Modern day CCD camera’s used by amateur astronomers are amazingly powerful and capable devices.
Placed at the prime focus of even very modest small aperture telescopes, a good CCD camera can produce breathtaking colour images that far outstrip anything that can be produced with a similar instrument using film, or that can be seen visually.
Here at IanKing Imaging we aim to demonstrate that taking high quality CCD images does not require lots of expertise or high end instrumentation. But by sticking to a set of basic principles virtually anyone, after a brief period of familiarisation, can be taking images similar in quality to those that often adorn the pages of the popular astronomy magazines.
One of the first questions beginning amateur imagers often ask is “which CCD camera to purchase first?”
Of course this will partly depend upon budget, but we hope that this article will give the reader some insight into how to choose a CCD camera.
CCD cameras offer numerous advantages over conventional film. The CCD is able to capture images with a far greater dynamic range than conventional film, giving the amateur the possibility of capturing high quality data even under light polluted skies. CCD cameras produce digital images.
A decent cooled CCD camera has certain advantages over uncooled low cost devices.
Most CCD CAmeras feature high graded CCD’s with very low noise characteristics. By cooling the CCD to approx 30 degrees below the ambient temperature, these camera’s are able to produce higher quality images than low cost uncooled camera’s.
Some of the cameras we sell cool the sensor even further to approx -50 degrees celsius below ambient.
There are a number of factors to take into consideration when choosing a CCD camera.
1) The Physical size of the CCD. This will determine the field of view provided with any given focal length telescope. The larger the CCD the wider the field of view.
The following calculation will determine the field of view of a given CCD camera.
Field of View in arc minutes = 3438 x CCD Size in mm/focal length in mm
An example is as follows using a Starlight SXVR-H9 and 80mm F6 refractor
3438 x 9/480 = 64.4 arc minutes or a little over 1 degree
2) The Resolution or number and size of the pixels on the CCD. For example The SXVR-H9 has 1392 by 1040 6.45um pixels. Compare this to early CCD camera’s which typically used CCD’s with 375 by 244 25um pixels. You can see from this that the modern SXV-H9 will provide far higher resolution images and can be used very successfully with short focal length instruments.
Conversley this sensor appears small compared to that found in a typical DSLR camera. However as many customers who upgrade from a DSLR to a typical cooled CCD Camera find - the much greater sensitivity, much lower noise, greater dynamic range and ability to make full use of narrow band filters allows a CCD Camera with a much smaller sensor to easily outperform a DSLR.
The resolution per pixel on the CCD is known as the sampling rate. The sampling rate in arc seconds can be determined by performing the following small calculation.
206265 x pixel size/focal length of telescope in mm
An example is as follows using an SXV-H9 and an 80mm F6 refractor
206265 x .00645/480 = 2.77 arc seconds
Camera’s with small pixels and lots of them, produce high sampling rates and give much better performance when used with short focal length small aperture instruments.
Remember! It is far easier to obtain quality images with perfect tracking by using a high quality short focal length and small aperture instrument, providing you use a CCD camera that provides a good enough sampling rate.
Imaging at much longer focal lengths is more demanding, particularly in terms of tracking accuracy required, and often takes much longer exposures.
3) CCD Noise Characteristics
Most deep sky targets are faint. Even the showcase targets like M42 and M31 have very faint outer components. Uncooled low cost imagers perform quite poorly when imaging faint data. High quality cooled CCD cameras are much lower noise devices and perform to a much higher level.
For example the current cameras from the likes of Starlight Xpress or ATIK using ultra low noise SOny Exview sensors have very low noise cooled CCD’s. Under typical conditions with these cameras it is not even necessary to take a dark frame and subtract it from your main image providing you dither whilst guiding
Autoguiding is a very important consideration. Due to their sensitivity CCD camera’s are capable of showing up small guiding errors with fiendish accuracy. Many imagers have found that the best results can be obtained by using an Autoguider to guide the mounting. Autoguiders come in various shapes and forms, from simple webcams to elaborate dedicated guiders. However most webcam based guiders have low sensitivity and struggle to guide on anything but bright stars.
Choosing a CCD camera with an optimised autoguider can make the difference between success and failure. Modern autoguiders are available in various forms. Guiding with a guidescope works well when imaging through telescopes with fixed and rigid optics. Current highly sensitive guiders such as the Starlight Xpress Lodestar work very well with Off Axis Guiders and using an OAG with a Lodestar is a superb way of achieving a self guiding camera and dispensing with the need to use a Guidescope.
The SBIG Self Guiding cameras now feature the guide sensor within the filter wheel and not behind the filters thus overcoming the main weakness of the original self guiding design where the guide sensor was behind the filters thus making guide star aquisition difficult at times.
5) Use Of Filters
Some of the easiest to obtain and best quality images are being taken by amateurs operating in highly light polluted locations using small aperture instruments and a CCD camera equipped with narrow band filters.
Narrow band filters allow only narrow select wavelengths of light to pass, but when used with a CCD camera that is sensitive to these wavelengths will produce very spectacular images that are completely impervious to light pollution or even in most cases moon light.
It is important therefore to choose a camera that has good sensitivity at the important narrow band wavelengths of Hydrogen Alpha (656nm) and Oxygen III (499nm and 501nm)
There are many hundreds of spectacular emission targets in the sky that emit strongly in these wavelengths and if imaged using the above filters can provide a spectacle that is almost impossible to obtain when using conventional LRGB filters or colour cameras.
Most of the modern CCD Cameras feature sensors that are ideal for imaging withg narrow band filters. The very popular Kodak 8300 and SOny 694 sensors for example are both very sensitive to the important narrow band wavelengths.
And lastly no article on CCD imaging would be complete without mentioning the necessity to choose a decent quality mount.
With todays crop of high resolution CCD camera’s, many amateurs are finding that they have much greater success in producing high quality spectacular images by employing camera’s such as the SXV series with small aperture instruments placed on good quality portable mountings.
For example, a modern CCD Camera will produce highly resolved images even at the prime focus of an 80mm aperture refractor. Place this combination onto a decent mount and well guided images are virtually guaranteed.
Imaging with longer focal length telescopes is more demanding and requires a lot of guiding precision.
Mount technology has exploded in the last few years. Decent budget astro imaging mounts like SKywatchers NEQ6 have opened up doors to amateurs who previously would not have been able to afford a mount with enough precision and load bearing capacity for deep sky imaging. Advanced mounts likw the belt driven Avalon range or the encoder based 10 Micron range are at the forefront of modern mount technology and have the precision to cope with longer focal lengths.
Likewise Paramount have been the leaders in ultra high quality mounts for astro imaging for some years and their current MX and forthcoming MEII mounts are superb in every respect.