Star testing

Choosing a mirror is very difficult, but not as difficult as giving advice on the subject. What are the criteria to choose between the large American manufacturers and their mirrors about which purchasers often complain, English manufacturers whose prices are unbeatable but whose quality is so to speak unknown (we would appreciate your feedback), French manufacturers with a reputation of regular quality but very high prices, and craftsmen whose reputations are sometimes overrated and sometimes justified? Messages on the Eclipsia newsgroup give clues not to be disappointed by the optics you will buy: first, ask for a bulletin of control from the manufacturer, and negotiate a payment whose balance will be done after control by an independent optician. A typical agreement between professionals is a front 30%, and 70% after conformity checking performed by an independent laboratory. Though this solution is certainly the best, it is also difficult to implement: find the optician who will perform control, negotiate this mode of payment with the manufacturer, pay for the control and transportation, etc. (refer to "Optical quality and large diameters" for more details). Another solution is to perform the test through an association, providing the control material (control by foucaultage, contrast of phase...) and competence to do it are available, then to negotiate with the manufacturer in the event of nonconformity. Another possibility is to perform startesting, recommended by Harold Richard Suiter in "Star testing astronomical telescopes", and which doesn't require particular equipment.


Startesting is very sensitive, allowing detection of many flaws of the mirror, but also of the telescope itself, for example in the supports. Nevertheless, interpretation of the test results is difficult. We can only give you here an overview of the process, but please refer to the book for a complete method, which will even prove that you must remain very careful, due to the diversity of problems you can encounter. Let’s describe that part.


The background acquired through practice, and moreover the contribution of those with experience, are a major help in this process. First, do not forget that several flaws can combine on the same apparatus. The image is then even more difficult to decode. You must try to separate and analyze those flaws one by one. When beginning the test, the first step is to put the instrument at the right temperature. Then, take advantage of a weak turbulence. A method to set the temperature is to target a star (polar, by example) and focus-unfocus so that you see a regular surface, with the shades of the secondary and the spider. If " waves " are seen on the mirror surface, the temperature is not correct. Fans behind the primary allow a faster temperature lowering. Of course, turn them off during observation. If turbulence is too strong, the only solution is to… delay the test. The basic principle of startesting  is simple: observe the unfocused image of a star, using a magnification at least equal to the mirror diameter, in mm. For example, use a 500x for a 500 mm diameter.


Unfocusing must be moderate. You should see between 5 and 10 diffraction rings for the whole test, except for specific flaws described later. A yellow or green coloured filter will allow you to select the most favorable bandwidth, for your eye. In white light the diffraction rings  tend to mix ones with the others. You will examine the intra and extra focus images, checking that the distance to the focal plan is the same, for example + or - ¼ button rotation. Also check that you have a perfect collimation. Moreover, you should use an ocular free from any of the flaws you try to detect. This is a very important point. Be cautious, not all simulations images take turbulence into account !

If the primary or secondary is flawed or incorrectly set, it can create artificial defects, such as spherical aberration, even with a perfect mirror. So, remember that you test the whole system, not only the primary mirror. You should be convinced about the quality of the other parts before judging the mirror. Concerning the support, the easiest flaw to detect comes from a constraint optic.

Target the polar, which is almost immobile in a non motorized Dobson. If you get an image like in figure 1, the mirror is constrained. Take into account that a thin mirror can easily be deformed.


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figure 1


Perfect Mirror.

Figure 2 shows the intra and extra unfocused image of a perfect mirror, with a 33% obstruction rate. Such an image shows that the diffraction is exactly the same with intra or extra focus: a perfect mirror !

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figure 2


Spherical aberration

Usually designed by l , this is the most common criterion expressing a mirror quality. However, there are others, and most criteria are important. The following figures show an under correction, with a 33% obstruction rate. The images on the left are intra, the images on the right extra. Inversely, an over correction would swap those images.


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figure 3


l / 4 mirror


figure 4

l / 2 mirror



Astigmatism can have several causes, such as:

- primary support with constraint on only one axis,

- secondary too spherical,

- deformation of the primary, caused by bad polishing,

- astigmatism of the eyepiece,

- astigmatism of the observer himself…

 Astigmatism seems to decrease when you defocus. On the other hand it is very apparent with a low defocusing.



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figure 5


Turn down edge

Turn down edge is a specific zone defect. It creates a contrast much more apparent in extra focus between the diffraction rings. To assess it, you must defocus up to 20 diffraction rings. You should so target a more brilliant star.

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figure 6

turn down edge with 25% obstruction


Roughness is apparent in the star test. Its effect is similar to the atmospheric turbulence, put aside the movement. It is thus necessary to:

- take advantage of a very steady sky,

- remain long enough at the eyepiece, to eliminate all defects induced by the movements of the atmosphere (for a fine survey of these defects, see the optical quality page).

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figure 7


For more details see Lequevre's  website.