Gta 4 Best Patch For Performance

[Idara Viengxay]

Chengets all of the credit for explaining how to tune a strain wave mount for optimum performance and I simply applied what Chen describes and the results for my mount are nothing short of incredible!

I received my AM5 about three months ago. The QC report seemed reasonable, although I really had no idea at the time what the important data was that the graph was showing. Chen explains all of that in the recommended post.

I was very happy with the AM5 performance initially. I'm using it with the ASIAIR+, ASI120mm mini, ASI2600MC pro, WO Redcat 51 and WO Uniguide 32. I started off using the default guide settings of the Air+ and the results were acceptable. I really didn't spend much time changing anything other than tuning the aggression settings and guide camera exposure time between 1 and 2 seconds. Typical settings I was using and their results looked like this.

As I spent more time using the mount for many hours of astrophotography sessions, I noticed intervals when the guiding numbers would become erratic. These rough spots where somewhat short in duration and didn't seem to effect data quality, but they also seemed periodic in nature. They would look something like this:

Looking around on the net, I found that many others were experiencing similar issues with this. Discussions covered the usual trouble shooting suggestions. Accurate polar alignment, seeing conditions, wind, calibration, tripod stability and vibration, etc. I was meticulous checking off all of the variables, but the erratic patches continued regardless. The periodic and repeatable nature of the behavior is what I kept coming back to. I concluded that there must be something mechanical about the mount that was causing the issue. I also figured that maybe it was something that was just luck of the draw. Maybe I should just accept what I was getting and be good with it. Was I ever wrong!

Browsing around here on the ZWO forum a few days ago I came across the post where Chen explains the idiosyncrasies of tuning a strain wave mount. I was skeptical, but optimistic to try something different. Besides, Chen clearly backed up every claim with solid facts, examples and even citations! I went out a few nights ago with no other agenda but to tinker with guiding accuracy. Working strictly from memory of a first reading of what to do, apparently I got it wrong. Nothing much changed. I decided to read everything again the next day very carefully. As the old saying goes, the devil is in the details. I decided to go out last night and try again, This time starting from scratch with settings that seemed so extreme, surely it would be comical. Spot On! I could not believe this was the same mount!

As you can see, I used settings that are quite different than default. I slewed to Procyon for the initial calibration because of its current proximity to just east of the meridian and along the equator. I initially set the aggression of each axis to start off at 5 (the lowest possible) and it only took a few seconds to dial in the numbers you see here. I guided here for an hour and it remained very stable. The total stayed between .2" and .5", averaging around .35"! Unfortunately, I didn't know that PHD logging on the Air+ only saves when using a plan or autorun, so I don't have that data to show the hour long run. I'll get more complete data the next time out.

I then slewed to three more general quadrants of the sky to check for consistency. I went west of the meridian along the equator, east near the north polar area and west near the north polar area. They all looked good and I made a few minor tweaks to the aggression settings along the way to dial in things a little better.

Finally I ended the test with a five pixel dither between each exposure to check settling repeatability. Again, I made some minor tweaks in aggression, but nothing major. Small changes in aggression settings can make a significant difference, so I found a slow and methodical approach here is beneficial.

I hope this information can help some others get the most from their AM5. I believe this mount has near unlimited potential within its designed capability and I'm very happy and impressed with mine. There are some pretty significant differences in the approach to tuning a strain wave mount and current software is not optimized for this technology. By having a better understanding of how this technology works, dialing in the best performance becomes more intuitive. Many thanks again to @w7ay for taking the time to explain the details behind the design, function and optimization of this amazing technology.

I have to try this. Amazing performance. I am still adjusting to ASI Air+ and AM5 mount and even with all the default settings I am really satisfied with guiding. I have to note that after v2.1 my mount started to misbehave, and I had to restart it a couple of times to get things back to normal.

My AM5 report states PEC 15.5/6.3 Arc-secs, and here is a guide screenshot after some 3h on the target. It shifted from 0.3-0.5 during my 6h imaging session yesterday.

That looks very good. Your experience would suggest that the slope(rate of change) of the various harmonic curves of your mount may not be as steep verses other AM5 mounts that experience wider fluctuations in guiding accuracy. The amplitude(Max and Min numbers on that report) of the fundamental PE is not really of much importance with regard to guiding. The zoomed graph at the bottom of the report will give a better idea of predicting which mounts will be more or less challenging to guide. That graph depicts the various harmonic curves and most important, how steep(slope) the rate of change in those curves is over time. This is where more frequent sampling(guide camera exposure rate)/correction pulses can dramatically improve the guiding of certain AM5 units.

Here are three figures to illustrate what you described. This first one for zero harmonic distortion (never going to happen with stain wave gears, I can assure you). I have given the fundamental periodic error (blue curve) a 50 arcsec p-p (25 arc sec amplitude).

The green curve is the first drivative with is ordinate scale on the right. This will in the final analysis (you will have to wait for the white paper) determine the minimum possible RMS error of the guiding -- in this case 0.37*0.707 = 0.26 arc sec/sec. So, 0.37 arcsec for 1 second guide exposure.

The above shows (again blue line) a periodic error that really does not have that much large harmonic terms (that I also scaled to 50 arcsec p-p, to be fair. But look at how ill behaved the derivative has become. The peak of the first derivative is now more than twice the one of the "smooth" mount.

Notice that the peak derivative is now over 1 arcsec. Good luck guiding such a mount. It may work fine most of the time, until you hit that part of the hour angle where the first derivative shoots up.

For now, just use a ruler-and-pencil way to estimate the worst case slope. There is no way to beat that; so no need to bang your head against the wall to try to get better guiding with the mount that your have.

Even in the case where the slope of the periodic error curve is 1 arcsec/sec, you will need less than 1/7 second pulse to correct it. I.e., assume that you choose 0.5x sidereal rate as the guide rate, that is equavalent to 7.5 arcsec/sec. To correct a 1 arcsec error, you would need only a 133 ms pulse, not anywhere close to a 2000 ms pulse.

Slow mechanical flexure can cause some error, and so can atmospheric refraction if the mount does not support King tracking rate. But neither are large enough to cause harm (any large mechanical flexure should be fixed independently, in any case). Most mounts support King rate, but since ASIAIR does not support KIng rate, I don't know if ZWO's mount does:

Those additional illustrations above really get your attention. The magnitude of the accumulative peak when several of the harmonics all cycle in to sync is impressive. Thanks for the additional discussion.

Notice that the derivative has an "N" term in the multiplying factor. I.e., the firth harmonic term creates 5 times the problem from the fundamental. Fortunately, the fifth harmonic distortion of most gears are also down by a lot -- it is up to QC to reject the high harmonic content ones (everything boils down to QC).

In the part shown here, the periodic error always exert a positive "pressure," even when there is a pulse. The pulse just happens to have a much larger negative slope (of the order of 7.5 arcsec/sec, vs the periodic error (of the order of 0.1 to 1 arcsec/sec).

When the pulse is running, the actual telecope is moved downwards (in this case) at a slope of the slope of the periodic error - the slope of the guide rate (7.5 arcsec/sec). Once you let the foot off the pulse, the telescope now moves again at its periodic error rate, for the rest of the exposure frame.

Notice that the guide star is also making the identical movement. I.e., during a 1 second exposure, the guide star will move up and down between 0.1 and 1 arcsec (so the centroid computing part had better be ready to handle that). For a 0.5 second exposure, the problem is halved. Thus the need to use the shortest exposure possible.

At some point, you can no longer use shorter exposures (unless you use larger aperture guide scopes). But here is where the N-pulse per exposure come in. Instead of ptting all the correction above at the start, we apply only half of it at the beginning, and apply a second one at the center of that exposure. The sawtooth doubles in frequency, while the sawtooth amplitude halves! If the slope does not change that rapidly (I have the also shown the second derivative in my white paper to show that it is indeed small) this is equivalent to halving the exposure time, without having to actually shorten the exposure time.

The penultimate solution is to use a single pulse, whose durration is the same as the exposure time, and whose slope is the exact opposite of the mount's slope! The amplitude of that green curve above collapses to zero (or somehing that is very small). That makes a strain wave mount as smooth as any worm geared one. Not all mount protocols can do that (control the guide rate to such fine grain). My RST-135 can control the guide rate from 0.01 x sidereal rate to 0.99 x sidereal rate, so I have at least something to experiment with in the future.

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