More eratic tracking - any ideas

[coenie777]

After polar aligning tonight to a point where Astrotortilla told me my errors were 15.60" too high and 5.90" too west (not sure if these are good numbers??), I proceeded to take some two minute exposures.

I took 40 of M20 of which I could use 16. Small tracking errors on every second exposure about.

I then tried three minutes and found this very weird patern. Can anyone perhaps tell me from this picture what it is that I am doing wrong or is wrong with the mount?

The three minute resulted in small crecent moon shaped images.

I moved to another part of the sky and took a 2 min. Then I got what looked like slipping in the tracking with the exposure starting then streaking a bit only to then track it there again.

I am really stumped by what is going on here and would appreciate some views. I took these with my 80/400 SW achromat so excuse the optics. Just want to try and solve the tracking with the small scope before I lump the 8" newt on it again.

Here are the crecent shaped image: (crop from the middle part)

And here the slipping one: (crop from around the middle, different part of the sky)

[pyrasanth]

These images show field rotation. It basically means you are not aligned with the celestial pole. If you can imagine the rotation of celestial objects around a central point, the pole, you are not central with your alignment hence the star trails in an arc which is similar to the photographs you see in magazines where a fixed camera catches the arcs of star trails. The cure is good polar alignment which takes time & effort. You could use the drift method of alignment. The center crop which you show is indicative of this issue. The double stars being shown is a result of this rotation as more light falls on part of the arc so you get the effect of 2 arced blobs.

[Thalestris24]

Hi

What mount? It looks like you may have balance and unwanted movement (bounce?) problems. Also out of focus. I take it you are unguided? 2 and 3 min subs unguided at 400mm are a bit optimistic unless you have a high quality mount. Also, is your mount on firm ground? If you are on decking, for example, you have to keep well away from the mount when imaging.

Louise

[coenie777]

Hi Louise, I need to update my auto signature  I am using an NEQ6 mount.

It is on firm ground and I tried to balance the lot before the session. I also sat next to the mount when these were taken and can confirm that nothing bumper it and there were no wind.

It is unguided though and 2 min may be pushing it a bit. On 90 sec I got 16 usable images out of a total of 40 that I took. Small trails on all the dumped ones.

The focus I still need to work on. I need to make a bahtinov mask for this scope still. Tried APT's focusing assistant but it is obviously not spot on yet.

Pyrasanth, if those arc are field rotation, is it the altitude or azimuth that is not yet spot on? Or both? As mentioned, AT showed me that I was off by 15 and 6" only which were the best I mustered so far. If it was due to field rotation and under the exact same conditions I would have auto guided, would that have resulted in better images?

Lastly, the mount has some "stickiness" in both its axis. RA is smooth but you need to push it a bit to move it. DEC is easier to move but on one point through a 360 degree movement there is a little "bump" that it feels like it need to ride over before it go smooth again. Could any of these contribute to the images shown?

Oh and while I am at it, what is the norm on an NEQ6 mount for 40 shots, should all of them be on target it everything is set up correctly or will you have the odd one that did not track too well?

[wxsatuser]

Lastly, the mount has some "stickiness" in both its axis. RA is smooth but you need to push it a bit to move it. DEC is easier to move but on one point through a 360 degree movement there is a little "bump" that it feels like it need to ride over before it go smooth again. Could any of these contribute to the images shown?

It really ought to be smooth all the way.

If it were mine I would take it apart and investigate why it's doing it.

I have just replaced all the bearings on my HEQ5 Pro.

It's a year old but a couple of the main ball bearings were a bit rough.

The taper rollers were nice and smooth and the tiny worm bearings were not to bad

but did the lot to make sure.

[Physicist13]

I'd say its almost certainly periodic error showing up. Do lots of short images (say 10sec) on a bright but unsaturated star for total of about 10mins. Stack with no alignment and youll see the PE. The slipped image looks like balance shift or vibraton/knock.

P

[Thalestris24]

Hi

How old is the NEQ6? The RA axis always seems a little 'stiffer' than the DEC axis (this is the case with my heq5 though I've no experience of the NEQ6).. However, there shouldn't be any 'bumps' like you mention. That probably needs further investigation. I'm not sure you can rely on AT for PA when you are so out of focus...  When using AT for PA can you see east or west, and south? Can you drift align manually to cross check?

Louise

[ollypenrice]

I tend to disagree with the analyses above but I could be up a gum tree.

Because the distortions are the same across the image I would rule out polar misalignment. This usually gives distortions in the corners that, if projected in each direction out of the frame, would make a rough circle. 

The 'double blob' effect has never been associated with polar misalignment in any image I've ever taken. It has usually been created by backlash. One star image is produced with the OTA resting on one side of the backlash and the other is produced when it shifts to the other side. However, you could have a periodic error which would mimick this effect if it tended to 'dwell' in two positions but oscillate fairly quickly between them.

The idea of being at a PA alignment precision of about 16 and 6 arcseconds per axis strikes me as improbable. Those are excellent values and may be correct, but if they are you were mighty skilled or mighty lucky.

Olly

[Thalestris24]

I agree Olly - it seems like a mechanical problem with the mount which is why I asked how old it is.

Louise

[steppenwolf]

There doesn't seem to be any sign of field rotation in your image so I believe that you have nailed your polar alignment.

The second image looks like a cable snag or a bit of debris in the worm gear causing a sudden jolt although if you had 'perfect balance' this could be associated with backlash. Have you tried imaging with the 'rising' side of the mount (i.e. the East side) made slightly heavier? This simple method keeps the RA axis under constant drive on just one side of each tooth - often, a perfect balance leads to teetering which can give the effect we are seeing here.

[Owmuchonomy]

I would agree it's nothing like field rotation. My constant fiddling with my HEQ5 suggests that the PA error quoted (if correct) is excellent. Therefore I am suspicious about these numbers. At less than 30" error I can get 90 seconds unguided and keep most of the subs. The second shot I have seen looks remarkably like when my cat takes a leap at the cables on the mount. It's either been knocked or there is something close to the mount to disturb it. Not sure about the first shot but It suggests something is slipping/moving or swarf in the drive (quite common). Remember, when it's tracking it is only doing so in RA. Try and equate the movement on your subs to the orientation of the sensor in your optical train. You can decide which way the mount is failing, North, South, west, East or a combination.

I will say that I have had my mount in bits 3 times, removed the Chinese molasses, replaced it with Molybdenum grease and replaced two worm drive bearings. It's better for it, so much so I don't want to part with it, sentimental old fool I am.

[Thalestris24]

Yeah, I think severe backlash and/or gunge in the worm gear seems likely. It's an NEQ6 carrying an ST-80 so I don't think PA or balance are likely to be factors in the image problems shown.

Louise

[r3i]

The OP mentioned he was imaging unguided, so surely there's not going to be anything controlling the mount that could introduce backlash?

Sounds like it needs a bit of TLC on its insides.

[Thalestris24]

The OP mentioned he was imaging unguided, so surely there's not going to be anything controlling the mount that could introduce backlash?

Sounds like it needs a bit of TLC on its insides.

True! But there could still be a problem with the motor gears?

[Viewmaster]

Note that on the double star images the top image has been given more exposure than the bottom image (less bloat)

So something is kicking back to retake the star again just over half way through the exposure.

[steppenwolf]

so surely there's not going to be anything controlling the mount that could introduce backlash?

Teetering?

[r3i]

Teetering?

You've lost me there [emoji1] please elaborate

[Tinker1947]

The balance should be a tiny bit heavy on both axis to stop any "Teetering" .........

[ollypenrice]

You've lost me there [emoji1] please elaborate

If the mount is close to perfectly balanced, as is likely, random effects will cause it to oscillate. A bit of wind, a vibration, cable movement, a slight jerk in the drives, all manner of things. Something in near perfect balance will 'teeter on the brink' of falling either way. You're right that guide inputs can provoke backlash oscillation but in the real world so can other small effects. You can't balance a pencil on its point. I thought Steve's single word reply was pure genius myself!   

The balance should be a tiny bit heavy on both axis to stop any "Teetering" .........

This works for RA, heavy on the east, but not necessarily for Dec unfortunately. When the OTA is vertical a front-back OTA imbalance reduces to zero effectiveness. This is why backlash in Dec is very bad news. Usually the solution on guided rigs is to run slightly polar misaligned and to disable one of the guiding directions. This does indeed work, but here we're unguided.

Olly

[Thalestris24]

Hi

Wouldn't it have to be a pretty big random effect to cause so much movement on an image with a 400mm scope (though I assume the above examples are cropped)? I can get 3mins unguided at 300mm on my AVX.

Louise

[r3i]

Yes I guess the term backlash is more encompassing than I realised.

[ollypenrice]

Hi

Wouldn't it have to be a pretty big random effect to cause so much movement on an image with a 400mm scope (though I assume the above examples are cropped)? I can get 3mins unguided at 300mm on my AVX.

Louise

Well, it wouldn't have to be much of an effect, I don't think. A larger scope with more inertia  might well oscillate less and need a bigger random effect to displace it. But there would need to be a lot of backlash. A sticky or contaminated drive does look like a good suspect.

Olly

[Dave_D]

i've had the 'double star' problem a few times, it's always been associated with a sudden 'jump' on my dec guiding trace. what's odd about mine though is the trace jumps, but then stays at the same level above the zero line for a while before any corrections seem to have any effect.

not sure if it's teetering as i've had it when the scopes been at roughly 60 degrees alt.

that's with a Vixen GP btw

[coenie777]

Thanks for all the responses so far. Really appreciate it.

The mount was bought second hand. The owner claimed he had it for 18 months which would have put it at around 2012'ish. I then discovered that the local agent in SA who sell these did not get any new stock for at least the last two to three years. So my copy could be a pre 2011 sample with all the issues that I have read about.

Overall the majority of solutions seem to hint at opening up and servicing. I am dreading that since I have never under taken any opening up of something working with grease and bearings etc. I have the Astro baby guide printed out in colour and need to still source white lithium grease locally which seem to be an issue. Any suggestions on an alternative grease that will not corrode the bearings and gears?

I am almost 100% sure that nothing bumped against the mount on that jumped shot. Balancing as some have mentioned in that light scope is very easy. I leave only one weight on the bar and move it all the way up to the mount stop to have it balanced. One thing that I did not do was balance the DEC axis when the camera were in focus. I balanced it with the focusing tube retracted (as for visual viewing) and had to rack it out quite a bit to get focus on the DSLR. So maybe that could have put the DEC axis a bit off balance.

[coenie777]

Is there any solutions that I can try at the scope to try and confirm what is wrong? There were a suggestion of taking 10 10sec stacks to confirm PEC. I will try that.

Any other suggestions on how I can determine if one of the suspected issues are the culprit?

What for instance can I do to determine if opening up the mount is the next step?

And can I assume that my issues is now no longer PA? I am sure my results were pure luck and maybe in reality not exactly what the software told me but it seem to be close enough now for 60 odd sec unguided shots. So the problem seem to be in the mount somewhere now.

Should I run the backlash and PEC functions that I see in the handset perhaps and see if things improve?