Over the past 4 years, I have been steadily assembling equipment for the purpose of photographing deep sky objects (DSOs). These are dim, sometimes large, diffuse cloudy patches when viewed using binoculars or a small telescope. Starting with the Andromeda galaxy (see below), I used a simple, long tube telescope fitted to a tracking mount (that tracks the DSO as it moves across the night sky). This allows a camera to collect light over a long period of time (often up to several hours) showing features that would otherwise be far too dim to detect with the naked eye.
Although I personally find this type of astrophotography very rewarding, there are some major drawbacks. The first and most severe is how sensitive this type of photography is to unwanted light. Since photographing DSOs requires a really long exposure, any stray light from streetlights or the Moon simply washes out the object’s detail. The second is the sheer time it takes to collect the light to produce these types of photographs. The main issue is that as I take these long exposures (typically I take over 30 shots each lasting up to 5 minutes), any disturbance such as a gust of wind or nearby movement can cause the image to smear.
Mainly for these two reasons, I decided recently to branch into a different type of astrophotography – webcam astrophotography. Although I don’t have any pictures to show yet (I haven’t finished modifying the webcam yet, but watch this space!) I will briefly discuss the principles of this form of astrophotography and how it can be achieved with very limited equipment (and budget).
Put simply, webcam astrophotography involves taking a video of a bright night sky object (such as a planet or the Moon) and using the best frames of that video to produce a high quality image. There are several advantages to this approach. Firstly, since the object you are photographing will be bright, the exposures are short. This means that movement of the telescope will not cause image smear. In addition, the telescope mount does not have to track the night sky very accurately since each frame of the video is taken over a fraction of a second (normally 1/30s). Secondly, bright objects far outshine artificial light pollution, which makes this form of astrophotography very suitable for people living in towns and cities.
So, with a webcam and a cheap telescope mount, some impressive astrophotography can be achieved (see this link). However, I have neglected to mention anything about the telescope. When photographing bright objects overcoming chromatic aberration (CA) is a real problem. This optical aberration occurs when light being focused through a lens splits into its constituent colours and these colours focus at different points. The colour fringing effect caused by CA is shown on the right.
So, I decided to use a telescope design known as a Maksutov-Cassegrain (modelled by my lovely fiancée Sarah) that avoids CA through an ingenious use of lenses and mirrors. There are three important advantages of using this telescope. (1) The light entering the telescope does not split into different colours (as mentioned). (2) Light is also neatly folded up so the actual telescope is conveniently small. Finally, although the telescope is short, it is capable of imaging at high magnification – an important feature if you wish to image small Moon craters or the great red spot on Jupiter.
In hindsight, perhaps I should have started my astrophotography hobby with webcam imaging; I would have saved a lot of time and money and not developed an irrational hatred of streetlights or the Moon!
Post by: Daniel Elijah