Starry Vistas by Don J. McCrady

Flat Fields with a Light Box

Sky 90 with Light Box - Click for larger image

Using a flat-field image to calibrate my astro-images has been a huge benefit, mostly to help remove central hotspot gradients like in the following example:

Gradient Comparison

Note how the uncalibrated image on the left has dark edges and a bright central gradient.  This is an unavoidable result of any optical system, no matter how good it is.  The calibrated image on the right has used a flat field to remove this gradient and has a much more even background.  (Note, however, that a flat field will not remove other gradients like those caused by light pollution.  As this example illustrates, there are still a few gradients, notably toward the right edge.)

Flat field calibration is also used to remove dust motes.  However, I regularly blow off any dust from my camera lens, so I do not use them for this purpose.

There are several ways to shoot a flat field calibration image.  They all require an image of an evenly illuminated surface.  An exposure is made such that between 1/3 and 2/3 of the camera's dynamic range is used.  (More than that, and you risk clipping highlights.  Less than that runs you into noise issues, and also exacerbate any gradients that may be present.)  Averaging many exposures is always a good idea to reduce noise, and if the exposures are of significant duration it's a good idea to subtract a dark frame from them as well.

Finding an evenly illuminated surface is the real trick to getting a good flat field calibration image  I've had good results by taping a white piece of paper to a wall and taking shots of it through my telescope on a cloudy day.  This is a simple method, but the problem is finding a wall without gradients.  Take a look around your house or apartment; you'll be surprised how many you'll see if you look hard.

So, I Made a Light Box

So I decided to make a Light Box, like the one pictured above, using basic plans gathered from the internet.  My light box is loosely based on this one.  I used all the same electrical parts, which I repeat here:

  1. 4 x 2.5v flashlight bulbs (Radio Shack #272-1132)
  2. 4 x base for bulbs (Radio Shack #272-356 or 357)
  3. Optional potentiometer (Radio Shack #271-265)
  4. 9v Battery
  5. White duct tape
  6. Hot glue gun & glue
  7. Wire, soldering iron, solder

Foamcore is the material of choice for the box, since it's sturdy and easy to work with.  For measurements, I kind of flew by the seat of my pants, but basically I measured the diameter of my Takahashi Sky 90's dew shield, and used that as the basis for all other measurements.  This image at the right shows the interior of the box.  The central box fits over the telescope dew shield, and uses a milk-white plastic as a diffuser layer.  (Actually, the material I used had some linear streaks in them, so I used two layers, with the streaks at 90° angles to each other to help cancel out the patterns they formed.)  I used quilter's templates, available from knitting/sewing craft stores as this diffuser layer.  The picture on the left shows how effective this material is in diffusing the light evenly.

You can also see the four 2.5v lamps in each corner, wired in series, and hooked to a potentiometer.  The potentiometer is optional, especially since I always crank the bulbs up to full power anyway.  The lights are powered by a 9v battery.

The corner wedges, also seen in this image, serve two purposes.  They add stability to the box, and they also help bounce the light back to the plastic diffuser layers.  It goes without saying that the inside of the box is white, including a copious amount of white duct tape.  All the pieces were glued together using a hot glue gun.

A lid fits over the end snugly to keep stray light out.  I keep it on all the time, but it can be easily removed to change burnt-out bulbs or perform quick fixes.

Thanks to PAO's Garden Spot Observatory  for his great site that served as my inspiration during this project.

Taking the Flat Field

If you have a monochrome CCD camera, you'll have to shoot flats for all configurations of the camera you image with.  That is, if you shoot LRGB, you'll have to shoot a flat field without a filter, another with the red filter, another with the green filter, and yet another with the blue filter.  As I stated above, the goal is to expose the sensor to about 1/2 of the camera's dynamic range.  This is a good compromise to ensure that your flats have high signal-to-noise, and also to ensure that your exposures do not encroach on an AGB camera's non-linear response zone.  You should shoot several exposures in each configuration and average them to reduce noise.

If you have a one-shot colour camera, like my Canon Digital Rebel, you have to take into account the fact that the lights used here are not white lights.  They produce images that are decidedly red.  Fortunately, there's a simple way to fix this using the camera's custom white balance.  First, take a test exposure that uses 1/2 of the camera's dynamic range.  Then use this exposure to set the camera's custom white balance.  Then, using this new custom white balance, shoot a set of flat field images.  They should be nice and gray with the colours evenly balanced.