WFieldPhotoGuy
super member
Reged: 09/05/04
Posts: 127
Loc: Texas, USA
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I haven't had much chance to get out and practice everything needed to get better at my imaging process, (do the clouds always hover over your house too?) so I have been reading some on guiding. Every reference I read mentions selecting a guide star close to the center of the frame I'm phogtgraphing. My question is if I'm guiding to reduce or eliminate RA error, (I only have a RA motor) why would it matter where the star I use was located. Isn't RA error, RA error. I mean if due to any number of factors, PE, Backlash, Motor running too fast/slow you have to adjust the RA rate, why would it matter where the star was.
Now I understand atmospheric conditions are specific to the area you are pointing, but if my objective is to reduce or eliminate the RA error my mount is producing, would it still matter where my guide star is? 
This may be elementary, but somehow I think others just starting out might be confused over this as well.
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Home Made Barn Door Mount
Apogee HD Mount (CG5 Clone)
Apogee 80mm AT
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4534
Loc: Pleasanton, CA
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You don't want the guide star too far from the object you're imaging. The distance is a function of focal length (no, I don't have a hard and fast rule).
The reason is that the stars don't trace perfectly concentric circles at a constant rate as earth rotates (??? you say?) because of atmospheric refraction. So at long focal lengths, if you track a guide star far away from the object of interest, you'll still get drift and field rotation.
When using a guide scope I generally try to track on a star in the same FOV as the main scope/camera combo and never end up with problems that way.
Suk
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WFieldPhotoGuy
super member
Reged: 09/05/04
Posts: 127
Loc: Texas, USA
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Suk, could you elaborate a little more. It's still not clear why the location of the guide star matters. Unless atomspheric refraction is the main cause, I don't think I quite get it.
I guess another reason I ask, is there always isn't a fairly bright star in the FOV to use as a quide star. Tracking on dimmer stars can be done, but it looks like guiding manually could be a real pain. I haven't tried it yet, but it looks like some type of auto guiding system might be in my future. 
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Home Made Barn Door Mount
Apogee HD Mount (CG5 Clone)
Apogee 80mm AT
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
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Isn't it also true that stars of greater Dec (closer to the pole) subtend smaller arcs across the sky than do stars nearer the equator?
So, pathological case, if you were guiding on a bright star near the pole (say Dubhe in the Big Dipper) but imaging on something near Dec 0° (say M42), then Dubhe would "move a shorter distance" across the sky than did M42 while you were guiding, thus possibly throwing off your guide corrections?
It seems like I've heard or read this somewhere, and I could be totally off base.
Jimbo
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Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
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WFieldPhotoGuy
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Reged: 09/05/04
Posts: 127
Loc: Texas, USA
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That makes perfect sense to me. Although I wasn't thinking of trying to guide on something 80-90 degrees away from what I was imaging.
For instance, if I have a FOV of approx. 7deg's with a 300mm lens trying to image say the Rosette Neb. The brightest star in that field is somewhere around mag 4.3, but 10deg's away there is a 0.45mag star. Much easier on the eye to guide on that for 30min, than the mag 4.3 star.
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Home Made Barn Door Mount
Apogee HD Mount (CG5 Clone)
Apogee 80mm AT
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
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What I'm trying to say is that (I think) it depends a *lot* more whether those 10° are in Dec or in RA. 10° in RA, probably not a problem probably. 10° in Dec, probably sad.
In other words, near the Rosette, given the choice of guiding on Betelgeuese or Sirius, pick Betelgeuse (because it's only about a degree or so north, compared to 10° south for Sirius).
BTW, guiding on really bright stars often throws me off when I'm manually guiding; I like a star that's a nice sharp point but still bright enough to see without straining. Bright stars are often too "blobby" for me to get a good feel for their drift.
YMMV, I am not a doctor, IMHO, hope that helps.
Jimbo
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Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4534
Loc: Pleasanton, CA
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Both the effect I described and the one Jimbo described can become quite a problem when you're taking very long exposures on a big sensor (i.e. a big piece of film  )
10 degrees is quite a bit, for a long exposure (30 minutes+) you might experience some field rotation. It's another reason over/under guidescopes are a little better than side-by-side (although I personally prefer side-by-side for relatively short exposure CCD astrophotography for other reasons having to do with the ability to mount really large OTAs next to each other): the offset of the guide-scope and imaging scope from the DEC axis exacerbates tracking problems when the guide star is far from the imaged object. Think about different arcs and the offset from the DEC axis when side-by-side and it should become apparent that corrections are going to drive side-by-side scopes to look at very slightly different portions of the sky.
And, if you haven't manually guided for 30 minutes yet, you're in for a real treat. Especially the first time you carefully guide for 30 minutes and then realize you didn't trip the shutter. I didn't really start up the astrophotography learning curve until I bought an autoguider, life is just too short for me to sit outside manually guiding for hours at a time!!!
Cheers,
Suk
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Blueshark928
Carpal Tunnel
Reged: 02/19/04
Posts: 2853
Loc: Crownsville, MD
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Quote:
I didn't really start up the astrophotography learning curve until I bought an autoguider
Same here. The longest i've manually guided a photo was 10 min. I bought an ST-4 right after. Been using it ever since.
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John
SV-102APO
SV-80/9d
Losmandy G11
HX-916
Edited by Blueshark928 (03/04/05 04:52 PM)
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
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I admit that 20 minutes manual guiding was about 5 minutes too long for me.
But I accept astrophotographers of all faiths, and I have to say that there's a certain level of respect that goes out to manual guiders. 
Jimbo
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Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
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WFieldPhotoGuy
super member
Reged: 09/05/04
Posts: 127
Loc: Texas, USA
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Thanks guys. I guess the best way to learn is to do it. I haven't bought a scope yet, but intend to soon. I think i'm leaning towards modifying my CG5 for autoguiding with the kit mentioned at this site. http://www.store.shoestringastronomy.com/products.htm
And use the camera mentioned here http://www.supercircuits.com/index.asp?PageAction=VIEWPROD&ProdID=4166
And finally the Apogee 80mm Richest field for a guide scope.
Total package about $300 delivered. Any suggestions for something better/cheaper, or more suited to the use of a guide scope, let me know. I don't intend to do much in the way of visual observing through the scope. My primary interest is for a "cheap" but usable platform for widefield work, using film. Thanks for all the help.
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Home Made Barn Door Mount
Apogee HD Mount (CG5 Clone)
Apogee 80mm AT
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Anonymous
Unregistered
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uuuuhhm..... you all started me thinking (whoops! )
when drift aligning you use stars at or near the celestial equator because drift is fastest to be seen there.
so when trying to polar align on stars at other DEC, results will be less good.
doesn't the same go for guiding? as long as you use a guide star on the same DEC you're ok?
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
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In short, that's the general idea, yes.
Jimbo
--------------------
Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
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