Starlight Xpress SXV-M7C - Manual (Page 21)

Handbook for SXV-M7C Issue 1 June 2004
21
‘Alternative Slew Mode’ available, which uses the correlation of image areas, rather
than a single star. This mode can be better in dense star fields.
Another option is ‘Auto remove dark frame’. This is advisable with S&S images, as
the slewing will mis-register the images with a single dark frame that is applied to the
finished sequence. To use this option, you will need a dark frame, taken with the same
exposure time as a single image from the sequence. This is stored on drive C with the
name ‘dark.def’
Now click on ‘Take Picture’ and the sequence will begin.
Using the High-Resolution modes:
Up to this point, I have assumed that the ‘Fast’ imaging mode is being used. ‘Fast’ is
fine for most purposes, but it does not offer the full vertical resolution of the
SXV_M7 CCD. Because of the CCD structure, it is impossible to read out all of the
pixels in a single sequence and so we must use some form of ‘double readout’ to get
the full 752 x 580 pixel resolution. This can be done in two ways, as follows:
1) Progressive mode:
In progressive mode, the exposure of both image fields takes place at the same time
and then each field is read out consecutively. To compensate for the extra exposure of
the second field, caused by the readout of the first, the second field is deleted at one
‘readout interval’ into the exposure and integrates again from that point.
Exposures taken in Progressive mode will be less dense than those in Fast mode and
‘STAR 2000’ guiding cannot be used. However, the total exposure time is not greatly
increased from that in ‘Fast’ mode and any motion or time dependent effects in the
subject will have a minimal damaging effect on the image.
2) Interlaced mode:
In this mode, two ‘Fast’ exposures are made, using a one full resolution line
displacement of the imaging fields from one exposure to the other. The images are
then combined to form the full resolution image. This mode gives a good image
density, but is more likely to show line to line variations due to differences in the
exposure timing etc. It can also show time related changes as ‘jags’ in the image,
although this is usually due to guiding errors between the two half frames.
As any exposure time errors, or variation in transparency, can cause line-by-line
brightness errors, interlaced images often show ‘Venetian blind’ patterns. The
‘vertical smoothing’ filter is provide for correcting these errors, or it can be enabled to
run automatically from the ‘Program Defaults’ menu.
Interlaced mode will work with STAR 2000 guiding.
Taking and using a flat field:
Flat fields are images, which display only the variations of illumination and
sensitivity of the CCD and are used to mathematically modify a wanted image in such