Riporto nel forum i due link che mi hai mandato potrebbero essere utili ad altri
http://web.telia.com/~u41105032/kolli/kolli.html http://propermotion.com/jwreed/ATM/Coll ... hesire.htmEd aggiungo la parte che riguarda gli S/N
Collimating a Schmidt-NewtonianA Schmidt-Newtonian differs from a regular Newtonian in that the primary is spherical, not paraboloid. To cancel the resulting spherical aberration, there is instead a corrector plate at the tube opening, a thin glass disc figured to a complex shape. This means that coma is reduced (approx. halved) compared to a paraboloid mirror of the same f/ ratio - and in addition, the closed tube may be less sensitive to tube currents.
But it also means that the collimation procedure must differ in some respects. I don't have a Schmidt-Newtonian to try, so this is based on a theoretical analysis - so if you try to follow these guidelines, I'd appreciate a mail about your experiences.
In the regular Newtonian, the optical axis must be accurately centered at the optical center of the primary. In the Schmidt-Newtonian, it must be centered in the corrector instead (this aspect seems to have eluded the author of the manual of one common line of such telescopes!). In practice, the centering of the optical axis at the level of the secondary and the corrector is determined by the position of the secondary and the focuser. However, there is no way to decide this or do the adjustments until you have established an optical axis first.
So here is an outline of the steps I believe are needed:
Step 1: See step 4 of regular collimation - tilt the secondary mirror to center the focuser's axis on the primary's center spot, using a laser collimator or crosshairs sight tube (it has been reported that the spot isn't always accurately centered, and you may want to check this once and for all, by removing the mirror cell and measure the position with a ruler).
Step 2: See step 5 of regular collimation - tilt the primary, using a Cheshire or Barlowed laser. Now you have an optical axis established, and you can check its position at the level of the corrector plate.
Step 3 - check the centering of the corrector plate. I see no practical way of using the center, as it is hidden by the offset secondary and its holder. Instead, you can use a peephole cap (or any cheshire or sight tube that is not too long!) and check the centering of the corrector plate's rim with respect to the primary mirror's edge. Looking at a brightly lit ceiling or the sky (avoiding the sun), you should see the inside of the corrector rim, at least if you rack the focuser fully in.
You may find that the rim looks wider in one direction - identify this direction by holding a finger just outside the rim where it appears widest.
If this is towards the focuser, it means the optical axis is offset towards the focuser and needs to be moved away from it. The optical axis will always pass the point at the secondary where the focuser's axis (and laser beam, if used) strikes. To move it, you would like to move the secondary away from the focuser, but this may not be possible - the offset is factory-adjusted. Instead, you can achieve the same thing by moving the secondary in the direction out of the tube. The necessary distance is half the difference in width of the rim at its widest and narrowest, but you will have to try your best guess. Loosen the 3 outer adjustment screws just a little (all by the same amount), and tighten the center one. If the widest rim is away from the focuser, move the secondary in the opposite direction, but be careful not to loosen the center screw all the way! Also try to keep the tube horizontal with the focuser above in case of accident, and be careful not to rotate the secondary holder.
If it is to one side or the other as seen from the focuser, adjusting the secondary won't help. You will have to shim the focuser instead, aiming it a little towards the narrower side of the rim.
If both, try first eliminating the larger of the errors.
This done, go back to step 1 and repeat - as many times as needed.
You may check the centering of the secondary as in the regular steps 2 and 3, but you will have to accept the decentering you may find - it should be negligible in practice, as the mirror should be reasonably correctly offset in the design.
Step 4: star test the collimation. What to go for is a symmetric diffraction pattern near or at focus - if you defocus enough to clearly see the shadow of the secondary, the test is likely not as sensitive as it should be.
By small adjustments of the primary, you should be able to remove or reduce coma. This is possible even if the original optical axis wasn't accurately centered at the corrector plate, but I believe you can get astigmatism that is not possible to cancel until you have got the axis centered.
Auto-collimating for wide-field photo
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facendo 2 conti, la camera di ripresa è la 350D, pixel da 6.4 micron.. vuol dire che le stelle sono da 15 micron a 36 micron.. guardando questo frame il cerchio ottimale (come curvatura) dovrebbe essere di circa 10x10mm