EQ6 Belt Mod (Belting Online)

[RobertM]

Lots of good work there Mark.  It's a bit difficult to see just how peaky or smooth the curve is for guiding purposes - any chance of showing an expanded single worm cycle ?

I agree with you about the belt teeth, if they have right angled edges rather than beveled or rounded then they are likely to catch on the cog teeth.  If so then that should ease over time or you could just run the motors for a few hours to bed them in.

Robert

[MarkS]

Hi Robert,

Here's an expanded single cycle of the worm:


The data comes from PERecorder and webcam at 10 frames/sec (each frame 1/25sec exposure).  For this plot I have shown a moving average over 1 second.

Here's the raw data without any kind of averaging:




It is impossible to know how much of the spikyness is caused by the seeing (which wasn't good that night) or by the mechanics of the mount.  That is always the problem when analysing PE data.

In any case, here's the same data with my complicated PEC removed from it:



Now it might be a bit simpler to separate the seeing related spikes from the mechanical related spikes.

Finally, here is the above graph again but with a 1 second moving average.  This should remove most of the seeing noise leaving just the mechanical.



Mark

[RobertM]

Considering that's removed most of the short term turbulence it isn't bad at all.  I think you really are into bearing swarf territory now.  Not only the worm but also shaft bearings could cause a lot of noise.  I also wonder how far you can go with the discrete movement of stepper motors, they will also cause tiny vibrations through the mount depending on load.

I assume that you'll only be guiding in RA because of precise polar alignment so aren't concerned with the dec axis ?

Robert

[MarkS]

I assume that you'll only be guiding in RA because of precise polar alignment so aren't concerned with the dec axis ?

That's a very interesting point.  Once I got my alignment bang on, I found there was very little difference whether or not the Dec axis was guided.  I still think Dec guiding is worthwhile, if only to counteract any residual slight drift.  Just need to work out the optimum Dec settings to prevent it trying to guide out random noise.

[The Thing]

Everything I read about guiding says you want PA to be off slightly in so that guiding corrects the DEC drift in a consistent direction avoiding any back and forth bounce movement between worm groves and gear teeth. There is probably an optimum PA error and direction of misalignment for each object position though as drift will be different. And I suppose it depends on how precisely unbalanced each axis of the mount is as well. I had to add a finder to the side of my OTA the other night to unbalance the DEC axis a bit more to stop it bouncing as shown on the guide graph.

[MarkS]

Thinking about it, of course you need to guide in Dec.
Otherwise how will dithering work?

[The Thing]

You can do RA only dithering in PHD2. It's on the Global tab in the Brain along with a dither scaling factor option.

[The Thing]

In your humble opinions, is it worth doing a belt mod to my HEQ5? It's already been rebuilt with new SKF bearings everywhere.

[RobertM]

If your guiding is good then is there much point ?  Don't forget that good guiding is also very dependent on the conditions so it may only when there's little turbulence that you might notice.

The new SW mount is belt driven but I don't know how much of a sales ploy that is.

Robert

[MarkS]

In your humble opinions, is it worth doing a belt mod to my HEQ5? It's already been rebuilt with new SKF bearings everywhere.

You are imaging at a long focal length so your guiding needs to be as accurate as possible.  Every little helps.  For me, the belt mod made a significant difference because it removes the transfer gear with its associated periodic error component.  It means I can now usually guide at 0.5arcsec RMS, though it depends on seeing conditions.

It's worth preparing a Periodic Error chart like Ivor has been doing and that will uncover the mechanical source of periodic errors and give some indication of likely improvement.  If the transfer gear is not contributing to the error then there is no point.  On the EQ5 the belt replacment is a really simple job so there's not much to lose.

For my 2nd EQ6 mount I already have on order another belt mod kit with a 12:48 ratio instead of the usual EQ6 12:47.  It's a special order and quite expensive because the 48 tooth pulley must be specially made.  It means I can no longer use the handset (it will be EQMOD only) but it does mean I'll be able to properly use PEC (since all gears will be turning at integer multiples of the worm speed).  I'm expecting to get guiding accuracy well below 0.5 arcsec RMS using PEC which means the seeing will be the main limiting factor for my long focal length imaging.

Mark

[The Thing]

Mark, how have your belt mods worked out? Was the 48 tooth pulley a worthwhile move? I know that my HEQ5 has been transformed by a belt mod.

[RobertM]

Thanks for the reminder, I must do this when I've finished my other projects.

[RobertM]

Just done mine, it sounds like it should have done in the first place now

Proof of the pudding though comes at Kelling.

[MarkS]

Just done mine, it sounds like it should have done in the first place now

Proof of the pudding though comes at Kelling.

A lot quieter and more refined?

[RobertM]

If say so !  Only the steppers are audible now and they are quite quiet.