The usual question: off-axis driving or tele-driving?

I'm setting up an RC8 to mount in my small remote private observatory. I have documented a lot about the guide for important focal lengths such as the full native one of the RC 8. Most recommend an OAG ….. I think I understand that to do this, especially from a remote observatory, you need a sensor up to it, so not inferior to a Lodestar or an ASI 174 MM mini. This is because remotely you must be sure to see the star always, not being able to act on the prism in both rotation and angle. We are therefore talking about top but expensive CCD. Now to start I will definitely rely on the guide with tele-guide and the trusty SSAG, then we'll see ….. My question is: will I have problems at 1.624 focal length to get the round stars? Making sure that there are no flexions between the guide ota and the main ota, nor flexions of the fok of the tele-guide because in general the guide with tele-guide is considered more poor than the OAG? Often with the OAG you drive on deformed and low quality stars ….. so I would like to understand well if there is a technical / scientific reason …. I know with the OAG you drive a star almost inside the sensor field , while in the other way no ….. but what changes?

Hi Galassia,
After sifting through your information, I can offer only this. For accurate guiding with a separate guide scope (without all the annoying math) the guide scope must be at least half the focal length of the imaging scope, and obviously, aligned well with no flexure between OTAs. But the best way, hands down, is with an off-axis guider. Yes, the OA guider uses a small pick-off mirror that barely intrudes into the field of view, but enough to catch several stars for you to choose from. And also, PHD2 is fine for an autoguiding program - and it's free. It has worked for me for a very long time.

Just for experimentation, I tried guiding a 2800mm telescope with a 480mm refractor - no good - not happy with results. Off-axis guiders are not that expensive, really, and neither are small guide cameras (from Orion, etc.)

This is simply the best way to go to eliminate any real issues. I've been OA guiding for many years, and would never opt for any other method.

Hope this helps a bit.

- - Steve

I think that what a lot of folks don’t fully appreciate is the precision required to produce even 2” star images, which should be achievable with your 8” RC. Remember that an arc-second is an extraordinarily small angle. It’s the angle subtended by a dime at about two miles! At that level, everything in your scope is made of rubber. That’s right…all of your seemingly stout aluminum parts, the optical mounts, and connections between them can mechanically flex by an amount that’s compatible to the precision that you need to achieve to produce a high quality image. That is why it is almost essential to guide through the same optical path that is producing the image. To be fair, you can go to extraordinary efforts to get good results with a guide scope with some scopes but it is usually far easier and more reliable to simply guide through the scope—particularly when you are using a Cassegrain type telescope. OAG and ONAG are pretty easy to set up and once configured, very easy to use. Getting this stuff set up and running is just part of the process of configuring a system to produce high quality images so enjoy the ride.

Good luck with it!

John

For large telescopes (and I mean LARGE, not 8") at high image scale there might be really little option but to guide off-axis, which have the extra advantage of actually guiding on the same optical path of the main sensor, so any mirror flop or sidewise movement is corrected. For anything else the option of using a guidescope is a just as good and allows using smaller sensor (e.g. cheap) and shorter focal length scopes (up to effective pixel scale 1/10th of the imaging scope). I have always used guidescopes since the days of film and I'm not looking back. A OAG puts a number of restraints on smaller scope in several situations which frankly are unwarranted for. Well, unless you use SCTs but there the issue is mirror flip-flopping.

Andrea,
I think that the cross over point for guiding through the scope happens in the 8"-10" range, but of course that depends on the mechanics, the available BWD, the field correction, median seeing conditions, and the capability of the mount. As you pointed out, Celestron and Meade SCTs suffer from mechanical issues that make it almost essential to guide through the scope. In order to push the imaging capability right up to the limit of seeing, guiding through the scope for most two mirror systems is a good idea. Two arc-seconds is two arc-seconds whether you have a 10" scope or a 24" scope. Heck I even use OAG on my 130 mm refractor. It's cheap, it's reliable, it's easy, and it works really well.

John

Thanks guys, so from what I'm understanding, if I hypothetically want to get closer to the resolution limit of my ota, I will necessarily have to rely on an OAG. Okay, I'll start saving my pennies for a good guide sensor…….

:happy-3

nce the optical train has been set up with the off-axis guide, so with a sensitive guide camera I will have no problem finding the guide star immediately …. I mean I will not have to have other accessories such as the field rotator, etc … working remotely? At the limit, as suggested by others, the shot shifts slightly.

I use a Lodestar II, which works very well, but is a bit pricey. I have several imaging friends, one of whom uses a ZWO ASI120MM-ss planetary camera, and another who uses an Orion planetary camera (can't remember the model). Both are guiding 2800mm Schmidts off-axis with wonderful results. Both also use PHD2 as their guiding software.

In years past (film days), I've used various off-axis guiders, and have only had difficulty finding a guide star a handful of times, primarily because the starfield containing my imaging target was unusually sparse, not through any defect or inadequacy with the OA guider.

Good luck and mush on!

Steve

GALASSIA 60:
Thanks guys, so from what I'm understanding, if I hypothetically want to get closer to the resolution limit of my ota, I will necessarily have to rely on an OAG

Absolutely not. If you want to achieve that you need lucky imaging, if your scope is large/fast enough (and the seeing is good enough). All the other ways of guiding have been used to achieve good guiding results, off-axis or on-axis.

BTW - The Schmidt I use is an 11" Celestron EdgeHD, which has a lockable primary mirror. From reading the above contributions, this is a consideration, although one of my above-mentioned friends has a regular Celestron 11" and doesn't seem to have difficulties. The trick is to get 'behind' focus first, then focus 'up', so the primary is resting against and being pushed up by the mirror's focus cradle. This usually ensures that there won't be mirror flop or shift during the exposure.

Another mention - which I'm sure you already know - is to allow plenty of time to let the mirror cool to ambient temperature. This is critical, especially in summer, when daytime temps can vary 30-50 degrees from nighttime temps, depending on where you are.

There, I think I'm done.

Steve

BTW - The Schmidt I use is an 11" Celestron EdgeHD, which has a lockable primary mirror. From reading the above contributions, this is a consideration, although one of my above-mentioned friends has a regular Celestron 11" and doesn't seem to have difficulties. The trick is to get 'behind' focus first, then focus 'up', so the primary is resting against and being pushed up by the mirror's focus cradle. This usually ensures that there won't be mirror flop or shift during the exposure.

Steve, my new OTA is an RC8 (Ritchey-Chretien configuration), so I shouldn't have any imaging shift and / or mirror flop issues. I previously had a Celestron C8 XLT, an excellent all-rounder tool, but for these reasons and also for the presence of the front corrector plate which fogged up very frequently, I stopped using it for deep-sky astrophotography ....

You need both! OAG can be more precise, but it doesn't always work.

CS

Hi Galassia

Use a guidescope.

People confuse main scope resolution used for image formation, which is limited by the seeing, with the star tracking capabilities of a guidescope. The resolution of the guidescope/sensor combination (in arcsec/pixel) is nowhere near the resolution of the main scope (in arcsec/pixel). But this is not the full story: 1) what you can image is limited by the seeing but alas the guidescope resolution still falls short of seeing; but 2) remember the PhD2 folks say you can detect to a 1/10 of a pixel the location of the center of the "less resolved" star and this is well inside the seeing resolution and by a factor of 4 or more than what is needed.

Numbers (example):
• 60mm F4 guidescope with ASI294 (2.9µm pixels) has 2.5 arcsec/pixel resolution and therefore 0.25 arcsec/pixel tracking under PhD2
• 1600mm F8 RC8 with ASI1600 (3.8 µm pixels) 0.5 arcsec/pixel but seeing limited to 1-2 arcsec/pixel.I use a guidescope

Because my RC8 and guidescope are super stiff then my 3 OAGs sit on shelves no longer used. I really detest using/configuring OAGs. They are up there with mosquitos in detracting from a great hobby. Remember with a guidescope you have more stars to choose from and there is multistar tracking.

Rod

Hi Rod!
Your considerations make me very happy, also because they are documented by technical calculations …… So far I have photographed with a Newton 250/1200 and Atik 383 L +, driven by a SW 70/500 and SSAG. I had about 21 Kg. of weight on the poor NEQ6. And I can tell you that I managed to make well-chased shots of 15 minutes even with this set-up beyond the range limits. Now the mount is to do the tuning and mount the Rowan Kit. When I can get it back, I believe that the weight of the set-up is halved, and I hope the mount is even more performing. Since I photographed I have never had problems finding stars with this guide system and therefore I believe that I will not complicate my life ……. Of course I will make sure not to have any bending between the two systems, so with vixen bars / losmandy well anchored and rigid focusers. Then the results will give the verdict ….. but first I'll try to do what I've always done …… Clear skies.

Related discussions