HEQ5 Tracking fault

[Greenmjg7]

hi all, i'm at my wits end, I have a HEQ5 pro, but have never been happy with it, but didn't know how it should be, it was stiff in both Dec and RA,  but I took it apart and de greased it, then lightly greased and seems better but not purfick !, but i'm looking for help to get it tracking without trails, it was ok last year but now its a nightmare, thank you in advance 

[jambouk]

Can you post a high resolution image with the trails on for us to inspect and give details of the scope, camera, settings you use to image (including sub length) and how you polar align, and if you guide, and whether the trailing occurs in every image in a run or not.

[Greenmjg7]

this was just 86secs, at iso 800, im using a William optics Z73 with a modded Canon T3, polar align using a Pole Master

 

[jambouk]

The training seems uniform over the whole field of view, and uniform in nature and not jumpy which might hint at movement artefact from the camera.

If this is reliably reproduced on all subs, I would make sure the balance is good, and I would do the polar alignment routine with polemaster twice.

I would suggest the issue is a combination of imperfect polar alignment and reaching the limits of the mounts for unguided imaging.

James

[vlaiv]

Polar alignment is source of such errors only if PA error is very large - in normal cases, you should not be able to see that much drift in minute and a half.

What you will see, however is periodic error. Heq5 is known to have as much as 35" peak to peak periodic error. It has worm period of 638s so it can drift as much as 7-8" per minute and sometimes even more if periodic error is not smooth.

Take for example this image:

It is pecprep view of periodic error of EQ6 mount (main period being 479s). Main points to see are stats on left bottom - Peak + is 12.35, Peak - is 13.10 - that gives 25 P2P error. Also important parameter is MaxRate which is 0.32"/px.

If you have such rate of 0.32"/px and you have exposure of 90s - your drift can be up to 0.32 x 90 = 28.8 arc seconds.

You are imaging at 2-3"/px. This means trails for that much drift would be 10-15 pixels long. This is worst case scenario, and in practice drift rate is never at maximum for long (it resembles sine wave) and in your image, trails are about 4-5px long?

What can you do about these trails? Two things:

- periodic error correction

- guiding.

I think that you need computer control of your mount + EQMod for both, but I'm not 100% sure. PEC (periodic error correction) could be available via hand controller as well, but like I said, not sure.

I have Heq5 also and have done both of above and yes it helps. There is one more thing that can help but it won't eliminate problem 100% - doing a belt mod.

Belt mod significantly reduces P2P error and hence max drift rate. There could still be some trailing but if it is small enough - it won't be seen (about a pixel or two at the most).

Hope this helps.

[jambouk]

vlaiv, in the absence of a guiding set up, or the ability to tune the mount or undertake a belt mod, is it worth the original poster doing a 10 minute sub with low ISO and maybe a filter to get a longer run of the trailing to see it in more detail?

Is periodic error correction that useful if you have a non-permanent set up and are having to do a fresh polar alignment each night?

James

 

[vlaiv]

12 minutes ago, jambouk said:

vlaiv, in the absence of a guiding set up, or the ability to tune the mount or undertake a belt mod, is it worth the original poster doing a 10 minute sub with low ISO and maybe a filter to get a longer run of the trailing to see it in more detail?

You don't have to do long exposure to see what is happening, if you already have sequence of short exposures. All it takes is to stack images without alignment. Maximum stacking method is better for reading star trails than average because average "weakens" star signature (averages with background when star moves between subs).

22 minutes ago, jambouk said:

Is periodic error correction that useful if you have a non-permanent set up and are having to do a fresh polar alignment each night?

It can be very useful, but it really depends on what is the main mechanical issue of the mount. Periodic error correction just reduces maximum P2P error and removes frequency components of mount periodic error that are harmonics of worm period. It can't remove other frequency components - like short period error of gear system, or very long period error of RA shaft. Later is not important as it is changing very slowly - period is usually 24h - or whole RA circle, but former can be real problem. It is usually much bigger problem if you guide as it is also harder to guide out fast changing error. Belt mod helps there, as it smooths out reduction between motor and worm.

Only issue with periodic error correction that I've found so far is fact that you must park your mount each time (when using EQMod and its VSPEC - variable speed periodic error correction, not sure if hand controller even has PEC and if it does - how it works and if this applies to it as well) or risk loosing sync between PEC and mount. Heq5 does not have encoders and position is tracked via stepper ticks - each time you power off it just resets counters and if you are not parked to same position you started when you recorded PEC information - sync is lost - you need to record pec again.

I had to do it couple of times so far - once due to power failure - mount stopped and it was not parked. Once because of laptop upgrade (reinstall of drivers and PEC file lost on old laptop).

If done with any care - drift due to polar alignment is very small. For example, using this calculator: http://celestialwonders.com/tools/driftRateCalc.html

You can see that for 6' - or arc minutes of error in polar alignment (that is 1/10th of a degree or about 1mm at arms length - 1/5th of full moon), drift rate is about 1.6"/minute - less than a pixel of drift when imaging at 2.5"/px and using 90s exposure.

But we don't have to assume it is either PA or PE error - we can see from the sub. If trails are in RA direction - it is due to periodic error, if they are in DEC direction - it is polar alignment error.

For the end, I'd like to show you an animated gif that I once made.

Guess which way is RA and which is DEC?

[jambouk]

vlaid - excellent. 

Greenmjg7, are you able to stack say 10 or so consecutive images with the alignment function turned off? If not, if you can upload them to Dropbox or the like, one of us can do this.

James