After capturing a few shots of the recent flyby of asteroid 2015 TB145, I thought I would look into software tools for astrometry to determine the location of the object and confirm it was in fact TB145 in my images. This analysis is used to discover, confirm and observe new asteroids and comets and to produce observation reports.

The image processing and planetarium packages I own do support astrometry, but only at higher license levels than I purchased. So I looked up a program I had used quite some years ago called Astrometrica, and was very happy to see it was still available – and very much supported!

Astrometrica is a shareware package created specifically for analysis and reporting of solar system objects, written and maintained by Herbert Raab from Austria. You can download and try out the package for free for a generous 100 days, and it is well worth the purchase price of 25 Euro.

The images from the session taken early in the morning on Halloween were acquired using Maxim DL and saved as FITS files which can be read right into Astrometrica. First, though, it is vital to enter the image parameters as Program Settings in order to correctly interpret and fit the images. The first settings are for Location which are key for reporting observations (my actual values changed somewhat):

The next set on the CCD tab is critical as it defines the scale and orientation of the image. These are used to convert the location of detected stars and objects to the coordinates of the star catalog used:

The focal length is that of the lens or telescope used in the imaging. I have entered the nominal value of 750mm +/- 5% for my 6″ Netwonian. After calibrating an image this could be set to a more accurate value and a lower variation for a faster fit, but this should work for a first pass. I’ve also set the Pointing error quite high, as I know the object is somewhere in the frame but don’t know the image center location very accurately. So we’re asking it to do a pretty wide search.

The CCD chip is 5.2 x 5.2 microns for the Canon DSLR, with no binning. Another key factor is the Position Angle that specifies the orientation of the camera on the telescope. Note that this can depend on whether the mount has flipped or not. Also, the image may need Horizonal or Vertical flipping depending on the type of  scope used and attachments. I spent quite a long time trying to work these out but could not get a fit at all with the settings I was expecting.

Then I remembered I had taken some shots of the Moon earlier in the session. Looking back at those, Tycho is on the left hand side of the images, and rotating 90 degrees counter-clockwise puts that and the other features in the “right” place with North at the top. As is, the image has East at the top, so the Position Angle is set to 90. All of the images could be rotated to have North at the top, but it’s probably best to leave them as is and have the program work out the orientation of reference stars. I was expecting that the image should also be flipped but it looks like that is not needed, so the Flip check marks are off.  Perhaps the camera takes care of mirror flipping the image, since it would need to do so for the camera lens as well.

There are other settings to get the correct time from the file, and the remaining tabs have other parameters to adjust for fitting, but these seem to be OK as is. So next I loaded in the set of 3 images from the second location used in the session.

The program has a great blinking tool, and that can be run without all of the setup details above. Simply select the Blinking Tool icon or menu item and this will extract objects and then align and display an animation of all of the select images in a new window. You can set the blinking rate, and stop and step through to take a closer look at anything moving. The images in this first set show a clear streak moving across a fair portion of the display towards the right hand side of the section below:

But to determine the actual coordinates of the streaker, we need to run the Data Reduction tool to fit the star field. From the Astrometry menu, we select Data Reduction and are prompted for a location. We can enter the known or estimated center of the image, or have the tool look up a given object and estimate the position for us. So we enter 2015 TB145 (after loading the MPCOrb database) like so:

The location could be off a ways for our quickly moving neighbor, and it is not at the center of the screen, but we’ve asked for a pretty wide search in the settings. Now we select OK and Astrometrica will run the fit. To do so, it needs a detailed star catalog. These can be downloaded and installed locally, but the tool can now fetch stars for a given region using an Internet service from a configured source – provided we are on line!

After that works its magic for a while we see:

This result indicates a poor fit, but we do have over 50 reference stars aligned. If we are way off in the settings, we would only see one match or a handful which would indicate we are probably lost! In this data set, the images have a lot of distortion and are not calibrated for brightness, so the fitting is pretty rough. If we accept the automatic fit we get a summary table:

Not sure how to interpret all of this, but it looks like we can determine locations in the images to about 1/3 of a minute in RA and Dec which seems pretty good to me! So now we can select any point and see its estimated position. If we select a star for example:

This object was identified as a potential star found in 2 or more images but was not included in the fit, presumably because it did not meet the entered fit parameters. But this is not the asteroid – we want to find the streak on each image and mark it’s position at the center of the streak like so:

Next we look up known objects in the area by selecting the button next to Object Designation and see:

We select the one known object in the vicinity, enter an optional note and then Accept to generate the observation for the selected object. (Some of the sets also indicated a nearby bright asteroid but that appeared to be just off of the frame). After selecting the Known Object K15TE5B which seems to be the official designation of TB145, it is labelled on the screen:

After doing this for the 3 images in the set, we can view the results, formatted for submission to the Minor Planet Center. To ACTUALLY submit the observations, I would have to be vetted and assigned an Observatory Code, but we can see just what the submission would look like:

COD XXX
COM Long. 72 07 24.2 W, Lat. 41 07 24.2 N, Alt. 30m
ACK MPCReport file updated 2015.11.16 21:41:42
NET PPMXL
K15TE5B C2015 10 31.39037 05 19 38.34 +18 56 12.5 12.1 V XXX
K15TE5B C2015 10 31.39209 05 19 57.11 +19 02 17.4 11.8 V XXX
K15TE5B C2015 10 31.39337 05 20 10.47 +19 06 39.7 12.1 V XXX
—– end —–

This shows 3 observations for K15TE5B or 2015 TB145 on Oct 31 with the time expressed as a fraction of a Julian Day, followed by the RA and Dec values and the estimated brightness or magnitude around 12. The default Observatory Code XXX is used for this example.

The other sets were fitted in a similar way, resulting in 16 observations of the asteroid over the session.

The positions determined from the fit were a ways off from the estimates provided by the program, so how do we confirm this was in fact TB145? The Minor Planet Center site has a number of tools to check results.

The MP Checker takes a time and location and reports any known objects in the area. We can enter the details on the form including a limiting magnitude and get

This shows our expected friend TB145. Note that if we increase the limiting mag to 20 the service returns a couple of dozen minor planets in this limited area – so there’s always a lot of them up there in the sky!

So it looks like we did find it! But how close are the positions we determined from the fit? One way to check is to compare the observed position with the result from the MPC Ephemeris  service page. We can enter a list of objects and desired time and get calculated positions based on the current orbital parameters. The MPC presumably received lots of accurate observations from the flyby, so we can assume the position should be pretty well known. After getting a list of positions at each minute over the session, here is a comparison of the estimated positions vs my observed position in the first set of 3 images:

The Observed positions (Obs above) are compared to calculated positions on the surrounding minutes. The coordinates are all between the bracketing positions, so this looks fairly good! At a glance, there is considerable variation between the observed results and the interpolated positions, so there is definitely a fair amount of error in my results. But this looks quite good for something moving so fast and close by!

So next I think I will try this out on some of the brighter and slower Main Belt minor planets, and further explore use of Astrometrica.

Earlier this week, I started seeing a lot of a buzz about the “Spooky” asteroid, 2015 TB145 set to pass close by to Earth on Halloween day. It was only discovered quite recently as it has an unusual orbit for an asteroid, well outsize of the ecliptic plane.

In a previous post, I discussed the likelihood of a cometary collision. Comets are relatively few and far between, so impact probability with these is very low – though they are large and fast moving when they come in and contact would be catastrophic. There are many, many thousands of known asteroids or minor planets much closer to home and these are somewhat more likely to strike.

Of course, the bulk of the asteroids are in the Main Belt and circling in a safe Indy 500 pattern out between Mars and Jupiter – there are likely millions out there large and small. The Trojan asteroids are in stable orbits of various kinds and are generally OK. Then there are the rogue NEO asteroids. While many of these are safely out of range, there are a number that we do have the potential to cross paths with.

The NEO asteroids are generally smaller in size and don’t have cometary tails and so are quite hard to spot. A number of very powerful telescopes around the world are watching as constantly as they can to find and catalog minor planets of every type. (And they find most of the comets these days as well). But a new one could pop up at any time that could pose a threat. 2015 TB145 was found just a few weeks ago by the Pan-STARRS survey and this is an excellent example of why these surveys are very important.

Still, there is a lot of room out there and luckily this pass of TB145 will be at a safe 1.25 times the lunar distance. The object is estimated to be 600m across. That’s not enough to be very bright normally, but at the closest approach the apparent magnitude was projected to increase to about 10 or so. This is not bright enough to see with the naked eye but pretty easy to spot in a modest telescope.

At least, when the Moon is not out! The asteroid was projected to reach closest approach on Halloween day around 17:00 UT or 2 PM my time in the US Eastern timezone. But estimates of the brightness showed it to be visible on the approach around 4-6 AM local time, with a track along the top of Orion – but passing fairly close to a nearly full moon.

I wasn’t sure if catching the flyby would be doable, but I set up my scope the night before and aligned it as described in the previous post.

There were a number of articles on line with finder charts and tables giving locations for the passage. But with an asteroid passing this close, the apparent position against the star background varies significantly in different places due to parallax. So it looked like using a start charting program that can track and display solar system small bodies would be the best way to go. I recently upgraded my ancient copy of The Sky and used that, but a number of other packages have this capability including Stellarium.

So I started up The Sky and worked out how to lookup and import the orbital elements for TB145 from the database at the venerable Minor Planet Center. Here is what I imported the day before:

This very handily brings in all the values for you as-is without typing which is a huge help! But I wasn’t sure whether the program would give you the position of the body relative to the center of the Earth or calculated for my actual location. So I went to the JPL site to calculate the local ephemeris and had an hourly chart for that as well.

So after taking a few shots of the Moon as described previously, I pointed to the rough location of the TB145 and scanned a bit by eye just to see what would be visible. I could see a number of stars but not very many, so it was unclear whether I’d be able to see the asteroid or not. (At 600 meters across, I’m calling it an asteroid!).

I had also recently bought cables allowing my CGEM mount to be accessed by a computer. Celestron includes a program to do this, but a telescope interface is also supported by my version of The Sky, and this can be done by other packages as well. The hand controller has a telephone-style plug providing an RS-232 serial interface. I haven’t had a computer having one of these in quite some time, so I also had to get an adapter to convert to USB. I set this up the night before, and after installing the driver and rebooting, was able to find the telescope at a virtual COM4 and connected.

With The Sky, when you connect the telescope, it’s position is displayed on the chart as a yellow circle so you can see where the telescope is pointing. Or at least where it THINKS it is pointing!You can also enter an object or coordinate and slew to that position from the program. I had never used a computerized mount before, so this was awesome!

Of course, this is only as good as the alignment of the mount, but this seemed work quite well in the neighborhood where I had aligned to. The CGEM has functions to calibrate further but I thought I would give it a shot with the initial polar alignment and 2 star alignment. The smaller sized scope and DSLR give about a 1 x 1.5 degree viewing area, so getting in the ballpark would be good enough.

Now things were set to point the scope to the expected location of TB145 and take some pictures. To start, I slewed to the expected position and took two 10 second exposures there at 5 minutes apart. Then I had the computer slew again to the current expected position. Since that was actually a fair ways away from the first location, I decided to take a series of three exposures 2 minutes apart to better find a moving object. (Also after learning early on in science experimentation that if you can take 2 measurements of something you may as well take 3!)

So I ended up taking 5 series of shots from about 05:15 local to 6 AM. I decided to use the position provided by The Sky since that was easiest, and that turned out very well! Here’s what it looked like at the time (rolling the clock back):

The display shows RA/Dec in Topocentric and epoch 2000.0 coordinates. I wasnt sure whether this compensates for the parallax or not, but it did seem to. At least, I did find the body in all of the shots given the position it provided!

So I took and saved the shots, naming they by the group and approximate time of capture. I glanced at the images and could see a number of stars. There were a fair number of them and they looked in good focus so I just saved them without looking further. I guess I was expecting the asteroid to have a pinpoint star-like appearance with only a 10 second exposure. At least that was the case when I imaged a few main belt asteroids a while back – but those are much farther away!

So I came in from the cold to have a look and tried looking at some images side-by-side. I could see bright pixel sized spots that appeared to move, but then noticed there were a lot of them and they would move or disappear at random! So these are hot spots and I guess you get more of these in an uncooled stock DSLR. Maxim DL comes with the blinking utility SN Search designed to look for supernova. That looks at pairs of images placed in 2 separate folders. So I took a few files and made copies to try it. After fiddling for a bit I worked out how to do the blink comparison and saw a faint streak that appeared to move in the first pair I took. I was able to find similar streaks in all 5 pairs, so it looked like I had it!

Here are sections of the third group I took around 05:30 local having the streak:

The stars around the streaks can be easily aligned across the 3 images, but the streaks clearly travel in a line across the stellar background, so this appears to be TB145 passing by at a good clip.

So this looks to be it, or at least some kind of fast moving object! I’ve yet to calculate the positions and confirm this is TB145, but I was able to see it in each group around where it was expected. Next, I’ll look into how to calculate the position based on nearby stars and compare against the expected locations.

This was very exciting to see and well worth getting up very early for. I was also watching reports on Twitter and saw a number of other people chasing TB145 early  that morning and saw some great results. And I could not resist posting the above picture myself!

While this did turn out well, there were a couple of lessons learned for me. For one, if I had looked more at the info provided by The Sky or other sources, I could have seen that the rate of movement of TB145 at the time was an expected 2.1 arcsec/sec in RA and 2.6 arcsec/sec in DEC. That gives a total movement of over 3 arc-seconds per second! So a 10 second exposure would show a movement of 30 arc seconds or 20 pixels or so at the resolution used. So I should have expected that the body would streak at this exposure time. Taking images 2 min apart would show a travel of 200 pixels which was a pretty good guess, though using a 1 minute interval and taking 4 or more would have given a more interesting collage. And with this rate, it would have been worth taking some time to look over the first set while I was at the telescope, as the streaks would have been pretty noticeable.

But this turned out great all in all, and I’m very happy to be getting a handle on using the CGEM mount – with a small scope at least!

After getting a few fair shots of the recent Lunar Eclipse, I thought I would try taking some more pix of the Moon. One thing I noticed from the previous session was variation in the focus of the stars around the moon. I had refocused with each shot by looking through the viewfinder, but was obviously not getting that very consistent!

Friends in my local astro club recommended using a Bahtinov mask to help with this problem. It’s a very clever focusing aid having a series of grid patterns at different orientations. These create diffraction lines that converge to a common center when your telescope or camera is in focus, and diverge when off focus. This provides a very sensitive indicator of when you are at the very right spot.

My clubbies offered to make a mask for me, but I found some reasonable ones on line and went the lazy way. The mask I got is plastic with some movable plastic screws you can adjust to fit around the end of a telescope or dew shield. It stays on pretty loosely but that seems to work out OK.

I tried playing with this a couple of times and it seemed to work nicely, but I did not set up to take pictures. Then I went out very early in the morning yesterday to catch the Halloween asteroid 2015 TB145. Since a very big, bright moon was in  the way, I took a few shots of that while waiting for the main subject to come up above the horizon.

I had setup the scope the evening before and did a rough polar alignment and a single-star alignment. Since there were a number of bright stars visible in the early morning Fall sky, I was able to try and complete a two star alignment of the mount. That was the first time I was able to do so at home as I have so many trees around the house. This turned out to be very helpful in catching the asteroid later on!

Then, I pointed to a bright star and placed the mask over the 6″ Newtonian used previously and the Canon DSLR attached. Looking through the viewfinder, it was pretty easy to focus the scope and get the diffraction lines to cross near the same point. I plugged the camera into my laptop and started Maxim DL to acquire an image. Limiting the image to an area around the star, I was able to tweak the focus a few times and get the pattern pretty close. Here’s what it looked like:

This looked reasonable to go with, so I slewed over to the Moon and tried taking some shots. The Moon was about 4 days past full but still very bright and up near the zenith at 4 AM. I tried taking a number of exposures down to 1/200th. Looking at the brightness histogram in Maxim DL, 1/100 seemed a reasonable exposure so I took a few shots at that time.

Here is one, cropped, saved out to JPEG and reduced to 40%This looks pretty good and you can certainly make out craters and other large features. Looking at a section of this at 100% resolution

You can see some smaller features but they are not very clear!

Looking at this at first, I thought there was a problem with the capture – perhaps it was still out of focus some or getting blurred through vibration. But maybe the detail is about as good as it can be with this setup. The 6″ Newtonian used has a focal length of 750 mm, which is only several times higher that a decent zoom lens. At this resolution, the Moon spans about 1350 pixels. With a diameter of 3500 km, the resolution is about 2.5 km / pixel at the center. So any feature spanning 5-10 pixels the eye can make something out of would be very big indeed!

One way to increase the resolution is to get a longer focal length, so re-trying this with my C-8 would be worth doing for sure. Another approach is to take a picture of the magnified image through an eye piece – which is what you do when you look through a telescope. I could give that a try as well with the SLR but would also like to try getting a bracket for a phone camera and playing with that.

After getting a few Moon shots I then tried taking some pictures to try to capture the “Halloween asteroid” 2015 TB145. That came out way better than expected, as described in the next post!

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