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How to perfect your astro-photos.

In my last post I discussed why astronomers take multiple identical photographs of the same astronomical object in order to reduce the effects of random noise. I discussed how this noise arises and gave examples of the improvements gained by stacking multiple photos together. Of course, reducing random noise within your image is an important first step but, if you really want to obtain the perfect astro image, there is still more to consider. Both your camera and telescope can introduce a number of inconsistencies in your images, these occur to the same extent in every photograph you take, meaning they cannot be cancelled out like random noise can. Here I will discuss what these inconsistencies are and the ways astrophotographers remove them.

So…what are these inconsistencies? Well they come in three types and each must be dealt with separately:

The first of these is a thermal signal which is introduced by the camera. This tends to look like an orange or purple glow around the edge of an image. It develops when heat from the camera excites electrons within the sensor. As we take a photo, these heat-excited electrons behave as though they have been excited by light and produce false sensor readings. This effect gets stronger with increasing exposure time and temperature. The best way to remove this is to take an equal length exposure at the same temperature as your original astro image but with no light entering the telescope/camera (perhaps with the lens cap on). The resulting picture will contain only the erroneous thermal glow. This ‘dark’ frame can then be subtracted from the original image.

fig_1

Figure 1. The original exposure (showing the constellation Orion at the bottom) shows a strong thermal signal in the top left. By taking a dark frame of equal exposure, we can subtract out the thermal signal, giving a better result.

The next inconsistency is known as bias. This constitutes the camera sensor’s propensity to report positive values even when it has not been exposed to light. This means that the lowest pixel value in your picture will not be zero. To correct this, it’s necessary to shoot a frame using the shortest exposure and the lowest ISO (sensitivity) possible with your kit then subtract it from the original frame. For most modern DSLR cameras, this subtraction has a very small effect but it does increase the contrast for the faint details in the picture – which is particularly important when shooting in low light.

Finally, and arguably the most important image inconsistency of all – uneven field illumination. This problem occurs when the optics within a telescope do not evenly project an image across the camera’s sensor. Most telescopes (and camera lenses) suffer from this problem. A common cause of uneven illumination is dirt and dust on the lens or sensor, which can reduce the light transmitted to parts of the sensor.

This is the objective lens from my telescope before and after cleaning. Although small specs of dust do not seriously affect the overall quality of the image, they can contribute to uneven brightness in the image.

This is the objective lens from my telescope before and after cleaning. Although small specs of dust do not seriously affect the overall quality of the image, they can contribute to uneven brightness in the image.

The final cause of uneven illumination is vignetting, this is a dimming of the image around its edges. Vignetting is normally caused by the telescope’s internal components such as the focus tube and baffles (baffles stop non-focused light entering the camera). These parts of the telescope can restrict the fringes of the converging light from entering the camera. So how do we combat this…keep cleaning the lens? Rebuild the internal parts of the telescope?…no. The answer is simple; take a ‘flat’ calibration frame. All you need to do is take an image of a evenly illuminated object (such as a cloudy sky, white paper, or blank monitor screen). Since you know the original scene is uniformly bright, any unevenness in the brightness of this image must be due to issues with the telescope. You then divide the brightness of the pixels in the original image by the pixels in the flat frame and magically, the unevenness is gone.

For your enjoyment, here’s some examples of flat frames taken from across the Internet, the middle image is from my scope. There are some diabolical flats here; I wonder if it’s even possible to conduct useful astronomy with such severe obstructions in a telescope!

Some examples of flat field frames taken by different telescopes. All these frames show were light is being blocked from reaching the camera sensor. My telescope’s flat frame is the middle picture; it looks good in comparison.

Some examples of flat field frames taken by different telescopes. All these frames show were light is being blocked from reaching the camera sensor. My telescope’s flat frame is the middle picture; it looks good in comparison.

By applying the flat frame correction, the background of the image becomes more even, and dark patches due to dust disappear! No need to clean your scope! (Image taken from http://interstellarstargazer.com).

By applying the flat frame correction, the background of the image becomes more even, and dark patches due to dust disappear! No need to clean your scope! (Image taken from http://interstellarstargazer.com).

For many people starting to turn their cameras and scopes to the heavens, all of this does sound rather arduous but there is software out there that will automatically combine your star images with the three calibration images and spit out what you want (see Deep Sky Stacker). I was amazed that for reasonably little effort and no extra money, I could improve the quality of my images significantly.

Post by: Daniel Elijah

 

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The brain bank comprises a group of scientists from the North West of England eager to enthuse and entertain with their scientific banter. To learn more about who we are see the our 'about' page. You can also find us on twitter @brainbankmanc or email us brainbankmanc@gmail.com.
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