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How can you measure the seeing in arc seconds?


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I've seen threads mentioning that the seeing is such and such a value in different places and on different days, but how can you tell? I'm asking this because I've seen recommendations on PHD guiding settings based on the seeing. Thanks.

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'Seeing' in this context usually refers to the effect of atmospheric turbulence on the sharpness and steadiness of objects viewed or imaged through a telescope. These effects cause a star image to twinkle and to jump around to a lesser or greater extent depending on the degree of turbulence.  In a long exposure image, the net effect of this jumping around is that star images are smeared into small disks rather than tiny points and it's the size of this disk that provides a measure of seeing.  Because stars of different brightness produce disks of different size, the usual measure used is FWHM (full width at half maximum).  That's a way of measuring the spreading out of the star image that is more or less independent of star brightness. 

Many image-capture programs (e.g. Maxim DL) have a function for measuring FWHM of target stars so you can monitor seeing conditions by looking at this value in successive short exposure (1-2 sec) images of a star. 

When it comes to guiding, an important thing is not to attempt to make rapid small guide corrections to follow every tiny jump of the guide star caused just by these atmospheric effects; i.e. don't 'chase the seeing'.  So knowing roughly what the seeing steadiness is (from FWHM measures) is helpful in setting the guiding parameters that determine the sensitivity and responsiveness of the guiding system, in order to match the prevailing conditions.

I don't use PHD, but I believe it has a function that allows you to display in real time the graphical profile and FWHM of the guide star.  Usually, FWHM is displayed in pixels and there will be an option somewhere to convert this into arcsec by entering the focal length of your scope and the camera pixel size.

Adrian

Edited by opticalpath
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Good explanation Adrian. :)

I use Meteoblue to check what might be happening. Note MIGHT. Although it is pretty accurate.

To reply to the first comma, yes it changes by the hour or less but I'm not an AP imager and as such tend to rely on what I feel at ground level. I live in a dark location.

Rich

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I've seen threads mentioning that the seeing is such and such a value in different places and on different days, but how can you tell? I'm asking this because I've seen recommendations on PHD guiding settings based on the seeing. Thanks.

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The above recommendations are good but IMHO you should not bother with this value . In practical terms with UK seeing it does not make a lot of difference to your guiding. Imaging is something else.

A.G

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Thanks. Are you saying that if you convert the FWHM into arcsecs that is the seeing or is it not that simple?

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If the seeing is said to be, say, '4 arcsec', what's meant is that the FWHM of stars in an image is 4 arcsec and that number provides an indication of the level of atmospheric unsteadiness.  It suggests that movements of, say, half of that value would be difficult to distinguish between tracking error and atmospheric effects, so you might set the guiding program parameters to ignore guide star deviations smaller than that, in order to avoid chasing the seeing.

However, seeing can change quite quickly and it's only one factor to consider when setting guide parameters.  There isn't a simple formula, so don't get too hung up on this.  Most imagers will use a bit of trial and error to find out what settings work best for their setup and will monitor actual guiding performance on the night.  They will look for evidence of under- or over-correction, and whether corrections are well-damped or oscillate wildly, and will adjust guiding aggressiveness and 'maximum move' parameters accordingly to optimise.

Adrian

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