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Spacehead

One last try - some more data on mount error.

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Hi All,

I have an EQ5 motorized on RA with a 200p.

For 2 years I did fine with tracking, gradually it got so the stars started trailing a bit so I decided to look into it.

I'm fine with the PA and focus.

I have replaced the motor, the thrust bearing on the main shaft, the main bearing on the main shaft, swapped the RA worm with the hardly ever used DEC worm, re-greased, I can see no wear at all even on close inspection.
I have experimented with different tensions on the Worm drive to main ring and the motor cog to the worm cog.  Nothing makes a blind bit of difference to the outcome no matter what I try.

The following first two pics show my mount tracking with the mount rotated off PA by about 20 degrees clockwise - but in essence - I am deliberately trailing the stars whilst off-tracking so I can see if there is a pattern.
So the first pic is an 11 min sub - a green line through a star trail shows the straight path the star SHOULD take.  The second image is a zoomed in image of an approx 1 minute section of that long trail.  The third image shows what the stars look like when tracked using this mount with strong PA, basically egg shaped (kind of) in the direction of the "bumps" on the trail - as we would expect.

There is no "drift" over time - (well - about 7 pixels per hour) - what is happening here is the telescope is falling behind so the star moves "forward", then the scope catches up again and the star moves back into position creating a wavy line (mainly) but you'll see on the long line image that even this is not consistent.

I am close to trying guiding now - but having replaced pretty much everything on the mount which moves - I am really confused as to what caused this - and more importantly, will this mount even work with guiding.

Any advice/thoughts appreciated.

Cheers


 

t1a.jpg

t1b.jpg

t1c.jpg

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I think that should guide out reasonably well.

BTW you have a PM.

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Periodic Error ?

Redo your PA and then run PHD2 Guide Assistant for 10 minutes, that will demonstrate your unguided RA performance.

Michael

 

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Although it is not stated here, the OP isn't guiding, just tracking, so yes, it does look like PE, but PHD2 isn't on the agenda just yet. 

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8 hours ago, almcl said:

the OP isn't guiding

In that case Spacehead has come up with the next-best method of displaying the sinusoidal shape of PE. 

Sorry I can't think of a way to improve his mount. 

Michael 

Edited by michael8554

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2 hours ago, michael8554 said:

In that case Spacehead has come up with the next-best method of displaying the sinusoidal shape of PE. 

Sorry I can't think of a way to I prove his mount. 

Michael 

It isnt a PE error - that occurs periodically and measurably - this is all over the shop and pretty much constant no matter the position of the gears.

 

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4 hours ago, Spacehead said:

It isnt a PE error - that occurs periodically and measurably - this is all over the shop and pretty much constant no matter the position of the gears.

While that's quite true, it may still be possible that the ripples on your star trails are in part caused by periodic error.  There is almost always more than one component to PE and the periods are often not harmonic with each other.  Here's the unguided appearance of my EQ5 (stepper motors rather than SW motors):

 Uncorrected.jpg.7b3b771bbf0345de3b22c2604054f30a.jpg

It's not that regular, but it is largely down to 5 or 6 quite large but independent frequencies superimposing on each other, the largest being the worm period at 300 seconds:

frequencies.jpg.36b153cc30aeb569dba85f282e0a7473.jpg

The good news is that with guiding, most of this can be smoothed out (it wasn't a night particularly good seeing so some of the short term random movements are still there):

guided.jpg.25e9a96321a6bd89bb5bfb2c6174bde1.jpg 

 

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Looks to be a likely periodic error, which you’d expect.

The thing which I’d worry about is balance throughout the whole imaging session. A 200mm reflector is a hefty and long scope and the standard EQ5 will struggle to offer a solid and reliable base for it. But crazier things have been done.

Good luck.

 

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