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AstroLab: How to Create a 3-Colour Image

Telescope images are black and white. This is because the camera only measure how bright something is. Placing filters in front of the camera lets us measure brightness in a particular colour (like B for blue).

We can combine different filter images of an object and create a colour picture. Red, green and blue are usually used. This makes colour pictures similar to what the human eye will see.

 

Please be warned that AstroLab does not automatically save what you are doing. We are using software in your Internet browser. If you accidentally hit refresh, you will lose the progress you've made!

 

Get Data of a 3-Colour Observation

Option A

  1. Go to "Go Observing".
  2. Select an observing programme with "Colour" in its name (e.g. "Galaxies" > "3 Colour Galaxies", or "Deep Sky Objects in colour").
  3. Choose an object and submit an observation.
  4. Go to "My Observations" to check the status of the request.
  5. Select "View Images" when the data is available.
  6. Select "View all these images in AstroLab" or select each observation code individually to download the data.

 

Note: the observing programmes you see depend on your user type. But any with "colour" in its name will get you the data you need.

If you need further guidance on using Go Observing, we have information and video tutorials in the Get Started section of our website.

 

Option B

  1. Go to "Archive of Observations".
  2. In the "... Taken for this Observing Programme" dropdown menu, select one with "colour" in its name (e.g. "3-colour galaxy").
  3. Click the "Search the Archive" button.
  4. Choose an object and click on one of the observation codes.
  5. Select "All observations for this request".
  6. Select "View all these images in AstroLab" or select each observation code individually to download the data.

 

If you need further guidance on searching the archive, we have information and video tutorials in the Get Started section of our website.

Open and Scale in AstroLab
  1. Open the images via the "View all these images in AstroLab" button or the "Open an image" tool in the "File" menu of AstroLab.
  2. Open the "Scaling" tool in the "Display" menu.
  3. Adjust the "Maximum value" to see fainter parts of the image. You can use the slider or type a number into the box.
  4. Adjust the "Minimum value" to reduce the noise (grey fuziness). You can use the slider or type a number into the box. Note: this number usually won't need to be increased much.
  5. Click on another image and repeat.

 

See our help guides for more information about: 

Assign an Image to each Colour
  1. Open the "3 Colour Image" tool in the "Colour" menu.
  2. Click the drop-down box next to "Red image" and select the image ending in "000".
  3. Click the drop-down box next to "Green image" and select the image ending in "001".
  4. Click the drop-down box next to "Blue image" and select the image ending in "002".

 

AstroLab does this process automatically for images opened via the website. Simply check the file name assigned for each colour to make sure it's correct.

Sometimes there might be a 4th image (ending in "003") in your dataset. This depends on which telescope and instrument were used for the observation.

If you want to remove it, press the white "x" button for that image box on the side. A pop-up box will appear asking you to confirm the removal. Make sure to check the filename before clicking "OK". You don't want to accidentally delete an image you need!

Switch between Viewing Options

You can switch how the images are displayed using the "Show..." buttons in the "Three-Colour Image" toolbox.

Certain viewing options may be useful for particular steps. This will be mentioned in the instructions.

  • "Show Red, Green and Blue": This is the default option. It displays the main image and small image boxes on the side in their assigned colour (e.g., the image selected for "Red image" will now be red).
  • "Show 3-Colour Image": This displays the 3-colour image in the main window. The small image boxes remain their assigned colour.
  • "Show Original Colours": This displays the main image and small image boxes in black and white, as they appeared before a colour was assigned to them.

 

When switching between viewing options, you can keep track of the image colours by looking at the "3-colour toolbox". The background will change to match the image's assigned colour. So, if the red image is selected, the background will become red.

Check Image Alignment
  1. Select the "Show 3-Colour Image" button.
  2. Look at the stars in your image. They should mostly resemble single white circles. Seeing red, green or blue circles that have only overlapped slightly (or not at all) means the images haven't aligned properly.
  3. If the images have aligned, you can skip ahead to the Adjust Colours section. Otherwise, select the "Align images" tool in the "Display" menu.
  4. A loading circle will appear as AstroLab does this task. Once complete, check the image alignment again. If this is correct, you can skip ahead to the Adjust Colours section.
  5. If further alignment is needed, follow the steps in the Shift Image Position section. This will allow you to move each image into the correct place.

 

Note: You might spot tiny coloured pixels or lines in the 3-colour image. This could be due to cosmic rays hitting the telescope detector during an observation. The camera reads this as light, so the pixel/s in that image become bright.

Even if cosmic rays hit during 2 or more observations, it's very unlikely that the same pixels will be affected across the images. This means you can have random pixels or lines of red, green or blue in the final 3-colour picture.

Shift Image Position
  1. Select either the red, green or blue image box at the side of the main window.
  2. Check the background colour of the "3-Colour Image" toolbox (e.g., if you want to move the red image, check that the background colour is red).
  3. Use the arrow buttons in the toolbox to move the selected colour image horizontally or vertically.
  4. Use the rotate buttons in the toolbox to rotate the image clockwise or anti-clockwise.
  5. Select another colour image and repeat until all 3 are aligned.

 

Note: If you want to reset an image back to its original position, press the centre button (the one with all the arrows pointing inwards). This will remove all the movements you've made for that image.

Adjust Colours

The amount of red, green and blue in the 3-colour image can be adjusted. You can experiment with the colours until you are happy with the result.

 

  1. Select "Show 3-Colour Image" in the "Three-Colour Image" toolbox.
  2. Select either the red, green or blue image box at the side of the main window.
  3. Check that the correct image has been selected by looking at the background colour of the "3-Colour Image" toolbox (e.g., if you want to adjust the amount of red, check that the background colour is red).
  4. Open the "Scaling" tool in the "Display" menu.
  5. Adjust the "Maximum value" using the slider or by typing in a number. The lower the number, the brighter the colour will become.
  6. Adjust the "Minimum value" using the slider or by typing a number. This will reduce the amount of 'noise' or fuzziness in the background. *
  7. Select another image colour and repeat. **

 

* Note: switching to "Show Original Colours" may help with this step. You want to limit the amount of greyness in the background and make it as dark as possible without affecting the brightness of the image too much.

** Note: switching to "Show Red, Green and Blue" may help with this step. You can compare the brightness (i.e. the 'amount') of each individual colour with the others.

Download the 3-Colour Image
  1. Select "Show 3-Colour Image" in the "Three-Colour Image" toolbox.
  2. Click "Download Picture" in the "File" menu.