Finder guiding issues with a Lodestar

[MarkS]

Any thoughts about the error 4?

I've got a note of it at home, from looking at the source code.

Mark

[MarkS]

Here we are - this snippet defines the error codes:

enum FindResult
    {
        STAR_OK=0,
        STAR_SATURATED,
        STAR_LOWSNR,
        STAR_LOWMASS,
        STAR_TOO_NEAR_EDGE,
        STAR_MASSCHANGE,
        STAR_ERROR,
    };

So 4 equates to Star Too Near Edge

Mark

[Ivor]

Thanks surprising not well documented I would have thought the error code would have included which edge of the box it left to help with adjustments.

Tonight looks hopeful so I'll try and produce the PEC file, mind you it's my 18th wedding anniversary today and I don't think I can get away with the line "have a romantic night looking at the stars" as my wife is wise to this now

[Ivor]

Why or why are the only clear nights in a row at the full moon so unfair!!!!

I produced the PHD file for PEC analysis last night it can be accessed from the below link
https://drive.google.com/file/d/0B4aWTDNICIFvaE1UZjFoWDZJWjQ/edit?usp=sharing

I will have to watch the PEC tutorials again so I can understand what's going on and maybe a little help from my friends

Once this as done I went back to M13 to take a few more subs and test out the guiding adjusting the max pulse up to 2000ms as it max'ed out last time. This time the guiding only max'ed out 3 times so I will increase it to 3000 for the next run. No guiding errors this time so I'm the last time was just clouds. The overall stats came out better than before which is pleasing, I feel I'm getting there.

Peak2Peak   13.56
RMS   0.66
arcsec/pixel   2.42
Average guiding (RMS*arcsec/pixel)   1.59 * I'm guessing this is the right calculation
Max swing   6.95


However the stars are still slightly elongated, whilst pondering this on the way into work I wondering whether this could be blooming, there is a setting AA5 for it but I don't it enabled. I shall try the next batch of shots with it set to medium.

[MarkS]

Check the subs for M13 - I think you'll find most have that elongation - it's guiding not blooming.

Here's your PE graph from the unguided run:


This is slightly better than before but it's a terrible PE chart for an EQ6 - it's difficult to see the usual periods in that chart.

This is confirmed by the PECPrep Fourier analysis:


The peaks on that chart correspond to amplitude of the various periods it finds in the data.
The usual culprits for an EQ6 are visible:
478.7sec Worm Drive
366.6sec Transfer Gear
182.2sec Transfer Gear 2nd harmonic
239.4 Worm 2nd harmonic

However the usual culprits are swamped by some other very high amplitude periods:
The biggest has a period of around 132sec
Another peak at 66sec
Another at 33sec
Notice the peiod halving - 2nd and 4th harmonics of 132sec

These periods do not correspond to anything I know of in an EQ6 but they are giving your PE chart its extraordinary profile which is more or less impossible to successfully guide away.  They weren't present in your previous PECPrep but your previous one was complete garbage anyway - very difficult to get anything out of it.

Just possibly this is some random effect but if you perform another unguided run and those frequencies are still present then it is definitely telling us something.

Out of interest, the replacement bearings you put in - did they have the same number of balls as the original EQ6 bearings?  I wonder if that is a possible explanation.

Mark

[Ivor]

Thanks for this Mark, I went through the PECPrep video tutorials last night and I was perplexed by the results as well so I'm glad I'm not going mad but concerned by the confusing results.

I used the SKF 6008 Series 2RS bearings as they were sealed I didn't check the number of bearings however these were recommended to me by Graham at AstroTEC so you'd think they would be right.

In an effort to isolate the cause, below is the list of issues/concerns I feel are outstanding with the mount.

• I've not replaced the worm bearings and at least one is a bit ropey, I was previously advised this wouldn't be a problem as it was away under load.
  With the clutches off the mount doesn't move as freely as before, with a little bit of force I can move the telescope but if misbalanced it wouldn't move on it's own.

I think I'll order the worm bearings today it's not going to do any harm replacing them and it will rule out of of the possibilities.

[MarkS]

If we can find what mechanical component gives the 132second period then the problem will be diagnosed.

Nothing in the gear train corresponds to this; the main bearings turn too slowly; so that leaves the worm bearings.  But 132sec does not correspond to the usual worm bearing ball period.  Is it possible that your worm bearing is not standard and has smaller balls?  Who knows?

If you look at the PE graph knowing to expect a 132sec period then it is actually very obvious in the graph.

Mark

[RobertM]

I think I'll order the worm bearings today it's not going to do any harm replacing them and it will rule out of of the possibilities.

Definitely do the worm bearings, in my opinion they make a massive difference.

I also can't understand why the axes appear so tight - are you sure something isn't rubbing and that there is a tiny gap between the two faces that separate the axis with body (if that makes sense).

Robert

[Ivor]

The bearing are ordered and shipped so if I'm lucky they'll arrive tomorrow so I can fit and test the same day.

I also can't understand why the axes appear so tight - are you sure something isn't rubbing and that there is a tiny gap between the two faces that separate the axis with body (if that makes sense).

If you mean between the RA Scale ring and the main body then yes. It feels like the worm gear isn't slipping against the outer casing, neither gear has any grease on it as I thought they weren't meant to is this a correct understanding?

[The Thing]

I used the SKF 6008 Series 2RS bearings as they were sealed I didn't check the number of bearings however these were recommended to me by Graham at AstroTEC so you'd think they would be right.

I did my worms first ('you can get a pill for that' said the missus) and it made a big difference, convinced me to do the rest. Did you change the thrust bearings as well? In my HEQ5 they were the worst bearings and were bumpy and sticky which caused the guiding to jump every so often.

And I put lithium grease on all the contacting surfaces and squeezed it into the thrust bearings.

[MarkS]

I'm pretty sure the 132sec period and it's higher harmonics are caused by the balls in the worm bearing.

This thread (http://stargazerslounge.com/topic/100401-eq6-belt-mod/page-4) reckons the ball period for the 608-2RSH (worm) bearing to be 139sec and NOT the 174sec period indicated by PECPrep.  I used the bearing frequency calculator on SKF's website to confirm the 139sec figure given a period of 478.8sec for the worm itself.  I'm not sure why yours differs slightly (132sec vs 139sec) but it is close enough to be significant and all the higher harmonics will be due to the fact there are multiple balls.

I've also decided to replace the worm bearings on one of my mounts.

Mark

[Ivor]

Thanks for investigating further, it makes me feel happier knowing there is now a quantifiable reason for my problems. I've only replaced the RA worming bearings at the moment so I can isolate the results; Thursday looks promising so hopefully I can put it through its paces then.
I have to admit I'm struggling visualise how the harmonics work on the worm bearings as it has multiple parts playing roles. Each ball bearing within the worm bearing has a rotation which combines to give the rotation of the inner ring of the worm bearing itself, how does this relate to 139secs?
I really need to find the time to read up on harmonics theory it's too many years since I did it at Uni.

[MarkS]

Think of it like this:  there is an inner and outer part to the bearing with balls filling the space between them.  It could be designed to have tiny balls or big balls.  For a given rotation speed of the bearing as a whole, tiny balls would have to rotate very fast but large balls would rotate a lot slower.  Any irregularity in the balls or debris inside would lead to irregularities in the rotation of the bearing as a whole.  The reason for the harmonics is a lot more difficult to explain.

[MarkS]

I've just had a look at the log of my belt modded EQ6 using PECPrep and I can see distinct periods of 133sec and 68sec which I reckon match the periods you are seeing (given the accuracy of the frequency analysis).  But mine have a fairly low amplitude - it is the worm and its harmonics that dominate that dominates.  However, since they are definitely visible I think it may well be worth me replacing those worm bearings.

[Ivor]

Patch cloud here so still undecided about whether to do a test run.

I got my worm bearings from here http://www.qualitybearingsonline.com/608-2rs-skf-8-x-22-x-7mm/