Seeing and Imaging...

[RichieJarvis]

Hi All,

I've been wondering. On the clear nights we have had, I've heard some say that they haven't bothered to get the scopes up and running because the seeing is bad.

Now, I know we've got the jetstream above us, which makes the atmosphere worse, and I check the seeing forecasts on Meteoblue before I go out, but how do you actually measure seeing as a human being?

What I mean is how can you tell? I believe theres a scale of 'seeing' which I've heard some people use, how is it measured? Do you need a fancy gizmo?

I've said before that the nights when the stars have been visually twinkly have been pretty good for me, imaging wise (or I thought so), so that doesn't appear to be a good indication of when to roll out the camera.

Thanks,

Richie

[centroid]

The most obvious sign of poor seeing, is how badly the stars are twinkling. This is indicative of an unsteady atmosphere.

Not always so noticeable, is the presence of very high thin cloud.

A high moisture content in the air, can also degrade things.

Dave

[Trevor]

Richie

Damian has a good article on his site here:http://www.damianpeach.com/articles.htm Trev

[narrowbandpaul]

the way to tell properly, rather than use the eye, is to measure the full width half max of a unsaturated star, that is, fit a gaussian profile over the star. This gives the standard deviation. The FWHM then equals 2sqrt(2ln2)*stdev. There are programmes that do this for you, or you can take the profile into excel, and fit a curve yourself

this only really works well if you are sampling at about one arcsec or less, ie the seeing spans a few pixels or more. If you are undersampling (using a shorter focal length, so that the seeing spans less than a pixel or so, then you can image whenever. If you image at about 2"/pix then 90% of nights are fine to use

using the eye is subjective, but twinkling is a good indicator, CCD's dont lie.

hope that helps

paul

ps the arcsec/pix = 206xpixel size (um)/ focal length (mm)

[RichieJarvis]

Thanks guys.

Damian's articles I had read before, but I think they make a little more sense to me this time.

One thing I don't get, is the Pickering Scale. Is this a visual check? If so, I've never seen an Airy disk - am I right in saying you only see one when the scope is exactly in focus? Also, wouldn't a CCD show an Airy disk?

As for imaging size, I use an SXV-H9 - it ranges from 1.65 arcsecs per pixel to 3.39 arcsecs per pixel depending on the scope I am using. If I barlow then I get real problems with focusing, which I am guessing is down to seeing, as that brings the minimum down to 0.82 arcsecs per pixel.

Thanks,

Richie

[narrowbandpaul]

without the barlow, i doubt you are seeing limited.

a CCd will show an airy disk. A perfect focus star will have most of the light within the central max, meaning the rings are hard to see, a bit out of focus and the light gets thrown to the rings, making them brighter.

of course dont measure FWHM for an out of focus star!

paul