Is a Premium Mount Worth the Premium Price? The Astro-Physics AP1100 Reviewed

The AP1100GTO-CP4 mount (Crescent Nebula backgound). Credit: Rouzbeh Bidshahri

Astro-Physics has been around since 1975, catering to advanced amateurs with their precision mounts and telescopes, with a motto of “We are dedicated to craftsmanship!!” 

I’m using the “standard”, non-encoder version of the mount, the AP1100GTO-CP4. There is also an encoder version, the AP1100GTO-AE. CP4 is the latest iteration with updated electronics and the newest gearbox. This is Astro-Physics’ mid-range mount with a base price of $9,650. In this review, we’ll look at how it works and performs, and some of its key features.

The author’s AP1100GTO-CP4 carrying the Planewave CDK12.5 telescope. Credit: Rouzbeh Bidshahri

Design & Operation

Astro-Physics (AP) German equatorial mounts are quite versatile, serving well for both visual observing and high-end imaging, whether in the field or in an observatory. Twelve-volt DC operation makes field use practical with several optional tripods. The mounts will track for hours past the meridian as long as they’re clear of the tripod. Visual users can opt for a keypad (a minicomputer, really), for an additional $890.

The Astro-Physics keypad controller. Credit: Astro-Physics

AP mounts are modular, with three components. The base/Right Ascension axis, the Declination axis, and the electronics. This design facilitates upgrades and replacements, when required. I pass all my cables through the mount to ensure they don’t get tangled up.

The mount broken down into two parts. Credit: Astro-Physics

This modular design makes transport more convenient, and the hollow design makes it very light compared to its payload capacity. The AP1100 only weighs 48lbs (22kgs) but has an impressive instrument capacity of 110lbs (50kgs). I’m also able to fold it and fit it into a compact carry case.

The AP1100 fits nicely into a medium sized case. Credit: Rouzbeh Bidshahri

All the electronics are in the external control module, “the brain”, and it’s included with the mount purchase, and I’m using the newest CP4 version – that has a series of ports, including Wi-Fi. I update the firmware myself, and the entire module can be upgraded if a newer version is released or needed, for about $1,200.

The CP4 controller. Credit: Astro-Physics

Modularity also means I can buy and set up the system for my needs, and it makes for easy replacement should a part get damaged. The downside is that you need to buy a lot of additional parts, like saddles, counterweights, keypad, tripod, power supply, polar alignment scope, and more. Expect a much higher total cost than the base price!

Polar Alignment

The AP1100GTO can be aligned to the pole at latitudes from 0 to 78 degrees. Side knobs are used to adjust in azimuth +/- 9 degrees from the center.  Interestingly, the mount can also be used in alt-az mode for specialized research or terrestrial applications.

I also got the optional polar alignment scope, RAPAS, for $390. It plugs into the back of the mount, and you simply position Polaris in the illuminated reticle grid displayed by the software (or phone app) based on your time and location, which can be entered manually or from an external GPS unit.

Polar finder scope and a simulated view using the software. Credit: Astro-Physics

Being used to camera based polar alignment methods (Polemaster, Ipolar, and Sharpcap), I was a bit reluctant to use something new like the RAPSAS at first, but quickly realized its merits, especially for field use. The 90-degree scope is easy on the neck and I’m able to polar align in minutes just after sunset without any power, computer, or electronics. Not having to wait for dark skies saves imaging time and is a welcomed feature for visual observers. The accuracy is enough for visual use or auto-guided imaging.

New Gearbox 

There have been a few versions of the AP1100. The newest iteration, with the CP4 controller and revised gearbox,  has levers on the gearboxes that completely detach the gears from the axis. This is great for fine balancing with zero friction. There are clutches that can be used, too.

Gearbox lever engagement. Credit: Astro-Physics

When engaged, the worm is spring-loaded and perfectly meshed into the worm wheel with constant pressure. This means there is no slack, and virtually no backlash, allowing for much better guiding with the mount responding instantly to very small corrections. I never have to “adjust” the gear spacing, either.

Worm gear animation. Credit: Wikipedia (User: Catsquisher)


Tracking accuracy is measured by periodic error (PE), the number of arcseconds the mount deviates from the sky’s motion during each revolution of the worm. 

The non-encoder version of the mount that I have, the 1100GTO, is rated to have less than +/- 3.5 arcseconds of error (7.0 peak-to-peak, i.e., from one side to the other) in a 6.4-minute cycle, which is very low. The AP1100GTO-AE with absolute encoders has a claimed error of 0.2 arcseconds, but add $5,000 extra for the encoders!

I’ve heard AP’s specifications are conservative, so I tested my own mount with the useful PEMPro software (included):

Periodic error test plot with PEMpro. Credit: Rouzbeh Bidshahri

This graph above shows real sky data with a 70 -lb payload on the mount. There are six worm cycles, all of which measuring an average error of +0.6 / -1.1 arcseconds. That’s 1.7 arcseconds (peak-to-peak), far better than the rated value of 7.0.

Data from Pempro can be upload into the mount to further correct for the measured periodic error, which results in under 1.0 arcsecond error (this is Periodic Error Correction – PEC). This, along with modeling, is what I’m doing for imaging unguided at 1700mm with 5-minute subs.

Below is my typical guiding error with rough polar alignment and no modelling rate correction. The total guiding error is 0.36 arcseconds (RMS), part of which is caused by atmospheric seeing.

Guiding performance plot using PHD2. Credit: Rouzbeh Bidshahri


There is an ASCOM driver for the AP1100 that controls basic functions like slew speed (max speed is 1200x), park position, tracking rate, and etc.

There is also advanced software, APCC, that is now included with the mount. This software has a host of features like setting meridian and mount limits, real time 3D simulation of your scope position, and much more.

Most noteworthy is that APCC can make an autonomous all-sky model. This allows the software to correct for almost all pointing and tracking errors, including weather conditions and atmospheric refraction. Tracking is so well corrected by the sky model that the user can image unguided when autoguiding is not desired or, as in my case, without encoders.

While unguided imaging is working for me, tracking may not be as robust as using an off-axis guider, especially with a large reflector and subs over five minutes. The off-axis guider method is easier to get consistent tracking results with. The encoder version of this mount should perform better unguided.

APCC has a lot of advanced capabilities, but it can be a bit bulky and overwhelming for the average user  (the manual is 287 pages!). But APCC is optional, and not required to operate the mount.

Final Thoughts

The design, fit, and finish of the AP1100 are excellent, with all CNC machined aluminum parts powder coated in classic AP white. Astro-Physics mounts are known for their reliability and mine hasn’t skipped a beat to date. I must say the performance has exceeded my expectations.

The biggest challenge with these mounts is the relatively high cost of a complete setup, and perhaps more importantly, the availability. It’s not uncommon to wait up to a year or more for one, but after using one I can see it’s well worth the wait!

An unguided image of the Bubble Nebula using the AP1100GTO-CP4 mount. Credit: Rouzbeh Bidshahri

Unguided stack result:

3hrs of unguided subs test:



MSRP: $9,650

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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