Service Note #3 Information on the Mark IV drive system Author: Tom Droege Date: Mar 19, 2002 [ this note originated in E-mail between Rob Creager and ] [ Tom Droege in March, 2002 ] > 1) What is the step count between limits for ra/dec/focus drives? I > calculated 1076 from the 70 degree range and .065 step size for dec. Just > read limit.c, and it seems now that we're at 160 degrees (+60 -90) for 2308 > steps. Just out of curiosity, why limit the ranges (+50 -20)? There is a conversion between steps and angle that I will have to look up for the RA drive. It is somewhere around 3 steps per pixel. So roughly 7/3 arc seconds per step. I can get you an exact number when I can find the right log book. But you don't need to know it unless you plan to single step the RA drive. This can be done, but in general I don't know why you would want to do it. The Decl. drive has a conversion factor. I think it is 90 steps per degree. But it is going to change when I get the new motors. So just put n steps per degree into the code and be prepared to assign n some new value. For the RA drive you do things by time. When you turn it on it goes 15 degrees per hour. You can fast forward and fast reverse it. Then it goes at 32x the sidereal rate. You can change this fast rate in powers of 2 but no faster than 32x. i.e 16x, 8x ... are possible. Note that there is a dac that provides a +/- 2% trim on the sidereal rate. So part of every location is a dac value for dac 12 that sets this correction. There is a 10 turn pot that gets you into the ballpark. Don't touch this unless you have a period meter to test it or you have a fun time trying to get it back in range. Note it is roughly at 0.14 seconds per step. With this and knowing that the radius arm is 12" and the screw thread is 24 threads per inch will get you steps to angle conversion. Then a few pictures at known times will allow exact correction. Note that this is one of the things to be built into the control program. A way to take a few images and make adjustments to the RA drive trim to get it right. I have been patiently waiting for this code. Also needed to be built into the code is a focus scheme. There is only one limit on each drive. The Declination drive is set to find the limit moving towards the horizon from above the horizon. The limit sense goes from 0 to 1 when at the limit. The declination motion can go from the southern horizon to somewhat past the pole. For the Declination drive, the best thing to do is to move it away from the limit constantly testing the limit switch until it goes to 0 state. Then move it towards the limit switch (slowly) testing after each step until the limit goes to !. If you don't care for an exact position you can move to the limit at full speed if you test fast enough. There is only one limit on the RA drive. But it can be moved in 1/12 radian steps. You can actually move through the limit switch. So if when you turn on the motor decides to be in a state where it is moving towards the limit switch and you are in the limit switch and are not testing it, you can go right through the limit switch and grind the motor when it reaches the physical limit. Further, if you (without looking at the telescope) try to home the RA, it is going in the wrong direction and will just never get there. So the first thing you want to do when turning on power is to stop all the motors. Just look at the init in the QBasic code. I figure this is self correcting. After you replace the chamois in the RA drive a few times you will figure out how to make sure this does not happen. Note that there are many more channels available for adding limit switches. They are cheap. So if you all want to add more protection, it can easily be done. Well, you may have to drill more mounting holes in the RA drive and figure out how to glue a limit switch in place for other things, but I just got tired of worrying with it. There will be plenty of opportunity for you all to improve your Mark IVs. > 2) I notice that in radec.h, the home position is defined as -17.5 degrees, > yet the software limits are at +-16 degrees? I have not measured it. I think the home position can best be determined by an experiment measuring a star field and knowing the time and how long it has been since the telescope was at the home position. Note, in general, if you want to know where things are you want to take out the backlash. So you always want to perform the same sequence of moves to the limit switch so the backlash is always in the same direction. > 3) Are there limit switches on both sides of travel for the mechanisms > (ra,dec,focus)? Seems not from limit.c. If I do a (and is > this -17.5 or -16 degrees), and then track siderealy, what happens when I > reach the end of travel? Is this a very bad thing to do and I must prevent > this from happening? Same for focus. Seems like dec has a single limit > switch tripped at both extreme's of travel. A well adjusted chamois RA drive will overpower the drive motor. So it will not be damaged (usually) if you hang at one limit or the other. There is only a limit switch at the most east position. When you reach the end of the travel, the mechanism just hangs with the motor trying to drive it further. The only really bad thing about this (if you don't wreck the chamois drive) is that it may not have enough torque to get back off the end of travel. So when you try to reverse it, it may just sit there. Then you have to go to the telescope and unjam it by hand. When I am doing a focus run, I always move fully into the limit switch and then out to the test position. The trombone is not as rigid as I would like and this gives you a better chance to get a good focus. Sigh! I wished I had use 1/2" shafting instead of the 3/8". > 4) Can the scope be damaged by attempting to go beyond limit switches, > either by direct command (dec/focus) or by letting it track too long (ra)? > If so, I'm thinking of a simple watchdog program to keep track of the > functionality of TAIT, and/or maybe adding a spawned task to TAIT to keep > track of ra position when tracking sideral rate or moving. The focus motors have different problems. There are stops on the focus trombone so the camera cannot fall off the end on to the floor. (probably damaging the TEC in the process). At the close in position, there is a limit switch and if you try to move too far past the limit, it just jams and stalls the motor with mostly no damage. If you don't have the hard stops set correctly on the trombone, the drive screw could pull out of the motor. Then you would have to go to the telescope and command movement toward the limit switch and push the drive screw into the motor to get it started. > 5) Why no ra abs/rel movement? Is there a repeatability problem? Backlash? Yes, there is backlash. I don't know how much. Just begging to be measured by some simple experiments with star fields. OK, possibly Michael can stuff this into the "service notes" so I can avoid answering these questions yet again.