Just looking through the viewfinder and expecting to get good focus with your eyes won't work very well. Eyes adapt too well to different views, and camera viewfinders are too dim to see well enough to achieve critical focus.

There are more than a dozen different ways to focus your camera. Some are easy, some are hard. Some work well, some not so well. We'll only go over a couple of easy and proven methods here.

Live View Focus

In section 408, we have already discussed, in detail, Focusing Camera Lenses using Live View, so here we will concentrate on focusing a DSLR camera when it is attached to a telescope.

Most all new DSLR cameras now come with Live View where you can preview the image on an LCD monitor on the back of the camera. You see what the sensor sees through the lens or telescope in real time, just like a video camera. You access the Live View settings under a custom menu. You turn on Live View on the Canon 1000D (Digital Rebel XS) by pressing the "Set" button on the back of the camera.

On the Canon 1000D (Digital Rebel XS), you access the Live View settings under the Custom Functions menu.

Unquestionably, the easiest way to focus is with Live View. It's also one of the best ways because it takes the actual image off the sensor and you can view it at high magnification. You simply turn Live View on, center a bright star in the box displayed on the screen, press the plus button on the back of the camera to magnify the image to 5x and then press it again to magnify to 10x. Then watch the image, and rack the focuser in and out slowly, just like as if you were focusing an eyepiece.

This is a video of the focusing process using Live View video mode on a Canon 1000D (Digital Rebel XS) to focus on the bright star Vega.

In the video at right, you will see the Live View start at 1x magnification with the magnify box in the center of the screen. If the star is not in the box, you can either move the telescope to center it, or move the box itself in the display with the direction arrow buttons or joystick button on the back of the camera. Pressing the plus button on the back of the camera magnifies and enlarges the box to 5x. Pressing the plus button again magnifies the view to 10x.

The video shows the star starting out of focus when it is first magnified. Focus is then racked in, going through the point of best focus until the star is out of focus on the other side of focus. It is best to go back and forth through the point of best focus several times, in smaller increments, so that you get familiar with what the point of best focus looks like. Then finally stop at that point.

If you are shooting a faint deep-sky object, you won't be able to see it on the Live View display. It will only work with bright stars, planets, or the Moon.

You can set Live View so that it will attempt to display the image with exposure adjustments for shutter speed and ISO. So if you have the shutter speed at 1/1000th of a second and the ISO set to 100, you are not going to be able to see a star. Set the shutter speed to bulb. Set the ISO to 1600. Use the brightest star you can find that is conveniently placed for viewing on the LCD on the back of the camera.

Live View is very sensitive to focus. If you are way off to start with, you won't see the star at all. Simply find a star and get it in the field of view of the camera, and then visually focus it through the viewfinder to get it as close to focus as possible. Then use Live View to magnify the star and focus it on the LCD on the back of the camera.

If you pick a really bright star or planet, you may find that you can achieve better focus if you use a shorter shutter speed. If you have the "simulate exposure" function turned on, the brightness of the object will change as you change the ISO and shutter speed. For a really bright object, use a shorter shutter speed. For faint stars, use ISO 1600 and the bulb setting. If a star is too faint, you may not be able to see it at all. The brightness of the star you will need will depend on the aperture of your scope or lens. You will need brighter stars with smaller apertures. If you try to focus on a star, and you have the scope reasonably close to focus to start with, and you can't see it at all at with the camera set to ISO 1600 and bulb, you will have to find a brighter star.

If you are shooting with a refractor, there may be times when the object you are shooting is overhead, and the LCD on the back of the camera is pointing down to the ground. It may be at a very uncomfortable position to view. In this case, you have several possible solutions:

• Focus on a star that is lower in the sky at a more comfortable position for viewing the LCD screen on the back of the camera. Then lock the focus, and move back to the object you want to shoot.

• Output the signal through the camera's video out port to a portable monitor or computer.

• Use an eyepiece diagonal with the tubes removed and hold it up to the LCD screen, or velcro it on. Some people have also made home-made mirrors to reflect the screen display.

• Buy a DSLR camera with an articulated LCD display screen like the Nikon D5000 or Canon 60D, 60Da, or T3i (600D). This screen pivots and swivels so you can adjust it for different viewing angles.

Live View will heat up the inside of the camera, which will increase the thermal noise, so turn it off as soon as you are done focusing.

When the star is focused, lock the focuser down so it can't move.

If you have a DSLR camera with Live View focus, you probably don't need anything else to help you focus your camera. Some folks think a Bahtinov mask helps with focusing. If you don't have Live View, and have to use trial-and-error, then a Bahtinov mask will certainly help.

Bahtinov Mask

Bahtinov Mask

Focusing with a Bahtinov Mask

A Bahtinov mask causes an "x"-like diffraction pattern with a line across the "x" that moves as the focus is changed. When your camera lens or telescope is at correct focus, the line will split the "x" exactly in half.

If you have Live View, you can use a Bahtinov mask to help you with focus if you have trouble judging when the stars are in focus. If you don't have Live View, you can simply take some test exposures, and then display the image on the LCD on the back of the camera and enlarge it to judge the location of the central line. Take more test exposures and vary the focus each time until the line splits the x. Then you are at focus.

Don't forget to remove the Bahtinov mask after you have achieved focus and before you take the actual images.

Astrojargon has a web page generator to make your own Bahtinov mask and Spike-A sells a commercial version.

Autofocus

If you are using a camera lens, you may be able to use autofocus if the lens is fast enough, and the star is bright enough, and you can get the star exactly on one of the little focus detectors on the viewfinder, which is not as easy as it seems.

This usually works better with longer focal length lenses that have fast focal ratios. and doesn't work that well with wide-angle lenses.

You may need to illuminate the viewfinder to position the star exactly on one of the autofocus spots. These are usually indicated in the viewfinder.

You will not be able to use the camera's autofocus to focus a telescope.

Special Considerations

• Curved Focal Planes - The focal plane is not flat in most scopes, it is curved. But the imaging sensor is flat. This can cause the stars in the corners of the frame to be out of focus. Unless you are using a field flattener, you will experience this problem to one degree or another.

  A field flattener is the best solution to this problem. If you don't have one, you can focus on a star about 1/3 of the way out from the center to the edge of the field. This will average out the focus so that more of the field will be closer to focus and the stars in the corner will be better, but the stars in the center will be worse. This method is a compromise that you will just have to try to see if you can live with the results.

  Another alternative is to focus on the stars in the center of the field and crop out the out-of-focus corners later during image processing.

• Mirror Movement - Most Schmidt-Cassegrain telescopes focus by moving the mirror in the tube. When the focusing direction changes as you rack the focus in and out, this usually leads to a shift in the mirror tilt angle, which makes the star jump in position in the field. This shift can also cause the position of focus to change. This can make focusing with a Schmidt-Cassegrain difficult. This problem be overcome by always coming to focus and stopping from the same direction, or by installing a third-party external focuser and locking down the primary mirror.

• Focus Change Due to Temperature Change - Because the metal parts of the telescope contract and expand when the temperature changes, so will the focus. Focus can also change during the night as temperatures drop. This can affect all types of telescopes. Monitor the temperature, and refocus periodically through the night.

Right Angle Finders

Canon's Right Angle Finder

The right-angle finders are sometimes considered as a method of focusing because they offer a little additional magnification, such as 1.25x and 2.5x. They do offer a more convenient way to look through the camera if it is positioned at an awkward angle.

But while these finders work ok in the daytime, they are difficult to use to achieve accurate focus through a telescope or camera lens at night. They are also expensive.

The real problem is that the view through them is incredibly dim. Also, you have to center your eye exactly in the sweet spot of the optics to be able to see a really sharp image. Because this is so hard to do at night, you can easily be fooled into thinking that you have achieved an accurate focus when you really haven't.

Note that you will not be able to see any deep-sky objects through such a finder and usually only the brightest stars.

I do not recommend trying to use a right-angle finder to achieve accurate focus.

Focusing - The Bottom Line Focusing is critical for good astrophotos. It is not that hard to do with a newer DSLR camera with Live View, or with an older camera with a Bahtinov mask. With Live View: Use manual exposure and manual focus. Pick the brightest star or planet you can find in the sky. Get reasonably close to focus first with the viewfinder. Set the camera to ISO 1600 and the shutter speed to the Bulb setting. Magnify the view to 10x in Live View. Go through the point of best focus several times so you know what it looks like, and then stop at it. Lock down the focuser after you have achieved best focus. With a Bahtinov Mask: Focus visually until the moving spike bisects the "X", Take a test exposure. Enlarge the test exposure to 10x on the LCD on the back of the camera and inspect the Bahtinov cross. Adjust the focus and take another test exposure until focus is correct.