STAR TESTING YOUR REFRACTOR:
Method: Use an eyepiece and the North Star. The easiest way to find the North Star is to look for the Big Dipper. Draw an imaginary line along the two end stars in the bowl of the Big Dipper. The first star you come to along this line is Polaris. It is best for this purpose that your telescope not be polar aligned, in fact point the mount head due east or west. This is because German Equatorial Mounts have a blind spot around the pole. Also turn off your motor drive if you have one attached to the mount. Use your lowest power (largest number eyepiece) to acquire Polaris, centre it using your slow motion controls. Now switch to your next higher power eyepiece, while keeping the image centred. The in-focus star image will have a bright innermost point, a slightly fainter inner ring and a fainter still outer ring that is hard to see. If it doesn't look like this, or you can't reach focus then start with: take out your star diagonal and look at the image slightly out of focus, this will allow you to better judge the pattern.
What you should see inside and out outside of focus.
Much confusion continues to surround this subject, so lets look at the major defects we will test for:
Chromatic Aberration
Definition:
A lens will not focus different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the dispersion of the glass. Always present to some degree, the question is how bad is it? Note that the longer focal ratio achromat yields less color at the same focus distance (0.2 mm) on either side of focus.
Solution:
Use of an interference filter such as the Sirius Optics MV-1 or Baader Fringe Killer can yield a great improvement.
Spherical Aberration
Definition:
Incoming light rays which are parallel to the optical axis but at different distances from the optical axis fail to converge to the same point. For our purposes the lens doublet fails to correct this as it should be designed to do. If it is more noticable on in focus or if more on out focus then you have spherical aberration, as it is noticable, it will affect image quality, especially fine planetary detail.
Solution:
Spherical aberration is present in all designs of telescope to a certain degree, but with good lens design it should be unoticable to the observer at focus, the trick is whether you find it acceptable in your telescope, the alternative is to return it to your dealer and opt for another (better) scope.
Astigmatism
Definition:
This defect in an optical system in which light rays from a single point fail to converge in a single focal point results in a lateral or longitudinal blurring of the image depending whether you are inside or outside of focus. Be sure to change eyepieces to ensure that the fault is in the objective and not the eyepiece.
If you are already diagnosed as astigmatic and wear glasses for this purpose, you must wear them at the telescope, particularly when star testing.
Try this test, if any of the lines appears thicker than the others then you have astigmatism.
Solution:
Again return the telescope to your dealer and opt for another (better) scope.
Coma
Definition:
Mechanical misalignment of optical parts, so that optical axis is off center from the mechanical axis.
Solution:
Discussed in collimation article...Refractor Collimation
Perfect Test
Definition:
When all optical elements perform so as to keep aberrations at a minimum to allow high definition viewing with sharp images. Star testing must be carried out under conditions of good to excellent seeing in order to be able to clearly judge results.
Images generated with Aberrator Version 3
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