Planet Hunt
LTImage Brightness Measuring Tool
You can use this tool to measure the brightness of an object. (You can find the tool in the Astro menu in LTImage).
Astronomers call this Photometry.
Summary
To measure the brightness of an object:
- Move the mouse pointer over the centre of the object.
- Click and hold the left mouse button.
- Move the mouse pointer until you have drawn a circle that is a little bit larger than the object.
- Release the mouse button. Two other, larger circles should appear (although they may be too big to be seen on your screen).
- Click on the Calculate button in the Tool Window.
- The brightness will then be calculated and put in the Results box.
A Bit More Detail
You can calculate the brightness of an object in your observation by adding up the values of all the pixels that make an object. (You can see the values of individual pixels using the Examine Tool).
The Problems with the Sky
Although it may seem quite simple just to add up the values of all the pixels in an object, everything is made a bit more complicated because the sky is never totally dark. That means that some of the "counts" in each pixel are not from the object itself and if we include them in the total, we will think that the object is brighter than it actually is.
LTImage solves this problem looking at the values of pixels in a ring just outside the object itself. By taking the average value of pixels in this ring and subtracting that value from every pixel in the circle around the object, we should be left with just the counts from the object itself.
An Example
| Stage 1: Choosing an object |
 First we choose the object we want to measure |
|
When choosing your object, you may want to Zoom in a bit using
the options in the Display Menu.
|
| Stage 2: Making the Object Circle |
 Now let's put a measuring circle around it |
|
To make the circle surrounding the object, move the mouse pointer right
over the centre of the object, hold down the left mouse button and drag
the cursor until the circle is just a bit larger than the object itself.
All the pixels inside this circle will be used to find the total counts for the object. If you get it wrong, just click on the centre of the object and start again.
|
| Stage 3: The Sky Ring |
 LTImage then creates a ring to measure the background sky |
|
When you release the mouse button, the Object Circle will turn blue and two new,
green circles will appear. These show you where the Sky Ring is.
Pixels inside this ring (between the two green circles) will be used by LTImage to work out how bright the sky is.
|
| Stage 4: Calculating |
 Now LTImage will calculate and display the object's brightness |
|
When you are happy with the size of the Object Circle, click on the
Calculate button. LTImage will then do all
the calculations for you and put the answer in the Results box.
Warning: For large Object Circles, this can take quite a long time.
|
| Stage 5: More Details |
 We can find out more with the 'Show Details' option |
If you want to know a bit more about the calculation, click on the
little box next to Show Details. You will then be given more
information (as you can see in the picture above). As well as the
sizes of the two Sky Ring Circles, you also get all the numbers
that LTImage has used in the calculation:
|
