Autoguiding Deep Dive Into.

Autoguiding Deep Dive Into

Contents Introduction Brief on Basic’s Guide Technical Blurb
PHD Guiding Software PHD Advanced Settings EQMod Pulse Guiding Conclusion/Tips/Closing

Introduction Thank you for the opportunity to speak to you all today, my name is Anthony Grimshaw and I am an Astrophotography Amateur and have been in the hobby for about 18 months now. I work in IT and have done for about 12 years, prior to this I was in the NAVY as a marine technician. I have found the combination of my professions have helped me greatly in the endeavours of my hobby. Let me start by stating I am no expert on Astrophotography and everything I have learnt to date has been through my time in the field, speaking with others and reading information from books and the Internet. During this time I have found that Autoguiding tends to be a sticking point for many (including myself), so today I am hoping to share my experiences with Auto guiding (specifically PHD), how it works and the settings you have available to make it work best for your specific set up.

Death by Power Point Introduction Con’t
This presentation is not a silver bullet session to fix all your problems, however I do hope the following information will assist in getting guiding working for you and ultimately getting you good images. I apologise in advance; these are complex tool sets with numerous settings and interfaces. In order to cover this adequately it will be Death by Power Point

Brief on Basics In my presentation earlier in the week I did a discussion on Auto Guiding basics, this covered basic concepts about the how’s and what’s of guiding. In a nut shell from that discussion: Auto guiding is the use of an electronic tool or system to assist your telescope mount from drifting or moving off the intended imaging target during long exposures. Auto guiding does this by tracking a star in the same or close proximity to that of your imaging target and sending adjustment commands to your mounts control system to correct for any apparent movement. Guiding is required because even the best mounts cannot achieve the level of accuracy required to track celestial bodies without error (especially at longer focal lengths).

Brief on Basics Con’t This is due to:
the drive gears in other areas of machining of the mount play in the ball bearings that permits their rotation Movement or Flexing in mount and brackets Misalignment from the celestial pole What Auto Guiding can do for your images.

Technical Blurb Most Modern telescope mounts use a control system called an “open loop” controller (also called a non-feedback controller) this is a type of controller that computes its input into a system using only the current state and its model of the system. A characteristic of the open-loop controller is that it does not use feedback to determine if its output has achieved the desired goal of the input. This means that the system does not observe the output of the processes that it is controlling. Consequently, an open loop system cannot correct any errors that it could make and it cannot compensate for disturbances in the system. Open loop controllers are chosen as they are simpler to manufacture and therefore much cheaper to make and subsequently cheaper to purchase.

Technical Blurb con’t To avoid the problems of open-loop controllers, a feedback circuit can be introduced. The feedback circuit effectively closes the loop and creates a closed-loop controller. Closed loop controllers have the following advantages over open loop controllers: disturbance rejection (such as unmeasured friction in a motor). guaranteed performance even with model uncertainties, when the model structure does not match perfectly the real process and the model parameters are not exact (e.g. poor polar alignment) unstable processes can be stabilized reduced sensitivity to parameter variations improved reference tracking performance Autoguiding is for all intents and purposes a system that provides the components required to create a closed loop system and provides the above benefits.

PHD Guiding Software There are a range of products and tools available for Auto Guiding, however today I am going to discuss the details of PHD Guiding, primarily because it is free, however it’s also a very powerful tool that will meet the requirements of most amateur Astro-Photographer’s. PHD “Press Here Dummy” was written by Craig Stark of Stark Labs, Craig has a PH.D in his own right; however he found it difficult to manually guide his scope so he wrote PHD as his answer to the problem. It can be found at I have a copy if anyone requires it? PHD literally stand for: Press Here Dummy So it’s supposed to be easy to use.

PHD Guiding Software Con’t
When you open PHD you are presented with the following screen It comprises the menu bar at the top: Of interest is the Mount drop down and The Tools Menu The command buttons at the bottom: Of interest is the Camera selection button Telescope connection button Loop exposure button PHD Target button (Start PHD Guiding) Stop button to stop PHD Exposure Time and The Brains Button

The mount selection list allows you to chose your mount type. The three main ones in common use are: ASCOM (Connects into EQMod or ASCOM) GPUSB (Connects to GPUSB Box) On-Camera (Uses on board camera control) All the settings discussed in part one cover PHD and all of the above connection types. Part two will cover the connection to EQMod and how you can fine tune PHD’s outputs further.

The tools selection list allows you to choose to enable some options and features. The two options you should enable are: Enable Logging Enable Server Optionally enable the Bullseye or Grid to help track stars movements, or to assist in pointing error adjustments/drift aligning. Optionally enable the Graph to see the Guiding outputs.

If you want a quick start guide have a look under the help menu for the “Impatient Instructions”. This does not cover any specifics but is the basics to get PHD running.

Most importantly the PHD Brains Button, this sets the advanced values for how PHD talks to your mount/software, It has the following values: RA Aggressiveness RA Hysteresis Max RA Duration (ms) Search Region (pixels) Min. motion (pixels) Calibration Step (ms) Time Lapse (ms) LE Port Force Calibration Log info Disable guide output Dec guide mode Dec Algorithm Dec slope weight Max Dec duration (ms) Star mass tolerance Noise Reduction Camera gain % LE Read Delay Dither scale RA-only dither Use subframes Defaults are displayed

PHD Advanced Settings So what do these settings do and what do they mean? RA Aggressiveness – The RA Aggressiveness setting is used to adjust the % of correction PHD makes to the mount, for example when set at 120 PHD will apply 120% of the correction to the RA drive to that detected by PHD and the stars movement. Usually a setting from 80 to 100 would be suffice, I would not recommend anything over 100. Default = 100 Recommended = 100 or less if oscillating occurs.

PHD Advanced Settings Con’t
RA hysteresis – Hysteresis (% Value) is the dependence of a system not only on its current environmental state but also on its past environment, meaning PHD can evaluate its past corrections and compensate for these corrections in the future. The idea here is that a quick change in the measured error in guide star position is probably due to a bad measurement (perhaps due to atmospheric disturbance), so the correction is "diluted" by mixing in some of the recent correction trends. In addition to sensitivity to seeing conditions, this setting is influenced by the characteristics of the mount. A good mount will not have sudden large changes in the RA position, so a large hysteresis setting is appropriate to filter out bad measurements.

RA hysteresis con’t – If your mount has a high rate of reproducible error, a number such as 30 – 45 is probably okay, but for a lower quality mount a value of 10 is likely a better as sudden large errors can occur and you want PHD to react to these. Default = 10 % Recommended = Start at 10 and increase if your mount allows.

Max RA Duration (ms) – The Max RA Duration setting determines how large the maximum correction PHD is allowed to make (in milliseconds). The idea here is to limit the damage caused by bad measurements of the guide star position. Allowing a very large correction (in one cycle) might require a large correction in the other direction later. If there really is a large error the exposure is probably ruined anyway, but in any case it isn't going to be much worse by breaking up the correction into multiple cycles, which is what happens when the max duration limit is hit. Unlike parameters specified in pixels, the proper setting for such timing parameters depends on the guide rate set in the mount. With the typical guide rate of 0.5X (50%), the max RA duration should be set at something like 300 milliseconds.

Max RA Duration (ms) con’t – Another consideration is that in most mounts (especially using an ST-4 guide port) the guiding process is delayed for the duration of the correction pulse, so allowing a large correction in one axis may delay a necessary correction in the other. ***NOTE*** Theoretically, guiding through a serial link to the mount (usually via an ASCOM driver) could make the correction more quickly, but this is generally not done. The next guide exposure cannot begin until the mount movement is done. Default = 1000 Recommended = Start at around 300 and slowly work up if constantly reaching this limit, if however you hit the limit constantly you probably have other issues guiding cannot compensate for such as poor alignment.

Search Region (pixels) – Search Region (pixels) defines the size of the area to search for the star to find its new position (the little green square around your selected guide star). The default of 15 pixels (i.e., a 30 x 30 area) is generally adequate. Larger areas take more time to process and if the guide star moves that far off you've got a serious problem that guiding is probably not going to fix anyway. Default = 15 Recommended = 15

Min Motion (pixels) – The Min Motion setting is the value that sets the minimum amount the guide star is allowed to move “without” triggering a correction, this setting applies to both the RA and DEC axis guide commands. If the value was set to 0.25 as an example the guide star would be allowed to “move around" a quarter pixel without PHD sending corrections commands. A setting of 0.05 means if PHD sees movement of a 20th of a pixel movement it will send a correction command and this is likely far to aggressive.

Min Motion (pixels) con’t – The default setting of 0.15 is typically suitable for most mounts, however it will depend to some degree on the relative magnification of the guide system / imaging system as well as on the seeing conditions and the quality of the mount. Default = 0.15 Recommended = 0.15 Can be more relaxed if the guide scope is of higher magnification when compared to the imaging scope.

Calibration Step (ms) – The Calibration step value is the length of a pulse in milliseconds that PHD will send to your mount during the calibration process. There is no easy recommendation for this parameter that will fit all telescope mounts and image set ups as it depends heavily on what the guide scope magnification is and what the guide rate is set in the mount. If you wish to adjust do the following: During calibration, look at the status bar to see how much the selected star has moved from the initial position. If the guide star is moving just a few pixels per iteration you could speed things up by using a larger setting for calibration step.

Calibration Step (ms) con’t – On the other hand, if it exceeds the goal in just a couple of calibration steps the calibration may not be as accurate. ***NOTE*** a high degree of accuracy in calibration is not always necessary. If it takes at least 4 iterations it's probably close enough. ***Note*** that this setting also defines the duration of pulses used to manually guide the mount. After calibration is done you can safely change this setting to the duration you want for manual guiding, but remember to change it back before you calibrate again. Default = 750 Recommended = Start with the default setting of 750ms and adjust only if you have issues getting a calibration to work properly.

Time Lapse (ms) – Despite the name indicating some sort of time lapse recording, the Time lapse setting provides a delay after each guide command before the next guide camera exposure starts. This is used because in some cases the mounts movement may not be complete or may not have settled when the correction is complete. Setting some delay here will provide time for the mount to settle so that the next exposure is done entirely at the new position. Default = 0 Recommended = 0 (Only change if you have a lot of oscillation in your guiding and only after you have trimmed the Aggressiveness settings down.

Dec Guide Mode – The DEC guide mode has 4 settings “Off/Auto/North/south”. This setting essentially disables or restricts how PHD can send Declination adjustments to the mount. Normally you would leave this set at "Auto" however if your polar alignment is excellent you might prefer to set it to “Off” to avoid spurious DEC corrections. Likewise if your DEC was drifting in one direction you can note which way it's drifting and enable corrections only in the direction required to compensate. This can be advantageous because it avoids sending DEC corrections that aren't really necessary. Default = Auto Recommended = Auto

Dec Algorithm – The DEC Algorithm setting has two available values “Lowpass filter/Resist switch”, Resist switch means that PHD will make corrections normally if they are in the same direction as recent corrections, but will ignore small errors in the other direction until they become large. The lowpass filter option effectively averages the corrections so that most spurious movements are avoided, but small steps to correct for drift are applied. If DEC guide mode is set to "North" or "South" a setting of "Resist switching" has no effect, so lowpass filtering would be the better choice. ***NOTE*** The Lowpass Filter setting is reported not to work correctly in the current versions of PHD, I cannot verify this however. Default = resist switch

Dec Slope Weight – The DEC slope weight setting does not have a lot of information available for it, however as I understand it the Dec slope weight changes the damping of the low pass filter and only comes into effect if you've selected "low pass filter" for the Dec mode. Again I cannot verify this setting so I suggest using the default value of 5. Default = 5 Recommended = 5

Max Dec Duration (ms) – Max Dec Duration setting determines how large of the maximum correction PHD is allowed to make to the Declination axis (in milliseconds) in one adjustment. 150 is probably adequate unless things are considerably bad, such as massive periodic error or poor polar alignment. You can raise it to, say, 800, but If PHD needs to send an 800ms pulse then you've most likely got serious polar alignment problems that you're asking to guider to accommodate for that are probably unreasonable. I'd recommend rather than increasing that number too high (300 should be the maximum for almost any mount), learn how to better polar align or fix the problems with the mount that are causing large movements rather than adjust this value too far. Default = 150 Recommended = 150 (Max 300 if reaching this limit frequently)

Star Mass Tolerance – The Star Mass Tolerance setting is designed to ensure that it is guiding on the correct star, PHD checks the "star mass" (how bright it is) on each exposure. If the stars brightness changes by a factor greater than the specified tolerance, a warning will sound. A value of 0.5 is reasonable, but you may sometimes get false warnings due to various conditions that affect the apparent brightness. In this case, you can avoid the warnings by setting a higher tolerance. A setting of 1.0 disables the feature. Default = 0.5 Recommended = 0.5

Noise Reduction – The Noise reduction setting has 3 available options (none/2x2/3x3), this is normally set to "none" however if you're working with a dim guide star and can't increase the exposure time, you might want to try one of the other settings. What it does is trade off resolution for sensitivity (like binning, but done after the capture rather than in the camera). This can be especially useful for cameras that have very small pixels. Such cameras may provide more resolution than is useful, but suffer from low sensitivity and noise that degrades the accuracy with which PHD can calculate the centroid of the guide star. Default = None Recommended = None (however play with this if you are having difficulty finding guide stars and maintaining them.

Camera Gain % – The Camera Gain setting is only available with some types of cameras and the recommended setting depends on the camera model. In general, you want to use the setting that provides the best signal to noise ratio for your guide star. This setting multiplies the amount of output signal from the camera per photon registered. Default = 95% Recommended = 95% But depends on the camera and it’s sensitivity.

LE Port and LE Read Delay – The LE port and LE read delay settings apply only to guide cameras that use parallel or serial ports on the computer to control the "long exposure" mode of a modified webcam; I have not used or modified these settings in any way so I cannot comment on the effect on the guiding system. Default = Port 378 Default = 0 delay Is there anyone here who has experience in this area that would like to comment?

Force Calibration – The Force calibration check box is used to force PHD to rerun a calibration cycle before it starts guiding. When unchecked, PHD will use the previous calibration data and begin guiding immediately when the target icon is clicked. PHD automatically “checks” this item when first launched and “unchecks” it when a calibration is successfully completed. It is recommended to check this if you move to a different part of the sky to force a new calibration cycle.

Log Info – The Log info check box when checked will write all PHD calibration and guiding information into a log file. I recommend leaving this checked so that you can later check how well the guiding performed. Not that you may do any checks, it’s still good to know that you can investigate any problems later if necessary. The log files are very small so do not worry about leaving logging running. ***NOTE*** PHD creates a new file for each date in which logging is started. This is much better than simply overwriting the old file (which older versions did), but it can still be confusing if you start logging after midnight on one night and then start it again before midnight the next night.

Log Info Con’t – The default location for the log files are the “My Documents” folder on windows PC’s and they are saved in the following format: PHD_log_DDMMMYY.log Default = Unchecked Recommended = Checked

Disable Guide Output – The Disable guide output check box when checked tells PHD to do everything the same with the exception of sending the correction commands to the mount. This is useful because it allows you to study the behaviour of the mount without guiding. In particular, by disabling the output and enabling logging you can create a log file that can be imported into a program to analyse the mount's periodic error and generate a periodic error correction (PEC) file. Default = Unchecked Recommended = When guiding! Naturally unchecked 

RA-Only Dither – The RA-only dither check box tells PHD to only send dithering commands to the RA axis only not DEC, under normal conditions dithering is applied to both RA and DEC. Dither changes only in RA is a good idea if you have problems guiding in DEC because it might take too long for DEC to settle after a dither move. For those that don’t know dithering is a command that can be sent to PHD to move the guide star X and Y distance from its current position and is used to help remove noise from your image stacks. ***NOTE*** The Enable Server Option must be enabled for software to send these commands to PHD. Default = Unchecked Recommended = Unchecked

Use Sub Frames – The Use subframes check box allows Cameras that support this feature to read out from pixels from only a portion of the image captured. If selected and your camera supports the feature, PHD will use it to decrease the amount of time it takes to download an image by reading only the area around the guide star (the search region). Default = Unchecked Recommended = Unchecked (unless supported by your camera).

PHD Advanced Settings Conclusion – PHD has a number of options available to you to assist in your guiding, while PHD is generally quite good when set at the defaults, however it is set this way to work on a wide range of mounts and you can tune PHD for your specific set up to enhance the guiding experience. As a best practise I recommend you only change one setting at a time and give leave PHD for 5 minutes to assess the impact this change had. This will ensure you do not get caught in the trap of chasing settings to fix issues. Additionally; it is important to remember that PHD is a complimentary system to your mount that monitors changes to apparent movement of stars, it is not a fix for other mechanical problems such as poor polar alignment, poor balancing or poor quality mounts.

Break Time Before we step into how PHD interacts with EQMod would anyone like a 10 minute break to stretch their legs, have a smoke or grab a coffee etc.?

EQMod Pulse Guiding EQMod is a powerful tool that integrates into the ASCOM Platform allowing enhanced control over Synta EQ mounts such as the EQ5 and EQ6. I am not going to cover all of EQMod’s settings as we would be here all night but I will discuss the specifics of pulse guiding via EQMod and how this differs from ST4 direct pulse commands. I will state up front that there are no “performance” benefits to EQMod pulse guiding over direct ST4 commands, you do however get some additional “on the fly” control not provided by PHD that allow you to tune your guiding for better results.

EQMod Pulse Guiding Con’t
In order to use EQMod’s pulse guiding capabilities you will need to be connected to your mount via an EQDir cable and be controlling the mount via EQMod. I have made a few assumptions at this point: You have properly aligned and balanced your mount. You know how to connect to EQMod and control the mount. You are interested in further fine tuning capabilities for your guiding. You want to use EQMod pulse guiding because you do not have an ST4 output to connect to your mount.

First and foremost to enable Pulse Guiding via EQMod you must select the ASCOM mount from the PHD menu bar as pictured left. This will direct PHD to talk to ASCOM/EQMod when you click the Telescope connect icon at the bottom of PHD. A this point you are prompted for the mount type, Select the EQMOD ASCOM EQ5/6 and click OK.

Once PHD is talking to EQMod and you are able to start guiding; there are a number of adjustments available in EQMod that override PHD’s input. These are: Autoguider Port (ST4) Guide Rate RA and DEC Rate Minimum Pulse Width (ms) DEC Backlash (ms) Pulse Width Override RA and DEC Gain

As mentioned earlier ST4 guiding works using a hard wired interface connecting your PC/switch box/guide camera to the mount's Autoguider Port. The interface consists of control lines (RA+ RA- DEC+ DEC-) to control each direction the mount can move. In the absence of a guiding signal the RA axis will move at the tracking rate that has been previously selected and the DEC axis will stop. ***NOTE*** With ST-4 based guiding EQMod plays no active part in the guiding process. You can however set the rate at which each axis will move when the associated guiding signals are sent to the ST4 Autoguider port.

The following settings are available:

An ASCOM Pulse guide message contains two parameters, a Direction to move and a Duration. On receiving such a message EQMod will instruct the mount to move at a pre-configured rate for the requested duration. Once the duration expires, the RA axis DEC Axis are returned to their nominal tracking rates - so if tracking at sidereal rate the RA Axis is set to sidereal rate and the DEC axis is stopped. If Lunar, Solar or custom rates are in effect then it is those rates that are applied.

Guiding Enable Check Boxes – The Checkbox associated with each Axis Pulse Guide Rate enables or disables the guiding on the associated axis. Guiding Rate – The Pulse Guide Rate sliders can be set between x0.1 to x0.9 of the nominal tracking rate at increments of 0.1 independently on each motor (RA or Dec). The rate sliders will determine how quickly the mount can move to correct a guiding error.

Pulse Width Duration Override – The “Duration Override” option allows you specify a fixed duration correction regardless of the input by PHD. This override is only applied if the associated tick box is checked. Minimum Pulse Width – The Minimum Pulse Width slider specifies the minimum length of time a correction will be applied and overrides any request by the autoguider for a shorter duration. EQMod always imposes a minimum pulse width of 50ms as it is not practical to accurately measure shorter periods.

Autoguider Calibration – PHD includes runs a self calibration routine during which a guiding pulse is initiated and the apparent movement of the guide star is monitored to determine the guide camera orientation and resolution. These calibration routines will typically require a minimum movement to be achieved and will fail if this is not achieved in a given time. This can lead to a potential problem as the optimum settings for guiding my not deliver “fast” calibration. If you do experience “guide star didn't move enough” type errors during autoguider calibration you could also change the RA/DEC Rate sliders in EQMod to a higher value to force quicker movement but please remember that the rate settings resulting in fast calibration may not be the best for active guiding.

The Pulse Guide Monitor – EQMod includes a Pulse Guide Monitor tool to help you monitor the effectiveness of your guiding and to allow you to dynamically adjust the strength of guiding being applied if you think it is either under or over correcting. The Pulse Guide Monitor is accessed by pressing the EQASCOM “DISPLAY+” button until it appears.

Interpreting the Pulse Guide Monitor Graphs – Pulse Guide Oscillations on the RA side basically means that there are correction overshoots. To fix this lower the RA Width Gain settings. Purely west side corrections. This means that there is a small amount of RA drift towards the east. Purely East side corrections. This also means an RA drift towards the west. ZERO Duration corrections - MAXIMDL usually issues this kind of command. It simply means a correction abort which is perfectly normal but does not apply to PHD.

Interpreting the Pulse Guide Monitor Graphs – Eastward drift correction. - In cases like this, multiple corrections are issued only on one direction with duration value getting smaller at each correction instance. It means the duration value issued by PHD is very small. You may have to compensate by increasing RA Width Gain settings. North side corrections - Successive corrections on the North side would mean a DEC drift towards the south. Pulse guide Oscillations on the DEC side - Definitely a must to lower the DEC gain settings. Southward Drift Correction - Just like in #5, you may need to increase the DEC width gain settings.

Adjusting the Pulse Guide Gain Slider bars – The horizontal slider bars labelled "RA Width Gain" and "DEC Width Gain" adjust the correction durations as issued by PHD represented as a percentage of their original values. This is useful when using guiding applications that do not permit the changing of parameters once guiding has started. ***NOTE*** Autoguiding calibration should always be performed with the sliders at 100% gain settings.

Focal length of guide system considerations – The relationship between the magnifications of the guide system and imaging system is somewhat flexible, but cannot be totally ignored. Magnification is also referred to as "image scale" and is expressed as arc-seconds per pixel. A guiding system can have lower magnification than the imager because PHD can find the centre of the guide star to much finer resolution than an individual pixel. But if you take this too far guiding performance will suffer. I would recommend that the image scale of the guide system be no more than two times that of the imager (half the magnification).

Conclusion We have covered a great deal of content, remember:
PHD is generically set up for “all” mounts and can benefit from tuning, the defaults are sometimes very aggressive. EQMod allows for fine granular “on the fly” adjustments to guiding that PHD cannot make. Make changes slowly and understand the impact the change will make to the system. Learn to read the monitoring tools you have available to diagnose problems you may be facing, these can assist greatly in fixing other faults (such as mechanical issues). There is an undocumented feature of PHD and EQMod called PEBKAC. Problem Exists Between Keyboard and Chair, meaning if it’s working don’t touch it, you will sometime get dud subs move on and take the next one and don’t fiddle.

Tips Autoguiding does have it’s limitations and is not a fully closed loop system, as such it is: Still important to make sure you have setup, balanced and aligned your mount correctly. Your mount is stable and does not have any significant malfunctions (such as excessive play in bearings or gears). Autoguiding require a stable and rigid mount so there is no flexing between the guider and the main scope. Ask people already doing AP for advice and guidance on how to use Autoguiders, There is a wealth of knowledge out there. Research before buying things to make sure you understand how they work and that it will work for your intended purpose and equipment. Test your equipment at home in the backyard before you go out to the Dark Sky site to avoid problems in the field.

Closing As with all thing astronomy share your ideas and findings with others to help them along their journey Remember you don’t need to spend a fortune to get a guiding system running as there are many tools that are very cheap or free to use, such as PHD. Please support these developers by making a donation if you can afford to, even $5 could ensure they are around tomorrow providing free software. Credit goes to the following for their information assisting in making this presentation: Greg Marshall EQMod Developers Neil Heacock Craig Stark Pray for clear skies  Any questions???

Autoguiding Deep Dive Into.

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