Starlight Xpress SXV-H9C - Manual (Page 15)

Handbook for the SXV-H9 Issue 1 June 2002
15
Achieving a good focus:
Your starting point will depend on the focus aids, if any, which you are using. With
the par-focal eyepiece, you should slip the eyepiece into the drawtube and focus
visually on a moderately bright star (about 3
rd magnitude). Now withdraw the
eyepiece and carefully insert the camera nosepiece, until it is bottomed against the
drawtube end, and then lock it in place.
With the flip mirror unit, all that is needed is to swing the mirror down and adjust the
focus until the star is sharply defined and centred in the viewing eyepiece. Now lift
the mirror and you are ready to start imaging.
SXV_H9C has a focus routine that will repeatedly download and display a 128 x 128
pixel segment of the image at relatively high speed. This focus window may be
positioned anywhere in the camera field and can be displayed with an adjustable
degree of automatic contrast stretching (for focusing on faint stars). To use this mode,
start up the software and select the SXV camera interface (File menu). Set the camera
mode to Binned 1x1 and select an exposure time of 1 second. Press ‘Take Picture’ and
wait for the image to download. There is a good chance that your selected star will
appear somewhere within the image frame and it should be close to a sharp focus. If
the focus is still poor, then it may appear as a pale disk of light, often with a dark
centre (the secondary mirror shadow in an SCT, or Newtonian). Now select the ‘File’
menu again and click on ‘Focus frame centre’; you can now use the mouse pointer to
click on the star image and the new focus frame co-ordinates will be displayed. Now
return to the camera interface window and click on ‘Start’ in the Focus frame. The
computer will now display a continuous series of 128 x 128 pixel images in the focus
window and you should see your selected star appear somewhere close to the centre.
A ‘peak value’ (the value of the brightest pixel) will also be shown in the adjacent text
box and this can be used as an indication of the focus accuracy. Although the peak
value is sensitive to vibration and seeing, it tends towards a maximum as the focus is
optimised. Carefully adjust the focus control on your telescope until the image is as
sharp as possible and the peak value reaches a maximum. Wait for any vibration to
die down before accepting the reading as reliable and watch out for bursts of bad
seeing, which reduce the apparent focus quality. Quite often, the peak value will
increase to the point where it is ‘off scale’ at 4095 and in this case you must halt the
focus sequence and select a shorter exposure if you wish to use the peak value as an
indicator. Once you are happy with the focus quality achieved, you might like to trim
the settings of your par-focal or flip mirror eyepiece to match the current camera
position.
Although you can reach a good focus by the above method, many observers prefer to
use additional aids, such as Hartmann masks (an objective cover with two or three
spaced holes) or diffraction bars (narrow parallel rods across the telescope aperture).
These make the point of precise focus easier to determine by creating ‘double images’
or bright diffraction spikes around stars, which merge at the setting of exact focus.
The 12-16 bit slider control allows you to adjust the contrast of the focus frame for
best visibility of the star image. It defaults to maximum stretch (12 bits), which is
generally ideal for stars, but a lower stretch value is better for focusing on planets.
Taking your first astronomical image: