Right Declination Astrophotography

Buying the correct Telescope & CCD camera

Field of View

The field of view (FOV) that your camera will see through a given telescope is determined by physical size of the CCD sensor and the focal length of the telescope.

Note that this has nothing to do with the number of pixels.

A CCD that has 512 x 512 pixels (262,144 pixels )that are 20 microns (4.47 x 4.47 uM) square will have exactly the same field of view as a CCD with 1024 x 1024 (1,048,576 pixels) that are 10 microns (3.16 x 3.16 uM) square even though the latter CCD has four times as many pixels.

This is also why binning 2x2 or 3x3 affects resolution but does not affect the field of view of the CCD. Larger CCDs have larger fields of view at a given focal length.

You can change the field of view of a CCD only by changing the focal length of the telescope, by using a focal reducer you shorten the effective focal length of the telescope.

You can increase the field of view (and make the image brighter in the process). By using a barlow or eyepiece projection you effectively lengthen the focal length of the telescope and decrease the field of view (and make the image dimmer in the process).

In order to determine the field of view for a given CCD, note the CCD's length and width dimensions (or diagonal) in millimeters and use the formula to determining the field of view for that CCD through any telescope as follows:

(135.3 x D ) / L = Field of View in arcminutes

where D is the length or width dimension of the CCD in millimeters, and L is the focal length of your telescope in inches.

You can use the same formula to find the diagonal field of view if you know this dimension. So, for example,

if you wanted to know the diagonal field of view of the STF-8300 when attached to a 5" F/6 telescope you would first determine the focal length of the telescope by multiplying its aperture, 5 inches, by its focal ratio, 6, to get its focal length, 30 inches.

The diagonal dimension of the CCD is 22.5 mm. To calculate the field of view multiply 135.3 x 22.5 = 3,044 and then divide by 30 = 101.5 arcminutes. By way of comparison, the diagonal field of view of the STXL-11002M through the same telescope would be 135.3 x 43.6 = 5,912.6 divided by 30 = 197.1 arcminutes, about twice the field of view.

PART 2

The image scale of your system (FOV) is dependent upon only two factors- your CCD camera's pixel size, and your telescope's effective focal length.

A low number for image scale (like 0.5 arcsec/pixel) means high resolution, and a high number for image scale (like 10 arcsec/pixel) means lower resolution.

This makes sense- if 10 arcsec worth of data (light) are represented by only one pixel, you have essentially crammed all of that detail into one point! Calculation of image scale (in units of arcsec/pixel) is easy:

FOV = 206 x pixel size (in microns) / effective focal length (in mm).
= 206 x 20 / 1200
= 3.43 arcsec / pixel

So what is the best image scale to aim for (in other words, what pixel size should we want in our CCD camera, for a given focal length telescope)?

Surprisingly, it's not 3.43"/pixel at my site. The Nyquist theorem suggests that in order to efficiently record this information and convert it into digital format, our system should be sampling the image more aggressively, by operating at an image scale of about 1/3 times the seeing,

For example, 1/3 x 3.43", or 1.17"/pixel. (Actually, it should be 1/3.3 times the seeing, but who's counting).

This means that my CCD camera/scope combination should ideally have an image scale of 1.17"/pixel, in order to take full advantage of my (suboptimal) seeing conditions and produce a final resolution of approximately 3.5"/pixel.

For example: Exchanging Camera

I have a Takahashi 106ed , focual lenght is 530mm, and the F is a 5, apature is 106ed. My camera is a SX964, but I am thinking in buying
a FLI 16803 for deepspace narrowband with a larger FOV. Lets do some numbers of both combos

Current setup:

SX694 Pixel size : 4.54um
Focal Length (mm):530

=206x4.54/530= 1.76" arcsec per pixel resolution, if we apply the 1/3 = 0.59

Remember what I said :A low number for image scale (like 0.5 arcsec/pixel) means high resolution, and a high number for image scale (like 10 arcsec/pixel) means lower resolution.

I have an excellent 0.6 and to take full advantage of a high resolution you will need from 0.5 to 1

Thinking to change camera to 16803.

The FLI 16803 pixel size is : 9um

same scope : 206x9/530=3.49" arcsec /pixel divide by 1/3 = 1.16

This combination will result in under smapling, which means that it will result in softer images, althrough the wide field is great. You can put a barlow 1.5 to low to increase resolution (lower the 1.16) but why do that, you will reduce the FOV.

If you when to buy a larger pixel camear think first that the telescope might not be the correct one.