Centre / align on a star wait two hours then goto same star...

[g__day]

This is something strange I experienced. I am trying to get my polar alignment to better than an arc minute or two of the SCP (and I'd say its very close).

So I power on an do a 1 star align on Achernar - its dead centre on the Canon Camera's central finder dot. I move off the star a degree and issue a goto Achernar - and it lands dead centre on the star again.

After that I check the hand controller time - its right to the second - so I don't touch the hand controller for the next two hours...

Now I remotely take 120 shots of 3 second duration - one minute apart and look at the drift. There is no DEC drift but the mount (or SCT's mirror) make it appear that things are running slightly fast in RA. The drift on my images from beginning to the end shot over two hours is 9mm on my images (whereas the diameter of the full Moon is 186mm) - So that tells me the mount is running 1.5 arc minutes fast every two hours - around 45 arc seconds and hour say.

But out of curiosity I look into the Camera and issue another Goto Achernar - and expect nothing to happen - but no - the gears engage and the star moves back half the distances say 45 arc seconds - so its now only 45 ac seconds of the Camera's central finder spot!

How can that happen? The SkySensor2000-PC says its tracking Achernar - no adjustments are made - then two hours later (its risen from say 40 degrees elevation to 70 - so no Meridian flip) and when I re-issue the goto it moves back!

Love to know what I am experience there!

[RB]

LOL Matthew, you crack me up !
I don't have a clue mate, but good luck with it, hope you resolve whatever it is you're resolving.

:thumbsup:

[bojan]

Now, that looks like your RA stepper motor is skipping a step or two now and then...
Perhaps you should have a look at gear mesh - maybe you have too much friction somewhere? Or bad balance?

[g__day]

Goodness you have to be tenacious in this field. I'm on a crusade to get my gear to its peak performance and iron out all wrinkles. Lack of documentation and in-experience are my two biggest challenges. Maybe I expect too much - but consistency is really high on my wish list - and until I exhaust all possibilities I will keep experimenting and throw ideas out there.

Perseverance!

[bojan]

Here you can find info on how to dismantle your mount.
http://www.beevo.com/

[bojan]

More closely, the info is around here:
http://www.beevo.com/dec_disassembly.htm

[g__day]

I dismantled the DEC once to tighten the slop - tricky but very doable - the motors and gears are hard to remove to get to the screws.

There is significant difference between an Orion Atlas (eq-6 variant) and the much larger Vixen Atlux!

Right now I am trying a 3 star align (which even in polar aligned mode) should run the polar mis-alignment routine in both motors to correct for pointing error - will know in 100 minutes if its helped tracking rate at all!

* * *

Well no - not with those alignment parameters - dec drift now - arg.

Iteration 2 manually set the 3rd alignment compensation drift parameter from 000 to -005 - halved the drift! Onto something here - set it to -010 next - bit less drift again but this time in opposite direct so tomorrow I'll try -008, its about 3 star diameters an hour drift at present - if I could get it to 1 or less I'd be ecstatic!

:)

[bojan]

For some reason I thought you have EQ6.. sorry :shrug:

However, if mount does not come back to the same position after slewing to and fro... and if it does not have encoders, then steppers are skipping when slewing.
All steppers have reduced torque at higher speeds, and if this is combined with load (friction or imbalance), they will skip, or even stall in worst case.
If possible, try to reduce the slew speed. If the problem disappears, then you will know what the problem was.

In some cases, it may even be a software problem.... but most likely not here, it would have been a common knowledge by now, I am guessing...

[g__day]

Not sure how to check the motors being steppers or encoders - there is a wheel on the motor with stripes and an optical reader - if that helps...

Big success - set the third tracking adjustment parameter to -007 - now tracking seems perfect - slight DEC drift (maybe one star diameter) after 180 minutes - no RA drift whatsoever!

I knew this mount could do it - teasing this level of performance out of it was a total marathon! Also on power on my first goto from last recalled position was maybe 15 arc seconds off - now that's performance!

A very happy,

Matthew

[bojan]

OK..That means you have servo system.
So my comments on skipping are not valid here.

I am glad you sort it out :-)

[Wavytone]

At the levels of drift you are talking about, you will start to notice the effects of atmospheric refraction, which mostly amounts to an error in RA rate for objects east/west, and produces an offset in declination for objects north/south. It can amount to 15 minutes of arc low down. You will also run into errors arising from the angles between the RA, dec, and optical axes of the scope not being perfect right-angles, and there will also be errors due to the varying flexure of your mount and tube assembly as it moves across the sky, which may easily amount to much more than 15' and having a scope which you assemble/disassemble each night means any calibration you make must be repeated each night, if you are to have any chance of correcting these.

Then there are the errors in your gear train, for which periodic gear correction is OK short term (minutes) but not longer term if your mount has only a small worm wheel with relatively few teeth

Correcting for all of the above to achieve accurate blind pointing better than 15' is well beyond the capabilities of most small scopes - Meade's LX200 achieves 6'.

[g__day]

Refraction is accounted for by the hand controlller using the King rates I understand - and its consistent repeatable error - now very small - so it may still be adjustable out!