by ACIS, you can measure its X-ray color.  With AstroVirgil, you can count how many photons were in a low energy band versus how many were in a high energy band.  It doesn't even matter what some astronomer who made the observation was trying to measure because Chandra records every photon.  What optical astronomers try to accomplish using filters, X-ray astronomers accomplish with data analysis.  The optical filters eliminate photons and thereby restrict what questions an observation can answer.  An X-ray observation is a list of data on individual photons that can be analyzed differently to answer different questions.  Maybe even your question.

The HRC Detector

    The second detector on the Chandra telescope is the High Resolution Camera (HRC).  It also detects every single photon.  Unlike ACIS, it doesn't just periodically read the photons out but detects the precise moment they arrive.  Unfortunately, it doesn't measure how much energy each photon has.

Grating versus Non-grating

    There is only one other system on the Chandra telescope we need to discuss.  It is called the "grating" and it works like a prism.  These devices spread out photons based their energy.  You can take sunlight and run it through a prism to create a rainbow.  From that rainbow, you can measure how much, for example, red light there is and how many shades of red are present, measure a blackbody temperature or identify emmission and absorption lines.  However, you no longer make a picture of the object that emitted the light.  Instead you have a spectrum and can study the spectrum.
    On the Chandra telescope the grating device is used to create a high resolution spectrum.  It is mounted sort of like a toliet seat that can flop down or lift out of the way.  When flopped down, it creates an "X-ray rainbow" on either the ACIS or HRC detector.  By studying the spectrum, you can identify specific elements.
    Most of the observations in the Chandra Archive are non-grating.

Chandra Non-grating Data Files

    The Chandra telescope is orbiting the Earth.  Data from observations is sent, via NASA's Deep Space Network, to the Massachusetts Institute of Technology and then on the Harvard-Smithsonian Center for Astrophysics (CfA).  Naturally, all the data from Chandra ends up in disk files.  Astronomical data is stored in a file format called FITS for Flexible Image Transport System and the filenames generally end in ".fits".

    Scientists don't directly use the data Chandra sends back.  It has to be processed, verified, calibrated.  Data from the telescope is called level 0.  The cleaned-up data used by scientists is called level 2.  When you are downloading data from the archive, you want the files with a 2 near the end of the filename, that is, files that end in "2.fits"

    For a non-grating observation, there are two types of level 2 FITS files you can download and read into AstroVirgil.  One type has information on every individual photon and is called an "event file".  Here "event" is short for "photon event".  Level 2 event filenames end in "evt2.fits".  These files contain several dozen bytes of information on every single photon.  Since observations record anywhere from 100,000 to over 1,000,000 photons, these files aren't small.  Event files range from a couple megabytes to well over 100 megabytes.  Generally, they are over 30 megabytes.
    The other type of file you can download is called an image file.  An image file doesn't contain any information on individual photons.  Instead, it just holds a picture (like a jpeg or bmp file) in fits format.  Therefore, an image file is reasonably small; always around 4 megabytes.  As part of generating level 2 data, the good people at the CfA create two image file for each observation.  One image file has a picture made from all the photons in the observation.  These files end in "_full_img2.fits" where "img" is short for "image".  The other automatically generated image files just uses photons from near the center of the image to provide a zoomed-in view.  These files end in "_cntr_img2.fits".  Since image files don't contain data on every photon you are very limited in the amount of analysis you can do on these files.

    Chandra data filenames start with the name of the detector used, either "acisf" or "hrcf".  I don't know why the "f" is there, but it is.  After the detector name is the observation number (or ObsId).  For example, observation 198 (of CasA) was taken with ACIS.  Therefore the filename begins with "acisf00198".  The event filename ends in "_evt2.fits".  Between the beginning and the end is some stuff I don't understand and have never worried about.  Generally it is "N001" or "N002" and sometimes "N003".  So, the entire file name for ObsId is 198 is "acisf00198N002_evt2.fits".  From the name we know it is a level 2 event file containing ACIS data from observation number 198.  Similarly, the file named "hrcf01021N002_evt2.fits" is a level 2 event file of photons recorded by the HRC for observation number 1021.

Chandra Grating Data Files

    Grating observations are used to make spectrograms.  For these observations there are level 2 event files.  However, the good people at the CfA do additional processing on grating observations to make them easier to study.  For every grating observation, they create something called a "pha2" file.  This file doesn't contain data on every photon.  Instead, it is basically a graph of the line the spectrum forms.  Because of this, these files are relatively small, only a couple megabytes.  AstroVirgil can read in these files and display the graph.  For grating observations, you probably won't have any need for the event or image files and can just download the pha2 file.  These filenames end with "_pha2.fits".  The rest of the naming convention is just like event or image files and includes the detector's name and the ObsId.

    The AstroVirgil release includes with three calibration files that compensate for Chandra's response to photons of various energies.  For example, the mirrors tend to focus more photons with an energy of 1 kev than those with an energy of 10 kev.  With these calibration files AstroVirgil can more closely display the true spectrum of the source and not just the spectrum that was detected and is stored in the pha2 file.