PHD2 guiding accuracy

[scotty38]

I decided to put my scope out and we have a bit of clear sky here so all good but to my question....

I've had my fair share of guiding quirks but, of late, things seem to be working ok and tonight, for example, I've had this:

This is with a WO GT81, 0.8x flattener, 294MC Pro and an AZ-EQ6 which gives a fairly easy to guide 2.5"/pixel but how accurate is my guiding above do you reckon?

The reason I ask is I see many posts mentioning getting down to, say, 0.3" with expensive mounts etc so should I really be seeing 0.4" with my setup?

Just curious and very interested on folks' thoughts.

[wimvb]

Looks good, but here are a few observations. You have a guiding rms error less than 0.1 pixel (which  I think is as good as you can hope for), but the smallest movement that PHD will react to (MinMo) is 0.19 pixels. (Your graph confirms this with just 3 small correcting pulses being sent in 100 possible.) PHD isn't sending any guide pulses. So in effect you're not guiding at all. Also, you have a Y-scale of +/- 8 arc seconds. You can safely set the Y-scale to +/- 2 arc seconds.

Finally, there are no DEC pulses at all. Your polar alignment must be spot on.

Edited January 3, 2022 by wimvb

[scotty38]

Thanks but not guiding at all? I've not done anything with the files yet but the previews in NINA all look fine and they're 5 minute subs andI think my mount would need guiding for that length of exposure wouldn't it, or am I missing something?

Changing the scale to 2 won't make the 0.40" change, just that the graph will be more "jagged" as it were.

Edited January 3, 2022 by scotty38

[ONIKKINEN]

Not really an expert in good guiding and definitely never seen a graph like yours on my setup, but a few things come to mind.

What sort of guider do you have? Setting the focal length wrong in PHD2 settings can give bogus readouts and also, if you have something like the 30mm miniguider its possible PHD2 just doesnt pick up the errors and so the graph is flat.

Another option is that the periodic error correction is doing a good job and it really is doing that well. This is what i hope is the case as a soon to be user of the AZEQ6 😁.

Also, could be a combination of all of the above + shooting in high DEC. At 73 degrees of Declination my EQM35 sometimes reports RMS of under 0.4 too. Definitely not truly that good but that is what PHD2 and my guider can pick up at high DEC.

Edited January 3, 2022 by ONIKKINEN
Forgot dec

[vlaiv]

What is your guiding resolution and what is your DEC?

[alacant]

2 hours ago, scotty38 said:

interested on folks' thoughts.

Hi

With only a tiny piece of guiding, without the log and -more importantly- the images which were produced as a result, it's very difficult to comment. 

One general point would be that what the guide camera sees and what the main camera sees can be quite different, especially with separate guide telescopes.

If you're satisfied with the images, I'd  say stay as you are. Look at the images, not the graphs.

Cheers

[scotty38]

200mm guidescope with 120mm mini so 3.87"/pixel and yes DEC very high as was imaging NGC281 but during calibration at nearly 0 it was showing 0.5/0.6 ish but I didn't grab any screenshots there as I didn't think much about it until I saw the 0.4" afterwards.

I realise it's much easier to guide there but really my question is can I believe the 0.4, is that what would be expected from this level of kit etc etc.

[vlaiv]

1 minute ago, scotty38 said:

I realise it's much easier to guide there but really my question is can I believe the 0.4, is that what would be expected from this level of kit etc etc.

Can happen. I was able to guide at 0.38 RMS with Heq5

DEC around 60°, and mount is heavily modded, but guide resolution is ~1"/px - so I'm fairly confident in numbers.

[scotty38]

3 minutes ago, alacant said:

Hi

With only a tiny piece of guiding, without the log and -more importantly- the images which were produced as a result, it's very difficult to comment. 

One general point would be that what the guide camera sees and what the main camera sees can be quite different, especially with separate guide telescopes.

If you're satisfied with the images, I'd  say stay as you are. Look at the images, not the graphs.

Cheers

Yes exactly and with my scope and camera I can guide way, way, way worse than this and still get round stars so my questions are more for my interest rather than whether I need to change anything if that makes sense.

[scotty38]

Here you go 13 stacked subs with just darks but, for me at least, the stars look ok I think.... I just finished building a mini pc today so tonight was for testing it all out and never really intended to image.

 

 

Edited January 3, 2022 by scotty38

[wimvb]

3 hours ago, scotty38 said:

Thanks but not guiding at all?

You are guiding all right. What I meant was that since there were no tracking deviations larger than MinMo, there were also no correction pulses being sent to the mount in the time frame you showed. In that time frame, you would have had the same tracking results even without guiding. This isn't bad. In the end it's the quality of your images that matters.

[wimvb]

2 hours ago, scotty38 said:

200mm guidescope with 120mm mini so 3.87"/pixel and yes DEC very high as was imaging NGC281 but during calibration at nearly 0 it was showing 0.5/0.6 ish but I didn't grab any screenshots there as I didn't think much about it until I saw the 0.4" afterwards.

I realise it's much easier to guide there but really my question is can I believe the 0.4, is that what would be expected from this level of kit etc etc.

Guiding RMS is in my experience usually better at high altitude, because there is less air mass to interfere with the light, so generally a smaller effect of seeing.

[wimvb]

3 hours ago, scotty38 said:

Changing the scale to 2 won't make the 0.40" change, just that the graph will be more "jagged" as it were.

Correct, but it will be easier to see if anything unusual is happening. With my AZ-EQ6 I usually set the Y-scale to 2" so I can se any residual error. It turns out that despite using PPEC guiding, I have a residual periodic error of 0.1 - 0.125" in my guiding.

[scotty38]

Agreed, I suppose I can maybe try Orion as a better test as that should give the guiding a harder time given where it is. Having said that, again, this wasn't a test just that I noticed the low number and thought wow 🙂

[scotty38]

1 minute ago, wimvb said:

Correct, but it will be easier to see if anything unusual is happening. With my AZ-EQ6 I usually set the Y-scale to 2" so I can se any residual error. It turns out that despite using PPEC guiding, I have a residual periodic error of 0.1 - 0.125" in my guiding.

Yep understood and I realised what you meant some time after I replied.

[wimvb]

Enjoy it while it lasts. 🙂

I've had numbers this low, but it's very dependent on where I point my scope and how sky conditions are. I always get the best guiding when clouds are about to move in. Most likely the decreasing transparency also quiets down the seeing.

[scotty38]

Well yes agreed, I'd never seen a number this low before hence why I took a screenshot and made a big song and dance 🤣🤣

[wimvb]

Maybe time to load that mount with a longer fl and make it work for a living. 😄

Galaxy season is coming soon.

[alacant]

9 hours ago, scotty38 said:

I'd never seen a number this low before

Hi

The guiding depends heavily on seeing. One night you may have 0.40. the next night, on the same target could be double, or more. 

10 hours ago, scotty38 said:

the stars look ok I think.

And that's all that matters; your satisfaction:)

Here's an analysis which would be better done on linear data, but it's quite flat. If you don't like it, you can always have a go at it in software. There's a trend ATM which seems to want to obliterate stars.

I like your shot. It has natural looking stars. We spend €thousands on gear to take images of stars. then reduce them to oblivion!

Edited January 4, 2022 by alacant

[scotty38]

12 minutes ago, alacant said:

Hi

The guiding depends heavily on seeing. One night you may have 0.40. the next night, on the same target could be double, or more. 

And that's all that matters; your satisfaction:)

Here's an analysis which would be better done on linear data, but it's quite flat. If you don't like it, you can always have a go at it in software. There's a trend ATM which seems to want to obliterate stars. A pity!

Thank you, much appreciated and it's nice to see it's flat as I did not know that. I've not been at this very long but even I know that issues are never very far away, usually on the next clear night.