DSLR Focusing

DSLR cameras have a viewfinder that sees the same field of view as the chip, which can be used for framing and focusing. Unfortunately, it can be difficult to look through the viewfinder when the camera is attached to the telescope. In addition, the human eye can itself adjust focus (accommodation). This means that it may look in focus to the eye, but is still out of focus to the camera’s sensor.  This problem is particularly acute with faint astronomical targets.

To make matters worse, the DSLR's built-in autofocus will not work for almost all astronomical targets. It is possible to get a rough focus through the viewfinder and a relatively bright star or the moon and planets. It is also possible to get a right angle viewfinder attachment with 2x zoom to make it easier to view. But critical focus is best achieved using software to evaluate focus. In fact with a computer controlled ASCOM-compatible focuser, a closed loop autofocus is possible through software control.

Focusing through image downloads is the only option for classical CCD cameras, which do not have viewfinders. Therefore the manufacturers of those cameras provided subframing (reading out just part of the image) and binning (reducing resolution by combining adjacent pixels) to make rapid image display possible. This makes focusing much easier. Unfortunately DSLR manufacturers design their cameras primarily for viewfinder focusing and/or autofocus, so they do not provide subframing and binning. The only option you have to speed up download is to use JPEG compressed format. The higher quality "RAW" downloads take much longer but are higher quality; switching to JPEG for focusing makes the process much faster. In MaxIm DL the readout mode is selectable on the Expose Tab.

Software focusing tools are also available. The Camera Control window can display focus quality both graphically and numerically. Of particular interest is the Half-Flux Diameter and Full-Width Half Maximum measurements, which tell you the diameter of a star image. The focus is simply adjusted for the minimum star size. These tools are not as effective on unbinned raw images because of the effects of the Bayer array, so you should either bin or select a different readout mode when focusing. For convenience, the Preset feature allows you to switch all these settings quickly.

In the past, manual focusing was the most tedious and time-consuming part of imaging sessions. However, this has changed with the advent of reliable autofocus techniques. The SharpStar™ autofocus feature can reliably focus as well as or better than manual focusing, but does require the use of a highly repeatable digitally controlled focuser, such as a RoboFocus, JMI SmartFocus, or Optec TCF-S. FocusMax http://focusmax.org is a highly recommended and widely-used autofocus tool that is compatible with MaxIm DL.