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  #1  
Old 28-02-2011, 10:46 PM
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[1ponders] (Paul)
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Question Movement between Subs. Any thoughts? - Latest result???

I'm trying to work out why I and getting drift between subs atm. The individual subs look pretty good, but there is a distinct movement between them to the south and west.

The star I'm guiding on is pretty much in the middle of what i'm imaging, besides there doesn't seem to be any rotation in it. Just movement.

Could this be another indication of flexure?

Polar alignment? (though I checked it a fortnight ago and it required minimal adjustment)
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Old 28-02-2011, 11:01 PM
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Octane (Humayun)
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I'd imagine that's flexure biting you in the rear end. : (

H
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Old 28-02-2011, 11:07 PM
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As a test, try stacking a few hours of guided images without aligning them.
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Old 28-02-2011, 11:24 PM
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[1ponders] (Paul)
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Yeah that's my thinking too Humayan. Long extensions back from the focuser to get cameras to focus. And now that I look at it its obvious which way the camera (particularly the guide camera is going to sag. S&W

While I haven't stacked them unaligned James I have had a number of them open at one time and then quickly closed each one (sort of like a poor persons animation) and the movement is pretty much a straight line in one direction, no rotation that I could see, or none large enough to really worry over.
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Old 28-02-2011, 11:33 PM
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Octane (Humayun)
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Paul, last winter, something similar happened to me.

I was shooting the Rho Ophiuchus complex with the modified 40D on an ADM Accessories ballhead mount for Losmandy plates. It was mounted on top of the ZenithStar 80 at the time.

I got 7 hours of data or something close to that. Only to realise when it came time to stack, that, little by little, there was movement as the camera continued to sag throughout the night. Wasted data. : (

H
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Old 28-02-2011, 11:36 PM
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Yep that's what its like. There is still a couple of connections that are thumbscrewed and not threaded. I'll work on those for the time being
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Old 01-03-2011, 08:09 AM
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Along the same lines, I elliminated that flexure by using an 1 1/4 diagonal attached to the end of my 70mm WO guidescope. The extra extensons to bring the guide camera to focus were thus elliminated.

Setup being WO 70mm, 1 1/4 diagonal straight into the rear then the guide camera, worked perfectly.

Cheers
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Old 01-03-2011, 08:20 AM
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When I was imaging some time ago this used to happen to me as well, however although the slight drift the images were quite stackable, (if that is a word) and had no effect on the out come of the final image.

Well I didn't think so anyway, and my images turned out pretty reasonable, I think.

Leon
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Old 01-03-2011, 08:34 AM
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John, the DSI with all those extension tubes on the right of frame look very familiar. : P

H
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Old 01-03-2011, 08:46 AM
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Quote:
Originally Posted by Octane View Post
John, the DSI with all those extension tubes on the right of frame look very familiar. : P

H


Got a few pictures of your DSI setup, H........

No wonder it bent under it's own weight .

Cheers
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Old 01-03-2011, 09:01 AM
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Oh, nah, that one was fine. I used to make sure it was done up very tightly.

It was the DSLR with 200mm lens on the ballhead mount that killed the Rho Ophiuchus data. : (

Feel free to email me the piccies at work. : )

H
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Old 01-03-2011, 11:34 AM
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Quote:
Originally Posted by [1ponders] View Post
Yep that's what its like. There is still a couple of connections that are thumbscrewed and not threaded. I'll work on those for the time being
Paul, I'm experiencing the same thing on my Titan. It is a slow but consistent drift in one direction from one sub to the next. I've listened (read) to what's been said about flex but somehow, that explanation seems to be a bit shy of convincing. This is a progressive movement - i.e. it is an ongoing and cumulative process. If the guidescope was indeed sagging under the weight of extensions and the like, and therfore unstable, I'd expect wobbles in the odd sub as a bit of wind or whatever affected it and made it move, but with guiding going and no meridian flips involved, there's no great lumps and bumps happening. Why does it happen in every frame in the same direction and why is it cumulative?

Peter
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Old 01-03-2011, 01:45 PM
pjphilli (Peter)
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Hi Paul

I have always had a problem with this when using my DSI2 to the extent
that for a long imaging session I must manually align. The end product does not appear to suffer though. I spent a lot of effort in chasing flexure etc but with no improvement. I still get a little bit of movement when using my Canon 400D but not enough to prevent auto alignment. I use a QHY5 guide camera.

Cheers Peter
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Old 01-03-2011, 01:56 PM
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Combination of both sag and polar alignment. I don't get this anymore because I worked hard on my PA with several nights of refinement under T-point. And; I only guide off axis now. It took a while to convince myself that OAG is the way to guide, but after a while you see consistent results. Flexure is a beast best left aside. I would recommend an AOG and work in your PA a bit more.
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Old 01-03-2011, 03:01 PM
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I have a Celestron Radial Guider for sale in the IIS classifieds!!!

Hehe - opportunist!

DT
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Old 01-03-2011, 03:05 PM
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Paul,

this is an "out there" thought but here it is, FWIW.

I have noticed that when I image with Nebulosity and guide with PHD on the same computer, there is always a time during which Nebulosity announced that it is "Not Responding". I think this is usually during download and loading the image in Nebulosity. During that time, it seems that Nebulosity is using pretty well all the clock ticks that the CPU of the computer can provide - i.e. PHD struggles for resources to continue doing what it does for that short interval between subs. Is it possible that the slip occurs during these short periods when guiding is not able to get access to the CPU cycles it needs to send the necessary corrections to the mount and does not resume doing that for sufficient time for the image to slip?
Others with better knowledge of how PHD uses CPU cycles could no doubt tell me I'm barking mad and many who lack that knowledge will probably do so anyway. But what the heck!

peter
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Old 01-03-2011, 03:17 PM
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You may be onto something Peter.

PHD seems rather processor intensive - it certainly takes a long time to respond to keystrokes when it is guiding.

I think Maxim disables guiding whilst downloading of the image is occuring and then waits until guiding is stabilised again before starting to image again. Tends to suggest that processing power may be limited.

DT
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Old 01-03-2011, 03:38 PM
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Hmmm, not sure if that would be the case Peter unless you are using a single core system to guide and capture. In that case I can see how things could get a bit locked up and slip a bit, however PHD should still notify you that you have lost lock a little. With dual core processign I cannot see how this would be a problem.
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Old 01-03-2011, 03:59 PM
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Thanks for all the info folks. Much appreciated.

I should say that the drift doesn't appear to affect my final images. The still stack np, I just have to crop the image a bit after stretching. Its more an annoyance than anything.

I've also taken a bit closer look during an 'animation' and the general direction is S/W but there are the odd frames where the drift is only S and others only W.

John that is how I used to guide exclusively, star diagonal and guide camera. It always worked a treat for me. I can't remember why i changed. Probably because I put a flip mirror into the equation.
Paul, yes I'll check on my PA. I was very confident when I first set the AP up, but found I had to tweek it a couple of weeks later. Settling ? But another check certainly won't hurt. I'll persevere a bit before going the OAG route. Tried it when I first started imaging and swore I'd never go back there. Though I've learnt a thing or two since then. Yes i saw that David. I still shudder.

I'll look into another guide prog Peter. The AP came with Pulse Guide but I've not loaded it as I've always been happy with phd. Just another new thing to learn. Oh and i use either the latest Maxim or M4.5 (depending on the night, still not a big fan of the new camera control interface compared to the old one)

Plenty of food for thought, thanks everyone.
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Old 01-03-2011, 05:24 PM
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Quote:
Originally Posted by DavidTrap View Post
I think Maxim disables guiding whilst downloading of the image is occuring and then waits until guiding is stabilised again before starting to image again.
DT
It does, with SBIG anyway. Paul, you said the subs were "pretty good" and the movement was between subs. So do you see any elongation at all in subs?.

If subs are OK, and the movement is only between subs, then flexure is not a problem (assuming the sub exposure times are far longer than the time between subs). How can flexure be a problem if 15min subs are OK and 20 seconds between subs show movement?.

A few years back I had this problem. On asking lots of experts (some of whom had seen this), there was no answer. I wondered if it could be guide settle time between subs, dither etc, but in the end the subs were OK, so it wasnt really a problem, I ignored it, although it was a puzzle..

If there is some elongation in *stacked subs* and more movement between stacks, then it makes sense the movement between subs is negated by the sub align processing, accentuating the difference between stacks. Flexure then could be the cause.
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