Telescope Reviews: My annual curves thread

Bryan, it may help to compare the histogram of your image to dough, if you roll it out (stretch) in small steady amounts it will stretch quite far before it breaks (clips).
If you try and roll it to far to fast it comes apart (again clips).
One way to help with this is to use anchor points, and the arrows on the key board, or type in values.
Stardaug;
When you open your image in PS, you should first create a new layer. An easy way to do this is to open the layer palette and grab the layer (called background) and drag it to the icon that looks like a turning paper, beside the garbage can.
Then do the initial stretch, (like Erik and Tonk suggest), this brightens your image, so you can tell your target from sky background.
Along the row of icons that you used to duplicate the layer you see a circle that is half black and white, click on it and you will get a info balloon that has a list. Choose the threshold ( second to last), your image becomes black and white and you get a window showing you a histogram with a white slider below it, slide the white marker to the left, close to the beginning of the histogram bulge, the image will be going white with just the edges showing black, if you have used flats then this darkest part of your image. If you didn't use flats then this is probably some vignetting and I would move the slider back towards the right a little to expand the black area a bit farther from the edges.
When you place your cursor over the image now it becomes a eyedropper tool, look for a place that is black without stars (you can zoom in if you need) after you found the spot you press left click the mouse while pressing the Shift key, this puts a sampler on the image there, that is your black spot.
Now you move the slider to the right and repeat for the brightest spot on the image, that sampler is your white spot.
You can cancel out of threshold and if you want you can drag that layer to the garbage can to delete it.
Your image comes back with the sample points on it, from here open your information palette and you have the readings for these areas.
To place these marks on the curve line, when curves is open just place the cursor over your sample point and Alt/click,
it will show as a black point on the line.
You can place 4 of these samplers on an image, usually the black, white points and 2 in the target area.
Hope this helps.

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Clear Skies
Lawrie

Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D