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Stub Mandrel

What Constitutes Good Guiding?

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I have seen lots of figures bandied about for guiding performance, so many that i am confused about what sort of performance I should be aiming for.

As I recounted here:

I was getting some nights with about 1.40" RMS and some nights with about 0.80" RMS using the EQ3 loaded with the 130P-DS.

With the, admittedly heavier, 150PL on the HEQ5 and after some tweaking to get better reliability, I am still stuck at 0.80" RMS.

I saw recent post by @Olly that suggests 0.25" is well beyond the reach of an EQ6 PRO, presumably a better mount than the HEQ5?

Should I be trying to get better results, and if so how much better can I expect to get? Surely (hopefully!) the HEQ5 can beat the EQ3?

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Posted (edited)

I think to be really honest the best you can get is round stars..

I can spend many many hours drift aligning, balancing fiddling with phd. I have managed to get down to .40" but average .50" - .75"ish but at the end of the day its the images I produce that are important not the numbers.

Still considering trading up my old qhy5 as I am convinced this would help bring the numbers down.

 

Mark.

Edited by spillage
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I agree with Spillage above... round stars is a good indicator of good guiding. Generally if your guiding is less than the angular distance per pixel in arc seconds than you will have subs as good as you can expect, even with lower guiding error.

The guiding accuracy is dependent on your atmosphere stillness, for example I don't know how the HEQ5 compares to the CGEM but with my CGEM I get 0.4" at best, but most of the time I can expect 0.6-0.8".

 

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+1 for round stars.  Often as not my PHD graph looks like an ECG, but if the stars come out OK then it’s doing it’s job...

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Nope - round stars - not good indicator. All that tells you is that guide error is randomly distributed between DEC and RA (same RMS in both, and both errors being random).

There are two indicators of good guiding, and none is perfectly reliable - guide RMS, and star FWHM.

Star FWHM will be subject to seeing, but if seeing is relatively constant (or similar between nights) - star FWHM will be smaller when guiding is good vs when it's not good.

Guide RMS is good indicator provided that your guide resolution is sufficient to resolve guide errors and that seeing is not totally poor (not worth imaging, or some serious low level disturbance like chimney exhaust right on optical path).

For most part seeing is not causing poor guiding - if you visually observe at high resolution, you will notice that majority of time seeing disturbances are very high frequency - it either blurs target (for example planetary) - this means disturbances are too quick for eye to see as shimmer of you can actually spot waving / shimmering / wobbling (it shows well on Moon) - but you will notice that it is also high frequency - changing position many times per second - such effect averages out in duration of normal guide exposure (2-3 seconds). If you suspect seeing is culprit - just use longer guide exposure - if you have fast changing PE it will worsen RA stats, but it should improve DEC stats/graph (this is how you will know - it will start to look more saw tooth).

If you have well behaved mount (no fast PE component - so smooth PE, and not very big) - neither PE nor PA will cause big guide RMS (provided that PA is reasonably good - even if you don't drift align).

Most guide errors are caused by mount mechanics surfaces being rough (poor bearings, rough surfaces, mount being loose) - to lesser extent, and wind and other factors (cable snag) to greater extent. You can see this by looking at DEC graph. There is no reason for it to jump around if PA is only reason for DEC to go out of normal position - but DEC tends to jump around pretty much - since DEC is usually not moving when tracking it can only be due to rough mechanic surfaces and external influence like wind. Otherwise it would slowly drift out of position just to be brought back (saw tooth pattern).

There is another thing that limits HEQ5/EQ6 class mount in guide precision - that is stepper motor micro step precision. Best you can hope for HEQ5/EQ6 mount, if tuned (belt mod) on windless night of good seeing is around 0.4-0.5" RMS - you simply will not be able to do better with these mounts. On such night you will often see DEC error being a bit larger than RA (if you have your PEC in place and mount is not suffering any short period PE component). It is because RA is in motion, and changes micro step very fast, about 100 times per second or so, and if microstep is missed it will not show in 2-3s guide exposure (mount inertia will make sure such miss steps are smoothed out). DEC on the other hand is in dynamic balance - and stepper micro controllers are known to sometimes miss micro step if under load. So DEC will tend to "oscillate" around position where it needs to be (misses micro step - there is stronger pull to get it back to where it should be - there is small jolt due to air motion, or vibration - it overshoots on other side, guide pulse reacts, sometimes it undershoots, sometimes it overshoots - a sort of random oscillation forms).

There is another component that is impacting guiding (if you are using OAG) - how well scope is attached to mount - that includes any slack in tube rings, is there even minute flex between scope and dovetail bar - how well dovetail bar sits on mount. Scope length will play a part in this due to arm momentum.

Bottom line - for HEQ5 / EQ6 class mount, best you can hope on a good, still night with tuned mount is around 0.5". A bit of breeze will bump this to 0.6-0.7" or above, depending on scope size, weight and connection to mount. A bit stronger wind and you are looking at 1" or above of guide RMS.

For guiding below 0.2" RMS you want premium mount (preferably on solid pier, shielded from wind - then you can go as low as 0.1").

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7 hours ago, vlaiv said:

Bottom line - for HEQ5 / EQ6 class mount, best you can hope on a good, still night with tuned mount is around 0.5".

Thanks Vlaiv, it helps to have an actual 'target' I can aim for. Not there yet, but looks achievable.

A belt mod is very tempting, if only because it is such an elegant solution.

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I hear the 'less than pixel scale' comments, but with the 130P-DS at 1.7" per pixel and an RMS of 1.4", I saw exursions up to +/- 8", enough to give stars a tiny 'pip' on some subs.

The challenge with that mount was getting rid of that sawtooth DEC behaviour and tuning was very fussy.

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Vlaiv is perfectly correct that the round stars test is almost meaningless but at least with round stars you can produce an attractive image. When setting up the first Mesu we always had round stars but by tuning the guide parameters they got smaller and smaller!

I don't think there's any reason to believe that the HEQ5 is less accurate than the 6 provided both are within payload.

Any backlash adds to the guide error and the worse the seeing the more it does so. (When corrections are few there is a reduced tendency for the mount to be sent oscillating across the backlash.) So losing the backlash is a very good idea. The other ways to combat backlash are to run slightly east heavy and slightly polar misaligned. East heavy keeps the drive in 'push' mode and the misalignment means you can disable guiding on one direction in Dec, letting the other 'push in the direction of correction.' This does work and won't stop you from doing 15 minute subs.

The duration of guide subs is also something to tune in the light of prevailing conditions. Our EQ sixes thrive on short guide subs if the seeing is good because they have pretty rapid PE. However, if the seeing is bad this has them chasing it, so we do better with longer subs which average out the position of the guide star's image. (It's worth remembering that the guide trace has no way of knowing where the real star is. It only knows where its image is. So very short subs do give a better trace but is that a trace of the real star or just its image?  Curses!! )

The other variable within the guide trace is the position of the scope between corrections. Again we have no means of knowing, but Avalon claim that with their backlash-free belt drive the correction is fed in faster so that, between guide inputs, the true location of the scope is on target for more of the time. With any backlash oscillation in play it's a good bet the situation between inputs will be worse.

Olly

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Thanks Olly.

I do aim to keep things slightly unbalanced.

Something I have noticed is that guiding is noticeably worse when low down (<20 degrees) but this may be a combination of bas seeing and poor contrast due to LP.

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6 hours ago, Stub Mandrel said:

Thanks Olly.

I do aim to keep things slightly unbalanced.

Something I have noticed is that guiding is noticeably worse when low down (<20 degrees) but this may be a combination of bas seeing and poor contrast due to LP.

I think it's the seeing. It's exactly the same for us even with pretty dark horizons.

Olly

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On 14/06/2018 at 00:32, vlaiv said:

Nope - round stars - not good indicator. All that tells you is that guide error is randomly distributed between DEC and RA (same RMS in both, and both errors being random).

Maybe so.  I still maintain that if your guiding is giving you reasonably tight, round stars then actual "readings" are of secondary importance.  My own mount is an un-modded, 10 year old NEQ6 and it nearly always guides out at between 0.5" (at best) and 3" . And that variation is a night-to-night thing - that range can occur when pointing at the same object at the same declination, just on different nights.  I usually image at 600mm focal length and by and large, star shapes in the resultant subs are just fine for me - or at least my pictures are pretty enough for my liking, anyway.  If stars seem a bit bloated then longer guide exposures can often help to tighten things up.   My NEQ6 seems to work best at short guiding exposures for some reason, usually 1 to 1.5 seconds. but sometimes cranking up to 3 seconds helps things if the seeing is a bit shaky.  I haven't had to mess around with any other settings in PHD/EQMOD for the past year (touch wood), pretty much irrespective of target.

I would say that the answer to the OP's opening question is "find what usually works for your set-up, then stick with it".

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OK, I'm in guiding Nirvana now... aside from two 'star loss events' and going behind a tree at the end (these are excluded), guiding of 0.99" for over 3 hours. You can see it was even better after 11:53 when the moon went down:

image.thumb.png.3c5b7a7b7f7e88b205d4cb071acb31ff.png

I only spent 40 minutes on the next target, for 0.65". If I can get this reliably on dark nights, I will be happy.

image.thumb.png.456abafe65b796ebe41c67d904287249.png

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Looking good.  Now can you resist the urge to tweak.....?

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On 14/06/2018 at 01:32, vlaiv said:

PEC in place and mount is not suffering any short period PE

I've always been unsure about PEC. Do I need to record it during guiding and then apply it or create a PEC file before in guiding and then apply it when I'm actually guiding. A little tweak extra towards better guiding. 

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1 hour ago, Gerry Casa Christiana said:

I've always been unsure about PEC. Do I need to record it during guiding and then apply it or create a PEC file before in guiding and then apply it when I'm actually guiding. A little tweak extra towards better guiding. 

My experience is with EQMod (there has been some debate if PEC helps or whether guiding is fighting PEC, but for EQMod it seems that PEC helps quite a bit) and here is how I do it:

I turn off guiding and use PHD2 to create log file (there is option in PHD2 to disable guide commands). I usually create about hour / hour and a half of log file. While doing that - at some point I press time stamp button in EQMod. EQMod auto PEC is also turned off. Next I load PHD2 log data into PecPrep and create PEC curve.

After that I just tell EQMod to use that PEC curve and work "normally" from then on. I usually recreate PEC curve on significant change in system, or if sync is lost (HEQ5 does not have absolute encoders, and one must park each time after session to preserve mount position to PEC synchronization - if there is power failure or something and sync is lost - PEC curve needs to be generated again).

In general PEC is meant to be recorded on "raw" mount data / motion - so don't have anything running that will disturb this - like guiding or whatever.

 

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On 24/06/2018 at 23:16, Hallingskies said:

Looking good.  Now can you resist the urge to tweak.....?

The Rowan Belt Mod looks well thought out...?

Actually was guiding low and with the scope on the on the east side last night, wasn't as good at about 1.03" Took a bit of fiddling  with settings to get that - mainly reducing the aggression as it was overshooting.

I think problem was that it was west heavy making the RA tracking 'jumpy'.

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Posted (edited)
2 hours ago, Stub Mandrel said:

The Rowan Belt Mod looks well thought out...?

Actually was guiding low and with the scope on the on the east side last night, wasn't as good at about 1.03" Took a bit of fiddling  with settings to get that - mainly reducing the aggression as it was overshooting.

I think problem was that it was west heavy making the RA tracking 'jumpy'.

In my limited experience I think it does improve things but it might be easier to get a pole master and get the improvements that way? Rowan mod isn't easy by the look of it. 

4 hours ago, vlaiv said:

My experience is with EQMod (there has been some debate if PEC helps or whether guiding is fighting PEC, but for EQMod it seems that PEC helps quite a bit) and here is how I do it:

I turn off guiding and use PHD2 to create log file (there is option in PHD2 to disable guide commands). I usually create about hour / hour and a half of log file. While doing that - at some point I press time stamp button in EQMod. EQMod auto PEC is also turned off. Next I load PHD2 log data into PecPrep and create PEC curve.

After that I just tell EQMod to use that PEC curve and work "normally" from then on. I usually recreate PEC curve on significant change in system, or if sync is lost (HEQ5 does not have absolute encoders, and one must park each time after session to preserve mount position to PEC synchronization - if there is power failure or something and sync is lost - PEC curve needs to be generated again).

In general PEC is meant to be recorded on "raw" mount data / motion - so don't have anything running that will disturb this - like guiding or whatever.

 

That sounds like a plan. Great. I'll certainly take the principle that PEC needs to be created raw. Unfortunately at the moment I don't have a permanent setup but I could in theory set up early let it record what the mount is doing and apply the curve once I start guiding?  

Edited by Gerry Casa Christiana

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9 minutes ago, Gerry Casa Christiana said:

In my limited experience I think it does improve things but it might be easier to get a pole master and get the improvements that way? Rowan mod isn't easy by the look of it. 

I thought it looks a doddle - but I am familiar with that sort of job.

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I would agree with @Stub Mandrel the belt mod will remove allot of the mechanical issue from the mount and spending some time with drift align will iron out the PA. The polemaster will only improve your PA and the original mechs will still give you the same issue even after adjustment.

I would also bear in mind that the belt mod is about 1/3 of the price of the polemaster.

 

Mark.

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55 minutes ago, Gerry Casa Christiana said:

In my limited experience I think it does improve things but it might be easier to get a pole master and get the improvements that way? Rowan mod isn't easy by the look of it. 

That sounds like a plan. Great. I'll certainly take the principle that PEC needs to be created raw. Unfortunately at the moment I don't have a permanent setup but I could in theory set up early let it record what the mount is doing and apply the curve once I start guiding?   

I don't have permanent setup either, but for PEC you don't need to have it. PEC records imperfections in manufacturing of different components of drive train. It can't correct all imperfections - only those that are periodic in nature (like different gears not being perfect circles) - and only if they are harmonics of certain base period. EQMod PEC can only work with harmonics of base worm drive period.

I've seen some question raised about viability of PEC when using different scopes on same mount. I think if one models PEC of mount - it should not matter what sort of scope you have on it - PEC needs to correct for drive error irrespective of the load. I use it with both small and large scope and it works.

You can do PEC in early evening (not astro dark yet), or during full moon when you would not otherwise image. Do it only once and use it for subsequent sessions. You don't need to do it each time. I've done it just 3-4 times so far (after fiddling with mount - belt mod or regreasing / changing bearings). I need to do it now once more since I changed my laptop and lost PEC file (I actually saved file but sync was lost because of fresh EQMod install). You can even do it when there is high level clouds - as long as you select bright enough star and PHD2 is able to track it long enough (without star lost message). One more thing - best to select star near/on Equator - PE will have the largest amplitude there and most precise recording of it will be possible. PecPrep takes into account DEC of guide star (either read from log file, or you need to enter it manually, depending on guide app you are using - PHD2 its automatic).

53 minutes ago, Stub Mandrel said:

I thought it looks a doddle - but I am familiar with that sort of job.

I'm a bit confused, what mount do you have? In your signature NEQ3 is listed. As far as I know, Rowan belt mod is available for HEQ5 and EQ6 models only.

Other thing that you might consider is changing bearings on your mount (irrespective of model). They are pretty much standard - get high quality SKF and replace each bearing - it will also help smooth out mount tracking.

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11 minutes ago, vlaiv said:

You don't need to do it each time.

Except for those of us who find it a major effort of will to remember to park the mount before switching it off...

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12 minutes ago, vlaiv said:

I'm a bit confused, what mount do you have? In your signature NEQ3 is listed. As far as I know, Rowan belt mod is available for HEQ5 and EQ6 models only.

Other thing that you might consider is changing bearings on your mount (irrespective of model). They are pretty much standard - get high quality SKF and replace each bearing - it will also help smooth out mount tracking.

I've got an HEQ5 as well now.

I'm a bit of a bearing upgrade nerd (I've done my lathe and my mill and fitted needle thrust bearings to the EQ3)  have a mate who used to import them in a big way (still does but limited range), so he can advise on best choice (I suspect he would recommend Nachi).

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17 hours ago, Stub Mandrel said:

I thought it looks a doddle - but I am familiar with that sort of job.

I did the upgrade (Rowan belt mode) on my HEQ5 in 30 minutes without any issues and I am all thumbs! Just make sure to get the "Pinion Gear Extractor" ... It was well worth the price !

Here is a noise comparison I did in mid-ugrade...

 

 

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16 hours ago, Stub Mandrel said:

Except for those of us who find it a major effort of will to remember to park the mount before switching it off...

If you can't remeber to park then put some thought into how to setup a resynch encoders position (maybe some distant terrestrial object you can see with a reticule eyepeice or a physical indicator on the worm shaft (EQ6Pro have a flat on the end). All you then have to do do is create a custom park with the mount at the position and whenever you want to reysnc just move the mount there (manually or under EQMOD control), select the resync as you current park option and hit the resynch encoders button.

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On 26/06/2018 at 17:48, Gerry Casa Christiana said:

I turn off guiding and use PHD2 to create log file (there is option in PHD2 to disable guide commands). I usually create about hour / hour and a half of log file. While doing that - at some point I press time stamp button in EQMod. EQMod auto PEC is also turned off. Next I load PHD2 log data into PecPrep and create PEC curve.

So just so I understand. In PHD2 you set up guiding on a star near the equator leave that going for a hour and a half then create a log file. Then load that into Eqmod pec prep? Sorry never done this but would like to try. So PECPREP is part of Eqmod right? I'll have to google this maybe @chrisshillito has already done this as I know he designed it. 

Gerry

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