tuning my AZ-EQ6

[wimvb]

This story started about nine months ago, in the middle of astro season. The tracking performance of my mount degraded, and those times when I didn't give up, the guide rms could be close to 2" (at an imaging scale of 1.2"/pixel). Especially DEC guiding was pretty bad. Removing the backlash did improve the tracking to some extent, but not enough, and my images where still blurry. If seeing isn't perfect, a guide rms of around 1" can be acceptable, but most of the time, the stars in my subs were elongated.

During summer recess, I took apart the DEC stage of the mount and cleaned and regreased everything. The inside looked quite clean, but there were some particles that imo, shouldn't be there. There was some wear on the DEC belt, so I decided to replace it. I was very careful to remove backlash during reassembly, so the mount should now have a properly functioning DEC stage. And it did, DEC guiding was now a lot smoother, and with proper balancing and polar alignment, DEC rms was good.

But now, RA was bad, and just before the start of astro season, I opened up my mount again, and cleaned the RA stage. This looked cleaner than DEC, and there wasn't much to do except remove excess grease in some places and regrease in other places. Again, I was careful to remove any backlash during reassembly. Even after the strip down and cleaning, RA guiding was still around 1 - 1.2", not low enough for my imaging scale. As it turned out the RA belt had always been too loose in my mount. Tightening it removed a 10 second period and made guiding a lot less erratic. The main period of my mount is now 120 seconds (one full rotation of the timing pulley), and not 10 seconds (one advance of each tooth on the timing pulley). Most of this period can be guided out using PPEC in PHD or the Ekos guide module.

While I tested the mount, I noticed that the RA timing pulley wobbled. This was partly because the end cap of the pulley has a very poor fit. But that shouldn't affect performance. (What was a bit disturbing became apparent later on, when I had removed the pulley from the motor shaft.) Anyway I decided to replace the pulley with a better one.

 

So I ordered two 12 tooth 2.5 mm pitch timing pulleys form Beltingonline, together with a spare belt. These pulleys don't have the right bore for the SW stepping motor shaft (5 mm), and I had a colleague drill a 5 mm bore with the lathe we have at my school.

About a week or two ago, I replaced the original brass timing pulley with an aluminium one. And voila, guide rms went down to 0.63", with occasional 0.57" 'dip'. Guiding is now definitely seeing limited (with a 3 seconds exposure time for the guide camera).

After removing the original pulley, I could examine it more closely. Skywatchers evidently decided that it is more economical to press a timing pulley into a brass tube that will act as a shaft adapter, than to make one from a solid piece of brass. The pulley is capped with a thin piece of metal in quite a sloppy manner. But this doesn't matter much. What does matter is how the toothed section of the pulley is pressed into the shaft adapter. As can be seen in the first image, this section isn't pushed straight in, and this causes the 120 s period in the tracking error. The slanted pulley causes the tension in the belt to vary over one revolution (120 s). Since this error is smooth, it can be guided out. But, imo, it is always better to avoid an error than to correct it. And that is why I decided to replace the pulley.

The high tension in the belt puts a lot of stress on the timing pulley, and I hope the aluminium can stand this stress. If I find that tracking deteriorates again, because of the pulley, I can always put the brass DEC pulley on the RA motor shaft, ad the old RA pulley on the DEC motor shaft. Hopefully, this won't be necessary.

 

[gorann]

Scary how simple and sloppy it is manufatured. Amazing that it works at all. I wonder if I would like to know what my SW mounts look like inside, but I porbably have to one day.....

[wimvb]

2 minutes ago, gorann said:

Scary how simple and sloppy it is manufatured. Amazing that it works at all. I wonder if I would like to know what my SW mounts look like inside, but I porbably have to one day.....

If it ain't broke, don't try to fix it. With your RASA at the imaging scale you have, you don't need super accurate guiding. But if you ever put your new toy scope on the NEQ6, you may need to clean and adjust it first.

Imo, mounts are like cars, they need regular service. We don't expect a car to run year after year without changing oil and adjusting the brakes. So why should we expect a low cost mount to function at sub arcsecond accuracy without any TLC? I've had this mount for more than four years now, so service was overdue.

[gorann]

Yes, but with cars the manufacturer calls them back for free service if they have made a detail that could malfunction (has happened to me twice the last years).

[newbie alert]

12 hours ago, wimvb said:

 

Imo, mounts are like cars, they need regular service. We don't expect a car to run year after year without changing oil and adjusting the brakes. So why should we expect a low cost mount to function at sub arcsecond accuracy without any TLC? I've had this mount for more than four years now, so service was overdue.

Look at how fast the gears actually turn, not much at all, its speed is all in the slew pointing to the target.. once it's tracking hardly turning at all.. that's why there's grease in the gears rather than oil that's in a car.. no brakes in a mount.. 

The difference in manufacturing tollerance is clear from a skywatcher to a Avalon.. it's been said that they share the same components, but like opposite ends on performance,  and a Avalon is said to be maintenance free

Well done on your upgraded spindle, I've seen many wobbly pulleys on gear spindles when people have done the belt mods, if the aluminium one wears, I'm sure that one could be turned in brass

Edited October 18, 2020 by newbie alert
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