Pickering Scale Observations

While waiting in anticipation for my webcam to arrive I did some tests to evaluate the seeing at my observatory.

I was wondering if the quality of seeing here varies depending on whether I look East, West, and so on.

I was also interested in seeing if I could determine how ( if ) seeing improves later into the night.

Here are the results of my observations made on 24 May 2004.

These results are in this text format to help being able to see trends.

                19 h 00    20 h 30    21 h 30
                -------    -------    -------
Regulus         Alt: 50         38         27
Mag: 1.3              4          4          4

Denebola        Alt: 48         48         42 
B Leo                 3          5          3
Mag: 2.1

Spica           Alt: 53         70        too 
a Spica               4          6       high
Mag: 0.9

Beta Crux       Alt: 50         55         55
Mag: 1.2              5          5          4

Musca
HIP: 59929      Alt: 46         47
Mag: 4.0              4          4

Vela
HIP: 45496      Alt: 53         --
Mag: 4.3              3

Kappa Vela      Alt: ??         44
Mag: 2.4              4          5

Even though this data is sparse and incomplete, I managed to learn a lot from it. Even more important, my next seeing scale test will be better planned and exectuted. ( This was my first ever seeing quality evaluation, so I have only just started learning,)

This is the first time I ever used the Pickering scale. I learned about it on this excellent webpage. It has useful gif animations showing the different scale levels too.

There is a sufficiently large gap in seeing quality between each scale level for it to be used accurately.

I focussed accurately on the first star for every session, locked the telescope mirror and did the minimum refocus - if required - on the other stars during the same session.

Observations based on table above:

- I did the 19h00 observations covering all 4 wind directions. All stars were roughly on the same altitude. The seeing seems to randomly differ based on direction. There is no clearly obvious steadier skies in any direction - spanning 90 degrees wide. I only made 6 observations - roughly 60 degrees apart. Next time I will make observations roughly 45 degrees apart - having 8 observation directions.

- At 20 h 30 Spica reached a 70 degrees altitude. It had the best seeing scale for this observation test. Even if 6 is high on the Pickering scale, I would not call the image quality great at this point. The official designated desciption of the scale 6 is - fair to good - I think this description is quite accurate. The image stability is fairly OK, but not really good.

- The seeing did not improve substantially from 19h00 to 21h30. It roughly stayed the same. I rated 3 stars at the same level, and 2 other stars improved one level.

- I started doing a third set of observations at 21 h 30. The seeing did not improve as I expected.

Regulus stayed at the Pickering scale 4 level, even though it dropped to 27 degrees altitude from 38 degrees an hour earlier. Strangly enough, the nearly Denebola (Beta Leo) dropped to level 3 from level 5. It dropped from altitude 48 to 42 - an 'insignificant?' amount.

Finally Beta Crux (Southern Cross) stayed at altitude 55 degrees. Its quality went down from 5 to level 4.

Improvements planned for next measuring-seeing observation:

Have at least 8 stars - therefore covering every 45 degrees: 45 * 8 = 360 degrees.
Compare difference (if any) in how easy to evaluate seeing when using just eyepiece, and when using 2x or 3x barlow.
Do observations spanning more hours.
Select stars at altitudes: 30 degrees, 50 degrees and 70 degrees for every direction.
Compare difference (if any) in how easy to evaluate seeing based on the magnitudes of stars.
Since stars rise and set in altitude over many hours, the same set of stars cannot be used throughout the observation session. Different representative stars might be picked every hour based on altitude and direction (azimuth).

At 21h30 I noticed heat wave coming from the mirror, so it was still cooling off. It might be that the results above are actually a measure of how well the mirror has reached thermal equilibrium, and not of improved seeing during the first few hours of the night.

A smaller telescope that cools of significantly faster will be a more accurate instrument to measure how seeing improves the first few hours at night, since it cool down time is mostly removed from the equation.

My observations raised the question: What is the 'official' way of measuring seeing - using a 2x Barlow on a white star at 75 degrees altitude, measured 3 hours after astro-twilight ends, or how else? Do you measure seeing in all 4 wind directions or only angularly 'close' to the object being studied? What must the stars' magnitude and color be?