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# Examine the CCD Images using XVista

Extract Basic Image Information

Now that you have a reduced moon image to work with, you need to derive some basic information about this image. This includes

1. image size in pixels
2. image pixel scale (arcsec/pixel)
3. image field of view
4. minimum and maximum intensity values represented in the image.

Proceed as follows:Image SizeDisplay the image in XVista. What is the image size in pixels (e.g., size horizontally x size vertically)?Pixel ScaleThe Kodak CCD has a pixel size of 9-microns (square). The camera was located at the f/16 focus of the 0.305-meter Tinsley telescope. What is the pixel scale in units of arcseconds/pixels?Image Field of View (FOV)Combine your image size and pixel scale to compute the field of view of the image. Remember that the long axis is oriented East-West, and the short axis is oriented North-South. Express your FOV dimensions is both arcseconds and arcminutes.Image Pixel ValuesUse the abx command in XVista to analyze the pixel data values, as follows. If the image is in buffer 1, type

```   abx 1 all
```

1. What is the minimum pixel value in the image? Which pixel is it located (give row,column coordinates)?
2. What is the maximum pixel value, and where is it located?
3. What is the average pixel value in the image?

Now, use the XVista histogram command to make a histogram showing the distribution of pixel data values in the image. Suppose that you found that the minimum and maximum pixels in your image were min=100 and max=1523, respectively (I made these up, your values will be different). If your moon image is in buffer 1, you would type

```    histogram 1 xmin=100 xmax=1523 nolog
```

which will plot the histogram of pixel data values. Add the hard command to the line above to make a hardcopy and add this to your notes. Answer the following questions:

1. What is the mode of the pixel values (peak of the histogram)? Is it easy to define this quantity unambiguously in this image?
2. Label the mean you computed above on your plot of the pixel data value histogram. What do you think the “mean” represents in this particular image?

Make a hardcopy of the image using the IMPOST command. For example, to make a PostScript version of an image in buffer 1, displaying intensity values between 0 and 100 and pixel scale 0.54-arcsec/pixel (I made these up! – your values will be different!), type

```    impost 1 z=0 l=100 scale=0.54 file=mycrater.ps positive
```

This will create a PostScript file with your image in the file mycrater.ps in your working directory. You should replace “mycrater” with your lastname, just so we know whose file it is. The positivekeyword ensures that the picture comes out as a “positive” print.

To print the PostScript file from inside XVista use the command:

```
\$ lpr mycrater.ps
```

(or whatever you called the print file). The default printer is the laser printer in MP4042. As long as you are using the ast350 lab account, this printer will be the default printer (no guarantees for other accounts).