Update: 5 May 2017

Telescope Service / GSO 150/600 Newton

Un petit télescope très abordable destiné à la photographie du ciel profond à grand champ.

Telescope Service / GSO 6-in F/4 Newton telescope

A small, very affordable telescope intended to wide-field, deep-sky imaging.

The small telescope is sold as an Optical Tube Assembly only or with a mount. Since it is intended to image deep-sky objects with a large field, it is better to use a serious, motorized mount. I added a vintage guidescope on top, directly screwed into a M6 thread (the rings offer two of them on top, and two Kodak threads at bottom). The tube comes with the two rings, but with no dovetail. The focuser is far fom the aperture because the long, front part of the tube acts as a light shield and dew shield. Long-exposure images showed no reflection at all and there is no need for baffling. The dovetail has to be long enough to minimize problems while balancing the full assembly. There is little room to add accessories once the rings are installed. The telescope comes with all necessary dust covers; the aperture cover helps to get dark frames. The optical tube with rings, focuser and finder weighs 5.7 kg. The tube is made of aluminium and its length is 580mm for a focal length of 600mm. Prime focus is easily reachable, located outside the tube to offer a long backfocus for a filter wheel, a DSLR, a CCD camera, and so on. Due to its fast F/D ratio of 4, the newtonian telescope requires a coma corrector. The telescope is delivered with an extension tube and a 2-in-to-1.25-in reducer.

The OTA delivered by Telescope Service comes with a good 8x50 viewfinder with coated optics and a standard, Skywatcher-like attachment system. Ajusting the orientation is achieved with a smart (hereafter widespread) X-Y system of plastic screws and a spring. Unfortunately I broke the fragile, plastic screws after some nights, then I replaced them with steel screws (large knurls should be more handy). In my opinion, the O-ring is a classic bad idea; its diameter is a little bit too small and I thickned it with adhesive tape. The center of the image is quite correct while the borders suffer from distorsion. Anyway the viewfinder really helps to find faint objects, and the 50-mm objective perfectly fits this kind of telescope.

The mirror cell, external view. (1) locking screws. (2) push-pull screws. (3) the mirror is exposed to air for better cooling. Given its modest size, no fan is necessary to speed up cooling. On the other hand, we have to be aware of parasite light sources such as a computer screen or a street lamp which can penetrate into the optical tube and alter the image calibration: flat frames are unable to fix parasite light slowly drifting while the telescope tracks a celestial object. A noticeable impairment is that the push-pull and locking spanner adjusters exceeds the length of the tube: they may be exposed to accidental twisting during transportation. I must admit that the small optical tube does not offer room enough to protect them without interfering while adjusting the alignment of the mirror.

Once the mirror cell is unassembled, the bottom of the optical tube shows three long removable screws with springs (1) acting as simplistic push-pull screws. (2) The stops for the locking screws. The system is simple but relatively efficient and stable given the small size of the mirror. However, a F/4 newtonian requires very accurate collimation, and the operation reveals itself to be rather hard with such a simple cell. I spent many nights to achieve a proper alignment of the main mirror. There is no need for a complicated, nine-point, astatic cell with a 6-inch mirror, but this F/4 optics deserves more precise mechanics: I often turn and turn again the knurls with little effect on the diffraction pattern, likely due to the too small springs. To be honest, my 10-inch, F/4.7 newtonian is easier to adjust.

In order to secure the mirror, the clips (1) are usually strongly tightened for delivery from the supplier. This is not the case here, ensuring that the moderate pressure does not apply mechanical constraint on the mirror, so I did not detect any trefoil. Note the very clean bevels (2) on both sides of the mirror. GSO, for Guan Sheng optical, is a corean manufacturer renowned for the constant quality of his products. Until 2016, the precision was even mentionned. However, the F/4 GSO mirror shows a by far better quality than my previous Vixen F/4 mirror. The classic, SiO2 coating offers a reflectivity of 93%. Note the baize pads (3) acting as shock absorbers. (4) One of the three locking screws. This mirror is made of BK7 glass, its thickness is 20mm.

The precision, monorail Crayford focuser is remarkable, with no slack at all. The only concern is the limited load capacity (this is normal for a Crayford-style focuser). I use a 1.3-kg equipment in a awkward position (coma corrector + cooled camera + filter wheel) and the microfocuser is barely able to adjust the focusing even if the sturdiness adjustment screw (1) is strongly tightened to achieve a sufficient pressure on the monorail (4). Since the optical tube is a newtonian, I successfully managed the problem by orientating the imaging train at the exact opposite of the counterweight shaft; hence the load is parallel to the ground and the microfocuser can act correctly. The focuser has two symmetrical knurls (2) and a 1:10 demultiplication knurl (3) for precise focusing.

The locking screw of the focuser (3) is very reliable, with no shifting while thightening. A double scale (2) is engraved, both in inches and in centimeters. All screw threads (2) of the focuser are absolutely flawless, along with the ones on the extension tube and 1.25-in reducer. The eyepiece holder, the extension tube and the reducer have a brass compression ring to preserve the equipment.

An adhesive ring (1) to facilitate the collimation stands at the center of the main mirror, but I don't exploit it. The secondary mirror is 63-mm large to prevent from vignetting when the telescope is equipped with a 24x36 reflex. The obstruction of 42% is quite impressive for a newtonian, but this is necessary for wide-field imaging. (2) and (3) are screws for collimation of the secondary mirror: not very handfull because this require a screwdriver. The four-vane spider is very thin, 0.5 millimeter (4). The diffraction spikes are discreete and elegant. The secondary mirror holder is seriously designed; it is massive and stable. The secondary is glued rather than be maintained by clips, a nowadays standard solution to avoid both trefoil and masking: this garantees a clean diffraction pattern.

Voici une image prise avec un correcteur de coma GPU et une caméra ZWO ASI071MC-cool.

Here is is an image taken with a GPU coma corrector and a ZWO ASI071MC-cool camera.