## An Example System

As an example, it is decided to build a speckle system around a typical Meade or Celestron 8 inch, f/10 Schmidt-Cassegrain telescope. For these systems, /obj =

2000 mm. This will be used in conjunction with a CCD video camera that has 540 pixels across a scan line and 15 im pixels. In addition, an eyepiece where feye = 13 mm is available for use. First, calculate the necessary f effective to get the central core of the Airy disk to span about the right number of pixels. To get the core to span 10 pixels, the Airy disk core needs to be about 150 i in size. From equation (10.1),

Dm D 150 um 203.2 mm = (ยก4)1= (2 44) 550 nm = 22712 ^ Using equation (17.4),

eye 2000 mm

To calculate the field-of-view (fov) in seconds of arc along the scan line,

(Number of pixels) x (pixel size)

feffective

. v 15 um 1 rad 206225 arc sec

1 pixel 22712 mm 1 rad

As can be seen from equation (17.4), to shorten d eye for a given f^tm, either use a shorter focal length eyepiece or a longer focal length telescope. One can use a Barlow lens to get a longer effective focal length. In certain cases, changing to a longer effective focal length or shorter focal length eyepiece is not practical. In these cases, one may have to replace the eyepiece in the projection setup with a microscope objective. Because microscope objective focal length parameters are not described like those of an eyepiece, using one typically involves a bit of trial and error. Start with the lowest power (typically x5) objective. If the image scale is still not quite right, change to a higher power objective. A bit of simple machining will be required to make an adaptor to fit the objective into the projection apparatus. Remember that the threads of the objective are pointed towards the camera.

Figure 17.5 shows a photograph of the prototype amateur speckle camera, built and tested in early 1992. It uses a microscope objective and a Philips monochrome CCD video camera. A description of its performance can be found in Turner et al.16 (visit http:// www.chara.gsu.edu/~nils/1992cadm.conf.577T.pdf).

Figure 17.5. The amateur speckle camera built in early 1992. Photo courtesy of Sky Publishing Corporation.

Figure 17.5. The amateur speckle camera built in early 1992. Photo courtesy of Sky Publishing Corporation.