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Autoguiding with PHD2: This open-source freeware can help you get perfectly tracked astrophotos.

In the good old days last millennium, serious astropho111 tographers hand-guided film astrophotos to ensure a sharp result. They spent countless hours monitoring a single star in a reticle eyepiece to ensure a perfectly tracked photograph. Otherwise, small-scale details in images were lost to blurring in the final result. Thankfully, those days are history. The advent of computer-controlled autoguider cameras has opened up deep-sky astrophotography to more amateurs than ever before. While autoguiding support is built into many high-end imaging programs, there's only one standalone program that does it for free.

PHD stands for "Push Here Dummy," a humorous, self-deprecating name for the highly regarded autoguiding program originally written by Craig Stark (available to download at Stark literally made auto-guiding as simple as pushing a couple of software buttons.

The latest version of the program, PHD2, works on Macs and PCs and includes some nifty new features. Multi-threading, new visualization tools, an improved drift alignment tool, and declination compensation have been added to eliminate the need for recalibration when switching targets. Here's how to use it.

First Things First

Before you get started autoguiding with PHD2, you first have to address all of the mechanical issues with your equipment that let you achieve accurate guiding. Here are some important things to consider:

* When using inexpensive mounts, limit the weight of your gear to half of the mount's stated payload capacity.

* Make sure to polar align your equatorial mount accurately, because a typical 2- or 3-star Go To alignment routine merely improves your mount's pointing model, but does not improve its polar alignment.

* Don't try to guide a large reflector or catadioptric telescope with a piggyback guidescope--use an off-axis (or on-axis) guider whenever possible.

* Minimize differential flexure between the guidescope and imaging scope. Pay special attention to the guidescope's focuser and its mounting rings.

* Always use guidescope rings with at least two alignment screws spaced 120[degrees] apart. Tubes in single set screws can pivot.

* Secure all dangling cables. They can subtly drag on your scope, sway in the wind, and ruin your exposures.

* Slightly offset the balance of your mount in right ascension so that it's lifting the weight on the east side of the meridian. This ensures the gear is always engaged. Be sure to rebalance after a meridian flip.

Getting Started with PHD2

Be sure to install any drivers required to operate your mount and autoguiding camera. Set your mount to a lx guide rate.

Once you've installed and open PHD2 for the first time, a "first light" wizard opens. This guides you through selecting your guide camera and inputting other equipment details, particularly how the program will communicate with your mount. Once that's completed, the program opens a History graph as well as a toolbar along the bottom of the window.

The toolbar contains icons to connect equipment, loop exposures (for calibrating and focusing), begin guiding, stop guiding, and change the camera settings. There's also a drop down menu to change the autoguider exposure and a slider to change the brightness of the image display.

Many additional settings are located under the menus at the top of the main window as well as under the advanced settings "brain" icon in the toolbar. Most of these settings can be left on their defaults.

Start by clicking on the Tools menu at the top of the screen and select Enable Star Image Logging. This can help you diagnose most guiding problems later.

Clicking on the brain icon in the toolbar opens the Advanced setup window, where you can change all of the settings made when first opening the program. You can also enable advanced features such as dithering (or offsetting) the guide star between exposures.

Once you've plugged in all your gear and are ready to begin, click the Connect Equipment icon and press the Connect button to the right of the camera and mount dialogs. When both have successfully activated, press the Loop icon to start an exposure cycle, using between 1- to 3-second exposures. Focus your guidescope or off-axis guider and then lock everything down.

Next, simply click on a star in the display window, and hold down Shift and click the Begin Guiding icon to calibrate the program to your equipment. The calibration routine will issue a series of move commands in right ascension and declination and then re-center the guide star. This will tell PHD2 how much the mount will actually move for a given amount of time. When calibration is finished successfully, PHD2 will start guiding automatically.

At this point, open the Guiding Assistant from the pulldown Tools menu. When launched, this will turn off guiding and monitor the guide star for a few minutes, then report back useful information such as your polar-alignment accuracy. It will then offer recommendations for how long your guide exposure should be, as well as minimum motions and backlash settings. These should then be automatically input in the Advanced Setup dialog.

Additional Tips to Improve Guiding

Now that you're autoguiding, here are some valuable suggestions to get the most out of your nights.

Don't chase the seeing! When imaging in unstable seeing conditions, set the guide exposure to around 3 to 5 seconds. This will produce a better average of the guide star's location. Also try reducing the right ascension aggressiveness settings in the Advanced Setup window, located in the Algorithms tab.

Disable unnecessary declination corrections. Some of the biggest guiding problems come from backlash in the declination gear, producing overshoot and oscillations, especially with inexpensive mounts. You can determine which way your guide star is drifting by simply turning off declination guiding altogether at the bottom right of the History graph for a couple of minutes and observing the drift direction. Then enable declination corrections only in the direction needed to correct for this drift.

Turn down the aggressiveness setting in right ascension in the Advanced Setup / Algorithms tab. This is often set too high, especially in poor seeing conditions. Adjust it until your right-ascension oscillation readout is between about 0.5 and 0.3 in the Guide Stats window. A readout of 1 means the program is changing directions after every exposure, so reduce the aggressiveness. A readout below 0.3 means you're not making enough corrections, requiring you to increase the aggressiveness.

Choose a guide star with the correct brightness. One that's too bright might saturate, leading to less accurate centroid calculations. Too dim, and it might disappear if conditions deteriorate. Select View > Display Star Profile from the pull-down menu. If the top of the peak is flat, then the star is being overexposed. Correctly set the calibration step size. During calibration, PHD2 will issue a move command in milliseconds in right ascension and declination to see how far the star moves. It will do this a number of times to get a good average. If you have the calibration step size set too low, it will take a long time to calibrate. Set it too large and it might not make enough steps to get accurate calibration data. Aim for about 10 to 15 steps. If you correctly input your guidescope focal length and autoguider camera pixel size, PHD2 will automatically calculate a recommended step size with which to start.

Change the minimum move parameters. The minimum move (MnMo) setting tells PHD2 how far a star is allowed to move before a correction is issued. If this is set too small, it will try to chase the seeing. Too large a value, and the software might not make needed corrections. Remember, the MnMo readout is for the guidescope camera but needs to be configured in relation to the resolution of your imaging scope and camera. If you're shooting with a longer focal length than your guide-scope's, the MnMo needs to be set to a smaller number.

Recalibrate only if necessary. If your mount reports the declination, you don't need to recalibrate when you move to a new target. If your setup is permanent, or the same every time out, and you orient the guide camera reasonably close to the last time you calibrated, you can save the calibration and use it again--you don't need to calibrate initially or recalibrate for a new target or after a meridian flip.

Accurately input your camera and guidescope parameters. Be sure of the focal length and guide-camera pixel size. This will give you a tremendous amount of feedback as to how well your setup is guiding. Note that you can set the History graph to measure corrections in either pixels or arcseconds. Setting this to arcseconds makes the graph readout appear much worse than if set to pixels, even though the guiding is exactly the same. Set it to arcseconds so you can see how you are really doing in relation to your imaging rig's resolution.

Align your autoguiding camera. While PHD2 doesn't need the autoguider camera aligned with right ascension and declination axes to do its job, it does help you when diagnosing problems after a night of imaging. If stars are trailed east-west, there is a problem in right ascension. If stars are elongated north-south, the issue is in your declination guiding. Aligning your guide camera makes it easy to determine where the problem resides.

Interpreting the readouts. RMS (root mean square) is a mathematical average of the guiding accuracy that gives you an indication of how well the guiding is performing. You can judge it in relation to the seeing. For high-resolution work under excellent seeing with top-end equipment, your RMS should be about V4 of the FWHM (full-width, half-maximum) you hope to achieve. When imaging with the shorter focal lengths found on camera lenses, guiding requirements are much more lax.

In Closing

Autoguiding can significantly improve your astrophotos by allowing you to shoot longer untrailed images--and more of them--to achieve a much better signal-to-noise ratio. Auto-guiding with PHD2 frees you from the tedious chore of having to do it manually.

Remember, however, that the map is not the territory. The ultimate judge of your guiding should be the size and shape of the stars in your images. It doesn't matter if you have 0.25 arcsecond of total RMS guiding error if your stars aren't round in your light frames. Likewise, if your stars are tight and round in your lights, and the history chart looks like a foreboding mountain range, don't obsess about this. Simply tell your scope to push here, dummy!

* Contributing Editor JERRY LODRIGUSS has been photographing the night sky for more than four decades.

* For more helpful astrophotography tips, visit Jerry's website at

Caption: PERFECT GUIDING Unless you use an extremely high-end telescope mount, you'll need to autogulde every deep-sky astrophoto to ensure round stars in your images. This deep exposure of Thor's Helmet (NGC 2359) is made up of more than 7 hours of exposures guided using PHD2. Facing Page: PHD2 includes several ways to monitor your guiding. The main image from your autoguider is seen at top left of this screen, with the chosen guide star marked with a green box. Additional windows monitor the profile of the guide star, guiding statistics, and the History graph that shows what corrections have been made in both axes. The toolbar is visible below the guide image.

Caption: CONNECTING EQUIPMENT Clicking on the USB icon in the toolbar opens a window where you connect your autoguider camera, mount, and auxiliary mount. Additional connections for camera rotators and AO devices are available when clicking the More Equipment button.

Caption: DITHERED GUIDING Clicking on the "brain" icon opens the advanced setup dialog. The Global tab includes an option to do "dither" (or offset) guiding, which improves the overall signal of images when shooting multiple exposures.

Caption: A CAMERA SETTINGS The Camera tab allows you to change exposure settings if necessary. These should be set automatically when choosing your camera during initial setup.

Caption: GUIDING Make advanced adjustments of the program's guiding parameters using the Advanced Setup Guiding tab. Input the focal length of your guide scope (or off-axis guider) and click the Calculate button to access the Calibration Step Calculator.


This advanced tool computes how much PHD2 moves your mount in both axes to make corrections, training the program to get the best results from your telescope mount.


The Algorithms tab in the Advanced Setup window lets you change the mode that PHD2 uses, though the default settings work well. Here is where you can adjust the aggressiveness values when imaging under unstable seeing conditions.
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Author:Lodriguss, Jerry
Publication:Sky & Telescope
Article Type:Product/service evaluation
Date:Nov 22, 2017
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