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.
Canon 450D DSLR camera (left) and a Logitech 4000 web camera (right)
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 a 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 case), and then point it to the north (or south, if you are located in the southern hemisphere) perhaps using a compass.
Point the polar axis to the celestial pole (see red arrow). Shown here is a Kenko NES mount. Note that the mount’s controller is also a DIY project which I will discuss in detail in future posts.
Adjust the telescope’s balance. You’ll be surprised that 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. With the ‘telescope side’ and the ‘counterweight side’ pivoting on the mount’s polar axis, a telescope pointed to the east must have a slightly heavier ‘counterweight side’ and a telescope pointed to the west, must have a slightly heavier ‘telescope side’.
We will now polar-align the telescope using the drift-alignment method. It is important that the mount is perfectly leveled. Launch the guiding software (GuideMaster). 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, orient the guiding camera such that the east-west direction corresponds to the horizontal line, and the north-south direction corresponds to the vertical line. Orient the web camera’s base toward the south, with its top towards the north.
Swinging the telescope in an east-west direction must move the star along the yellow path. Swinging the telescope in a north-south direction must move the star along the red path. The camera must be properly oriented, otherwise, autoguiding will not be possible.
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:
1. For a star in the zenith: a drift along the north-south line (red path) means that the mount’s polar axis needs to be moved horizontally, to the left or to the right (azimuth).
2. For a star on the eastern or western horizon: a drift along the north-south line (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. The image below shows how the mount’s polar axis may be adjusted horizontally (azimuth) and vertically (altitude).
Screws A, B, and C allow most equatorial mounts to move the polar axis horizontally (A and B), and vertically (C) in an altitude-azimuth manner.
To illustrate, let me give an example.
Suppose we are currently pointed to a star near the zenith and we have observed that it drifts vertically (either 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, 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), otherwise, if the drift has worsen, then we move it instead to the opposite direction (in this case, to the right), and continue adjusting until the drift is finally corrected.
We now point the telescope to a star on the eastern or western horizon, and we have also observed that it drifts vertically (again, either upward or downward, 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, 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), 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), and continue adjusting until the drift is finally corrected.
Better polar alignment is achieved by repeating this method several times. Permanent observatories are polar-aligned in this 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), we 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. For objects located to the west of the meridian requiring what is called the ‘meridian flip’ (i.e., flipping the telescope 180 degrees along the declination axis upon reaching the meridian), you must invert the RA signals by clicking ‘Telescope‘ and ‘InvertRA‘.
Now look for the nearest bright star (mag 5 or 6 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 the ‘Guide’ button (Note: The ‘Guide’ button is clickable only after the ‘Focus’ button has been clicked twice; click the ‘Focus’ button once to allow focusing, adjust focus if necessary, then click it again to deactivate it and allow guiding.).
Upon clicking on the ‘Guide’ button, a selection box will appear. Choose ‘Lock on actual position‘. The mouse pointer will then change into a ‘+’ (plus) sign. You will need to identify the guidestar using this pointer.
Click on the guidestar. The guiding software will lock onto it, and the guiding operation will start immediately. A tab which contains useful information about the current status of the autoguider will appear. A screenshot is shown below:
The original position of the guidestar is marked with a red cross-hair, while its current position (if in case it drifts) is marked by a yellow cross-hair. 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. The autoguider will keep the guidestar from drifting horizontally, but it may however, drift vertically, which implies you have not achieved precise polar alignment yet.
The software’s sensitivity to the guidestar’s movement may be adjusted through the ‘Aggressiveness‘ slider. Note that we are only interested in the RA slider. If the aggressiveness is set too high, the guiding software might attempt to correct for errors not related to tracking (e.g., errors related to seeing). Adjust the RA slider to the left or to the right until a balance between sensitivity and smooth tracking is achieved.
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 (or just press F1).
It is now time to take a photo. Set the DSLR to ‘bulb‘ setting and have the cable release/remote shutter ready. 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 an hour or more, and still keep the stars perfectly round (without trails), but you won’t need to expose that long since typical sub-exposures only last 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 sub-exposure times. A single 10-minute exposure produces better image quality than a stack of 10 separate images worth 1-minute each.
The following photos were taken with the help of the home-built DIY autoguider using a 4-inch refractor and a 60 mm guidescope.
Located 1500 light-years from Earth, the Great Orion Nebula (M42) presents a cross-section view of a galactic bubble of gas and dust. Sky-Watcher Equinox 100 ED, Canon 450D DSLR, and a home-built autoguider. Image processing done in IRIS. ISO400, 10 x 6 min exposure (1 hour total). Photo Credit: Anthony Urbano. More Orion Nebula images here.
M51 (Whirlpool Galaxy) along with its companion NGC5195. Sky-Watcher Equinox 100 ED, Canon 450D DSLR, ISO1600, 8 x 240 sec exposure. Photo Credit: Anthony Urbano
At a distance of 2.2 million light-years, Andromeda Galaxy is the most distant celestial object visible to the naked eye. It is listed in astronomical catalogs as M31 or NGC 224. Sky-Watcher Equinox 100 ED, Canon 450D DSLR, ISO1600, 1 x 240 sec exposure. Photo Credit: Anthony Urbano
M33 (Pinwheel Galaxy) Sky-Watcher Equinox 100 ED, Canon 450D DSLR, ISO1600, 2 x 90 sec exposure. Photo Credit: Anthony Urbano
M81 (Bode’s Galaxy) and M82 (Cigar Galaxy) Sky-Watcher Equinox 100 ED, Canon 450D DSLR, ISO1600, 4 x 240 sec exposure. Photo Credit: Anthony Urbano
Omega Centauri Globular Cluster. Image taken in UP Diliman, Quezon City, Philippines using a 4-in f/9 refractor and a Canon 450D DSLR. 30-sec exposure, ISO 1600, tracking mount. Photo Credit: Anthony Urbano
M13 Globular Cluster in Hercules. Image taken in UP Diliman, Quezon City, Philippines using a 4-in f/9 refractor and a Canon 450D DSLR. 30-sec exposure, ISO 1600, tracking mount. Photo Credit: Anthony Urbano
Pleiades (M45), also called the Seven Sisters, is a galactic star cluster (open cluster) in the constellation Taurus, about 415 light-years away. The faint bluish glow surrounding each star is visible in this photo. Image taken with a Sky-Watcher 100 ED on Kenko NES mount, Canon 450D DSLR prime focus at f/9, ISO 1600, 1×240 sec exposure, December 2011, Basud, Camarines Norte. Photo Credit: Anthony Urbano
Beehive Cluster (M44). Image taken with a Sky-Watcher 100 ED on Kenko NES mount, Canon 450D DSLR prime focus at f/9, ISO 1600, 1×240 sec exposure, December 2011, Basud, Camarines Norte. Photo Credit: Anthony Urbano
Just one degree west of the bright star Antares (2 moon diameters), M4 is one of the easy-to-find globular clusters. It is 100 light-years in diameter and only 7000 light-years away (rather close in terms of globular cluster standards). Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 60 secexp, IS0 1600. April 6, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M6 is an open cluster in Scorpius also known as the Butterfly Cluster. To the naked eye, its stars are on the verge of visibility, making an illusion of a ‘flying butterfly’ as the stars in the cluster twinkle. The brightest star in the cluster, BM Scorpii (an orange star), varies brightness from 6th to 8th magnitude in a period of approximately 28 months. Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 60 sec exp, IS0 1600. April 6, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M7, also called the Ptolemy’s Cluster, is an open cluster in Scorpius, with its stars spread in such a large area (1 degree), and thus, best viewed with a pair of binoculars. With a small telescope and on a dark clear night, fainter stars in the cluster may be observed. Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 60 sec exp, IS0 1600. April 6, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M92 globular cluster in Hercules imaged using a Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 90 sec exp, IS0 1600. April 9, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
NGC2024(Flame Nebula) and IC434 (Horsehead Nebula) Sky-Watcher Equinox 100 ED, Canon 450D DSLR, 3x120sec exposure, ISO1600. Photo Credit: Anthony Urbano
M1 (Crab Nebula) Sky-Watcher Equinox 100 ED, Canon 450D DSLR, 2 x 180 sec exposure, ISO1600. Photo credit: Anthony Urbano
M20 is a cloud of ionized hydrogen gas some 25 light-years in diameter. Dark lanes visible in 3-4 inch telescopes may be seen extending from its center towards the west, northeast, and southeast, effectively dividing the nebula into 3 distinct patches, hence the name ‘Trifid’ which means ‘split into three,’ was derived. M21, an open cluster, is also visible in this photo (upper left of the nebula). Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 240 sec exp, IS0 1600. April 6, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M8, the Lagoon Nebula, is a cloud of ionized hydrogen gas some 50 light-years in diameter, located about 5000 light-years away. It is a region in space where new stars form. To the east of the nebula is the open cluster NGC 6530, also visible in this photograph. Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 2 x 240 sec exp, IS0 1600. April 6, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M57, the Ring Nebula, is a planetary nebula in the constellation Lyra. It is a cloud of cold gas expanding away from a small hot central star that provides energy for the gas cloud to glow. The nebula is relatively easy to find and is visible even with 3-4 inch telescopes. Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 10 x 60 sec exp, IS0 1600. April 5, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
M27, the Dumbbell Nebula, a planetary nebula in the constellation Vulpecula imaged using a Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 5 x 90 sec exp, IS0 1600. April 5, 2012, Camarines Norte, Philippines. Photo Credit: Anthony Urbano
A 30-minute exposure with an OIII filter shows the fine thread-like structure of the Veil Nebula, a supernova remnant in the constellation Cygnus. The narrow-band filter helps cut through the severe light pollution in the city. Sky-Watcher 100 ED 4 in f/9 refractor, Kenko NES mount, Canon 450D DSLR, 4 x 480 sec exp, IS0 1600. April 15, 2012. Photo Credit: Anthony Urbano
If you have questions, feel free to leave a comment. Clear skies!
Related articles:
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)
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© Anthony Urbano (Manila, Philippines)
