Let’s talk equipment because to take care of the five ‘F’s we do need the gear to work with. You’re probably expecting me to start with the telescope but you’d be wrong. You don’t even need a telescope for astro-photography! What you need is a sturdy mount ……
The Mount
The most important component for successful astro-photography is the Mount. Unless your imaging system is solidly mounted no matter how good the camera/lens/telescope combination is, you will not be able to take good images. The sturdier the mount the better your images will be and all that hard work Finding, Focussing and Framing your image will be amply rewarded.
For exposures of more than just a few seconds, it is necessary to use a motorised mount that will ‘track’ your chosen object. There are essentially two kinds of mount, the Altitude/Azimuth (Alt/Az) and the Equatorial (EQ) mount. A typical Alt/Az mount is the ‘Fork’ mount so often seen supporting a Schmidt Cassegrain Telescope (SCT), and a EQ mount is normally seen supporting either a Newtonian Reflector or a conventional Refractor. However, for astro-photography, an Alt/Az mount has to be modified to mimic the actions of an EQ mount by the addition of a ‘wedge’ to tilt the altitude axis. A modified Alt/AZ mount and an EQ mount perform the same function - namely to be aligned with the rotation of the Earth around it’s axis. Why? Simply to allow the mount to move in one axis only to accurately track your chosen object.
You may be wondering why moving an unmodified Alt/AZ mount in both up and down and from left to right directions doesn’t achieve the same result? Well, it does up to point and an unmodified mount can be made to follow the movement of a celestial object across the sky. BUT long exposure images taken using a mount like this will suffer from ‘Field Rotation’ in which the image will rotate around the centre of the field of view (FOV) resulting in star trailing as you look toward the edges of the frame.
An equatorial mount is orientated in such a way that it’s ‘horizontal axis’ (in reality it is not horizontal at all but tilted at an angle matching your latitude) points at the North Celestial Pole (NCP) which is an imaginary point in space representing a point on the celestial sphere directly above the Earth's north pole. This orientation is called ‘Polar Alignment’ and the name of the axis that you align is called the Right Ascension (don’t ask!) axis.
Accurate Polar Alignment is vital to ensure accurate tracking of your chosen object and there are various methods that can be employed to ensure this. Details of these procedures will be included later but for now, refer to your mount’s manual.
Once the mount is polar aligned and you have Found you object, Focussed and Framed it in the viewfinder you need to Follow it’s movement and that means rotating the RA axis at the same speed of rotation as the Earth. This speed of rotation is one rotation per 23 hours 56 minutes and 4 seconds and is known as Sidereal Rate.
Assuming that you have a suitable gear train and motor drive, you would think that it would be easy to track an object moving at this relatively slow speed - BUT, of course, it isn’t that straightforward. Why? Well, it is simply a matter of manufacturing tolerances - no gear is perfectly formed and no matter how accurately made, the motor and gears will not move at exactly the correct speed ad infinitum. Regular errors are introduced by these imperfections called ‘Periodic Errors’as they happen over regular periods. Periodic Error Correction (PEC) can be applied to the mount to help in this regard but there will also be errors that are not periodic in nature and these will also need to be corrected. These two types of error conspire together to stop the mount from Following the chosen object correctly but a combination of PEC and ‘Guiding’ and it is vital that these errors are resolved or star trailing will result and this will ruin your images.
Guiding
There are essentially two methods of guiding a telescope whilst imaging:-
Manually using a second telescope mounted on the same mount or an off axis guider attachment and a crosshair eyepiece, making changes to the RA and DEC by pressing the relevant buttons on the mount’s hand controller in response to the apparent movement of a chosen ‘guide star’
Automatically using a second telescope on the same mount or an off axis guider attachment and a second CCD camera and control software making changes to the RA and DEC by computer control of the guide port on the mount in response to the apparent movement of a chosen ‘guide star’
The second method is highly desirable to avoid the very tiring process of squinting down a eyepiece for extended lengths of time but does require additional hardware and software to implement. Additionally, it is necessary to set up the system each session to obtain careful calibration of the control movements and this can be time consuming to get right.
So, we have a sturdy mount but what to put on it? A Telescope or a suitable Lens.
The Telescope
This article will concentrate on equipment suitable primarily to Deep Sky Objects. It may surprise you to realise that this type of imaging often requires relatively low magnification as many of the objects that we seek, although small in photographic terms are relatively large in astronomical terms. This is not to say that a long focal length is not suitable but a ‘faster’ telescope will be an advantage on many occasions. What do I mean by 'fast'? - I mean a telescope with a large aperture in relation to it's focal length.
Most types of telescope can be pressed into service for DSO imaging although Refractors with achromatic lenses are less than ideal because of the chromatic aberrations that these lenses will introduce.
SCTs tend to have long focal lengths and are often used with focal reducers to achieve a wider field of view - a very popular focal reducer being the 0.63 X.
Regardless of the type of telescope you wish to use, you will need a suitable method of attaching your camera to the focus tube of your telescope. For prime focus photography, the eyepiece is removed and the camera installed in it’s place. Many focus tubes come equipped with an industry standard 42mm male ‘T’ thread and this means that a camera equipped with a ‘T’ adapter can be easily attached. ‘T’ adapters are available for most makes of DSLR but if your focus tube does not have a ‘T’ thread on it then a different type of adapter will be required for your DSLR, namely one with a 1.25" nosepiece. Attaching an astro-CCD normally requires the use of a 1.25" nosepiece attachment usually supplied with the camera.
The Camera Lens
Earlier on I mentioned that you don’t even need a telescope for astro-photography – and this is perfectly true. Many objects are so large that only a very small amount of magnification is required to capture them in their entirety. A telephoto lens of 200 – 300 mm focal length will yield some amazing images of larger objects like star clusters and nebulae. Additionally even a ‘standard’ 50mm camera lens can be put to good use to capture wide field images of whole constellations – just try a 2 minute exposure of the Milky Way using a 50mm lens – you will be astounded at the beauty it will capture!
In January 2007, I was awarded ‘Photograph of the Month’ in Astronomy Now magazine for an image taken with my standard Canon EOS 300d and a Canon 200mm telephoto lens so it can be done!
A camera and lens can be fitted in two different ways - either directly onto the mount head or (more usually) piggy backed on top of an existing telescope. The same requirements of being able to track the object you are imaging still apply, of course, so it makes sense to attach the camera/lens combination to a telescope that is already set up and polar aligned. It is important that this attachment should be solid or vibration could be introduced which will spoil your images. I recommend making up a suitable bracket along the lines of this one.
The Camera
For the purposes of this article I will be considering two kinds of camera, a CCD (Charge Coupled Device) camera and a DSLR (Digital Single Lens Reflex) camera both used at prime focus.
CCD Camera
CCD cameras come in two flavours, mono and single shot colour. Both types require a PC and suitable control software to operate. The more advanced versions have a method of cooling the sensor called ‘Peltier Cooling’ and this is a very desirable attribute as the colder the sensor, the less electrical ‘noise’ it will generate.
Mono CCD
A mono CCD camera simply takes black and white images but by the use of coloured filters, the images can be combined using suitable software to produce a colour image. The main advantage of using a mono camera in this manner is that an increase in sensitivity is achieved over a colour CCD and, theoretically, a better resolution. This advantage is offset (especially in the UK with our fickle skies!) by the need to make three separate exposures to achieve a colour image although it is possible at a push to ‘create’ the Green channel from an existing Red and Blue filter. Imagine the frustration of collecting, say, Red and Green data only for the clouds to roll in …… Talk about the ‘F’ word!……
The most commonly used filters for generating colour images from a mono camera are Red, Green and Blue (RGB) and these three ‘broadband’ filters will produce a full colour image when combined. Often, a Luminance image is taken as well - this is just an image taken with no filter (although it is a good idea to use an infra-red (IR) filter - see below) and combined with the RGB filters to produce an LRGB image. The Luminance ’channel’ gives additional detail to the overall image. It is common to take different length exposures with each of the different filters to achieve a specific colour balance or to accentuate a particular colour. Sometimes, rather than a plain or IR filter being used for the Luminance channel, a Hydrogen Alpha (Ha) filter can be used on emission objects to enhance the red Hydrogen output of these objects.
As an alternative method, some imagers use Cyan, Magenta and Yellow (CMY) filters for a similar effect but this type of image has to be processed in a different manner.
For some special purposes, the RGB filters are replaced respectively with Hydrogen Alpha (Ha), Oxygen III (OIII) and Hydrogen Beta (H beta) to allow you to do tricolour-imaging of emission line objects even from very heavy light polluted locations. These alternative ‘narrowband’ filters cannot be used for non-emission objects like galaxies etc.
Single shot Colour CCD
A colour CCD is actually just a mono CCD with a thin layer of filters coating the individual pixels, laid out in specific pattern known as a 'Bayer Matrix'. In this matrix, there are repeating groups of four filters arranged in a square - one Red, two Green and one Blue filter in each square block. Special software is then used to decode the Red, Green and Blue data to construct the colour image.
A CCD camera is usually over-sensitive to infra red (IR) radiation so it is common to use an IR filter with a sharp cut-off at the ‘visible’ light end of the spectrum to avoid ‘light splatter’ on both a colour CCD and for the Luminance channel in an LRGB image.
Digital SLR
A Digital Single Lens Reflex (DSLR) camera is essentially a single shot colour camera in a self contained case. However, the sensor itself may well be a CMOS device rather than a CCD device but from a user’s point of view, although the two technologies are rather different, the way they are used is identical.
The Canon range in particular have proved very popular with the astro-photography fraternity despite their use of CMOS technology which theoretically is less suitable than CCD technology for astronomical use but this does not appear to be borne out in the excellent results achieved using Canon cameras.
All the information above pertaining to the single shot colour CCD applies with the exception of the need for a PC to control it (but more about this later) and the fact that these cameras already have an IR filter installed on top of the sensor.
DSLR cameras generally have larger sensors and more pixels than pure astro cameras and this fact can be put to good use in the taking of wider FOV images than most affordable CCD cameras would allow. Being self contained, they are quicker to set up in the field.
DSLR cameras ‘off the shelf’ represent extremely good value for money but unlike an astro CCD camera they are not optimised for astronomical use as they are not cooled and they tend to have rather ‘wide’ IR filters which tend to attenuate the important red portions of the light from emission objects like many nebulae. However, a DSLR will still take some amazing astro-photographs despite it’s apparent limitations and for may people it is all the camera they will need.
Because of the design of a DSLR, there is a risk of vibration from the mirror that flips out of the way during exposure upsetting long exposure photographs. A desirable feature to have on your DSLR is, therefore, ‘mirror lock’ which locks the mirror out of the way of the light train several seconds before the shutter opens. This ensures that any vibrations caused by the mirror’s movement will have been damped out naturally before the imaging itself takes place.
There are improvements that can be made to a standard DSLR and several companies offer the service of removing the wide IR filter and replacing it with a more astro-friendly narrow filter but this service is rather expensive and will invalidate your original guarantee. The resulting images from these modified cameras are excellent with much improved sensitivity to the red portion of the spectrum.
Unlike an astro-CCD, a DSLR does not need to be connected to a PC to be operated although there are some good reasons why this is a desirable way of controlling the camera. Using the right software, very flexible camera control can be achieved using a PC. Remote connection can assist with ensuring accurate focus and scripting the taking of multiple images during a session without manual intervention.
Of the two functions that a PC can carry out with a DSLR, focus is the most important in my opinion as focus is one of the prime requirements for a good astro-photograph. By using suitable software a PC can make focussing a DSLR a finite process.
Now you’d think that focussing a DSLR would be easy, just look though the viewfinder and focus - well if you could try this but you would be very disappointed! A DSLR viewfinder is designed for daylight use but on a tiny, dim object it fails miserably! Using an astro-CCD is a little easier as the only way to view what you are imaging is by viewing a captured image and you can adjust brightness and contrast to assist in checking the focus - and that of course is what you need to do with your DSLR too! There are several software tools to assist in this endeavour but I recommend a program called ‘DSLRFocus’ - the clue is in the name ….
If you simply cannot have a PC out there with you when imaging with a DSLR, there are some alternatives. An obvious method is to focus on a very bright star by adjusting the focus mechanism until the chosen focus star appears at it’s smallest. You can then move back to your final object and start imaging. Some people use a ‘Hartmann Mask’ to assist with this but from my own experience, the severe dimming of the image that this system causes and the fact that just at the critical point of focus, the very elements that you are bringing together merge as one but are now dimmer than without a Hartmann Mask means that for me, it just doesn’t work!
No matter what process you use, good focus is paramount to success so take your time and get it right!
There is a limit to how long you can expose an image for and there are several factors that decide this for you:-
Light Pollution - the greater the light pollution, the shorter the exposures you can take before the image becomes ‘fogged’.
Tracking - the better your tracking accuracy, the longer you can expose for.
Sensor Noise - the less noise your sensor produces (and ALL sensors produce some noise!), the longer you can expose for.
Object brightness - the brighter the object, the quicker the image will ‘burn out’ resulting in an overexposed image and a loss of detail and colour.
Peripheral object brightness - as above!
You might think that not being able to image for long periods of time will mean that not enough data will be collected but there is a software solution at hand called ‘stacking’. This works by taking multiple images one after the other and then combining them using special software (some of it free - see Registax). This stacking process results in all the best parts of each individual image being brought together to make a single high quality image.
Sadly, once the stacking process is completed, it is highly unlikely that your work is finished! Post processing is where the real work and the success or failure of the image will be decided. It will also be an opportunity for you to stamp your own interpretation on the finished image. Click below for more information:-
Copyright Steve Richards 2008