You had one of your favorite pictures printed and it just has been delivered! Full of excitement, you unbox the image and contemplate your work in large format. So many details you didn't even notice during the post-processing! Such a breath-taking result. Faint nebulae, dark background, vivid colors, oval stars!

Wait, what? Oval stars? OVAL STARS?

It... it can only be a printing mistake... or maybe it is... it must be one of those...

Guiding issues

The "obvious" causes

Polar alignment

Let's first demystify one point: A not-so-good polar alignment shouldn't have a massive influence on the autoguiding quality. However, a bad polar alignment can create strange banana-like pattern. The further the field is from the guide star, the more visible the phenomenon is. 

Trick: always try to align your guide scope and your telescope. By doing this, the alignment error shouldn't be detectable, unless your misalignment is huge (several arcmin)

Cheap mount mechanics 

Here, no secret: A good mount combined with a careful maintenance is key for success. 

Trick: Turn both axles and ensure there are no "sticky" spots

Wrong telescope balancing

This point look easy to solve, but it's not. Most of the astronomers balance the telescope in horizontal position only and it's a mistake. In the example below, the telescope is properly balanced in horizontal position, but the center of gravity is below the DEC axis. Consequently, the telescope is unbalanced in vertical position. Even worse, it seems to be balanced at 180°, but in an unstable position.

The less obvious causes

Wind gusts

Gusts can make the guiding deviate by more than 20 arcsec. Those are rapid errors, practically impossible to control with software. The best way is still to protect your gear from the wind and avoid any equipment that can make your scope move. For this reason, we decided not to install any automated dust cover on the tube, frequently amplifying the wind effects. The optical tube is kept as simple as possible.

And please, please! Remove this Telrad you haven't used since you started doing exclusively astrophotgraphy! :-)

Bad seing

Under our dark Austrian sky and during a clear night, our standard FWHM is close to 1,5 arcsec. This mean that the star size is approximately 3 arcsec. For this reason, our guiding limit is set at 3 arcsec. Whenever the guiding star deviates by more than this limit, the image is stopped and a new one starts.

Surprisingly, a concrete floor or any other thermal inertia can affect your seeing dramatically, by creating thermal disturbances in the air. Very large concrete blocks look very professional and steady, but in our case, 300kg of concrete were more than enough. However, if you poured 2 tons of concrete, it is recommended to paint it white.

Belt tension (or gear adjustment)

Unless you have a costly direct-driven one, most of the mounts are driven by gears or belt.

The reason I prefer the belt is described in the page "The right mount", exactly for its ability to perform a good autoguiding. Still, this type of mount isn't backlash-free and needs to be tensioned regularly, ideally twice a year (hmm. OK, let say every year).

Now, let's combine a loose belt, with an unbalanced telescope previously described, and let see with an example how it performs

In this example, we try to stabilize the orange mass on the green cross. First, the gears turn CCW and the upper side of the belt is tensioned. The driver pulley starts turning in the opposite direction (2), release the tension and starts to tension the lower part of the belt (3). Due to the resulting overshoot, the orange mass misses the green spot and continues its movement

And now you get it: it is not just about the proper balancing, but also how well the entire transmission is tuned. 

This phenomenon easy to identify: it usually happens close to the meridian flip or after a dithering, and usually affects one axis only (for our mount: usually the DEC axis). 

Cable management

When I started doing astrophotography, I don't know how many times I have red: "Take care of your cable management".

Until I understood it the hard way. At that time, we used a Skywatcher 80 without any guiding issue. We purchased a 120ED and only when we were close to the horizon, we started to have massive guiding issues, even leading to a fatal guiding error.

We investigated the issue rapidly and it turned out that the guiding pulses were becoming longer and longer, the closer we were from the horizon. We looked at the refractor carefully and understood what was wrong: The cabling was the same, but the overhang was longer and pulled the camera down.

That being said: let's optimize the cable management

• Bring the cables as close as possible to the axes of rotation 

• Don't let cables drag

• Use cable clamps 

Guding parameters

Here my only piece of advice is: slow and steady!

We usually reuse the same guiding calibration during one or two months, and it seems it's good enough. Ekos calculates normalized RA and DEC calibrating values in ms/arcsec, therefore, no need to start to new calibration for each object.