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guiding issues - star trails and image shift


craigg

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Hi folks,

I've just updated my setup to use a SW ST80 guide scope having previously used a finder/guider setup.  Here's what I'm using:

Mount: HEQ5Pro

Imaging Scope: WO ZenithStar 71mm

Guide Scope: SW ST80

(both above connected with an Ambermile side-by-side bar)

Imaging Camera: Modified Canon EOS 600D

Guide Camera: QHY5II-L-M

I'm using lin_guider guiding software (essentially PHD for Linux) which was working just fine before using the 50mm finder and the same QHY guide camera.  I've updated the guider settings to give it the 80mm aperture and 480mm focal length of the ST80, also tweaked the gain and exposure to suit it. 

So, when I calibrate and start the guider, I'm getting reasonably good guiding corrections in the guide chart (I think so, anyway, it seems similar to how it looked before with the finder).

Now, here's the issue:- I take around 30 images at about 8 min exposures.  If I cycle through the raw images from start to finish I can see each one slightly shifts towards one side of the image compared to the previous one.  If I zoom in on the images I can also see a small amount of star trailing in the same direction as the drift.

Any hints of tips on what to check would be much appreciated as I'm all out of ideas :-(

Thanks,

Craig

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Have you got the software only making corrections in one direction, or is it correcting in both...north and south...

also, when you changed your guide set up, did you alter the Calibration step size to match the new set up, as with an ST80 you would need a much smaller step size than with the 50mm Finder.... :)

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10 minutes ago, craigg said:

80mm aperture and 480mm focal length of the ST80

I doubt whether it'll be this but try 80 and 400 respectively, make sure the bar it at perfect centre of gravity and don't forget to balance the bar relative to itself -by balancing both telescopes in the clamps so doesn't tip either objective or camera end- as well as ra and dec. HTH

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1 hour ago, SkyBound said:

Have you got the software only making corrections in one direction, or is it correcting in both...north and south...

also, when you changed your guide set up, did you alter the Calibration step size to match the new set up, as with an ST80 you would need a much smaller step size than with the 50mm Finder.... :)

lin_guider seems to be quite good in this respect - in the video settings you just give it the aperture, focal length, sensor size and pixel size and it works the rest out for itself.  I'll have another look though, when I have it running again, just in case I've missed something.  Thanks for the help.

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1 hour ago, alacant said:

I doubt whether it'll be this but try 80 and 400 respectively, make sure the bar it at perfect centre of gravity and don't forget to balance the bar relative to itself -by balancing both telescopes in the clamps so doesn't tip either objective or camera end- as well as ra and dec. HTH

They're reasonably well balanced I think - I've got the two scopes on the dual mount bar positioned so the centre of balance is around the middle of the bar (i.e. perpendicular to the scopes) with both cameras attached and focus tubes around the same position as for imaging.  I've also positioned the bar slightly off centre in the mount's puck as the WO is quite a bit heavier than the ST80, so it stays put when I move the Dec axis with the clutch released.  One thing I was thinking was whether to slightly off balance the counter weights so that it's leaning slightly 'into' the gears, if you know what I mean.  I read somewhere this can help. 

Interested in why set the focal length to 400mm?  I can give it a try by all means, but just curious what the thinking is with this?

 

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Not sure about the Linux version, but the focal length of the guidescope determines the length of calibration step size during the initial calibration routine, this is very critical for accurate guiding, well that's with PHD anyway... :)

i used to use an ST80 for guiding and had the calibration step set to around 750ms, it is the length of the pulse to send to the mount, to move the scope in any given direction, as you don't want it moving too far in each step, or too little either..

the larger or longer the focal length the shorter the pulse, and the shorter the focal length the longer the pulse, needed to move the same amount across the sky.

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3 minutes ago, SkyBound said:

the focal length of the guidescope determines the length of calibration step size

That's where I think the OP has set the wrong focal length. Most if not all st80s are 400mm f5. I don't know of a 480mm version. 

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I would seriously suspect differential flexure...even just a tiny fraction of movement (imperceptible to the human eye) can cause this.

Once I had everything bolted down securely at two points (front and back) the issue went away. Metal to metal contact where possible and no scope rings unless they are clamshell types...

Just my thoughts.

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Thanks for all the suggestions, that's given me a few more things to check.  I may be misremembering the focal length, might be 400mm that I have set, for some reason I thought 480mm.  That's what happens when you get old.  Anyway, I'll go back to verify, maybe it's that simple.

StuartJPP - I'll also strip down the dovetail, clam shells, etc and make sure everything as tight as can be.  Doesn't hurt to double-check.

Looking at the forecast, I'll have plenty of down-time to do this stuff before I get to test it out again.

 

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10 hours ago, michael8554 said:

Hi Craig

I can't explain why, but I find that image shift over a series of long exposures, all in the same direction, means my PA is not quite right.

Michael

If your mount is rotating about an axis that is not centre red on the celestial pole, then instead of following a circle with all the stars rotating about the same point, the stars are rotating about a different point.

If you have long exposures, then the stars are following a different path to the camera - hence the stretched stars.

I understand it - not sure if I explained it.

 

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The ST8 needs to have the whole rear assembly nice and tight in the main tube. It's held in by three radial screws. The focuser also needs to be locked up very tight once set. (Fine focus is not necessary for guiding.) Finally, whatever extension you're using to reach focus must also be tight.

When looking to fix guiding issues the first thing is to have the imaging camera aligned along RA and Dec so you can see what's what with the elongation. Just take a 5 sec sub while slewing slowly and look at the angle of the star trail. Rotate the camera and repeat till the trail's horizontal or vertical and note which is which axis.

To test your PA take an early and late sub and combine them in a stacking programme using star alignment. With a perfect PA the stars and the frames will line up. If PA is out the stars will align but not the frames.

If your version of PHD is like PHD2 it will have the guiding assistant tool which you run for a few minutes. It will suggest adjustments to the parameters which you can then click to apply. I find this a formidable tool.

Olly

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Thanks for all the suggestions.  PA could be an issue - I've had it on a SW pillar mount which has been polar aligned reasonably accurately.  However, I have assumed it's remained aligned over a few weeks, maybe it's moved a bit.  I'll check it next time I can see some stars, whenever that may be.  I must admit, the field rotation you describe makes sense, but wouldn't I be seeing significant adjustments in the guiding chart if that was the case?

I've temporarily bolted the ST80 rings directly to the side-by-side bar, so no clamp or dovetail bar.  It's rock solid, so that should rule out any flex in the guide scope.  The focuser seems rigid, there doesn't seem to be any give in it and the focus lock screw is tight - the extender is a skywatcher kit barlow with the lens removed.  I don't think there are any issues there either.

Fingers crossed it works (and for the cloud to clear).

Thanks again for all the useful comments.  I'll let you know how it goes...

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1 minute ago, craigg said:

Thanks for all the suggestions.  PA could be an issue - I've had it on a SW pillar mount which has been polar aligned reasonably accurately.  However, I have assumed it's remained aligned over a few weeks, maybe it's moved a bit.  I'll check it next time I can see some stars, whenever that may be.  I must admit, the field rotation you describe makes sense, but wouldn't I be seeing significant adjustments in the guiding chart if that was the case?

I've temporarily bolted the ST80 rings directly to the side-by-side bar, so no clamp or dovetail bar.  It's rock solid, so that should rule out any flex in the guide scope.  The focuser seems rigid, there doesn't seem to be any give in it and the focus lock screw is tight - the extender is a skywatcher kit barlow with the lens removed.  I don't think there are any issues there either.

Fingers crossed it works (and for the cloud to clear).

Thanks again for all the useful comments.  I'll let you know how it goes...

Hi

Make sure you're balanced in RA, DEC - and vertically. After balancing you can make things a little 'east heavy'.

Louise

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