Diffraction Effects – 18 May 2009

Location: at home in Tomball, TX
Cloud cover: 0%
Transparency: above average (4/5)
Seeing: est. avg (3/5)
Darkness: no moon, city glow
Wind: light, less than 5 mph
Temperature: 65º+
Humidity: 45%-60%
Dew Point: 45º-55º
Time: 9:00 pm – 10:15 pm CDT
OTA: 8″ SC

Started by observing M13. Some faint individual stars appeared in averted vision. Seemed larger apparent diameter than other GCs I recall.

Next stopped at Saturn. Rhea and Titan were easily visible. Two stars (mag 10 and mag 12-13) also in the field that I mistook for Iapetus and Dione (respectively).

I wanted to spend some time observing an airy disk to estimate seeing conditions. I started with Arcturus. Very bright center, yellow-orange color. Ring pattern was usually discernible. Focusing was somewhat challenging due to variations in distortion. Radial streaks or spikes also visible. Needed 7.9mm ep to observe diffraction rings.

Also noticed there was a diffraction line across the entire FOV when using the 45º erecting prism diagonal. Line appeared with different EPs, did not appear with 90º reverse-image prism diagonal.

Finally tried observing airy disk for Spica. Blue color was striking contrast to Arcturus’ yellow-orange. Observed similar variations and distortions of airy disk as observed with Arcturus. Also noticed that the tips of the radial spikes in one direction had an orange tint. I moved the star across the FOV to see if the effect was an off-axis abberation, but the orange tips retained their orientation independent of Spica’s position in the field. Spica was about 45º altitude, so it didn’t seem like it would be an atmospheric effect. My other guess was that Spica might be a double star and that the secondary might be spectral type G or K. A quick google search shows Spica is indeed a binary double, but I haven’t yet found any info on the separation or the spectral class of the companion.

I’d say the seeing was about 4/10 on the Pickering scale, often varying between 3 and 5.

Entry for June 05, 2007 – Observational Notes

Cloud cover: 5%-10% – Cirrus
Transparency: below avg.
Seeing: below avg (4/5)
Darkness: city sky glow, no moon
Limiting Magnitude: 4 (unaided), 8 (bino)
Wind: none
Humidity: 85%-90%
Temperature: mid-70’s

Instrument: Bushnell 12×50

Compared to last night, tonight was a a little “hazy-er”. Not sure if this is due to worse transparency or higher humidity. Last night’s transparency might have been better than I rated it.

Jupiter was a bright disc surrounded by atmospheric glow that encompassed its moons. Callisto, Ganymede, Io, and Europa were all visible.

Antares appeared much as last night, yellow-orange with a lot of atmospheric sparkling that included green.

Vesta formed ~120° angle with HIP 80793 and phi Oph.

Spica was bright and blue; Arcturus yellow-white.

I could just make out the bright oval of Saturn, but no ring separation and no moons.

Venus large and very bright orange-white. Could not discern phase, perhaps due to brightness and/or haze.