Seeing and Transparency 2

Seeing and Transparency 2

Postby edward-fraini » Tue Aug 05, 2014 7:49 am

True Story –
When I was just starting my Messier program I was somewhat confused as to the current sky condition so I asked the observer next to me what he thought and he replied “About a three.” On hearing this comment a second close observer interjected strongly “No, it is at least a six.” Observer one countered “No way, besides there is no six.” It turns out they were both correct. Between the discussion that followed and some internet searching I learned that there are at least many classification systems used with no consistency on span of the scale or the direction of improvement. In the Antoniadi system the scale is one to five with one being the perfect, in the “Astronomical Seeing” system the scale is again one to five but in this system five is perfect. A third system is the Pickering and its scale is one to ten with ten being perfect. Believe it or not there are a couple others out there that are used less frequently. It is a good point to mention that you absolutely must have good collimation before you can accurately measure Seeing and your mirror has to be close to the ambient temperature. If you haul your scope to Columbus in a trailer or in the trunk of your car the temperature can be pretty close as soon as you set up.

For Reference each of the three formal Seeing systems are detailed below.

Antoniadi Seeing System:
Focus your scope on the brightest star you can find close to zenith. Work the focus until the Airy Disk is as small as you can make it. With good collimation you should see diffraction rings and even four spikes with a DOB from the spider. Take it slightly out of focus to the point you do see spikes radiating from the star. Now rate the amount of dancing you observe from one to five where:
1 = Perfect steadiness with no quivering of the out of focus image;
2 = Slightly quivering, with moments of calm lasting for several seconds;
3 = Moderate quivering, with larger tremors moving entire image;
4 = Constant, troublesome quivering and shaking;
5 = Severe quivering with images dancing across the field of view such that detailed observing is difficult.

Astronomical Seeing System:
This system is based on the amount of magnification that the sky will support. So like in the Antoniadi Seeing System you want to focus on a bright star near the zenith or better yet a planet, I have tried this with the moon at less than 50% illumination and thought it worked well. Using a low power eyepiece make an observation. Use increasing magnification until you note the object is badly distorted. Apply this scale:
1 = Very poor seeing: Severely disturbed image, shaky at low power.
2 = Poor seeing: low power was pretty steady but medium power is not.
3 = Good seeing: You can use about half the magnification of your scope and still have a steady image. (50 times of the diameter of your mirror in inches would be the max). Most likely you can see only two bands on Jupiter.
4 = Excellent seeing: Medium powers are stable and planetary details quite obvious, high powers are good but a little mushy. More and more planetary detail is observed.
5 = Super seeing: The conditions will support the shortest focal length your mirror will allow.

Pickering Seeing system:
First focus on a bright star using 75 to 125X magnification, focus very finely until you see the airy disc and diffraction rings. As with Antoniadi Seeing System three key things to remember; collimation, collimation, collimation! Now rate the observation based on the following:
1 = Big messy blob
2 = Medium messy blob
3 = Brighter at center
4 = Airy disk often visible
5 = Airy disk always, short arcs frequent
6 = Short arcs always
7 = Long arcs sometimes
8 = Broken circles in motion
9 = Inner ring stationary
10 = Complete pattern stationary

I polled a group of Astronomy League Coordinators for input on what method of reporting they would like to see with submissions. There was no consensus at all. They did have this request in common. When you submit your logs they would like to see a table of the seeing scale you did use. One of them did suggest a Seeing system that seemed interesting to me. He suggested the following.

1 = stars are rock steady to the horizon or within a few degrees of the horizon
2 = stars are twinkling up to about 20 degrees
3 = stars are twinkling up to about 45 degrees
4 = stars are twinkling up to about 60 degrees
5 = stars at the zenith are twinkling

Poor seeing is caused by light moving at different speeds in air of different temperatures. Things around you that soak up heat in the daytime and release it after the sun goes down are going to create columns of poor seeing around you. Those big rock formations at TSP are not your friends. Think about what you plan to observe. Deep sky objects are not that affected by poor seeing much but solar system objects and double stars are. If you are going to work on an Astronomy League observing program that involves solar objects or the Double Star program for example you might want to think about not setting up on a big hot square of concrete, which would be a pad at the Columbus site. With these programs you would be better suited to wait till the these objects are well above the 2X air mass line, approximately 30 degrees above the horizon.

Key take away - to gain points with the observing program coordinator include with your submission the seeing scale you use. Secondly use any method you want, formal or informal, that is simple for you to execute repeatedly so that overall there is consistency in your observations.
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