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Astrophotography tips from a keen astrophotographer

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Category: Specialist

Looking to the stars - If you think you've photographed everything on earth why not turn your lens to the sky.

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Astrophotography is the photography of stars, planets, the Sun and Moon, and other celestial objects in the sky, such as galaxies, stars clusters and nebulae. It can be easy to get started shooting simple things but it will get progressively harder when you start to turn your attentions to the smaller, fainter objects.
 
If you've never done this before try shooting the moon. You don;t have to do it in the middle of the night either you can get some beautiful shots at dusk,” said Jerry Lodriguss who's images of nebulae and galaxies have appeared in publications all over the world.
 
When it comes to equipment a simple camera and lens on a tripod will work or you can substitute the camera's lens for a telescope. A DSLR works best but you can use a compact with the cameras lens shooting through the eye piece of a telescope for shots of the moon or if you're feeling brave you can try a much more complex set-up: “Put a camera with a telescope substituting the camera lens on an equatorial mounting that is polar aligned with the Earth's rotational axis.  Which is driven by a computer when using another telescope mounted piggyback on the prime telescope to track the stars with high accuracy.
 
No matter if you choose the complicated or easy route, a tripod is essential as is a remote release as exposures can be from a few minutes to several hours.
 
I have used everything from ISO200 at 1/4000sec at 1000mm focal length for the Sun and up to 175 minutes (nearly 3 hours) of total exposure at ISO1600 for nebula. For faint stuff, typically you use ISO1600 with a low-noise DSLR camera, and individual sub-exposures of about 5 minutes, and then stack many sub-exposures into one longer integration which, when combined, can last one to several hours.
 
For those who live in towns and cities light pollution will stop you seeing the sky as it truly is so to get the best astrophotos you need to take a drive out into the country away from the city lights. Also if you intend on photographing the sun remember it can burn your retina and cause blindness so always ensure you have proper, safe solar filtration. Patience and a love for nature and an appreciation of the beauty of the night sky are also something Jerry recommends you have before you step foot outside.
 
As with all photography a little post-production can be a good thing and astrophotography is no exception.
 
For long exposures of faint deep sky objects, there is a lot of post production. Images must be calibrated with dark frames to remove thermal signal before they are demosaiced to produce colour from the bayer sensor. This requires a special astronomical image processing program.  Then after calibration, individual sub-exposures are stacked to increase the signal-to-noise ratio between the object and sky background. Then image correction and enhancement techniques are applied to bring out all of the faint detail.

What is usually considered noise in long exposures is really thermal signal which can be removed with dark frames. This is the same basic concept as in-camera long-exposure "noise" reduction. But we shoot many dark frames to create a master dark frame to remove thermal signal because this introduces less true noi
se when the master dark frame is subtracted from each individual "light" frame. And in-camera noise-reduction wastes clear dark sky time that is better spent gathering photons to increase the signal in the signal-to-noise equation. Darks can be shot in your garage on a cloudy night, and a library of darks created for different temperatures. These darks can then be used later for calibration depending on the ambient temperature when the lights are shot.

Real noise, true noise, can not be removed. It can only be mitigated by increasing the true signal from the object you are interested in, and this can only be done by using longer total exposure times, either with longer sub exposures, or longer total integration times with shorter sub exposures.
 
Jerry has been an amateur astronomer and astrophotographer for more than 35 years.  For more advice or to learn more about his work visit Jerry Lodriguss' website.



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