TAL 1 User Guide
This user guide was written by David Randell for the 110mm TAL 1 Newtonian telescopes available for loan to society members. Although fairly specific, much of the content is relevant to all small telescopes. A Review of the TAL 1 can be seen in the Buying a Telescope section.
These notes are intended to supplement the existing information contained in the manual that comes with this instrument. If you have any comments on the content or further suggestions that you think a novice or newcomer using this telescope might benefit, please use the appended blank sheets or add some additional sheets yourself. At periodic intervals these notes will be revised to take these comments and ideas on board so that we can ensure members can get the most out of this instrument.
A separate observing logbook is included. Please use this so that members using this telescope can really begin to see what can be observed with this instrument, and who to contact if some object logged by another simply cannot be located. Similarly a scan through the log book will bring to someone's attention if some object another could not find or some feature they could not see, was observed. Often comparative observations mean different telescopes have been used, so its difficult to see what one might realistically be able to see, or image.. In this case, however, we have the same instrument, so any record of observations would be of immediate interest and will help members in deciding what to observe, and what to expect.
The main emphasis of these notes lie towards the practical hands-on aspect of astronomy. You can supplement or research subjects of interest by first checking out the references and recommended software. To this end we have also included a planetarium program that can be used to help you plan your observing sessions.
3. The finder
10. Finder Charts
11. Coloured filters
13. Glasses wearers
The three feet bolt to one end of the pier, and the equatorial head screws into the the other end. The latitude has been set for Stratford and its environs. In use try to find a level region of ground before polar aligning, and once you have polar aligned, mark the position of the three feet so that you can set up again very quickly in the same spot. For ease of transportation you may find that temporary removal of one of the three feet is sufficient to negotiate restricted passageways or whatever.
The optical tube assembly (OTA) fits to the mount via a felt-lined cradle. Moving the tube while secure in the cradle will not scratch the paintwork. For ease of tracking, remember to balance the telescope first. Simply slide and adjust and lock the telescope in the cradle until the OTA can be freely swung through Declination and remain in balance. Similarly adjust the counterweight on the polar axis so the OTA can be similarly swung through Right Ascension. Basically you want to be able to unlock the RA and Dec axes and be able to swing the telescope into any position, let go of the tube, and make sure that the telescope maintains that position as far as is practically possible. The two large knobs are for fine adjustment in RA and Dec, the other two much smaller knobs are for locking down the axes.
The telescope can be rotated in this cradle during one's observing run. This is an intended design feature. As the telescope is swung to different locations in the sky, the tube effectively rotates with respect to the ground, thus sometimes putting the eyepiece in an awkward position. By simply easing off the cradle tension and rotating the tube, it makes the eyepiece easier to get to. The best single position to lock the tube in the cradle is to align the tube axis and polar axis so that they are parallel, then rotate the tube in the cradle so that the focuser points straight up. That is to say in this position to look through the eyepiece you would stand over the tube and look straight down.
There is another benefit of this feature. The diagonal mirror in held in place by a four vane spider mount. The vanes create a diffraction effect and can be seen when looking at bright stars with low to medium magnifications – one sees a star with four radial spikes – just like the archetypal star. If you are trying to see a faint companion star or satellite of a planet that happens to fall on a diffraction spike, it may be rendered invisible. But by rotating the tube one can 'rotate' the spikes around the object and thus making the otherwise invisible or obscured object easier to detect.
The collimation has been checked and the telescope should not go out of alignment in normal use. Obviously, sudden knocks are to be avoided! If the collimation is known to be out, or your images are always poor, or you are simply not sure, bring the telescope along to the society and ask for advice. Basically, looking through the focuser and without the eyepiece in place, you should see reflections of the perimeter of the main mirror and the diagonal mirror and the perimeter (in this case the one end) of the focuser tube all concentric to one another, and with the reflection of your eye in the centre.
You can check this under the stars too. Centre the telescope on a bright star and use a medium magnification. Then slowly defocus, then re-focus again while watching the image in the eyepiece. When defocused you should see a circular patch of light form with a central black hole (formed by the shadow of the diagonal mirror) together with the shadow of the four spider vanes that hold the mirror. The black hole should remain in the centre right through to focus. If the black hole is way off-centre, misalignment is indicated. If the scope is misaligned you will not get good images – do not be satisfied with anything else, get it collimated...
The finder has been pre-aligned with the scope, so no additional adjustment should be necessary, apart from fine tuning the focus by rotating the eyepiece. If the finder goes out of alignment, simply get a bright star centred in the eyepiece, and then re-align the finder to centre the star on the cross hair using the set of six adjustment screws. The finder has an 8 degree field of view and produces an inverted image (as does the main telescope). Enclosed in these notes are a few finder charts specifically matched to the finder field of view, use these to home in on your first elusive targets if you are new to astronomy. To match the orientation with the field of view through the finder, first compare the region of the chart with the general area you are interested in using the naked eye and then simply rotate the chart through 180 degrees, before matching the central region in the finder to that shown in the central circular region – see below.
Figure 1 gives you an idea of the relative field of view for the finder and the supplied 25mm eyepiece. Its useful to compare these at the telescope to give you a clearer idea of their relative sizes – use a rich area of the sky for this, e.g. the Pleiades (M45) is a good target to give you a clear sense of scale.
Figure 1 – The field of view of the finder (8 degrees – large circle) compared with that seen the through the telescope using the supplied 25mm eyepiece (1.6 degrees – small circle). The cluster of stars shown to scale is the Pleiades (M45) in Taurus.
The telescope uses an equatorial mount. This type of mount uses two axes perpendicular to one another, the one inclined at an angle to the ground called the polar axis, and which corresponds to your angle of latitude. By inclining the one axis thus, and pointing it due North – it will point to that point in the sky where all the stars appear to rotate around – namely the celestial North pole. Once set, the telescope can be made to track the apparent motion of the stars across the sky, by now rotating the telescope about this single axis as the stars slowly circle overhead. This set up makes it much easier to track objects in the eyepiece than the alternative alt-azimuth mount so popular with user of terrestrial telescopes. Here the corresponding motion is a series of step movements, and where both axes (as opposed the single axis of the equatorial mount) have to be continually adjusted.
The equatorial mount opens up the possibility of astrophotography, and finding faint or other wise difficult objects using a set of setting circles (supplied with this scope). Once set, the user can 'dial up' the object by first getting its co-ordinates from a star atlas, and then rotating the telescope about the two axes until the pointers on the graduated circles align. It also makes it much easier to re-find some object you may have been looking at, and which has temporarily moved out of the eyepiece field, since all that is required is movement to the polar axis only, and which will centre the object in the eyepiece once again.
The process of aligning the mount so that the polar axis points to the North celestial pole is called polar alignment. In the Northern hemisphere we have one bright star currently fairly close to the pole, namely Polaris in the constellation of Ursa Minor, the Little Bear – see Figure 2.
Figure 2 – Finding Polaris: use the Plough asterism – part of Ursa Major (abbreviated as UMa) – and follow the pointers to Polaris – in this case the circled star to the right of the figure, and forming the tail end of the constellation Ursa Minor (UMi).
We can then use this guide star to polar align. The easiest way to do this is to first study the finder filed for the region around Polaris – see Figure 3. This view matches the finder view of the telescope. Polaris is clearly marked with a line connecting Polaris to another star, and that star to another (but not visible here). Obviously this line is a visual aid only, and part of one of the classic line figures used by astronomers to link together the bright constellation stars to make specific groupings of stars more obvious. What you see here is part, and the 'tail end' of the constellation of Ursa Minor – cf. Figures 2 and 3.
Figure 3 – Finder chart for polar alignment. Note the central triangle of fainter stars that contain Polaris. The celestial pole is very close to the centre of this triangle – aim for this spot.
To polar align the telescope you will first need to identify Polaris in the sky, then level the mount as much as possible while pointing the polar axis roughly in the direction of Polaris. Then get Polaris in the finder scope, and matching the view in the eyepiece with that shown in Figure 3, adjust the altitude of the mount and gently move the mount to the left or right, until you (a) match the field in the drawing, and (b) are able to see the bright stars 'rotate' forming virtual circles in the eyepiece field as you freely rotate the scope about the unlocked polar axis. When done, you are polar aligned. You can of course simply, roughly point the polar axis to Polaris for a very quick rough and ready alignment, but you may well find that you will have to make adjustment to both axes. Its up to you, and what you intend to do, e.g. do I simply want a quick look at a few objects without using the setting circles, or do I want to track down that elusive object or do some guided astrophotography? If the latter appeals, accurate polar alignment is required.
Use low powers first, then increase the power as necessary. With lower powers you'll see a larger portion of the sky (or real field of view) and a correspondingly smaller field of view as you increase the magnification. Remember also the higher the magnification, the more you will have to adjust the telescope to keep the object centred in the eyepiece owing to the diurnal motion of the stars, and the more atmospheric turbulence will be seen to affect the image.
The supplied eyepieces fit into the barlow lens and in use one first pops I the barlow lens, then inserts the eyepiece. As a guideline use the following magnifications, based on the supplied eyepieces and barlow lens.
Wide fields under dark skies (large galaxies, Milky Way, diffuse nebulae, open clusters)
Unless you're handy with maths or already have used a telescope, one of the biggest unknowns is anticipating how big an object appear in the eyepiece? The planets for example subtend very small angles, while some loose clusters and other deep sky objects subtend much larger angles. Below are a few classic targets – the Pleiades in Taurus, the Moon, and Jupiter – scaled to show the corresponding regions of sky for different eyepiece/barlow lens combinations. This will give you some idea of what to expect.
In Figures 4 and 5, the four circles centred on the object (working inward) respectively correspond to the real field of view using the following magnifications and supplied eyepiece set: 32X (25mm eyepiece), 54X (15mm eyepiece), 96X (25mm eyepiece + 3X Barlow) and 161X (15mm eyepiece and 3X Barlow). In Figure 6 only the innermost circle is shown owing to the relative size of the planet – as I said the planets will not appear very big in the eyepiece!
Figure 4 – The Pleiades
Figure 5 – The Moon: shown in phase – the dotted line representing the part of the Moon cast in shadow.
Figure 6 – Jupiter and its four main satellites
For the planets use as high a power as practically possible, though centre the object with a low power first. Jupiter will show its four main satellites and some banding on the ball of the planet, and Saturn its main ring system, faint banding, the shadow of the rings on the ball of the planet, shadow of the ball of the planet on the back of the rings and a few satellites; but in both cases the images will be small. Mars, although typically much smaller still, should still show large scale surface detail and the polar caps when well aspected. Uranus should show a very small greenish disc, but Neptune will appear almost star like. The Moon on the other hand will easily fill the eyepiece and show a considerable amount of detail – mountain ranges, valleys, craters, rilles, lava flow marks and so on. But remember, for some faint objects and especially for faint planetary markings, one has to be patient and learn to observe. And of course it also depends in part on the acuity and sensitivity of your eyes too.
For deep sky, do not religiously keep to low magnifications if your skies are light-polluted. If the object can be framed in the eyepiece, use a medium or high magnification – this will increase the contrast, will show fainter stars and can often bring out some faint detail that would otherwise be invisible. The best policy is to try different eyepiece combinations and see what works for you. Remember to use averted vision if the object is faint, i.e. look slightly to the side of the object but concentrate on the target. This will optimise the more light sensitive regions of the eye and assist detection. Another trick is to hold the eye as still as possible and fixate on the object, and allow the eye's natural short integration time to 'build up the image'. This is best seen when looking at globular clusters, where suddenly outer stars will appear to resolve from out of the stellar matrix.
Be Very Careful! Best to project the image keeping the eyepiece in-situ using the supplied solar projection screen. Either use the off-axis aperture mask, or if using the full aperture, keep the viewing session to very short periods. This reduces the heating up of the eyepiece where the heat is concentrated, and minimises the chance that the cement holding the elements will not melt! Remember to cover up the OG of the finder scope, you will not need this.
Never ever attempt to look through the telescope at the Sun under any circumstances – even if the Sun is very dim from being obscured by thin cloud or fog. Value your eyesight – you will not get another chance! The scope came with a solar filter, but following general recommendations, this has been discarded in the interest of safety.
To safely observe the Sun first put on the end cap and pop in the 25mm eyepiece. Put the solar projection screen on the end of the polar axis and align the telescope and tray so that the eyepiece points directly to the tray and is perpendicular to it. Then to align the scope simply point the telescope in the general direction of the Sun and look at the shadow cast by the OTA on the ground. Aim to get the shadow formed by the tube to be as small as possible and forming a perfect circle. This will mean that the axis of the tube is pointing directly to the Sun and the telescope will be more or less aligned. Then remove the end cap. With the solar image now projected on the screen, simply adjust the focus until you get a sharp image, and track using the slow motions if necessary. The clips on the tray are for holding paper or card. With this you can plot the positions of sunspots and another other solar detail visible. You can also photograph the solar disk projected onto the paper or card using this method.
Finder charts for a few classic objects are appended: The Pleiades (M45), the Orion Nebula (M42), The Andromeda Galaxy (M31), the Beehive Cluster (M??), the Ring Nebula (M57), and a few classic double stars: Mizar and Alcor, Albeiro...
Use these for planetary viewing. Some swear by them, others not. Try them out on a few planets and see whether it helps you single out particular features of interest. The grey filter is a Moon filter and used to reduce the brilliancy of the image at low powers if need be. All the filters clip over the front of the eyepiece unlike most other commercially sold filters that screw into the end of ones set of eyepieces.
You can use the telescope for simple guided astrophotography. In this case one uses the telescope to guide one while making the exposure. The camera fits (via a standard 1/4 20 threaded screw) onto the supplied bracket, which in turn threads onto the end of the polar axis. It will be best to use a 35mm SLR camera with say a 28mm, 50mm or 135mm lens and cable release. You will need to experiment to see what camera/lens combination works the best for you. Apart from differing sky conditions and films used, cameras and lenses vary in weight (thus affecting the balance) some users will better polar align than others, some will manage to track better, be better at minimising vibrations, and be more patient than others. All these factors will affect the net result. Just remember that the longer the focal length of the lens of the camera, the smoother and more accurate one needs to be with the tracking.
You will first need to polar align the telescope with the camera attached and make any necessary adjustments with the counterweights for balance. The plate can be easily locked so that it runs parallel to the axis running through the telescope; this made easier by resting a rule on the plate and eyeballing the rule and top of tube so that they appear in line.
Next point the telescope to some pre-determined object, and then align the camera so that the optical axes of both are now coincident – i.e. the target in the telescope is centrally place din the camera viewfinder. Put the reticule cross-hair in the 15mm eyepiece, and the eyepiece in the barlow lens. This will give you a tracking magnification of 161X. Pop in the eyepiece/barlow unit. Centre the cross hair on a reasonably bright star close to the centre of the field you are interested in. Defocus slightly until you can clearly see the cross hair standing out and centred on that guide star. Open and lock the shutter, and then gently track using the slow motions, keeping the reticule centred on the defocused star for the duration of the exposure. When done release the shutter and log the details, i.e. the camera and lens used, film and exposure used; and of course the subject or RA and Dec co-ordinates of the object.
If you use prescription glasses, find out if you suffer from astigmatism of the cornea. If you do and its marked, you may need to keep your glasses on all the time when observing. But typically most persons suffering from a tad of astigmatism can see decent images through the telescope without their glasses when using high magnifications. This is because only a very small part of the deformed cornea is actually intercepted by the small cylinder of light as it leaves the eyepiece, and so reduces the distortion.
To see if you suffer from astigmatism, take your glasses off and hold your glasses away from your face at, say reading distance. Look through each lens in turn and through the lens at some square shaped pattern. Rotate each lens back and forth about its axis. If the perimeter of square pattern appears to wobble and move in and out and distort as you do this, you have astigmatism for the corresponding eye, otherwise not. If you simply suffer from long or short sight in its pure form, you can get by without glasses period when looking through the telescope – just remove your glasses and adjust the focus as per normal.
If you have not already got a star atlas, get hold of a copy of Norton's Star Atlas 2000.0 or a copy of the Collins Pocket Guide: Stars and Planets. Both will give you a lot of information. You may well find the you can get hold of a copy of Nortons (albeit not the most recent edition) from you local library. This has a very reference section – packed full of useful practical and background information; while the latter is an excellent pocket guide to the stars.
See the Software section for information about obtaining astronomical software.