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 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.
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.
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).
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.
If you have questions, feel free to leave a comment. Clear skies!
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© Anthony Urbano (Manila, Philippines)