Orwell Astronomical Society (Ipswich)
Review of Helios ST80 Wide-Field, Short-Tube Refractor
The Helios StarTravel range of telescopes comprises instruments with apertures of 80 mm, 102 mm, 120 mm and 150 mm. The StarTravel 80 is the smallest in the range; it is a portable 80 mm, F/5 wide-field refractor. Two variants are available: one optimised for astronomical use and the other for terrestrial use. The former is the ST80; it is equatorially mounted and is supplied with a 90° star diagonal. The ST80 retails between £150-£200 and is the subject of this review. The variant optimised for terrestrial use is the ST80T; is uses the same optical tube assembly as the ST80 but comes with a 45° prism and rings/adapter to enable mounting on a standard camera tripod. (Note: a tripod is not included!)
I ordered an ST80 from Warehouse Express (http://www.warehouseexpress) in Norwich and it was delivered the following day. All components arrived in a box weighing around 5½ kg. The package included a screwdriver, spanner and simple instruction sheet to assist assembly.
The equatorial mounting has setting circles in both axes (Right Ascension or RA and Declination or Dec) with a fine adjustment screw to set the latitude to a particular location (51.5° for Ipswich). There are slow motion controls for both RA and Dec. The ST80 can be upgraded with a motorised RA drive as an accessory item. Use of tube rings rather than a fixed mounting block means the tube assembly and finder can be easily rotated to the most comfortable observing position. With slow motion control extension knobs attached, the movement in both RA and Dec axes was reasonably smooth with no serious slop or backlash at any point.
After using the ST80 on a few nights, I made two simple improvements to the equatorial mount to make the instrument easier to use when seated:
The 80 mm objective lens has an orange coating and performs very well considering the fine manufacturing tolerances required to produce such a short focal length. Defocused bright star images (e.g. Vega at 133x) either side of the point of focus displayed identical circular diffraction patterns showing that the objective is well corrected. The focussing movement is always smooth throughout the range.
The removable 6x30 finderscope provides bright clear images with a wide field of view. This makes the ST80 particularly easy to use and therefore suitable for beginners as well as more advanced observers. I found it provided a superior field of view to the 6x30 finder supplied with my vintage 60 mm refractor. A minor inconvenience occurs when locating objects at or near the zenith: a 90° eyepiece adapter for the finder would make its use on such objects much easier. An O ring supplied with the finder fits over the recessed part of the finder tube assembly which is inserted backwards (eyepiece end first) into the mounting bracket. The O ring pivots the finder at the front of the bracket allowing it to be aligned with the optical axis of the main optical tube assembly using two alignment screws located towards the rear of the bracket. A third spring-loaded point pushes the finder tube against the two alignment screws.
A universal camera mounting thread on the focuser allows virtually any 35 mm SLR camera to be attached to the scope. This is achieved through a T ring adapter (not supplied) which converts between a particular SLR camera thread and the threaded part of the focussing assembly.
Two 3.2 cm eyepieces are supplied with the ST80: a 25 mm Super Modified Achromat (SMA), an enhanced form of the standard Kellner eyepiece, and a 10 mm Kellner eyepiece. I was impressed with the optical quality of both eyepieces. The 25 mm compared very well with a 25 mm Plossl. The 10 mm eyepiece is fairly sharp but has shorter eye relief as is to be expected. The 25 mm and 10 mm eyepieces provide magnifications of 16x and 40x respectively; these double to 32x and 80x with a 2x Barlow lens. I found the ST80 performed well at all magnifications up to 133x, which I achieved with a 6 mm Plossl eyepiece and a x2 Barlow (133x was the highest magnification that I tried). Eyepieces plus Barlow lens attach to the ST80 via the 90° star diagonal which inserts into the main optical tube assembly.
The field of view using the 25 mm SMA eyepiece (without Barlow) is around 3.5° providing excellent wide-field viewing! One of the strengths of the ST80 is clearly its capability as a portable wide-field low magnification instrument. The Pleiades easily fit into the field at 16x and pretty well at 32x using the Barlow.
The 90° star diagonal provides an upright image with the left and right hand components inverted so it can also be used for daytime terrestrial observing with those constraints in mind.
I made observations initially at Orwell Park Observatory and later at my home in north Ipswich. I used the following telescopes for comparison purposes: at Orwell Park Observatory, the OASI 250 mm Dobsonian with 15 mm eyepiece (circa 100x) and, at home, a Prinz (Dixons 1960s vintage) 60 mm F/15 refractor with 25 mm Plossl eyepiece (40x magnification).
I made the following observations at home.
The Helios ST80 is an excellent value-for-money, portable telescope capable of providing good results on deep sky objects and planets if its limitations in aperture and focal length are appreciated. I feel it would suit beginners as well as more advanced observers who may want an additional portable scope. Although its short focal length means that it is not necessarily optimal for planetary observing at high magnifications, I found that it worked well at 133x when seeing conditions permitted. I'd therefore recommend an additional eyepiece (6 mm Plossl) to support lunar and planetary observing.
Readers will no doubt be interested to learn that the optical tube assembly is identical to that used in the Celestron NexStar 80 mm computerised refractor scope called the NextStar 80GT. The same optical tube manufactured by Synta has also appeared under other brand names including Orion (USA), Skywatcher and Konus (Vista).
Comparisons with other Synta ShortTube scopes are more difficult to quantify without direct experience. Reports from a variety of internet resources suggest that the 102 mm variant (in this case, Helios ST102) is excellent for low magnification wide-field observation of deep sky objects providing images noticeably brighter than the ST80. However, colour fringing on bright objects is slightly worse than the ST80 and performance degrades significantly at magnifications in excess of 150x. The conclusion on the 120 mm variant is that its performance is inferior to larger aperture Dobsonians available for around the same price (£350). I have seen no reports on the 150 mm variant. Strongly recommended are the 80 mm and 102 mm (ST80 and ST102). Of the two, the ST80 is the more popular scope.
http://www.scopereviews.com/ - review of Celestron ST80 (similar specification to Helios ST80) and comparison with Televue Pronto (costs around £700 more!) Other reviews include Meade, Takahashi, Astro Physics and Celestron telescopes plus various eyepieces and accessories.
http://www.cloudynights.com/ - another site with many telescope reviews including the Celestron FS80WA - similar specification to ST80 but no longer manufactured since superseded by NexStar 80GT.
http://www.weatherman.com/ - lots of different scopes covered - recommended! Many links to other telescope review sites.