Sensor Size, Focal Length and Field of View

Once you decide on an object to photograph, you need to figure out how you are going to frame it. If you have a collection of different camera lenses, you can pick the focal length that most aesthetically frames it. If you only have one lens, or a telescope, you will have to use that!

Generally, you will record the most detail in an object if you fill the frame with it, rather than shooting loose and blowing it up later.

The field of view you will record is dependent on only two things, the focal length of your optical instrument (lens or telescope), and the size of the sensor in the camera.

For a given focal length, a larger sensor will record a wider field of view.

For a given sensor size, a shorter focal length will record a wider field of view.

Formulas for calculating the field of view based on sensor size and focal length are located in the appendix.

Most astronomical planetarium programs also have the ability to plot a frame on a star map showing the field of view for a camera based on the sensor size and focal length.

When to Focus

Once you have found a method of focusing that allows you to achieve consistent excellent focus, you have to decide at which point in the object acquisition and framing procedure that you want to focus.

If the object is bright or has bright stars in it, you can acquire the object first and then focus.

If there aren't any bright stars in the field to focus on, you will have to move the telescope to a bright star nearby and focus, and then move the scope back, re-acquire, and re-frame the object. Yes, this is a pain.

Remember that if you have a telescope that suffers from mirror movement or flop, the focus can be thrown off if you move the scope a large distance after the initial focusing. In this case use a star as near to the object as possible to focus on.

Locate the Object

Most deep-sky objects are faint. They can be difficult, if not impossible, to see through the viewfinder of the camera when it is hooked up to the telescope. Live View is not sensitive enough to show deep-sky objects. How do you locate a deep-sky object that you can't even see in the camera? There are several different ways.

• Computerized Go To - If you have a computerized Go To mount, you can simply punch in the catalog number or coordinates of the object and it should appear in the field of the scope. You may have to move it slightly to center it perfectly, depending on the accuracy of your mount and alignment.

• Setting Circles - You can look up the coordinates of the object and then use the mount's setting circles to acquire it in the scope.

• Star Hop - If you don't have setting circles, you can use the time-honored tradition of star hopping to the object of interest.

• Guidescope / Finder - If you take the time to accurately align your finder or guidescope with the main scope, you can use them to find the object. The finder may or may not be able to show the object, depending on its size and brightness, but it should be able to get you close. Then you should be able to detect it at least in the guidescope. Since your guidescope is aligned with the main scope, the object should then be in the main scope.

• Main Scope - Just use an eyepiece in the main scope to find the deep-sky object and then replace the eyepiece with the camera. This can be a problem if you have shot flat-field frames and can't change the orientation of the camera because removing the camera makes it very difficult to replace in the exact same orientation.

After you think you have found the deep-sky object, shoot a test frame to be sure.

Frame the Object

You can try to center and frame the object through the eyepiece of the camera body, but if you're using a refractor or Schmidt Cassegrain, and the scope is pointed overhead, you're going to find this difficult.

You can also try to center and frame the object with a right-angle finder accessory that attached to the back of the camera's eyepiece, but the field of view will be very dim. Even if you can see a faint deep-sky object in the eyepiece of your telescope, you almost certainly won't be able to see it through the right-angle finder. Even identifying star fields to match with a star chart will be very difficult.

It is possible to use a flip mirror, to switch between an eyepiece and the camera. The object is found in the telescope with a regular eyepiece, and then the mirror is flipped to send the light from the scope to the camera. One problem with flip-mirror devices is that they require a certain amount of back focus distance between the focuser and camera, and can cause vignetting on certain optical systems.

Align the camera parallel to the right ascension (east - west) or declination (north - south) of the scope, depending on which way the object is elongated.

Don't change orientation if you are shooting flat field frames.

Shoot a test exposure to check the framing.

Special Considerations

If you are shooting flat field frames, do not move change the camera's orientation to the telescope tube after shooting them. In this case, if the object requires special framing different than you have been using, you will need to acquire the object, frame it, move to a focus star, focus, shoot flat-field frames with a light box, re-acquire the object, and re-center.

Test Exposures

With a digital camera, we have the luxury of being able to review a test shot on the LCD on the back of the camera, or even download the image to a laptop. This way we can check to be sure the object is centered and framed exactly as we want it to be. The test exposure doesn't have to be very long, perhaps a minute at the highest ISO the camera is capable of. Then a strong contrast stretch in software will reveal any faint nebulosity or galaxy details to see if it is correctly framed. This frame can also be used to check the focus of the telescope or camera lens.

After acquiring and framing the object, another test exposure, or series of test exposures can be made to determine the best exposure for the equipment and conditions. Determining the correct exposure will be discussed in a later section.

Plate Solving

Software can be used to "plate solve" an image that you have shot. Commercial programs such as PinPoint Astrometric Engine, and freeware programs such as AstroTortilla and Star Locator Elbrus will examine patterns of stars in your image and match them to index files, and determine the exact size of your field, and the exact location of the center of the field in terms of right ascension and declination.

If you know the exact center of the field you want to shoot, you can then adjust the pointing of the telescope to match these coordinates.

Some of these programs, such as AstroTortilla, will interface with camera control programs such as Backyard EOS and AstroPhotography Tool to exactly center your telescope on an image of the same field that you have shot previously, even on a different night. It works like this:

• You shoot an image of a particular field.

• You solve that image with the plate-solving program.

• The program determines the exact center of the field for that image and stores this information in a bookmark.

• The next night you go out to shoot that same field, you call up the bookmark in the plate-solving program.

• The plate-solving program then tells the camera-control program to take a short exposure of wherever the scope happens to be pointed.

• The plate-solving program then solves the exact location of this test exposure. It now knows exactly where the scope is pointing, and it knows where you want it to be pointing from the bookmark.

• The plate-solving program then instructs the telescope to move to the location of the field you have previously shot and stored as the bookmark.

• The plate-solving program instructs the camera-control program to take another short text exposure, and it solves that frame.

• If the pointing is not exact, it moves the scope and takes another test exposure and solves it.

• This process is repeated until the scope is pointing exactly the same as the original image. You can specify how accurate you want this pointing to be in the plate-solving program. It usually only takes a couple of test exposures to get the pointing quite accurate.

• If your camera is oriented exactly the same as it was on the night of the original exposure, it should be framed exactly the same.

The trick to using a plate-solving program like AstroTortilla is to experiment with it on a cloudy night and let it solve the original image then, not under clear dark skies when you should be shooting light frames. Short exposures of 10 to 30 seconds seem to work best for plate solving. There are a number of parameters that usually need to be adjusted inside the program to get it to efficiently solve images. It also helps to know the field size and input this into the program before doing a plate solve, as this will greatly speed up the plate-solving process.