One of the first things a budding astrophotographer discovers is that focusing a telescope at night through a regular 35mm camera is not a very easy task. Astrophotography is perhaps the most demanding type of photography there is in terms of intolerance of poor focus. Being just slightly away from perfect focus can lead to fuzzy and bloated stars on the film. A 1/32nd turn of the focus knob can make the difference between a mediocre shot and a magazine quality shot. To make matters worse, finding that precise focus is extremely difficult in the typical astrophotography conditions of near total darkness, often freezing temperatures and the fatigue one feels at 2:00 AM.
As I will explain in this article, knife edge focusing is really the best way to reliably achieve precise focus in astrophotography. However, I also recognize that as a person gets into astrophotography they are not likely to jump right into knife edge focusing. Focusing through the camera using the focusing screen is probably the most common way novice astrophotographers focus their shots. There are several techniques and some equipment that can be bought or made to dramatically help in achieving a good focus on a focusing screen. I think it is important to make the most of the equipment that one has and to take one step at a time on the path towards astrophotography excellence. It is important to recognize that it takes a lot of effort and time to master any skill, and astrophotography is no exception.
2- Focusing screens
All 35mm Single Lens Reflex cameras (SLR) have a mirror in the light path
that redirects the incoming light up to a piece of glass or plastic that
has a matte surface at the exact distance as the film plane. It is important
that this focusing screen provide both a surface for the light to be focused
upon while at the same time be clear enough for the image to be seen in
the viewfinder which is on the other side of the screen. In daylight photography
there is such an abundance of light that a densely matted screen is ideal.
With astrophotography where there is very little light to work with the
normal daylight focusing screens defuse the light too much. The most obvious
thing to do when confronted with the extremely dark screen of a typical
35mm camera is to install a brighter focusing screen.
I had the bright idea when I started to do astrophotography of removing the focusing screen all together on a really old Minolta I bought used in high school 15 years ago for $45. I shot a test roll of film of the moon with this setup. The image through the viewfinder looked just like it did looking through an eyepiece which made focusing extremely easy. Of course as soon as I got the film processed every single shot was WAY out of focus. Without a focus screen for the image to lay on my eye adjusted a tremendous amount and so what looked to be perfectly focused for my eye was not even close on the film.
The standard daylight focusing screen is too dark for astrophotography work while not having a focusing screen at all simply permits the eye too much latitude in judging the focus point. The thing to do then is to find a screen that is a compromise. If you do not have the type of camera where you can change the focusing screen, you will have to make due with what you have, or use another method of focusing the telescope besides through the camera (See the section on Knife Edge Focusing).
With my Olympus OM1, I have experimented with 4 different focus screens. One of the main reasons that I got my Olympus OM1 was for this feature.
The 1-12 Olympus focusing screen is a very bright and very clear
screen. In fact the central region of this screen is completely clear with
the exception of cross hairs on the focal plane. The rest of the screen
has a very fine Fresnel grading. It is a joy to use, and a joy to focus
on as it looks almost as bright as looking through an eyepiece while looking
through the camera viewfinder. The joy usually stops when the film comes
back from the processor however. I have tried to use this screen now for
about a year. It is possible to focus with it using the Fresnel grading,
but it is very difficult to know when you have a good focus and so it is
impossible to have any confidence in your focus using this screen.
The 1-13 Olympus focusing screen is the screen that came with
my camera. It is great for daylight focusing as it has a central prism
and a nice matte surface, but it is clearly too dark for astrophotography.
The 1-8 Olympus focusing screen is the screen I most often use
after struggling with the 1-12 for so long. It is a completely matte screen
with no Fresnel grading to distort the image or trick your eye into thinking
it is well focused when it is not. When I reach focus with this screen
I can be fairly confident that it will be in focus on the film as well.
It is not nearly as bright as the 1-12, and so framing my shots, and even
seeing what I am photographing can sometimes be difficult, but it is a
good compromise screen.
The Bettie IntenScreen Plus Is perhaps the best focusing screen available for astrophotography. It is slightly brighter than the Olympus 1-8 but far darker than the Olympus 1-12. If you don't plan on ever getting into knife edge focusing, this is the screen to get. I personally just could not justify the cost of the Bettie compared to the quite comparable Olympus 1-8. The Bettie costs about $80 while all the Olympus screens are only $35. I've got the Olympus 1-12 and the 1-8 for less than the cost of the Bettie screen.
Getting a brighter focusing screen will make a lot of difference in your ability to focus for astrophotography. However, there is a catch 22 involved with using a focus screen at all for astrophotography. A good focusing screen must defuse the light in order for it to display an image on the screen. This isn't a problem with daylight photography, but in astrophotography where one is trying to focus on stars which are essentially point sources, defusing the light is the last thing you want to do. Finding the exact focus by looking at a star on a focusing screen is a very hit and miss operation with any focusing screen.
The human eye is also not well suited for focusing at night on point sources of light. It has a marvelous ability to adapt to small deviations of focus even through a focusing screen. Just looking through the viewfinder on a brighter focusing screen like the Olympus 1-8 or Bettie IntenScreen will rarely permit you to achieve truly perfect focus. The focusing screen is still defusing the light and the eye is just too good at compensating for small deviations in focus where the film has zero tolerance for deviations in focus.
3- Focusing Masks and Magnifiers
When looking at a slightly out of focus star image on a good focusing screen it is still often difficult to tell exactly where the best focus is. There are two ways to help in determining the best focus on the screen which work even better when used together. One way is to magnify the image on the focusing screen and the other is to create a mask to place over the front of the telescope.
There are a number of ways to magnify the image on the focusing screen. If you have a camera with a removable prism (like the Nikon F series), you can make or buy a magnifying viewer which takes the place of the prism and looks directly down on the focus screen. If you can't remove the prism you can still magnify the image on the focusing screen by looking through the viewfinder with a 6x finder scope, or small spy style telescope. This can be quite awkward and you must make sure the small scope is focused on infinity. I generally use the Olympus Verimagnifinder as it takes the image from the viewfinder at a right angle and it has a switch which can select 1.2x or 2.4x.
Putting a two holed mask over the front of the scope can help in finding focus as well. I made my mask out of black cardboard. It doesn't have to be pretty and does not have to be precise. To the left is an illustration of what the mask should look like. It should be big enough in diameter to cover the full aperture of your scope, and the holes should fit on either side of the secondary mirror. If you have a refractor, just make the mask so it fits over the objective lens and has two holes on either side.
Below, you see an image of what the slightly out of focus stars look like with the full aperture double holed mask on the front of the scope. It is simply a matter of moving the focuser to bring the double star images together. When the two stars are perfectly centered, like you see on the right, you know you are very close to best focus. To help even further, magnify the star images. You want to be able to see the slightest difference between either side of focus and the rangefinder effect of the mask helps quit a bit. The mask also works very well with extended object like the moon or the larger planets like Jupiter.
4- Knife Edge Focusing
Knife Edge Focusing is the best way to focus for astrophotography since
it does not rely on your eye's ability to distinguish a single point of
focus but instead creates a highly sensitive pattern that is easy to see.
Knife edge focusing is used by all the top astrophotographers in the world
because it consistently achieves an unquestionable precision of focus without
relying on one's ability to focus one's eye well in near total darkness,
and does not rely on the questionable focus achieved looking through ground
glass.
As I mentioned earlier, a focusing screen has a frosted surface which defuses the light slightly. When trying to reach a very precise focus
on a point source like a star, the diffusion actually makes distinguishing
the true point of perfect focus very difficult. As you can see in the figure below, there is a subtle difference between slightly out of focus and
perfect focus. In the field, this difference is even harder to distinguish
than the illustration would suggest. Even when looking at the star image
under magnification (like described above) it can be something of a guess
where the exact focus point is.
A knife edge focus works by intercepting the light cone creating an
apparent dark line on the image seen past the knife edge. Focus is achieved
by wideing the pattern created by the star, until it takes up the whole
field and fills the circle. Once the pattern fills the field of view,
one moves the scope from side to side with the slow motion controls.
If the pattern blinks off when moving the scope one way, and then blinks
back on when moving the scope back it is perfectly focused. If the circle
has a line move across it when the scope is moved, it is not
in perfect focus yet and it is necessary to make a fine adjustment to the focus
until it simply blinks on and off.
I have removed the Ronchi screen from my SureSharp and have placed two
very thin razor blades just a hair width apart in its place. I was careful to place these razor blades exactly on the same plane as the old Ronchi screen to keep the focal plane to be exactly the same. This
gives me a much cleaner pattern and at only about 1/200th of an inch separation,
gets me much closer to perfect focus before I have to move the scope to
see if the image blinks off and on.
Sky and Telescope's Test Report. Better Ways to Focus Your Telescope.
Chuck Vaughn's article on knife edge focusing.
Build your own knife-edge focusing
camera body by Rockett Crawford.
A very good article on Doc G's webpage about focusing for astrophotography.
The SureSharp.
There are several different commercial knife edge focusing tools available
on the market. Probably the most popular is the Spectra Systems SureSharp Click here to see their webpage.
It costs a whooping $135 and an additional $49 for the right angle viewer which is all but essential for refractors and SCT's. It technically does not
actually use a knife edge, but rather something called a Ronchi screen.
A Ronchi screen is a pattern of parallel black and transparent lines spaced evenly apart. It works on the exact same principle as the
knife edge, but gives a more distinct pattern when the star is way out
of focus. I see little use for the additional lines of the pattern for my focusing, as when I rough focus through the camera I always get only a single line anyway. I found the Ronchi screen included with the SureSharp to be too course and the image produced too fuzzy. The spaces between the lines are too
wide in my opinion (at about 80 lines per inch). I made a Ronchi
screen that was 150 lines per inch but the results where disappointing because
even a 600 dpi laser printer just didn't make a sharp enough boarder between
the line and the space and so the pattern it created was very soft and
thus difficult to use. In fact looking at the Ronchi screen
that comes with the SureSharp, it appears to be a pretty poor edge as well
and was quite likely made using some type of printer on clear plastic instead of being laser etched like a quality Ronchi screen should be.
Having a razor sharp edge is critical to the creation of a sharp and distinct pattern. 
The illustration here shows what the SureSharp looks like. It
is basically a piece of machined aluminum in the shape of a bell. One
end fits over the custom T-Ring which places the Ronchi Screen (or razor blades) inside at the exact same focal plane as the film in the camera.
On the end opposite the T-Ring is a window where one looks through to see the pattern produced by the screen. To use the SureSharp,
the camera is removed and the focusing is done with the SureSharp in place. One then simply
looks through the SureSharp at the pattern created as the star light passes
through the Ronchi screen (or razor blades) while adjusting the focus.
Once focus is achieved, the SureSharp is removed and the camera is attached
to the T-Ring as normal with the shot perfectly focused.
Other articles on the Web that deal with astrophotography focusing:
