1. Star shape in the live view of a DSLR camera and manual adjustment: This can be tough. Depending on your camera, it could mean pressing some buttons to zoom in to a star. Your telescope starts to move, it is dark - you hardly see the control elements of your camera, it is cold - your fingers are either stiff or in gloves. And finally, success is not guaranteed.

2. Bahtinov mask with live view of a DSLR camera and manual adjustment: Such a mask creates a diffraction pattern which depends on the focus - but you need a really bright star to have an idea, what the pattern looks like. And again...it could mean pressing some buttons to zoom in. Your telescope starts to move, it is dark...

A typical diffraction pattern from a Bahtinov mask. Focus is good in this case, as the middle spike is very close to the center.

3. Bahtinov mask, astro camera and live view on the PC with software evaluation: This combination now provided much better results. With a sufficient exposure time of a couple of seconds, the spike pattern was well visible. The Astrophotography Tool provides a Bahtinov Aid module which calculates the quality of the focus based on the pattern.

But...it is dark, it is cold, right hand on the Crayford focuser, the other hand holding the laptop. Depending on the star brightness, there are still some seconds required for image capturing, not to forget the time the telescope needs to settle after manual adjustment.

4. Electronic focuser: This is where the fun begins. A stepper motor with 5V power supply via USB port with manual adjustment or computer adjustment and a temperature sensor.

Consider - the focus zone of my setup is about 70 µm, which corresponds to 1.8° on my Crayford - tough exercise at -10° C when focussing manually.

The ZWO EAF can be ordered with a manual control box, but it can also be controlled by ZWO software or the Astrophotography tool. No ore touching the scope for focussing.

5. Electronic focuser with software automatization: The Astrophotography Tool provides a Focus Aid Module which moves the focuser with the stepper motor, takes an image, analyses the image (e.g. based on stars' shape or image pattern frequencies) and calculates the best focuser position.

This is a must, if

• your setup is sensitive to temperature changes,
• you are expecting mechanical focus changes caused by change of the scope orientation during the night,
• you are using different filters (the thickness of my Skyglow filter is different from my dual-band filter, moving the focus by 340 µm which is about 5 times my focus zone, and it took me an hour to find the issue).

The Astrophotography Tool provides several parameters for continuous update of the focus, e.g. by Nth image, by change in temperature, by filter change or continuously with the temperature.

Always remember...imaging time can be precious. You do not want to fiddle away with your focuser setting.

So finally, the ZWO EAF controlled by ATP became my tool of choice.