- The Earth is a rotating object. We need to use reference points in the sky to match our scope rotation to that of the Earth.
- Think of telescope alignment loosely like building a house.
- In Astrophotography each preceding layer is vitally important to the next one above it as we setup the scope from the ground, up.
- The table below reads from the bottom to the top.
- Polar alignment serves for those with German Equatorial (EQ) mounts or GEM and I don't believe play any role in AZ/EL (up, down, left, right) type mounts.
- For beginners setting up EQ mounts, the process of polar alignment can be overwhelming and appear impossible. You've probably watched YouTube videos where PHD is tracking +/- .5 arc seconds from zero. While having the minimum deviation from zero is ideal, this may not be possible with your particular mount.
- Without good polar alignment it won't matter how many alignment stars you have you've programmed into the controller, the particular angle or arc it takes to get to those stars will be incorrect.
- Care should also be taken with tripod location. Changes in ground state under tripod legs will effect alignment (soft grass, freezing ground)
- Also avoid bumping the mount once it is setup and aligned. Some mounts can be nudged no matter how tight everything appears to be.
- Polar alignment can be done a number of ways each with varying costs:
Polar Alignment Methods Type Description Pros Cons Cost All-Star Polar Alignment Software in controller points to a series of bright stars, Prompts user to center the objects in view. Controller provides error rate, and prompts user to adjust RA and DEC knobs to correct. Built into mount controller software
Can be used when Celestial North Pole (CNP) is blocked by trees, houses, a Sasquatch
Can be time consuming Free (built into mount) Polar scope Scope with star positions etched in the glass inserted into the mount. Aligns to Polaris and surrounding stars in their respective positions.
Handy to use with Clay's Kochab Clock method.
Requires clear view of CNP
Reports of wet knees from kneeling down to look through the scope
~$35 USD Drift Alignment Computer software process of calculating star drift pixel by pixel and providing input on RA and DEC adjustments Built into a number of imaging applications.
When performed with PHD2 software, a graph will indicate amount of error
Can be time consuming.
Requires imaging camera, computer and potentially guide scope
Free depending on software Polemaster Add-on camera that connects to mount. Software prompts user to rotate mount, adjust RA and DEC knobs. Very quick and very accurate polar alignment.
Polar alignment can be achieved in under a minute Great for those of us that setup and tear down every night.
Requires unobstructed view of CNP and computer. Requires mount adapter
~$300 USD for camera plus ~$30 for mount adapter
- I've personally used all the methods above. My greatest success was with the Polemaster due to it's ease of use and speed.
- Unfortunately I cannot use the Polemaster in my observatory due to height of walls and neighbor's tree blocking the Northern sky.
- Since my scope is setup permanently, I use drift alignment with PHD2. I perform subsequent alignments as I notice guiding deteriorating either due to bumping the mount or drastic temperature shifts.
- Now that the foundation of our telescope is stable and aligned, it's time to align the optics to celestial objects.
- Star alignment is the act of instructing the scope to slew to a specific celestial object in the sky, then visually aligning the object in the center of the scope.
- A number of factors play a role in successful alignment and subsequent star guiding:
- Scope balance
- When mounting the OTA, attach all accessories you plan on using and balance the scope both horizontally and vertically.
- Poor balance can strain worm gears, belts and potentially overload structural components of the mount and OTA.
- Poor balance can also make it hard for the mount to stay in position between guide pulses
- My method of balancing:
- Balance the RA axis first with the counterweights
- Turn my RA axis to the left until the counterweight bar is level
- Check the DEC axis for balance
- Repeat the last 2 steps until all axis can be easily moved with 2 fingers and do not turn on their own.
- Payload weight
- Make sure you're not loading to much gear onto your mount.
- There is much debate on this subject. A number of forums posts recommend not exceeding 50% of the mount's max weight value.
- This seems to be different per mount manufacturer
- Like an unbalanced scope, an overloaded mount will be hard to keep in position between guide pulses
- Mount quality
- An EQ mount valued at $1000 USD will most likely not be built to tight tolerances of that worth $3000 and up. It's one of those cases where you get what you pay for.
- Quality of components such as bolts, clutches, motors and gears play critical roles in overall mount performance.
- Backlash also plays a role in the overall mount quality and is the result of spacing between gear teeth. All gears have some level of backlash, it's just a matter of how much is acceptable and how you cope with it.
- For example on Celestron mounts, they recommend finishing a target slew by pushing Up and Right on the controller. This appears to work in practice and has stopped my mount from running away from a target in view.
- Users of other types of mounts such as Losmandy say that this is not a factor.
- Cable routing
- Easier said than done in some cases. Locate cables in such a way that they won't bind on anything during extreme mount movements. Cable snags are bad and should be avoided at all costs.
- I personally wrap my cables in plastic flex tube and run them through mounts, making sure there is enough slack for meridian flips, etc.
- Extreme temperature can stress motors, lubrication in gear meshes and effect guiding electronics.
- Refer to the manufacturer for thermal limits.
- Electrical Power
- Good, clean power is important for smooth guiding and consistent results.
- Mount components can be damaged if the power supply is under-rated for the task.
- Electrically noisy power supplies can cause controller software to crash and as well as inject noise into USB ports, effecting imaging computers.
- Watch for any voltage differentials between anything powered.
- If powering with outside with house current, assure you have a good ground.
- I chased a USB problem with my camera for months until I discovered my extension cord had a bad ground pin.
- Buzzing in fingertips or worse yet, shocks can happen without proper grounding.
- Scope balance