To prevent flexing, the tube assembly is made of 1.5mm thick metal tubing. A two-inch Dual-Speed Crayford focuser is used which will support CCD and DSLR cameras with the usual accessories. Internal knife-edge baffles reduce internal reflections to give increased contrast. The secondary mirror has a diameter of 50 mm giving a 26% obstruction. It employs a low-expansion Pyrex primary mirror and multi-coated Schott BK7 front meniscus lens. The Skywatcher Explorer 190 Maksutov-Newtonian DS-PRO, f/5.3, telescope is, costing typically £1,050, great value for money and should give images comparable to a 150 mm apo refractor at a fraction of the cost. These are provided by three companies, Skywatcher and Orion (USA) manufactured in China by Synta (and thus likely to be very similar) and Intes-Micro, manufactured in Russia. I have the f/6 version and the lunar image whose taking is described at the end of this article is of equal quality to those taken (and described elsewhere in the digest) with my 127 mm refractor even having a 21% obstruction. At a cost of ~£1,600 this is considerably less than the current cost of my 127 mm apo refractor. So the latter would have an effective ‘apo’ aperture of 126 mm. A ‘rule of thumb’ suggests that the effective diameter of a reflecting telescope (as compared to an apo refractor with unobstructed aperture) is the primary mirror diameter minus the secondary diameter. The planetary version has a focal ratio of f/8 and employs a secondary mirror of 26 mm diameter producing a 17% obstruction. The more general purpose version has a focal ratio of f/6 and employs a secondary mirror of 32 mm diameter giving an obstruction of 21% of the diameter of the mirror. It is interesting to note that Intes-Micro produce two versions of their 6 inch (152 mm) aperture Macksutov-Newtonian. The diameter of the secondary mirror required becomes smaller the longer the focal length of the telescope so for planetary observing a longer focal length telescope would be best. As Macksutov-Newtonians are perhaps better suited to lunar and planetary observing (along with small angular sized objects such as planetary nebulae, globular clusters and distant galaxies) they tend to be specified with relatively small secondary mirrors so that, perhaps, only the central 10 mm diameter of the image plane is unvignetted. So if one were to specify a secondary mirror for planetary observing and imaging, where the image can be kept in the centre of the field, a secondary mirror just large enough to avoid vignetting in the field centre would be used. However the larger the diameter of the secondary obstruction to the light path, the more light is taken away from the central disk of the Airy pattern and moved into the surrounding rings thus reducing the effective resolution of the telescope and the micro contrast in the image. One might simply specify a secondary mirror size so that no vignetting occurs and this may well be the case when wide field observing and imaging is the prime task of the telescope. The smaller the secondary mirror, the greater the vignetting that will be observed. A view of a Newtonian mirror seen from the centre of the field of view and offset from the centre where the image will be vignetted. This means that away from the centre of the field of view the image will become less bright – an effect termed vignetting. As one moves one’s eye away from the centre of the focuser it is likely that less of the mirror will be seen as in the image below. If, without an eyepiece, one observes from the centre of the focuser, one will see the whole of the mirror, so that all the light collected by the mirror will help to form the image there. In both Newtonians and Macksutov-Newtonians there is always a design compromise in specifying the diameter of the secondary mirror. The fact that the secondary mirror is not supported by a spider eliminates the diffraction spikes that would be seen in a Newtonian − helping to increase the ‘micro contrast’ of the image. This corrects for the spherical aberration of the primary mirror but also significantly reduces the coma that would be seen towards the edge of the field of view of a low power eyepiece. The Maksutov-Newtonian will be the same size as an equivalent aperture and focal length Newtonian but, instead of using a parabolic primary mirror, employs a spherical mirror allied to a thick, curved, meniscus lens at the front of the optical tube. A downside is a greater weight, so perhaps a more expensive mount would be needed. With apertures of typically 150 to 190 mm they would cost significantly less than the same aperture apo refractor. A Maksutov-Newtonian can provide an excellent alternative to an apochromat refractor for lunar and planetary observing and imaging.
0 Comments
Leave a Reply. |