Astrobobby acquired a 10.000 USD 10Micron and still, the result is as bad bad as with his former Skywatcher EQ5. 

Astrobobby should have red this article first. 

How to select the right mount according to your gear, needs, and budget? Which criteria are relevant and which ones are not? Let see...

Mount type

Most of the astrophotography mounts are equatorial. However, it is not rare to see heavy telescopes mounted on a massive alt-azimuthal mount but in this case, a field rotator is mandatory even for short exposure. 

As an advantage, only the one axis (Right Ascension) will rotate, provided that your mount is properly polar aligned. And I can't repeat it more: don't underestimate the negative effect of an inaccurate polar alignment. As an example, our pier has a drift of approximately 1 arcmin every 2 years. Such a misalignment is enough to transform your beautiful round stars into ugly bananas. Our target is to keep an alignment error lower than 15 arcsec, ideally less. 

Maximum load

No secret here. The more load, the heavier the mount. If your telescope weights already 10kgs including cameras, filter wheel, etc., it is highly recommended to select a mount capable of 20% to 50% more, ie. 12 to 15kgs. You won't regret it.

About periodic error

Ideally, the RA axis of an equatorial mount perfectly compensates the earth rotation with 1 revolution every 23h 56mn and 4s, at a constant speed. Unfortunately, the speed isn't exactly constant. To make it simple, each rotating component adds a small periodic error. The closer we are from the motor, the "faster" in the cyclic error. The periodic error is nothing but the sum of all cyclic errors, also called harmonics.  

The periodic error may give a good indication on how good the mount is. There is no universal rule, but an excellent mount can have less than 5arcsec of periodic error, while a periodic error more than 30 arcsec may indicate a poor mechanical design and/or a lack of maintenance (damaged gears, old grease, etc).

• But is it right the say that a mount with 30 arcsec periodic error can't be used for astrophotography? 

Well, it depends. Without any autoguiding, it is certainly true, unless you shoot with a 30mm wide angle lens or you shoot at very low exposure (Moon, Saturn or Jupiter for example). 

If you are targeting long exposures, it may become an issue.

• Yes but I'm using autoguiding! What's the problem? 

Actually, the gradient of this periodic error is more critical than the error itself. Look at the following example:

Here, the red curve shows very steep rising and falling edges. Even if the autoguider sends the maximum compensation pulse, it may not be sufficient and the star will starts to deviate. 

Even worse, if the mount encounters backlash issues, it might be impossible to compensate such an error, even with a very aggressive controller. 

Mount precision and accuracy

Let's consider now that our periodic error is in an acceptable range, and properly compensated by the autoguider. An other well-known issue is the precision and the accuracy of the mount. Short reminder about what is what :-) 

Let's assume we start our mount every day at 10PM, during 10 consecutive nights. Each night the mount will slew at a different point. If we don't correct this error with the astrometry, it may result into different field of views, and the hardly acquired 12 Mpixel image will be cropped at 4 Mpixel. 

Is astrometry the miracle?

What if I tell you there is a magical tool solving the inaccuracy of your mount?  This function is called astrometry.

In short, the astrometry captures an image of a starfield, and asks a solver to determine the exact RA and DEC position of its center. It repeats this operation until the target is inside the acceptable range. 

In the example below, the picture is off-centered by 15 arcsec, which is within the tolerance of 45 arcsec. For the Peepoodo Observatory, we decided to set the limit at 20arcsec, which seems to be a good compromise between time to align, and lost pixels. Since we dither at 3x3 pixel, we loose few pixels anyway.

But is Astrometry the answer to the Ultimate Question of Life, the Universe and Everything?

Unfortunately no, and this in two particular cases:

1. The astrometry only corrects the several inaccuracies and lack of precision of your gear. Unfortunately, if the difference between theory and reality is getting too high, the mount could slew to a tree or a building, and no astrometry will be possible. In the case you start imaging close to your horizon, this could create problems. 

2. For a picture close to the meridian flip, it may happen that the mount and your automation disagree, if the pointing error is too high. This results into very strange behaviors where the mount executed a meridian flip without noticing the automation. The RA axis will then slew in the wrong direction and... you have no chance to reach the target. 

3. Astrometry without rotator doesn't correct the rotation. Once, our filter wheel wasn't screwed on correctly. We saw the field rotation, but too late. Two nights of images were lost. 

Short conclusion: the best astrometry algorithm will never replace a good polar alignment and a good mount model, at least good enough to capture the first picture, and good enough to avoid strange patterns on long-time exposures. However, it is one of the most time-saving feature we could ever use. Strongly recommended!

Mobile or stationary observatory?

Choosing the right mount also depends on the observatory type. Intuitively we'll rather select a light mount for a mobile observatory, but weight isn't the only parameter.

Even more if the observatory is far away, maintenance-friendliness is a crucial parameter. Our gear stays under ambient conditions without any thermal isolation and we could see how important it is to purchase a qualitative mount. Our Skywatcher AZ-EQ6 Pro has almost no corrosion issues, except at the DEC brake but nothing critical yet.

Gears are quite insensitive to temperature, but create backlash issues and periodic errors. For this reason, we decided to purchase a mount with belt transmission, but this requires ideally a summer and winter tuning. Well if I'm honest, we do it only when we have guiding problems, ie. approximately once a year.

No matter if your setup is mobile or stationary, a good polar alignment tool is crucial: it must be quick in the case of a mobile setup, and extremely accurate in the case of a stationary one. We could see significant improvement between the EQ5 and the AZ-EQ6! Note that an assisted polar align with NINA, EKOS or any of the automation software that supports this feature, is very helpful and will make you save precious time.

Drivers and interfaces

It sounds obvious, but please ensure that your favorite automation software has a driver for your future mount! It is also good to visit forums (Cloudynights, Astrosurf, etc.) and see what are the usual issues, and how stable the driver is. This is a general remark for any astrophotography equipment.

In our case, we were very disappointed with ASCOM drivers, since it is very difficult to troubleshoot anything. This was one of the main reasons to go for EKOS and INDI open source drivers, with a very active and helpful community. Now we can fairly say that both driver and automation software are quite stable and include all features you may need