This page is being updated to reflect the current changes and upgrades in my setup. I am now using other alternatives such as a GPUSB or an Arduino as a means to establish communication between the telescope and the mount since the parallel port (the port used in the original version of this DIY) is now becoming obsolete.
This part of the DIY guide focuses on the actual guiding operation and the drift-alignment method for precise polar alignment.
We begin by first assembling the telescope along with the guidescope. We also attach the imaging and the guiding cameras and connect all the necessary cables leading to and from the computer.
Roughly polar-align the telescope. For perfect polar alignment, polar finders are helpful but not a necessity (if your telescope has a polar finder, then it would be an advantage). In my observation site, I have a 20-degree obstruction which blocks my view of Polaris, but I still manage to polar-align my scope each night that I conduct an imaging session (my setup is not permanently-mounted). In fact, it is possible to achieve perfect polar alignment even without actually seeing Polaris. The actual drift-alignment method will be performed during guiding operation. For this step, you may just adjust the inclination of the polar axis to match the latitude of your observation site (e.g., approximately 15 degrees in my location), and then point it roughly to the north (or south, if you are located in the southern hemisphere) using a compass.
Adjust the telescope’s balance. A telescope needs to be thrown a bit off balance in order to achieve better tracking. To ensure that the gears inside the mount always mesh perfectly, the mount should always slowly lift its load up, and not slowly lower it down. A telescope pointed to the east must have a slightly heavier ‘counterweight side’ and a telescope pointed to the west, must have a slightly lighter ‘counterweight side’ (or simply put, the east side should always be slightly heavier).
We will now polar-align the telescope using the drift-alignment method. It is important that the mount is perfectly leveled. Run the guiding software. Point the guidescope to a bright star near the zenith (overhead). Adjust the focus/camera settings if necessary. With the image of the star displayed onto the video feed, rotate the guiding camera such that the east-west direction corresponds to the horizontal line, and the north-south direction corresponds to the vertical line. Make sure that the web camera’s base is oriented towards the south, with its top towards the north.
Set the mount to tracking mode to begin tracking the star. An error in tracking may cause the star to drift along the east-west line (yellow path) while an error in polar alignment will cause the star to drift along the north-south line (red path). First, we disregard the east-west drift as it will be corrected later by the autoguider.
We now look for any signs of north-south drift (any drift along the red path). The drift-alignment method requires us to observe the drift of 2 stars: one in the zenith and one on the horizon. It works in a rather simple manner:
- For a star in the zenith: a drift along the north-south line (a star moves along the red path) means that the mount’s polar axis needs to be moved horizontally, to the left or to the right (azimuth).
- For a star on the eastern or western horizon: a drift along the north-south line (a star moves along the red path) means that the mount’s polar axis needs to be moved vertically, higher or lower (altitude ).
By observing the drift with each adjustment made, it is possible to determine if the most recent adjustment helps correct the drift or not.
To illustrate, here is an example:
Suppose we are currently pointed to a star near the zenith and we have observed that it drifts vertically (the star moves upward or downward, it doesn’t matter which direction). Since the star is near the zenith, it means we need to move the polar axis horizontally, to the left or to the right, but we do not know yet which of the two directions (left or right) will lessen the drift. We arbitrarily chose to move the polar axis, say, to the left (westward), and observe if it corrects the drift. If yes, then we continue to move it to the same direction (in this case, to the left, westward), otherwise, if the drift has worsen, then we move it instead to the opposite direction (in this case, to the right, eastward), and continue adjusting until the drift is finally corrected.
We then point the telescope to a star on the eastern or western horizon, and we have also observed that it drifts vertically (again, the star moves upward or downward, and again it doesn’t matter which direction). Since the star is on the eastern or western horizon, it means we need to adjust the polar axis vertically, pointing it higher or lower (increase or decrease altitude), but we do not know yet which of the two adjustments (higher or lower) will lessen the drift. Again, we arbitrarily chose to point the polar axis, say, a bit higher, and observe if it corrects the drift. If yes, then we continue to move it to the same direction (in this case, moving the polar axis a bit higher thus increasing the altitude), otherwise, if the drift has worsen, then we move it instead to the opposite direction (in this case, moving the polar axis a bit lower thus decreasing the altitude), and continue adjusting until the drift is finally corrected.
Better polar alignment is achieved by repeating the drift-alignment method several times. Telescopes which are permanently-mounted in observatories are polar-aligned in a similar manner. At first it may seem difficult, but through practice, it is possible to drift-align in less than 10 minutes.
As soon as an acceptable polar alignment is achieved (no drift within 5 minutes or within the exposure length you are intending to image), you are now ready to start with the actual guiding operation.
We now turn our attention to the main imaging scope (with the DSLR attached). Point the imaging scope to the area of the sky you wish to photograph, frame it properly, adjust focus, then begin tracking. To avoid complexities, I strongly suggest (if this is your first time to do this) that you try to image targets located to the east of the meridian (objects that are in the eastern side). Objects located to the west of the meridian (objects that are in the eastern side) require what is called the ‘meridian flip’ (i.e., flipping the telescope 180 degrees along the declination axis upon reaching the meridian). In this case the guiding software must invert the RA signals. There are different ways of telling various guiding software that a ‘flip’ was made. In PHD Guiding, there is a setting called “Flip Calibration Data” (under Tools>Modify Calibration) that must be ‘checked’.
Now look for the nearest bright star (mag 3 perhaps) which could serve as a guidestar. Point the guidescope to this guidestar. At this point, the main imaging scope is now pointed to the target you intend to image while the guidescope is pointed to the guidestar. With the guidestar at the center of the field, click on it and start guiding by clicking on the ‘Guide’ button (in PHD2, it is the ‘target’ or ‘dartboard’ icon).
The guiding software will lock onto it, and the guiding operation will start immediately. A tab with some useful information about the current status of the autoguider will appear.
During guiding, the computer sends signals to the mount (to either speed it up or slow it down) in order to keep the guidestar centered onto the red cross-hair. While the autoguider keeps the guide star from drifting horizontally, the guide star may still, however, drift vertically. If that happens, it means have not achieved precise-enough polar alignment (to solve this, repeat the drift alignment process).
Most of the settings are normally left with the default values, but should you wish to learn more about these settings, you may refer to the ‘Help’ tab.
It is now time to take a photo. Set the DSLR to ‘bulb‘ setting and have the cable release ready (alternatively, you may opt to connect DSLR onto a laptop, using a dedicated DSLR software EOS Utility for Canon). Always double check the focus. As soon as you are ready, click the DSLR’s shutter button (and keep it pressed) for the whole duration of the exposure. An autoguider setup would allow exposures up to several minutes or more, and still keep the stars round (hopefully, without trails). A typical exposure would last only for about 5 to 10 minutes. The exposure time is limited by the local light pollution, thus, it is advised that you consider traveling to a dark-sky site to achieve longer exposure times.
The following photo was a test shot taken with the help of the home-built DIY autoguider setup. I have yet to take a deep-sky image with this new setup. If you are interested to see previous images taken with an older setup (one which used a parallel port instead of a GPUSB, click here)
If you have questions, feel free to leave a comment. Clear skies!
DIY Autoguider (Part 1: Introduction)
DIY Autoguider (Part 2: Setting-up the Guiding Software)
DIY Autoguider (Part 3: Wiring Diagrams)
DIY Autoguider (Part 4: Autoguiding and Polar Alignment)
For featured photos, click here.
For tutorials on how to get started with astrophotography, click here.
For DIY astronomy projects useful for astrophotography, click here.
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© Anthony Urbano (Manila, Philippines)