A good focus is critical to achieving the sharpest details in an image viewed through a telescope whether using an eyepiece for visual observation or a camera for EAA or astrophotography. Because the human eye is capable of focusing over a wide range of distances it is not difficult to achieve a satisfactory focus for visual work simply by adjusting the focuser until the image looks sharpest through an eyepiece. Our eyes can adjust to slight focus offsets or changes in focus with temperature without much difficulty. But for astrophotography or EAA the Critical Focus Zone (CFZ), or distance over which focus is limited by seeing, tube currents, diffraction, can be a fraction of a mm and a camera cannot adjust for deviations from this like the human eye. The CFZ is defined by equation below and depends upon the focal ratio of the telescope and the wavelength of light observed:
CFZ = 4.88 x (f-Ratio) ^2 x L
where the CFZ is given in microns and L is the wavelength of light in microns. Since L doesn't change much over the range of wavelengths we observe (~0.4 to 0.6 microns) we can use the wavelength for green light, 0.5 microns. For a telescope at f/10 we have:
CFZ = 4.88 x (10)^2 x 0.5 = 244 microns
while for f/2 (for example using Hyperstar on a SCT) we have:
CFZ = 4.88 x (2)^2 x 0.5 ~ 10 microns
As you can see, the CFZ is not only very small (244 microns = 0.244mm), it reduces dramatically with focal length because the light cone becomes much steeper at shorter focal lengths thereby compressing the distance over which focus can be achieved. You can see how challenging achieving a sharp focus can be at very short focal lengths. This is why focusing aids are critically important for EAA and astrophotography.
While focusing by eye is not sufficient for EAA it is at least good enough for an initial rough focus. To make life easier in the dark it is a good idea to focus during the day time using a distant object like a power pole or tree. This can save a lot of frustration later since one can be centered on a star but not even know it because the light from a badly out of focus star is spread out so much as to be nearly invisible. If like me you do not have sufficient line of sight from your backyard to a distant object for focusing you can use the craters on the moon to obtain the sharpest possible image. If the moon is not visible but Jupiter or Saturn are, another good method is to adjust the focuser until the planet's moons become visible. But this method will have to wait until the sky is dark. Likewise, rough focusing at night can be achieved by observing a star cluster and adjusting the focuser until the number of stars in is maximized.
To simplify the process for future nights, make a note of the number of turns from full clockwise or full counter clockwise rotation of the focuser needed to achieve focus. Do this for each optical configuration such as at each focal ratio for your telescope. This way, you will only need to do this once and will be able to quickly achieve rough focus with your telescope in any optical configuration at night without wasting time or getting frustrated.
Mechanical Focus Aids
To achieve the best possible focus, one or more focusing aids are necessary. The most common and simplest is a focus mask. Focus masks of one form or another have been around for quite some time. In 2005 Pavel Bahtinov invented what has become the premier and most widely used focusing mask. This mask consists of a set of three grids etched into a thin plastic sheet which, when placed in front of the entrance to the optical tube, creates a diffraction pattern from the light passing through it. The diffraction pattern consists of an "X" with a vertical line passing through the "X". Precise focus is achieved by adjusting the focuser until the vertical line bisects the "X". The vertical line will move left or right of the center of the "X" as the focuser is moved in and out of focus. A point source such as a star is required to create the diffraction pattern. Use a bright star and/or an exposure of 1-2 seconds to obtain a bright and large diffraction pattern for greater sensitivity. Also, use the camera's zoom feature to magnify the diffraction pattern to achieve the best sensitivity.
Bahtinov masks are readily available in sizes to fit most telescope apertures and even come with a center cut-out to accommodate the secondary mirror on an SCT. Many make their own Bahtinov masks as I did meticulously cutting out the pattern in a piece of cardboard for my 14" SCT when I could not find a ready made one at that size. A thin but rigid plastic sheet is a better choice, but my cardboard Bahtinov mask lasted many years until I sold the 14" SCT.
Most telescope focusers have a rough and fine focus knob to help achieve a sharp focus. If the focuser that comes with your telescope does not have a fine focus it may not have sufficient sensitivity to achieve the desired sharp images and may need to be replaced with an after market focuser. This is especially true for SCTs which do not have a fine focusing control. There are a number of manual fine focus replacements from companies like Starlight Instruments which are made specifically for SCTs. Also, JMI sells motorized focusers for SCTs which can be controlled by a hand control or via a computer. Both Celestron and Meade provide motorized focusers with fine adjustment control which can help to achieve sharp focus.
Since EAA entails the use of a camera and, most likely, software to operate the camera, automated focusing is an excellent way to go. There are many different software available for automated focusing either as a stand along function or as a utility in a larger software suite.
FocusMax is a stand alone software which automatically adjusts the focuser on both sides of focus to obtain a V-Curve from two intersecting lines which define a precise focus where the lines intersect. FocusMax uses the Half-Flux Diameter (HFD) to determine the best focus position. The HFD is defined as the diameter of the circle containing half of the star light (flux), which is spread out due to the Gaussian nature of starlight caused by seeing. The smaller the HFD the better the focus and the better the seeing. Since FocusMax connects to and automatically adjusts the focuser, an electronic focuser which can be recognized and controlled by the software is required. A manual focuser will not work in this case.
@Focus3 is an excellent focusing utility in The Sky X software. Like FocusMax it requires an electronic focuser which is connected to The Sky X software and @Focus3. It uses the Full Width at Half-Maximum to determine the best focus position. Much like FocusMax, @Focus3 adjusts the focuser on both sides of focus to generate a curve of light intensity versus focuser position. But instead of a V-Curve it produces a bell-shaped curve. Measurement of the width of the curve at half the maximum height defines the FWHM which is then used to find the peak of the curve for the best focus point.
Many EAA'ers use SharpCap for real time viewing, live stacking and on the fly processing. SharpCap offers 6 different focus utilities. There are 3 different utilities for deep sky objects, two of which use the FWHM metric to define the best focus. One uses measures the FWHM on a single star and the second obtains an average FWHM on multiple stars in the field of view. The third deep sky focus utility requires a Bahtinov mask and determines the best focus at the point where all three lines created by the diffraction pattern intersect. SharpCap generates a focus score for each focusing utility with the best score giving the best focus. There are also 3 different focusing utilities optimized specifically for planetary viewing which derive scores using measurements of contrast or detail in the image to generate a focus score. SharpCap's focusing utilities can be used with a manual focuser or with an electronic focuser controlled by SharpCap. In either case, the observer adjusts the focuser either by hand or through the SharpCap software. Unlike FocusMax and @Focus3, the process is not completely automated but still works quite well.
As the night air cools, the telescope tube will shrink causing the telescope to go out of focus over time. This is especially true for an Al tube compared to a graphite tube. For this reason it is important to refocus throughout the night. One technique is just to refocus at a fixed time interval such as every 30 minutes which can be done manually or automatically if the right software is used. A more sophisticated method is to use a temperature sensor connected to software to automatically refocus for every half a degree change.
Achieving the best possible focus need not be difficult nor expensive. For those wanting simplicity a Bahtinov mask is the way to go. For those wanting automation there are many electronic focusers with compatible software which can make the process practically invisible to the user. Obtaining and maintaining a sharp focus throughout a viewing session will insure the best possible images and the most detail limited primarily by the seeing conditions.