JimH123 Loc: Morgan Hill, CA

Just started reading about a method of guiding called On Axis Guiding. Everyone is familiar with the use of a Guide Scope for tracking. And there is also off-axis guiding as in:

http://www.telescope.com/Astrophotography/Autoguiding-Solutions/Orion-Deluxe-Off-Axis-Guider-for-Astrophotography/c/4/sc/60/p/5521.uts

But On-Axis guiding is another method altogether. I don't have this, and I have only been reading about it. It is termed "ONAG". A company, Innovations Foresight, has developed a product containing a special 45 degree angle mirror that reflects all light from 350nm to 750nm, and passes all Near Infrared (above 750nm) allowing a camera to collect the visible light and a tracking camera to watch the IR portion of the spectrum. This unit attaches to a SCT type scope.

http://www.innovationsforesight.com/education/on-axis-guider-onag/

A review of the product:

http://www.star-watcher.org/onag-rev.html

PDF on ONAG
Attached file:
(Download)

SonnyE Loc: Communist California, USA

Yeah, I was reading about it over <on another forum>.

:twisted:
Look, I just got my PHD (no, not a doctorate) and I'm guiding for hours and hours on end.
Last session I picked my own star for Orion's tracking and was on for 5-6 hours straight. (Probably nOri, or 31 Ori)
And part of that time I fell asleep while the camera kept imaging, and the mount kept tracking like the entire Universe had stopped.
I keep track with PHD's bulls-eye sight, and also zoom in on Stellarium to see the drift of the little telescope icon. Compared to the target DSO, the bobbing around is so marginal that it is barely perceptible.

And I have discovered another tiny trick: If for any reason PHD isn't acting properly, I found that shutting down the program (PHD) and restarting it after a slew or meridian flip, has cured all ills. Think of it as a reboot. ;)
It works Miracles I tell ya. Miracles

Now why on God's green earth would I wanna mess with that? :hunf:

Besides... I am a refractor guy. I'll leave this for you bucket heads (SCT's). :lol: :lol: :lol:

OK, sirius now: (Sirius, get it?)
I tried for months to get an OAG to work for me. Never would. As soon as I hung the 50mm guide scope on, BAM, things started working.
Know why I tried so long to get an OAG to work? Because it makes the most sense. Give the guidance system the same FOV as you are trying to image.
In my own reasoning, give the brain running the tracking an eye on what you are looking at. A guidance system should be just that, vision for the mount.
I've recently learned that there are CCD cameras that guide and image at the same time. Perfect!
OK, hold on to your hat, it only takes around $5 - 15 K to get one. :shock:
So, for now, I'm really happy with where I'm at.

There will always be something new coming out.... ;)

Oknoder Loc: Western North Dakota

Axis guiding eliminates a problem that occurs when using a two optics system called flexure. Its had to diagnose because it looks so much like drift or field rotation due to not being polar aligned properly. I have read up on the on axis guiding and it is intriguing but there have been a few complaints mostly due to inner reflections, but those should be correctable with properly mastered flats. When I first read about it, last year I think I though of trying to DIY one using a flip mirror system but never got passed the dream phase of production.

Bloke Loc: Waynesboro, Pennsylvania

I've never reached the stage of trying to guide a scope, but I have often wondered about that... It seems only logical, if you want to guide so you can image 'X", then why can't you guide *on* "X"??? Anything else just complicates the procedure, surely?

Oknoder Loc: Western North Dakota

in theory that is what you are striving to do in reality though that is much easier said than done. normally you must just be content with tracking an object before long the same declination as the main object you are trying to image.

Matthew

SonnyE Loc: Communist California, USA

I think what it amounts to in guiding a telescope is compensating for the rotation of wobbly old Earth.
So when we are zeroing in on a DSO, the scene isn't moving, the stars, if moving, are moving much slower than us here on the Earth.
So... aiming at a star or DSO in the field we want to observe, the guide scope can lock onto many different stars than our target, and the DSO target will stay stationary in our guided scope.

To support this, my guide scope can and will lock onto most anything in it's field of view, and over the course of hours and hours the main telescope will stay locked on pretty close to the DSO we wanted it on.
PHD 2.5.0 has a part in it (Tools) where it will Auto-select Star. And usually it will pick something distant from the actual targeted DSO.
It doesn't matter so much because if the two are moving in space, they aren't going to move much in relation to how much our spinning planet will move.

One real wild variable to this is we minions drag out a bunch of telescope equipment, plop it down and do our best to set it up level and plumb, then whine because we did or didn't get a good alignment on it.
Good grief, there are so many variables in play that it is a wonder we actually get what we do.
Only in the last week or so have I been able to get a good tracking on Orion's Great Nebula. But it isn't for aiming at the heart of my intended target, but at some lesser star distant from the target.
I think that targeting the heart of the nebula it is so bright that the guidance gets confused, and you can't pull a decent picture for the mount bobbing around.
Set your guidance on a lesser star, and you get finer guide pulses.
Make sense?

JimH123 Loc: Morgan Hill, CA

Yes, this does make sense. And for the same reason, I don't use a bright star to focus my camera. It is too fat, and I can't tell what is best focus. But on a dimmer star, it is much easier to tell.

Bloke Loc: Waynesboro, Pennsylvania

Ok... It seems that, the further apart the target and guide star are, the more variation there will be in their motion. If you look at pictures of star trails, stars nearer the pole will have a shorter arc than those far away. This is most obvious in those pictures which actually show Polaris, but it will also apply to any other stars which are not both situated on the equator, or *above* the equator technically.

Tracking should be more accurate, the nearer the guide star is to the target object. Taken to the extreme, the closest it can get is to *be* the target. This is talking about stars, however, point sources. I realize that tracking on a nebula or a galaxy would present issues.

Oknoder Loc: Western North Dakota

You're right bloke, but only for stars in declinaction does that apply. If the stars are on the same right ascension plane then they will move at the same rate. this is called Siderial which is the same speed at which the Earth spins given your latitude but just in the opposite direction.

If we are being technical then in all reality every star is moving at a slightly different rates and some even in different directions, upwards of over 100 kilometers a second.

Guiding can be tricky for a few different reasons the main one being the sensitivity of your guide camera along with the focal length of the scope you are guiding with if different than the scope your imaging with. rule of thumb says that your guide scope should be 30% longer in focal length than your imaging scope. I can't speak for anybody else but I do not follow this rule and some of my images it is evident in.since I mainly image at well over a thousand millimeters and guide with only a 400 millimeter scope I will on occasion develop stars they're slightly oval or egged.this is all dependent on that nights seeing and how cooperative the mount wants to be.

Matthew