In prime focus astrophotography, the telescope is used as a large telephoto lens. Prerequisites for prime-focus astrophotography are:

• a telescope on a motor-driven equatorial mount;
• a guidescope , an off-axis guider, or a self-guiding CCD camera;
• an illuminated reticle guiding eyepiece or an autoguider (unless using a self-guiding camera);
• a single lens reflex camera  or a cooled CCD camera; and
• all necessary mounting devices to rigidly connect all of the components together.

If using a digital SLR camera, it is recommended that the camera be powered from a large external battery to avoid the internal batteries dying during an exposure.

Planning

Prior to any photography session, it is necessary to spend time in planning. Many deep-sky objects will not be visible in the finderscope or on the focusing screen of the camera, so the image must be centred using star patterns taken from a good star atlas or a planetarium computer program. Print out the star field with the camera frame located in the intended position, or if the program does not do this, draw the camera field of view to scale by hand so that you can refer to it during the photography session. Because of the length of exposures needed to achieve quality astrophotographs of most dep-sky objects, it would be unusual for more than two objects to be imaged during any one session, and sometimes a single object could be imaged over two or more nights, particularly if imaging time is limited by moonlight or clouds.

If more than one object is to be photographed, estimate the time at the start of each set of exposures based on the planned length of the previous exposure and about 10 to 20 minutes between exposures to allow for locating the object, finding and centring a guide star and refocusing when necessary. Preplan the use of filters and the duration of all exposures. WRITE EVERYTHING DOWN. If, when your images have been printed, you cannot remember what you did, you won’t know why they turned out to be so good or so bad.

Always have spare batteries on hand for any piece of equipment that relies on battery power. Photography without an illuminated reticle in the guiding eyepiece is impossible and the absence of illuminated crosshairs in the finderscope will make centring objects difficult. The same goes for a battery-powered telescope drive.

Alignment

An absolute requirement for prime-focus astrophotography is precise polar alignment. Imprecise polar alignment requires frequent tracking corrections and large corrections will lead to poor guiding accuracy. It also results in field rotation if the misalignment is more than minor. If your telescope mount has an accurate polar alignment scope, then reasonable alignment will be readily achieved. If not, the star drift method should be used.

The star drift method of polar alignment is both quick and easy (or reasonably so). If you can visualise the path in the sky followed by the telescope, and the path of the stars for the telescope pointing towards the eastern horizon and pointing towards the celestial equator at the zenith, you wont need to memorise any rules, as corrections become obvious. If not, then remember that:

• for the telescope pointing towards the eastern horizon (15 degrees altitude is acceptable), if a star drifts north in the eyepiece, the telescope declination axis is pointing too low, and if the drift is south, the axis is pointing too high. (Reverse these if looking towards the western horizon.)
• for the telescope pointing towards the celestial equator near the zenith, if the star drifts north, the telescope azimuth is pointing too far east, and if the drift is south, it is pointing too far west.

Keep making corrections until there is virtually no north-south drift in a five-minute period.

To use this method it is necessary to know which direction is north or south in the field of view. The best way of finding out is to nudge the telescope about its declination axis towards the south whilst looking through the eyepiece. Stars will appear to move north.

Comfort

Another requirement if manually guiding is comfort. Mosquitos can be a distraction so having access to insect repellent is useful and nights can be cold – even those that start out mild – so, apart from coastal locations in the summer months, warm clothing will be necessary. Crouching or stretching to get your eye in line with the eyepiece will not result in the best-guided photo. Sitting in a good posture is ideal, however, if this is not possible, then standing erect is almost as good. Also, give your eye a break now and then (but hopefully not when the mount is at an inaccurate spot on the gears). By positioning the timer where it is easy to see, you will always get to look away from the eyepiece for a moment as you check on the number of minutes to go. (Time can seem to go very slowly at times.)

Focusing

It is not possible to regularly achieve perfect focus using the unmagnified focusing screen of an SLR camera. When you do focus perfectly, it is more due to good luck than to good management. A magnifying right angle viewfinder attachment will help and some SLR cameras have the ability to display a highly magnified view of part of the image, which allows reasonably accurate focusing. However, the most consistently accurate focusing method is by an automated focusing routine using a computer to control a motorised focuser on the telescope.

If focusing by eye, always aim at a reasonably bright star at least 30^o above the horizon (to avoid too much scintillation) and ensure that it is not a double star. Having achieved a good focus setting at the start of a session, you cannot assume that focus will be maintained over the next three or four hours of photography. At least two things may cause loss of focus. First is mirror shift (in a Newtonian or a Schmidt-Cassegrain telescope). Second is thermal expansion or contraction. If you focus at, say, 7 pm when the temperature is 18^o and by 11 pm the temperature has dropped to 3^o, the telescope tube will have contracted enough to cause loss of focus, particularly if it is aluminium, and especially if it is a Schmidt-Cassegrain. In a refractor, the tube will shorten with a drop in temperature but the focal length of the lens may lengthen, often resulting in an increased rather than a reduced focal length. A thin aluminium tube will respond quickly to temperature changes but a doublet or triplet lens will continue to cool for a long time after the temperature may have stabilised. Even a one degree change can affect precise focus. The focus tolerance for an f6 system is about 0.07 mm – well able to be reached by thermal movement or mirror shift.

In summary, if you do not want your stars looking like blobs, or worse still, like doughnuts, then you must make the effort to achieve near-perfect focus and remember to check and if necessary, to refocus from time to time.

Guiding

Along with focusing, guiding is essential to creating a good astrophotograph. People have been known to use hand-operated slow motion controls to guide during long exposures but it isn’t something you would want to do too often and the resulting photos would be the sort you would look at when nobody else is around! A good quality drive and drive corrector are essential, along with an illuminated reticle eyepiece. An autoguider is even better.

Although it is preferable to have electric drives on both the right ascension and declination axes, hand operated slow motion control of the declination is acceptable if good polar alignment is achieved requiring a small correction only every four or five minutes. Great care must be exercised in making corrections, as it is easy to cause unintended movement in excess of that being corrected, particularly if the mount is not very sturdy.

The number one enemy of good guiding is flexure – flexure of any part of the telescope and guiding system relative to the path followed by the light that falls on the detector. During a 20-minute exposure, the right ascension axis rotates 5^o, which is enough to alter the gravitational forces acting on every part of the assembly. Movement could occur from flexure of the telescope tube, flexure of brackets holding a guidescope, flexure of the guidescope, flexure of a mirror mount, flexure of the rack and pinion focuser, flexure of the off-axis guider, etc. Flexure of brackets is probably the most common source of the problem.

The best way to overcome flexure is to use an off-axis guider or a self-guiding camera. Each uses light picked up from the edge of the beam outside that which falls on the electronic detector. Advantages of these methods are:

• the full telescope aperture is used for viewing the guidestar which means a fainter star can be selected, thereby increasing the number of potential guidestars; and
• the hassle of centring the object in the camera viewfinder, then struggling with adjustment screws to move the guidescope to a suitable star – an exercise that can sometimes nudge the object off centre or right out of the picture - is avoided.

A disadvantage if you are using a Schmidt-Cassegrain telescope is that the guidestar images at the edge of the field are like blurred seagulls, not the crisp, round images you would like to see. Also, when photographing from the city where the low contrast between the bright sky and stars eliminates faint guidestars, finding a suitable guidestar will often dictate an off-centre image.

Having composed the photo on the camera screen, found a suitable guidestar and, if manually guiding, adjusted the two axes of the telescope until the star is central in the guiding eyepiece, the task of guiding begins. (It is assumed that an autoguider is not being used.)

Most illuminated reticle eyepieces have a 0.2 mm square box etched in the centre, often as two pairs of 0.2 mm spaced lines at right angles that cross the centre. Keeping the guidestar in that box is sufficient if photographing through a 135 mm telephoto camera lens but most certainly not sufficient for prime-focus photography through the telescope.

It has been recommended that star images should not trail by more than 0.04 mm on the detector. This can be represented as an amount of drift off line using the formula S=AF/57.3 where S is the image size (in this case the drift), and F is the focal length of the telescope. The number 57.3 is simply 180/pi, to convert radians to degrees. For example, for a 1500 mm focal length telescope, and after rearranging the formula, the drift A = 57.3 x 0.04 / 1500, that is, 0.0015^o or 6 arc seconds approximately. Now the same formula can be applied to find the size in arc seconds of the 0.2 mm square box for the focal length of the telescope used. If a guidescope is being used, this will most likely be different from the photographic focal length. In this example, we will assume that an off-axis guider is being used, therefore the focal lengths will be the same. Thus, the box size A = 0.2 x 57.3 / 1500, that is 0.0076^o or 28 arc seconds and our maximum drift is therefore 20% of the box. Guiding within such a tight tolerance is made practicable by the magnification of the eyepiece.

Guiding is necessary because no telescope drive is perfect – even the best observatory telescope. A good mass produced drive may have a tracking error of 20 arc seconds or more and a bad one, 120 arc seconds. The “best” tracking error is one that varies in a sinusoidal fashion; the worst is one that makes sudden periodic excursions.

Always have the brightness of the illuminated reticle as low as possible in order that the star stands out clearly and so that, if the star goes behind one of the cross-hairs, you can detect the glow even if not the actual star image which, for a faint star, is tiny compared to the 0.02 mm or thereabouts width if the etched line. Do not guide using an out of focus star, as the small drifts that must be corrected are easy to detect only if the guidestar is well focused.

There are many things that can go wrong. Most astrophotographers have, at some time or other, guided carefully for 20 or 30 minutes only to find that the camera shutter had been set at 1/125 second or a similar brief time rather than at the 'B' setting! Clouds can be a nuisance when they move across unexpectedly, as can trees when the object being photographed sinks behind a branch before the exposure is complete. Aircraft, with cabin lights on and landing lights blazing, will ruin any photo if they cross the field of view. When things go wrong, cut your losses and start again.

To begin an exposure, first hold the hand controller so that pressing the right hand right ascension control button moves the guidestar to the right, then position the sharply focused star in a corner of the box, start the timer, open the shutter and lock the cable release and you are on your way to creating what could be a magnificent photograph.

Results

Eta Carinae

Helix Nebula

Omega Centauri

Orion Nebula

All images by Max Kilmister.