John Hayes:
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 What mount modeling software are you using? As long as there is little flexure and as long as the model is based on perhaps 6 or more stars, it should point very well. As you add stars to the model you should see the cone and other values converge. Celestron mounts use up to 6 stars and do very well, but other mounts using only 3 cannot be expected to get the cone value right. A high end mount should not need physical shims to handle what software could be doing! Except in this particular case where it actually impacts the imaging. Any mount should point well without shims as long as its behavior is repeatable and it is modeled with a good number of stars. Frrank
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Guatentee it's cone error as others have stated. I've personally had this problem and seen the improvement when cone error is corrected using ConeSharp. The rotation caused by cone error is always much worse near the poles.
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Hi, I know this is an old thread, but I have this issue with 20 hours of Iris Nebula data. Polar alignment was always under 1‘, mostly in the low 10“/20“ during the sessions, the spider vane is a fixed aluminium ring from Backyard Astro, is this possible to fix in post processing? I have BlurXterminator, StarXterminator and PixInsight so I hope this is fixable… 🥲 Because otherwise the data is perfect
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Eduardo Saborido:
Hi
A good leveling of the E/W mount is as important as the polar alignment. I remember seeing one of Adam Blocks videos where he had a data set with the same problems. He mentioned in a side note, that this is related to a not leveled base. @Eduardo Saborido said what I thought all the time while reading through this thread. But I haven't thought this through, yet. I was a bit surprised to hear Adams comment on this.
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Christian Großmann:
Eduardo Saborido:
Hi
A good leveling of the E/W mount is as important as the polar alignment.
I remember seeing one of Adam Blocks videos where he had a data set with the same problems. He mentioned in a side note, that this is related to a not leveled base. @Eduardo Saborido said what I thought all the time while reading through this thread. But I haven't thought this through, yet. I was a bit surprised to hear Adams comment on this. Could be the reason - I was imaging in the field, I tried to level it as good as possible but I can‘t guarantee the mount didn‘t sink in unequally.
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I also had this problem with my Newton ASA.
All you need to do is position a few millimetre shims to raise (or lower) the telescope nose (depending on the corrections you need to make to your equipment).
I used my TPoint model (TheSkyX pro) to make sure the problem was corrected.
For the target that had been made (Valentine nebula), I just took my stars before the meridian turn.
So I took half the exposures. No more problems and the exposures were sufficient to get my stars colored.
Translated with DeepL.com (free version)
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Mina B.:
Hi, I know this is an old thread, but I have this issue with 20 hours of Iris Nebula data. Polar alignment was always under 1‘, mostly in the low 10“/20“ during the sessions, the spider vane is a fixed aluminium ring from Backyard Astro, is this possible to fix in post processing? I have BlurXterminator, StarXterminator and PixInsight so I hope this is fixable… 🥲 Because otherwise the data is perfect Cone error. After registering all the frames use only one subset with uniform spike orientation for the stars and add to the starless version.
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andrea tasselli:
Mina B.:
Hi, I know this is an old thread, but I have this issue with 20 hours of Iris Nebula data. Polar alignment was always under 1‘, mostly in the low 10“/20“ during the sessions, the spider vane is a fixed aluminium ring from Backyard Astro, is this possible to fix in post processing? I have BlurXterminator, StarXterminator and PixInsight so I hope this is fixable… 🥲 Because otherwise the data is perfect
Cone error. After registering all the frames use only one subset with uniform spike orientation for the stars and add to the starless version. Thanks, I‘ll let the nights stack seperately as well then, or do a seperate stack with alright stars! Would be a pity to waste that amount of Data, 20h on Iris in Bortle 3 are quite something
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This is due to spider banes that are not exactly symmetrical with respect to the Dec axis. If you think carefully about the orientation of the sky vs the scope, before/after a flip the vanes have been rotated by 180 degrees. Any inaacuracy in the vanes geometry will show.
You can prove it for yourself by choosing a field of bright stars at the zenith or meridian - take a short exposure that won’t show drift due to poor PA - long enough to show spikes - say 30s, then flip the mount and repeat on the other side.
Superimposing these will show the same issue.
It’s one advantage of a refractor, SCT or maksutov - no vanes, no spikes.
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Just wondering if stacking the pre flip subs and post flip subs separately and comparing might help narrow it down?
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It will - especially if you try what I suggest above.
If there’s no issue for targets at low declination either side of the meridian this implies the vane arrangement is good.
Another source of error is a non-perpendicular error in the mount between the RA and Dec axis. For a target near declination 0, the spikes from the vanes will be rotated by 2 x the NPE.
Conversely if it occurs only near the celestial pole it is likely to be cone error.
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Mina B.:
Thanks, I‘ll let the nights stack seperately as well then, or do a seperate stack with alright stars! Would be a pity to waste that amount of Data, 20h on Iris in Bortle 3 are quite something It doesn't matter if the star removal leaves no trace of the spikes. Just create a stack with your best stars all with the same spike orientation. And use the stars from that stack, rememberin to use it as the reference frame when you add all of them together.
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Well, there is an alternative solution which would eliminate the whole issue - modify the scope by replacing the straight vanes with curved (circular) ones, bent to match a circle of the same diameter as the primary mirror.
No more spikes.
You then should be able to stack images either side of a meridian flip without issues.
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Thomas LELU:
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 Looks like some cone error to me.
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