Getting the best performance from my AM5: Page 2

Getting the best performance from my AM5

w7ay

raawe Would this avoid the unavoidable guide spikes?

The only way to avoid the quide star from moving is to apply that last paradigm which I mentioned above -- ie, to guide by using different guide rate (Pulse Amplitude Modulation; if you are an EE) instead of pulse duration (Pulse Width Modulation) as we do today. I.e., if the correction slope (we come back to slopes again :-) is the same as the periodic error's slope, so they cancel each other out, and the guide star becones stationary -- in practice, the slope is not exact, and you will notice the slight movement, and apply a slightly different guide rate in the next frame, etc.

It is a large leap, and I don't know how many mount protocols can accept variable guide rates. However, that second paradigm, which is to issue multiple distributed shorter pulses, whose accumulated pulse duration is equal to that of the original pulse. Think of it as PWM at a higher pulse rate.

This would reduce the movement of the guide star (lower amplitude sawtooths), but not eliminate it.

Once you get used to the appearance of that sawtooth, it will become obvious that it is impossible to avoid the guide star from tracing a line while a guide exposure is being taken. Once you grok that, the solution(s) become obvious.

Whatt you descibed if a good way, too. I.e., changing the guide dynamics. Albeit, requires the periodic error curve for the entire revolution of the RA axis -- about 24 hours worth of recorded periodic error curve.

Chen

StarzLite

tempus

I haven't used PHD2, but when you say "complaining", are those warnings something you can disregard and still proceed with using the settings you would like? If so, I can see no harm in experimenting with the settings we are discussing.

The entire premise of what w7ay is presenting here is that the current guiding software platforms are not optimized for use with strain wave mounts, so the settings discussed here are a work around.

Dave

tempus

I use Phd2. When I try to lower the Max RA Rate and Max Dec Rate below 1500ms, Phd2 keeps complaining that those values are too low to make the needed corrections. Especially after a dither. I can't see Phd2 working well at the low values suggested here. What am I missing?

StarzLite

raawe
I did not change the guide rate. As you said, I believe the default is set to 0.5x sidereal.

The guide camera is the ASI120mm mini. The gain is set to High(48) and Bin1(Bin2 is off) mode.

Exposure is set to .5sec(2FPS).

Good luck with tweaking in a little more guiding precision.

Dave

StarzLite

MrAstro

Yes, ASI120mm mini set to High gain(48) and .5sec exposure.

Dave

mikenoname

I think tempus meant max duration not max rate because I am getting the same error messages.

I got out last night briefly and tried the 150ms max duration settings, lower aggression and 0.5s guide pulses and did not see much improvement. In fact, in some cases it was worse. PHD also intermittently complained that with the duration set so low it could not keep the guide star in the FOV.

Only an hour of play time is not enough to draw any conclusions, so I am looking forward to the clouds going away again...

w7ay

@mikenoname

What is the worst case slope on the periodic error curve that ZWO included with your mount?

From the first posting in this thread, Dave has one of the better ZWO mounts out there, with magnitude of slopes similar to my RST-135. My RST-135 has larger p-p periodic error, but smaller harmonic distortion. That is why I jumped into this discussion with more "color."

As in everything in this hobby, don't blindly use some other person's number. Measure that slope carefully and then set up the max pulse duration. If you have one of the worse ZWO mounts, as I mentioned earlier, there is nothing you can do.

For example, if the slope is over 1 arcsec/sec and you are using 1 second exposures, then the RMS guding error (just of the RA axis) could never be always less than .35 arcsec. If the slope is worse than that, the RMS error will also be worse.

The only thing you can do is to reduce the exposure time to 0.5 sec by using a reasonably sized guide scope. And if the slope is still too large to match your plate scale, then you will just need to move to a different mount.

By the way, also check where your error is coming from, the RA axis or the Declination axis. There were some postings in this forum complaining about the large backlash in their ZWO mounts (the strain wave gears do not have much backlash, but the rest of the mechanical design can add backlash).

If you are driving the guide rate at 0.5x sidereal rate, you can never get a backlash in the RA axis (except when you dither)since the RA axis is being tracked at 1.0x sidereal. A 0.5x sidereal pulse will only change the track from 0.5x sidereal to 1.5 x sidereal, and since both are positive numbers, the pulses can never produce a backlash.

The story is a little different in the declination axis. If you are seeing a large declination guiding error, introduce some polar alignment error (that's right, you read that correctly) so that there is a declination drift rate that is larger then the slope of your declination axis. Unfortunately, ZWO does not publish curves for them, so you will have to use some trial an error. If you remember, a polar alignment error can be measured by watching the drift of a star in declination (check out "drift alignment" on the web). So, just add enough polar alignment error so that the declination drift is greater than the curve of your declination gears. Your exposure duration may be limited by the field rotation, but that is what one has to sacrifice when there is lots of error in the mount.

Chen

mikenoname

w7ay

I seem to remember you detailing how to determine the slope of the curve from ZWO's diagram, but can't seem to locate it now. Can you point me to it?

Mike

w7ay

mikenoname I seem to remember you detailing how to determine the slope of the curve from ZWO's diagram

Place a ruler at the steepest part of the curve. The ordinate of the chart gives the vertical scale in arcseconds. The horizontal scale can be approximated (as units of seconds) by assuming that the fundamental period is about 400 seconds long.

The ruler line then gives you units of arcsecond per second.

Chen

mikenoname

w7ay

Don't know what 'fundamental period' means in this context. Since the mount rotates 360 deg in one day ideally, then one degree is 24*3600/360, so basically 240 seconds. From my zoomed in section (hopefully my chart will upload below) from the first peak to the bottom of the next trough looks like ₁.9 degrees to me (30.7-31.6). Or should I just be measuring one individual short slope?

In any event, by my calculations that comes out to be 456 seconds. The peak looks to me to be ~+6.9 and the trough at -7.7 for a difference of 13.6 arcsec.

Therefore my resulting number would be 13.6/456 or 0.03 arcsec/sec correction required.

Am I close? [Waits anxiously for his paper to be graded]

StarzLite

mikenoname
This will give you a starting point.
It's an example where w7ay did a worst case slope calculation on @Kevin_A's report:

https://bbs.astronomy-imaging-camera.com/d/15711-very-large-backlash-in-new-am5/19

Dave

mikenoname

StarzLite Thanks Dave. I knew I had seen him post it somewhere but I couldn't find it again when searching.

Mike

Kevin_A

Yes, your PE numbers are very similar to mine but your slope is a bit different.
I would expect your mount to guide well and 1s exposures should be adequate to tame guiding. Mine guided between 0.4 to 0.7 rms right out of the box without tweaking at all. So my aggression and max durations were set too high and you could tell by the jagged results, but that was just a baseline test. Once the skies clear I plan on dropping down to 30-50% aggression and max durations around 150-200ms to start. I will also monitor how dithering recovers after… with such low settings.

mikenoname

Kevin_A

Yeah, our max & min PEs are nearly identical but the harmonics are clearly different. I tried settings like those last night and did not have good success, so I'm eager to see your results.

Mike

w7ay

Kevin_A max durations around 150-200ms to start. I will also monitor how dithering recovers after… with such low settings.

No need to wait for night sky, Kevin; just use simple deduction.

If each pulse is 150 ms long and you are using 0.5x sidereal rate (7.5 arsec/sec), then the most the mount will move with each exposure time is 1.1 arc second.

So, if you had asked for a 3 arcsec dither, it will take at least 3 exposure times to get there, if there is no backlash. A strain wave gear has vanishingly small backlash, but the mechanical design of the mount can add plently of backlash. This is why it behooves you to not ask for a large dither in RA to avoid a backlash. It is harder to avoid backlash in the declination direction through.

Chen

Kevin_A

mikenoname unfortunately at this time of year even the clear nights have bad turbulent seeing which makes guiding erratic in itself. So it is hard to be scientific with skies that suck. Our mounts seem to be on the better side of the PE bunch, but each is different. Time will tell but we have to still remember that these are not considered high end mounts even though our guiding numbers are still very decent. We are paying a premium for portability not for high end precision.
I look forward to further testing when the skies settle down enough to give more accurate results that I can use to fine tune this tiny wonder.

mikenoname

OK, putting together the info from several different posts/threads I realized that Chen wanted just the steepest part of your curve extrapolated out so that you can get the steepest pitch (slope) calculated. From that I got the line drawn on my chart below (sorry for the crappy photo).

From that we can see that the x-axis displacement is a bit less than one unit, which is 0.6 degrees, so I am calling it 0.5, while traversing a y-axis displacement of 17.5 arc seconds. Assuming again that 1 degree is 240 seconds, then my mount needs to overcome 17.5 arc seconds of error in 120 seconds at worst. Which means 0.146 arcsecs/sec. More in line with the results he has calculated for others, and my slope is more gentle than Kevin's so that also fits.

w7ay

mikenoname More in line with the results he has calculated for others

I suspect that the worse case slope for your mount is a bit higher. In that frame of 3 fundamental cycles, the solid green line shows a higher rate slope:

I had duplicated the solid green line, and flipped the horizontal axis into the dashed green line, so it easier to eyeball.

I didn't try to look for even worse cases, and that is why a numerical table would have been useful, we can just numerically differentiate that curve.

I have definitely seen much worse just in the ads that ZWO has put out (they are apparently oblivious that this is what limits autoguiding; some of those examples should have been QC into the dustbin).

For now, that is a rough guess of the best you can do to guide the RA axis. The part that I had earlier explain with the diagram:

I.e., while you are guiding the mount, the sky will be taking the path of that sawtooth. Unless you can reprogram your autoguider to split that guide pulse into two shorter guide pulses, and that was how we began this discussion.

Chen

Kevin_A

w7ay thanks again for all your contributions!

mikenoname

w7ay I suspect that the worse case slope for your mount is a bit higher.

Thanks, Chen.

It's looking like Kevin's and mine are nearly twins (except for harmonic effects) since the steeper result you are seeing probably puts my slope in the 0.15-0.16 range. Looks like I'll be going back to 1 sec guide pulses which did seem to work well with it in my previous sessions.

Mike

ASquecco

In the Windows Paint app, I found it's possible to measure lengths in terms of pixels.
So for my mount I tried to calculate the worst slope like this:
I hope 0.15 arcsec/sec it's not too far from the truth !

raawe

ASquecco Interesting. I'll have to do mine as well. Just by eyeballing it, I think I'll have to reduce dither from 10 to 5, maybe even lower.

Kevin_A lol, you are going to catch a cold

Kevin_A

I always dither small amounts like 2 pixels with my noise free 2600/533 cameras and get great results. However, my dithers have been fast and small deviations, quick to return to zero but then sometimes huge and slow return. I am not sure if the belts are set tight enough on my drives and may have some slop that is causing this… not sure. I really have not been able to test anything due to constant clouds and snow. So i can neither confirm nor deny anything at this point. I am ok with 3-20 pulses to get my dithers back on track as it matters much less than better guiding control. So I still plan on using 150ms as a starting point for testing and see how it goes. Snow, wind and clouds are my biggest issues now! Haha

w7ay

Kevin_A I am not sure if the belts are set tight enough on my drives and may have some slop that is causing this… not sure.

Kevin, if the RA dithers are less than 7.5 arc seconds, there should be no backlash from the strain wave gears themselves, but it could come from other sources, like you said, a loose belt, or a cheap pulley (the ones that come with the ZWO EAF for example, are not what I'd consider good pulleys :-).

Now, if you limit each RA travel to be less than 0.75 arcsec (e.g., by setting max RA pulse to 100 ms), then a 3 arc second dither could take 4 exposure times to reach the destination, and longer, if it is not smart enough -- the ASIAIR, for example, does a slow exponential decay because it is lazily using PHD2 guiding to recover from a dither -- instead of appling a long enough pulse to directly drive the guide star to the new position and then restart guding.

Still, as you say, not a big deal when you can achieve better guiding.

Chen

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