On Fri, Jul 10, 2020 at 10:33 AM andy pugh <bodge...@gmail.com> wrote:
> > Sprockets for T2.5 belts are available with 10 teeth, so that would be > a 2.3m table for the 300:1. Which is also probably too big. I suppose > that the design criterion is that one stepper step should be less than > the angular resolution of the telescope. So what is that, in this > case? > We don't know the size of this 'scope but let's assume about 12" diameter. then it is on the order of 1/2 arc-second You need to move at 1/2 the resolution otherwise you can see the steps. So let's say each step would be 1/4 arc-second Stars move 15 degrees per hour or 15 arc-seconds per second. So he needs to step at about 60 Hz. Let's assume 1/64th micro steps these are roughly 100 arc-seconds per step. He needs about 400:1 gear reduction. This is getting unreasonable because the telescope will NEVER achieve the theoretical resolution because of the atmospheric blurring Lets say we do 1 arc-second steps and 100:1 gears. The traditional professional-level method is to use very nice and very expensive brass wheels and stainless steel worms and even more expensive bearings and shafts. But nowadays with computers, we can place a camera such that it looks at the image of a highly magnified star then we measure the X,Y location of the image on the CCD sensor and use a PID feedback loop to keep the star image from moving. So the servo loop tries to keep drift to zero. They measure drift with units called "mas" for milliarcseconds. But this is a "Dobsonian" scope and the goal of these is to be cheap and portable, exactly the opposite of a professional level scope. Most of these use a door hinge as the axis that is roughly aligned by eyeball with "north" and the drive is a hardware store threaded rod turned by a DC geared DC motor and the operator sets the tracking speed by eye and hand. These things reduce the drift to what can be tracked with the eye. It seems our OP is trying to do better than the common door hinge and threaded rod tracker but still wants a 'scope that fits in the back of his car and costs less than the car. So sub-arcsecond pointing is going to be hard. -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
