Excelsis Ratings/Reviews - Votes:Astro-Physics AP900 QMD

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Astro-Physics AP900 QMD

This is AP quality and the best of the best in terms of carrying your mount with engineering precision and style. They typically sell used for about $3,300, and are not easy to find, but occasionally out there. The downside is that by the time you do the azimuth adjustment upgrade, the Dec worm upgrade, buy some counterweights, a mounting plate, and a pier, you'll be out at least double the used price. But even so with all the accessories a contemporary AP900 GTO will cost substantially more, you'll need the accessories for it too, and be north of $10k. These are excellent mounts fully capable of photography.

Although the mount is definitely a big step up from the Losmandy G11, it has several downsides. First, the altitude adjustment handle is a nightmare compared to Losmandy systems. Difficult to get torque on the adjustment handle from behind the mount while polar aligning: AP even makes a little handle to be used as a lever, a real pain to use because it can't go all the way around forcing short turns. AP's latest mount, the Mach 1, wisely decided to imitate the Losmandy format for altitude adjustment.

The second defect is that the digital setting circle system is poorly designed. It has to be completely removed in R.A. in order to polar align. That's OK, but the process of removing and re-insertion puts contradictory demands on the engagement of the encoders with the mount. The aluminum wheel AP uses to make a pressure fit around the encoder shaft is a poor solution. After not much use, the ductility of the aluminum stretches the aluminum and causes slippage of perhaps twenty to thirty arc minutes. If you're using a wide field refractor it's not such a big deal, but in other applications, like a C11 or C14, or a long focal length Mak, it is a big deal indeed.

It is possible to fix this problem but too detailed to recount here. A complete photo essay is on the Argo_Navis Yahoo groups. The sum of it is that after using a variety of systems I have found that, as a digital setting circle guy, the Losmandy G11 is actually better designed.

I use this mount, I love this mount, but it took quite a bit of doing and some money to get it to meet my requirements. If you do get this mount, fix the encoder wheel problem (which requires a user modification of the OEM part), and combine it with Argo Navis you'll get pointing accuracy superior to the AP go-to systems, which at this writing still do not include mount modeling. You might also consider upgrading to 10,000 tic encoders since the 900QMD series was all 4,000 tics.

AP has not had as much customer demand for pointing accuracy as one might think because the clientele favors wide field refractors and relatively bright objects that don't put a demand on pointing accuracy. The people that DO want pointing accuracy are typically imagers and they invariably have worked around the problem by putting T-point or similar software on their laptops.

So, at least of this writing, you are actually better positioned to get pointing accuracy with the older mounts if you don't want to use a laptop in the field. The AP paddle system in the go-to mounts does not (yet) rival the capabilities in Argo Navis and in PC based systems like T-point.

The mount is very steady under high load and does something my G11 could not do: carry a C14+102mm refractor.

It's a wonderful system but I can't quite bring myself to give it an "overall" rating of 10 because of the flawed digital setting circle design, which put a premium of looks at the expense of functional accuracy. On this limited dimension, and the altitude adjustment, the AP takes a back seat to Losmandy (superior on steppers and worm, which are more critical). And the AP digital setting circle design is certainly better than some of the improvisations made in the 90s, such as the JMI hardware for Vixen. (Never get JMI digital setting circle hardware for Losmandy mounts). It's just that on this point the AP design is a very definite second best and that's somewhat surprising. In addition, it is a chore, every time one sets up, to screw the polar scope in and then take it out again.

On the other hand, with Argo Navis and 10,000 tic encoders you will get pointing accuracy of 3 to 5 arc minutes, and that's about as good as it gets.

Overall Rating: 9
Performance:10 Ease of Use:8 Value:10
Weight: 5 (Veritable Vote)
Date: 05/27/2008 06:34:07 am PST
By: gnowell
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=577398

Reply Replies: 0

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