Test the telescopes optics



Let's begin testing the optical quality of your scope. To do that you have to choise a good seeing night and point your scope to a bright star high on the horizon. The star should be of magn.2-3; use big magnification, about 2.5 or more times the diameter of your scope in mm and take the star at the center of the field; don't use barlow lens and use only good optical quality eyepieces; if your eyepiece has not good quality, the test, called star test, won't say you the true. Remember that the quality of an optical system is given by the less quality element; you can have an excellent telescope but if you use a 15$ H chinese eyepiece, the quality of the image will be given by the eyepiece. Have a big magnification means to have about 250-300X for a 100 mm scope, 600 for a 25 cm scope and so on. You have to analize 3 different patterns (called diffraction figures); 1) a star defocused (intra-focal) 2) a star defocused (extra-focal) and 3) a focused star; you'll see a pattern like these:

intra-focal diffraction pattern

Extra-focal diffraction pattern

Focus diffraction pattern; the bright spot is called Airy disk


this strange and defocused pattern give you many information about the optical quality of the scope; look at it very carefull.

1)  a perfect optic will show a perfect and circular diffraction rings, equal in intra and extra focal, like that:

If your telescope shows that, you own a perfect one!Unfortunately this is very rare, especially for commercial achromatic refractors,so let's go to analize the possible aberrations present:

2) scollimation; this is not exactly an aberration, that is, an optical defect, and it is easily correctable. When you telescope is not good collimated (this means that the mirrors or the lens are not perfectly centered), you will see the extra-axial aberrations at the center of the field; these aberrations are especially the coma and maybe even the astigmatism; but the coma is always present, so it is very important to collimate accurately the telescope to avoid this aberration even at the center of the field. You will see a pattern like these :

These images show a strong scollimation of the optics; it is very difficult to have a so scollimated mirrors!


3) Spherical aberration; this defect is unfortunately very common in commercial scopes, especially in achromatic refractors and some little newtonian (which both uses spherical lens or mirrors). As the word suggest to you, this aberration is due to a spherical mirrors or lens, with no corrective elements in the optical path. The optics laws tell us that a spherical lens or mirror won't focul the image on a unique center point (the focal point) but there are more focal points, due to the fact that the light coming from the edges of the mirror will focus farther (positive spherical aberration) or closer (negative aberration) to the focus point. This optics fault is very harmful and will give you few contrast hi-resolution images and don't let you to reach the resolution power of your scope. So a scope with visible spherical aberration is completely useless for hi-resolution observations (or imaging). The spherical aberration is reduced (theorically should be absent!) in newtonian reflectors with parabolic primary mirror and in some good SCT which have a spherical mirror but the aberration is well corrected by the front lens. The spherical aberration is easy to see through the star test:


4) astigmatism: this aberration is maybe more harmful than spherical, but fortunately is less common. If the spherical aberration is a direct result from a spherical mirror or lens, the presence of astigmatism at the center of the field means a fault in the optical system, a defect of your lens or mirror; the astigmatism is instead natural at the edge of the field using some optical configurations. This aberration produces a double plane where the light will focus; the 2 planes are perpendicular and this means that you have a double focus points; so you'll never have a sharp image, never. if your scope suffers of astigmatism you'll see a pattern like that:


5) mirror (or lens) deformation; this is a fault in your optics, but fortunately this fault is many times corrigible; the deformation indeed is caused many times by too tight lens or mirrors in their cells; with much attention loosen a little the screws of the mirror cells, and many times this aberration will disappear:


6) rough optics; this aberration is not common in commercial scopes, but could be present in old scopes with deteriorated mirrors. With rough mirror you'll see a diffraction pattern a bit blurrer with few contrastated rings: this optics fault causes only low contrast images and should not influence the resolution power.



7) chromatic aberration; this aberration is present only in fully lens scopes, that is, every refractor; this is not an optical fault, but the consequence of the optics laws. These laws tell us, that in a lens the blue radiation is focused before the red light, so you have one focus point for each wavelenght. The shift between blue and red image is inversely proportional to the focal lenght of the scope, so to reduce the aberration we need a long focal refractors.The aberration is completely reduced if the focal lengh is about 15 times greater then the lens diameter, or better if the ratio F/D=f is about 15. this is easily achievable with small diameter scopes, but if you wanna a big diameter scopes, you'll have a long and heavy tube. For this and other reason all the biggest telescopes of the world are not refractors.

8) coma; this is an extra-axial aberration, and it is always present in every telescope; it is indeed not a defect of the optics but the result of the optics law; it is proporzional to the ratio F/D of the scopes, so an f4 scope has much more coma than an f10 one, or rather, the coma become visible closer to the optical axis. If the light goes through the lens off axis, it won't focus to a point and look like a strange circle. The farther off axis, the larger the circle, giving objects a comet-like aspect.

The images at the edge of the fields look like small comets, so the name "coma" (like the cometary tail). This aberration is not important for small field observations, but it is for wide field ones especially in the imaging; the only way to avoid that, is to use a special accessory called coma corrector (expensive and usable with not all telescopes).