Good morning all, I am currently encountering, with another astrophotographer, a rather intriguing problem with our Newtonian telescopes. I currently have a Newton ASA 10", very well collimated and very robust at the mechanical and mirror level. My colleague has an EPSILON 130ED which is also very well collimated and configured. When we image targets close to the north celestial pole, we notice a significant field rotation between, before and after, the reversal at the meridian. Results, when stacking the egrets are split.   On the other hand, with lower targets, no problem.... !I recall that there is no rotation of the tube in its rings, no rotation at the level of the optical gear etc.... We both use NINA software for acquisitions. Thank you in advance for your help and opinion on the matter. Good day
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You need to polar-align very well to avoid this. Field rotation induced by polar alignment error is the worse near the pole.
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Thanks a lot for your answer.
The polar-aligment is < 00° 00' 02" in AZ and ALT with my PARAMOUNT MOUNT in fixed position. Also confirmed by my TPoint model under TheSkyX pro.
In addition, my friend checked his station setting with the pole master THEN finished with the PHD drift method over 30 minutes!
Hard to be more specific.
Do you still confirm that it can still be due to the polar alignment error?
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Thomas LELU:
Thanks a lot for your answer.
The polar-aligment is < 00° 00' 02" in AZ and ALT with my PARAMOUNT MOUNT in fixed position. Also confirmed by my TPoint model under TheSkyX pro.
In addition, my friend checked his station setting with the pole master THEN finished with the PHD drift method over 30 minutes!
Hard to be more specific.
Do you still confirm that it can still be due to the polar alignment error? How about somekind of mirror shifting ? when i go into zenith my stars are very sharp,when i go on a low altitude,my stars are less sharper. just a shift of 1 milimeter will show it in large newtons. I had tilt in my 10 inch f4,ruins all the images.
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Thank you for your answer but no problem concerning that for our 2 different configurations.
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Hm interresting! Years ago I read a thread about bringing optical axis and mount axis together. May this be an issue near the pole?
Best
Mike
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Thomas LELU:
Thanks a lot for your answer.
The polar-aligment is < 00° 00' 02" in AZ and ALT with my PARAMOUNT MOUNT in fixed position. Also confirmed by my TPoint model under TheSkyX pro.
In addition, my friend checked his station setting with the pole master THEN finished with the PHD drift method over 30 minutes!
Hard to be more specific.
Do you still confirm that it can still be due to the polar alignment error? Could it be that the PA is too precise? I have a full metal not fixed pier and probably because of heat/cold and other reasons my PA goes off a tiny bit over time. When I image targets close to the celestial pole and they get close to the meridian, I usually notice that the target moves a little bit in the frame but not a field rotation.
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Thomas LELU:
Thanks a lot for your answer.
The polar-aligment is < 00° 00' 02" in AZ and ALT with my PARAMOUNT MOUNT in fixed position. Also confirmed by my TPoint model under TheSkyX pro.
In addition, my friend checked his station setting with the pole master THEN finished with the PHD drift method over 30 minutes!
Hard to be more specific.
Do you still confirm that it can still be due to the polar alignment error? Quite possibly. However, I was not precise enough. Below I will explain more. First, I calculated the maximum possible field rotation in 1 hr of exposure when: a. you image a field at Dec = 89 deg. b. you have a polar alignment error of 2". The answer is 0.5 arcmin of rotation in 1 hr. I believe this field rotation induced by PA error is very small, definitely smaller than what's shown in your picture. So simple PA error cannot explain it, if your PA error is really as small as 2". However, I believe in most cases, we aim for the unrefracted pole. The 2" error you mentioned must be referring to the unrefracted pole. That works the best for the overall sky. However, if we zoom into the small area around the refracted pole and only care about that small area, the sky there rotates around the refracted pole instead. From this point of view, your PA aiming at the unrefracted pole, is off. Typical atmospheric refraction is around 1 arcmin, depending on where you are, of course. If we assume a refraction of 1 arcmin, your very precise PA suddenly is off by 1 arcmin for the pole area. If using this PA error instead, the maximum field rotation that can happen in one hour becomes 30 arcmin for a declination of 89 deg. I don't know the time difference between the two pictures you show, but a rotation of 30 arcmin is definitely noticeable. So, although I cannot rule out other causes of field rotation, the apparent PA error caused by atmospheric refraction remains a highly plausible reason.
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Besides accurate polar alignment, you also need to eliminate cone error. Cone error behaves like polar misalignment. The mount can only make up for it by moving in RA and DEC, not by moving Alt and Az.
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Stupid question, but I will ask it anyway: Has your camera remained connected and not been rotated throughout this imaging session? The only time I ever saw this phenomenon was when I removed the camera and didn't quite get it back in at the same angle.
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Am I correct in believing that after polar alignment you should plate solve away from the pole ideally mid sky on the meridian before imaging near the pole? Ditto for calibrating PHD2 or whatever you are using to guide?
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Is this due to the fact that we track in Ra and Ra tracks in a circle? With a larger angle of rotation the effect would be most noticeable at the poles. Polar alignment would contribute but even with perfect alignment you still have that larger rotation. Have a look at my 3 panel mosaic of the Iris Nebula and notice the diffraction spike angles in the left panel and the right, quite a large rotation and the wider the field the more obvious the angle becomes.  On second thought most likely different to your issue. Wim's response makes sense and cone error might explain my rotation too.
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I agree with Wei Hao and Wim van Berlo. A small amount of cone error combined with small pointing error with respect to the refracted pole position could cause this sort of problem. An easy solution is to put some "wiggles" on the spider supports to mask the diffraction spikes. Then it wouldn't matter.
John
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Andy Wray:
Stupid question, but I will ask it anyway: Has your camera remained connected and not been rotated throughout this imaging session? The only time I ever saw this phenomenon was when I removed the camera and didn't quite get it back in at the same angle. In Thomas' images, you can see that it is the diffraction spike (caused by the secondary spider) that rotated against the sky. Whether the camera rotated is irrelevant.
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Hey Thomas, I had the same problem a while ago and in my case it was definitely a cone error. https://www.astrobin.com/ij53jp/In my case it arose after my Newton hit the tripod after a boisterous flip. In my case it was -3° before and after the flip. Raising the Newton slightly in the front mount helped. Sorry about my english, And Congtrats to IotD, btw. CS Jan
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A friend had the exact same problem after a flip, with a rather expensive brand new truss Newtonian. in his case the solution was to re align the spider vanes. they were moved about 1 degree in rotation.
I've seen the data before and after, that solved it for him completely.
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Ah…good one Dave. Yes, the spider vanes also have to be symmetric about a flip! Again, getting rid of the diffraction pattern is probably the easiest way to fix this problem when working near the pole. The other "best" solution (that's not cheap or easy) is to get either a L-series or a fork mount to do away with the flip altogether. I've become so used to my L-series mount that I'm completely frustrated with my new refractor on a Mach2. The flips and dead spaces in the sky are maddening. It has the most trouble pointing straight up, which of course is the best place to image!
- John
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Hello
I am the second person to have this problem, my spider is not adjustable because it is in one piece on the Takahashi epsilon, on my side I have already checked everything, no camera that rotates, primary and secondary very well fixed, the epsilon are like roques! on all the poses made before the reversal I have no problem, it happens as soon as the reversal takes place but on all those made after no difference either, just between the one before reversal and after.
sorry for my very bad english, I use google translate.
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Andy Wray:
Stupid question, but I will ask it anyway: Has your camera remained connected and not been rotated throughout this imaging session? The only time I ever saw this phenomenon was when I removed the camera and didn't quite get it back in at the same angle. No, I am in a fixed position and the optical train has not moved. Wei-Hao Wang:
Andy Wray:
Stupid question, but I will ask it anyway: Has your camera remained connected and not been rotated throughout this imaging session? The only time I ever saw this phenomenon was when I removed the camera and didn't quite get it back in at the same angle.
In Thomas' images, you can see that it is the diffraction spike (caused by the secondary spider) that rotated against the sky. Whether the camera rotated is irrelevant. Yes and I recall that the phenomenon occurs at the reversal at the meridian. On the one hand, everything is nickel. The stars are well aligned. On the other side the stars are well aligned too. The problem is after stacking where we notice that the spikes are split. Thank you all very much for your replies and opinions. I will try, first of all, to resolve the cone error.
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Has anyone ever considered that the secondary mirror spider could be slightly twisted and asymmetrical. I'm having the same problem right now. with an image field rotation, the effect would have to be continuous and not suddenly with the meridian flip. I would be interested to know what you think of it.   |
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If you have unsymmetrical spider vanes you won't need field rotation to see that.
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Then I got it right. Thanks
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Hi
A good leveling of the E/W mount is as important as the polar alignment.
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This is an interesting problem and I think Wim's answer regarding cone is correct. An eq mount can point at any point in the sky in either of two ways with a meridian flip - ignoring collisions with the tripod. In this case, apparently, when doing that flip with the dec axis close to the pole, and then adjusting ra and dec to re-center the object - the field will appear to have rotated a bit. If that is true and the rotation would be noticed immediately after doing a flip - then you know it has nothing to do with polar alignment or level because the sky hasn't moved at all in that time. The problem is strictly in the mount and optics.
I say optics because cone isn't just a mechanical issue and includes any offset of the optic axis relative to the saddle, in RA. It is sometimes referred to as "collimation error" - I guess in cases where the mount is assumed to be well made and any such error would be dominated by the OTA. It doesn't mean the OTA isn't itself collimated - it's just that the optic axis is a bit off from the mechanical one.
If it is a Paramount, do you have a good mount model that can tell you the cone value? And note that the value will include any contribution by the optics. Even if it is below a degree, as long as you are imaging with the ota axis somewhat close to the polar axis, there could be a big rotation in RA required to compensate. And this rotation would be much less at lower dec - as observed.
I see many people wanting to fix cone with shims and it never made sense to me because with a good pointing model, the mount can compensate for it on GoTo. And cone has minimal impact - if any - on guiding. But for this case of having spider vanes *and* doing a meridian flip - this is a good case for using shims and nulling out cone.
So - do you have a mount model that can tell you the cone value? Can the other person you mention with this problem also find the cone value? I would give shimming a try, and you can dial it in with the mount model. Try one piece of foil or something and see how much it changes - and that should tell you how much more to use.
Oh - and what dec. were you centered on? Your latitude shouldn't matter because all that matters is the cone value and how far off the dec. axis is from the polar axis.
Frank
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Not all pointing models handle cone error very well. My AP1600 could point to an object pretty well on one side of the meridian but it was really sloppy on the other side of the meridian. After shimming the saddle to reduce cone error, the pointing accuracy improved considerably and both sides were pretty much the same.
John
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