Bogeys

After getting back into astronomy recently, one new development I’m really enjoying is the great abundance of astronomical events, exploration and research on social media. Following progress, discoveries and images from the New Horizons mission in near real-time on Twitter, for example, has been a thrill and quite addicting!

But there has always been a fringe element around this subject, and that comes up in SM as well.Last month I saw some fear mongering about a major comet or asteroid due to hit the Earth in September, and subsequent rebuttal and denial by professionals. I don’t even want to post a link to the sites purporting this nonsense – but you could look it up! But it’s got me thinking about whether a really big celestial object could catch us by surprise..

I’ve also re-joined a local astronomy group and am looking forward to getting out with them again. Many of the members have changed, but they still have a core group of very dedicated observers. One of the former members I knew was a dedicated comet hunter, and would go out on many clear nights with a wide-field telescope or binoculars and scan the sky for fuzzy objects.

To hunt for comets by eye, you have to become acquainted with existing nebula, galaxies and other fuzzy objects. In fact, Messier developed his catalog for this very reason! He had identified over 100 objects (and these have been added to), and today there are a number of more comprehensive catalogs for all of the “fixed” deep sky objects visible in the sky.

The club also co-hosted a talk by Thomas Bopp a while back, who, of course, is one of the co-discoverers of the spectacular Hale-Bopp comet. He described stargazing with friends and noticing a faint fuzzy object he did not recognize. After checking start charts for known objects he suspected he had found a comet. After watching it for a while, he noticed movement, confirming that it was not a nebula or galaxy. He did not have a way to take a picture, so he sketched the comet relative to nearby stars and was able to work out a location for the new comet. Then he sent in the discovery the old-fashioned way – by telegram!

Alan Hale is an avid observer and hunter of comets and had also noticed the new visitor and sent in his observations as well. The comet was confirmed to be new and was named for the two co-discoverers.

When Hale and Bopp first viewed the comet, it was determined to be about 9 AU out from the sun or well over 1 million km from Earth. The comet was one of the largest and brightest seen in recent times and would surely cause major damage if it happened to hit the Earth, but how likely is that?

After a newly discovered comet or minor planet is observed for a few days, it’s quite straight forward to calculate it’s orbital path. The motion of any body in orbit around the Sun was determined by Kepler and others in the 16-17th centuries and explained by Newton. Any body captured by the Sun will travel in an ellipse – or a perfectly round circle which is a specific type of ellipse. The orbit of the planets is roughly circular for the most part, but long period comets like Hale Bopp move in a very elongated or eccentric track. Hale-Bopp makes a trip around the Sun every 2500 years or so and when it comes to visit it approaches 0.93 AU before traveling way out to the fringes of the Solar System at 370 AU. It’s thought that it used to go further out before it’s orbit was brought in a bit closer by Jupiter.

Given the orbital parameters for a comet, one can work out the probability for impact developed by Opik over 50 years ago and refined by scientists in the field since. The probability is determined by whether the orbits cross and how often, and the chance that both objects will be in the same place at the same time. This gets fairly complicated as the orbit of a comet or other smaller body is subject to precession as well as perturbation by other bodies. So the orbits can align from time to time increasing the chance of a crossing. But since there is lots of room out there and the visits infrequent, the collision probability from a long-period comet is estimated to be in the tens or hundreds of millions per year per potential impact.

Perhaps people imagine the probability of these collisions to be more likely than this very small number because of the pictures we’ve all seen of the Solar System. These are fine conceptually but impossible to draw at actual scale. As this page from the NOAO shows, if the Earth were represented as a small peppercorn, or about 1/10 of an inch across, it would be located about 25m from the Sun. And Hale Bopp would travel about 10,000 m away on every orbit, coming back every 2500 year for another shot. Another awesome, live scale model was made recently by Wylie Overstreet and Alex Gorosh in the Nevada desert and posted here.

So I don’t see how a big comet could take us by surprise, but we’ll soon see! Anyhow I’m still waiting for Planet-X to end us, which was another doomsday hoax propagated on Usenet a while back. Perhaps things have not changed so much after all..

Sol

I’ve been wanting to get a new telescope mount for years – something that tracks better for taking astro pictures. Of course, I would go online from time to time and window-shop at the high-end professional mounts – but I don’t have the five-figures to spend on one or even enough sky to make use of it if I did!

I’ve been eyeing the Celestron CGE line for a while and was doing some price comparisons recently. The DX model was listed for under $2000 at all the sites I checked, including a major on-line retailer I frequent. That was too good to resist so I just had to click on Buy!

I’ve also been tinkering with an old telescope I have since it looked to be a good match for my Canon DSLR – an Orion SkyView 6″ Newtonian. I think it’s a great first telescope and I’ve always found the views to be quite crisp – especially after getting a decent right-angle mirror and an eyepiece or two..

When I tried attaching my Canon 450D to the scope I found it could not quite reach focus. Not surprising as the focal plane on these is usually pretty close in to the tube. I had a focuser I had bought for this scope and never got around to putting on, so I removed the old one, drilled holes to bring the new one in a little closer. I also had to drill out a new CGE dovetail plate to mount it, but then was good to go!

I have a nice solar filter for this scope I got for watching the Christmas Solar Eclipse in 2000, so I thought I would try this out first on the Sun. Here’s the setup:

Orion SkyView 6The CGE mount will let you align on the Sun if you enable that option in the setup. So I angled the mount to point roughly North and ran a Solar System alignment on the Sun. The display prompts to center the object in the finderscope, which I didn’t have on of course. So I did a rough pointing by looking at the shadow cast by the telescope tube and then the hinges of the tube rings. That was enough to center the Sun in the eyepiece (with a filter on the telescope of course!) and then confirmed the alignment. The mount tracked quite well East to West but needed a little nudge to the North from time to time, but this was good enough to get some pictures!

Visual observation showed one little sunspot on the visible surface, so it was a pretty boring sun photo – but it worked! I tried taking pictures at various exposure from 1/125 though 1/500. I just took the photo directly through the camera after viewing the histogram on the camera display, without any remote software. I pressed the shutter button manually and re-focused at each shot and a few of these came out OK. Here’s an example:

Sol201509061-croppedOne feeble spot is pretty clearly visible at the bottom center.

The 6″ Newt is an f5 at 750 mm focal length. We can calculate the field of view using [Covington, Astrophotography for the Amateur]:

FOV = 206,265″ X array-size(mm) / FL(mm)

With an array of 22.2 x 14.8 mm and a focal length of 750mm that works out to 6105 x 4070 arcsec or 102 x 68 arc min. Not bad!

With a width of 4070 arc seconds and 4272 pixels, that works out to 1.4 arcsec / pixel. Seems this is classically considered a good match, though perhaps it depends on what one wants to take pictures of. In any case, this should be a nice setup for brighter wide-field objects, so there will be plenty of things to try to catch.

M57

Spent an evening a while back seeing what I could capture with my Canon EOS XSi camera through a zoom lens I bought with it: a Canon 55-250mm F4-5.6. I setup my CI-700 mount with a Celestron C7 scope and mounted the camera to the scope. I just let the mount track without guiding and took various exposures at different focal length settings.

Lyra was fairly low to the west so I zoomed the lens to about 180mm and pointed it to cover beta and gamma Lyrae. Exposures where taken at 20, 30, 60, 90 and 120 seconds.

The mount was very roughly aligned and I generally setup on my back deck. The deck is fairly sturdy (as the previous owner had a large hot tub on it!) but it’s certainly subject to vibration. So with this setup, there is a lot of motion blurring at 60 seconds and above. But quite a few stars are visible at 20 seconds, and 30 seconds seems to provide the best balance between intensity and sharpness.

Here is a reduced version of one of the 30 second shots:

Even reduced about 5 fold, one can make out a bluish-green object roughly along the line between the 2 brightest stars (β and γ Lyrae), a bit below the mid-point. This is M57 or the Ring Nebula.

At 100% resolution, the ring structure is quite apparent:

The stars show a fair amount of streaking, but I was quite surprised at the sensitivity. I need to get a decent star atlas but it seems that stars are visible well below mag 12.

I figure if I can pick up the Ring Nebula through a camera lens in 20-30 seconds, I should be able to use this setup to pick up other objects in the Messier catalog! Though maybe a somewhat larger lens would help.

Some other wide field shots:

Milky Way

M31