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#1
18-01-2018, 09:15 AM
 Bart Don't have a cow, Man! Join Date: Apr 2013 Location: Adelaide, South Australia Posts: 724
How do I calculate how good my guiding needs to be

How do I calculate how good my guiding needs to be for the scope and camera I am using?

Is there a formula that you can use or an online calculator.

When I'm watching my guiding graph in MaxIm, should I be observing the scale in arc-secs or pixels?

#2
18-01-2018, 11:49 AM
 codemonkey (Lee) Registered User Join Date: Jul 2013 Location: Kilcoy, QLD Posts: 1,719
Hey Bart,

Ray (Shiraz on here) once proposed the following formula to approximate the final FWHM:

SQRT(seeingFWHM^2 + scopeFWHM^2 + (2.35*guidingRMS)^2)

Playing around with these numbers should give you an idea of the impact of guiding on your final images. Seeing and guiding aren't really independent though but that should give a reasonable starting point.

One key thing is that RA and DEC RMS are similar. If they're different you'll get increasingly elongated stars / "motion blurred" images.

If RA and DEC are close in terms of RMS, as would be expected by a good mount, you will at some point become seeing limited (i.e. given enough focal length + aperture and small enough pixels). This now becomes a question of sampling... you don't want your sampling rate (arc-second per pixel) to be so high that you're consistently throwing away SNR chasing details you can't get due to your seeing.

The general rule is to view guide logs / charts in arc-seconds. Consider that your guide camera probably has different sized pixels than your imaging camera. This means that a 0.5px RMS guide error is meaningless without further information, because what you really care about is the magnitude of error relative to your imaging camera. If you set the guide software to show in arc-seconds then you can more easily interpret the results relative to your imaging camera, and hold more meaningful discussions with people since you're comparing apples with apples.

Cheers,
Lee
#3
18-01-2018, 06:47 PM
 Bart Don't have a cow, Man! Join Date: Apr 2013 Location: Adelaide, South Australia Posts: 724
Ok, lets pretend I am mathematically incompetent! (Not hard)

How do I measure the FWHM values presented?

The RMS calculation is simple enough.
#4
18-01-2018, 11:57 PM
 Camelopardalis (Dunk) Drifting from the pole Join Date: Feb 2013 Location: Brisbane Posts: 4,637
You want your guide error to be small enough that your imaging camera doesn’t really see the error.

My rough guesstimate would be...less than half the pixel resolution of your main imaging camera...so if each pixel can see 2”, aim for your error shown by PHD to be ~1”.
#5
19-01-2018, 01:14 PM
 Bart Don't have a cow, Man! Join Date: Apr 2013 Location: Adelaide, South Australia Posts: 724
1.23" seems to be the resolution so I need to aim for 0.6" or better?

QHY9 with 5.4 pixels size at 899mm.

If I was getting the odd jump to 1", would that affect my image clarity and stars sizes too much?

I'm trying to decide if I'm going to pull down my AZ_EQ6 and clean it out/tune it as it seems to guide well for a bit then have a little jump and then run well for a bit again.
#6
19-01-2018, 06:02 PM
 Camelopardalis (Dunk) Drifting from the pole Join Date: Feb 2013 Location: Brisbane Posts: 4,637
The only way to tell is to test it out and see how it goes. You will likely find that your guiding behaviour varies from night to night because of stability (or lack thereof) in the atmosphere, without having touched the rig at all.

If you get nice round stars, then great!

It’s too easy to get caught up in guiding parameters that don’t necessary translate into a visible difference to the end result over time. That’s when you know your guiding is going OK and there are likely other things at play preventing you getting smaller stars, notably the seeing from the equation Lee quoted.

Of course, this assumes good and stable focus of your scope.

The effect of the odd lurch away from ideal is hard to quantify. If it only impacts, say, one frame in ten, then when you stack them the trace/trail in the duff frames will get rejected. But if it’s ruining every frame you probably want to tackle it sooner rather than later.

With the lube in these mounts...I typically swing it about a bit in both axes to ensure it moves smoothly (clutches disengaged) before reverting to the index position and locking up the clutches. Not sure how much it helps, but occasionally mine will inexplicably seem to get stuck, where in one axis it looks like it’s going for a long walk, then after a short while the motors overcome the stiction and it resumes normal service.

I also find that PHD sometimes has a mind of its own. It’s always good to check the guiding calibration data to ensure it’s not out of whack. After all, the software is trying to correct for mechanical unpredictability, and it’s not an exact algorithm...
#7
20-01-2018, 07:22 PM
 Bart Don't have a cow, Man! Join Date: Apr 2013 Location: Adelaide, South Australia Posts: 724
Thanks for your thoughts Lee and Dunk, appreciated.
#8
21-01-2018, 07:54 AM
 codemonkey (Lee) Registered User Join Date: Jul 2013 Location: Kilcoy, QLD Posts: 1,719
Hey Bart,

Apologies for the late response. Your question re calculating the FWHMs is a good one, and not an easy one for me to answer--your math is probably as good as mine!

The scopeFWHM will be affected by the aperture of the scope, the quality of the optics, even the size of the pixels. My intention in posting that formula was more that if you plug in some vaguely reasonable estimates you can see how one variable affects the overall picture and then make some reasonable decisions.

Seeing in typical locations in Australia will range in the 1" - 3" so let's use 2" for seeingFWHM.

I have no idea what scope you're using and even if I did there's a lot of variables in scopeFWHM that I don't know how to account for, so let's just plug 1.23" in there (you're sampling rate).

Let's take the aforementioned 0.6" RMS for guiding and then we get:

sqrt(2^2 + 1.2^2 + (2.35 * 0.6)^2) =~ 2.73" FWHM

Now let's see what happens if our guiding RMS jumps to 0.8":

sqrt(2^2 + 1.2^2 + (2.35 * 0.8)^2) =~ 3" FWHM

And dropping the guiding RMS to 0.4" gives us:

sqrt(2^2 + 1.2^2 + (2.35 * 0.4)^2) =~ 2.5" FWHM

This doesn't entirely answer your question which was "How do I calculate how good my guiding needs to be" but it tells you, given all other variables remain the same, the impact that RMS guiding error has on your images, from which you can make a decision on what's good enough for you.

If your image scale is 1.23" and your peak guide error is only 1" then I'd suggest your images are likely to be excellent and I certainly wouldn't go pulling my mount apart if I were you.

Cheers,
Lee
#9
21-01-2018, 10:03 AM
 Camelopardalis (Dunk) Drifting from the pole Join Date: Feb 2013 Location: Brisbane Posts: 4,637
Nice examples, Lee
#10
21-01-2018, 06:32 PM
 Bart Don't have a cow, Man! Join Date: Apr 2013 Location: Adelaide, South Australia Posts: 724

I'm using a QHY9 through an FLT132. Using CCD Calc and other various web calculators, I get 1.23. I run a Tak TOA35 flattener that gives me the minutest amount of reduction, 0.998 or something.

I'll read it and let it sink in and do some thinking.

Thanks again, guys!

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