Sky-Watcher 254mm f/4.7 Dobsonian Telescope
In recent years, Dobson fever has increasingly invaded amateur astronomers aware that the strongest sensations are felt with large diameter telescopes. The observation of the planets is indeed limited to Jupiter, Mars, Saturn (and the Moon) whereas the clusters, nebulae and galaxies are counted by tens in a 150mm instrument (see hundreds and thousands for the largest diameters under a sky dark).
Without motors or electronics, Dobsonians are indeed the instruments providing the most light, at a given budget, compared to any other type of instrument (mostly motorized and Go-To). Sky-Watcher designs solid tube Dobsonians appreciated by beginners and amateurs for many years through a range that extends from 150mm to 305mm.
The 200 and 254mm will be appreciated by amateurs because they are able to show many more objects with more details. They enter the range of instruments that can be kept for many years before having reached their limit. It is also possible to combine them with O-III or UHC filters to see the most beautiful nebulae from a different perspective (more contrast, less light pollution). The 305mm offers great views on thousands of objects (NGC/Messier catalogs). Nebulae, globular clusters and galaxies will be easily accessible.
For the beginner in visual observation it is the best price/performance ratio! Treat yourself to a "real" telescope without delay. Surprises and discoveries will be there for a minimal investment!
The Newton design offers several advantages over other families of instruments (receivers and Cassegrain telescopes).
First of all, the manufacture of mirrors is more economical for an equal diameter and its focal ratio is generally very low. You then benefit for a very attractive budget from a bright telescope which provides a virtually coma-free field on the 150 and 200mm models (the 254 and 305mm with their short focal ratio, can be supplemented with a coma corrector) . The central obstruction is weaker than the Cassegrain type telescopes, if the collimation (adjustment of the mirrors) is well respected, the image quality is good in planetary. The Dobsonian mount also considerably reduces the price of this telescope compared to motorized equatorial mounts.
Au contraire des fabrications très bon marché ou les télescopes Newton de petits diamètres, les Newton (Dobson) Sky-Watcher de 150, 203, 254 et 305mm disposent d'un miroir primaire parabolique. Les miroirs sont polis sphériquement puis retaillés pour atteindre une forme paraboloïdale (plus coûteuse et difficile à obtenir). C'est un critère de choix indispensable pour les instruments au-dessus de 130mm et à f/D court.
Unlike very cheap manufactures or small diameter Newtonian telescopes, the 150, 203, 254 and 305mm Newton (Dobson) Sky-Watchers have a parabolic primary mirror. The mirrors are spherically polished then recut to achieve a paraboloidal shape (more expensive and difficult to obtain). This is an essential selection criterion for instruments above 130mm and with a short f/D.
Sky-Watcher does not only offer telescopes at very advantageous prices but also easy-to-use instruments as shown by the tilting system which is fitted to all Dobsonians from 150 to 305mm. The Sino-Canadian firm has in fact developed and patented (United States Patent No. 6,940,642) a handle that allows you to tighten or loosen the fork on the optical tube. Thus, depending on the inclination of the tube and the weight of the accessories present in the eyepiece holder (binocular head, eyepieces of different angles), the observer can increase or decrease the friction. The tube is then sufficiently supported not to tip downwards as is the case with other constructions and free enough to be easily oriented towards another place in the sky.
Focusing is achieved through a bi-flowing focuser. A 31.75mm adapter is supplied as standard to use eyepieces of the same format from the 50.8mm output. A clamping screw located at the top of the focuser maintains focus according to the orientation of the optical tube. A 9x50 achromatic finder on its support is also delivered as standard.
In terms of the material used for the primary mirror, the 254mm and 305mm Dobsonians benefit from a Pyrex glass which has the effect of reducing the effects of dilation due to thermal changes. Indeed, its coefficient of thermal expansion is lower than standard glass, which reduces its warm-up time and ensures better image quality.
- 1 Newton Sky-Watcher 254/1200 optical tube
- 1 Dobsonian azimuth mount
- 2 flowing eyepieces 31.75mm: Super 10mm and Super 25mm
- 1 finder 9x50
- User manual in French