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Apologies, it's surprisingly difficult to Google an answer  In Nina my auto guiding shows (for example) 0.3 (1.8) I'm I correct in thinking the (1.8) is the RMS error ? If so, what is the 0.3 ? |
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I don't know Nina but I know math. Most guiding software reports errors in both PV and rms. The smaller number is typically the rms and the larger number is typically the peak to valley (PV) of the guide error. For randomly distributed values, the rms typically ranges from 1/4 to 1/6 of the PV. Just be sure that you know the units. It is common to be able to express the errors in units of either pixels or arc-seconds. Arc-seconds are the most useful when comparing the performance of your system with other systems and when you report numbers here, arc-seconds are generally the most useful units to use. John |
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TiffsAndAstro: The 0.3 is probably the error in guide camera pixels, and the 1.8 would be the error in arc seconds, assuming you have entered your guide scope focal length and guide camera pixel size correctly. The 1.8 is the more important number, as John has stated, since this allows you to directly determine how this error would impact your subs. As an example, if your image scale is 1.4"/pixel for your main scope and camera, the 1.8" error tells you that your errors, on average, are not restricted to within a single pixel, which will result in distorted stars and loss of edge resolution of whatever it is you are trying ti image. |
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 In NINA, the first number is the guide error in PIXELS. The second number is ARCSECONDS. The clue is the " which means seconds. The drop down menu is specifically for the graph display. Generally speaking this should always be in arc seconds, but if you use an OAG and have the same image scale for your guider AND imaging camera, then it can be useful. |
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| @TiffsAndAstro if you get stuck on something, I actually upload screenshots to Chatgpt and find some surprisingly accurate answers. It tends to fail with highly specific or contextual questions though. posting your initial question, this was the response. The forum will likely get you the best possible answer, but if you can't wait... ****chatgpt's reply**** In the scenario you described using N.I.N.A. (Nighttime Imaging 'N' Astronomy) with PHD2 for autoguiding, the notation "0.3 (1.8)" likely represents two specific measurements related to the performance of your guiding system:
Thus, in your example, "0.3" indicates the current error is quite low at the moment you checked it, suggesting good immediate tracking performance, while "1.8" as the RMS error suggests that over time, the average error has been a bit higher, which might prompt adjustments in your setup or conditions for improved long-term accuracy." ********************* |
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Nick Grundy: That answer is quite wrong. With all due respect, please don't use ChatGPT for Astro. It's so incredibly clueless 95% of the time. |
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John Hayes: been told it's pixels(RMS) Going to reset phd2 to defaults as it was better then  |
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Arun H:TiffsAndAstro: great explanation Ty. My image scale is 2.02 and id like half that for my RMS. I have a sw gti so that might be overly optimistic of me. I've managed my cables a bit now, can balance my gear better and have a bit better focus on my guide scope. will test asap, maybe tonight, maybe in 6 month's time ;( |
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Brian Puhl: Couldn't agree more. ChatGPT seems incredibly confident when it gives you a wrong answer. It is also very apologetic if you point out that its answer is wrong. I wouldn't use it for anything important. |
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Brian Puhl: not sure I actually noticed the " but to be fair I'm old and laptop screen is small |
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Ty all for your help, really explanatory. I'm going to reset phd2 as I've made some changes to wiring and guide focus which should help. |
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chatgpt's explanation was inaccurate, but interestingly, came to the same conclusion that 1.8 total RMS was the important figure and most impactful. That's with pretty limited input. (and available immediately) The AB forums are a great resource for expertise, but everyone sleeps sometimes. (and you can sometimes get very contradictory opinions and solutions) But obviously, it's just one source. I'd always try to get more. For phd2, the google group has good resources as well https://groups.google.com/g/open-phd-guiding |
