Which Autoguider?

I'm going to see what kind of luck I have with a Celestron Off-Axis Guider. Obviously, it will require an autoguider but which one? I've searched this forum and I'm not having much luck finding information on these things. When checking them out on the vendor sites, it looks like you could spend anywhere from a few hundred dollars to a few thousand. If an inexpensive one will work, what's the point of an expensive one? Is the tracking that much better, or are we just splitting hairs at that point? The ones I'm considering (which are on the low-end, $-wise) are the Celestron NexGuide AutoGuider and/or the Orion StarShoot AutoGuider. Any thoughts?

(Hopefully I'm starting to getting low on hardware-related questions)

Here you are, back in the deep end again

The downside of the off-axis guider is that you are looking at a very small patch of sky (compared to a guidescope) and where you can look for a guidestar is dictated by how you want to frame your subject. That means that you have much less chance of finding a bright star to guide on.
For example, if you are imaging the Horsehead nebula, there are 5 or 6 stars in the right area for the OAG but they are mag 7-8. With a guidescope and typical guide camera, you can guide on a nearby mag 1 star.

So a better guide camera gives you more opportunity to successfully find and guide on a star. For quite some time, the Lodestar camera stood head and shoulders above the others. Now there are cameras like the QHY5L-ii and the ASI120 that are closing that gap. The bottom line is that the more you spend, the more flexibility you have in framing your subject.

Steve.

SteveInNZ wrote in post #17944622
Here you are, back in the deep end again

That made me laugh. And I'm sure I'll be underwater for quite some time.

I understand. (This was actually also brought to my attention in another post.) My thought was that maybe I could give the off-axis guider a go and if I'm not happy with it, I could add a guide scope later and use the autoguider in whichever configuration made sense. Although I'm hemorrhaging money at this point. I need to put the brakes on a little. Maybe it would be better to return the OAG and go the other route.

The Lodestar looks interesting but yes, it is a little pricey. At about half the price, the QHY5L-ii and the ASI120 look very promising. And to be sure I understand, all of these will work with the OAG I've got, right? I get a little confused when I'm reading reviews and people are talking about using it for videos of the moon, etc.

I saw this review of the ASI120 on Amazon: "Bought this as a guide camera. Using it with my off axis guider and 8" SCT I am able to guide on stars down to the 11th magnitude with 1 second exposures. I just love the sensitivity of it, amazing!" The review about the sensitivity sounds good, but 1-second exposures? Why even use an autoguider for a 1-second exposure?

It sure would be nice if there were a better way to evaluate these things. :-/ Consumer Reports for Astronomy?

That's one second exposure for the guide camera. The imaging camera might be doing a 5 minute exposure but the guiding system checks the position of the guide star every second and adjusts the mount accordingly.

The OAG solves an issue that is particular to the SCT design in that the primary mirror moves as the telescope changes angle. Because the guide system is looking through the same optics, it can correct for that. A separate guidescope can't correct for that.

One thing I like about the QHY5L-ii is that it will go into a 1.25" eyepiece holder so with the OAG I can match the focus position of the imaging camera and the guiding camera quite easily. I expect that would be a bit more fiddly with the ZWO one but I've never tried.

The QHY, ZWO and Lodestar cameras are also good planetary/lunar/solar imaging cameras. I'm not sure if the cheaper options you mentioned can also be used in that manner.

Steve.

The Orion Star Shoot Auto-Guider (SSAG) is extremely popular (possibly the most popular). It's not as good as the Lodestar, but if the Lodestar price tag is not in the budget, then the Orion SSAG is a pretty good choice.

The SSAG has a 1.25" nosepiece (just like an eyepiece) but the OAG will have a male t-thread. It turns out... the barrel of the SSAG unscrews and the threaded end that remains on the camera end is actually a female t-thread.

But you're not in the clear.... when you use an off-axis guider the whole contraption is mounted to the back of the scope and that means when you adjust focus you are adjusting focus on both your imaging camera and the guide-camera at the same time. This means you cannot focus independently. The distance from the pick-off mirror on the OAG to the guide image chip has to be the same as the distance from the pick-off mirror to your main imaging camera's image chip.

To make this work, you may have to pick up some spacer rings (they come in various lengths... 1mm, 2mm, 3mm, etc.) and they have a male t-thread on one side and a female t-thread on the other side (I bought mine from Agena Astro products website) you'll need to measure or work-out the distance that you need to compensate for and buy the appropriate spacers.

I just got off the phone with a guy from OPT. I broke down and ordered the Starlight Xpress Lodestar X2 Autoguider. He kind of reinforced my mindset of going big first. He said that "At least when you buy expensive first, you only cry once."

I did ask about spacer rings to focus the autoguider on the OAG and he told me that the Celestron OAG I'm getting has some type of helical method to change the distance to the pick-off mirror and focus the camera.

Wise choice. Painful, but wise.

Steve.