PHD2 guiding log - interpreting and improving?

[tooth_dr]

5 minutes ago, michael8554 said:

So not really a solution, but good news, not Tracking Rate.

Tracking Rate is the unguided RA rate, usually Sidereal.

Guiding Rate is a fraction of Tracking Rate that PHD2 applies to the Tracking Rate to guide, plus to move west, minus to move east.

0.5x is good, you can try up to say 0.9x to chase backlash if you had it.

Belt mod was someone else on sgl.

Can't see your post on Open Phd so can only say that sub-pixel guiding involves micrometre moves, so a hairs breadth sag in your guidescope setup will show on your guiding

Michael 

Thanks Michael.

It's look like it is 'ok' for my 500mm focal length imaging, but when I put on my 1200mm reflector I might have to reconsider my strategy of using the finderscope, and switch to an OAG instead.  I'm loathe to do the belt mod after reading the headaches one of the guys is currently having.

[Stub Mandrel]

I'm not commenting... I changed a few things last night and I can't get below 1.00".

I think seeing was part of the issue, and I suspect some DEC backlash has crept back in.

[RolandKol]

I have quite a different problem....

Usually I start the session with 3sec exp. guiding in around 0.55 -0.7 Total RMS, I use NEQ6+130PDS+MonoSetup+PiggyBack Samyang with Canon, - guiding via ZWO OAG and 120MM-mini.

My session starts around 1h before  Meridian, - Flips, guides nicely, but around 2h after the Flip "Rock&Roll Starts"... Graph shoots UP and guiding ends up around 1.7 Total RMS.

I guess it's a balance issue as it started since I PiggyBacked the Canon...  My back garden is not windy and the cat is quite calm  PA around 1", - on a tripod.
 

Been "dancing tango" with my mount for several weeks, - balancing, re-balancing.... no luck....  
 

Funny, but once the Mount starts behaving, - I drop guiding exp to 1sec and it calms down.... If I go back to 3sec, - disco starts again. So not all works as written in the "vocabulary"

Not a big problem... But I cannot go to sleep once imaging during a weekdays... Have to keep an eye all the time, - annoying.

[kirkster501]

How do I find out what the total RMS is - the figures in the little box to the right?  They change about quite a bit I have noticed.

Ever since I have been doing AP, for 6 years or so now, I have always battled with guiding.  Seems to be an art.  Strangely, I had better results when I was taking down and setting up every night with the original PHD than I now get with my observatory and a MESU mount.  That said, after some advice from vlaiv I seem to have settled it down.  I need clear sky time to work on this.

[RolandKol]

12 minutes ago, kirkster501 said:

How do I find out what the total RMS is - the figures in the little box to the right?  They change about quite a bit I have noticed.

Ever since I have been doing AP, for 6 years or so now, I have always battled with guiding.  Seems to be an art.  Strangely, I had better results when I was taking down and setting up every night with the original PHD than I now get with my observatory and a MESU mount.  That said, after some advice from vlaiv I seem to have settled it down.  I need clear sky time to work on this.

I always thought, once balanced and have a descent PA, - you do not have to dance around Mesu, like I do with my NEQ6
I am glad you managed find a right path to calm it down... Curious to see the results.

As per Total RMS, - you never have it constant, thats why everyone tells "around".

[kirkster501]

THanks.

So in that example the 0.13 number (arc seconds [I think] or pixels?), is it that number or the TOTAL that is used in the 0.5 x imaging resolution equation?  I think the two will be close anyway.  One thing for sure; at one stage I was nowhere near 0.5 and was way over, with blobbier stars.

Sometimes my DEC is dancing around.

I am pretty convinced after my adjustments suggested to me my vlaiv that I now have this sorted because on two targets after i adjusted my algorithm and the aggression my guiding was flatlining almost like in the screenshot and only on the third night - with much poorer seeing - did it start to dance about a bit (bit not as bad as before I made the changes).

EDIT:  Ah, I think the two numbers are the pixels AND then the arc seconds.  Tot 0.16 (0.63").

[RolandKol]

20 minutes ago, kirkster501 said:

EDIT:  Ah, I think the two numbers are the pixels AND then the arc seconds.  Tot 0.16 (0.63").

Correct
 

[RolandKol]

Also, I am not sure if you ever tried to keep an eye on RA (OSC) value, which is 0.26 in this picture, just under the red highlighter.

This value shows the speed of of your RA. 0.5 is the midpoint speed value, if it goes Up, - speeding.

as PHD2 recommends, it should be somewhere around 0.3 - 0.4, slightly lower than the mid point value.

[kirkster501]

Many thanks.  Ok so another question (sorry to the OP for maybe slight head hijack but this is still very relevant to the original topic).

I see people mention the mount can guide at 0.5 or 0.3 of sidereal.  What is that all about?  Why doesn't it guide *at* sidereal?  Why the fractional value and what purpose does that serve?

[kirkster501]

I started a similar post when guiding was causing me grief.  I think I am on top of it now but need more clear skies:

 

[michael8554]

10 minutes ago, kirkster501 said:

I see people mention the mount can guide at 0.5 or 0.3 of sidereal.  What is that all about?  Why doesn't it guide *at* sidereal?  Why the fractional value and what purpose does that serve

Like I said before, these are plus and minus speed changes to the sidereal tracking rate. 

When your mount drifts east, PHD2 sends plus changes, so that the mount briefly speeds up and goes back to where it should be. 

And vice versa.

You could go 1.0 times correction, but then the RA would briefly stop tracking when minus corrections were applied.

 Then the mount would have to take up backlash before tracking again.

So up to 0.9 max perhaps is best.

Michael 

[kirkster501]

1 minute ago, michael8554 said:

Like I said before, these are plus and minus speed changes to the sidereal tracking rate. 

When your mount drifts east, PHD2 sends plus changes, so that the mount briefly speeds up and goes back to where it should be. 

And vice versa.

You could go 1.0 times correction, but then the RA would briefly stop tracking when minus corrections were applied.

 Then the mount would have to take up backlash before tracking again.

So up to 0.9 max perhaps is best.

Michael 

Got it. Thankyou.  Sorry, I've only just seen your previous explanation a few posts back.  I missed that.

[vlaiv]

12 minutes ago, kirkster501 said:

Many thanks.  Ok so another question (sorry to the OP for maybe slight head hijack but this is still very relevant to the original topic).

I see people mention the mount can guide at 0.5 or 0.3 of sidereal.  What is that all about?  Why doesn't it guide *at* sidereal?  Why the fractional value and what purpose does that serve?

Pulse guiding works by issuing a pulse with certain duration that makes mount track at different speed then sidereal. We are talking about RA corrections for the moment.

So mount is tracking at sidereal - 15"/s, and it starts to lag for example. Pulse will speed up mount tracking to catch up, but what will be the speed of mount while it's catching up? If you have your guide speed set to 0.5x sidereal, then mount will be running at x1.5 sidereal for duration of pulse (1.0 + 0.5 = 1.5). If mount needs to slow down because it went too far ahead, then tracking speed will change to slower. How much slower? Depends on guide speed. So for given 0.5x sidereal, mount will track at 0.5 sidereal for duration of pulse (1.0 - 0.5 = 0.5).

There are two important considerations when choosing guide speed. Duration of pulse and stiffness of the mount (and scope on it). If your guide system needs to correct for 1" of error and you have your guiding speed set at 0.5x sidereal, pulse duration needed for this will be 1" / (15"/s * guide speed) = 1/ 7.5 = ~133ms. If your guide speed is set to 0.1 x sidereal, this duration will be 0.666ms.

Slower guide speed means longer pulses. There is "lower limit" on pulse duration in some cases. EQMod for example sets that to 20ms. This limits how small correction you can make. There is also timing precision when issuing guide pulse (I think that used to be an issue with older computers, but it might still be issue in responsiveness of mount electronics).

There is another thing to consider. If one is for example using 1.0x guide speed. When issued guide pulse to "slow down" - mount will effectively stop for duration of guide pulse. Mount has inertia and any change in tracking speed will introduce vibrations and shake. Large guide speed needs very stiff mount that will not shake much on guide pulse.

I'm advocating low guide speeds, as this allows for smaller corrections to be made, and it does not introduce too much shake into mount when issuing a guide pulse.

[kirkster501]

Super explanation vlaiv, many thanks  

[RolandKol]

yep Micheal...

And thats why, I would suggest to experiment with your Ra Agr and reduce it to, for example to 30, and see how it goes on your Mesu, your last screen-shots showed your Ra(OSC) was above 0.5 and corrections shooting both directions.

I would lower it completely and start increasing to the best position

[michael8554]

To add to vlaiv's concise answer - loved the math ! 

If you're fortunate enough to have a tightly engineered mount like the Mesu, then low guide fractions make sense for the reasons he stated. 

The less fortunate amongst us may have mounts with say 2000ms dec backlash.

Slight PA error with dec balance and one direction dec guiding can work.

But nipping that backlash  in the bud may involve exposures lengths that are pushing close to seeing eg 1.5 secs, and big guide fractions.

Michael 

[Stub Mandrel]

I wish @Vlaiv would write a book on the maths of astrophotography for complete idiots ?