On the road to automating telescope rig for astrophotography, an autofocuser becomes an important tool to deal with focus changes through the night due to temperature, mirror flop, flexure, and changing filters. The ZWO EAF (electronic automatic focuser) and PrimaLuceLab’s ESATTO are two different classes of focusers that meet different sets of needs. Which one is right for you?
ZWO’s EAF does not sit on your optics train, but instead couples to a focus knob. For astronomers with very little backfocus to spare, the EAF will serve you well. There are a variety of telescope models that are supported by a variety of different mounting brackets from refractors to Newtonians even to Schmidt-Cassegrains. The EAF comes in the box with a bracket, screws, and flexible couplings to fit many common telescopes, and additional brackets can be purchased for particular models of telescopes, such as Schmidt-Cassegrains. The author uses an EAF on her Takahashi FSQ-106N refractor with the supplied bracket.
The EAF supports both ASCOM and INDI, as well as ZWO’s ASIair mini-computers, and has an additional port that can either be used with a temperature sensor or ZWO’s hand controller. The new version of the EAF does not require an additional 12V power supply, but instead pulls 5V power over USB.
The EAF performs very well. Takahashi refractors are known to change focus dramatically with ambient temperature, and the EAF makes re-focusing frequently during the night a snap. The author has had no communications troubles with her computer, and no backlash has been apparent. The EAF has a movement capacity of 5 kg (11 lbs) while remaining a small, lightweight box, and retails for $199.
Rather than coupling to a focus knob, the ESATTO can replace the focuser on some telescopes (such as refractors, Newtonians, and Ritchey-Chretiens) or provide a more precise way of focusing Schmidt-Cassegrains. It comes in 2, 3, and 4-inch apertures, has a draw tube length of 2 inches (15mm), can support 5 kg (11 lbs) of weight, has an internal temperature monitor as well as an external probe port, and can move as little as 0.04 microns (1.5 millionths of an inch) per step for precise focusing. It has direct WiFi to be controlled via a smartphone or computer (in addition to USB), and has a special port for PrimaLuceLab’s upcoming ARCO rotator. A variety of telescope-side and camera-side adapters are available to match many types of rigs. The author uses the ESATTO 2” with her Celestron 8” Schmidt-Cassegrain, which does not have mirror locks.
The ESATTO supports ASCOM, and the Focuser Manager software only runs on Windows. It will also run on PrimaLuceLab’s EAGLE mini-computer. It can be powered by USB alone or with an additional 12V power source, although the author found that it worked more reliably with the 12V power source.
The ESATTO performs very well. Focusing Schmidt-Cassegrains can be difficult because of their wide focus plateau, but autofocus using software is fast and easy. The author often has connection issues with the ESATTO, however – at the beginning of a night (so far, never in the middle of the night), about once every week or two, it has needed to be unplugged and replugged from both power and USB for the computer to connect to it properly. The focuser truly has zero backlash – focusing with a luminance filter, and then using a measured offset for narrowband filters, always results in perfect focus still. The ESATTO 2” retails for €625 ($751, at the time of writing).
The ZWO EAF and PrimaLuceLab ESATTO fall into two classes of focusers, and both have excellent performance and usability. The author has used both for the last year and has had success with both and minimal issues. The kind you get will depend on your telescope setup, backfocus requirements, and budget.
ZWO EAF $199 as tested
PrimaLuceLab ESATTO as tested $751