Astro photography

Welcome to my Astro page. When I got my first telescope as a kid and stared into the sky, I was forever obsessed. In the library I have read so many books about the universe, and I loved all those stunning pictures. I always thought that such pictures were only for the professionals with huge expensive equipment. Until I discovered that it was actually also accessible for the amateur, and not expensive at all. 

If you are also interested I may be able to help you on your way.

Some of my photographs

Star tracker

A star tracker is a motorised, equatorial mount that fits on top of a tripod and allows a camera to follow the stars (or a planet, a comet, or anything else) as they are carried across the sky by the rotation of Earth.

Why are star trackers such a good choice for astrophotography? A camera and tripod can only take exposures of a few seconds before Earth’s rotation causes objects in the sky to produce star trails. You want to take exposures that are minutes long, with no trailing, thus producing much more detailed and colourful images. 

I use the iOptron SkyGuider Pro myself. It is very compact and easy to carry.

The main benefit of a star tracker is that it will allow you to take multiple, identical images of the same object, which you can then sandwich or integrate together in free image-processing software to make a single image with incredible resolution and detail: a sorcerous process called stacking. This can be done with short exposures on a ‘static’ DSLR too, but better results are achieved with longer, tracked exposures.

Concept of stacking

The concept of stacking in astrophotography is simple, by combining multiple images together, the signal-to-noise ratio can improve. In other words, increasing the signal that you do want and reducing the noise you don’t. Over time the noise level grows at a slower rate than that of the signal so very long exposures produce higher signal-to-noise ratios resulting in smoother, cleaner, more detailed images. Objects with a wide dynamic range like the Andromeda Galaxy and the Orion Nebula have intensely bright cores and much fainter outer regions. A single image of these could easily reach saturation on the brightest areas before the dimmer details have registered at all. But when you stack several unsaturated images together, the dimmer values accumulate into higher values, bringing fainter objects over the bottom limit of the dynamic range (in other words, you can start to see them), while at the same time the brighter values increase as well. Stacked images, therefore, display a wider dynamic range.

Calibration frames

There are unwanted signals generated by the camera’s sensor itself like:

  • Thermal noise as the sensor warms up during long exposures.
  • Variations in pixel-to-pixel sensitivity.
  • Shadows caused by dust particles.
  • Vignetting of the light cone.

This additional degradation of the image is tackled by a process called calibration. Which involves capturing extra one-off frames like bias, dark, and flat frames that are included in the stacking process to ‘subtract’ noise.

Light frames: 

All the individual shots of your target image are called light frames.

Bias frames: 

Removing the readout signal from you camera sensor – even when a pixel has not received any sort of signal there is still variation in how the camera reads data off the sensor. To produce a bias frame you will need to put the cap on the lens and take the shortest exposure possible.

Dark frames:

Correcting the variable dark current generated in each pixel as the sensor warms up during long exposures that can produce what are known as ‘hot pixels’. To produce some dark frames you must cap the lens again and take exposures that match the length and temperature of your image captures (which are known as light frames).

Flat frames:

Correcting the variable light sensitivity of pixels across the sensor. Correcting vignetting. And removing the shadows cast by dust particles. To produce a flat frame you need to make an image while the telescope is pointing at a uniformly illuminated light source with the focus set at the same position used for the light frames.