Weather here in St. Louis has been outstanding this week. Highs in the 70’s and lows in the 50’s. Perfect autumn weather. And with it, crystal clear skies that made for some very nice observing during Jupiter’s opposition last night. Since Jupiter is a nice bright target, I headed out to Broemmelsiek Park to observe1.
Since I arrived just as the sun was setting, Jupiter was low on the horizon so I took some time to hit a few other targets. I started off with Saturn which was at an excellent height for me. Typically targets in the $20-30º$ altitude range are quite comfortable for me as it puts me at the right position on the step-ladder I use without having to squat or twist too much. In contrast, aligning on Polaris ($\approx 38º$ altitude) always has me between steps2.
I ended up taking $12$ observations of Saturn before doing a couple stars in Sagittarius and one in Capricorn, each $3-4$ times. By then, Jupiter was high enough it was an easy target. I also try to avoid targets too low on the horizon because of atmospheric refraction causing the object to appear out of position.
For Jupiter I took $16$ observations but ended up tossing one out. Taking back to back observations makes a pretty consistent pattern. Objects rising tend to have about half a degree of increase in altitude and azimuth every two minutes, which is about how frequently I can take measurements on my own. One measurement stood out as it was notably higher than this pattern suggests and, sure enough, the altitude for the next one was back in line, so I tossed one observation out.
If you’ve been following these observation posts, you’ll note that I’ve been working on finding better ways to correct for and prevent known errors. On the prevention side, this has meant re-leveling the quadrant if I slew more than $\approx 30º$ and checking the level frequently. On the correction side, I have long tried to take into consideration the offset of Polaris from the true north celestial pole, but figuring out what to do with an azimuth ring that was never correctly aligned on Polaris anyway has been trickier.
Previously, I have tried to check the alignment more frequently and then re-set the azimuth ring. But since I can’t necessarily tell which of the positions was truly better until after the fact, this didn’t make for good procedure and it didn’t give a consistent set of numbers for me to adjust against since the scale itself had moved.
So in one of my more recent observing sessions, I got the bright idea to check the alignment more frequently but not adjust the ring. Instead, I would simply record the offset and figure things out on the back end.
For last night’s observations it would look something like this:
- $19:00$
- Az Reading of Polaris: $0.0º$
- Polaris Az: $+0.72º$
- $19:39$
- Az Reading of Polaris: $+1.10º$
- Polaris Az: $+0.76º$
- $20:11$
- Az Reading of Polaris: $+1.90º$
- Polaris Az: $+0.80º$
- $20:53$
- Az Reading of Polaris: $+1.75º$
- Polaris Az: $+0.82º$
My method would then be to break the observing session up into sections and average out each of these pieces.
So for the period from $19:00 – 19:39$ I would first consider the average of Polaris’ azimuth ($+0.74º$). Since I had set the zero point of the azimuth ring on Polaris to start, this means I would have to turn the ring back $0.74º$ to be pointing due north which would result in a $+0.74º$ increase to all readings.
Next, I considered the average of the reading of Polaris on the ring at each of the two times ($+0.55º$). To correct for this, I would need to subtract that amount from each reading.
Putting the two together, the total correction was the combination of these two factors resulting in a final correction during that time (which was my observations of Saturn) of $+0.19º$.
To help understand if that was correct, I compared the azimuth of each of my observations with the values given in Stellarium. Overall, the error in azimuth based on that was $0.08º$, so this method would give me a correction that’s right about $\frac{1}{10}º$ which tends to fall in line with what I expect the instrumental uncertainty to be anyway. So it looks pretty solid.
For the stars and Jupiter it gave even better results. For the stars, I came up with a correction of $-0.72º$. Stellarium said I was high by $0.79º$, so subtracting $0.72$ would have made my average off by only $0.07º$. For Jupiter, my correction was $-1.02º$ and Stellarium said I was high by $1.08º$, so took my average to only $0.06º$ high. It’s nice when the math lines up!
Ultimately, after averaging the observations for Saturn, my RA was high by only $0.04º \pm 0.12º$ and my declination low by $0.14º \pm 0.12º$ which is pretty decent.
For Jupiter, the RA ended up being dead on: $0.00º \pm 0.15º$ and the declination was low by $0.13º \pm 0.10º$. Overall, entirely reasonable results.
As always, the data is available in my Google Sheet.