yes please! i would like to see

Ok, see below. Sorry for the poor quality screen shots. I took them with my camera aimed at the screen ;^)).

Top image is the raw periodic error of a typical microstepper driving a precision worm. You might think the worm is not great, but the reality is that the two pulleys have runout and produce the majority of the large scale periodic error. The worm itself actually has only about 2 arc sec error. The rest is caused by the commercial pulleys which have a fair amount of runout. This is basically what you get in almost all imported non-encoder mounts that use belts and microsteppers.

The plot is taken at 100msec sample rate so you can see what is really happening during a typical worm cycle. Normally when you see a PE curve, the sample rate is 10 times slower, so you don't see the fast moving errors on PE data sheets.

The real issue is not the large scale PE, which can be handled by guiding (as long as it is slow moving and not too large an excursion). It is the fast moving errors at the 1 arc sec level that cause problems for guiders. The second plot shows in detail that microsteppers with belts have two fast moving errors. The belt teeth cause a ripple every few seconds (in this case once every 12 seconds at approx 1 arc sec P-P). If your guiding cadence is fast enough you might be able to guide it out successfully. However, if your guide rate approaches once every 3 - 4 seconds, you will see a residual error that modulates with time.

The faster moving error repeats every 2 seconds with another 1 arc-sec P-P error. This one cannot be easily guided out because the guider is always a step behind and can actually amplify this motion. The error is caused by the stepper motor itself which has 5% angle variation for each winding step point. If your camera pixel scale is 2 - 4 arc sec per pixel, you may not see these kinds of errors. If your guider is exposing at 2 - 3 seconds, the guide graph will average out the tiny fast motions and will record perhaps very low rms values. However, the stars that you are recording on your imaging camera are doing this tiny back and forth dance in RA and slightly smearing the resolution of your image.

The third graph below shows how the absolute encoder tames all three errors.

just wondering up to how much focal length would the Mach2 be suitable unguided.

Focal length has nothing to do with how long to do unguided. Unguided depends on how well your model is made and how much drift you are willing to accept in your image. In the image that I posted the drift rate was approximately 5 arc sec per hour. Each 10 minute exposure had approximately 0.67 arc sec of drift, more or less. After stacking, the resulting stars had a value of .065 for flatness in MaximDL (see attachment below). Some of the exposures had more, some had less, and I simply stacked them all with Median Combine.

5 arc sec per hour might seem high, but the setup I used was purposely offset from the pole and had very high drift rate. Before modeling the RA drift was measured at 48 arc-sec/hr and the Dec was 96 arc-sec/hour. I would expect that one could get modeling down to +- 1 arc-sec/hour with good polar alignment.

Roland

-----Original Message-----
From: Seb@stro <sebastiendore1@...>
To: main@ap-gto.groups.io
Sent: Thu, Jun 4, 2020 10:40 pm
Subject: Re: [ap-gto] NGC5371 and surrounding galaxies

Interesting! Thanks for the 101 course on mount design tradeoffs Rolando.

Just wondering up to how much focal length would the Mach2 be suitable unguided. Do you think it would do it at say 1400mm FL (8in Edge+FR)? What about 2000mm (no reducer)?

Any feedback from the lucky folks that already got their hands on the Mach2?

Sebastien

Hi Astronuts,

I joined AstroBin and posted my first image. Hope the link works:

https://www.astrobin.com/full/vsz056/0/?nc=user

This is an image taken with the Mach2 and 160EDF refractor, unguided over a period of 2 hours. I modeled a path that the object would take with 3 data points and let MaximDL take a set of images until the scope ran out of room and stopped at the limit. I'm shooting in very light polluted skies, the Moon was up and bright, so i was pleased that i was able to record some very faint stuff.

No guide scope was used, no field flattener, just the 160 refractor straight to the 8300 chip. Larger chips would need a flattener of course.

Rolando