British Astronomical Association - Computing Section - britastro.org/computing

(This is no longer important because we now generate red-on-black versions of our charts automatically, but it may be of interest.)

This process enables a chart to be displayed while observing, to assist in finding the relevant object. We will describe the process in Windows but if you are using a different operating system there will be very similar steps; you just have to find the corresponding menu options. Also we refer to Adobe Photoshop but other image processing applications can do this, perhaps with different menu options again.

1. Save the chart as an image file on your disc
   Either (a) right-click on the link in one of the tables or lists above and from the pop-up menu select "Save link as..." (in some browsers it may be "Save target as...") or (b) click the link to open the chart in a new window, right-click on the chart and select "Save image as...". You will see that the image name is quite descriptive, eg, 27Euterpe2011Oct4(J2000).png
2. Open the file in Photoshop
   The file type is Portable Network Graphic, a web standard that most general-purpose imaging applications can open. It is PNG-24 which means it has 3 colour channels (red, green, blue). 24 because each channel has 8-bit depth.
3. Invert the contrast
   Ctrl-I is the short-cut key in Windows/Photoshop, corresponding to the menu option Image/Adjustments/Invert. The image should now be white on black.
4. Remove the green and blue channels
   
   1. This requires the Channels window to be on display (select it from the Window menu).
   2. It also requires the background colour to be set to black (at the bottom of the floating tool bar).
   3. Select the whole image (Ctrl-A).
   4. In the Channels window select (highlight) only the green channel and press the Delete key. The green channel becomes entirely black (zero).
   5. Similarly select (highlight) only the blue channel and press the Delete key.
   6. In the Channels window select RGB to see all channels again and now the image is red on black.
5. Save the red/black image on disc