AutoGuiding: Off-Axis versus Piggyback OTA

[SteveL]

I`m thinking about getting a bit more serious with my imaging, and looking at autoguiding in the next few months. The two ways I can see of doing it is either with a seperate OTA piggybacked onto my NS8GPS or using an off-axis adapter. Other than the cost difference of the extra OTA (or not) and mounting rings, issues I can see are:

• 
  [li]Weight/balancing of the OTA[/li]
  [li]Light attentuation and possible distortion for prime focus passing through the off axis adapter[/li]
  [li][/li]
  

Would an off-axis adapter be a good "half way" on the road to autoguding, or should I just go the whole way and look at a seperate refractor (which would also be useful for other stuff).

Any help would be greatly appreciated.

[Kaptain Klevtsov]

With the SCT there is a chance of getting image shift due to mirror flop where the primary mirror moves very slightly as the 'scope pans across the sky. Because of this I would go for the off axis guider rather than a separate guide 'scope.

I understand that finding a guide star close to the target and just off the edge of it is the "fun" part of OAG.

BTW there is no attenuation or image degradation with the OAG as most of the light just goes through the hole in the middle. Some light that wouldn't have landed on the imaging camera is borrowed and redirected to the guide camera.

Captain Chaos

[SteveL]

I did not know that... you learn something new every day, thank you.

[centroid]

With the SCT there is a chance of getting image shift due to mirror flop where the primary mirror moves very slightly as the 'scope pans across the sky. Because of this I would go for the off axis guider rather than a separate guide 'scope.

Captain Chaos

Steve

The LX200GPS has a 'primary mirror' lock to eliminate this problem, does the Celestron not have this facility?

Dave

[barkis]

With the SCT there is a chance of getting image shift due to mirror flop where the primary mirror moves very slightly as the 'scope pans across the sky. Because of this I would go for the off axis guider rather than a separate guide 'scope.

I understand that finding a guide star close to the target and just off the edge of it is the "fun" part of OAG.

BTW there is no attenuation or image degradation with the OAG as most of the light just goes through the hole in the middle. Some light that wouldn't have landed on the imaging camera is borrowed and redirected to the guide camera.

Captain Chaos

Does the C8 SCT not have a mirror lock similar to the Meades, or at least a hole you locate a threaded rod to lock the mirror and fit a Crayford focuser. Should have directed this to SteveL, but I don't think he will mind if you answer for him KK.

Sorry, just read Dave's bit.

Ron.

[Kaptain Klevtsov]

If there is a mirror lock, then disregard what I said, as the problem doesn't exist. Either way will work fine.

Captain Chaos

[SteveL]

No mirror lock as far as I know

Been reading some reviews, and its pretty evenly split between "use an OAG" and "use a seperate scope".... both have their pros and cons. Best I wait for a wedge first though

[Nessy]

Hi Steve, i'm going down this route also (as you probably guessed) but possibly not as quickly as you so would be well interested in what you do (being I'm also using the NS8). Have you decided which wedge yet?

Thanks

Chris

[SteveL]

I`ll be borrowing MartinB's Celestron wedge to try it out, and take it from there i think

[MartinB]

Celestron claim that the mirror on there NS8s are sufficiently well engineered not to need a mirror lock. The focus on mine is certainly rock solid. Having been grappling with the off axis system used by the sxv adaptive optics system I can confirm that it is a bit of a head ache. You have to get the guide camera par focal with the main camera. Getting a guide star at F10 is a challenge. No doubt easier with focal reducers. Cheap option though and without the problem of flexure. Frustration potential 9/10

[SteveL]

Looking like OAG is the prefered option (at least here). Its also the cheaper option to start with, although I am sure I`ll end up with a piggyback OTA at some point in the future.