We all having attempted to adjust a mount have suffered the “backlash” or roominess in the mechanical chain. This problem is mainly serious in astrophotography. Our practical article shall help you to solve it.
What is the cause?
The shaft of the motor is linked to the shaft of the telescope by means of a series of gears (normally a reducing gear box at the outlet of the motor, any gear or cardan joint for linking it to the endless screw and ending with the endless screw and the ring gear). Among all the gears, there is some space or roominess since otherwise they would not be able to rotate. The result is that since the motor starts to rotate up when the movement is transmitted to the telescope, some time is elapsed (the motor has to rotate a series of degrees for “eating” the roominess). Once marching, and provided the rotation direction does not vary, the effect is not again observed. Nevertheless, when changing the direction, the process starts again.
In the practice
The effect of the backlash are pernicious in two senses:
- They reduce the precision of the GOTO; the computer cannot exactly know when it is pointing up, since as far as it is concerned, the rotation of the motor coincides (in function of any relationship) with the rotation of the telescope.
- The guiding becomes very complicated: in the declination axis, because the one of right ascension must be slightly “loaded” in such a way that the motor always push, and whenever possible is guided at a speed lower than the sidereal speed, so the direction of the motor is not inverted, it only reduces speed.
How to correct
Fortunately, the computerized mounts are used to have an adjustment of “backlash” for every axis. Furthermore the astrophotography programs also include adjustments in the guiding modules.
The problem is to succeed with the appropriate parameters, mainly in a visual way. An excess of correction shall cause the declination axis to “dance” when trying to guide. To the contrary, if the adjustment is too low, it shall be slow when correcting, and it could be that the calibration is not as precise as we wish.
The Lunático solution
In Lunático we found a very simple solution for solving the problem: if we could move the mount during a concrete time interval, first in a direction and then in the opposite, while we inspect in some way the movement of the telescope, it would be easy to reach the desired precision level.
We are to explain that with an image obtained during the execution of the program:
Image 1 : Declination, lack of adjustment
Let us select any trace; starting from the thickest point, we see that the trace goes up, then it goes right, it goes up again, to the left and at last it goes up a little.
This image has been obtained whilst the program:
- It waits some seconds for leaving an initial mark (the “thick” point)
- It moves the AR axis during 3 seconds towards west (first section upwards)
- It moves the declination axis during 10 seconds to north (first section at right)
- It repeats the movement to west
- Idem but now to south (it inverts the movement)
- At last, it moves a little to west
Thanks to the program we see, in addition to the fact that the camera is not perfectly oriented with respect to the axis, that the adjustment of the backlash in declination is a little high: the vertical traces must be in line or in any case the last a little to the right (in the image) of the first (the excess of correction is worse than a default).
Image 2 : Declination, almost perfect
We speak in a pertinent way with our computer, astro-computer or similar, and we tell it to decrease the correction in declination. After several iterations, we must arrive to an image similar to image 2.
Anyone of those images shows a mount reasonably corrected. Anyway, it is easy that the first image is similar to Image 3.
Image 3: Frame with problems
We see in the photography (Vega, if I remember it, eclipsing the remainder of stars in the field), that the mount needs some attention. In this case, the movements are, starting from the initial mark, and since the right ascension is to be corrected: north (short), west (long), north (short), east (long but not so much and with a curve), north (short).
What can we observe in this image?
First of all, the west trace is much longer than the east one; the correction in AR must be increased.
Second, the observed curve is due to the fact that the declination axis goes back to its position after the second movement to north (back to south), taking approximately half a second (remember that the long trace to west is of 10 seconds). I do not know exactly, the reason of that because it does not happen after the first movement to north, although I suspect that when inverting the movement in AR (to east), the declination axis made use of that for going back to a more comfortable position. Nevertheless, the problem disappeared by adjusting the backlash in declination.
|The program is available to be downloaded free: Click here for download the program and a brief instructions manual.|