Imaging Primer

DIGITAL PRIME FOCUS ASTRO-PHOTOGRAPHY OF DSOs

and an outline of how to process them

plus the equipment required

This article is a basic primer that may be of use to those interested in photographing Deep Sky Objects (DSOs) at prime focus using either an astro-CCD (Charge Coupled Device) camera or a standard DSLR (Digital Single Lens Reflex) camera. No mention will be made of other forms of digital imaging like web-cam imaging, afocal imaging (photographing through a telescope eyepiece with a non-DSLR Camera) or eyepiece projection, although a subsequent article will cover these forms. The following observations and comments are based on my own experiences of imaging DSOs at ‘prime focus’.

Photographing Deep Sky Objects (DSOs) is one of the most challenging forms of photography you could imagine. Think of what is involved:-

    1. You will be photographing an object that appears to be moving
    2. The object that you will be photographing will normally be very dim - so dim you probably can’t see it with the naked eye.
    3. The object will be relatively small so you will need a long focal length lens to capture it
    4. The only way to capture enough light from this object will be by taking a long time exposure through a wide aperture

To take DSO astro-photographs, you’ll need to invoke the Five Fs!

Find, Focus, Frame, Follow, Film

FIND YOUR OBJECT

This can be harder than you’d think as many DSOs are difficult to see even through a telescope. Star hopping or the use of a GoTo telescope are the best methods but in any event, you do need to plan in advance what object(s) you wish to image during the session.

FOCUS YOUR OBJECT

In many ways, focus is the most important of the five ‘F’s - no matter how good the Framing, tracking and exposure of your image, if it is out of Focus it will be a failure.

FRAME YOUR OBJECT

Once you have a good focus on your object you must ensure that the composition is right so you need to adjust the direction of the telescope to get the object in the right place in the frame.

FOLLOW YOUR OBJECT

We have already determined that the object will be moving so you are going to have to follow that movement! The RA motor drive on your mount will go a long way towards this.

FILM YOUR OBJECT

Well, film is not really the right term as we are using a digital sensor here but the sensor is a replacement for the film after all and it does start with an ‘F’! ‘Photograph Your Object’ would have sounded right but would have been really cheating.

That’s the five ‘F’s for you but what about equipment and post-processing

Copyright Steve Richards 2008