RoundStars
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Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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What's the most likely cause of the target drifting across the field of view over a series of exposures with autoguiding? This would be with a guidescope/guide camera piggybacked on the imaging scope. I assume it's flex, in spite of using quality rails and rings, everything firmly snugged down -- but I can't seem to get my mind around how it happens. Thanks!
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walt r
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Reged: 02/13/07
Posts: 3463
Loc: Doylestown, PA
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It is common for either camera to move in the focuser with all the cables attached. What method does your focusers have to clamp the camera nose piece?
I've read about others having this problem when using a single screw on the focuser. Some drilled and tapped a second thumbscrew hole about 120° around the focuser that cured the problem.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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John Miele
professor emeritus
Reged: 05/29/05
Posts: 659
Loc: North Alabama
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It could also be your polar alignment is off. If this is the case, you will drift between frames even though you are autoguiding.
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C11/Atlas Mount/Denk II BV
SV102ED/AT Voyager
DSI IIc, DSI II Pro
ST-80, Mallincam Hyper Plus B/W Video Camera
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QueueCT
super member
Reged: 06/25/08
Posts: 100
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I think the question is whether you're guiding on both axes or just RA. If just RA then it's likely that polar alignment is off.
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RoundStars
member
Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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I'm guiding in both RA and Dec. I'm sure that polar alignment is off at least a little, but the mount is responding to commands and keeping the guide scope right on the guide star, without a great deal of movement on corrections. The pier is level, and the scopes are balanced. Camera cables are looped loosely to the pier so that their weight doesn't interfere with equipment movement. I usually make 5-second guide camera exposures. Stars are almost always egg-shaped, in the same direction that the image drifts across the FOV.
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DaemonGPF
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Reged: 03/22/08
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Loc: New Mexico
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I agree on the polar alignment bit. It doesn't have to be off by much to get a gradual drift in the images, even though your guidstar is probably still dead center on your guiding software.
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-Josh
http://cleardarksky.com/c/AlbuqNMkey.html
My AP Gallery
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RoundStars
member
Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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OK, guys. I'll do a careful drift alignment, add a guide camera setscrew, and try again. Thanks for the input!
-Joe
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jmasin
sage
Reged: 12/22/08
Posts: 489
Loc: Murphy, TX (DFW)
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Quote:
I agree on the polar alignment bit. It doesn't have to be off by much to get a gradual drift in the images, even though your guidstar is probably still dead center on your guiding software.
How's this?
I'm not being difficult, but the only way I can see this happening is if the FL of the two scopes (main and guide) are very different. This is probably always the case I guess.
If guidescope FL = main scope FL (and camera pixel size is about similar) I can't imagine any significant drift if you are guiding in both axes. Rotation sure.
Again, not trying to be difficult, just trying to get this in my head.
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Cheers,
Jon
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Orion Atlas 8" (203mm f/4.9 on Atlas EQ-G)
Tasco 60mm (~35 years old and still kicking!)
Nikon D300, 80, 70 & many lenses
http://www.pbase.com/jmasin/astro
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 25206
Loc: NE Ohio
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Quote:
If guidescope FL = main scope FL (and camera pixel size is about similar) I can't imagine any significant drift if you are guiding in both axes. Rotation sure.
Yup. In fact, the guide FL can be up to 5 times less (with the same pixel size) without problems. The only thing I can think of is mirror shift in one or the other telescope, or relative flexure in the mountings of the two telescopes. If the guidestar remains stationary but the imaged stars move, then the two fields of view are moving relative to each other.
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John C
Urban Observatory
A&M/Astreya 76mm F/6 APO
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Tak FSQ-106N F/5 APO
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Meade ETX-125AT
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walt r
Post Laureate
Reged: 02/13/07
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Quote:
... and the scopes are balanced. ...
Stars are almost always egg-shaped, in the same direction that the image drifts across the FOV.
Ok, that's a little more info.
If the scopes are perfectly balanced then it could be gear backlash in the RA axis. Try unbalancing the scope so its heaver in the eastern direction. That way the unbalance maintains gear mesh contact as the RA motor drives the scope westward.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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s58y
Postmaster
Reged: 12/12/04
Posts: 5510
Loc: Eastern NY
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Perhaps you could tell us more about the imaging scope and guidescope, and exposure time.
For example, if you're imaging with a 16-inch f/13.5 SCT and 45-minute subexposures, it would almost take a miracle to get perfect stars, unless you has some super high-end setup.
Does your autoguiding software report the guiding errors (RMS error, etc.)? Are both the DEC and RA guiding errors much smaller than 1 pixel (when scaled to the pixels over on the imaging scope)? If you turn off the DEC autoguiding (or DEC aggressiveness = 0), does the star elongation get a lot worse?
Is the autoguider pointed to a guidestar within the imaging camera FOV? If so, is the guidestar round, or are all stars elongated in the same direction and with the same length across the whole field?
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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RoundStars
member
Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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More info on the troublesome setup: The imaging scope is an 8" SCT. The guide scope is an 66mm APO piggybacked on the SCT.
I try to slightly unbalance the setup to the east and south to minimize the effects of backlash, but sometimes do notice guiding attempts in two directions. I may have too light a touch in that department, and will try unbalancing more heavily.
I am having problems understanding how flexure or mirror shift could cause smooth, uniform drift across the FOV. It seems those would cause irregular and intermittent movements. The smooth drift and elongated stars look more like the guide camera and scope commands chasing the mount's corrections, or vice versa. That's the part that I can't get my head around. Maybe it's something like my corrections are too aggressive. Does that make sense?
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RoundStars
member
Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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I'm imaging with a Canon DSLR on an 8" SCT at f 6.3, and guiding with a 66APO and an Orion autoguider, all on an Atlas EQ-G goto. My subs are typically 180 to 300s. The guide star is within the imaging camera's FOV, or close by. Guide stars appear to be round, but the image scale is small enough that it's difficult to say for sure. At any rate, they're not grossly elongated. I would have to do some calculations to answer your question about the correction aggressiveness expressed in imaging-camera pixels, and I'll do that. I use PHD Guiding, which can generate a text log file, though I haven't studied the numbers to understand what's happening. I haven't attempted to turn off dec autoguiding to see if the drift or star elongation gets worse, and will have to experiment with that on the next night I can see stars.
The star trailing I mentioned elsewhere, when it occurs, is pretty uniform across the FOV. I think it's associated with the object drifting, as the elongation occurs in the same direction as the target's drift. I think that if I identify and eliminate the drift, I'll take care of the elongation too.
Edited by RoundStars (10/13/09 06:28 AM)
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Miguel Lopes
professor emeritus
Reged: 01/04/07
Posts: 695
Loc: Portugal
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You need to go OAG.
It is not a polar align problem for sure, as your guide star is close to the target.
I would blame flexure, mirror flop and other unkown issues. OAG solves that.
I would also say that a 66mm small FL guide scope is not enough, but that is a reason for bad guiding, not drift.
Try OAG. It is difficult to find a guide star but it will solve all the issues you have experienced.
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Astrology is the science for ignorants. Astronomy is the science for those who feel ignorant. - Miguel Lopes
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jmasin
sage
Reged: 12/22/08
Posts: 489
Loc: Murphy, TX (DFW)
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With due respect I don't agree with above. An 8"/f6.3 (1300mm) main scope with a 66mm APO guidescope (~400mm??) should be doable. I guide my 1150mm newt with a 9x50 with no issues.
Have you adjusted settings in PHD? Aggressiveness etc. may make a difference. Maybe you are under-adjusting and what you are seeing is in fact polar drift that you aren't fully guiding out.
The other possibility is if your polar alignment is off enough such that during the calibration there was significant drift. Try adjusting calibration steps to see if you get better results.
I think what could be happening here is poor guiding and you don't realize it because the image scales are different. At ~400mm you don't observe significant shift, but at ~1300mm you do. This is still a doable setup, so if it were me I would start adjusting PHD settings.
What I would definitely do is during the PHD cal, take exposures. Take an exposure during RA E/W cal, then take another one DURING DEC N/W cal. You'll have two exposures with really long lines for stars, but you will definitively know which direction in the frame is RA and which is DEC. Then you can determine which way things are "drifting".
Edited by jmasin (10/13/09 03:19 PM)
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John Wunderlin
Vendor - Spike-a Focusing Mask
Reged: 10/01/04
Posts: 699
Loc: Mineral Point, Wi
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I frequently guide my 2500mm scope with my 600mm ED80 without issues. Polar alignment is my vote as well.
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John Wunderlin
Vendor - Spike-a Bahtinov Focusing Mask
Empire Ranch Observatory
Iowa County Astronomers
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Miguel Lopes
professor emeritus
Reged: 01/04/07
Posts: 695
Loc: Portugal
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It's doable, but it depends on the mount and on the particular setup.
Most errors come from flexure at that focal length.
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Astrology is the science for ignorants. Astronomy is the science for those who feel ignorant. - Miguel Lopes
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DaemonGPF
Post Laureate
Reged: 03/22/08
Posts: 3580
Loc: New Mexico
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Quote:
Quote:
I agree on the polar alignment bit. It doesn't have to be off by much to get a gradual drift in the images, even though your guidstar is probably still dead center on your guiding software.
How's this?
I'm not being difficult, but the only way I can see this happening is if the FL of the two scopes (main and guide) are very different. This is probably always the case I guess.
If guidescope FL = main scope FL (and camera pixel size is about similar) I can't imagine any significant drift if you are guiding in both axes. Rotation sure.
Again, not trying to be difficult, just trying to get this in my head.
You're right, I wasn't thinking clearly when I typed my response. Rotation is what I had in mind, drift is what I was typing to. Complete disconnect on my part.
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-Josh
http://cleardarksky.com/c/AlbuqNMkey.html
My AP Gallery
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Hermit
member
Reged: 10/26/08
Posts: 57
Loc: Northern California
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If you stack your images without aligning (DSS can do this . . . check the 'no alignment' box in the alignment tab), does your image look something like this? I had some cables dangling from my guide scope, and after I secured the cables back along the guide scope to the mount, the little curved star tracks mostly went away. I think the weight of the cables while the scope tracked across the sky was enough to pull the guide scope off center along a curve.
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Meade LX90 LNT 8" Schmidt-Cassegrain
Standard Wedge, LX90 fork, LX200 tripod
Alan Gee II telecompressor f/5.6
Baader short 1.5cm T-adapter/T-ring
Astronomik CLS-CCD clip filter
Self modified Canon XSi with Baader UV/IR filter.
Meade Model 277 60mm refractor with Orion SSAG
Losmandy V-dovetails for balance and guide scope.
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RoundStars
member
Reged: 07/09/08
Posts: 60
Loc: Hutchinson, KS
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Followup . . . .
I did a drift alignment, found a little error, and corrected it.
After doing some test images using my small 66APO, I have litle or no image drift, and improved star shape. I think now that the drift was caused by flex. The moment with the weight of the C8, DSLR, guide scope, guide camera and everything else, must be large, and is probably twisting the mounting rail. I think mounting the OTA with good rings would help here. I need to experiment more. Thanks to everyone for the helpful comments.
-Joe
-Joe
Edited by RoundStars (10/17/09 08:39 AM)
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