Part 2: Exploring Advanced Techniques for Deep Sky Object Photography with Telescopic Mounts
Welcome to the second installment of our blog series dedicated to capturing Deep Sky objects through telescopic photography. In this segment, we will delve into the critical aspect of choosing and configuring the right mount, with a specific focus on the type employed in our astrophotography endeavors. Additionally, we will guide you through the setup process and alignment procedures, offering insights to optimize your experience.
At the heart of successful deep-sky imaging lies a capable mount that can effectively track celestial objects by compensating for the Earth's rotation around either the North or South Celestial Pole. For our readers situated in the Northern Hemisphere, the presence of Polaris serves as a valuable reference point, streamlining the initial alignment process. Various mount types exist, but the German Equatorial and Fork mounts stand out as the most prevalent choices.
The German Equatorial mount emerges as the predominant style for astrophotography applications, boasting a wide array of models from numerous manufacturers. Despite brand variations, the fundamental principle remains consistent—a dual-axis mount with perpendicular axes. One axis aligns with the Celestial pole, referred to as Right Ascension, while the other supports the telescope and counterweights, called the Declination. The convergence of these two axes occurs at a T intersection, further stabilized by a pivot facilitating latitude adjustments.
Upon achieving precise polar alignment, which is discussed in more detail later in the article, the Right Ascension (RA) axis seamlessly tracks celestial bodies at the sidereal rate, while the Declination (Dec) axis empowers the telescope to be directed to any declination point in the sky, spanning both the northern and southern hemispheres. Once the targeted celestial object is within the telescope's field of view, the RA axis exclusively handles the tracking process.
To accommodate the dynamic nature of astronomical observations, most mounts are equipped with adjustment motors on both axes. These motors facilitate minor corrections essential for maintaining accurate tracking throughout the imaging session. Historically, these adjustments were manually executed, requiring careful intervention by the observer. However, with advancements in technology, these fine-tuning processes are now seamlessly executed by an autoguider system.
The Autoguider system represents a pivotal evolution in mount control, automating the intricate adjustments needed for precise tracking. This technological innovation enhances the overall efficiency of the tracking process, allowing astronomers and astrophotographers to focus on capturing the awe-inspiring details of the cosmos without the need for manual interventions.
The fork mount stands as the second most prevalent choice for amateur astrophotography, offering a distinct design compared to the German Equatorial mount. Fundamentally an Altitude-Azimuth mount, akin to a Dobsonian in its basic configuration, the fork mount undergoes a transformative enhancement with the incorporation of an adjustable wedge. This modification elevates its functionality beyond mere tracking, rendering it suitable for astrophotography applications.
The introduction of an adjustable wedge serves as the pivotal modification, effectively converting the Azimuth into the RA axis, and the Altitude assumes the role of the Dec axis. Through skillful adjustment of the wedge angle, observers gain the capability to align the azimuth bearing with the latitude of their observing site. This enhancement ensures optimal positioning and orientation of the telescope, facilitating precise tracking and imaging of celestial objects throughout their apparent motion in the night sky.
Regardless of the mount chosen, the imperative step in your astrophotography setup involves performing a precise polar alignment. This aligns the Right Ascension (RA) axis with the Celestial pole corresponding to your hemisphere. The Northern Hemisphere enjoys a distinct advantage with the presence of Polaris, which is conveniently close to the North Celestial Pole, simplifying this alignment process significantly.
In the accompanying diagram, the crosshair denotes the celestial pole, while the circle with calibration marks serves as a guide to facilitate accurate polar alignment for the Northern Hemisphere. Embracing the convenience afforded by modern technology, enthusiasts can use mobile applications such as Polar Finder Pro, as shown below. This app provides a polar clock displaying the position of Polaris within the calibration circle. Aligning Polaris with this designated spot ensures a fundamental polar alignment, particularly well-suited for instruments with shorter focal lengths.
For photographers in the Southern Hemisphere, the process of achieving polar alignment involves utilizing the stars surrounding the much fainter Sigma Octans as a guiding asterism to locate the pole. Employing a polar clock application is instrumental in determining the orientation of this asterism for the selected date and time. By rotating the reticle in the polar scope to correspond with the figure displayed on your mobile device, you establish the foundational alignment. Unlike Polaris, under suburban skies this group of stars can be difficult to locate.
Once this initial adjustment is completed, the next step involves locating the asterism and fine-tuning the Altitude-Azimuth (Alt-Az) controls located on the bottom of the equatorial head. These adjustments aim to position the stars within the circles delineated on the reticle, ensuring an accurate alignment that facilitates optimal tracking for celestial observations.
While the procedure may sound straightforward in theory, the practical execution can be challenging, often eliciting a range of emotions, perhaps even colorful language, especially during the initial attempts. Rest assured, persistence and familiarity with the process will contribute to a smoother experience over time. Practicing polar alignment is important and can be done on the evenings when the sky condition may not be optimal for photography. It is one of th fundamental skills required for astrophotography and mastery of it will ensure your enjoyment of the hobby.
We have covered the mount and we have covered the basic polar alignment, now let's look at putting it all together.
To enhance user-friendly navigation, the setup process is conveniently broken down into a series of sequential steps. These steps are thoughtfully arranged to facilitate the smoothest possible setup experience. It's worth noting that, while you may discover alternative orders for setup as you become more familiar, ensuring that all steps are covered will ultimately lead to a successful and efficient polar alignment
Begin by placing the Equatorial mount onto the tripod top, ensuring the azimuth pin aligns with the azimuth bolts on the equatorial head. Proceed to securely fasten the equatorial head to the tripod by tightening the primary locking shaft. Tighten both azimuth bolts on the equatorial head until they meet the azimuth pin on the tripod. Attach the eyepiece brace onto the primary locking shaft and elevate it into position using the large screw nut. This serves a dual purpose, functioning as both an eyepiece holder and a brace to prevent tripod leg movement during use.
Lower the counterweight bar and remove the stopper bolt located at the bottom of the shaft. Proceed to affix the counterweights; if you have two, position one in the middle of the shaft and the other at the base. In the case of three counterweights, place the third at the top of the shaft. After securely fastening the weight screws, reinsert the stopper bolt at the bottom of the bar to complete the process.
Loosen the bolts securing the dovetail saddle on the top of the equatorial head. This enables you to effortlessly position the telescope onto the mount. Lift the telescope onto the equatorial mount and firmly secure it in place by tightening the dovetail saddle bolts.
Arrange all the accessories intended for the visual/photographic session on the telescope. This step ensures effective balance on the mount. Include cables, finder and guider scopes, camera or eyepieces, and your control box (e.g., ASIAIR, EAGLE).
Ensure all bolts securing the telescope to the mount are tightened, guaranteeing a secure setup. While securing the telescope or counterweights, Loosen the RA clutch bolt on the equatorial head to allow the telescope to swing around the main axis. The objective is to assess the balance between the scope and counterweights. If the scope tends to swing downward, the mount may be imbalanced, potentially too heavy at the scope end. Conversely, if there is no movement, it indicates a potential imbalance with the mount being too heavy at the counterweight end.
With the RA clutch bolt still loosened, rotate the scope around the RA axis to the 90-degree position where the counterweights and telescope are level. Gently release pressure and observe the mount's tendency to rotate.
a. If the telescope rotates down, indicating an imbalance with the scope end being heavier, manually return the telescope to the top of the mount's rotation and lock the RA clutch. Adjust the balance by lowering the counterweights on the shaft. Place the second weight closer to the lower one, approximately half the distance, and secure it with the thumb screw. Loosen the RA clutch and return to step 7. Continue adjusting the position of the second counterweight until achieving a proper balance. If the scope remains unbalanced with two counterweights together, consider introducing a third weight if available. If three weights were initially placed on the shaft, lower one until balance is achieved.
b. If the counterweights rotate down, indicating an imbalance with the counterweight end being heavier, manually return the telescope to the top of the mount's rotation and lock the RA clutch. Correct the balance by raising the counterweights on the shaft. Move the lower weight up closer to the middle weight, about half the distance, and secure it with the thumb screw. Loosen the RA clutch and return to step 7. Adjust the position of the lowest counterweight until achieving a proper balance. If two counterweights together still result in imbalance, consider removing one of the weights.
c. If the telescope and counterweights remain stationary, congratulations, you have successfully achieved balance.
After achieving balance in the RA axis, proceed to balance the scope in the Declination axis:
a. While holding the telescope, loosen the Declination clutch and rotate the scope 90 degrees to position it at right angles to the main shaft of the mount. If the top end of the telescope tends to rotate down, indicating an imbalance, manually return the telescope to the starting position and lock the DEC clutch. Hold the telescope assembly and then slightly loosen the dovetail saddle bolts securing the telescope to the mount, allowing the dovetail bar on the telescope to shift lower in the mounting. Adjust it approximately 1cm and secure the dovetail mount bolts again. Once the telescope is secured, repeat step 10. Continue adjusting the position of the telescope dovetail bar in the bracket on the top of the mount until balance is achieved.
b. If the back end of the telescope rotates down, indicating an imbalance with the back end being heavier, manually return the telescope to the starting position and lock the DEC clutch. Slightly loosen the Dovetail Mount bolts securing the telescope to the mount, allowing the dovetail bar on the telescope to move higher in the mounting. Adjust it about 1cm and secure the dovetail mount bolts again. Once the telescope is secured, repeat step 10. Keep adjusting the position of the telescope dovetail bar in the bracket on the top of the mount until the balance is achieved.
c. If the telescope remains stationary, congratulations, you have successfully achieved balance in both the RA and Declination axes.
Return to the RA axis and verify that the adjustments made to the Declination axis have not affected the balance. Recheck the balance of the RA axis and, if necessary, repeat steps 7 to 10. Once both axes are satisfactorily balanced, you are now prepared to commence your polar alignment.
Connect the main power cable to the designated connection on the equatorial head, ensuring there is some slack in the cable to accommodate the mount's movement throughout the night. Attach the hand controller of the mount to the labeled port and power on the system briefly to confirm the correct connections. Avoid leaving it on to conserve power, especially if you are operating from a portable power supply.
At the base of the equatorial head, where it meets the tripod, locate the gauge ranging from 0 to 90 degrees. This gauge signifies the altitude angle of the main RA shaft of the mount. Consult your mobile phone or computer to determine the latitude of your observation location. Utilize the altitude adjustment screws to raise or lower the mount angle to match your specific latitude. Once this adjustment is made, you can maintain this altitude setting for subsequent observations at the same location.
Utilizing a compass and considering the current magnetic deviation, adjust the azimuth bolts to rotate the main shaft of the mount, aligning it precisely with your relevant pole.
The standard setup of the mount is now completed including the rough alignment. To finish the polar alignment, you will need to wait till dark as you will need to see the stars around the pole itself
This concludes part 2 of Shooting with a Telescope. The next installment will be about the Telescope itself.