When it’s mated with a good quality apochromatic triplet, does a carbon fiber telescope tube give you any advantages over a standard aluminum one? We learn more with the Explore Scientific CF 102mm.
Pros: Good optics, lightweight, optimized for imaging
Cons: Basic manual focuser
Apochromatic (APO) triplet telescopes are held to be the instrument of choice for small-aperture astro-imaging. Explore Scientific offers three ranges of APO triplets. Mine is the mid-range (“Professional”) one that uses HOYA FCD100 for the middle lens element.
The normally-perceived benefits of a carbon fiber tube over an aluminum one are reduced weight and lower coefficient of thermal expansion. In the 102 mm size, the carbon fiber option weighs 24 ounces (0.7 kg) less than the aluminum option, a weight-saving I find to be a worthwhile on my lightweight setup.
However, the tube itself is only part of the assembly, which has an aluminum lens cell and focuser and, of course, a heavy glass lens. Taken as a whole, the low thermal expansion of the carbon fiber tube confers no advantages over aluminum, because the latter compensates for thermally induced focal length changes in the glass.
The carbon also has a lower thermal conductivity and greater thermal mass than aluminum. This means the telescope will have a longer cool-down time, but it also means that will be less affected by temperature fluctuations during an imaging session. Being black, the tube is a relatively good absorber and transmitter of heat; it will get very hot if you leave it in the Sun, and mine frequently accumulates frost on clear nights. Even though the dew-shield extends 4 inches (10 cm) beyond the front of the lens, a heated dew-strap is necessary if you observe where humidity is even moderately high.
The focuser is a very basic dual-speed (10:1) offering, with a graduated scale on the draw-tube which (provided you keep a record) makes it quicker to attain focus if you use different camera setups or swap over to visual use. If you want vibration-free focusing, a third party motorized focuser is an essential accessory. Explore Scientific also includes two 1½ inch (37.5 mm) extension tubes for increased adaptability (e.g. imaging with or without a flip-mirror in the optical train).
The mounting rings incorporate a standard Vixen Optics Sky-Watcher dovetail and a carrying handle that is slotted so that it doubles as a mounting bar for accessories such as a guide-scope.
I have been impressed by the image quality. I use mine with an Atik 414EX camera and do not notice any focus fall-off towards the corners of the 11.2 mm diagonal sensor, but users who have cameras with larger sensors find a field-flattener to be useful. This also reduces the focal ratio from f/7 to f/5.6, which is advantageous for deep sky imaging.