Scope Doctor
Our equipment specialist cures your optical ailments and technical maladies
I struggle when travelling with my 200P Dobsonian. Do you have any advice for transportation, or is it time to downsize? ALAN DOWNE To get the most for my money, I’m considering buying a second-hand scope. What should I consider before making a purchase? BARRY FUDGE
Dobsonian reflectors represent some of the very best value instruments as you get a lot of aperture for your money and a no-frills but very stable mount. However, an 8-inch (200mm) reflector is a relatively large and cumbersome instrument – as you have discovered – so it may be worth simply downsizing a little so that you don’t lose too much of that valuable aperture, but have a more manageable instrument to move around. A 6-inch (150mm) reflector would be an excellent compromise here, as this would still provide a very generous aperture but would be just that little bit smaller and easier to transport.
Unfortunately, the Skyliner 150P Dobsonian – made, like your current telescope, by Sky-Watcher – has the same 1,200mm focal length, so there would be no saving in overall length. However, Sky-Watcher does have a standalone version, the Explorer 150P, which is available in a shorter focal length of 750mm, and this could be satisfactorily mounted on a suitable altaz mount.
The 150P has the same focuser as the telescope that you are used to (a singlespeed Crayford design), but in all other respects it is just a scaled-down version of your existing optical tube.
Mounts suitable for the 150P would include the Sky-Watcher AZ4 and the Vixen Porta II mount, both of which come equipped with lightweight but stable aluminium tripods. Buying second hand is a popular choice as astronomers generally respect their equipment to ensure they get the best out of it. But there are some safeguards that you can implement to further ensure that your purchase goes smoothly.
If the telescope has been used for astrophotography, ask the vendor to send you a recent photograph captured through the telescope and examine the star shapes closely, checking for any unwanted aberrations like astigmatism or flare.
Small marks on the outside of the optical tube are normal, but be very wary of dents as they’re an indication the instrument has been dropped. Check the action of the focuser, ensuring that it is smooth with no flop in the focus tube. Examine the optics carefully under ordinary daylight, not with a torch.
General dust is quite normal, but look for any scratches or blemishes on the lens surface. If there are any signs of dust, moisture or fungal growth between any of the elements, walk away!