The focal length of the telescope determines the field of view (FOV) of
the CCD chip and the size of the object you are imaging. The longer the focal length, the
smaller the field of view and the larger the
object on the chip. Often you will hear a telescope such as the LX200
described as a 250mm (or 10 inch) f/10 telescope. The 250mm refers
to the aperture of the telescope and the f/10 is the f ratio.
The f ratio is just the telescope focal length divided by the
aperture. So we can deduce that the 250mm f/10 LX200 has a focal
length of 2500mm. For telescopes of the same aperture, the lower the
f-ratio the larger the FOV and the smaller the size of the object on the chip.
It is very important to adjust your focal length
so that the CCD camera can correctly sample the object you want to
image. A correctly sampled image is one which has recorded
the finest detail that can be resolved by the telescope and has done so
using the minimum number of pixels. For example,
most medium sized telescopes can resolve details as small as 0.5
arcseconds. To correctly sample this level of detail will require
each pixel to cover no more than 0.25 arcseconds of sky. You could
have pixels covering less than 0.25 arcseconds but no more detail will be
visible and you will have increased your exposure time. This is
known as oversampling. If you used pixels covering more than
0.25 arcseconds some detail will be lost. This is known as undersampling.
atmospheric turbulence (or seeing) at most locations limits the resolution
of any telescope to 3-4 acrcseconds during long exposure imaging. To correctly sample this level
of detail requires each pixel to cover no more than 1.5-2 arcseconds of
The number of arcseconds per mm at the focal point of any telescope is
given by the formula 206265/(focal length in mm). If you know the
dimensions of the CCD chip you can then calculate the FOV of the chip. The
number of arcseconds of sky covered by each pixel is given by the formula
206*(pixel size in microns)/(focal length in mm).
The table below shows the FOV in arcminutes and arcseconds/pixel for
the MX516 (10 micron pixels) , SBIG ST-7E CCD camera (9 micron
pixels), and Canon 10D SLR (7.4 micron pixels) on the LX200 at various f-ratios.
From this table you can easily see that long exposure images of deep
objects where seeing limits resolution to 3-4 arcseconds are best carried
out at f-ratios of either f/6.3 or f/3.3. For planetary, lunar or
solar images, the
short exposure durations used (tenths of seconds)
can freeze the atmospheric turbulence so that resolutions approaching the
limit of the telescope (0.5 arcseconds ) are possible. For these
images a high f-ratio of f/20 or f/40 should be used to ensure that the
planet is a reasonable size on the CCD chip and maximum detail is
captured. Increasing the size of the planet on the CCD chip
also spreads out the light received. At f/10 a bright planet such as
Jupiter could saturate the pixels even with a minimum duration exposure.
The f-ratio of the LX10 and LX200 can be reduced to f/6.3 and
f/3.3 using focal reducers available from Celestron or Meade. The
f-ratio can be increased to f/20 or higher by placing a telenegative (also
know as a barlow lens) in front of the camera. I use a Celestron f/6.3 Focal
f/3.3 Focal Reducer and Meade
in my imaging.
As an alternative to reducing the focal length of the telescope to
achieve correct sampling you could use the on-chip binning capability
provided on the ST-7E. The ST-7E allows you to create 18 micron and
27 micron pixels by using the 2x2 and 3x3 on-chip binning
modes. The 2x2 binned 18 micron pixels on an f/10 LX200 cover
the same arcseconds as the 1x1 binned 9 micron pixels at f/5 and the 3x3
binned 27 micron pixels cover the same arcseconds as the 1x1 binned 9
micron pixels at f/3.3. The disadvantage to this is that the images
produced look very small on the screen.
Most popular deep sky objects will fit within the FOV of the MX516 or
ST-7E at f/6.3 or f/3.3. To image larger objects (e.g. M42 or the Andromeda
galaxy) you will need to take images of different parts of the object and
create a mosaic. Alternatively you could use a smaller focal length
telescope or camera lens piggybacked on the LX200 OTA. For wide field images I
use a Takahashi FS-60C 60mm apochromatic refractor piggybacked on the
LX200. The FS-60C has a focal length of 355mm at f/5.9.
An f/4.5 focal reducer can also be used with the FS-60C to give a focal
length of 270mm. The FOV table above for the Takahashi FS-60C is:
I have also used a Canon 28-130mm zoom lens with the Canon 10D SLR
camera piggybacked on the LX200 (this gives a FOV of 46x31 degrees)
For a very good discussion on focal lengths, image sizes and an excellent list of deep
sky objects and their sizes visit Doc G's