How to Star Test a Telescope and Fix Optical AberrationsA star test allows you to measure the quality and accuracy of your telescope and identify if you need to calibrate or collimate it.
By performing a star test you will be able to tell how accurately aligned your telescope optics are. This optical alignment requires that all the mirrors and/or lenses are positioned precisely and pointing in the right direction along the optical path. If your telescope is out of alignment you will suffer from poor view, blurriness and inability to achieve sharp focus.
If you have a reflecting telescope check your collimation and make any adjustments that may be required.
Before You Start
To perform a star test you will need to set your telescope up to point to a bright star - such as Sirius or Vega - and you will need a high magnification eyepiece, ideally in the 10-20mm range. A tracking mount is very handy to keep the star in the field of view, but it's not a requirement. If you don't have a tracking telescope it may be worth trying to observe a star close to Polaris, or Polaris itself, as this will greatly reduce the amount of movement.
Before you start check how good the seeing is. The atmosphere needs to be as still as possible when doing a star test. If you look up and the stars are twinkling like crazy then the seeing isn't good enough for a star test. If the seeing is good, set your equipment up and wait for it to acclimatise.
The Star Test
The star test involves looking at your chosen star when it is in focus and out of focus. As the star becomes out of focus you will see the star transform into a large circle, and depending on the type of telescope you have, it may become doughnut-like in shape. These out-of-focus shapes are called Airy discs.
Ideally, the star you are observing should be high in the sky to avoid atmospheric disturbances and anything that could get in the way, such as tree branches or buildings. You should also perform the test during a night of good seeing - when the stars are not twinkling.
Allow your telescope to cool down to the ambient temperature as you would do for normal observation. This will keep the disturbances to an absolute minimum. You should leave your telescope for 30-60 minutes to cool down properly. You can try observing the star with a warm telescope, and if your telescope needs to cool down further you may notice long streaks coming from the point of light when it is out of focus.
Once your telescope is cooled down you can start the star test. Adjust the focuser so that the star is inside focus and slowly keep adjusting until the star is in focus, and keep going till you get out of focus. Pay attention to the pattern of rings around the star when it is inside and outside focus. In a perfect telescope, the pattern of rings will be perfectly concentric and the same when inside focus and outside focus.
If the rings are not concentric your telescope has aberrations. These can be difficult or impossible to fix and depend on the type of telescope you have.
If the rings are not clearly defined then you could have dirty lenses or mirrors, or they are not perfectly smooth. You can clean the mirror or lens but if the surface is not smooth there isn't much you can do at home to fix it.
Most telescopes, if not all, will have some kind of aberration. Even the most expensive telescope are not 100% optically perfect.
If the concentric rings are not perfectly round, either egg-shaped or ellipses then there is most likely a distortion on the mirror. This distortion is called Astigmatism. If the direction of elongation changes as you move from inside to outside focus then it is probable that the mirror clamps are too tight. Loosen them a little and try again and hopefully, this problem is resolved.
Spherical Aberrations
Another problem frequently encountered is spherical aberration. This is when the outer parts of the lens or mirror don't quite have the same focus as the centre parts. Cheap telescopes often suffer from spherical aberration and it makes viewing faint objects quite difficult and can make pin-sharp views impossible.
There are two types of spherical aberration, under-corrected and over-corrected. If you have over-correction the concentric rings inside focus will not be sharp, yet the outside focus will be. If the inside focus is sharp and the outside focus is blurry then you have under-correction. If the outermost ring has spikes or streaks it indicates that the mirror may have a turned-down edge.
The methods for fixing these aberrations, or if they are even possible to fix, depends on the type of telescope you have and the amount of adjustability it offers. You should check your handbook that came with the 'scope.
