Imaging in Alt/Az mode


Can I do astrophotography with an Alt/Az mounted LX90?

 Yes! but there are a couple of things to be aware of.


There are 2 issues to contend with when it comes to taking long duration photos through your alt/az mounted telescope.

1)   1) 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 Issues
When you are tracking an object in Alt/Az mode, your telescope is being driven by 2 motors. The Declination / Altitude motor 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. These errors 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 downloaded from Mike 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 objects.

Figure 4: Example of Triffid Nebula -  20 stacked images, each 30 secs duration
Its 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. Its 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 duration