New HEQ5 Pro - best algorithm?
JST200
- manual PA
- EQMOD Multi-point Align
- Calibration
- Guiding Assistant run
- Drift Align
- Guiding Assistant run
- 3hr imaging run
- meridian flip
- 1hr imaging run
bw_msgboard
Hi Jim. It’s good to hear you’ve made a lot of progress and have been able to get some satisfactory images – that’s really what this hobby is all about. J I think you might have a misconception about what we mean by “seeing” – it really has nothing to do with the brightness of your night sky. Here’s one simplified explanation, an excerpt from the tutorial on analyzing guide logs:
You can’t get very far looking at guiding performance without coming to grips with astronomical seeing. This is a complex subject, not something to try to deal with here. But the Cliff’s Notes version goes something like this. “Seeing” is the term given to the positional jitter and sudden brightness changes of stars we see (or image) through a telescope. It is atmospheric turbulence, caused by the movement of thermal cells in the Earth’s atmosphere, and there’s basically nothing to be done about it. Light is refracted as it passes through each atmospheric cell, so when you look at a star, you’re really looking through a column of air that is behaving like a column of little lenses. That might be ok except that the refraction of the light by each cell depends on the temperature of that cell, and the cells generally have different temperatures. And of course, the atmosphere is very dynamic, so these elements are all moving around at various speeds, coming into and then leaving the column of air you’re looking through. Thinking of it this way, it’s a wonder we can image anything. Particularly with longer focal lengths, this atmospheric seeing is the single biggest source of the guide star movement we see, and we’re stuck with it. Can’t we guide it out? The short answer is ‘no.’ The longer answer is also ‘no.’ The movement of the atmospheric cells means the guide star position is changing at rates of 10’s to 100’s of times per second. You aren’t going to be able to measure it and react to it nearly fast enough, even using amateur-grade adaptive optics devices. Professional observatories are able to do it to a large extent by employing very expensive measurement devices, artificial stars, and mechanisms that can both deform the mirror and shift the image at very high frequencies. That’s not us.
For many amateurs, the worst sources of bad seeing are in the area right around the telescope – convection from hard surfaces and rooftops, tube currents, things like that. To some extent, those can be improved with a certain amount of attention.
Your guide log shows you’re generally getting guiding accuracy right around 1 arc-sec or so, and you should typically be getting nice round stars in your main images. Your RA performance is generally a bit worse than Dec, a common situation. It looks like there’s an uncorrected periodic error in the mount of about +/-15 arc-sec, so you might want to look into applying a periodic error correction to your mount. You can consult the PecPrep documentation and look on one of the EQ-related forums to see how others have fared. You can also try using the PPEC guide algorithm for RA – if you do that, specify an initial period length of 680 sec and leave the auto-adjust period box checked. I don’t see any reason to change anything else, the Declination guiding looks pretty reasonable for a mount in this class.
Judging from where I think you are in the whole learning curve for astro-imaging, I wouldn’t advise spending a lot of time at this point trying to improve the guiding – or at least not until you encounter specific problems or find that you’re having to discard too many images. If you do run into problems, we’ll be happy to look at your logs and suggest ways to sort them out.
Have fun,
Bruce
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Chuck Porter
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bw_msgboard
Hi Chuck. I’ll answer for Jim since I just looked at his guide log. He did two GA runs and both reported Dec backlash of around 14 arc-sec or about 1 second of time at his configured guide speed of 0.9x sidereal. In subsequent guiding sequences, the backlash was handled well by the PHD2 Dec backlash compensation algorithm and there were no issues with guiding in both Dec directions.
Bruce
Chuck Porter
bw_msgboard
From:
open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Chuck Porter
Sent: Monday, September 24, 2018
9:22 AM
To: open-phd...@googlegroups.com
Subject: Re: [open-phd-guiding]
New HEQ5 Pro - best algorithm?
Thanks Bruce.....
Your comments seem to speak favorably toward the GA reported backlash with the belt drive system......it sounds so......as 1 sec beats my 4 seconds with worm gears. I guess we would need to know about the reported backlash prior to installation of the belts.
3.5 to 4 sec is apparently too long for the PHD2 backlash algorithm not to recommend guiding in one direction.
It’s a judgment call in the GA, there’s no well-defined upper limit. But long compensation guide pulses can allow the RA tracking to wander further off the mark before the next exposure and next RA guide command are issued. In any case, your guiding performance (based on the last log I saw) shows that Dec guiding, even in ‘auto’ mode, is not your limiting condition. So if you magically removed the Dec backlash and had much-improved Dec guiding, you’d have elongated stars in every image because of the RA errors. That’s why you usually have to look at mount errors on both axes and try to make improvements that keep things roughly similar.
Bruce
Chuck Porter
JST200
You are of course right... I did have a miss-understanding of the detail of "seeing". So thanks for the article - yet more to read and digest! lol I was mixing light pollution in.
Sorry, Chuck, I can't give you any data prior to the belt mod, as I bought it from Rother Valley Optics as an installed-by-them option.I did so because I wanted the mount to be as quiet as possible and also thought it would have a big improvement on backlash. As far as I am concerned, both appear to be true.
bw_msgboard
Sent: Monday, September 24, 2018
11:35 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] New HEQ5 Pro - best algorithm?
Hi Bruce,
Thanks for
you input and thoughts.
You are of course right... I did have a miss-understanding of the detail of
"seeing". So thanks for the article - yet more to read and digest!
lol I was mixing light pollution in.
What I meant to mention was that I live 10 miles from Manchester Airport (which is a major waypoint for a phenomenal amount of air traffic), so we can have 10 or more vapour trails across the sky at any one time). Might this affect my "seeing" and therefore the way I guide? The article seems to suggest not, but I thought that that was part of the reason for the ZFilter algorithm?
I don’t think this sort of thing is likely to have much effect on your guiding. Seeing is best dealt with by not “chasing” it – that is, by using guiding parameters that avoid corrections that are likely caused by high frequency seeing effects. All of the guide algorithms can do that, they might just take different approaches – but there’s no particular guide algorithm that’s necessarily better than another in the face of seeing. One quick way to judge this is to watch the level of guiding activity in Dec. If you’re seeing Dec corrections more than 20-30% of the time coupled with lots of direction reversals, you’re probably chasing the seeing. The simplest way to fix that is by increasing the min-move values until the Dec guiding activity subsides. This is *not* true for RA where guide corrections normally happen very frequently.
That said, I don’t see any evidence in your data that you were chasing the seeing. Your guiding performance is currently limited by RA, a pretty common situation with these mounts. Take a look at the graph showing RA (red) and Dec (green) performance over this interval:
You can see that the Dec movements generally stay within an envelope of +/- 1 arc-sec which we can say is mostly due to your seeing conditions. But the RA shows a much wider range of rapid movement, more like +/- 2 arc-sec. And there are a number of places where the guide star displacement was 3 arc-sec or greater. That’s why I suggested you focus on RA and look into periodic error correction.
Having acquired a mount that I thought might provide a visible periodic error (I think it was just lost in the jumble on my EQ3/2), I turned on PEC Recording in EQMOD, though not applied it. It did show a periodic error. So perhaps there would be no harm in recording it again and then applying it? I did have a look at the log with EQMOD PECPrep, which I think showed a cycle of 680s - though I confess, I'm dabbling in something I have't got to grips with yet. How did you identify that the 680s period?
The PHDLogViewer tool is what we use to analyze these logs. Here’s an estimated view of the native periodic error in your mount (right-click, Analyze selected, Raw RA):
This is a pretty strong signature and should be addressable by periodic error correction. If we use the frequency analysis tool in the LogViewer, we can see the major contributor is an oscillation in the 680-690s range:
https://openphdguiding.org/phd2-log-viewer/
I think this lines up with the native worm period on your mount.
I loved your comment "Judging from where I think you are in the whole learning curve...". I know I'm still at the bottom,cks when I read some of the posts on this and other astro forums!
I didn’t mean that in a negative way, not at all. We *all* started at the bottom of the curve and generally had to stop climbing at some point based on time, money, and interest level. I was trying to help you avoid the tar-pit of spending tons of time analyzing and adjusting your mount while getting nothing useful done in terms of taking pictures. I prefer the approach of focusing on whatever problem is limiting your satisfaction with your images – and oftentimes that can be post-processing, nothing at all to do with mechanics and gear. At least in my experience, that approach means you’re more likely to be fairly happy with how you’re doing and less likely to just give up in frustration.
Have fun,
Bruce
Thanks
for answering Chuck's question for me. I wouldn't have know where to look to
find the answer!
Sorry, Chuck, I can't give you any data prior to the belt mod, as I bought it
from Rother Valley Optics as an installed-by-them option.I did so because I
wanted the mount to be as quiet as possible and also thought it would have a
big improvement on backlash. As far as I am concerned, both appear to be true.
Sorry, your response seems to have raised a lot more questions. Hope that's OK??
Thanks, Jim
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JST200
I know it can be all too easy to take the written word the wrong way and the need for sensitivity. But I really didn't take your comment in a negative way. I truly found it amusing. So in turn, I apologies for perhaps given that impression :-) I really enjoy reading what I think of as the "team's" posts. Your patience and attention to detail is brilliant and you all deserve a comprehensive round of thanks!
JST200
Chuck Porter
JST200
I guess you seen the YouTube videos about the Rowan Belt, by them?
http://youtu.be/yhq1gpOTs0c
You still get the motor whine, but the terrible gear crunching has gone - so less noise to disturb the neighbours at unsociable hours! So has the backlash from the drive gears, though there is still the worm gear backlash.
Well worth the ~£100 in my opinion.
Cheers, Jim