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Found 2 results

  1. asgr

    Tracking drop

    Hi All, I'm getting a fairly repeatable problem where after a few hours of faultless tracking performance with my DDM60 pro, I start getting persistence position errors after about one minute of tracking. Checking the motor tuning I can see the RA axis (usually) is building up a tracking error over time until it stops tracking entirely. Again, at the start of the night I don't see this behaviour. Has anybody else had this trouble? Any advice would be appreicated (I'm currently assuming it is an issue with balance or tuning). Cheers, A
  2. Why do I get different configuration results with different pointing files one might assume that if you make 2 pointing files you will get the same result regarding the calculation of Polar Alignment (PA) error or encoder resolution and other factors. This is not the case. Why ? Mathematical background: There is a set of formula that describe the pointing error of the mount 1. Polar alignment error in Azimuth 2. Polar alignment error in Altitude 3. Collimation error 4. Mount error (axes not perpendicular) 5. Tube flexure 6. Encoder resolution error 7. etc. What Autoslew does when it gets a pointing file from you is to make a best fit of the variables in these formulas that give a minimum RMS pointing error in this pointing file. If we assume we have a perfect mount and a stiff telescope we can use the configuration and make the same pointing file again and we will get the exactly the calculated RMS error you received as result after optimization. This is definitely true but why is it that your configuration shows different values after every new pointing file ? Reason 1: Different number of stars in pointing file and thus different number of fitted formula When you add more stars in your pointing file Autoslew will automatically increase the number of variables that can be fit. If you take only few stars, it would not make sense to optimize more than PA and encoder resolution. When you add more stars, thus adding another formula to fit (like tube flexure) Autoslew will probably find out that the RMS error is smaller if it adds a certain amount of tube flexure instead of fitting all the error into an Altitude polar alignment error. Thus, you can have a different result now in PA since the extra Formula (in this case Tube Flexure) has Autoslew allowed to make a better fit. The more stars you use the more formula are used and the better Autoslew can find out which error really belongs to the PA (or encoder resolution). Certain errors can cause very much the same pointing error behaviour. As an example, when you slew from SouthWest 60 degree alt to SouthWest 20 degree alt and Autoslew measures 20.1 degree altitude there instead of 20 degree you can exactly explain this result by assuming that a.) You have some tube flexure b.) You have a Azimuth polar alignment error c.) Encoder Resolution is wrong Autoslew has only the chance to find out which of these errors might be the “real” one when you measure MORE stars between these 2 points because each of these errors will cause different slopes between the 2 above measured positions or use stars at different sky positions. For example if you continue the measurement exactly in South direction and measure the same error again the Azimuth alignment error CAN’t be the source because an Azimuth PA error would not be able to cause a Declination error at the Meridian. Of course Autoslew is not so smart to think like that. It does the same thing that all other programs like Pinpoint and programs that do pointing optimization for the very large telescope do: It varies the polar alignment factor, the tube flexure and the encoder resolution until it gets the minimum RMS pointing error of all the measured stars. As you might understand, when you measure only the 2 stars mentioned in above example the result will be random and chaos because it can fit this behaviour with any of the above formula. This changes when you add the 2 stars in South direction because the optimization will now give less RMS error when the Azimuth error is zero but everything is put in the Tube flexure (or encoder resolution...). When you add more stars at the right places, the optimization will also be able to distinguish between encoder resolution error and flexure and so on... Reason 2: The telescope behaviour cannot be perfectly fitted with the used formula This will nearly always the case. The ideal mechanical world will cause the telescope to flex like the assumed Alt-Flex=A*Sin(Z) where Z is the zenith distance. So if the telescope has a flex that is getting stronger towards horizon than the calculated flex constant A will be different depending on the altitude of the stars you choose in your pointing file. It might even try to fit the residual mistake into a PA error or another formula and thus adding 3 arc minutes PA error which in reality is not existent. The problem that you can never find formula that exactly describe YOUR telescope is also the reason why the optimized formula (=configuration) will change depending on which stars you choose to make your pointing file. The next typical question that now comes up is: WHY DON’T YOU ADD MORE FORMULA IN AUTOSLEW? As an example, we could try to fit the Flex by a more complex formula like : Alt-Flex=A*Sin(Z) + B*Z^2 + C*Z^4 etc. If we only add enough factors in this series we can be sure that we can fit ANY flexure behaviour that is purely Altitude dependent. But wait – does that really make sense ? Yes, in case you want to make a pointing file with infinite number of stars it would indeed make sense to fit 50 different variables (assumed that we find a mathematical optimization algorithm that is able to find the exact minimum of a function with 50 variables). If you have only a limited number of stars and you want to make the best of it trying to fit too many variables is a good recipe for a complete desaster. The result will be random and chaos at positions where you don’t have measurements. The rule is: If you have few stars, fit few variables and only the most important once. However you might have noticed that if you add even more stars (like 30) Autoslew will add another fit on top of the formula fit – the Fourier Fit. That is a very powerfull fit to model the residual errors that are left after the Formel Fit. But it makes only sense to use this if you have enough stars. If you do Automatic Configuration Calculation you don’t have to care about all this because Autoslew will calculated how many variables to fit. Reason 3: Hysteresis, non reproducible and random errors This is probably very easy to understand. If you make 2 measurements with different random results (like depending on where you have pointed before), the configuration will also have different results. Sounds logic ? Conclusion and Remarks: You can expect to get different results with every new pointing file. This is NORMAL. From my experience the major error when making pointing files are hysteresis errors followed by flexure problems that cannot be modeled by Autoslew. When you make a 20 star pointing file and you get a residual RMS pointing error of larger 1 arc minute it simply does not make any sense to correct a 5 Minute PA error that Autoslew will show you because the chance is high that even without any change the next pointing file will show a complete different result. In this environment, you would simply not NOTICE any improvement of 5 arc minutes polar alignment because your other errors (Hysteresis, Formula residuals etc.) are much more prominent than the residual PA error. As a rule of thumb the determination accuracy of PA is about 5x the residual RMS error. Another option to find out when to stop PA is when you make a 5 star pointing file in East and then do the same in West. If you get -2’ on one side and +5’ on the other side you should stop the PA process because your measurements are not accurate enough. So people who have such bad results like 1arcmin RMS should (and I am aware that a lot of you will not like to hear this again and again) improve the reproducibility of the telescope and / or selectmore stars to allow also the Fourier Fit to take place. Final remark about encoder resolution of DDM 60: The Encoder resolution of the DDM60 is only another formula (like PA or flexure) and the accuracy of the factor depends on the quality of your measurement. If you get different results you will usually not be hurt by that because the errors during your pointing and tracking will be the same errors that caused your measurements to be non reproducible and they will be larger than the error caused by the error in your resolution error. Tip: When you did a reasonable PA (+-7 arc minutes) make a big pointing file one time, save the resolution and never touch it again. You should only redo this procedure if you have the feeling that you enhanced your telescope flexure and hysteresis problems and thus allowing you to make more accurate measurements. For a manual how to meassure the Hysteresis and Time flex please click the icon below: SequenceHyst-Timeflex.pdf
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