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You have purchased a Multi-Schiefspiegler telescope 150 f/11 (or 170 f/10, 200 f/11), the optics uses an aperture of 150 mm (170/200 mm) and has a focal-ratio 1:11 (or 1:10).
The focal length f is calculated according to
f = 150 mm x 11 = 1650 mm.
By using eyepieces of different focal lengths (fo) you can obtain different magnifications (M) with your telescope. The magnification can easily be calculated:
M = f/fo
We will discuss below which range of magnifications is useful to apply with this telescope. The performance of the telescope is not defined by the maximum of magnification which is possible theoretically.
The optical performance of the optics is defined and characterised by the resolution limit to resolve objects of small angular distance (double stars etc.) and the ability to image details of different intrinsic contrasts (contrast performance). An example are the clouds on Jupiter, they have a low intrinsic contrast and their contrast differences are very small. A perfect unobstructed telescope will transform this details to an image where the contrast has already been reduced due to physical laws. A less perfect telescope (with central obstruction and some optical errors) will reduce the contrast-transfer further. That means, for example, that you can not distinguish between different shallow clouds on Jupiter anymore.
Both performance criteria – resolution and contrast are optimised within the physical laws by the Multi-Schiefspiegler design.
The angular resolving power AR is defined by the aperture D (here D = 150 mm) and can be calculated for visible light according to:
AR = 140/D = 0.93 " (given in arc sec)
That means that this telescope should be able to resolve a pair of stars (of equal brightness) which are a distance of 0.93 " apart.
The value obtained >from this theoretical calculation can be compared to the results of practical observations and allows to estimate, if the theoretical limit is derived by the telescopes optical performance.
The other meaningful quality criteria of the optics is the contrast performance. Here, all unobstructed optical systems, Refractors and Schiefspieglers are favourable. The optical quality of the surfaces has also strong influence to the contrast performance and should therefore be optimised.
To reach the theoretical limits of optical performance, the Multi-Schiefspiegler uses only high quality mirror materials (like annealed Pyrex) with highly accurate surfaces (PV value of the wavefront better than 1/10 wavelength). In addition, a high reflective coating of about 95 % reflectivity (maximum of 97% at greenlight) is applied to minimise light loss.
The mirrors have to be precisely adjusted to derive the maximum performance of the optics.
This adjustment was done by using an autocollimation test setup with a laser light source. This device is very sensitive to image errors due to misalignment and to surface errors of the mirrors. We adjust the optics very accurately to compensate all the image errors. This perfect adjustment is fixed by the mechanics and very stable over long time periods. Under normal conditions (observations and transportation of the telescope within the case) the alignment will be stable.
Only after heavy mechanical vibrations and shaking (the telescope falls down etc.) a readjustment might be necessary.
By the help of the following instructions each amateur will be able to readjust the mirror optics easily.
From time to time you should control if the adjustment of the optics is perfect.
Therefore, you should observe a bright star and locate it at the central part of your field of view, using an eyepiece of medium power. Adjust the focuser to a position where you see the sharpest image of the star. Than, you defocus the star by changing the focusers position by some mm to the outside (extrafocal) and also to the inside of the focus (infocal). In both cases you should see a uniformly illuminated disk of the star of circular shape.
Both defocused images of the stars should be almost equal in intensity and shape, which is a sign of perfect adjustment and quality of the optics. But if you observe a illuminated disk of elliptical shape than their might be astigmatism present, which has to be compensated by using the outer adjustment screws. For this purpose, 2 screws reach the outside of the tube (see image below). They control the mirror tilts within the telescopes plane of symmetry.
The star test should be performed under good seeing conditions (stable and good weather conditions) and only after the telescope has cooled down to the air temperature. Otherwise the stellar disks will be not stable and their shape and illumination will always be in motion.
A readjustment of the optics will be necessary, if you observe an elliptical shape of the defocused star. Try to analyse the orientation of the ellipse, that means the positions of the long and short axes. Then go from the infocal to the extrafocal position (or vice versa). You should see that the orientation of the ellipse has changed by a rotation of 90°. This is a characteristic sign of astigmatism, which has to be compensated. In extremely cases of astigmatism the ellipse will be deformed to a single illuminated line.
Before you turn the adjustment screws make sure that your eyepiece does not induce this image error by itself. You should rotate the eyepiece, if the ellipse also rotates your eyepiece is defect. Your eyes might also suffer from astigmatism and for this test you should wear your contact lenses or spectacles. After you have excluded this errors and astigmatism is still present, carefully read the following instructions for the readjustment.
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