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Instructions for final adjustment
(you need the adjustment key )
The telescope has been adjusted very precisely in an optical test setup (autocollimation test). Under normal conditions this adjustment is very stable and no readjustment of the telescope is required. We give a detailed description of final adjustment here for the cases that strong temperature differences and very heavy shaking of the tube induces a slight misalignment of the optics. The methods described below, can also be used to control the adjustment and optical quality of other telescopes.
A misalignment of the Multi-Schiefspiegler optics produces astigmatism. This error can be easily detected and corrected. For this purpose two adjustment screws reach the outside of the tube, one is located at the forefront (S1) and one at the backfront (S2), see the image below.
This adjustment screws change the tilt of the 2. and 3. mirror within the telescopes plane of symmetry (this plane cuts all mirrors at their centres). This tilt angles are responsible for the correction of astigmatism, all the other mirror tilts remain unchanged.
Additional adjustment screws are located at the mirror cells inside of the tube. They should not be turned without special knowledge about the precise optical adjustment procedure. They have been aligned by the manufacturer very precisely in an autocollimation test setup.
You should do the adjustment at the first time, using a virtual star at daylight (a laserpointer can be used in this case). The test uses an aperture mask and will be described below.
After you have obtained some experiences you can also use a bright star at night to perform final adjustment of the telescope. It will be comfortable to use a mounts with a motor drive to correct the motion of the stars.
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A very easy method to judge the image errors of a telescope can be performed by placing a mask, which contains some small holes in front of the aperture. At night, a bright star is observed through this mask. At daytime you can use a virtual star or a distant laser pointer to control the adjustment. The Multi-Schiefspiegler might show a small amount of non-compensated astigmatism. By using a mask with a hexagonal arrangement of the holes this image error could be easily detected. You simply have to examine the image of the star (through this mask) with your eyepiece. You should defocus the star (by a few mm way of focusers travel in both directions) and look at the hexagonal pattern. If astigmatism is present, the pattern shows a vertical or horizontal deformation. This method is very sensitive and is based on ideas of Hartmann and Vaisela [6].
For daytime adjustment you should install a virtual star or a laserpointer more than 10 m away >from the telescope. Orientate the pointer in such a way that the laserspot hits the aperture mask at the middle point. Then you should turn or rotate your telescope slightly so that you see the spot pattern in your eyepiece. The test should not be done at bright sunlight. Its better to locate the telescope and the laserpointer in a shaded area.
An aluminium cover containing this hexagonal mask is delivered together with the telescope.
For doing this test at night you should choose stable weather conditions and observe a bright star through the mask. You should use a motorised mount to compensate earth rotation.
Now you can analyse the pattern of the star by an eyepiece of medium power. Defocus the image by a small amount (infocal or extrafocal position). You will see the hexagonal pattern in your eyepiece. If the pattern looks symmetrical, that means all points have the same distance, than astigmatism is almost perfectly compensated. This situation is displayed in the following image. The image of the mask is symmetrical and shows no distortions. Now you should slowly bring the star to focus and all the starpoints should merge into a single point. It is surrounded by some rings, this is caused by diffraction of light at the holes of the aperture mask.
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But maybe you will see a non-symmetrical image of the aperture mask (distorted hexagon) like that in the following image (image in the middle or right side). Therefore a readjustment of your telescope is necessary to compensate astigmatism.
You should examine the pattern by slightly rack in the focuser to an infocal position. If you see a vertical distortion (vertical distance is longer than horizontal, image in the middle) then you should turn the adjustment screw at the front-side by a small amount (not more than ¼ turn) clockwise.
If you see a horizontal deformation (see below, image at the right side) for an infocal position of your focuser then you have to turn S1 (adjustment screw on the front-side) counter-clockwise.
Make a small adjustment step and look again at the image. It has been displaced to the right or left side of your field of view. You should centre it again in your eyepiece, by slightly turning the adjustment screw S2 (if you have turned S1 clockwise, you have to turn S2 counter-clockwise and vice versa)
If the image of the pattern looks more symmetric than before, make another adjustment step by turning S1 in the same direction by a smaller amount. But if the pattern looks more distorted, you have to change the direction of turning the screw S1.
There exist a certain position of adjustment screw S1, where astigmatism is cancelled exactly. Maybe you have to change the direction of turns for several times to find the right position of S1. The screw S2 (at the backfront) should always be turned in the other direction (S1 clockwise, than S2 counter-clockwise and vice versa). S2 does not affect the image error and is only used to centre the image again in your eyepiece.
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This adjustment procedure looks more difficult than it really is. With a little bit of experience the adjustment is done easily within a few minutes.
The method is very sensitive, because the observer can easily detect small distortions of the hexagonal pattern induced by astigmatism. You can test at daylight and not have to defocus a star at night. The defocused image of a star might always be in motion due to unstable weather and thermal effects. Sometimes it is therefore not easy to judge, if the defocused image of a star deviates from a circular shape. This deviation is also a strong indication for astigmatism (see the star test).
By performing the star test at stable thermal conditions you can get a slightly better correction of the image errors. It is more sensitive because the whole aperture is used for this test and the image of the star and the diffraction rings are smaller therefore. So you can use an eyepiece of high power to examine the image and to detect errors.
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Now turn the screw S1 by a small amount in some direction (for example 1/4 turn clock wise, not more). In most cases small turns of the screws will be sufficient. The motion of the screws was restricted to a total of about +/- 2 turns to make sure that the optics will not seriously be misaligned. Look through the eyepiece, the star has moved away from the centre of the image and might be out of sight. Then turn also the screw S2 at the backfront near your eyepiece) by the same amount but in the opposite direction (in our example by ¼ turn counter-clock wise). The star should reappear in your eyepiece, adjust the screw S2 to bring it back to the centre of your image field. Now you should check for astigmatism, defocus the star in both directions by changing the focusers position by some mm (not more). The elliptical shape of the star should have been reduced and changed towards a circular shape or it will look more elliptical. In the first case try to repeat the adjustment steps by smaller turns of the screws. In the latter case you have to change the direction of turning your screws and to repeat the adjustment steps. Always make the star to reappear in the centre of the image by turning the screw S2 (not by S1). The screw S2 should always be turned in the opposite direction with respect to S1 to locate the star in the eyepiece again.
Only the adjustment-screw S1 is responsible for correction of astigmatism, a certain tilt position exists, where the error will disappear completely. If you go beyond this tilt position, the error will grow again and you have to change the direction of turning the screw. Do not try to go beyond the limited turns of the screws, otherwise they might be damaged.
After you have almost obtained a circular shape of the disk of the imaged star, you can make the adjustment more precisely by using an eyepiece of smaller focal length (more power). Try very small turns of the screw S1 (1/8 turn will be sufficient) to remove the remaining error.
This procedure adjusts the optics very precisely and you will bring it to its optimal performance. With some experiences the adjustment goes very easy. If you can get the help of a friend it will be more convenient. He/She can turn the screw S1 at the forefront and you can always look through the eyepiece (to check for astigmatism) and readjust the position of the star by turning the screw S2 next to your side.
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For a small amount of astigmatism the defocused star will appear like an ellipse. Its long axis is oriented either horizontal or vertical. The latter reminds on an egg which stands upright.
If you see this egg (always that with upright orientation) for an extrafocal position (focuser is racked out beyond focus) than you have to turn screw S1 also to the outside (that means counter-clockwise)
If you see this egg (upright orientation) for an infocal position (focuser is racked in beyond focus) than you have to turn screw S1 also to the inside (that means clockwise)
You can refine the adjustment by doing this procedure in smaller steps.
By applying this rule you will always turn the screw S1 in the right direction and will systematically find its correct position.
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