Telescope Reviews: What will be the next revolution for ATM'ers?

Hello:

I've also considered the problem of having a big and really light mirror. Such a mirror would also likely be pretty fast. I think the solution is a Nasmyth Cassegrain layout. This way the eyepiece is close to the ground and the tube (such as it is) will be kept short. All the top end will have to do is support the secondary mirror since the eyepeice, focuser, and finder will be down at the bottom. However, the secondary might be as large as 12" for a 1 m scope, so that's not a trivial amount of weight. The longer focal ratio of the Cassegrain and the additional degree of freedom with the cc of the secondary will lead to some interesting combinations and trade-offs.

Regarding the comment on DMs, I think it'll be a while before many amateurs are using them. A few of us work with DMs professionally and are ATMers, and we don't even have them working on our scopes yet! Getting a DM working on an amateur scope in closed loop would take a lot of work, and wouldn't be particularly useful for many of the things amateurs look at.

First: Nearly all AO systems work in the infrared, but we look in visible light, so you'd need to share the few photons your scope is collecting between the wavefront sensor and your eyeball. Most AO systems work on 4+ m telescopes, which gather a lot of light.

Second: You'd need a REALLY stable mount so that you can lock onto the guide star and track it accurately (you won't want to burn DM stroke following tracking errors). Many DMs are mounted on a tip-tilt mount to offload things like tracking errors from the DM to the tip-tilt mount.

Third: You'd need to add a wavefront sensor to your scope, and bring along the power supplies, drive electronics, and computers to run the system. Hopefully your scope can be made to focus and balance with the wavefront sensor.

Fourth: No AO system I know of can use an extended object (i.e. the planets) as a guide source. The only thing that a conventional AO system would work on that amateurs look at are double stars. Given the time, effort, and money, is it worth it just to be able to split double stars? I'm working (here and there) on a wavefront sensor that could use the planets as the "guide star", and according to my calculations (famous last words) it'll work.

Fifth: Many amateurs with scopes in the ~8" and smaller rangs wouldn't see much benefit from an AO system since smaller scopes are often diffraction limited anyway. Bigger scopes, which are always seeing limited, would see the most benefit, and also gather enough light for the wavefront sensor to work. Also, AO works best in good seeing!

Sixth: Even small DMs are quite expensive (>$10K). The detector(s) in the wavefront sensor are also expensive and can be quite fragile. Some wavefront sensors use an ordinary CCD, and we know how much those cost. Others use APDs (up to 188 at a few $K each). There's a new avalanching CCD technology that looks quite promising, but I have no clue how expensive that'll be. Adding the cost of the power supplies, computer, drive electronics, wavefront sensor optics and other hardware, it's easy to see why these things tend to take many engineers and scientists years and millions of $ to build. Mass production for the amateur market would drive the unit cost down, but there's still the fundamental costs of the DM, the detector, and the optics, so I don't see even a relatively basic AO system for amateurs costing less than $20K-$50K any time soon.

Seventh: AO is a technology, which has it's limitations. It can help correct for seeing, but it won't completely correct for it, and it be able to achieve a high level of correction all of the time. Sometimes the seeing it just too bad and too fast for an AO system to keep up with. The users expectations will have to be adjusted accordingly.

None of these points are show stoppers, just things to consider. I hope that some amateurs will get AO working on their scopes some day soon. I suspect that it'll be someone who works on it professionally, who has the know how, and who has access to the expertise and spare parts to cobble something together in their lab.

Cheers
Mike Connelley
who dabbles with AO and DMs at from time to time....