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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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