I'm wondering why on some nights the stars are not perfectly round. I'm using a 10MicronGM1000 premium mount and the polar alignment is good and I created a 99 star model. Tonight, for example, NINA, which gets its data from the weather sensors outside, says there is 0 wind and 0 gusts. Is it possible that there is wind but it's much higher up in the atmosphere which would explain the stars not being totally round?
Sometimes the star shapes vary from sub to sub. Some subs they're perfectly round, and some they're not. It's not anything I can't repair in processing, but of course I'd rather start with the best data possible.
Thanks,
Jerry
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Jerry Gerber:
I'm wondering why on some nights the stars are not perfectly round. I'm using a 10MicronGM1000 premium mount and the polar alignment is good and I created a 99 star model. Tonight, for example, NINA, which gets its data from the weather sensors outside, says there is 0 wind and 0 gusts. Is it possible that there is wind but it's much higher up in the atmosphere which would explain the stars not being totally round?
Sometimes the star shapes vary from sub to sub. Some subs they're perfectly round, and some they're not. It's not anything I can't repair in processing, but of course I'd rather start with the best data possible.
Thanks,
Jerry Turbulence [wind shear], high, low or middling, would bloat (spread is the right word) the stars' shape and in fact they would be rounder, if anything (and I sit right in the middle of the jet-stream so I know quite a bit). Source of deviations from a round shape (other than the exit pupil not being round) can be few and all of them nefarious. Mostly have to do with radiative cooling and the consequence of that. Others might be down to guiding, even more so with an OAG. Check your PHD2 tracking.
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andrea tasselli:
Turbulence [wind shear], high, low or middling, would bloat (spread is the right word) the stars' shape and in fact they would be rounder, if anything (and I sit right in the middle of the jet-stream so I know quite a bit). Source of deviations from a round shape (other than the exit pupil not being round) can be few and all of them nefarious. Mostly have to do with radiative cooling and the consequence of that. Others might be down to guiding, even more so with an OAG. Check your PHD2 tracking. I have terrible seeing as well and fat, but still round. If guiding total rms is good, check that dec and ra rms aren’t significantly different. Cheers, Scott
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In my experience stars can distort in different directions due to the effects of seeing, in fact seeing is a lot more complicated than just good or bad, there are different types of good and bad seeing. The worst possible seeing usually produces perfectly round very fuzzy stars, but sometimes when stars have a low FWHM the atmosphere can still be bumpy distorting stars in some subs and others not (even with 1 second exposures).
All this becomes more obvious the larger telescope you have, a small scope is not as capable of displaying the nuances of atmospheric turbulence.
Clear skies
/Gabriel
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Gabriel Wiklund:
In my experience stars can distort in different directions due to the effects of seeing, in fact seeing is a lot more complicated than just good or bad, there are different types of good and bad seeing. The worst possible seeing usually produces perfectly round very fuzzy stars, but sometimes when stars have a low FWHM the atmosphere can still be bumpy distorting stars in some subs and others not (even with 1 second exposures).
All this becomes more obvious the larger telescope you have, a small scope is not as capable of displaying the nuances of atmospheric turbulence.
Clear skies
/Gabriel Wouldn't short exposures, like 1 second, be more likely to show seeing related eccentricity than longer exposures where the directional effect is more likely averaged out? Imaging at 2350mm, 5min plus exposure time, and seeing that varies from 2" (very best) to 7" or even worse, and can double/halve in a single night, I've not noticed anything but symmetric bloating. Also, in my experience, poor seeing tends to be more thermic in nature than due to air movement. The jet stream overhead doesn't help, especially when turbulent, but more local mixing of warm and cold air is the bigger culprit for me, I believe. It's why winter is a lot worse than summer. And sometimes ground level wind seems to actually help by smoothing out sharp temperature gradients. Cheers, Scott
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Scott Badger:
Gabriel Wiklund:
In my experience stars can distort in different directions due to the effects of seeing, in fact seeing is a lot more complicated than just good or bad, there are different types of good and bad seeing. The worst possible seeing usually produces perfectly round very fuzzy stars, but sometimes when stars have a low FWHM the atmosphere can still be bumpy distorting stars in some subs and others not (even with 1 second exposures).
All this becomes more obvious the larger telescope you have, a small scope is not as capable of displaying the nuances of atmospheric turbulence.
Clear skies
/Gabriel
Wouldn't short exposures, like 1 second, be more likely to show seeing related eccentricity than longer exposures where the directional effect is more likely averaged out? Imaging at 2350mm, 5min plus exposure time, and seeing that varies from 2" (very best) to 7" or even worse, and can double/halve in a single night, I've not noticed anything but symmetric bloating. Also, in my experience, poor seeing tends to be more thermic in nature than due to air movement. The jet stream overhead doesn't help, especially when turbulent, but more local mixing of warm and cold air is the bigger culprit for me, I believe. It's why winter is a lot worse than summer. And sometimes ground level wind seems to actually help by smoothing out sharp temperature gradients.
Cheers,
Scott I agree, it's more evident during short exposures, I never go longer than 20 seconds, and with my last scope 60 seconds. I can imagine taking a 5 min sub it is likely to be symmetric if tracking is accurate. /Gabriel
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For what it's worth I always take 3 minute subs (LRGB) or 5 minute subs (SHO).
Yesterday the roof was open for several hours and my scope was in sunlight. Though it was only 60F maybe the temperature cool-down contributed to the stars looking the way they did.
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I completely agree with Andrea. Seeing is generally an isotropic phenomenon that makes star images larger—and for long exposure imaging, generally makes stars more round; not less round. You can get out of round star images from seeing but the seeing generally has to be very good and you have to be using very short exposures (e.g. <2-3 sec) before you see that effect. For long exposure imaging, there are a lot of things that can cause elliptical star images including (but not limited to) tracking errors, vibration induced by wind or ground movement (yes, we get earth tremors in Chile), oscillation in the mount (most common in direct drive mounts), astigmatic errors in the optical system, and (possibly) diffraction effects. I went through a period of getting elongated stars that varied over the field and I tore my hair out trying to figure out why. I finally discovered that my filter wheel had lost its home position and was placing the filters at an angle, which caused diffraction spreading star images into an elongated mess. Re-homing the FW quickly resolved the problem.
The number one and number two suspects when you are getting out of round stars are guiding glitches and/or wind gusts. I’d recommend checking your settling criteria. Insufficient settling can cause double star images and/or elongated stars that may vary with the seeing and the responsiveness of your auto-guiding system.
John
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