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 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. |
figure 4 l / 2 mirror Astigmatism 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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