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Hello Fellows! My understanding so far is that tracking accuracy of an EQ-mount solely depends on the accuracy of polar alignment in that a three-star alignment won't outperform a one-star alignment in terms of tracking as good alignment delivers good GoTo pointing accuracy without increasing tracking accuracy. I am right or am I correct? 😎 Thanks so much and Cheers, Robert |
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The alignment is very important... that being said my mount has absolute encoders. I build a quick plate solve model... followed by a larger model.... usually 50 stars or so. I do very long subs with no guiding. Without going into too much detail all mounts are different. The bottom line in my opinion a good alignment in most cases is priceless. That same alignment is integral to your tracking. Once again my opinion and based on my experience. |
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It’s my belief that you are correct Robert. Polar alignment is critical for good guiding. My experience is that a three star model only outperforms a one star model for go-to accuracy and does not have any impact on guiding whatsoever. I use the Sharpcap polar alignment routine and it is simple and fairly quick to get the alignment done very accurately. |
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| In my experience the polar alignment is absolute crucial. Like @Michael Feigenbaum i am also using the SharpCap routine. I get the best results in guiding accuracy when the shown distances are below 10 arcseconds to the perfect alignment. With an EQ6 this is sometimes a bit tricky due to the mechanical inaccuracies, but with a bit experience i am mostly done in five minutes. |
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Hello all, please excuse me, but I have to disagree to some extend. Polar alignment is very often over estimated. Being in the range of a few arcmin leads to a remaining error (concerning FOV rotation) which is completely neglectable, since it is in most cases in the sub-pixel range. Chasing for an extream low PAE is not required concerning FOV rotation. Also it gets noticeable far beyond 10 Minutes exposures. Very often bad PAE is identified as culprit so it is not. Here is the link for the exact calculation according to Hook's equations which where published here: Journal of the British Astronomical Association, vol. 99, no. 1, p. 19-22 (Feb. 1989). You will find another essay on it here: http://celestialwonders.com/articles/polaralignment/PolarAlignmentAccuracy.pdf More readings are linked inside the the PDF. Also I want to reference the (unfortunately only in German available): "Handbuch der Astrofotgrafie" (Ulrich Dittler, Axel Martin, Bernd Koch; OCLUM, p. 177): Here you will find a diagram showing that PAE of 4 arcmin is sufficient for a 60 minute (!) exposure at 45° degree altitude without visible error of a guiding star in the very corner at medium pixel size. (Since I do not want to infringe any copyright I do not to simply post the page here). What is much more impacting tracking and guiding precision is refraction, since tracking speed is dependent on humidity, temperature and air pressure. Esp. the lower the altitude angle is. That is also the reason why some mounts track via both axis and use refraction correction e.g. 10 Micron. @Rhett Herring: I have seen you are using a 10M Mount. There is also a very good statement from 10M about this topic, which states there is no need to be obsessed about extreme low PAE (unfortunately the forum is not public, needs registration): https://www.10micron.eu/forum/viewtopic.php?f=16&t=1397 Here you see also that a PAE of approx. 4 arcmin is sufficient for 15 minutes exposure with a 4000x4000 sensor without any visible FOV rotation at 80° altitude. There is also a bunch of you tube videos where well repudiated APs and SW writers come to the same conclusion. Conclusion: Extreme low PAE is not required. Do not obsess about and waste time to minimize it too much. There are much more impacting effects (refraction, thermal effects, bending, cable drag, imbalances, flexure, play in focuser and mount, and many more....). Usually PAE is not the culprit, but of course it is not wrong to have a low PAE. It also depends on the mount's correction capabilities e.g. working with models, dual axis tracking or RC. With 1-2 arcmin PAE you are absolutely on the save side concerning FOV rotation for most use cases. |
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| @Ruediger this is interesting. I understand the reason for your (and other) conclusion, but it differs from my personal experience. I remember one evening where my guiding graph showed not so good values after polar aligning at about 1 arcmin. I stopped the session, realigned at about 10 arcsecs and the guiding graph was way better. I know that there might be some other reasons you mentioned responsible. Of course i do know well that personal experience is no proof. But i think i will stick to my arcsec-alignment - it does not consume a lot of time and i am on the safe-side :-) |
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Michael Timm: Hi Michael, please do not get me wrong: Of course, the less the PAE is, the better the results are. If you get better results when fine tuning your PAE you may have an other issue which multiply with PAE. PAE is only one screw to adjust and if it works for you it is perfect. But you might even get better results when you are eliminating the dominant problem. But of course, it is always a matter of effort which bug to crunch. Finally the result is the proof. I just wanted to bring some scientific facts and also some threshold figures, since this witch hunt for low PAE is a permanently repeated rumor and people spent uncounted hours to minimize PAE unsuccessfully and get frustrated since they do not get rid of their star's elongation. As said, that is not the number one trouble maker. |
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Ruediger: I totally agree! |
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With less than perfect mounts a low PAE is a ticket for good tracking corrections. I used to spend HOURS to get it right for a more or less permanent setup. The amount of peripheral field rotation is not the issue here but rather the mechanical errors (of all sorts) the act of moving 30kgs of stuff with sub-arcsecond accuracy with ordinary mounts that ordinary folks tend to use (e.g. NOT an ASA or 10Micron). Nowadays I use SharpCap to do that. Still not quite sure whether a 30" error is any better than 3' but certainly beats 30' ! |
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Ruediger: Very comprehensive explanation @Ruediger... I agree completely. The point I was trying to make was PA is qutie important but that adequate PA is not difficult to achieve and should not drive people nuts!! |
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Michael Feigenbaum: Thank you @Michael Feigenbaum . I totally agree with you. The basic question is / was: When is it good enough? I am usually satisfied if I am below 1' and happy when below 30". Much less PAE is only snake oil, because any small thermal changes will ruin this result. Just to give an example how wired things can be: I know following case which really was a tough one: One AP suffered from star elongation in his new obsy - but not always (high end mount on steel pillar, roll off roof). It turned out, it was the white painted wall of the observatory which reflected sunlight onto one side of the pillar and that heated up and cooled down during the night. This was enough to influence geometry asymmetrically. Depending on the temperature and when opening the roof the alignment was gone. So depending when he opened the roof he was lucky or not. Seriously, to obtain and maintain a real precision in arcsec range is with affordable amateur equipment is (almost) not possible and fortunately not needed. It is important to know the limits you can achieve. Also the scientist know and work with the remaining errors. You cannot not cheat physics.  |
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In low end mounts (like everything that I own  ) good PA is important because correcting it requires micromovements in DEC which low-end mounts suck at. If a low end mount needs to make many corrections in DEC, things go downhill pretty quick.That said, about 2 arcminutes is perfectly adequate if you have PHD and roughly 1 arcsec imaging resolution. 1 arcminute is stellar. Usually when we are talking about good PA we are talking about the improvement in PA when you use a computer assisted method or manual drifting, compared to using the mount's polar finder which tends to be horribly uncollimated. So it is more like 1-3 arcminutes vs 10-30. Also, since it was mentioned, I think PA errors like 10 arcseconds are completely unrealistic. Seeing, mechanical errors, guidescope aberrations, different star profiles easily make the same PHD polar alignment routine converge anywhere from 1 to 3 arcsecs of error with slightly different angles too, without me having moved anything in the mount. Cheers, Dimitris |
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Also, since it was mentioned, I think PA errors like 10 arcseconds are completely unrealistic. Seeing, mechanical errors, guidescope aberrations, different star profiles easily make the same PHD polar alignment routine converge anywhere from 1 to 3 arcsecs of error with slightly different angles too, without me having moved anything in the mount. Hi @dkamen totally agreeing. Maybe a short tip I want to share, which worked out quite well with any GoTo mount for me. 1. Clear all stuff (model, PA corrections, or what ever) and set proper time and location 2. Roughly point to north and level your mount 3. Slew to any bright star via GoTo. They mount will miss it. 4. Center the star with camera or high power cross hair only with Atl and Az knobs. 5. Repeat 3 and 4 one more with another star By that you usually have an PAE below 1 max 2 arcmin within only a few minutes. I even achieved 30 arcsec PAE in the very first run. Absolutely important is proper location and time settings (also temp, humidity and air-pressure for refraction correcting mounts). That are the key factors. Remaining tuning is for evenings where you are bored or you are invited at your mother-in-law. Than you can say "I have to fine tune my PAE - Sorry"  |
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Ruediger: Lol...🤣🤣🤣 |
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| @Ruediger @Michael Timm @dkamen Hello friends, Obviously, I have triggred an interesting discussion beyond the scope of my question :-) Actually, I merely like to confirm whether an accurate good GoTo (1-3 stars) alignment can improve tracking (good PA provided). I assume that the tracking accuracy depends on PA accuracy only (mechanical errors put aside). I believe that @Michael Feigenbaum has already answered my question in his first post :-) Cheers Robert |
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Ruediger: I belong to that forum. I have gone 60min with no issues. Too lazy to test beyond that. For me not having to guide is just one less thing to worry about and fewer wires. I have a refractor so even in breezy conditions I don't worry. In the past I had an SCT, I would guide with that when needed. |
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I didn't know 4 arcminutes of error were this forgiving . I guess it depnds on your focal length but I am assuming you are not talking about 14mm lenses . Having experienced a far worse performance even with 2 arcminutes, this discussion actually inspired me to take a good look at the worm gears of my EQ35M Pro. Turns out DEC was too tight in one direction (explains why it had problems guiding South) and RA was too tight in one direction and with a small play on the other one (explains fits of rage in RA). Now if only the clouds went away... |
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I didn't know 4 arcminutes of error were this forgiving . I guess it depnds on your focal length but I am assuming you are not talking about 14mm lenses . Hi @dkamen , I am sorry but it is not.  FOV rotation is dependent on the FOV and only indirectly to focal length. The give parameter match for medium FL of 3000 mm.You can calculate it yourselves here: http://celestialwonders.com/tools/polarMaxErrorCalc.html An example:  |
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I didn't know 4 arcminutes of error were this forgiving . I guess it depnds on your focal length but I am assuming you are not talking about 14mm lenses . Nor sure its my turn to chime is but according to Mount Monitor I'm usually less or right around 1 arcmin |
