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Observing with Street Lights
Dark sky sites not always necessary to see the Milky Way (This image was taken ouside of a B&B in Julian, CA)
Saturday, August 13, 2016
Big Bear Solar Observatory Tour and Pleides Meteor shower viewed from the lake and discussion of amateur asteroid size determination by occultation timing
This week lets begin with a question submitted by Searching for Gravity Waves and New Drone Pilot, Dr Gary, who asked how the 20 km scale factor shown in last week's image of the Sky and Telescope's asteroid occultation measurement, was determined? Good question Gary.
My initial consideration when I first saw the magazine photo was that the scale factor was related to the rotation of the earth. Wrong! Remember that since the star behind the asteroid occultation is so much further away from us than is the asteroid that the occultation or shadow as seen by observers on the Earth is essentially the same size and shape as the asteroid. Now the rotation of the Earth, even at the equator is only about 1000 mph, so an asteroid of say 100 miles in diameter would be occulted for about 1/10 of an hour. This is not observed! The more appropriate movement to consider is the Earth's rotational velocity about the sun, which is about 67,000 mph. This velocity and the velocity of the asteroid in its orbit around the sun combines to make the apparent speed of occultation. Remember that the occultation as observed on the Earth is the third point of a line extending from the Earth through the asteroid on to the star. This pencil thin beam of shadow, about the same physical size of the asteroid, moves across the Earth and is seen by different observers at different times.
So, how is the scale factor determined? Hey, it seems like it is going to take a lot of math and trigonometry and orbital mechanics to work out all the details. No, I'm not going to go through all that work, but you can give it a try, or an even easer way is to check out the website of the International Occultation Timing Association (IOTA), at www.occultations.org. They provide predictions for the next occultation and a whole several hundred page document that describes all the gory details. Check out attached image, showing the recent prediction of an occultation that crosses the North American continent, but misses a big chunk of California. Now maybe some of you amateurs are in the right position or are willing to fly and drive to the right location and get there just in time to see the 2.1 second occultation. What is that you say, you're not up for that level of amateur commitment? Luckily, the IOTA has many members and volunteers around the world to do this for the rest of us.
I found it interesting that the 5.4 magnitude drop in light from the star is limited to that amount because the asteroid itself has its own visible magnitude, so the amount of light doesn't just drop to zero, but instead drops to the amount of light reflected from the asteroid. If any of you amateurs has actually tried or succeeded in doing this sort of thing let me know.
Now this week was our chance to tour the Big Bear Solar Observatory, so Resident Astronomer Peggy and I drove up to Big Bear for a daytime tour and nighttime observing of the Perseid meteor shower. Now, it wasn't planning that got us to Big Bear at the same time as the shower, just plain darn luck. Anyway, we wanted to see the BBSO and were just a little bit worried that when it comes to our bad luck at getting observatory tours and viewing, like Mt Wilson 60-inch scope, we worried that maybe clouds or even the fire near Lake Arrowhead would interfere with our tour. See attached photos showing the waking path to the observatory. The big scope dome is located out in the lake, to minimize ground air heating effects, and connected by a walkable causeway. The big dome scope is supported by a smaller dome that contains a small scope for full field of view of the sun and another scope that monitors the Earth shine on the dark limb of the moon. Luckily the mostly clear and cloudless sky went well and we enjoyed being able to go inside the dome and climb the old stairs to the 1.5 meter scope.
Now, I took some photos inside the dome but found that I really couldn't capture all the details within a single photo. So, the best way for you to get some details of the scope and observatory solar research projects is just go to their website: www.bbso.njit.edu
The BBSO staff does not do any visual eyepiece observing as it is all done with cameras and the images are reviewed on computer screens in the control room. The 1.5 meter scope has a 83 meter focal length, which provides very good detail of the sun, but they rely on the smaller full view scope to identify what regions of the sun are of particular interest. Some of the images of the sun and flares and sunspots were truly amazing. Just for my own comparison purposes, I snapped a photo of the sun the day before the tour with my 600 mm lens and of course, the BBSO gets much, much, much better views.
I did ask the staff two questions. First, I asked how a scientist gets posted to this BBSO location. He said that you have to work for low wages and not screw up. Hey, I could qualify for one of those conditions!
Secondly, I asked how they go about aligning their solar images with the "north" rotary axis of the sun. I've wondered how to do that with some of my solar images and considered tracking sun spots over several days, but didn't quite know what to do. They use the latest solar ephemeris and calculate the orientation of the sun and then a device rotates the camera so that the "north" pole is at the top of the photo image. So, I guess if I looked up the ephemeris data which shows the orientation of the sun in some coordinate system, then I should be able to calculate the orientation of the sun in my image and correct for my observing location and pointing altitude and azimuth. Wow, it sounds like a lot of work so I might just skip this piece until later.
After the tour and dinner and site seeing and touring the Village shops and having found the fudge store, we retired to our hotel room which faced north with a lake view and settled in for the Perseids. We woke up about 2:00 AM and spent about an hour on our room patio looking for meteors. Resident Astronomer Peggy counted almost a dozen and I saw about 2/3 as many. It was pretty great, but their were a lot of lights around the lake so it was not quite as dark as we wanted. It was dark enough though for me to notice Casiopeia, which had risen at that time and also able to spot the Pleiades (M45) with the naked eye, something that is hard for me to do in OC city lights.
Finally, we had to end our mountain adventure and drive home. At the observatory, we found a big parcel post left on the doorstep. The package was from Celestron and my repaired AVX mount was (apparently, we will see) ready to go. Now last time, I mentioned that I had heard all sorts of horror stories about repair service, but at least based on my one data point, getting the scope back a mere 10 days after sending it in was pretty impressive. Thank you Celestron. Now, I lost one more excuse for not going out and doing some observing!
Until next time,
Resident Astronomer George
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