Help For HOU-IP Software
by Tim Barclay, TERC

Min/Max.

Change these values to get better contrast in order to see details.

1. How to change Min/Max.
2. Hints for getting best contrast
3. Default Min/Max settings.

4. Min/Max and Resolution.

1. How to change Min/Max.

Increasing the value for Max makes more of the image bright.
Increasing the value for Min makes more of the image dark.
Adjusting the value for both of these helps bring out detail in the image.

2. Hints for getting best contrast: This can be fussy.

3. Default Min/Max.

These are the initial values of Min/Max when you open an image.
The image processor determines these values by sampling the image pixels. It sets a Max value that will display the brighter objects in the image in white and a Min value that will display the background in black.

4. Min/Max and Image Resolution.

The smaller the brightness range between Min and Max, the better the resolution. Image resolution refers to how well you can see details. The 64 different hues in the greyscale palette (or 16 in some color palettes) are distributed between the Min value (black) and the Max value (white). You see a detail in a feature in the image when one part of it is a different hue than the part next to it. If the brightness range is smaller, more subtle brightness changes will be represented by different colors -- that is better resolution. To see this for yourself:

Slice.

Use this tool to create a graph of brightness versus distance along a slice line on the image.

1. How to draw a Slice line.

2. How to Work with the Slice Graph.

3. Slice Graph Options.

4. Saving or Printing a Slice graph. (Open and Save As are not implemented yet.)

Auto-Aperture.

Use this tool to find the brightness of an object.

1. How to use Auto-Aperture.

2. Explanation of the Results window display. Here is an example:

Aperture: M51.FTS;1 (158, 24)
  • Tool name: image name; image window number, and center (x,y) coordinates of the object.
Brightness: 22036, sky: 19
  • Total brightness of object in Counts, average brightness of background sky.
Radius: 5, Sky radius: 10
  • Radius for circle enclosing all the pixels of the object and Sky radius for perimeter of circle sampled to get average background sky value per pixel.


3. Trouble shooting. When not to use Auto-Aperture

This tool is designed to find the brightness of small, isolated objects in an image such as stars.

4. How Auto-Aperture and Aperture work.

The brightness of any pixel within an object, such as a star, has two components: the brightness from the starlight itself and the brightness from the background sky. Auto-aperture and Aperture measure the total starlight from a star by summing the Counts of all the pixels within a star and subtracting the Counts from the background sky.

The star radius is used to determine the circle of pixels that are assumed to be within the star. The sky radius is used to create a 3-pixel wide ring surrounding the star in order to measure the background sky. Auto-Aperture and Aperture measure the average Counts from the pixels within the sky radius ring. That value is subtracted from each pixel within the star radius circle. Then the remaining Counts for each pixel within the star radius circle (the Counts of starlight) are summed to get the brightness of the star.

Auto-Aperture automatically calculates a star radius and sky radius based on the quality of the image. Aperture allows the user to set these values which enables flexibility when measuring objects that are embedded in a galaxy or nebula or for objects that are very close together.

Zoom and Zoom Box.

Use this to magnify all of the image (Zoom) or a portion of the image (Zoom Box).

1. How to use Zoom

2. How to use Zoom Box

Zoom Box always doubles the Zoom factor.
Change the value in the box next to 'Zoom' to change the zoom factor in the Zoom Box window.

3. Why Use Zoom Box to Zoom

Zooming typically moves part of the image off screen. If that is a part you are interested in, you have to scroll to see it. Zooming also makes things look different - you really do see individual pixels. This can make it harder to find your way around when scrolling.
Zoom Boxing around the object or part of the image you want to examine makes it more manageable.
Zoom Box is necessary if you want to Print part of an image. Print only prints the part of the image that fits in the window with (0,0) at the origin (bottom left corner). It ignores any scrolling you have done to display the part you want to print.

4. Suggestions and trouble shooting.

You can move the Zoom Box window and change its size just as you would any other window.
The window is given a computerese name. There is nothing you can do about this initially, but if you do Save a Zoom Box, which is rare, you can give it a new intelligible file name.
If you do several Zoom Boxes, you need to keep track of Box window names and the corresponding boxes drawn on the image. Otherwise unraveling who goes with which can be a real puzzle.