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Drift Alignment with PHD2 - The Bookmark Technique

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#1 Jon Rista

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Posted 16 August 2014 - 07:32 PM

PART 1: Introduction

 

Welcome to the official tutorial on using PHD2 and the bookmark technique to dial in extremely precise polar alignments. Many of

you may already be aware of the fact that PHD2 can be used to perform polar alignment. What you may not be aware of is that polar

alignment with PHD2 can be extremely accurate, and that it can be done without necessarily having line of sight on polaris. For

those of you who have an obstructed view, or live near the equator, aligning with PHD2 is a free, easy way to achieve very

accurate polar alignment very quickly.

 

rTsojEr.jpg

 

I am also sure some of you have tried PHD2 drift alignment, and given up on it. You found it clunky, difficult to avoid cycling

back and forth between one bad alignment and another, etc. You may have even put money into another tool to solve your alignment

problems, tools that may rely on the (potentially inaccurate or imprecise) pointing accuracy of your mount for "iterative"

alignment. This tutorial aims to introduce you to the key feature of PHD2 that alleviates these problems, making polar alignment

easy, fast, and consistently effective...at dialing in VERY accurate PA: bookmarks.

 

Wmfczao.jpg

 

If your like me, you've noticed the bookmark feature of PHD2 throughout your time using it, but never realized with a use for

them. While trying to improve my PA a few weeks ago, early August 2014, I was struggling with PHD2 drift alignment while trying

to move the guide star the right amount, relative to it's previous position. It hit me at that point that bookmarks would be a

perfect way of guaging how far, and in what direction, I needed to move the guide star, in order to properly refine my alignment.

 

The key benefit of polar aligning with PHD2 is it requires only the adjustment of your altitude and azimuth adjustment knobs or

bolts. This is a benefit, in that it means you can dial in an accurate polar alignment without having to rely on the pointing

accuracy of your mount (which may be limited to 1-2'). This alignment technique does not require iteration between two stars, and

therefor does not require you to move the mount at all. At least, not beyond the initial pointing towards your alignment star

(near the meridian and CE for azimuth, or horizon and CE for altitude). Once pointed, all adjustment is done purely with the

alt/az adjustments on the mount.

 

All of the frustrations with PHD2 drift alignment can be eliminated by using bookmarks. By marking the current position of the

star before each adjustment with alt/az knobs, you have a clear guage of where the star was, and after a couple adjustments, you

know exactly what effect turning your adjustment knobs one way or the other has on the star position relative to where it was.

This takes a lot of the guesswork out of PHD2 drift alignment, which is probably the greatest source of frustration, and a key

reason why it can be just as timeconsuming as other methods. Bookmarks also help you define a shrinking range as you add more

bookmarks, guaranteeing that, somewhere between two of those bookmarks lies a perfect polar alignment.

 

With just a few iterations of drifting, placing bookmarks, and making smaller and smaller adjustments, you can dial in a polar

alignment less than 0.5' (<30") off the NCP, and if your persistent, within a few more iterations you could dial in a polar

alignment less than 0.1' (<6"). With a little bit of practice, performing fast, highly accurate polar alignment to ten arcseconds

off the NCP can be achieved in *less than 15 minutes.* This screenshot shows the kind of tracking and guiding performance that

is possible with a very accurate polar alignment:


Edited by nitegeezer, 21 August 2014 - 11:13 PM.
Edited at request of Jon

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#2 Jon Rista

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Posted 16 August 2014 - 07:35 PM

PART 2: Getting started

 

First things first, you need to get PHD2 ready to use for drift alignment. You will need to start it up, point at a star near the

meridian and celestial equator (for azimuth alignment, which the drift align wizard defaults to on startup), and calibrate PHD.

Calibration must be performed before doing drift alignment. If you forget to calibrate, the drift align wizard will only

momentarily display a message in the status bar indicating that calibration is required for the wizard to work.

 

After calibrating, choose a high SNR star (over 10) within the middle quarter of the frame. It is important to choose a star

here, as when you start adjusting, the star can move in any direction in the frame, by as much as a quarter of a frame initially.

Picking a star near the edge could result in adjustments moving the star right out of the frame.

 

fGW0Wvb.jpg

 

Before starting the drift alignment wizard, you should make sure that bookmarks are enabled (shown), and that you delete any

existing bookmarks. This will assure that you don't end up confused abot which bookmarks mean what (especially if you had tried

to drift align earlier, and ended up needing to repoint the mount.) It probably goes without saying, but saying it anyway for

good measure: Make sure you are pointed a bit ahead of the meridian, and any of your mount's stop limits, so you have time to

perform drift alignment without running into a meridian or horizon limit during the process. It also helps to make sure that

there are no trees or other obstructions that might find their way into the frame while your aligning.

 

Finally, set your exposure time to 1 second, so you can see an immediate response to any adjustments you make. It also helps to

set the history period to 200 ticks, and the y axis to 8. I've found that these settings are best for early drift checks...later

on, when you require longer drift checks, you might want to up the period to 400 ticks.

 

k60dsrT.jpg

 

Once all the above things are done, your ready to start the drift alignment wizard. You can find it in the Tools menu, Drift

Align. You should see a window like this appear when clicking on that menu option:

 

rTsojEr.jpg

 

The Scope Pointing feature will only be accurate if you are using ASCOM Pulse Guiding. If you are using an ST-4 cable to connect

your guide camera to the ST-4 guide port on your mount, the information here will not be accurate. If you are using ASCOM Pulse

Guiding, you can even control your mount from within the PHD2 drift align window, and it will show you how close you are to the

meridian and celestial equator, making it very easy to get right on top of it for really good drift check.


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#3 Jon Rista

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Posted 16 August 2014 - 07:43 PM

PART 3: Drifting and adjusting

 

Click the "Drift" button, and let PHD start the drifting process. PHD will automatically disable declination guiding, however it will guide in RA. Let PHD drift for a good 20-30 seconds...anything less, and the information is bound to be highly inaccurate. Especially early on in the drift alignment process, the first 10-20 seconds of drifting will be eratic, and the indicators (trendline and the pink circle around your guide star) will jump around, possibly quite wildly, until they "settle" and normalize around a given trend.

 

4mGhCqX.jpg

 

Depending on your luck, you may find that your polar alignment is quite good, or you may find that it is quite bad. As a general rule of thumb, if your drift is out by more than about 8', then you will need to tighten it up a bit before actually using bookmarks. The reason for this is an 8' or larger will often require moving your chosen guide star right out of the frame (larger sensors, like the Orion SSAG, may be viable up to 10-15'). I find it's best to try to get an initial polar alignment between 2-5' with whatever initial routine you use, as this is usually sufficient to keep the guide star within the frame throughout the rest of the process.

 

noVqdBc.jpg

 

Once you know the direction your guide star is drifting, and by how much, click the "Adjust" button in the drift align window. BEFORE you actually make any adjustments, click back into the PHD window, and on your keyboard type SHIFT-B. This will create a bookmark at the current position of your guide star:

 

lQgwRTz.jpg

 

For a better view of what a bookmark looks like, it's the cyan colored set of circles within the green guide star box:

 

7iGXAbA.jpg

 

Now you can make and adjustment with your azimuth knobs. You will see a pink circle around your guide star. This circle will follow the selected star once you hit align. This is supposed to be an indication of how far you need to move the guide star (with the azimuth knobs, not by moving the mount in RA or DEC!) in order to refine your polar alignment. It should be noted that this circle is only accurate when you are VERY close to the meridian/CE junction...within +/-5 in meridian offset and declination in the "Scope Pointing" box of the drift align window. If you are not using ASCOM Pulse Guiding, it may never be accurate, regardless, since it's accuracy depends on PHD having an accurate reading of where the mount is currently pointing. 

 

If you are close to the meridian/CE junction, you can move the guide star from the center of it's bookmark, to the edge of the pink circle, and that should dial in a fairly decent PA. If you are not close enough to the meridian/CE (which is probably the case, if you need a good 10-15 minutes to align, your probably at 0 DEC, and off the meridian by 15 degrees), then you will want to move the star about a quarter of the size of the frame at most. I have found that moving the star enough that it's drift "switches sides" helps you guage, with pretty high accuracy, where to move the star on the second move to quickly dial in a good, sub arcminute polar alignment.

 

qzUJLeK.jpg

 

To clarify, if you started out pointing to the east of the meridian, drifting two arcseconds in the negative (red plot in PHD drifts above the midline in the graph, and the "Polar alignment error: " indicator at the bottom left of the graph shows a negative number), you should initially try to adjust your guide star to drifting one to two arcseconds in the positibe (red plot in PHD drifts below the midline in the graph, and the "Polar alignment error: " indicator at the bottom of the graph shows a positibe number). (NOTE: If you are pointing west, these directions may be reversed.) Let it drift for about 30 seconds, and when the polar alignment error indicator and trend lines settle, place another bookmark.

Note: You may find that your first adjustment simply makes your original alignment of 2-5' even worse. This simply means that you need to adjust the opposite direction. It is important to look for this, as knowing which direction to turn your azimth or altitude adjustment knobs is important to quickly dialing in an accurate alignment.

You now have "bookends", as I call them. These two points should be your absolute outer boundaries. If you make any adjustment to the star that pushes it beyond these two points, your pretty much guaranteed to end up with a worse polar alignment. Somewhere between these two points is a perfect polar alignment. The task now is to refine your alignment, and use additional bookmarks to tighten the range within which you will find that perfect polar alignment.

 

EM6mSxH.jpg


Edited by Jon Rista, 16 August 2014 - 08:03 PM.

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#4 Jon Rista

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Posted 16 August 2014 - 07:47 PM

PART 4: Refinement
 
Now that you have your bookends, you need to make another adjustment. If you started out with your first bookmark at around -2' PA, and your second one at around -1' PA, then you know that roughly midway between those two points is an accurate polar alignment. The simplest approach to making the next movement of your guide star is to move it midway between the first and second bookmarks. My recommendation is to try to move it close to, but not exactly to, the midway point. If the star is currently at bookmark 2, move it about 3/8ths of the way back towards bookmark 1.
 
J8mMULy.jpg
 
Do some drift, and check your alignment. You may need to drift for a minute or two this time to get an accurate read. You should find that your PA is less than 1' now, and if you are lucky, it should be even less than 0.5' (30"):

 

is2Or5y.jpg

 

Click the Adjust button again, use SHIFT-B to set a third bookmark. You should now have three bookmarks, with the third inbetween the first two within the range for polar alignment. You should also notice that the pink PA adjustment circle is quite small, compared to originally, when it was quite large (in the case of this tutorial, it started out almost larger than the entire field of view at over 9' polar misalignment.) 

 

Kimdm6h.jpg

 

Now, which direction you move the star next depends on the direction it was last drifting. If you started out with -2' PA, then moved to +1' PA, then moved about 3/8ths of the way back to bookmark 1, you should be around +0.5'. That means you would still want to move the guide star towards bookmark 1. If, for whatever reason, you noticed that your PA was something more like -0.5', then you would want to move the guide star back towards bookmark 2.
 
Making this determination for which direction, between which two bookmarks, to move the guide star is critical each time you set a new bookmark, and are ready to make another adjustment. You need to know which two bookmarks define your "range" within which a better polar alignment may be found. With practice, it will become second nature to guage the direction in which your guide star should be moved to improve your polar alignment, but for now, the general procedure laid out in the previous paragraph is how you make that determination. Assuming that your PA was off by +0.5' at your last bookmark, make another small move towards bookmark 1 with your azimuth knobs. This move should be very small:

 

zTj0DgH.jpg

NOTE: You may find that you have trouble making such a find adjustment with your mount. Tuning mounts for better PA adjustments is beyond the scope of this article, but suffice to say, you may need to polish the contact plates between the mount base and pier/tripod, hypertune the adjustment knobs themselves, maybe even make some kind of PTFE/Teflon gasket to place between your mount and pier/tripod, etc. I know that I have to slightly loosen my center locking shaft on the Atlas to free up azimuth movement. Altitude locking bolts can often be difficult to adjust with lots of weight on the mount, so it might be useful to perform polar alignment with the lightest equipment you have (and possibly try to compensate in altitude for the normal weight you usually have, although in my experience, that is probably not as necessary as it sounds...I rarely drift in RA, and my PA is still extremely good overall.)

 
Your polar alignment, at this point, should be around 0.5' or less. As such, your drift will be very low, and may already be acceptable to you. If so, you can simply call it good here, and start working on your imaging session. If you want to really eliminate declination drift as an issue (you may have persistent issues guiding in dec, your mount may simply not guide in dec well as in the case of the Celestron AVX mount, etc.), you can keep refining. You will need to drift for longer. About two minutes drift or more with this level of accuracy is necessary to really see which direction your drifting, so you know which direction to adjust.
 
Drift for a few minutes, click adjust and place a bookmark, then make an adjustment in the appropriate direction. Each successive bookmark should get closer together, and the location of a true, perfect polar alignment should start to become clear. If it does, it should become quite obvious exactly where you need to place your guide star to get a highly accurate PA. Do so, and you should be able to forget about guiding in DEC at all for the nights imaging session. Here is the fifth move of the guide star, resulting in a polar alignment of 0.19':

 

IqoUh1O.jpg

 

I wanted to see if I could dial in an even better alignment, so I made one final move, and let the star drift for a longer time to get a better read on the alignment. Final PA for this session ended up being 0.05', or 3":

 

lzQ9ogY.jpg

 

Here are some full views of the PHD guide graph, showing the alignment and drift of the star over a period of several minutes:

 

7GuLTZ3.jpg

 

It can also be useful to check the target graph, which is point sampling plot of the star centroids over time. I like to set it to 400 samples, to get a good statistical read, and gauge how the samples are distributed. In this case, over a period of about three minutes, the rough peak-to-peak distribution of the plots is about 2", since the vast majority of the points fall within the 1" circle:

 

k2cUMIJ.jpg

 

A significant amount of the points in this plot are within the 0.5" circle as well. If your exposures are less than three minutes, then you probably couldn't hope for better than 1" P2P guided tracking. Finally, I also like to check my guiding RMS. If the overall is less than 0.5", I consider that very good for shorter focal lengths (<1000mm) (I honestly don't yet have enough experience with imaging at longer focal lengths to say how good it may be for imaging at say 2000mm or so.) In this case, it is 0.45". DEC RMS is 0.25", and RA RMS is 0.39":

 

Gsq55Qd.jpg

 

These are excellent for guiding alone. If you program and enable a PEC curve, these numbers should improve further, down into the 0.2x" range for everything, as PEC will take care of some significant sources of error, leaving behind more minor stuff. 


Edited by Jon Rista, 16 August 2014 - 08:31 PM.

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#5 Jon Rista

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Posted 16 August 2014 - 07:49 PM

PART 5: Caveats

 

Sometimes, once you start to dial in a really tight polar alignment, things stop behaving logically. You may expect that making a small adjustment in one direction will improve your PA...however once you make that adjustment, things seem worse. There can be a number of causes for this. For one, drift aligning on a night that has a consistent breeze, or any amount of wind faster than that, especially if it's gusty, can make drift alignment quite a frustration. I've found that trying to dial in a PA better than 1' on such nights is usually a fruitless endeavor, although with persistence, getting somewhere between 0.5'-0.75' can be done.

 

Sometimes, the relatively fast frame rate of 1s can become problematic at a finer drift scale. When your PA gets below 0.5' (30"), you may want to reduce your frame rate to 2s. This should help alleviate seeing as a factor, and your drift should be more consistent. You will need to allow the star to drift for several minutes at least when doing this, in order to get an accurate read, but it can and usually will be much more accurate. It just slows down the process.

 

Balance can play a big role in how easy it is to dial in polar alignments more accurate than 0.5' as well. You need to make sure that the worm and ring gear in your declination axis remain engaged. If your imbalance is not significant enough, you may still experience what I call "declination bounce", where your ring gear's teeth effectively "bounce" off the worm teeth. This becomes a greater problem when your pointed close to the zenith...so if your meridian/CE junction is closer to the zenith, you may find that you need a greater amount of declination imbalance to eliminate bounce as a problem. Bounce will usually manifest as an oscillation in dec around the midline of the PHD2 graph. If you let your star drift for long enough (many minutes), you'll eventually get an accurate read on your PA, and it may be quite good. Bounce simply increases the amount of time it takes to finish the drift alignment process, and in the long run, it can worsen your overal tracking performance in DEC (which kind of

defeats the point of having such a good alignment in the first place).

 

Finally, you may find that you have some drift in RA. A good drift align session will usually have bookmarks all lined up, probably along some diagonally oriented, but otherwise strait, vector. If your drifting a bit in RA, then your might find that your bookmarks also drift a little bit, along the diagonal oriented 90 degrees perpendicular to the angle of your azimuth adjustments. This isn't a huge problem, but if the movement is significant enough, it can make it a little more difficult and time consuming to find that perfect spot where a really accurate polar alignment exists.

 

If you plan to perform drift alignment in altitude, the last thing I'd stress is that you do your best to point as close to the horizon/CE junction as possible. If you point too far away from that point in the sky, PHD's readings tend to lose accuracy. Point more than about 20 degrees away from that point in the sky, and PHD's readings effectively become useless, as your drift will reflect both azimuth and altitude. Making "corrections" in altitude for that kind of drift has the potential to throw out your azimuth alignment.

 

It may simply be luck, but I have found that my altitude alignment is not as important as my azimuth alignment. It may simply be that I dialed in a very good altitude alignment in the past, and it has stuck (the dial on the mount itself certainly seems to be pointing at almost the exact altitude I live at, 39 deg 58 min north, just a hair less than 40 deg north.) If you do not have a good altitude alignment, the closer you can find a guide star near the horizon/CE junction, less than 20 degrees away, preferably about 10 degrees away to allow for some lead time before the star disappears below the horizon, but close enough for an accurate read, the better.


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#6 nitegeezer

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Posted 16 August 2014 - 09:41 PM

Wow, that is a great guide.  I just scanned through it, and decided a bookmark for this thread is necessary. 

 

Thank you for going to this effort.  I am in the early phases of learning to guide, and really looking forward to trying this.


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#7 rflinn68

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Posted 16 August 2014 - 10:56 PM

Very informative for sure. I also say lets make this a sticky on its own for a while and then later we'll move it into the Best Threads in B&II. Thanks for taking the time to post this Jon. I'm positive a lot of people will find it very useful. 


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#8 Footbag

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Posted 17 August 2014 - 06:20 AM

Awesome!  I don't use the bookmarks, but it sure seems handy!  May have to try it.  


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#9 Jon Rista

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Posted 17 August 2014 - 06:40 AM

I experimented with this a bit more tonight. I moved my mount, and I think my altitude is off now. I have no view of either horizon/CE junction at all from where my mount is now, so I decided to simply drift align right on my target. I ended up pointing to a few targets last night, but drift aligning down to >0.5' only takes about five minutes each time I do it now, and the nice thing about doing it this way is you fine-tune your declination drift for whatever target your pointed at. If you can't see the horizons to perform proper altitude drift alignment, this is probably the next best thing. 

 

Fundamentally, I think all that really matters is that your dec drift is low enough that it doesn't cause problems for your exposure length. Usually, getting PA down to >0.5' is good enough to have a very cooporative dec drift that can easily be guided. Getting PA down to >0.1' usually allows you to do without DEC guiding at all unless your at a very long focal length or doing very long exposures. 


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#10 tazer

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Posted 17 August 2014 - 07:18 AM

Excellent guide Jon. Thanks for putting it together.

 

This comment isn't meant to detract from your considerable and incredibly helpful effort, but it might be worth of a side note: Those guiding via OAG will (probably) find that the selected star jumps out of the frame on each adjustment until polar alignment is very close. What I do, via my OAG @ 731mm, is just select whatever suitable star is in the frame and keep going. So bookmarking may not be advantageous to them until the very end (at which time they may already be close to the precision limit of their alt/az knobs.)

 

Thanks again for your effort.

 

Edit: And since this is the _beginner_  & intermediate imaging forum, it might be helpful to remind people to make sure they have the correct focal length and guider specs entered into PHD2 so that the PAE PHD2 generates is accurate. I mention this only because I've done that before (I switch between OAG and guide scope sometimes) and been amazed at how accurate my polar is calculated to be. I am a perpetual beginner. :D


Edited by tazer, 17 August 2014 - 07:24 AM.

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#11 karambit27

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Posted 17 August 2014 - 09:20 AM

Thanks a lot for this Jon. I cant really see anywhere near the horizon/CE in either East or West for Altitude alignment. What is the best way for my to align the Altitude then?


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#12 telfish

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Posted 17 August 2014 - 10:21 AM

Excellent!


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#13 tjay

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Posted 17 August 2014 - 11:57 AM

Great guide Jon.  Perhaps this is something that can get incorporated into the PHD documentation.


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#14 Greyhaven

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Posted 17 August 2014 - 06:31 PM

Jon
Well done, I'm just learning PHD2 I'm using a SSAG mini set up and your efforts will help me greatly.
Thanks for sharing.

Grey
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#15 Jeff2011

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Posted 17 August 2014 - 07:32 PM

Well written tutorial Jon. My main concern is that I don't have a good view of the east or west horizon.  If I remember correctly, it is right around 20 degrees or so.


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#16 Jon Rista

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Posted 17 August 2014 - 07:48 PM

For everyone who does not have a good view of the horizon, I've found that is not really a problem. If you do your initial polar alignment (i.e. with a polar finder scope in the mount) well enough, that usually gets you close enough. This tutorial was all done with azimuth, which only requires a view of the meridian (the imaginary line bisecting the sky overhead) and celestial equator (which should be up in the sky somewhere unless you are very close to the poles.

 

So my recommendation is perform a good initial polar alignment with your polar finder scope, then just do azimuth drift alignment. And see how that goes. If you still have some drift, well, you can always just realign and minimize dec drift for wherever your pointing. With practice, this method is EXTREMELY fast. I redid PHD drift alignment three times last night, and within five minutes each time, my PA was less than 30 arcseconds off the pole. That's good enough, so long as you still guide DEC. It's not good enough to do away with dec guiding entirely...for that, you want to dial in an alignment better than 6 arcseconds (0.1'), and that takes more time, and it's best to drift align both azimuth and altitude properly for that kind of precision.

 

Additionally, to achieve it, you may have to tune up your mount...getting the kind of super fine alt/az adjustment knob movements required when your tuning to below ten arcseconds is not easy...my atlas has a lot of stiction where the mount meets the tripod, and I have to loosen my center locking shaft a bit to get enough movement freedom to really dial in my alignment...and even then, the locking bots themselves are not particularly precise on the ends where they meet whatever plate or nut or shaft that they press against to perform their adjustments. So you need to figure out how to fine tune your altitude and azimuth adjustments...reduce the friction between the mount and the pier or tripod, maybe polish up and round out the ends of the adjustment knobs/bolts, maybe even lubricate the ends of them or even lubricate the threads (i.e. graphite) to make sure that everything moves freely and smoothly. You'll be amazed at how fine an adjustment you might need to make when your trying to dial in a PA that does not require declination guiding. 


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#17 Alex McConahay

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Posted 18 August 2014 - 12:12 AM

First off, let me say that this was a very informative tutorial.

Since you seem to know the program well, could you tell me:

1. When it says at the bottom of the graph that "Polar Alignment Error:0.01' " (as it does on mine), then PHD says I have less than one arc second polar alignment error, (.01' equals .6 arc seconds, right?). That sounds better than I would expect it to be. Am I reading it right?

2. Why do the numbers change? My reading actually varies from 0.01' to 0.06' or 0.07' Is it just seeing, etc.?

3. If I am in fact less than 4 arc seconds off polar aligment (as the readings of .06 or .07 indicate--especially when most readings are substantially less), should I not be able to turn off dec guiding and not worry too much about drift?

Alex
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#18 Jon Rista

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Posted 18 August 2014 - 01:12 AM

PHD's polar alignment indicator is really just a rough gauge. Unless you let it drift for a really long time, it's likely that when you get down to polar alignment errors less than 0.1, the error estimate is within the margin of error. A lot of things can affect the polar alignment error readout. Wind, for example, which I had tonight (just a light breeze) was making my PA error jump around between 0.1 and 0.65, in both north and south. 

 

When you get down to really fine polar alignments, you should really ignore the PA Error indicator in PHD, and start watching the graph. If you can let it drift for three to five minutes with less than 1" of movement in declination, then your probably perfectly fine. Something that I've noticed, once I THINK I'm down to a really close polar alignment, is that it will drift REALLY nicely for a couple minutes...then, suddenly, it will tack one direction or the other a bit....after five minutes of letting it drift, my PA Error changes from 0.05' to -0.45'. I'm honestly not sure why it does that, and the interesting thing is it will drift along and seem stable in one direction for a few minutes, say -0.1', then after that it will slowly switch directions, and after five or six minutes of drifting, your now at +0.35' PA Error. Usually, once you see it switch like that, that is your TRUE polar alignment. If it is less than 0.5', I leave it be. If it is more than 0.5', I'll try to fine tune it (which can be difficult until you smooth out the connection between mount and pier/tripod. 

 

If your altitude alignment is off too much, that can lead to consistent drift in one particular direction. If you can't see the horizons, and that drift is 0.5' or less, my recommendation is just leave it, and guide it out. If you really do get down to around 0.1' PA error, then you should be able to do without dec guiding. There will be some drift, and eventually the dec plot in PHD's guide graph will move right out of the range of the graph...but it should move so slowly that it doesn't matter from the standpoint of the quality of your subs (unless your really doing 10 minute or longer exposures, in which case I say guide regardless.)


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#19 JJK

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Posted 18 August 2014 - 06:41 AM

You likely have to wait longer than the period of your mount's worm gear assembly to get accurate estimates of PA error.


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#20 Jeff2011

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Posted 18 August 2014 - 09:41 AM

When I first started out in this hobby, I remember reading somewhere that your polar alignment only has to be as good as it needs to be.  I guess Yoggi Berra could have said that if he was into AP.   In other words, to spend a whole lot of time trying to get the best polar alignment possible may be a waste of time.  For the focal length and length of subs that I image at, I usually find that an ASPA is sufficient especially if I do my alignment with a camera instead of a reticle eyepiece.  That is as long as my declination guiding is behaving.  I have not gotten into narrow band yet, but when I go for longer subs (10+ minutes), then a better polar alignment may be needed.  The other factor is that field rotation may be an issue for longer subs like 20 minutes or more if the polar alignment is not good enough.   The declination guiding issue with the AVX is also another reason to try to get a really good polar alignment. I was lucky once with my ASPA and got very close.  I was able to turn off dec guiding and could have taken 5 minute subs although my LP kept me to shorter subs.

 

Next time I get some clear skies, I am going to give Jon's method a try.  I will also try guiding through ASCOM rather than ST-4.


Edited by Jeff2011, 18 August 2014 - 09:44 AM.

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#21 Alex McConahay

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Posted 18 August 2014 - 08:23 PM

>>>>>When you get down to really fine polar alignments, you should really ignore the PA Error indicator in PHD, and start watching the graph. If you can let it drift for three to five minutes with less than 1" of movement in declination, then your probably perfectly fine. Something that I've noticed, once I THINK I'm down to a really close polar alignment, is that it will drift REALLY nicely for a couple minutes...then, suddenly, it will tack one direction or the other a bit....after five minutes of letting it drift, my PA Error changes from 0.05' to -0.45'. I'm honestly not sure why it does that, and the interesting thing is it will drift along and seem stable in one direction for a few minutes, say -0.1', then after that it will slowly switch directions, and after five or six minutes of drifting, your now at +0.35' PA Error. Usually, once you see it switch like that, that is your TRUE polar alignment. If it is less than 0.5', I leave it be. If it is more than 0.5', I'll try to fine tune it (which can be difficult until you smooth out the connection between mount and pier/tripod.

You know, I cannot tell if you are responding to my earlier post or some other post.

I guess what I want to know is whether there is any real value to a number for polar alignment error when that number can change so easily.

Alex
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#22 Jon Rista

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Posted 18 August 2014 - 08:31 PM

@Alex: Yes, I was responding to you. Sorry, should have addressed you.

 

As for the polar alignment error, there is definitely value to it when your error is much larger. When your performing the initial steps, setting those initial "bookend" bookmarks, you want to use it. In my case, it usually seems to start out around 3'. So I'll shift alignment to the other side to around 3', then I know that a very good PA is somewhere right between those first two bookmarks. Usually, that is exactly the case, and the reason why dialing in a good polar alignment does not need to take a long time.

 

It's when the PA error number in PHD gets very small that I think it loses it's accuracy. At least, for shorter drift durations. JJK is probably right, you probably have to wait for at least one worm period for it to be reliable once your down to a fraction of an arcminute to get an accurate read, and even then, I've seen my dec drift just take off after drifting fine for an hour. I have no idea why that occurs, maybe it's something with my mount. Anyway, it's better to ignore the PA error at small values, and just look at how your star actually drifts by using the red line in the graph. If it tracks along and doesn't drift more than a fraction of an arcsecond after several minutes, then your good to go. That's all you really have to worry about. 


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#23 nitegeezer

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Posted 20 August 2014 - 11:30 PM

I have pinned this topic as a sticky.  I think the topic will be of use to many new to autoguiding so this will be easily available toward the top of the list.  At some point if this is not used regularly, it will be unpinned and back to a normal topic.  


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#24 nitegeezer

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Posted 20 August 2014 - 11:33 PM

Jon, at the bottom of your first post, there is a comment of (Image here).  If you have it and can add it in, that would be great.  If is has been too long and you can't edit that post, if you will post it with instructions, I would be glad to add it in.


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#25 Alex McConahay

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Posted 21 August 2014 - 08:30 AM

Jon,

Thanks for the answer.

Alex
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