Focussing The D etector

To visually measure the changes in an image profile introduced by moving the focus position, and hence, decide the best focus position, histogram s of slices (or cross sections) through the image profile need to be displayed. The buildup display mode allows th e user

to select areas of an image from which individual rows or columns can be co-added to produce a cross section.

In practice the detector is focused by m aintaining a centroiding resolution of | of a CCD pixel and imaging a point source illum ination (or in spectroscopic applications, arc lines). The focus position is then found by one of two m ethods, depending upon the imaging optics placed in front of the detector. These are:

• A t m ajor telescopes such as the the AAT and the W H T the focus on spectrographs can typically be found by using a H artm ann test. F ig 3.7.

H a r t m a n n A F ro n t F ace O f D e te c to r LA M P H a r t m a n n B X

Fig 3.7. Obtaining focus using the Hartmann test

The m ethod consists of taking two images of the point source illum ination, one with H artm an A blocking off half the image and another w ith H artm an B blocking the other half of the image. If the image is focused, then the position of images I a and

Ib coincide w ith one another. If the two images do not coincide the focus position is easily found by observing the separation of I a and Ib for changes in x. The focus position is changed by moving (in x ) the telescope collim ator, b u t if focus cannot be obtained anywhere in the collimator range, either the detector has to be moved closer to th e collimator (by redesigning th e detector m ount) or stood off from the telescope using shims.

To display image cross sections side by side, the user m ust first change the display to buildup mode by executing R E S E T . This allows cross sections through an image to be displayed. If the user wishes to produce a cross section parallel to th e x axis then the user types B U IL D and then the build up num ber where up to 8 buildups can be defined. The program B U IL D prom pts for th e sta rt row and the num ber of

rows to be summed into a ID cross section. This buildup definition is then saved, and can be recalled at a later date by typing L O A D and the associated buildup num ber.

Similarly, if the user wishes to produce a cross section parallel to the y-axis the user types Y B U IL D to define how m any columns are summed together to make the ID cross section. The program Y B U IL D prom pts for the s ta rt column and the num ber of columns to be summed. The ybuildup can be recalled a t a la ter date by executing L O A D as before.

Once a buildup is selected it is displayed by executing S H O W . The range over which a buildup can be displayed is altered by either typing R O W S (and the row sta rt and end values) when displaying a ybuildup, or S T A R T and L E N G T H , when displaying a buildup. Up to six (y)buildups may be displayed on the m onitor a t any one tim e (see the section on buildup commands).

Images I a and Ib are displayed on top of one another by firstly taking an integration using one H artm ann and then executing S A V E to fix the image on the screen. An image is then taken w ith the other H artm ann and this becomes superimposed on top of the previous image. Several such cross sections can be saved on the screen at any one time.

• In the laboratory or on instrum ents th a t do not contain H artm ann shutters, a simpler bu t ju st as effective way of finding the focus position exists. This m ethod also involves taking a cross section through the a point source image, but this time measuring its full w idth a t half maximum (FW H M ). Focus occurs when th e image FW HM is a minimum and so in the laboratory (where in our case the source of illum ination is provided by a Baum projector), the projector position from th e front face of the detector is moved until the im age FW HM is a minimum. This is only found by moving the image through the focus position and observing an increase in FW HM either side of th e focus position.

3.4

T aking O bservations W ith T h e M IC D e te c to r

An observing program may involve the observation of very different types of object having vastly different sizes, wavelength coverage and intensity. The following procedures should

be carried out in order to observe each of these objects most efficiently;

1. If the object or background illum ination is expected to be bright, either neutral density (ND) filters or narrow transm ission band filters should be placed in front of th e detector. Before exposing MIC to light of an unknown intensity the intensifier front gap voltage Vî should be turned down to OV. W hen exposed, Vi is increased slowly whilst watching the real tim e display.

2. Set the initial d a ta acquisition form at to its m axim um size i.e. 256 pixel rows by 256 pixel columns.

3. Select the appropriate centroiding resolution using the program F O R M A T . A high centroiding resolution is required for closely spaced objects (like closely spaced stars or in the case of spectroscopy, closely spaced spectral lines). It is also required in order to adequately sample a single star profile, for example, or a single arc line.

4. If the double counting threshold (C hapter 4) needs changing then type T H R E S H ­ O L D followed by the double counting threshold. If the user enters a value of -1 then the double counting circuit is disabled. The threshold is preloaded w ith its optim um value and so it should only be redefined if testing the system.

5. Move the image onto a defect free or ’flat’ p a rt of the detector (usually th e centre).

6. Place a d a ta acquisition window around the object(s) in th e field of view. Minimizing the d a ta acquisition area not only helps to decrease the am ount of memory required to store the image but also increases the detectors’ dynamic range performance by decreasing the CCD frame tim e (C hapter 4). P rior to placing a window a short integration of the field using the maximum d a ta acquisition form at m ust be taken and then displayed on the m onitor. Up to 16 d a ta acquisition windows can then be defined by calling the program B O X , which places a box cursor on th e image display and allows the user to define subsets of the image for future integrations. The cursor can be moved around the image and its size changed.

7. Integrate on the image (by executing G O ) until the desired signal to noise ratio (C hapter 7) is obtained. The image can be displayed whilst being integrated by executing U N S C R A M B L E to initiate the d a ta unscrambling routine, and then

choosing the display mode, which is described in more detail below. W hilst an observation is taking place, a button box m ay be used in order to gain access to the com puter keyboard. This could be used for one of several reasons;

• S to p p in g A n I n te g r a tio n ; The integration can be stopped prem aturely by pressing the S T O P button. The duration over which the observation took place, is displayed on the screen and the d a ta collected w ithin this tim e is held in com puter memory. An integration can also be stopped from the command line by executing A B O R T .

• H o ld in g A n I n te g r a tio n ; The integration can be paused by pressing the H O L D button. The d a ta remains in memory, and the observation is restarted by executing C O N T IN U E , whereby the newly accum ulating d a ta is added to th a t stored in memory. This feature is helpful, for example, when cloud affects an observing run. The integration can be held, and restarted as soon as the cloud disappears.

• C h a n g in g T h e D isp la y ; By pressing the C H A N G E D IS P L A Y b u tto n , the com puter prom pt will appear on the screen. This allows the user to change the display whilst the integration continues as a background task. The continually updated image is returned to by executing S H O W .

8. If the detector is being used to directly observe astronom ical objects, use the sky background to obtain a flat fleld image using the same cam era form at, d a ta acqui­ sition windows and centroiding resolution. This ensures th a t all objects are ’flat fielded’ w ith sky background imaged over exactly the same area of the detector.

The detector status can be displayed on the VDU at any tim e by firstly obtaining the com puter prom pt and then executing L IS T . The program displays inform ation about the detector status obtained from variables stored in the global table e.g.

• The size and position of each d ata acquisition window.

• The current and previous integration times.

• The observers name, the current run num ber and the d a ta destination.