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Center the secondary mirror in the tube

You should offset the secondary mirror from the center of the tube, away from the focuser. Calculate the offset, and use a ruler, or else wait and adjust after step 6 is done. (If you cannot offset the secondary, e.g. because of the spider design, you may leave it centered, without serious consequences.) (This minimizes error type 4)

3 - Center the secondary mirror along the tube, as seen from the center of outer rays (this is the point where the main mirror appears to exactly fill the face of the secondary!). As previously explained, the secondary mirror should be offset both away from the focuser (this was done in the previous step) and towards the main mirror (but since it appears centered only when correctly offset, this is automatically taken care of in this step!).

You can center it in a simple or combination sight tube, as described below. You may also use a crosshairs sight tube or a laser collimator to do this, but then you need a spot at the optical center - or a holographic laser collimator. If you find that the secondary is offset "sideways", away from the tube axis, you may have to go back to either step 1 and shim the focuser sideways, or to step 2 and adjust the spider setscrews. (This minimizes error type 2)

4 - Tilt the secondary mirrorto make the extended optical axis hit the center of the main mirror .

Use the appropriate setscrews on the secondary mount - depending on the design, you may also rotate the secondary holder to center "sideways". You can use a laser collimator by making the laser beam hit the center spot, or a crosshairs sight tube, by centering the spot on the crosshairs.You could also use a simple or combination sight tube, by centering the main mirror within the sight tube end (this is what you must do if you don't have a center spot).

If you can rotate the secondary, you could get it skewed by tilting it one way, and rotating it the other. If you see the secondary or its reflection looking skewed, try straightening it up, then rotate it to get the main mirror roughly in line. Then start over with this step, but do not rotate.

If you have made significant adjustments, go back to step 3 (and possibly step 2) and check that the adjustments still are OK, or adjust if needed.

If the main mirror is poorly adjusted, part of its edge may be obscured by the tube opening. If this makes centering difficult, go forward to step 5, and make a coarse adjustment before going back to step 4 again. (This minimizes error type 1B.)

Going forward from here, do not skip step 5!

5 - Tilt the main mirror, to make its optical axis reflect back on itself.

If you have a mirror cell that holds the mirror very loosely (this is particularly common in Dobsonians), you may make the mirror settle by tilting the tube nearly horizontally, and then raise it vertically, before you go on.

Here you use the set screws to adjust its tilt (use 2 to adjust, and leave the 3rd), and thus the tilt of its optical axis. You could use a Cheshire or a combination tube, by centering the main mirror spot in the bright spot of the Cheshire (if you use the calibrated Cheshire, you will know that the error type 1A is within tolerances, when the black spot is completely inside the bright spot). You may even use a peephole with a semi-transparent lid as a primitive Cheshire, if you illuminate it from the outside.



If you can reach to adjust the collimating screws while looking into the Cheshire, this simplifies things enormously! I have built my own telescopes so that I can, but with most commercial telescopes, this is not possible. The next best thing is an assistant that you can ask to turn each screw in each direction while you note the effects. A simple trick is to put a sticker near the focuser, where you draw two arrows to mark the direction that the spot appears to move when you turn each screw inwards - this way, it is easier to decide what screw to turn in what direction.

You can use a laser collimator (and a perforated center spot!) to get close, but as the precision is limited, fine tune by using a Cheshire, or star collimate (see step 7) for the final adjustment.

If you have no center spot, you can use a double crosshairs tube - see here how.

(this step minimizes the critical error 1A)


Date: 2015-12-11; view: 750


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Error type 2 - the optical axis strikes the secondary mirror at a point away from the optical center. | Check the finder and adjust if necessary
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