There is no intuitive approach to adjust the optical axis of the reflector, how to move the optimal imaging area across the photo at will, in order to get the best imaging effect? This article suggests a method.
Based on the star and move the optimal imaging area to the center of the CCD negative always been tricky, the popular strategy is to use defocusing method to observe the extrafocal image of the star test, and adjust the extrafocal image to the concentric circles, which is difficult to achieve good results, because most of the time the extrafocal image in the center of photo is already concentric circles, but the actual imaging effect is still poor, the corners of the star test is still badly anamorphic, few people can successfully use this method.
What is the operation mechanism of the adjusting screw of the primary mirror? Firstly, we can conclude that the result of the adjustment is the adjusting screw moves the focal plane across the photo.
This conclusion is so obvious that it is well known to take a mirror and reflect a flash of sunlight on the wall. Do not move the mirror, and just turning the mirror makes the spot move a lot. The same is true for the adjusting screw of the primary mirror. By moving the primary mirror up and down, the focal plane can also be moved. Basing on this theory, we can move the focal plane across the photo.
Therefore, using star testing to calibrate the optical axis principle is very simple, as long as the optimal imaging area is moved to the center of the photo, which is the ultimate means of optical axis adjustment.
This method is not suitable for artificial star testing, because the number of artificial star is too small to see the various parts of the focal plane.
The practical application of the method is simple, after calibration of the eyepiece, imaging with the actual sky，after focusing to star, observe the imaging situation, find out the best imaging area and the worst of imaging area, adjusting screw of the primary mirror, and move the best image area down to the center of photo and the bad image area out of photol, for example, the star in the upper left corner is elongated and the star in the lower right corner is perfect, indicating that the good image field area is inclined to the lower right corner. My way is to estimate the distance that the image field needs to move to the upper left corner, select a star, use the adjustment screw of the primary mirror to move the star to the predetermined position, and then expose and observe the improvement of the imaging, then adjust depending on the situation until the result is satisfactory.
Since the locking screw of the primary mirror can also be slightly shifted, I usually use the locking screw of the primary mirror directly for adjustment, before adjustment, number 1 to 3 for each screw, and remember that the number of screws to adjust the direction of the star on the photo, the adjustment will be very beneficial, generally can make the best imaging area in the middle of the photo, the four corners of the star are of the same size. Unless the photo is tilted or the focal plane is wrong, it works. The adjustment time of this method is very short, usually 10 mins can be completed, but the effect is very obvious, and can make accurate calibration greatly simplified.
Comparison of adjustment effects in the same area of the photo:
Before the adjustment
After the adjustment