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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.
Basic’s
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.
For example the Starlight Xpress range of cooled camera’s 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.
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 an SXV-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 SXV-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.
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 has a sufficiently high resolution CCD. 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 SXV series of Starlight Xpress camera’s have very low noise cooled CCD’s. Under typical conditions with an SXV-H9 it is not even necessary to take a dark frame and subtract it from your main image as this camera has incredibly low dark noise. The SXV series also feature’s a USB 2 interface that allows these camera’s to have amongst the lowest read noise of any off the shelf CCD camera. This is very important as most imagers will work by co-adding may shorter exposures together, rather than take one single long exposure. And the degree of read noise inherent in the CCD camera will significantly effect the quality of the co-added resultant image.
4) Autoguiding
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. The Starlight Xpress SXV series of camera can be used directly with the excellent SXV guide camera. When controlled by programs such as Astro Art or Maxim DL this guider is amongst the most accurate and easy to use of guiders on the market.
Most of the SXV series and the MX series of Starlight camera’s can also use the self guiding Star 2000 technique. Where the guiding is undertaken by the main imaging camera. This technique can work very well when imaging with long focal length telescopes.
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 Starlight SXV series and MX series of monochrome camera’s are all sensitive at these important wavelengths and being such low noise devices will provide very spectacular images even from highly light polluted locations.
6) Mountings
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, an SXV-H9 or the new SXV-M8 camera will produce highly resolved images even at the prime focus of an 80mm aperture refractor. Place this combination onto a decent mount such as the Vixen Sphinx or GPDX or the Meade LXD75 and you are virtually guaranteed to be producing magazine quality deep sky images in a very short period of time.
Imaging like this can be great fun and a lot less demanding than imaging at long focal lengths. Yet some of the best images published, and contained within the gallery on this website, have been taken using very small aperture instruments and small but accurate equatorial mountings. Alternatively, some amateurs have chosen to piggy back a small refractor onto the back of say a larger SCT. Again this can be a lot of fun and produce image’s of a quality hitherto unknown before the introduction of high resolution low noise camera’s.
All this is now possible because manufacturers like Starlight Xpress have introduced high resolution camera’s that work perfectly at the prime focus of small aperture instruments.
