Taking Dark and Bias Frames

A dark frame is an image of your camera's noise. Dark frames are more essential on longer exposures, but any image can benefit from them. Keep these ideas in mind when creating a good dark frame.

• Take the dark frame and the light frame around the same time. This assures that they were both taken at the same temperature.
• Use the same exposure for the dark frame as you used for the light frame.
• Take multiple exposures (at least 3-5) and use the median combine function.

A bias frame is system noise that is always there regardless of the length of the exposure. So a bias frame is always the shortest exposure the camera can handle. They are used to scale the dark frames when the exposure times between the dark and the light frames are different. So if you use the same exposure time between dark and light frames you don't have to use bias frames.

Procedure:

1. Go to the TAKE IMAGE tab on the CAMERA CONTROL.
2. Set your series and exposure and choose the type of frame you wish to take under the IMAGE section.
3. Make certain the AUTOSAVE ON button is activated in the AUTOSAVE tab.
4. To do a median combine go to the IMAGE menu and click on COMBINE.
5. Under this menu you will find COMBINE FOLDER OF IMAGES. Click on that and choose your folder for the dark or bias frames.
6. Choose MEDIAN under HOW TO COMBINE FRAMES and REDUCE FOLDER.

Taking Flat Fields: Flat fields are used to correct for noise in the optical path of the telescope system. The use of a flat field is essential for a good image. Keep these ideas in mind when creating a good flat field.

• It must have even illumination.
• It must be taken with the same optical setup as the image you are going to apply it to (don't change the focus).
• It should be about one-third to one-half the saturation values for the camera.
• Multiple exposures (8 to 12) should be taken and averaged using the median filter.
• Take separate dark frames to use with the flats.
• Take flats for each color filter for color imaging.
• Check the pixel value across the image. The image should have variations of only 5-10 units.
  There are many ways to achieve a flat field, and I've outlined a few below.
  1. Dome Flat: Close the dome and with the lights on take an image of the painted white image of the shutter. Move the telescope a small amount to different parts of the shutter for averaging out irregularities. This method is good if you need to change the focus of the optical system during the night.
     
     
  2. Sky Flat: Work this one in twilight before the sky gets too bright. You will need to change your exposure to compensate for changes in twilight brightness. You need a similar average value for each exposure. Be careful about getting faint stars in your images as well. Move the telescope slightly so they average out.
     
     
  3. T-Shirt Flat: This can be done in twilight or the daytime. There is a white cloth, and a rubber band in the observatory box. Place the cloth over the front of the telescope pulling it tight. Secure it with the rubber band and place the dew shield around the telescope. Point the telescope toward the ground and take your exposures.
  Procedure:
  1. Start with a test flat. Go to the TAKE IMAGE tab on the CAMERA CONTROL.
  2. Set your series and exposure and choose FLAT for the type of frame you wish to take under the IMAGE section.
  3. Make certain the AUTOSAVE ON button is activated in the AUTOSAVE tab.
  4. Evaluate your flat by looking at pixel values across the image. There should be no more then a 5-10 pixel variation. To measure pixel variations in CCDSoft you need to move the cursor over the image and watch the brightness value in the status bar located at the bottom of the window.
  5. When you are satisfied, take a series of flat frames and do a median combine (IMAGE/ COMBINE/ COMBINE FOLDER OF IMAGES and choose MEDIAN under HOW TO COMBINE FRAMES).
  6. You will also want to take a separate set of darks for your flats. Select DARK in the IMAGE dialog box under the TAKE IMAGE tab and use the same exposure as your flats. Save them in the DARK FRAMES FOR FLATS ONLY file.
  7. Use a median combine on these as well.
  Image Reduction: After you take your series of bias, dark and flat frames you are ready for image reduction. From the CAMERA CONTROL panel, use the Image/ Reduction box and click on BIAS,DARK,FLAT.
  
  Taking Images and Processing - Deep Sky Deep Sky Imaging With The ST-2000XM Camera: Deep sky objects are easy to image and for most Messier objects, good results can be obtained in exposures done in as little as 30 seconds. Longer exposures would be better or even stacking images (see Registax) for a better signal to noise ratio. Taking the image is only part of the job; the real art comes in the processing. We'll begin with some basic processing, but check the manuals for further techniques.
  
  We'll assume that you have the object centered, and focused. We'll take a grayscale image here and go through the color process under the planet section.
  
  Imaging Deep Sky Objects:
  1. In CCDSoft, open up CAMERA CONTROL and TAKE IMAGE. Set your exposure or even a series of images.
  2. You can save images the standard way with the save button or use AUTO SAVE in the control panel. Select a file name prefix, starting number, and folder.
  3. Start processing with the HISTOGRAM tool. Slide the controls to set the brightness and contrast.
  4. Experiment with the other image tools. The Richardson-Lucy deconvolution tool is excellent for sharpening your image.
  5. Save your image and send to Mission Support.
  Taking Images and Processing - Planets: Planetary Imaging With The ST-2000XM Camera: The ST-2000XM Camera seems a bit more intuitive with deep-sky images then planetary. Your first results may seem to show that you can't get an exposure fast enough or a filter dark enough not to over saturate the chip. This isn't the case and by following the procedure outlined below you will be able to get some high resolution images of our closest neighbors. As always focus is critical for good resolution images so follow the focus procedures before attempting these.
  
  Imaging a Planet:
  1. Open up CCDSoft and obtain focus.
  2. Click on COLOR on the CAMERA CONTROL. Pay special attention to your exposures. Color for this camera and filter wheel should be a ratio of 1:1:1.6 for RGB. Don't worry that the image looks overexposed.
  3. Click on the HISTOGRAM tool and set it for PLANETARY. Adjust the sliders for the best contrast and repeat for the other images.
  4. Click on IMAGE, then COLOR, and COLOR COMBINE. The image may not be aligned properly. We'll fix that next.
  5. Highlight the color you want to realign. Now click on IMAGE and then ALIGN and SHIFT. Put in a HORIZONTAL and VERTICAL OFFSET. You may have to experiment with this. Click OK. Repeat for any of the other colors that may need to be realigned.
  6. When satisfied with your image click COMBINE.
  7. Save your image and send to Mission Support.
  Planetary Imaging With The STV Camera: The following instructions will allow you to use the STV Camera with the main telescope. The advantage is that it is easy to use and has a best sharp feature that works very well for planetary imaging. The disadvantage is that you will not have its use as an efinder and the resolution is not as good as the ST2000XM.
  
  Setting it up:
  1. Remove the STV from the bottom of the telescope and unscrew the long barrel (the efinder).
  2. Remove the SBIG ST2000XM Camera from the main telescope and unscrew the "T" adapter from the camera. Screw this on the STV.
  3. Remove everything from the back of the telescope and put in the STV.
  4. Turn on the STV unit in the observatory. Click on IMAGE, and then VALUE until the display shows 1.0 second. Click IMAGE again to take the picture. The image will appear on the screen and update every second.
  5. Using the IN and OUT buttons in CCDsoft, tighten up the image until the object gets into sharp focus.
  6. Click on the IMAGE button to stop the exposure.
  Imaging a Planet:
  • Center the planet on the screen on the STV unit or on the Hab Observatory TV (Channel B). You can access the STV unit on the Hab Observatory computer as well. Note: The video images will NOT show up on the computer - but will show up on the TV.
  • You should be on the IMAGE SETUP setting. Click PARAMETER to get to ZOOM.
  • Click PARAMETER to get to MODE. Click VALUE to get to BEST SHARP.
  • Click PARAMETER to get to EXPOSURE. Turn the knob under VALUE to get to the correct exposure time. The following exposure times are a good place to start- experiment with them for the best exposure for your sky conditions.
    Mercury - .04 seconds
    Venus - .01 seconds
    Mars - .06 seconds
    Jupiter - .03 seconds
    Saturn - .06 seconds
    
  • Click IMAGE to take the exposure. The screen will now show a split screen. The new image on the left side will refresh itself according to the exposure time. The sharpest image will be saved on the right side of the screen.
  • Let it go and enjoy. You can even watch it on the Hab Observatory TV (channel B). When you are happy with your exposure click IMAGE. The best sharp will now appear full screen. Saving and Transferring an image from the SBIG STV to the computer:
    1. On the Hab Observatory Computer: click DOWNLOAD and DOWNLOAD SCREEN IMAGE (image may appear dark - that's ok for now).
    2. On the computer: click FILE I/O and then SAVE AS BITMAP. The folder should be STV images.
    Processing the Image:
    1. Open CCDSoft.
    2. Click open the FILE FOLDER and click on your image in the STV IMAGES folder.
    3. IMPORTANT! Click on GRAYSCALE. Another image will appear.
    4. Close out the original image (can't use it in a bitmap format without changing it to grayscale so it's not needed).
    5. Click on the HISTOGRAM icon. Change the setting from ADAPTIVE to PLANETARY. Move the sliders to set the brightness range.
    6. Experiment with the tools in the IMAGE tab. When you are satisfied with your image save it as a jpeg and send it out to Mars Mission Support.