Autoguide flexures: the use of an autoguider to improve the tracking accuracy of the telescope mount not only requires a sensitive camera and a good guide telescope (or an off-axis guider) but also a properly machined mechanical system that is able of supporting it without problems or flexures.
I decided to write an article about it when, last week, I received a call for help by an amateur astronomer who could not guide: despite the guide graph that he observed in PHD Guiding software was good, without obvious peaks or tracking errors, the images were "moved" and the stars elongated. I do not have with me his pictures but here you have an example of how the image may appear.
After checking with him that the telescope was perfectly aligned and working, I made hime describe to me the guiding system and ..... surprise! He was using a guide telescope in parallel to the main one (an apochromatic refractor) with guide rings similar to these that have:
1) Plastic screws to lock the guide scope
2) Single screw to lock them on plates or support rings
Despite this kind of support have the great advantage of a very cheap cost, it is undeniable that it has some construction problems that, potentially, can generate autoguide flexures and then thwart the efforts we make in astrophotography . In fact, no high sensitivity autoguide or high precision mount can work perfectly if the mechanical system is not rigid. What happens is very simple: when we use rings similar to these, during extended exposures in deep-sky exposures, the parallel guiding telescope moves imperceptibly (but it moves ....) relative to the main one. The guide camera continues to adjust the position of the mount based on the star pointed by the guide telescope but it moves with respect to the main one: the result is a noticeable "shift".
That's why guide rings, also if simple instruments, must be carefully machined to avoid autoguide flexures. Let's see for example in our PLUS guide rings and see the differences from the previous ones. They're more espensive (although not so much) but they have large aluminum screws (with Nylon head to avoid scratching the guide telescope) and they have at the base 3 threaded holes to a more stable allow installation. Moreover the base (where there are 3 holes) is flat to allow an even more secure fastening.
Our PLUS guide rings can be installed on support rings or plates of all brands (thanks to the presence of a centrale threaded hole) but they connect to the best to all the elements of the PLUS system using, for each ring, 2 M6 screws. This way you will be sure that the telescope will be installed in parallel with high rigidity and that you will not have autoguide flexure problems that ruin your pictures.