"What is the field of view or sky coverage of my CCD/telescope combination?" This is a basic question asked by most beginning astrophotographers.  This is easy to calculate with the field of view formula. Since the original formula can be confusing to non-mathmaticians like me, we will be using a simplified version that still gives accurate results.

The field of view is related to the size of the CCD chip or film for that matter, and the focal length of the telescope. The longer the focal length, the less sky is covered by the CCD. Changing telescopes is like changing lenses on an SLR camera. The longer the focal length of the lens, the more magnified the object. To determine the FOV , you have to go through the calculations for each camera/telescope combination that you use.

You will have to have the following data to find your field of view and if an object will fit in the frame:

1. The x,y dimensions of your CCD chip in millimeters
2. The focal length of your telescope in millimeters
3. The size of the object you wish to image in arcminutes or degrees

Since CCD chips are small, you will probably work in arcminutes of sky coverage. The formulas work just as well with film photography 35mm film is 24mm x 36mm.

These variables used are expressed below:

• S = dimension of one side of the chip (or film negative) in millimeters
• f = focal length of the telescope (this will change with telecompressers or barlows)
• ArcMin = arcminutes of sky covered
• Deg = degrees of sky covered which is useful for wide field images.

The formulas are:

• ArcMin = (S x 3438) / f
• Deg = (S x 57.3) / f

Let's try an example with a CCD chip that is 13.8mm on one side and 9.2mm on the other and a 180mm aperture telescope working at f/9. To calculate the sky covered we have to calculate coverage for each side of the chip. If your chip is square the results will be the same for both dimensions. We will express the value in arcminutes, so we'll use the first formula. We want to see how the galaxy M96 will appear on the CCD chip. This helps you frame the image, and  visualize how it will look.  KStars  lists its size as 7.5 x 5.2 arcmin.

The focal length of our example telescope is 180mm x 9 = 1620mm. If your telescope's aperture is expressed in inches, you can convert it to millimeters by multiplying the aperture by 25.4 millimeters per inch.

So the values for the example telescope and the CCD chip's wide dimension are:

• S = 13.8mm
f = 1620mm

Plugging in the values into the ArcMin formula the result is:

• ArcMin = (13.8 x 3438) / 1620
• ArcMin = 29.3 arcminutes of sky covered in that dimension

The other dimension  of the chip will be:

• S = 9.2
• ArcMin = (9.2 x 3438) / 1620
• ArcMin = 19.5 arcminutes of sky covered in that dimension

So the area of the sky covered would be 29.3 x 19.5 arcminutes of sky. Since M96 is 7.5 x 5.2 in size it will fit in the CCD and be well framed.