Telescope Reviews: LX-800

Differential flexure is not a guiding problem or a mount problem.

By definition, differental flexure is a problem anywhere in the system between the guide chip and the main imaging chip. You can have absolutely perfect guiding and still have problems with differential flexure. Even if you have a perfectly rigid guide scope, the main telescope itself will flex. In fact, most people learn about flexure first hand shortly after learning how to do very good guiding.

Keep in mind that with a separate guide scope, you have two different optical systems looking at the sky. If they don't move perfectly in sync with each other, you will get star trailing. On 5 or 10 minutes exposures, it's usually not noticeable unless it's pretty bad. By the time you get to 15 or 30 minute exposures, it starts getting pretty noticeable and really hard to correct.

The problem is that it there are many sources where it can be introduced. Someone mentioned a beefy saddle, but this is not (at least in my experience) typically a problem. Common sources include sagging of the focuser on either telescope, shifting of the primary mirror on either telescope (even with mirror locks, you'll get some shift.) If either telescope tube sags, you'll have it. If the camera is connected to the focuser by something other than 3 lockdown screws or a tightly threaded connection, then the camera can shift within the focuser.

You may think that you can solve the problem just with rigid parts, but the amount of flex needed to cause a problem is at the microscopic level. In practice, it's a really tough problem.

This is why to beat it, people use self guiding cameras and off-axis guiders. SBIG is working on a technique that uses two guide chips and an artificial star. The idea is that one guide chip takes frequent exposures to deal with what most people think of as guiding. Then, the second guide chip takes infrequent exposures using the artificial star so that it can detect and correct for flexure. But they've been talking about this for several years and I don't think that they've yet brought it to market.

So yes, even if Meade has made guiding one-button easy and reliable, they are not trying to address differential flexure here.

To me, none of this means that Meade has failed. And it certainly does not make me think that they are ridiculous. Instead, I think that they've built a system for a specific market (beginning imagers). Of all of their claims for the LX800, I don't recall ever seeing them claim that they address flexure. I don't fault their marketing department from calling out things that they don't do.