Stars all over the shop - please advise before I bin the lot.

[Spacehead]

Hi All - I track with an eq5, RA motor I added, SW 200p, PA = perfect, balance perfect and tried different things.

I get stars deviating over time and then re-locating themselves in the correct position.

So lets say I do a 1 min sub - I will get this when tracking (see small pic with stretchy stars zoomed in).

If I point the north leg of the mount to east, then switch on the RA - I am tracking offset to the actual motion of the planet.  Thus, I can now see the deviation of the stars on the mirror over time.

But not all the stars show the same deviation / bumps!!!!! 

It is important to know that this is NOT drift.  My alignment is perfect and the end of a 1 hour shoot results in only 6 pixels of drift.  Whereas each shot can be 20 pixels out - but over time the stars come back into position - within a minute normally.

I've got two examples below with star trails and ive coloured in a straight line to show how the stars deviate to give the stretched / jumpy appearance in the actual tracked properly image.

I have stripped it (the mount) down - re-greased the worm gear and main ring, replaced the RA motor completely - but to no avail.



Totally at a loss.


 

[JamesF]

My initial guess would be that this is just down to the accuracy of the gearing and the amount of backlash in the gear chain.  The gearbox on the motor contains quite a few gears and there can be a fair bit of movement between them all.

If there's any breeze at all the 200P can be a bit of a sail, too.

What you could try is making the east side of the mount slightly heavier than the west, so it's slightly out of balance and the gears are always fully engaged.  Whether that's the counterweights that need to be heavier or the OTA depends on which side of the meridian they're on.

James

[Spacehead]

8 hours ago, JamesF said:

My initial guess would be that this is just down to the accuracy of the gearing and the amount of backlash in the gear chain.  The gearbox on the motor contains quite a few gears and there can be a fair bit of movement between them all.

If there's any breeze at all the 200P can be a bit of a sail, too.

What you could try is making the east side of the mount slightly heavier than the west, so it's slightly out of balance and the gears are always fully engaged.  Whether that's the counterweights that need to be heavier or the OTA depends on which side of the meridian they're on.

James

Hi James - I have replaced the motor and its gearbox with new, there is no backlash on the worm at all now - during experiments, I have - if anything - very slightly over tightened everything just to eliminate.  To no avail.
No - there is no breeze - during my testing I ensure dead calm only - if there is even the slightest breeze I do no testing.  Breezes tend to give a more random wobble too in my experience - but this error is consistent over many nights - always the same.
I have overloading the east side to varying amounts over various nights - no effect - the error still occurs.

I am thinking its the large main bearing in the RA head now - that is the only thing I have not managed to gain access to yet.

[Spacehead]

(I have even tested the ground stability!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!)

 

[pete_l]

Quote

I get stars deviating over time and then re-locating themselves in the correct position.

The repetition rate of deviations often provides insight into their origin. Do you have the option of producing longer exposures - say 20 minutes? But not with a 90° offset from the pole, but just a few degrees. That way the position of a star won't move it out of frame during the exposure.
The goal would be to see whether these deviations occur at regular times (and therefore are associated with the drive) or are random and therefore have an external source.

[Spacehead]

3 minutes ago, pete_l said:

The repetition rate of deviations often provides insight into their origin. Do you have the option of producing longer exposures - say 20 minutes? But not with a 90° offset from the pole, but just a few degrees. That way the position of a star won't move it out of frame during the exposure.
The goal would be to see whether these deviations occur at regular times (and therefore are associated with the drive) or are random and therefore have an external source.

Yes I had thought about doing EXACTLY that tonight!!! Because I thought "if only the star would stay in view longer I could get a repeat pattern!!" - I tried joining multiple exposures to achieve the same thing but it didnt work out very well.

The "error" direction (the angle of the star blips within a frame) - when polar aligned - is equal to the direction of the tracking - so I have to find a nice angle for the North leg which allows trails in long exposures, but also shows the error clearly, as it will - if the angle is too small, overwrite itself if you see what I mean!!
 

[almcl]

It does look a bit like Periodic Error in the RA worm. 

Hard to say without a guiding trace (and IIUC you are just tracking, not guiding?) but when I imaged using my EQ5 and self added motors, this was a bit of an issue. I switched to guiding and then to stepper motors and that did reduce the error, enough that I still use the EQ5 when imaging with a 200mm lens, where its errors are small enough not to show. 

[Spacehead]

I've stripped it all down now today and am changing all the bearings - will have to wait on delivery.  PE on the RA worms are intermingled with this error on the images - I have seen the extreme errors occasionally and am fairly happy with that.
Yes I am only tracking because I have no money.
This problem has been progressively getting worse (I was browsing my old images).
There is about 1mm or even less play on the main bearing (shown hanging on the shaft) - so I am replacing both bearings - see what happens.  Took me two damn hours to get that sealed ring off the shaft - what a bind!!!

[michael8554]

Why do people guide? 

Because there's no such thing as perfect PA, PEC, or perfect mounts, even with new bearngs. 

Particularly an EQ5. 

By all means fettle the mount to get the best from it, but be realistic about unguided performance........ 

Michael 

[Stub Mandrel]

Hi @Spacehead,

I'm afraid it's really unlikely the bearings are at fault - the play should be removed by adjusting the preload on the bearings carefully.

The errors are inevitable, most come from periodic error on the worm, but every gear in the drive will introduce some error. Even belt drive has errors due to imperfections in the pulleys.

They will get worse over time, not necessarily wear but  gears and bearings will gradually shift under the influence of cycles of heat and cold, slowly increasing backlash or causing binding.

That's why the very best mounts use plain spindles bearing on smooth discs and optical encoders (which can be made far more accurate than gears as they bear no loads).

When you consider that tracking with typical accuracy of 1 arc minute is the equivalent of firing bullets through the earth at Australia and getting every one of them to hit a target the size of an olympic swimming pool.

When I do my planetary imaging, I see a shift of several pixels just from me shifting my weight from one foot to the other near a tripod leg. Just picking my camera  timer up can cause a brief execursion of several arc-minutes when imaging.

The mass of a telescope is enough to cause variations over time.

So, the sort of variations you are seeing are inevitable in any mechanical mount affordable to mortals, yet it can be effectively eliminated by good guiding which uses feedback to keep the errors to about a scale of 1 pixel.

I used to get reliable 30-second subs, I could even keep most of them with up to 2-minute exposures.

Then I started guiding.

Then I discovered I had to put some more effort into removing backlash etc.

Now I can guide to pixel-accuracy for 5-10 minutes for every sub (if the sky stays clear!) But if I lose my guide star, most of my subs will have eggy or double stars.

 

[Spacehead]

The thing is that I write websites for a living - and nobody pays me - the most i get is £150 for two weeks solid work.
In other words im skint as - so guiding is out of the question.
 

[StarryEyed]

These mounts with all their imperfections can cause unpredictable behaviour when two or more of these errors decided to coincide. There's a point when your gains become so small that futher effort to reduce them seems pointless. You can get the mount to work well if your willing to care for it and work on it. Mine even with a polar alignment that would give you two minutes unguided in the DEC still jumped about in the DEC. The thing was infuriating. I had it in bits a number of times. In the end I accepted the mount for what it was. I have been happy with it ever since.

[Stub Mandrel]

5 hours ago, Spacehead said:

The thing is that I write websites for a living - and nobody pays me - the most i get is £150 for two weeks solid work.
In other words im skint as - so guiding is out of the question.
 

Guiding doesn't demand quality optics or super expensive cameras.

I assume you have a laptop then.

Get a cheapest astro cam off the web, cheapest you can find that will do 2 or 3 second exposures - do avoid the 'planetary only' ones. Or look for a used one on the classifieds here.

Combine this with a 0.965" adaptor and the cheapest toy telescope off e bay and you are good to go.

[david_taurus83]

You need to guide. Your tracking with a cheap mount (please dont take offence, I've had one myself) and imaging at a demanding pixel scale. 1 metre focal length will even require precise guiding.

[dph1nm]

I think the EQ5 has a 10min worm period, so it should be easy to tell if this is your issue as the stars will move back and forth in RA on this timescale. You will have periodic error, it is inevitable. It can be pretty large - e.g. several tens of arcseconds.

NIgelM