The quest for an airline-portable telescope for visual astronomy

2020 February 3

Jeremy Shears

Introduction

The era of low-cost flights has, for many, opened the possibility of escaping cloudy and light-polluted Britain to observe from much better locations, offering more comfortable climatic conditions and access to skies which are invisible from our northern latitudes. I generally carry a pair of 8×40 binoculars when I travel overseas as they do not take up much room and provide wonderful views of the Milky Way. However, there are some occasions when a telescope is required – for example, to provide higher-power views of the planets, globular clusters and double stars.

Over the years I have tried various telescopes for travel; most recently, a 66mm ƒ/6 refractor and 60mm Maksutov–Cassegrain. The short-focus refractor was great for wide-field scanning, but was found wanting for high-magnification views of planets. The Maksutov–Cassegrain was the opposite: good for high-power views, but disappointing for Milky Way vistas. With the perihelic opposition of Mars coming in the summer of 2018, I was keen to find something I could take on trips further south, both to observe the red planet and to enjoy wide views of the deep sky. I also figured that with Jupiter and Saturn frustratingly residing at southerly declinations for the next few years, I could turn the travel scope on them.

My requirement was therefore for an optical tube assembly (OTA) that was compact enough to carry on board an aeroplane with even the most stringent size and weight allowance, while being light enough to mount on a simple-but-sturdy tripod that I could take in my check-in luggage. Since I was intending to do visual observing only, an equatorial mount, drive or other electronics were not needed; these considerations would save a lot of weight. In fact, the only accessories I needed were a finder and some eyepieces. Although compact reflectors could meet the requirement – and I am aware of people taking components of 300mm reflectors as hand luggage – I did not want to fiddle with disassembly and reassembly. Thus, I began to consider a relatively short-focal-length refractor with impeccable optics that would take high powers and not require collimation. Whilst some 100mm (and even larger) refractors are airline-transportable, I figured that a telescope of about 75mm would be even more convenient.

Having had some positive experience with Takahashi refractors (I own a Takahashi FS102 and two FS60s), I was interested to learn that the Japanese company also produces a 76mm instrument, the Takahashi FC-76 DCU, the tube of which splits into two for ease of transport:. I took delivery of the OTA in 2018 June and this report presents my initial impressions after using it for six months.

The Takahashi FC-76 DCU

The heart of the Takahashi FC-76 DCU (Figure 1) is an apochromatic 76mm ƒ/7.5 doublet fluorite lens. This is a Steinheil design with the fluorite component at the back and all surfaces anti-reflection coated. The tube is 80mm in diameter and comes with a non-retractable dew cap. The tube and dew cap are 605mm long and can be screwed apart into two halves: one 330mm long, containing the lens, and the other 280mm, including the focuser (Figure 2a). The total weight is 1.8kg. To combat scattered light, the inside of the tube is matte black and fitted with four knife-edge baffles.¹

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