What on earth can cause this? Tracking, alignment and guiding problems

[Jannis]

So, as i was imaging yesterday, i was really struggelig for some reason with both alignment, tracking and guiding.

First i got alignment might be poor, then got alignment success but pointing accuracy was way off, even with several added stars. repeated polar and star alignment at least 4 times, but with no success. Managed to get it acceptable-ish in the end, after over 2 hours or work. Tried both 2 and 3 star alignment as i know i have some cone error at the moment, but still got better resulth with 2 star this time. What's odd is that even after adding a calibration star, if i chose to move back to that same star again, it would often easily miss it by quite a bit, but i managed to find my target after a while at least..

However, now my tracking wasn't good, object drifted quite a bit. And guiding didn't solve it either. I know i guide with a very short scope, 135mm and the SPC880, but it have worked perfectly before. It gives me a FOV a little larger then on my main scope.

I was imaging almost stright up, in ursa major.

Scope was balanced quite well (but not 100% to protect against gear slack), guide-cam was fastend properly, OTA was fastend properly, HEQ5 head was mounted properly, camera fastend good, and no loose bolts that i could find.

In PHD, it kept sending huge pulses to correct both in RA and DEC, as in over a 1.5 sec RA pulse and 0,4 sec DEC pulse with guide speed set at 0,75x on mount. Sounds rather on the exstreme side of you ask me, concidering the guide scope's focal lenght?

I used 3sec frames dor PHD to be able to see the guide star, and i think this should also be enough to even out any turbulense?

Main picture was jumping all over, and i rarly got nice looking stars. But just when i thought it couldn't get worse, the mount seems to have suddenly jumped quite a bit all of a sudden. Nothing was moving the scope physically, no cables holding it or anything, and it was while it was tracking and guiding. Obviously PHD lost guide star and started guiding like it was possessed, but what can cause it to jump like this? Luckily it happend only once.

I've just recently taken apart the DEC axis and relubricated it, and also adjusted the gears to go as smoothly as possible, but not too tightly either to prevent it from jamming. I think however i might have a small damage somewhere on the big DEC gear though, as i was a bit unlucky when i took it apart, but it must be extremly minor as i couldn't feel anything wrong at all with my fingers, and i Did check it extremly carefully.

Any ideas on what to try next other then 4 liters of fuel and a match?

Attached is the 120sec sub of where it jumped a whole lot suddenly. I'm imaging at 1000mm with the 550d (1.6x crop).

Also attached is a gif showing how the stars generally lookd like for the other subs. 100% crop.

[Merlin66]

IMHO it comes down to having a very good balance in all 3 axis......

[Jannis]

Hmm, i see. I've balanced my mount and scope is so well that almost no matter what position i put it in manually with clutches realeased, the scope stays put.

I've taken it only sliightly off balance with the gears in mind, but as i was imaging almost stright up, it might not have been enough off balance to hold it still?

Could that jump we see be as simple as the scope in a way "tipping over", as in going from leaning one way to the other on the gears?

But even with an overbalanced scope, that doesn't explain why my alignment and pointing accuracy suddenly is way off?

It worked perfectly last time i was outside with it. Bang on target with only 1-2 extra added stars.

Maybe i shuold simply take the whole mount apart again and recheck all the gears to make sure they're not too tight somewhere?

[George]

Hmm, i see. I've balanced my mount and scope is so well that almost no matter what position i put it in manually with clutches realeased, the scope stays put.

I've taken it only sliightly off balance with the gears in mind, but as i was imaging almost stright up, it might not have been enough off balance to hold it still?

Could that jump we see be as simple as the scope in a way "tipping over", as in going from leaning one way to the other on the gears?

But even with an overbalanced scope, that doesn't explain why my alignment and pointing accuracy suddenly is way off?

It worked perfectly last time i was outside with it. Bang on target with only 1-2 extra added stars.

Maybe i shuold simply take the whole mount apart again and recheck all the gears to make sure they're not too tight somewhere?

Alignment and pointing accuracy is nearly always down to poor polar alignment, you do say in your opening post that you had problems getting it calibrated and your findings since still point to the mount not being polar aligned. I'd work on the polar alignment before tearing your mount down and rule that out as your problem.

Is your mount permanently setup or do you bring the mount indoors between sessions?

[Jannis]

It's not permanent, so it's resetup every time.

I did recheck polar alignment several times as it just didn't seem right with my alignment problems, but i couldn't find anything wrong.

And i've just recently realigned the polar scope as well. Tested it on polaris just to check that nothing had happend to it, but it was still 100% lined up.

I also leveld off the tripod before i polar aligned.

But i agree that these problems does indicate poor polar alignment, so i will recheck that once again.

But what i don't understand is; after aligning, i added 1 star to pointing accuracy, then moved scope a little to add another star. Then if i now chose to go back to the first star again, it wouldn't be spot on anymore. In fact quite far off. Doesn't that sound a bit odd even if polar alignment was slightly off?

It's still strange if it's poor polar alignment though, as it's something i'm very used to and usually able to do in less then 3-5 min, and that night it sure sounds like i wasn't able to get it polar aligned after over 2 hours of work...

[George]

Just watched your video, seeing looks awful or the mount is overshooting guiding commands. Try disabling DEC guiding and see if that improves things, also do you have any backlash?

How quickly does PHD calibrate.

[Jannis]

Seeing was good(ish), it's guide commands going mad it seems, but probably not PHD's fault.

Like i said, PHD was sending almost up to 2sec guide commands to the mount if i let it. I had to limit maximum guide commands in PHD to get a somewhat usefull guiding.

PHD was correcting pretty much constantly, as in every 3rd sec.

i used these settings:

RA agressiveness 90

RA hysteresis 20

max RA: 1000

max DEC: 300

3s calibration steps

DEC guiding auto, resist switch

rest was at default settings.

I tried disabeling DEC guiding as well, but no success with that either, just got even worse (wich again confirm polar alignment failsure or something else wrong with the mount?)

Edit: it's easy to see on the 100% crop gif how the galaxy moves back and forth all the time between the frames - so the guiding does hold it in place somehow, it just can't keep it still.

[ollypenrice]

Max RA of 1000 seems high to me. What happens if you try 300?

Your image shows two sets of slightly offset round stars so something is right and something is wrong. I'd say backlash, but where is it coming from? Free play across both sides of mesh? Or is it the over enthusiastic guide command banging it first one way and then the next? Either way it seems to spend most of the time guiding at the right speed but in one of two different places. Only one axis is affected.

Olly

[Jannis]

Hmm, yes, 1000 seems high to me too, but as it kept maxing it out even with 1500 i thought it was because of very high movement, so i didn't try to limit it too much. But maybe your right and that it's the guiding commands banging the gears back and forward causing it?

I didn't try anything as low as 300, but i guess i could try.

The problem when imaging almost stright up seemd to be that scope isn't firmly leaning onto the gears in any of the directions, so it's kind of wiggeling a bit back and forward it seems.

I noticed a big inproovement after the scope did a meridian flip later in the night. Maybe i simply have to drag the scope quite a bit Out of balance when imaging stright up?

I will open it up again and check the motor gears to make sure they're good. If they are, i guess next step is to recheck the DEC gear that i suspected had gotten a minor damage when i took the DEC axis apart. Was hoping i wouldn't have to take the whole DEC axis apart again though.

[boris sputnik]

Hi Jannis !

Once I had a similar problem...Very similar. I finally found my problem. Maybe it's not the same for you ,but I noticed that one of my tripod leg was slowly sliding down and putting my scope off level.... It was not a lot ,not noticable at first look ,but when I put my level on I saw it was off. The change in temperature makes the tube of the tripod to contract so the initial tightening is not tight enough when it gets colder. Since then I always check if the tripod legs are locked tight enough and I very often have to tighten them as the night progress.

I hope this will help.

Cheers !

Maxx

[boris sputnik]

I forgot to say....When I balance my scope ,I alway make it a tiny bit heavier on the east side if my object to photograph is in the east part of the sky ,and a tiny bit heavyer on the west side if the object to photograph is in the western part of the sky. That way ,the gears are always in contact ,preventing the worm gear to turn without pulling the scope because to well balanced !

I hope my English is clear enough

Maxx

[boris sputnik]

I would like to correct what I just said before...It's always the eastern side that's a tiny bit heavyer...When I take a photo in the eastern sky ,the weights are a bit heavyer...When I take a picture in the western sky ,the telescope is a bit heavyer...So the bit of weight is always turning the scope eastward...

Sorry for the mistake

Maxx

[Jannis]

Thanks for all the tips so far. Will probably give it a go in a day or two if it's clear and see if the problem still exists with a few changes.

will change to shorter guide pulse,

make sure tripod legs and HEQ5 head are tightly fit,

unbalance scope a bit so the weights are a bit heavier then the scope, and the scope a little heavier on the east side (i asume just turning the OTA a bit around so the camera is a bit on the east side is enough?)

and maybe try a target that isn't almost stright up.

If that doesn't work, i'll take it apart again and readjust the worm gears. And if it's no good weather, i'll start by doing that anyway.

If it still doesn't work, i'll record a video of the whole thing from setting up the tripod to guiding so you can all see what i'm doing step by step to help spotting what i do wrong.

[boris sputnik]

For Unbalancing the scope a bit ,just use the counterweights ,you can even mark the counterweight bar...one mark for telescope heavyer ,and one mark for counterweight heavyer !

Maxx

[Jannis]

So you mean to make the whole telescope side of the mount heavier or lighter, and not just one side of the OTA?

I was thinking about unbalancing it in both DEC and RA, not just one of them.

Edit, i see now i wrote wrong earlier, i didn't mean to turn the OTA around to unbalance it, but to pull teh OTA a little out from the rings to make one end a bit heavier.

[themos]

It could be that you didn't tighten the azimuth clamp after adjusting the mount in azimuth? I know that I can be fooled sometimes by things like that.

[boris sputnik]

It's the RA that should be a bit heavyer eastward...the DEC should be well balanced ! The OTA itself should be well balanced !

[Jannis]

ok, will give it a try as soon as th weather lets me.

themos: could well be, as i use all the original bolts wich are not the best and most solid, so i'm a bit careful with all the bolts. Will try to tighten them a bit more.

[themos]

I was thinking of the central bolt that "hangs" under the mount. I tighten that after I adjust the azimuth but sometimes I subsequently find it's not as tight as I thought and the whole mount could wobble about.

[Jannis]

I'm recording a video now from setting it up to guiding and the final picture. first got alignment failed, then success, but with targets still off. Then started PHD with 300ms RA instead, and started a 500s sub to show how it looks.

PHD graph looks very nice now actually, but now i'm guiding on capella instead, a lot lower, and a lot brighter.

don't know when i'll be able to upload the video though

[Jannis]

Ok now, 500s sub of capella = pinpoint stars. PHD graph = as smooth as it can be. Will now give it a try in NGC 3718 again and see

[Jannis]

Ok, PHD graph is a lot smoother, but still get a quite strong pulses now and then. Tried 600s, and a bit unclear stars. tried 400s but still unclear stars ((actually probably even more unclear then at 600s). Will now give it a go to lower the guide pulses even more just to see. Will try half and see how it does.

[Jannis]

Here's a print screen of PHD graph, and how the stars now look in teh center at 100% crop. This is an 800 sec exposure, with guide settings: 150ms RA max, 75ms DEC max. all other settings at default.

Note though, my only available guide stars in this area is veeery faint (marked an equally bright star in red, probably can't even see it after uploading here, lol), and even with a 3-5 sec settings in PHD, it still looses teh guide star now and then for a few rounds. Usually less then 10 sec.

Could the faint guide star be one of the causes?

I guess i really need to mod my SPS880 to be able to guide on fainter stars with better signal to noise ratio... >_<

[themos]

You should try taking Dark frames in PHD. They will clean up the guide star signal somewhat. Shooting away from the Milky Way means that stars tend to be quite faint, I've found that to be a problem with unmodified webcams. Apart from that, check for flexure in the usual places.

[Jannis]

I've tried dark frames too, but didn't help too much for me. probably more of a help on a modded camera.

But for now, it seems like my guiding isn't teh main problem actually, as the 500s exposure of capella made perfect stars. I think i'll just have to take the mount apart agan and recheck all the gears.

I don't understand how any slight slack in teh gears can cause pointing accuracy to be way off though?