Handbook for the TRIUS PRO 694 Issue 19th April 2019
10
It is necessary to set up a good optical match between your camera and the
telescope. Most SCTs have a focal ratio of around F10, which is too high for most
deep sky objects and too low for the planets! This problem is quite easy to overcome
if you have access to a focal reducer (for deep sky) and a Barlow lens for planetary
work. As a guide, most CCD astronomers try to maintain an image scale of about 2
arc seconds per pixel for deep sky images. This matches the telescope resolution to
the CCD resolution and avoids ‘undersampling’ the image, which can result in square
stars and other unwanted effects. To calculate the focal length required for this
condition to exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the TRIUS-PRO 834 and a 2 arc seconds per pixel resolution, we get
F = 0.00454 * 205920 / 2
= 467mm
For a 200mm SCT, this is an F ratio of 380 / 200 = F2.33, which is much less than can
be achieved with standard focal reducers. However, moderate deviations from this
focal length will not have a drastic effect and so any F ratio from about F4.5 to F6.3
will give good results. It is clear from this result that the ‘Starizona Hyperstar’
adaptor is very well suited to use with the 694, as it operates at around F1.95, so you
might be interested in getting one of these.
The same equation can be used to calculate the amplification required for good
planetary images. However, in this case, the shorter exposures allow us to assume a
much better telescope resolution and 0.2 arc seconds per pixel is a good value to
use. The calculation now gives the following result:
F = 0.00454 * 205920 / 0.2 = 4674mm
This is approximately F23 when used with a 200mm SCT and so we will need a 2 or
3x Barlow lens. Barlow lenses are less critical than focal reducers and most types can
be used with good results. However, if you are buying one especially for CCD
imaging, I recommend getting a 3x or even 5x amplifier, or the planets will still be
rather small in your images.
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.
