Meade Field Derotator

This review is of the Meade field derotator. Its can be obtained from any Meade retailer and the currently purchase price is for $839.00 AU including GST. This will fluctuate with the rise and fall of the Australian dollar.

I bought the derotator several months ago with the hopes that I would not need to purchase a wedge and go through all the hassle of polar aligning the LX200 that I currently own for long exposure photography. The concept is very simple, as I will explain. As you are all aware the earth rotates about its axis every 24 hours. This rotation becomes evident when taking stills to form star trails. The star trailing is in fact the field rotation. So, when the Lx200 is mounted in Alt Azimuth mode any shot taken lasting longer than a few minutes will produce the effect of field rotation, to the point that the stars in the centre will be nice and crisp and all other stars moving away from centre will show increased trailing. This effect will be more dominant and happen much quicker as you shot near the celestial pole. Some shots will show signs after 30 seconds near the pole.

To eliminate the problem two solutions are present, the first is to mount the scope on a wedge and polar align. The second is to use a field derotator. The derotator does exactly that. The scope once aligned sends a signal to the derotator, which slowly counteracts the movement of the earth for any given position in the sky. There is a formula for working this out and I won’t bore you.

The field derotator is to my knowledge only designed for the Meade Lx200 range of telescopes. You can see from the shot below that it readily plugs into the control panel of the scope.

I have found that it is best to plug it into the second port from the right. For some reason it works best in that port.

As for assembly it is relatively simple. The derotator can be threaded onto a number of accessories with the same thread as shown in the pictures below.

Other appliances can be then fixed onto the derotator such as the zero image shifter and off axis guider.

The advantages of this configuration is that a quick set up can be made and the field derotator used to correct for field rotation thus improving portability of a large telescope. It is ideal for field trips of one night, where set up can be made early in the afternoon and imaging can commence straight after the alignment procedure. In fact I have found that when used to its full potential, the derotator performs extremely well. Images are generally free of field rotation and it is a breeze to fit and remove during any time of the evening after set up. And; I can see that it would be even better suited to that large LX200 systems which weigh a considerable amount more than the 10”.

However, there are some disadvantages. Two stand out as problematic. First, use of the field derotator will create a huge balance problem to the scope. The light train gets very long and of course this will upset the pivot point for the scope. Under such conditions balancing of the scope is essential and quite frankly the Meade balance set is not up to par. The scope will produce shots with stars in U shapes and squiggly lines. A 2-D weight set is essential if the scope is to perform, as it should. Without the 2D balance the scope will struggle against gravity and place enormous strain on the mechanical and electrical system. Something that should be avoided! In addition to this point, it should be noted that as a minimum, the Meade balance kit should be purchased at the same time. This is something that is not explained in the Meade sales brochure.

Secondly, the field derotator has trouble coping when the initial alignment was made under a limited field of view. Meaning if you cannot see a lot of the sky and have to align with two stars that are relatively close together, the derotator struggles with the calculations and still produces some field rotation.

That being said the photo below shows the sort of results that can be obtained with the derotator. This shot of M42 was taken with the use of the derotator over 13 minutes. With a person of much greater skill all trailing can be eliminated.

So if you have a LX200 scope and don’t want to go to the trouble of polar aligning your scope each time and potentially dropping it on the ground whilst mounting to the wedge, I suggest thinking about the use of the field derotator. I hope that this review will help you in your decisions for astrophotography.

Paul Haese