As we get older, most of us find that it is increasingly tiring to observe for long periods standing up. Standing is usually quite convenient for visual use of Newtonians, but not for refractors and Cassegrain-form telescopes, which are not normally mounted high enough for this, unless the telescope is being used at an unusually low altitude. Large, long-focus Newtonians, on the other hand, place the observing position very high, and need to be used with a platform, a ladder or a specially-made very high chair. Some kind of observing chair is a most desirable comfort for any observatory - or perhaps a selection of chairs at different heights, though this might cause too much clutter in a small observatory.
In the 19th and early 20th centuries, when the vogue was for long-focus refractors, a common kind of arrangement was an observing bench or couch, a bit like a weightlifting bench, with an adjustable sloping back, and an adjustable height. Elaborate examples, such as the one still used with the Northumberland telescope in Cambridge, England, took the form of a huge wooden arc on which the observing bench moved, so as to keep the observer's relationship to the telescope constant with changing eyepiece elevation. There are few contrivances like this left in use now, and the lying-down observing position seems to have gone out of fashion, most observers using refractors and Cassegrain-pattern telescopes preferring to sit in a normal chair and use a mirror or prism diagonal (the mirror type is optically better). The advantage of lying down is that it eliminates the light-loss and possible aberration from the diagonal, and it actually is comfortable, but it is hard to arrange in small observatories.
When it comes to visually using a large, long-focus Newtonian, or a very large, short focus one, we are dealing with mountaineering, and the risks that that entails. I would not put up with a large Newtonian on an equatorial mounting that did not have an easily-rotatable eyepiece position, or a choice of eyepiece positions. Without either of these, they are just impossibly awkward. The rotation can be accomplished either by rotating the whole tube in its cradles, which usually takes quite a lot of force, and therefore has to be done before an object is centred exactly, as the operation usually knocks the centring off, or by rotating only the top end of the telescope. Reflectors that have been constructed to allow this are rare, and it is sometimes claimed that they don't hold precise collimation when the end section is rotated. I haven't experienced them.
The best system short of this is to have, as I have with my 245 mm (9.5 in.) reflector (Fig. 8.1), fairly-loosely fitting cradles (which still hold the telescope firmly enough), a band of metal (known as a slip-band) clamped round the tube above the cradles, so that the telescope can be loose in the cradles but not fall through, and handles fitted round the tube to aid in turning it. In fact, handles are a tremendously useful, but utterly simple, addition to any moderate to large telescope that is not always used in GOTO mode. With a Cassegrain-pattern telescope they should be fitted near the rear cell, to aid moving the telescope by hand. They remove the temptation, particularly with inexperienced telescope-users who you might invite to use your equipment, of moving the telescope by yanking on the focuser.
With the classical large refractors the telescope handles took the form of a metal ring, like a ship's wheel, all round the back end of the telescope. I imitated this on my 254 mm (10 in.) Dall-Kirkham Cassegrain with a piece of 10 mm (0.4 in.) diameter copper tubing, such as is used in immersion heaters, which I bent to a circle of the correct diameter (the material is supplied already curved). I then soldered the ends together with a gas-canister-type plumbers' torch and mounted it on the telescope by holding it at intervals with copper plumbing p-clips screwed to the rear cell (Fig. 8.2).
I have found a cheap, plastic, self-assembly rotating office chair to be an invaluable addition to my observatory as an adjustable-height seat, to cope with most of the height variation of the eyepiece of a large Cassegrain telescope on a German mounting. It will not go high enough to cope with low-altitude observation, and there is an awkward range of intermediate positions for which the eyepiece is too high to use with the chair, but too low to use standing. Another cheap chair, a folding, aluminium and wood, high, bar-style one, copes with these intermediate positions. It is impossible to get a normal, adjustable
chair that is usable for all eyepiece heights encountered with this type of setup. I find the plastic office chair to be very useful as well, in that it is on castors, and so I can move from the observing position to the computer desk on it, without getting up. It is also small and light. I found steel office chairs, with or without arms, too big and heavy to be convenient in my small observatory.
I also have a small, aluminium, plastic-topped folding stool which I use in my run-off shed. It is stored in the shed when the shed is closed up. It is very light, but suffers from the disadvantage that, being metal, it conducts heat away, and the top, consisting of black vinyl plastic, radiates heat away, so if you are not sitting on it all the time, it becomes covered with condensation and has to be dried with a towel. Wooden or plastic chairs or those with fabric tops suffer much less from condensation. A wet bottom is not conducive to observing comfort.
These are ad-hoc, cheap solutions. Special, variable-height observing chairs are manufactured and sold by astronomical outlets. They are quite expensive. They are usually folding, for easy transport, and attempt to achieve a much larger height variation than can be had with ordinary adjustable chairs. Ideally, an observing chair should have a seat height range from about 30 cm (12 in.) to about 1 m (40 in.). I am afraid I simply haven't come across a specialised observing chair design that is completely successful (though I haven't tried them all), and I stick to my ad-hoc solutions. This is not to say that a successful portable observing chair is not possible. I don't know if it is.
One type is shown in Fig. 8.3. This is good in that it is made of wood, and so has little tendency to get dewed-over. However, the three bare, wooden slats that constitute the seat make it uncomfortable to sit on, and the seat itself is far too shallow for any normal human bottom. Were it deeper, the centre of gravity of the observer/chair system would probably fall too far forward, and both would fall over. The height is adjustable by fitting the seat to grooves on the front piece of the A-frame. However, it is restricted at the lower end of the range by the hinge, so it will not go as low as would be useful. Towards the top end of the range it becomes unbalanced and unstable-feeling, and there is nothing to lean one's back on, and, more importantly, nothing to put one's feet on, which are left dangling uncomfortably. Though the chair folds, it doesn't easily fold flat, as the seat will stick out unless removed from the frame completely. Overall, it is a poor design.
I have seen a picture of a chair manufactured in Canada, known as the Beer observing chair, which seems to avoid some of these faults. It is similar, but the top of the A frame is much higher and the seat slots in below it, so the hinge does not obstruct a large range of movement for the seat. It also has foot rests, and looks more stable. A not dissimilar A-frame design in metal and plastic can be bought, much more cheaply, from major stores, as an "ironing chair". These have been found useful by some astronomers, but they lack foot rests. As I say, I don't know if the perfect observing chair is achievable - the problem seems so far to have defeated human ingenuity - and the most comfortable, though not the most convenient, solution may be several normal chairs of various heights.
For visual use of a large Newtonian, another option is to place a high chair on a platform, but this is cumbersome. Some observers use step-ladders to observe standing up, which will be safe enough, provided one does not have to climb up too far, but tiring. It is fatiguing to have to stand on the narrow treads of ladders for very long. Easier to use are specially-constructed sets of wooden steps, equipped with a rail or post that is gripped for balance. These, however, will be bulky and heavy, and difficult to move around the observatory. Overall, the users of long-focus Newtonians do not have an easy time, and they also suffer from a much higher risk of damaging ancillary equipment, by dropping it from lofty heights.
Once consequence of the electronic revolution in amateur astronomy has been that it has become quite common for warms rooms, for telescope and imaging control, to be added to observatories. Of course, professionals have been using these for a long time. The warm room could be in the house; it could be, as I use, an outbuilding near the observatory, or in could be designed into the observatory itself. If the latter, it had best be separated from the observatory by a short, cold corridor to minimise thermal interference. It also should ideally be placed to the north of the telescope.
Personally, I would advocate the warm room being a separate building. However, people who have constructed run-off roof observatories with warm rooms as part of the same shed, on the north side, have reported that it does not cause any thermal problems, because, with the run-off roof open, the observatory is entirely at outside temperature. Certainly, the warm room should not be underneath the telescope, as I have seen in some observatories. A first-floor observatory always creates severe problems of telescope support and mechanical isolation. A pier rising to a first-floor level is never going to be vibration-free, if it be of reasonable dimensions. Having the control room underneath adds to this problem the problem of rising warm air.
For basic remote control of a telescope, the requirements are: firstly, a method of getting pictures from your telescope to a remote monitor or computer; secondly, a method of controlling the drives at both fast and slow slew rates remotely; and thirdly, a means of controlling focus.
The pictures can come from a video camera, a surveillance camera, a modified webcam, or a CCD camera. Popular video cameras for astronomy are made by Minitron and Watec. These use an S-video connection to a monitor, and such leads can be very long. To connect them to a computer, for recording images, an electronic box known as a frame grabber is required. Most other cameras used for astronomy now have a USB 1 or USB 2 connection, but a few use FireWire (IEEE 1394). In the past, parallel and serial connections were used. As mentioned before, USB connections can have problems over distances of more than a few metres or yards. A series of passive repeater cables might work, but, if not, active boosting will be required. Another, more complicated, possibility is to set up a computer network using ethernet connections, which can be extended a long way (hundreds of metres or yards) without trouble. In this scenario, one computer in the observatory connected to the camera, and possibly also to the telescope mounting, would communicate, over the network, with one in the control room.
The way in which remote telescope control can be implemented will depend on the type of mounting and drive motors. For older mounts, and altazimuth telescopes, a bespoke system will probably be required. The use of synchronous motor drives is not really a possibility, as they cannot be slewed fast enough to be of use for remote control purposes. All other types can be adapted. The telescope could be controlled from a specially-made control box, or interfaced with a computer. A non-GOTO stepper or servo drive system could just have its handset lead extended sufficiently far to make it "remote".
For remote operation, the telescope's pointing direction, obviously, needs to be known. Therefore shaft encoders need to be incorporated into the mount, or step-counting electronics need to be used with stepper or servo drives. Attaching encoders to Dobsonian mountings is relatively easy, because of their large bearings. For German equatorial mounts that have not been designed with this in mind, it is a non-trivial problem. The systems developed by American amateur Mel Bartels offer methods of upgrading to automatic pointing, and remote control capability, almost any type of home-built telescope or simple mounting, including Dobsonians and other altazimuths. In the UK, AWR Technology are specialists in providing bespoke drive and control systems for observatory telescopes.
Most commercially-made new mountings are more consistent in their operation and capabilities, and the setting-up of a remote control link is going to be far easier with an up-to-date mount, which has been designed to interface with a computer. The main issue for most is the very high cost of computerised mounts that are capable of taking observatory-size telescopes, and that are genuinely accurate enough in their operation to make remote-control a realistic option. The leading manufacturers of such mounts are the American companies AstroPhysics, Software Bisque, and Losmandy, and the Japanese company Takahashi. All these companies produce very high-quality, heavy duty observatory mounts integrated with the latest control technologies, which work with a wide range of proprietary and third-party software. While mountings further down in the price spectrum may claim remote-control capabilities, the extent to which these can be put into practice is, in my experience, limited.
Typically, modern mounts use an RS-232 serial connection to communicate with a computer, and to update their own internal software (firmware), the latest versions of which can be downloaded from the Internet. This is curious, as RS-232 is an old type of connection, and a not very fast one, which many PC manufacturers have now abandoned. It is necessary to buy another piece of hardware, a USB to serial port adaptor, to connect mounts to many modern computers. By this means, planetarium programs, showing the sky for your location in real time, and containing the co-ordinates of thousands of celestial objects, can send commands to the mount to slew to particular objects or particular co-ordinates in the sky. If using a USB to serial adaptor, a virtual COM port is created in the system software, which is allocated a number, and this number must be set to correspond in the planetarium, or telescope control, software, and the virtual port software (accessed on Windows computers though the Device Manager). This can be a source of problems, as the computer can change the numbering of these virtual ports arbitrarily, without telling the telescope control software. If you have several adapters, it can be difficult to determine which number corresponds to which function.
Clearly, this type of remote control will only be helpful if the slewing is accurate enough. The accuracy must be higher, in arc minutes, than the field of view of your camera, or you will be permanently lost. This is the main reason why a top-quality mount, with top-quality encoders and motors, and minimum flexure of both the mount, and the optical tube, together with excellent, rigid coupling between the two, is pretty much essential for remote telescope operation. A partial way around these rigorous requirements can be to have a camera on a smaller telescope on the same mounting, acting as a digital finder or guidescope, also connected to a computer, which gives a wider field of view than the main telescope/camera combination. This is a method I have used to facilitate remote planetary and lunar imaging with a non-GOTO mount, by setting the pointing approximately in the observatory, and then repairing to the warm room to make fine corrections using the view through the guidescope. However, this is only a very limited form of remote control.
RS-232 connections, using Category 5 cables, can be reliable over distances of 50 m (160 ft) or more, making practical the control of the telescope from the house. My warm room is only a few metres from my telescopes, however, and I have found that, in my case, an easier system is to use the USB connections, already set up between the telescopes and warm room to transmit images, for telescope control purposes as well. I use a USB hub at each telescope, with the serial to USB converter, for telescope control, connected into it alongside cameras, and then have these hubs connected together at a further hub, which sends all signals, through only one wire, to the control room. This system seems to work perfectly for controlling two telescopes, one long-exposure camera, and one webcam, simultaneously. Certainly, if one went too far in this direction, one could exceed the maximum USB 2 bandwidth. However, mount control signals require very little bandwidth; it is fast cameras (webcams) which need a lot.
My system is shown diagrammatically in Fig. 8.4. The reason there are two RS-232 connections from the first mount to the network is that the Astro-Physics GOTO mount I have allows this, and it is needed to control the mount with two pieces of software simultaneously, which can be desirable (a planetarium program
and a tracking-correction program, for example). Most mounts, however, do not have this capability. It might be noted that Software Bisque's Paramount ME allows direct USB connections (so far as I know, the only mount to do this), thus eliminating the USB to serial converters, with their annoying software complication. The reason I have the second of the first-stage hubs powered, but not the first, is that I find this is necessary, as my second telescope is further from the second-stage hub than is the first, and is connected to it with two chained passive repeaters rather than one, with more signal loss.
I can place my laptop either in the observatory, connected directly to the powered hub, or in the warm room, connected via the repeater cable. The system thus allows flexibility, and only one data connection needs disconnecting and reconnecting to make the location change. Additionally, guide cameras can be connected to the hubs to provide serial autoguiding capability for the mounts (of which more later) without the need for any additional wires. This is subject to bandwidth restrictions, and software compatibility of multiple cameras connected to one computer.
If you are going to be a long way from your telescope while it is operating, you do need to be certain that various other things are not going to happen out there, unbeknownst to you. You need to be sure that the optics will not dew-up, so you need to have all the optics that can dew equipped with dew heaters, switched on. On all catadioptric telescopes you will also need a substantial dew shield. If you are using a dome, you need to know that the telescope is not staring at the dome rather than the slit. You need to know that the telescope will not collide with its mounting or with anything else in its movements, that it will always behave correctly, and that it will not get tangled up in its own wiring and break something. Ensuring that none of these bad eventualities ensue is the most difficult part of remote telescope control, particularly if you are completely isolated visually and aurally from the telescope. There are many possible mishaps or even calamities in remote telescope operation that it is difficult to predict, and for that reason, control from a nearby shed or semi-detached section of the observatory is likely to be an easier and safer option than retreat to the house. In any case, many people will wish to be secluded well away from family, TVs, radios, and other interruptions while observing.
One issue with remote operation of the observatory is that audible cues for how equipment is working are lost. One does not realise how important these are until the sounds are not present. One gets used to hearing how the drives should sound when they slew properly and unhindered, or how a focus motor sounds moving a small distance, and all these cues are incredibly useful. One idea of mine, which I have not tried, is to put a microphone, connected to an amplifier, on the telescope mount, connected to a speaker in the warm room. This might make the remote-control experience more "immediate", and could potentially prevent problems.
One important tip I have for remote-control imaging using a computer-driven German mount, which also will prevent problems, is to prevent unnecessary meridian reversal or normalisation - in fact to have reversal never happen, if possible. Most computer-driven GEMs will insist on reversing or normalising when slewing to an object on the "wrong" side of the meridian, which is just what you do not want when using a complicated system of telescopes and cameras with wiring attached to them. You do not want the wiring getting wrapped round the telescope and pier, possibly snagging, and jamming the mount or breaking something, which can occur even if the wires are well-organised, as I have recommended elsewhere, using cable-ties. In general, one will be working short distances either side of the meridian, and, provided one has checked that the required latitude of movement is available to the mount without the telescope striking the pier, there is no reason for the reversal, which, anyway, makes collimation, guidescope alignments, and slewing all less accurate.
Astro-Physics mounts and their associated software allow the meridian to be temporarily offset, so the telescope will behave as if objects past the meridian have not crossed it, or, with a negative offset, as if objects that have not passed the meridian have passed it. Some other computer-controlled mounts can have the meridian offset feature enabled through control software on a PC. The cheaper mountings on the market cannot, which severely limits their usefulness for remote operation (but, in general, their slewing is not accurate enough to allow it anyway). An exception to all this is the Paramount, which is designed in such a way as to maximise slewing and tracking accuracy, but to allow very little traverse on the "wrong" side of the meridian. The Paramount will need to be normalised a lot; however, it makes up for this by featuring through-the-mount wiring, which potentially can eliminate all problems of wires snagging on equipment.
Software and hardware does exist for controlling domes, and for co-ordinating them with telescope movements. The leader in this area is Software Bisque, which offers full options for co-ordinating their mountings with domes, with cameras, and with motorised filter-wheels for taking tri-colour images, though their software offerings Orchestrate, CCDSoft, TPoint and AutomaDome. There is a web group dedicated to sharing home-designed dome control systems. However, this level of sophistication is not essential for most observers engaged in typical observational programs still to benefit from the comfort of remote-control for much of the time.
For instance, a typical procedure of a CCD imager might be to slew his telescope to a bright star in the area of the sky in which he wishes to work, while in his dome, then move the dome by hand to the correct position, ensure that the camera, and possibly filter wheel, are in place and connected, and then retire to the warm room. In the warm room, the image can be focused on the camera chip and the star can be centred, and then, because the identity of the star is known, the control software can be synchronised exactly with the telescope's true pointing direction. The software can then be instructed to slew the telescope to the real target, say, a nearby faint galaxy, the camera exposure can be increased sufficiently to reveal the galaxy, further corrections can be made, and then the image capture can be begun with the capture software, all operations taking place on one computer. This procedure eliminates many of the possible errors in automatic telescope pointing, by restricting operations to a small area of the sky. Usually a long sequence of exposures will be taken, possibly through different filters, which will mean quite a time in the warm room before it is necessary to venture into the cold observatory again. If several targets are planned close together, this interval may be even longer.
Amateurs specialising in survey work may need complete automation for the high efficiencies they require, for example, for imaging a large number of galaxies each night to monitor for supernova explosions, or a large number of star fields to look for comets or minor planets or new variable stars. But most observers are in a more casual league, and a partial level of remote-control will be sufficient for them. Although, as I have shown, multiple telescope control and imaging operations can be performed through one laptop, having more than one computer, or maybe one computer with more than one monitor, in the control room would make things easier in terms of being able to see imaging and tracking program windows simultaneously, and not having to shuffle open program windows so much to see what is going on. Some amateurs have quite a few computers in their observatories, dedicated to specific functions.
Remote focusing is an essential, and is in fact very useful in the observatory as well, particularly for high-resolution planetary imaging, where the large image scale means it is hardly possible to touch the telescope at all, without an absolutely first-class mount, in order to avoid losing the image. Electric focusing units in some cases connect mechanically to the focusing knob shaft that is already part of the telescope, and in some cases are separate add-on units, screwed or push-fitted to the back of the telescope (Fig. 8.5). They are usually battery powered. (Consistent with my policy of battery-avoidance in my observatory, I have wired mine to work off the main observatory 13.8 V supply.)
The lead between the focuser and control box, for most electric focusers, is a simple analogue one, carrying a DC signal and terminating in 3.5 mm jack plugs, and can easily be extended a long way. Some more sophisticated focusers use RS-232 control, and these can be connected to a computer, either directly through RS-232 leads, or via USB. Some mounts (such as the Astro-Physics models) allow the DC control focusers to be connected to them, allowing mount control software to control the focus using these focusers as well. Otherwise, a separate interface is required for this.
Most focusers, whether motorised or not, suffer from a certain amount of both focusing slop and backlash. The former means that the image shifts slightly when the direction of focusing is changed, due to a slight change of optical alignment. This can be a nuisance in imaging, though for visual work it is not usually a worry. The latter means that the focus does not change immediately when the direction of travel is reversed (similar to backlash in mount drive worm and wheel systems). An advantage of the add-on electric focusers, such as the Crayford types, particularly for telescopes such as SCTs, which normally use primary mirror movement for focusing, which generates both image-shift and backlash, is that these focusing irregularities can be eliminated for imaging purposes. A disadvantage of them is that their range is small, and does not allow access to the full range of focus that is gained in the moving primary mirror system. Systems exist for motorising the primary-mirror mechanisms of SCTs, made by JMI and Technical Innovations (the Robo-Focus system), both US companies. For ideal focusing remote-control of an SCT, possibly, two focus motors would be required, one to adjust the primary mirror position, and one to make shift and backlash-free adjustments using a fine Crayford-type focuser.
Remote-controllable filter wheels are very expensive, for what they are. I don't have one, so I go back to the observatory to change filters. However, automatic filter wheels can be connected to the PC as well (via the ubiquitous RS-232 so beloved of telescope accessory makers) and controlled with software, or they
could have their own simple control box. Making one cheaply should not be beyond the powers of someone with basic engineering skills.
With most commercial mountings, below the most expensive level, it is wise not to slew them at their maximum rates, to preserve the gearboxes, which in some cases only contain nylon gears. They are annoyingly expensive to have replaced or repaired after the warranty period has expired, and always inconvenient, as this normally means sending them back to the manufacturer, who is often on another continent. One buys more piece of mind with high-end equipment, as well as more accuracy and predictability.
The true accuracy of most mass-market GOTO systems is not much better than the diameter of the full Moon, 30 arc minutes, and therefore, with longer focal length instruments, or smaller detector chips, the focal length of the imaging system needs to be appropriately reduced to always be able to locate the desired object in the field. This is done by means of a focal reducer between the telescope and camera. This lens acts in the opposite way to a Barlow or Powermate lens, causing the light rays to converge more sharply onto the detector, simulating a short focal length telescope, and increasing the field that can be viewed. Focal reducers can, however, have the undesirable side-effects of introducing field-curvature (so stars at the edges of the frame are not sharp), and vignetting (where the edges of the field are dark, because they are not fully illuminated from the objective). Common f10 SCTs cannot have their f ratio reduced below 6.3 before these effects become apparent, with moderate-sized detector chips (8 mm, 1/3 in. chips or larger).
Another element in many remote imaging setups will be a camera connected to a guidescope (similar to, or the same as, the finder camera mentioned above), which is then connected to the PC again, to allow autoguiding software to send guiding commands to the mount. These commands can reach the mount by one of two possible routes. In parallel or ST-4 type autoguiding, which is a possibility with most modern mounts, whether computerised or not, they are sent from the parallel or USB port on the computer, via an interface box, to the mount, which they reach through a cable terminated with a telephone or modem-like connector, known as an ST-4-type guide cable (after the original SBIG ST-4 autoguider). In serial autoguiding, which works with computerised mounts, the commands are sent from the serial or USB port of the computer directly to the mount controller, using the same route utilised by planetarium-software control. This is the simpler option, using fewer wires. In both cases, the autoguiding software detects slight drifts in the tracking, due to mechanical inaccuracies in the mount, polar misalignment, and atmospheric refraction, by accurately monitoring the position of a guide star on the detector of the guide camera. It then automatically sends corrections to the drive motors to keep the pointing sensibly on track. This is absolutely essential for very long exposure imaging (five minutes or more), even with the best mounts. It works best if the effective focal lengths of the main and guide telescopes are comparable.
Autoguiding can in some cases be done by the main imaging camera simultaneous with imaging, eliminating the guide scope and camera, either by dual-use of the imaging chip (the Starlight Xpress Star 2000 system), or by use of a smaller chip within the main camera (as used in the latest SBIG units). It can also be performed with a guidescope and a stand-alone camera and controller system in the observatory, like the early SBIG ST-4, STV, and their successors, which were developed originally to control telescopes for long exposure film photography, ending the wearisome drudgery, for the imager, of staring at a star on crosshairs for hours on end, making the corrections to the tracking by hand. Such standalone autoguider units are still very useful for autoguiding DSLR shots without need for a computer. For computer-based imaging operations, however, the other methods of autoguiding are more convenient.
A guide camera, or a guiding chip, differs from a long-exposure imaging camera in that it has to be able to read off its images quite rapidly, typically, once every few seconds, or more frequently, to allow the drives to be corrected quickly enough for image-drift not to become a problem at long focal lengths. It therefore requires an adequately bright guide star to work with. A guidescope is not essential for autoguiding, even without imaging camera-integrated guiding, as a guide camera can also be connected to an image-splitting unit on the main telescope, known as an off-axis or radial guider, which sends most of the light to the main camera, and a bit to the guide camera. This method, with the integrated camera methods, avoids the problem of slight changes of the guidescope pointing with respect to the main telescope, due to mechanical flexure. However, these methods run the risk of there being no adequately bright star for guiding present in the main field. A separate guidescope has the advantage that it can be angled around quite a wide area of sky in order to locate a suitable candidate.
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