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1 Analyzing PHD2 Guiding Results – A Basic Tutorial Introduction At some point,

1 Analyzing PHD2 Guiding Results – A Basic Tutorial Introduction At some point, most imagers want to analyze the guiding performance they’re getting. They may be trying to identify and fix particular problems or perhaps just trying to get the best performance they can from their set-up. Either way, the PHD2 guide logs are the best source of data, and using them is far better than trying to look at the guiding graph in real-time. The latter is useful as a quick check on how things are going, but any serious problem diagnosis or tuning requires a longer time window than what is available in the real-time graph. In this document, I’ll be using Andy Galasso’s PHDLogViewer application as a way to show common problems and guiding behaviors. This is the tool we most often use for helping PHD2 users, and it is likely to serve you well. What I’ll discuss in this document is simply the result of my own personal experience and research. I don’t claim to be an expert on telescope mount mechanics, and I still encounter guiding behavior that I can’t explain. But I’ve been using some version of PHD since 2006, and I’ve analyzed hundreds of guide logs in the ensuing years, both my own and those from other PHD2 users. Hopefully, what I’ve learned can help you understand your guiding results or can at least accelerate your learning curve a bit. Basics – A Big Picture View Summary Statistics When looking at these graphs and statistics, you want to look at the measurements in units of arc-secs (a-s). If this isn’t obvious to you, take a look at “Image Scale and Measurements in Arc- secs” in the Appendix. To get a big picture view of your guiding, start by looking at the RMS numbers that appear to the lower right of the graph: 2 When PHDLogViewer loads a guiding session, the “RMS” value is the standard deviation of all the star movements in that session In this example, about 68% of the movements were below 0.59 a-s, the remainder were larger. Just as a rough starting point, you probably want to see your total RMS value be 1 a-s or lower. How much lower will depend on many things, especially your seeing conditions and the quality of your mount. Now look at the individual RMS values for RA and Dec to see how they compare. With most mounts I’ve seen, it’s common for the RA value to be somewhat higher than the Dec value, probably because the RA gear system is always running while the Dec gear system is not. There are also some seeing-related things that can tend to make the RA displacements a bit larger and more frequent. However, if the RA and Dec RMS values differ by a large amount, for example 2-3X, you are likely to see elongated stars in your images. So what happens if these two values are nearly equal but are also quite large? That’s likely to lead to bloated stars in your images, and your resolution and sharpness will suffer. It’s not enough for the stars to look round – you want them to be both small and round, where “small” is determined by your optics rather than by guiding errors. You’ll sometimes see forum posts from people who say “I get perfectly round stars even with 30 minute exposures.” Well, that’s probably a good thing – but how large are those star images, and how do their sizes compare to what they get with short exposures of 10-20 seconds? For many of us, guiding rarely runs all night without some sort of glitches. When looking at the overall statistics, you often need to filter out these events. You’ll probably need to address them at some point, but it’s good to get a feel for the overall guiding performance without having the numbers polluted by these unusual events. Consider this section of a guide log: Something bad happened shortly before 22:47, a huge excursion in RA that caused the total RMS to reach 2.6 a-s for the time period shown. But the guiding before and after that event looks much better. You can use PHDLogViewer to isolate those regions to see how things were going. When I did that on this data, the statistics looked much better: a total RMS of 0.5 arc-sec before and after the unusual event. This tells you the overall guiding was going pretty well, but you'll need to figure out the source of this large RA excursion. Doing that will be discussed in the section on “Gremlins.” 3 Seeing When you look at a guiding graph, most of the rapid star motion you see is caused by seeing, something you can’t do anything about. If this isn’t a familiar subject for you, take a look at the section on “Astronomical Seeing and Guiding” in the Appendix. If the rapid movements are large, you should check the exposure time being used – longer exposure times can help to reduce the envelope of seeing deflections you see (see Appendix). These rapid excursions are reflected in the RMS statistics and may even be the primary contributor to them. It may help to run the Guiding Assistant periodically to get a sense of the typical seeing conditions at your site. Poor seeing can’t be corrected by guiding and you’ll simply have to do the best you can. Over time, you’ll probably learn what to expect on most nights and will quickly recognize the effects of seeing when you look at the guiding graphs. However, if you’re “chasing the seeing” because of poor choices in guiding parameters, you’ll see that too many of these seeing-induced star movements trigger guide commands, and this will often result in a saw-tooth pattern in the graph. Identifying those sorts of problems will be covered later. What’s on the Graphs When you look at the details shown in the guiding graph, you’ll normally start by looking at two things: how far the guide star moved from one exposure to the next, and how PHD2 reacted to that move. Here’s a typical part of a guiding graph zoomed way in to show the details: The star movement is shown by the connected lines, blue for RA and red for Dec. The rectangles show the guide commands that were generated by PHD2, and their heights indicate the relative sizes of the guide pulses. Don’t over-analyze the up/down convention for displaying the rectangles. This orientation was chosen to reduce clutter and because it is more intuitive – you want the command to “push” the star in the direction opposite to its apparent movement. If you look closely, the rectangles always trail the star motion by a small amount because PHD2 is reacting to the move seen in the previous exposure. What you see here is pretty typical – sometimes it only takes one guide pulse to restore order while in other cases it may take multiple guide pulses in the same direction. You’ll see that some star displacements don’t trigger any guide pulses at all. That’s usually because of the “min-move” setting for that axis but it can also be due to intentional damping in the guide algorithms. You also need to pay attention to the scale at the far left to keep things in context. Beginners often look at these graphs and think the guiding is horrible because there seems to be so much motion. But the motion you see here is generally falling within a range of +/- 1 arc-sec, and the total RMS error for this section is about 4 0.6 arc-sec. In this example, the guiding was about as good as the seeing conditions would allow. Easy Problems To get warmed up, we can start by looking at some easy problems to diagnose. One of the easiest is something that looks like this: Wow! Absolutely perfect tracking, the guide star never moved! Um…… no. This is what happens when you’re guiding on a hot pixel, which is the same as not guiding at all. This should almost never happen, but if it does you can try the following: 1. Let PHD2 auto-select the guide star (Alt-s). It can be hard to visually distinguish a hot pixel from a faint guide star when you're just peering at the display. 2. Be sure you’re using either a dark library or a bad-pixel map. 3. Apply a 2x2 or even 3x3 noise reduction filter (brain dialog/camera tab). Now let’s look at another problem, one that’s more likely to happen if you’re using a guide cable attached to your camera and the mount's ST-4 guiding interface: 5 It looks like guiding got off to a reasonable start until the star took a big lurch to the west (down). But in this case, PHD2 was never able to recover. Look at the continuous stream of ‘east’ guide commands that were sent for the next two minutes – nothing happened! The guide star was never returned uploads/Management/ analyzing-phd2-guide-logs.pdf