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During the night, air temperature often changes. The air cools down very fast after sunset. Therefore a certain temperature difference between the telescope and the surrounding air exists at the beginning and also during observations. This causes air turbulence within the optical light path. Warm air emerges and rises up from the surface of the mirrors and at the tube walls. The turbulence will seriously effect the stability and quality of the images. The tube of the Multi-Schiefspiegler consists of metal and it will immediately cool down to ambient temperature. The mirrors made of low expansion glass need a larger time to cool down. Therefore it is preferable to mount the telescope and locate it outside and let it cool down for a while (1/2 h minimum) before starting observations (remove all tube covers). When you have the possibility to store the telescope in an almost unheated room this will help to reduce the time for temperature adaption.
The mirror surfaces will slightly change their figure due to temperature differences, but this effect is minimised by using glass of low thermal expansion.
You should protect the telescope by a large plastic bag against dust and rain if it stands outside for a longer time and is not in use.
In addition to the turbulence inside of the tube, there exist atmospheric turbulence (seeing conditions) which also influence the stability of the images and their quality.
Under bad seeing conditions you can not apply height magnifications to resolve the finest details of moon, planets, double stars etc. In this situation you can not reach the resolution limit of your telescope. A smaller aperture telescope will show the same details and is less sensitive to the turbulence. You can easily demonstrate this by reducing the aperture of the telescope with a circular ring.
Under good seeing conditions and after thermalization of the telescope you should apply a magnification of at least 150 to see all of the details which can be resolved by the optics. This value of magnification is called the standard magnification and depends on the aperture, its value amounts to the aperture in mm. In this case the exit pupil is 1 mm in diameter. The exit pupil describes the diameter of the parallel light bundle which emerges from the eyepiece and should enter your eye. The eye needs a certain magnification of the telescope to resolve the details delivered by the optics.
You should also try higher magnifications (up to about 300), this can result in a better awareness of image details. But there exists a certain limit of magnification (of about 300, depending on seeing conditions) and beyond this limit the image gets larger and darker, but shows no further details.
The maximum of meaningful power V(max) can be calculated by:
V (max) = 2 x diameter of aperture (in mm)
For the Wolterscope 150 we obtain:
V(max) = 300
You can easily calculate the focal length of the eyepiece (Fe) necessary to get a certain power:
Fe = f(telescope) / V
Fe = 5.5 mm (for V = 300)
Therefore an eyepiece of 6 mm focal length will give you the highest magnification. You should choose a type of eyepiece, which gives a high degree of optical quality, at least for the higher magnifications. Plössl - or Orthoscopic eyepieces are examples of high quality eyepieces. Another possibility is to combine a low power eyepiece with a Barlow lens (magnification factor of 2 -3 ) to obtain a higher magnification. The Barlow lens multiplies the focal length of the telescope with its magnification factor.
There exists also a value for the smallest meaningful power (minimal power). This value is reached when the exit pupil gets larger than about 6 mm. This value almost equals the entrance pupil of the human eye (it changes with human age). Larger exit pupils are vignetted by the eyes pupil and result in a light loss.
The minimal power (MV) can be calculated by:
MV = entrance pupil/ max. exit pupil
The entrance pupil is given by the aperture and we obtain for the Wolterscope:
MV = 150mm / 6 mm = 25
You can always calculate the exit pupil by the formula:
exit pupil = entrance pupil(aperture)/ magnification
Therefore the range of meaningful magnifications for the Wolterscope 150 should range from about 25- 300.
For the moon you can sometimes use very high magnifications above this limit because of the intense light and due to the high contrast of the surface details. Also stellar clusters should be magnified very high to resolve the inner core.
For Deep-Sky observations you should choose an observing place without much straylight coming from surroundings, to obtain a darker background of the sky. On a dark sky you should be able to see stars down to 6. magnitude. Deep Sky objects (Galaxia, nebula, comets) are often very faint and extended and you can not apply high magnifications.
You should also consider that your eyes need about 1/2 h for adaption to the darkness. By observing the objects not in a direct way (look a little bit to the side of your object) you obtain a higher light sensitivity and are able to see more details of faint extensive objects.
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