Cloudynights.com-Drift Alignment With PHD2 - The Bookmark Technique

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Drift Alignment with PHD2 - The Bookmark Technique

#1 Jon Rista

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 opolaris. 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.

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

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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.

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 betwee

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:

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.



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

delete any

existing bookmarks. This will assure that you don't end up confused abot which bookmarks mean what (especiallyif 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.

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:



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.

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 quitewildly, until they "settle" and normalize around a given trend.



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.



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:



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



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

unction...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.



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.

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.



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"):



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.)



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:



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. Iknow 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 imagin

session. Here is the fifth move of the guide star, resulting in a polar alignment of 0.19':

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":



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

several minutes:

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:

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":

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.

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 los

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 timebefore the star disappears below the horizon, but close enough for an accurate read, the better.



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