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Old 21-03-2012, 08:24 AM
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Guidescope alignment to Imaging OTA ?

How cirtical is it that the guidescope is aligned to the imaging train OTA ?

With the very wide 50 and 80 mm guidescopes we are now able to use, guidestars can be fairly far off axis from where camera is pointing.

How much of an impact does this typicaly have on guiding ? Is it better to ensure they are pointing at pretty much same part of the sky ?

What are the symptoms of not having them aligned ?

I aks becuase recently when Imaging objects nearer to SCP i am having eratic guiding, whereas objects nearer to dec zero seem to have no guiding issues.

2 nights ago I was imaging an object near scp. I could not seem to get round stars even though PHD was showing straight lines on the graph. Seemed like I was getting image rotation. I then switched to my internal guide chip on the ST-2000XM and guiding was perfect.

I double checked all connections on my external guidescope and camera. All solid so no movement or flex. Is it possible that guding on stars further off axis from imaging plane near SCP impacts guiding more than stars closer to imaging plane ?
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Old 21-03-2012, 09:00 AM
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Quote:
Originally Posted by cventer View Post
How cirtical is it that the guidescope is aligned to the imaging train OTA ?

With the very wide 50 and 80 mm guidescopes we are now able to use, guidestars can be fairly far off axis from where camera is pointing.

How much of an impact does this typicaly have on guiding ? Is it better to ensure they are pointing at pretty much same part of the sky ?

What are the symptoms of not having them aligned ?

I aks becuase recently when Imaging objects nearer to SCP i am having eratic guiding, whereas objects nearer to dec zero seem to have no guiding issues.

2 nights ago I was imaging an object near scp. I could not seem to get round stars even though PHD was showing straight lines on the graph. Seemed like I was getting image rotation. I then switched to my internal guide chip on the ST-2000XM and guiding was perfect.

I double checked all connections on my external guidescope and camera. All solid so no movement or flex. Is it possible that guding on stars further off axis from imaging plane near SCP impacts guiding more than stars closer to imaging plane ?
It isn't that critical that the guide scope and imaging scope have parallel axes. If they don't your polar alignment has to be better. I've seen posts elsewhere where they got good results for high declination targets with the guide scope axis 10-15 deg off the imaging scope.

The closer you get to the poles the harder it is to guide because there is less pixel movement in RA, so having the guide scope pointed away means any tracking error is easier to pick up. However it also puts the centre of any field rotation where the guide star is located which may be outside your imaging area, which is why we get back to needing good polar align.

Even then you might have to limit the exposure time.
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Old 21-03-2012, 09:26 AM
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mswhin63 (Malcolm)
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I just shoved my guidescope on and presto worked. I am sure it is a bit off.
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Old 21-03-2012, 09:54 AM
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Originally Posted by mithrandir View Post
It isn't that critical that the guide scope and imaging scope have parallel axes. If they don't your polar alignment has to be better. I've seen posts elsewhere where they got good results for high declination targets with the guide scope axis 10-15 deg off the imaging scope.

The closer you get to the poles the harder it is to guide because there is less pixel movement in RA, so having the guide scope pointed away means any tracking error is easier to pick up. However it also puts the centre of any field rotation where the guide star is located which may be outside your imaging area, which is why we get back to needing good polar align.

Even then you might have to limit the exposure time.
What if your guidescope is pointing west of the SCP but imaging to the east?
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Old 21-03-2012, 10:12 AM
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Personally can't see a problem as both would be pointing to stars that are all on the same polar rotation. Maybe not very practical.
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Old 21-03-2012, 10:15 AM
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Agreed, not very practical. Just interested and throwing a hypothetical out there.
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Old 21-03-2012, 01:23 PM
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Quote:
Originally Posted by cventer View Post
How cirtical is it that the guidescope is aligned to the imaging train OTA ?

With the very wide 50 and 80 mm guidescopes we are now able to use, guidestars can be fairly far off axis from where camera is pointing.

How much of an impact does this typicaly have on guiding ? Is it better to ensure they are pointing at pretty much same part of the sky ?

What are the symptoms of not having them aligned ?

I aks becuase recently when Imaging objects nearer to SCP i am having eratic guiding, whereas objects nearer to dec zero seem to have no guiding issues.

2 nights ago I was imaging an object near scp. I could not seem to get round stars even though PHD was showing straight lines on the graph. Seemed like I was getting image rotation. I then switched to my internal guide chip on the ST-2000XM and guiding was perfect.

I double checked all connections on my external guidescope and camera. All solid so no movement or flex. Is it possible that guding on stars further off axis from imaging plane near SCP impacts guiding more than stars closer to imaging plane ?
I'd actually try making your guidescope even more out of alignment when imaging near the SCP. See if that improves things
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Old 21-03-2012, 01:27 PM
Poita (Peter)
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From the PHD FAQ:

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Originally Posted by Poita View Post
Q. When near the pole PHD doesnít seem to guide or calibrate well.
A. When you guide near the pole there is less noticeable drift. and guiding becomes a lot easier. Calibration, particularly in Dec, becomes tougher. Think of it this way. If you were at the pole itself, you could turn your mountís motors off and image all night long without any error. Polaris takes 24 hours to move in a small circle. In terms of physical, linear motion across the sky per second itís darn near still. As you get closer to the equator you speed up and get to see your mountís real errors. Right near the pole any EQ mount can track. This is why, when measuring your mountís PE, you need to divide whatever PE you measured by the cosine of the starís Dec. At the celestial equator, this is 1 and the readings are accurate. As you move nearer the pole, your PE measurements are smaller than truth and you need to scale them up. By the time youíre at Polaris with a Dec of 89.17, you need to scale your PE measurements by 68x to get them to really reflect what the mount was doing.
Or, if that doesnít sit right with you, ever try moving your EQ mount manually to try to get to an object near the pole? As you get near the pole, even moving the scope manually to get to the right spot requires lots of effort - huge motions of the scope make little motions of the stars. Since the calibration process wants to see the stars move as a result of engaging the motors at guide rates, this gets to be a problem near the pole.
When imaging near the pole, you can increase the Calibration step and/or to turn off Dec guiding. You can also move away from the pole a bit more and calibrate there and then move back (donít go too far). Remember, if you recalibrate on the one side of the meridian you must always recalibrate if you move to the other side of the meridian.
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Old 21-03-2012, 01:29 PM
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That makes sense.
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Old 21-03-2012, 01:30 PM
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What is strange is that the guiding was perfect with internal guide chip, but horrible with external starshoot guider. Even though phd was giving me rms errors of only 0.13 pixels I had this wield rotation on long subs but none with internal guide chip.
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Old 21-03-2012, 01:46 PM
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Chris
I use a 72mm guidescope with an x/y shifter so that I can center the guide camera exactly to the imaging camera because without it I found the two were not exactly aligned when fully clamped down. So far there has been a GUidestar in the camera at least 90% of the time but if I want to find a better one then I can use the XY shifter which I find a real advantage.
Allan
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Old 21-03-2012, 03:24 PM
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Thanks everyone. Looks like alignment to scope is not the issue. Guess I need to track down what else this might be.

can someone also help with this questions. I am having a brain fart day and cant seem to solve th emost basic of maths problems.

My guidescope scale is 3.57 arc sec per pixel

My Scope and Camera is 2.8 arc sec pr pixel

If guide scope moves 1 pixel or 1/2 a pixel what movement in pixels or arc sec is this on my main imagers ccd chip ?
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Old 21-03-2012, 05:21 PM
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Quote:
Originally Posted by cventer View Post
Thanks everyone. Looks like alignment to scope is not the issue. Guess I need to track down what else this might be.

can someone also help with this questions. I am having a brain fart day and cant seem to solve th emost basic of maths problems.

My guidescope scale is 3.57 arc sec per pixel

My Scope and Camera is 2.8 arc sec pr pixel

If guide scope moves 1 pixel or 1/2 a pixel what movement in pixels or arc sec is this on my main imagers ccd chip ?
Chris,

1/2 guider pixel = 3.57/2/2.8 = 0.64 imager pixel

similarly 1 guider pixel = 1.28 imager pixel

PHD can guide at an effective value of around 0.1 pixel if the guide star covers more than 1 pixel, so a slightly defocused star normally works better.

It uses the light falling on the adjacent pixels to calculate the centroid. If all the light is on one pixel it can tell where on the pixel it falls, so can't do anything until it starts appearing in an adjacent pixel.

I wish the weekend weather would improve so I can practice what I preach.
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Old 22-03-2012, 09:12 PM
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I hate to rain on your parade. If your mounts polar alignment is not perfect any guide scope that is not colinear with your imaging scope will cause rotation in sequential images.

I have had fun proving this by establishing where my guidescope was and watching the inevitable rotation around the point where my guide scope was pointed in my imaging system.

I used to see this as a bonus as the noise would be smoothed out with stacking.

Bert
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Old 22-03-2012, 11:30 PM
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Bert that's what I head heard and suspected so glad to hear its true.

Although my alignment is pretty darn good as I can keep a star in a 20x20 box for 25 minutes with no issue.

I notice this slight deviation between my images and yes it does smooth out some noise.

So do you sight your guider and ota now to be exact?
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Old 22-03-2012, 11:51 PM
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Hi Chris,

can you post the pics with rotation and without, I'm having some serious rotation as well and I never bothered checking guidescope alignment to the primary ota. but I don't see the point if you use an x/y shifter or the 3 screw on the guidescope to find a star. once you adjust the guidescope to get a guidestar, both axes won't be parallel anymore.
I use an St80 as the guidescope and an 8" F4 and am struggling to figure out if it is due to my focuser not being square or if the ccd plane has a tilt.
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Old 20-04-2013, 08:23 AM
Daveskywill (David)
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About Autoguiding

Cventer:

Hi: I'm into this astrophoto disease thing too. And in my the New CCD Astronomy book by Ron Wodaski, he says that even with a german equatorial mount correctly aimed to the Earth's polar axis, there would be some star giggling pattern caused by backlash in the worm and worm gear.
That's if you're not using an autoguiding camera. But when you use that autoguider, it corrects for PE as well as tracking error due to not so correct polar alignment.

Maybe you already know that. But I too am trying to use PHD to make my pics better and no trail or gerk to my stars. Also, too I have an ST-2000XCM (dual sensor camera), but only got the main sensor to work with PHD. There maybe an ASCOM non complaince thing going on that won't allow me to use the guide CCD on my 2000XCM.

And yeah, I'm excited because soon I'll have my cool 80 ED from Orion semi APO scope. That should better allow me to do autoguiding.
In the mean time I've been trying to use a little 1/4-20 threaded mount on the top of my 10" Meade SCT, but it's bent wrong, so I've got to compensate with the DEC on the handcontroller to compose the picture. So I'm looking forward to the little scope with tube rings.

Hi, this is David again:

I've got the Meade LX200GPS 10" F10, with T2i and a newer CCD an ST-2000XCM, and I've tried M31 and recently (though n hemi attainable) M51.

But they still have star trailing in those pics. I'm trying to use CCDSoft too and PHD for autoguiding.

PS: that CCDSoft's autoguide screen shows you can invert the x-axis direction. And tinkered with that, and the CCD head position. PPS: I'm trying to use the ST-2000 through the main OTA and piggy back my T2i with a 70-300mm APO lens. Is that an okay setup? And what CCD head orientation should I use?

It corrects for somethings. Like the resultant T2i pic shows zig zag lines.
And I'm wondering if it's thinking backward or something. Like I've got an inverted scope. But my EOS lens is proabably non-inverted.

Any advice?

PS: here's my pic of M51, using a 214mm fl on my APO EOS lens, piggyback with T2i, on Meade.

Thank you.
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