Solar Filters. Never use eyepiece-fitting solar filters; they can overheat and fracture in use. This could cause blindness always cap or remove your finderscope for solar observing. There are three safe ways to observe the Sun with a telescope that is not specifically designed as a Solar Observing telescope, such as a Coronado PST or a Lund:
1. A full-aperture solar filter made of a material specifically designed for the purpose. It must be checked for pinholes, scratches or other damage every time it is used and must be attached so that it cannot accidentally come off.
2. Projection with a refractor or reflector telescope. Do not do this if any part of the optical train is housed in plastic. Never project with a catadioptric such as a Schmidt-Cassegrain (SCT) or Gregory-Maksutov (Mak) telescope; they can easily overheat.
3. A Herschel wedge. Ensure that you know how to use it properly.
Magnification Claims. Astronomical telescopes are designed to collect light; it is therefore aperture, not magnification, that should be your primary interest. Claims of 650x for a 60mm telescope are unrealistic and are an indication that the telescope is not a proper scientific instrument. You will rarely use a telescope at a magnification that exceeds its aperture in millimetres (i.e. 200x for an 8" aperture), and even that will require good sky conditions. You may exceptionally use it at twice this magnification, e.g. for double stars or for critical planetary observation, when the atmospheric conditions are ideal.
Research your purchase thoroughly. The best telescope for you is the one that you will use the most. If it is too heavy or bulky, takes too long to set up, or is awkward to use, you will end up not using it very much. Ideally, try before you buy. Your local astronomical society ( many UK ones are listed at http://www.fedastro.org.uk ) may have observing evenings where you can usually try out different telescopes and have their owners tell you about their characteristics. If you cannot do this, solicit advice from astronomy forums. However, there is no such thing as an "ideal" telescope, but there are many that can come close, so don't fall into the trap of never owning one because you can't find the elusive "ideal".
Testing Used Astronomical Equipment
Remember: No telescope will ever be perfect, but anything that is irritating indoors or in daylight will most likely become infuriating under the stars.
1. Give the telescope a thorough visual examination. Look for scratches on lenses and mirrors (best seen by looking at a shallow angle). Some scuffs on telescope tubes are normal, but excessive scuffing my indicate poor care. Dents are signs of impact, and it may be more than the tube that is damaged, especially if the dents are near the lens or corrector cell, primary mirror cell or spider or focuser. If there are dents near any optical part, ask the seller to remove the optical part so you can examine it for damage: what starts as a small crack hidden by a clip can develop into something much worse over time. Check that all bolts, mounting plates and accessories (such as finders) are securely tightened: if anything is loose, find out why and ensure that it can be tightened.
2. Ask the seller to give the telescope a good shake (explain that it will be taken over bumpy roads to your observing site). Listen for any rattles and investigate their cause. Primary mirror cells in catadioptrics are normally slightly loose, but this should not be excessive.
3. Check the focuser for smooth travel. There should be no tight or sloppy regions, and a minimum of backlash (zero, if it is a Crayford). Check for grease which may be disguising poor mechanics.
4. There is no substitute for star-testing the optics, but this is not always possible, in which case, use a high contrast distant object (such as a TV antenna against a bright sky). Make sure that you can achieve focus with any eyepiece you intend to use with the telescope, and that it "snaps" to focus: a range of "almost-but-not-quite focused" indicates poor optics. If the telescope is a refractor, check for false colour. Check for field curvature by moving a focused object to the periphery of the field of view.
1. Give it a visual inspection then check it for rattles, slackness, etc.
2. Ensure that any slow-motions operate smoothly, with no stiffness or slack spots, and a minimum of backlash.
3. Ensure that the clamps do actually clamp the shafts.
4. Ensure that you can balance the telescope on the mount. With the clamps slackened, you should be able to move the telescope smoothly to any orientation, then leave it without it moving.
Check for slackness, particularly where the legs attach to the head and to any spreaders/accessory trays.
1. With the telescope set up on the mount and tripod, use a medium-power eyepiece to focus on an object and give the telescope a sharp tap. Does the vibration settle down within a few seconds?
2. Ensure that all parts of the sky are accessible without the telescope tube or slow-motion handles fouling the tripod. (Note that with German Equatorial Mounts you may have to do a meridian flip to achieve this; do not expect to be able to track across the meridian.)
3. Ensure that you can achieve a comfortable viewing position for both horizon and zenith objects.