Can I do astrophotography with an Alt/Az
“Yes!” but there are a couple of things to be aware
There are 2 issues to contend with when it
comes to taking long duration photos through your alt/az
Field Rotation (applies to all Alt/Az mounted telescopes):
When a telescope is set up in alt/az mode, the stars will
progressively rotate around the centre of the field of view.
Figure 1: Example of field rotation
To demonstrate what I mean, imagine that you are standing up
with your body facing East and you are watching a small
group of stars as they rise on the Eastern horizon. As the
night progresses, you turn your body (ie, adjust your
viewing azimuth) & tilt your neck (adjust your viewing
altitude) to follow that group of stars. If you were to follow that
same group of stars until they set on the Western horizon,
they will appear to have rotated by 180 degrees (relative to
your eyes). You would also be very tired with a sore neck.
This is because your body will be pointing 180 degrees from where it started out
relative to the group of stars.
BUT. If you were to lay flat on your back (facing North –
South) and watch that same group of stars as they progress
from east to west, their position will remain exactly the
same (relative you your eyes) all night.
Figure 2: Following the stars in Alt/Az mode
2) Drive Tracking
When you are tracking an object in Alt/Az mode, your
telescope is being driven by 2 motors. The Declination /
which is in the right hand fork arm of the LX90 and the
Right Ascension / Azimuth motor which is located in the drive base.
Whilst these motors are perfectly capable of keeping track
of an object in an eyepiece for the purposes of visual
observations, they typically suffer from small mechanical
imperfections which, when it comes to long exposure
astrophotography, can introduce significant star trailing in astro exposures.
These imperfections are found on all
motor driven telescopes and can be split into 2 categories:
Periodic Errors - These are errors that
are caused by imperfections in the worm drive mechanism.
can be seen to re-occur in a predictable manner on a
periodic basis, typically once every revolution of the worm. In the case of the LX90, the worm revolves once
every 9 mins, 21 secs. These errors can be compensated for by
Periodic Error Correction (PEC).
The issue that LX90 owners hit here is
that PEC only operates on an equatorially mounted telescope
(ie, when you are using an equatorial wedge). However,
saviour is at hand. Rick Seymour has created a great
firmware hack for the Autostar that tricks the LX90 into
thinking that it's equatorially mounted. This patch can be
Weasner's Mighty ETX site.
Tracking Errors - These are errors caused
by imperfections in the machining of the gears and gear
teeth and can occur in the drive train (gearbox or the worm
gear). They are not periodic in nature and can therefore can
not be predicted.
Figure 3: Example of star trailing on the declination axis.
Having said this, it is entirely possible to take many short length
exposures (15-30 secs) and stack them using one of many astro imaging software packages. Some great images of
brighter objects (such as M20) have been taken this way. The
problem is however that you really need to take longer
exposures to capture the faint details of many deep sky
Figure 4: Example of Triffid Nebula - 20 stacked images,
each 30 secs duration
It’s also entirely possible to perform planetary imaging
with a modified webcam in Alt/Az mode as the exposure length
on webcam images is measured in fractions of a second. It’s
simply a matter of capturing hundreds or even thousands of
exposures as part of an avi video file and stacking them.
Figure 5: 350 stacked video frames, each 1/33 sec in