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Last updated: 11 March 2004
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Last updated: 11 March 2004 |
Sent: Friday, January 18, 2002 5:10:40 From: mjedwards@taz.qinetiq.com (Martyn Edwards) I've watched the skies for quite a few years through binos, and bought my first telescope - an ETX-90 - a few months ago. Before I bought it I had a good look around your site, and therefore knew of the excellent work that you, Dick Seymour and Clay Sherrod were doing. What really aroused my interest was Dick's patch to modify training values. I had always felt that the training method was a one-off single shot attempt, and I wasn't too happy with my results. When Dick posted his patch I started playing around, based on the following. As I understand it, training just takes up (calibrates) the backlash. Once the backlash is taken up during the training process then, providing your ETX is not a complete dog's breakfast, it shouldn't matter how far you slew to your target - and the same applies when you return to the target from the opposite direction. I felt the kick which is imparted by the Az and Alt Percent values was a bit of a red herring (as well as being somewhat subjective), but might just do something which was unwanted - therefore I needed a training procedure which started off with zero percent values and allowed me to take a number of readings to eliminate random errors. Dick's patch enabled me to do what I wanted, so I developed the attached spreadsheet. In order to use it you need to install Dick's patch; as a matter of courtesy I emailed him before contacting you, and he is happy for me to ask you to publish this post. The spreadsheet is based on calculating averaged errors for a number of Az and Alt training values, and then predicting the optimised Az and Alt training values which would lead to zero error. I have built it around four training values - the graphs at the bottom of the spreadsheet should be straight lines, and thus any gross error in user entered values will stand out a mile and allow the user to recheck that particular area of data entry only. Three values wouldn't have allowed you to identify the oddball, and five or more values gets boring! The values I have used are 20%, 40%, 60% and 80% of the training values that you start off with - for no particular reason ( you could use any four values set between zero and 32767 - it's just that these figures work for me. By the way, the spreadsheet has real figures from my own ETX in it). If you feel this email and the spreadsheet could help the ETX community then would you please post it under the Patches (ASU 3.0 ROM file) area of your Autostar Information page, in the hope that others might use it, abuse it and hopefully improve it! Best regards, Martyn Edwards
Subject: Training your ETX scope Date: 3/8/04, 11:45 From: Niall Saunders (niall@njs101.com) To: mjedwards@taz.qinetiq.com Hi Martyn, Thanks for your excellent spreadsheet, and for giving me another way of looking at what is happening inside the cute little Autostar. (And thanks also to Dick Seymour for patching the code to allow us to see what is actually happening when we train the drives - perhaps MEADE might realise the power of this information, and make it 'official' in a new release.) Firstly, I would like to send you a slightly modified spreadsheet - where the primary differences are in that the training exercise is repeated SEVEN times. I know that you state that four iterations are sufficient but, when a user is faced with a scope that has perhaps been 'playing up', then they will (and should) take the time to convince themselves that at least one reason for misbehaviours has been eliminated (or identified as a culprit !!). I have also modified the spreadsheet calculations slightly to indicate a 'Standard Deviation' figure and a 'Correlation' figure. Whilst Standard Deviation 'per se' might not be easily grasped by many users, the Correlation figure, expressed as a 'percentage', could be used instead - giving them at least a figure of merit for, or confidence in, the results of their recently completed training exercise. Also, to make the acquisition of the data easier, I have created a simple 'blank form' that can be printed, and used 'in the field'. In fact, the only part that I have NOT addressed is the creation of the INITIAL training values for each axis. The way I have been doing my excursuses (now implemented on both an ETX105 and an LXD55 8" SCT) is to first determine the CURRENT training values, then to carry out a 'normal' single-iteration "Train Drives" as proscribed by Meade. The only thing I do now make sure of, is that ANY 'Train Drives' routine takes into consideration the advice of Dick Seymour, whereby you bring the OTA 'on-centre' IN THE DIRECTION THAT THE OTA IS GOING TO MOVE OFF IN when the Autostar implements the next stage if the process (thus ensuring that the gearbox backlash has already been eliminated for the 'slew-away' phase of each of the four stages.) From the CURRENT, or SINGLE-ITERATION, training values I think that it would be better to provide a 25%, 50%, 70% , 100%, 133%, 200% 400% spread for each axis - thus generating good statistical data on each side of the 'expected' result. Finally, I found that, in order to be truly reliable, I had to make sure that I 'slewed away' by an amount equivalent to 3/4 of one of the 'segments' in the FINDERSCOPE each time. This meant that the calculations were based on AT LEAST 2 degrees of scope motion away, and two degrees of return drive, on both axes, for each stage of the routine. I also got into the habit of slewing away at no more than 'Speed 5', and came back at 'Speed 5' until I knew that the target would have been re-acquired in the 26mm SP eyepiece. At that point I dropped to 'Speed 3' and then 'Speed 1' for final location - remembering at all times NOT TO OVERSHOOT !!!!! And, of course, you CANNOT do this without a cross-hair reticule eyepiece. But, you can make one of these for NOTHING if you have some card, glue, very thin wire (or human hair) - and a little bit of ingenuity. Hint: Unscrew the chromed mounting barrel from your 26MM Meade SP, then also unscrew the next (matt-black) ring as well. At this stage, take care as you could end up disturbing the glass lens assembly - just be careful, and set down the remainder of the eyepiece so that it cannot be accidentally 'knocked over'. The matt-black ring has an internal 'ridge' at one end, the end closest to the chrome ring. The focal plane of the lens is just inside this ridge - at the lens side - and so if you place a reticule in this plane, it will be in clear focus whilst you are observing. Now all you need is a paper 'tube', with four tiny 'razor-blade' slits at one end. Wedge, and secure with glue (cyanoacrylate 'super' or 'crazy' glue seems best), one end of your two 'wires' (or 'hairs') into two adjacent slits. Stretch them, quite tightly, across the tube, and secure them in the slit at the opposite side. Fit the paper tube into the black ring, such that the reticule is closest to the internal ridge, and reassemble all the rings. Job done!! [Note: Mike, if you want a series of pictures showing how I achieved this, please let me know. Also, please ensure both Dick and Martyn are FULLY credited for their initial efforts if you feel that this addendum needs to be published.] Hope this helps. Cheers, Niall
Subject: re: ETX Training values Date: 3/9/04, 12:06 From: Anke I read your update article about training of the Autostar. I think the data you show give a very good picture of the dependency between the training values and the errors measured. The data range cover very well positive and negatives errors from under and overshooting. If I write you this e-mail is to suggest a further small improvement. If you look carefully the graph I think you would agree that a linear regression is not the best fit to the measured data. I believe a exponential relation may give a much better fit. As example I took the value for the AZ training. One of your data shows a very small error (-0.00055 o less than 20 seconds!) for a train value of 832. However your linear fit provide a optimum value of 936 (12.5% higher). If you adjust a exponential equation to the data you will obtain the following expression [AZ value] = 857 EXP(-4.42 x [Error]) (the R^2 = 0.997 if this make any sense to you). So if the error should be zero the optimum value is 857 (3% higher) May be you are familiar with the term R^2 to know how good is a regression. As close to 1 better is the regression. So, for the linear regression we obtain R^2 = 0.949 and for the exponential R^2 = 0.997. That's all! I am not a mechanical engineer but may be there is a practical reason why the errors follows better a exponential relation than a linear trend. I ignore it. Sorry to disturb you. I just want to help a little bit. DavidAnd:
Hi David, I am glad that the information was of some use. I cannot say for sure whether the data conforms to a linear or exponential regression. I simply took the data as supplied by Martyn, and extended the number of samples from 4 to 7. However, even at 7 samples there is probably not enough statistical information to determine whether the function is, or is not linear. What I can say is that, in practice (and this is no doubt borne out by Martyn) the Autostar behaves MUCH, MUCH better when trained using this method. I have implemented the training exercise on two separate telescopes - an ETX-105 and an LXD55 8" SCT. After using the suggested training values, the scopes both performed SIGNIFICANTLY better. In saying that though, moving between two Terrestrial Landmarks was still not 'perfect'. It may be that the exercise needs further refinement, perhaps with the OTA at a totally different angle (such that the gearboxes are in a totally different orientation, and hence different 'mesh'). The sample data on the spreadsheet example is NOT a record of the actual exercise I carried out on my ETX-105. (Sorry for any confusion!!). When I implemented my version of Martyn's spreadsheet exercise, I used what 'I believed' were a suitable range of training values, picking numbers that seemed suitable to give me a 'spread' around the Y-axis. However, I simply 'jiggled' some of the readings on my posted example of the spreadsheet, to fit with the fact that I have now entered formulae into the spreadsheet to give 50%, 66.7%, 75%, 100%, 133.3%, 150% and 200% of the 'initial starting values' seen by a user after the scope is first trained (using the 'accurate method' as described by Dick Seymour). The R^2 values are therefore NOT as good as I have seen from REAL exercises. For example (and you can simply enter these values into the small table at the bottom of the spreadsheet - making sure that you are working with a BACKUP copy, because you WILL be overwriting the simple 'lookup' formulae otherwise in these cells), here are the results from the two telescopes I have access to:- For the ETX-105 =============== Az Error Az Training 0.104027778 600 0.047083333 800 -0.000555556 1000 -0.025694444 1200 -0.081527778 1400 -0.125138889 1600 -0.1725 1800 Alt Error Alt Training -0.040972222 100 -0.026111111 150 -0.017916667 200 -0.004305556 250 0.00625 300 0.010416667 350 0.02375 400 Giving the following final solution:- Best Fit Az Intercept 1038 Best Fit Alt Intercept 283 Standard Deviation of Result 13.4 5.2 Correlation of Result (%) 99 98 (note that I have taken the R^2 value, and expressed it as a percentage, to suggest a level of confidence for non-statisticians!!) For the LXD55 8" SCT ==================== Az Error Az Training 0.015972222 300 -0.001944444 350 -0.009166667 400 -0.021805556 450 -0.0325 500 -0.040833333 550 -0.052638889 600 Alt Error Alt Training -0.070277778 500 -0.041944444 600 -0.008333333 700 0.016944444 800 0.040416667 900 0.068194444 1000 0.0925 1100 Giving the following final solution:- Best Fit Az Intercept 357 Best Fit Alt Intercept 748 Standard Deviation of Result 5.7 4.6 Correlation of Result (%) 99 99 (note that I have taken the R^2 value, and expressed it as a percentage, to suggest a level of confidence for non-statisticians!!) >From a purely mechanical point of view, it would seem logical to assume that the relationship between 'Error' and 'Training Value' would be LINEAR, but without access to the algorithm used by Meade when implementing the Training Values, it would be impossible to say. If you consider what is happening during the 'Train Drives' routine (some of the following comments being paraphrased from Dick Seymour, Dr. Clay Sherrod and Martyn Edwards' previous postings), then things may seem clearer:- 1.) Starting from 'scratch' you set out to "RESET", "CALIBRATE MOTORS" and "TRAIN DRIVES". 2.) The "RESET" stage is no longer as important as it may well have been in the earlier versions of the AutoStar code. Certainly, when I upgraded from v2.6Ec (and v2.6Ed) to Meade's 'official' v3.1Ee, the 'setup parameters' in the Flash Memory were apparently retained without any corruption. The same applied when I then subsequently upgraded 'sideways' to Dick Seymour's v3.1e4 patch. Even the values held in the AutoStar for the 'CALIBRATE MOTORS' stage were carried over without corruption. But, providing you are happy to re-enter all your user data, then, yes, I would recommend a full RESET - just to be on the safe side. 3.) The 'CALIBRATE MOTORS' stage is quite important - especially at any point where you switch to a different power source for your scope. Theoretically, it should NOT be that critical, as the action that 'calibration' has on each axis takes place on circuitry that has ALREADY been supplied with a 'constant' voltage source from an on-board regulator. More specifically, each axis has its own micro-controller (on the circuit board associated with the axis, over and above the microcontrollers and microprocessors that are in the AutoStar itself). What happens when a 'calibrate' command is issued is that each axis, in turn, has its microcontroller send a 'full power' voltage level to the LED transmitter that shines through the slotted wheel (fixed directly to the motor output shaft). Each axis also has TWO sensors to detect this incoming beam of light - the intention being that the detectors will sense the rotation of the disk. However, in order to save money, the disk is made from translucent plastic, and so the intensity of the LED output has to be 'cut back' until a happy medium is achieved. The microcontroller for each axis therefore spins the motor (at full speed) whilst reducing the power, in steps, until a 50% 'mark-space' ratio (or 'square-wave') is detected by the sensors. The microcontroller has 64 possible 'levels' it can use for this, and the source voltage, prior to being 'stepped down' is from the on-board 5V regulator feeding the microcontroller. (In actual fact, the microcontroller is regulating the CURRENT into the LED. which is what actually determines the intensity of output, but the theory is still the same). This happens twice, first for the RA/Az axis, then for the DEC/Alt axis. The values, once determined, are sent back to the main microprocessor, in the AutoStar, where they are stored in non-volatile Flash EEPROM memory. Probably the MOST IMPORTANT thing you should consider when 'calibrating' the motors is to do this IN THE DARK (or in the ambient level of lighting that you will most likely be using the scope in.) A higher level of ambient lighting causes the LEDs to need to output MORE light in order to produce a 50% square-wave pulse train. Calibrate your motors in bright daylight, and then use them at your dark skies sight, and you could well be 'over-driving' the LEDs, causing false pulses to be counted by the microcontrollers. By the way, if you are an LXD55 user, things are only SLIGHTLY better - at least you have a solid, stainless steel, opaque, slotted disc, instead of the cheap translucent plastic toothed wheel used on the ETX scopes. But a close inspection of this will show that it is almost certainly NOT running 'true' and will 'wobble' all over the place - again the reason for making sure that your CALIBRATE MOTORS exercise was carried out with SOME knowledge of what is happening inside the system. 4.) Now you are ready to 'TRAIN DRIVES'. Ideally, you will follow Martyn Edward's procedure, and will have used Dick Seymour's patch, and will have upgraded your AutoStar to v3.1e4. But you still need to carry out at least ONE 'Meade' training routine, to give you a good 'starting point' for my 7-stage adaptation of Martyn's process. But, when you set out to do this, you must be aware that the 'normal instructions' for the process are NOT GOOD ENOUGH. As Dick Seymour states, you must visualise what the AutoStar is going to ask your motor drive gearbox train to do AT EACH STAGE OF THE PROCESS. Normally, users will 'Train' the RA/Az axis first (it's the first menu option after all). An object is chosen, and centred, and then the AutoStar slews off in the '<' direction, and asks you to re-centre the object USING ONLY THE '>' key. For this process to be meaningful, the gear-train MUST already be FULLY MESHED in the '<' direction BEFORE the AutoStar 'slews away'. This means that you MUST initially 'centre' the reference object using ONLY the '<' key. If you overshoot during the initial phase, you will HAVE TO START OVER AGAIN, moving away at least quarter of the finderscope AFOV before slewing back to centre. The Training Values are also more reliable if you do not have to adjust the 'other axis' at the same time. Note also that, in later version of the AutoStar software, you CANNOT use any other direction keys when returning the object to centre - but you are NOT protected from this possible error when INITIALLY bringing the object on-centre. So, start by Training the RA/Az drive, and bring the object 'on centre' using ONLY the '<' key. AutoStar will the 'slew-away' in the SAME direction, and as you to re-centre in the '>' direction. The next stage is that AutoStar slews off in the '>' direction (you ought not to have to do anything between these stages, as you will ALREADY have centred the object by your previous actions, AND in the correct direction). You re-centre in the '<' direction. Then you work on the DEC/Alt drive, initially moving off in the '^' direction, before re-centering in the 'v' direction. AutoStar takes over, and 'slews away' in the same 'v' direction. You bring the object back to centre in the '^' direction. Finally, AutoStar 'slews away' in the '^' direction, and you bring it back in using the 'v' key. I only ever use speeds 5, 3 and 1 when carrying out ANY of my training - these speeds seem to give the most control. 5.) And if you do NOT have an eyepiece with a reticle, you will only get 'average' (at best, more likely 'poor') results. Take the time to roll up a piece of card and make a reticle for yourself. You will save yourself I don't know HOW many beer tokens - but it is so simple to do that it seems almost criminal to pay someone else to make a reticled eyepiece for you (unless you need specifically high accuracy, or an illuminated reticle for astrometric purposes). 6.) And, finally, the NEXT time you find yourself doing a Train Drives exercise, at least you will have the PREVIOUS results to compare against. If the values appear to be significantly different, or differ each time, then it would be wise to suspect an anomaly with the gear train of the appropriate axis. On the ETX-105, the motor control PCB is attached to the gearbox subframe (plastic assembly) with self-tapping screws, and these can, and do, work loose. Moreover, the gear-box sub-frame is attached to the cast aluminium chassis with further self-tapping screws - and on my ETX-105, these were almost falling out. The ETX-105 has a better method of adjusting back-lash on the worm drive than was apparently the case with the early ETX models (the new ETX-125 chassis uses the system developed for the ETX-105, or vice versa - anyway, they are both FAR MORE RELIABLE) - but it is still worth checking that the machine set-screws have not worked loose. If you have an LXD55 GEM mount, then it is definitely worthwhile checking all the bolts and screws in the gear mechanisms. There appears to be NO 'thread-locking' compound used on these, and a BRAND-NEW mount from MEADE exhibited 'slop' all over the place. Twenty minutes with a set of Allen keys and a Philips screwdriver will fix BOTH type of mount - PAINLESSLY. And a little bit of 'light-duty' threadlock might keep the problem at bay for years to come. Hope this helps. Cheers, Niall Saunders Aberdeen (Cloudy Skies, hence the time available to write this lengthy tome!)
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