AZ-EQ6 GT Significant Periodoc Error

[pstas]

Hi all first post here!

I've been reading your forum's post for a long time and I have found really instructive and educational information.

Last year, I bought my first telescope to fulfil my dream! A combination of a Skywatcher Black Diamond 120ED and an AQ-EZ6 GT mount. Last year I tried the visual part, to learn the basics and take some photographs (with mediocre results). This year I tried to improve my photographic skills.

The first thing I noticed was that I could not take a photo without extreme star trailing. As I've read, a bad polar alignment would be the cause.

So I tried carefully to improve it through the PA scope of the mount, or with the 3-star alignment of the mount with no good results. The strange thing I noticed looking through the eyepiece, was that sometimes the stars were drifting, after I had released the keypad button during the alignment.

Last month I bought a QHY5L-II camera to use it as a guiding camera. I connected the mount to my laptop using a custom made adapter (cost me 14€) and with the very good EQMOD drivers, I start using software solutions for Polar Alignment, including PHD2 and EQAlign (more accurate and friendly but a little bit buggy). For guiding I tried also the MetaGuide (the only one that allowed me to take a photo of 30 secs!! without trailing).

 

I soon realized that the mount should have significant Periodic Error on the RA axis.

Using PHD2 and these instructions http://eq-mod.sourceforge.net/docs/eqmod_vs-pec.pdf (appendix A), I measured the PE curve of the mount and imported the measurements into the PECPrep for evaluation.

The peak to peak value is extreme: 130arcsecs

Strange thing is that the period of the PE is not the period of the worm but a quarter of it: It is worm's fourth harmonic or Stepper motor's period (119.67 secs). A harmonic cannot have larger amplitude than the base frequency, so I guess something is wrong with the stepper motor or the motor pulley or the motor driver circuitry or maybe the belt. I changed the belt with a new one (a Contitech Syncroflex) and repeated the measurement but the results where similar. The contribution of the 116.67secs error is 3 to 4 times more than the error from the worm.

 

The resulting PE is really difficult to be handled by the PHD which after 5 to 10 secs fails to follow the movement. Even at the calibration phase it prompt errors regarding the orthogonality error.
 Drift align is almost impossible with PHD (target circle never gets steady but keeps growing and then shrinking and back again).

 

Any suggestions?

Thanks in advance,

Panayiotis

[Dave Lloyd]

Hi and welcome, loads of expert advice on here (not me?)

[Jonk]

My first response (I have the same mount) is to drift align using the DARV method (plenty online about it).

This should put you in the area of many minutes unguided when done correctly. I managed about 8 minutes last year without guiding, with minimal trailing, during testing.

Have you also opened the mount to see if everything's seated correctly, worm mating properly, bearings ok etc.?

[Ouroboros]

I think you've got a mount problem.  I have an AZ-EQ6 and it performs much better than yours even without going to great lengths to polar align using drift alignment or whatever. I use a 1000mm focal length telescope. Yours is 900mm I think. I've never bothered doing periodic error analysis because it never seemed necessary. Short exposures <30-60 seconds are usually trail free. Good polar alignment and guiding usually makes trailing acceptable for 5 minutes or longer.  OK I'm not as fussy as some with their images. I'm not trying to compete with the best. 

So what to do?  I guess the mount is beyond the 12 month warranty. Even so I think you might have a case to go back to your supplier.  If they're a good supplier they might be willing to help you find a solution. 

There are people in the UK with the skills to service mounts like ours. I don't know who they are, but someone on this forum will have contact details. Or maybe there are people like that in Greece.  If you're mechanically competent (I'm not particularly) there are some instructions for disassembling and maintaining these mounts on the web. 

I hope you solve the problem. 

[Dr_Ju_ju]

As other have said, I'm surprised your mount is behaving so badly, as I can easily get 180 sec subs unguided, even if I just polar align via polar scope... so I suspect there's something wrong with your mount alignment\balance.

If you feel up to it, you can follow Scott Cunnington's guide (attached) to strip down your mount & re-grease etc...  I've recently done mine & it makes a big difference to 'smootheness' of the mount

 

AZEQ6 Stripdown guide final.pdf

[Ouroboros]

DR Ju Ju Thanks for posting the link to Scott Cunnington's informative and interesting guide. Successfully frightened me off doing it I must say. But incredibly useful to the intrepid engineers amongst us I imagine. 

[kendg]

Thanks for the link to the strip down guide. I must have missed it.

[jambouk]

Are you sure it is being well polar aligned first? You mention the stars drifting out of the field of view with an eyepiece, that to me suggests VERY poor polar alignment. I use a focal length of over 7000cm and things take a long time to drft out of the FoV and I don't guide my AZEQ6.

Good luck getting it sorted.

James

[pstas]

Thank you all for your invaluable messages.

On 29/7/2016 at 14:28, Jonk said:

My first response (I have the same mount) is to drift align using the DARV method (plenty online about it).

This should put you in the area of many minutes unguided when done correctly. I managed about 8 minutes last year without guiding, with minimal trailing, during testing.

Thank you for the amazing hint. It is a very quick and precise method to detect any PA misalignment. It is stable and the results are repeatable. The only drawback I've found is that it does not provide an accurate correction method but rather an approximate one and then a new measurement is required to check for the results. It took me almost 2 hours to do two series of alignment, first with 2'05" exposures and then with 4'05" exposures. The results, although far from perfect, are shown with a star exposure of 721secs (12mins). It is clear the problem with the oscillation on the RA axis.

 

On 29/7/2016 at 15:23, Ouroboros said:

I think you've got a mount problem.  I have an AZ-EQ6 and it performs much better than yours even without going to great lengths to polar align using drift alignment or whatever. I use a 1000mm focal length telescope. Yours is 900mm I think. I've never bothered doing periodic error analysis because it never seemed necessary. Short exposures <30-60 seconds are usually trail free. Good polar alignment and guiding usually makes trailing acceptable for 5 minutes or longer.  OK I'm not as fussy as some with their images. I'm not trying to compete with the best. 

So what to do?  I guess the mount is beyond the 12 month warranty. Even so I think you might have a case to go back to your supplier.  If they're a good supplier they might be willing to help you find a solution. 

There are people in the UK with the skills to service mounts like ours. I don't know who they are, but someone on this forum will have contact details. Or maybe there are people like that in Greece.  If you're mechanically competent (I'm not particularly) there are some instructions for disassembling and maintaining these mounts on the web. 

I hope you solve the problem. 

Ouroboros, thanks for the suggestions. I have 2 weeks left, till the warranty expiration. I've already contacted the local supplier and he asked me to do a couple of tests, but with no good. I'm sure there is some sort of problem with the mount, so I'm trying to describe the problem the best I can, to help solve the problem with the manufacturer.

 

On 29/7/2016 at 16:57, Dr_Ju_ju said:

If you feel up to it, you can follow Scott Cunnington's guide (attached) to strip down your mount & re-grease etc...  I've recently done mine & it makes a big difference to 'smootheness' of the mount

AZEQ6 Stripdown guide final.pdf

Very nice guide. I have found another one (not so scary) photographic guide at another forum:

http://www.cloudynights.com/topic/534504-az-eq6-lower-dec-bearing/

Also, since I've used it many times I would not suggest the plain lithium grease, since it will require to dismantle and re-grease after a couple of years.

 

On 30/7/2016 at 07:47, jambouk said:

Are you sure it is being well polar aligned first? You mention the stars drifting out of the field of view with an eyepiece, that to me suggests VERY poor polar alignment. I use a focal length of over 7000cm and things take a long time to drft out of the FoV and I don't guide my AZEQ6.

Good luck getting it sorted.

James

James, I would exchange my mount with yours any time. I will include my scope and any equipment I already have!!!

[Owmuchonomy]

I have the same mount but I must say it is the second one. The first had a very obvious periodic error which Synta agreed with and the retailer exchanged it under warranty.  Synta provided a test protocol which I followed and supplied the results to them to prove the issue. I suggest you challenge your retailer if all else fails and before you break open the mount.

[Ken82]

This is normal for these types of mounts and can be reasonably corrected with pecprep and guiding . 

http://demeautis.christophe.free.fr/ep/eq6.htm

[Stub Mandrel]

You may have excessive backlash (one symptom is things drifting after an align), or you may have things TOO tight causing binding.

[pstas]

This weekend I spend almost both nights on trying to improve PA and measure the PE to see if it was decreased.

After spending almost 2 hours of DARV method polar alignment and the results shown in my previous post, the guiding performance of the PHD still not good. As shown in the first image, the peaks exceed 12 arcsecs at the graph and the PHD is struggling to correct them.

 

I run again the PE recording process and the results although better than previously are still not good. PE peak-peak = 95 arcsecs. The 119.6secs period plays a significant part in the total PE.

[Ouroboros]

You've no doubt checked and rechecked this .... but just to certain is the balance in RA right?  I find I sometimes get a lot of star trailing and, assuming I've polar aligned OK, I can only put it down to poor balance. I also find the AZ-EQ6 quite difficult to balance because the mount is so stiff even with the clutch well and truly off.  

[pstas]

Well, with this mount the DEC axis balancing is easy. For the RA axis, the clutch mechanism prevents it to rotate freely and there is no way to improve it.

I do a small trick to balance precisely the RA axis. I release the RA clutch and then rise and hold between my legs the tripod leg that points to the South. This way the RA axis becomes a little more horizontal. With the free hands I press the mount head away from the equatorial wedge to minimize the friction between the head and the clutch mechanism. For a second the mount rotates freely so I can balance it accordingly.

This works well with not very heave equipment and at lower altitudes.

[Ouroboros]

Pstas - sounds like you've got balance sorted. I do something similar by adjusting  the elevation of the RA axis on the mount to near 100°.  The mount swings freely in RA when I do that. 

[Owmuchonomy]

1 hour ago, pstas said:

Well, with this mount the DEC axis balancing is easy. For the RA axis, the clutch mechanism prevents it to rotate freely and there is no way to improve it.

I do a small trick to balance precisely the RA axis. I release the RA clutch and then rise and hold between my legs the tripod leg that points to the South. This way the RA axis becomes a little more horizontal. With the free hands I press the mount head away from the equatorial wedge to minimize the friction between the head and the clutch mechanism. For a second the mount rotates freely so I can balance it accordingly.

This works well with not very heave equipment and at lower altitudes.

Sounds to me like you have the RA adjustment much too tight?

[pstas]

Well, if I put the RA clutch handle to the previous hole, then the clutch cannot be tightened. When I release the clutch, the mount can move inside the RA axis.

[Jonk]

20 minutes ago, pstas said:

Well, if I put the RA clutch handle to the previous hole, then the clutch cannot be tightened. When I release the clutch, the mount can move inside the RA axis.

I think he means you have the worm too tight?

[pstas]

The mount will be returned to the South Europe Skywatcher's dealer in Italy for inspection and repair/replacement.

[Ouroboros]

2 hours ago, pstas said:

The mount will be returned to the South Europe Skywatcher's dealer in Italy for inspection and repair/replacement.

Good. I hope you get a satisfactory outcome. Let us know what happens. 

[pstas]

A note: The warranty for the mount in Europe is 2years.