III. Raw Data Reduction

When getting the data from an image, the data will be automatically logged into a text file. The software does a great deal of work automatically.

Arranging the files

Create a folder with the star name and observation double date in the title, (e.g., TOri 22-23 Jan 07)

Create folders for each filter data. (B Raw Data, V Raw Data, R Raw Data, I Raw Data). Put the flat field image for that filter in the corresponding filter folder.

Open an image file. Close the file, but not the image processing program. This is done to make sure the log text file (called ImageInfo) is created in the correct folder and then you can start with the file you wish.

Note: You can open the ImageInfo text file any time to check the log. You can also edit the log at any time and save the changes. Do not change the name or location of the file until all data has been logged in.

Calibrating the Images

1. Click on one of the image files in the new folder. The AutoStar image processing program will open the file.

2. Close the file, but not the image processing program.

3. Click on the Group pull down menu and select New.

Figure 1
AutoStar Image Processing Group Selection

Note: The Photometry selection is a bit misleading. It's designed to automate time series projects where you may have hundreds of images in each filter. For starting out, this can cause more problems than it solves. Ignore it for now. Also ignore the rest of the item from this pull-down menu as the mainly apply to astro imaging and not photometry.

4. Find the new file. Select all the images of a given filter (e.g., the V filter images in the V Raw Data folder).

Figure 2
Selecting V Filter Files to Calibrate with V Flat Field

Note: This will create a new text file called ImageGroup.lst. Do not move or rename this file. This is not a text file and cannot be opened. Just let it be as it defines the files in the Group.

5. From the Group pull down menu select Calibrate.

Figure 3
Calibrate Selection

6. A Calibrate window will be displayed where you can select and include a Bias, Dark Frame and a Flat Field image. If you have used the auto dark subtraction while taking images or if the exposure time is less than 1 second you do not need to use another dark frame. If you have used a dark frame when taking the images the Bias image is not needed.

Figure 4
Calibrate Window

7. Select the Flat Field you wish to use to calibrate with (e.g., Select Flat Field and find the flat field for the filter used for the images to be calibrated, e.g., FB). Click the Include Flat Field box and if desired the Include Bias button (after selecting the Bias image). Remember the Dark Frames have already been used on the fly when the images were exposed. Click on OK.

8. The program will now automatically calibrate all the images and save the new calibrated images with a new name (Calibrated_"file name"). The original images will be left untouched.

Figure 5
Calibrated Files

9. To organize the files make a new folder within the current folder. Call the folder "Cal Filter Name Data" (Cal V).

10. Repeat steps 3 through 9 for each filter. You now have sets of calibrated data ready to have the star magnitudes determined.

Differential Magnitude

The best and most accurate way to do photometry is to use differential photometry where program star magnitudes are compared to comparison star magnitudes and then normalized to produce a magnitude. This means you must know the comparison star magnitude accurately for each band.

Setting Reference Magnitude

1. From within the image processing program select File and Open.

2. From within the Cal V folder select the first image to be processed. Make sure the file name has a Calibrated_ in from of it.

Note: AutoStar automatically adjusts the display contrast so even if the images look faint, the data should be okay. If the star image is too faint you can adjust the contrast manually. the adjustment only affects the display. You can also expand the image to full screen if it helps.

3. Draw a small box around the comparison star. There will be a diagonal line across the box. The line should be approximately across the middle of the star. This is not critical as the software will find the center of the star.

Figure 6
Photometry Cursor

Note: The Photometry region cursor has three main components:

Magnitude/Centroid Area
Pixel List Area,
Profile Line.

The Magnitude/Centroid rectangle bounds the entire area used in the magnitude and/or centroid calculations. This is a bit confusing as to properly determine the magnitude of a star, only draw the small box to the lower left (Pixel List Area) around the star of interest. The Profile line delineates the pixels that will be displayed when the Draw Profile function is selected.

4. In the pop up window select Set Reference Magnitude.

Figure 7
Selecting Set Reference Magnitude

5. Set the Reference Magnitude for the comparison star for that filter. Click OK.

Figure 8
Setting the Reference magnitude

Note: This will set the reference magnitude for the comparison star. This must be done with each image. Other star measurements will now produce a magnitude referenced to the comparison star magnitude.

Star Magnitudes

6. Draw a box around the first program star. A widow will pop up. Select Determine Magnitude.

Figure 9
Selecting the Determine Magnitude for a Star

7. A Magnitude Determination window will show indicating the magnitude referenced to the comparison star along with other data.

Figure 10
Star Magnitude Determination and other Statistics

8. Click on OK. Do not click on Log as the data will be logged twice. Just click OK.

Note: A text log of all the data will be created automatically. The file will be called ImageInfo.

9. Repeat Steps 1 - 8 for each program star you wish to know the magnitude of.

10. As a double check and a way to indicate where you are in the log file, determine the magnitude of the comparison star as the last measurement. It should be the same as what was set for the reference magnitude.

Note: To continue with additional images with the same filter, repeat steps 1 - 8, but you will not need to re-enter the reference magnitude value as it is already set so just click OK for that step.

11. When done you should have data from all the images for the given filter. You should also have a single text file called ImageInfo. You can rename the text file now. Call it "B Data.txt."

12. Repeat steps 1 - 11 for the other filter images. A text log file called ImageInfo will be created.

ImageInfo File

The ImageInfo file is a text file that is automatically created when determining the magnitudes of the stars.

Figure 11
ImageInfo Log File

The X and Y Center data helps identify which star the data applies to. Of the rest of the data the Magnitude, which is relative to the Reference Magnitude, Flux and Max are the data of most interest. The FWHM (Full Width Half Maximum) number gives an idea of the shape of the star image. If this gets too far from around 6 there may be a problem with the data. The rest of the data may be of interest for determining the quality of the data.

Created 1 February 2007

Modified 13 February 2007

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