Manual Guiding Technique and Tolerances
Posted 11 October 2006 - 04:02 AM
Posted 11 October 2006 - 09:38 AM
Lee, and CF are actually telling you the same thing.
What they are saying is that field rotation in longer exposures WILL show up if you are aiming your camera too far off the DEC inclination to the guide scope. Plus in the config you posted the camera mounted to the focuser on the newt is OK, but the the other, if used for vary long exposures will indeed produce field rotation images, because it is not on the DEC axis.
The term *Piggyback* doesn't imply you can just put your camera any ole place on the OTA or mount.
Piggy back means you place your camera/imaging scope on the back of your main scope directly opposite the DEC axis of the mount. Opposite of the counter arm. In this way the camera will indeed be aligned to that axis. If you place your camera, say on the side of your scope at right angles to the DEC axis, you will have moved your camera's angle of arc from that of your guide scope. In short exposures this isn't really a problem as CF has pointed out. If using a 50mm-200mm camera lens for the primary lens for wide field images, this still isn't to much of a problem if exposures are conservative.
When shooting OFF axis, or when using a guide star when imaging those dark, I can't see'ems, you need to do as CF says and use a star very close to the DEC of the target. If not, well, as CF says, you will definately have FR.
Sure your target object will be spot on, but all the surrounding stars will look like they are rotating. Which is what they are doing.
Rob (what do I know anyway)
Posted 11 October 2006 - 09:51 AM
You need to be GUIDING in the same area of the sky you are imaging. Actually, it's the DEC that causes the most problem. You want the declination of the GUIDESTAR to be as close as possible to the same declination of the SUBJECT you are photographing. Objects in the sky with different DECLINATION will travel in different ARCS, therefore they need to be the same to prevent rotational trailing.
Good explanation CF.
But I think we are crossing our hairs, when it comes to explaining what the DEC/RA axii are on the scope.
The counterbalance arm is not part of the RA axis it is part of the DEC axis. It goes down the center of the DEC axis. However it, and the OTA rotate on the RA axis.
This can get comfusing for some, if we are not clear.
In this thread the counter arm, has been mentioned as both being on the DEC, and RA axis. Just because the counter arm rotates within the RA axis doesn't make it part of that axis.
Posted 11 October 2006 - 02:19 PM
Posted 11 October 2006 - 06:15 PM
I agree my terms were not 100% correct. I edited the posts to remove the RA reference when talking about the counterweight shaft. Thanks for catching that!
CF, we are here to dispence accurate information, and you are the best at that, for this subject. I just wanted to make sure we are all on the same page, so those who are new to the subject can learn with as little confusion as possible.
It's a tough subject any way, and you've got the lead on it.
Rob(just a helper)
Posted 11 October 2006 - 06:43 PM
Okay, I always thought that a camera riding piggyback could be mounted anywhere and could even be pointing at a different part of the sky. In my mind everything is moving at the same rate so how does field rotation happen (if the polar alignment is good)?
In an over-under situation (like your scope), you can get field rotation, when the two scopes are pointed at different places in the sky because the stars don't actually track in perfectly concentric circles
That's caused by atmospheric refraction, a prism effect. So not only are the arcs slightly different depending on where in the sky the star is, the tracking rates are different as well. So that's why even if you had theoretically perfect polar alignment and tracking rates, you'd still have to guide/autoguide.
So this setup in the link below is not really all that reliable?
And (question for Suk)if that config in the link is wrong, what is the part of the takahashi rings that protrudes outwards that I have my wide camera on for? I thought it was for piggybacking.
You're using it exactly the right way and this configuration is the most reliable from the point of view of minimum likelihood of field rotation. The disadvantage compared to side-by-side configuration is that as you add more stuff, you're getting more weight further out on the mount, making mount rigidity and tracking performance more critical.
For example, this is why Astro-Physics recommends over-under rather than side-by-side - less chance of field rotation. I'm citing Astro-Physics not because of any refractor claims (heading that argument off!) but because they (Roland) are very experienced imagers and it shows in the way their equipment is designed.