Observing Asteroid Occultations

You’ve probably seen me wittering on about asteroid occultations at some point or other, or you’ve seen the links to my results page on the left. Maybe you’ve even read my Asteroid Occultation 101 page and still have no idea how it’s done or why.

It’s rather complicated but the idea of this post is to try and explain how I observe asteroid occultations.

Let’s have a look at a schematic of my setup. Click to see the full size version.

Schematic of Asteroid Occultation Equipment

I told you it was complicated! Basically what we have here is a computer controlled telescope. The laptop controls the pointing of the telescope mount via software (Cartes du Ciel). This allows me to find and point the telescope at the star I want to observe. The gamepad is used as a remote control for fine movements to align and centre the star in the field of view.

Another piece of software (FocusPal) controls a motorised focuser on the telescope so I can keep the star in focus without having to touch the telescope which would cause it to move and vibrate.

The third bit of the jigsaw concerns the video recording aspect. A very sensitive video camera (Watec 910HX) records what it sees through the telescope, it’s sitting where I would put an eyepiece if I was observing visually. The camera is clever in that it can do internal frame integrations. It can take exposures of up to five seconds allowing fainter stars to be observed. Typically exposures of 0.16s or 0.08s are used.

The video signal passes through a Video Time Inserter (VTI) which adds a GPS accurate time stamp onto the signal. I want to record the video on the laptop so the analogue video signal passes via an Analogue to Digital (A/D) converter to the laptop. VirtualDub software is used to record the digital video signal and because the raw video would result in huge files a non-lossy compression codec is used to reduce the file size somewhat (Huffyuv codec).

Once this is all setup and working and I’ve found the star I want to observe I get a 728×576 25fps video recording of the event. I usually take a four minute recording centred on the predicted time. This allows for time errors and/or the possible discovery of asteroid satellites.

A still image from a typical recording looks like this.

I usually put the target star right in the centre of the frame, as I have done above. It needs to be visible but not saturated. Using the camera controls I can adjust the exposure, gain and gamma settings to get a good signal to noise ratio. The timestamp from the VTI can be seen lower left. The time this frame was recorded was 18:23:02.0866 on the 17th February 2015. Accurate timing is essential for asteroid occultation observations. It can be done with radio signals or even a synced PC clock but a 1 pulse per second VTI makes it a whole lot easier.

A few years ago if the observation was positive (the asteroid passes in front of the star) I would have to bother myself with complicated calculations of internal camera delays (see video exposure time analysis). Thankfully it’s a lot easier now with software which automates a lot of the processes.

Firstly I run the video through Tangra 3. This piece of software measures the light output of the target and comparison stars. If the result is negative the light output of the target star (flux) will essentially be a flat line (with random noise) and that’s the end of the processing. The result is reported to the PlanOccult mailing list as negative.

If the result is positive the light output of the star will drop to zero or to that of the asteroid alone. A light curve of a positive result is shown here.

The cyan line shows the light output of the target star. As the asteroid moved in front of the star the light output dropped to that of the asteroid alone. The yellow, green and pink light curves are for nearby comparison stars which are also visible in the video recording.

As this is a positive result I needed to take the data from Tangra 3 (basically a text file of light output values) and import it into the Asteroidal Occultation Time Analyser (AOTA) module of Occult 4 software. After setting some parameters this very clever piece of code automatically looks for occultations in the data, finds them and works out the time of disappearance and reappearance of the target star. It can then correct the times taking into account the internal time delays of the video camera.

The output from AOTA looks like this.

Hey presto, the times are calculated and all that’s left to do is fill in a report form with the observation details and result and send it off to the PlanOccult mailing list. A few days later the result will appear on the European Asteroidal Occultation Results page (euraster.net). Every so often the results are compiled and appear in a peer reviewed journal with my name in the contributers list for use by professionals and future observers.

Easy innit!

Well it is easy once you’ve got the hardware and workflow sorted. It typically takes me less than 20 minutes from the predicted time of occultation to be up and and running and ready to record. Having a permanent setup helps enormously. It means I have been observing many more low probability events that I wouldn’t have bothered setting up for before. I’m still waiting for my first <10% probability positive result though! In fact positive results are rare, about 1 in 20 of my observations have been positive. But the next one could herald a major discovery. The first set of rings around an asteroid were discovered in 2013 using pretty much exactly the same methodology as described above. Maybe I’ll discover the second…

An Insight to PixInsight

A colleague and fellow astroimager (Welford Observatory), with whom tips and tricks in the coffee lounge at work are exchanged, suggested I try PixInsight (PI) for processing some astroimages. I tried the demo version once before and I, like many others before me, found it very complicated and daunting and gave up quickly.

He pointed me towards Harry’s Astro Shed as he has posted various get you started video tutorials for PI. So I bit the bullet, paid for a licence (£lots) and got stuck in.

Yes, it’s still very complicated but with Harry’s help I also discovered it’s very powerful. The calibration routines for applying bias, dark and flat frames to the raw data are the best I’ve ever used. Move over Nebulosity 3 you’ve been outdone.

After practicing on some old images it was time to try it out on some new data using the recommended workflow.

The first thing to do was to take a new library of dark frames. They shouldn’t change from one imaging session to another so you can take them, create a master for different exposures times and they can be used over and over again. I did these on a cloudy night, a bit time consuming but the system can be left to do its thing.

The next night was clear so I decided to have a go at the region around the Horsehead Nebula in Orion. This is a region full of nebulosity which responds well to H-alpha filters. I took 36 exposures of five minutes each, not really long enough for faint nebulosity but I wanted to get as many exposures as possible in one go as a test of PI.

I’m often a bit lazy when capturing flats but I was careful to make sure I got good ones and then calibrated, registered and integrated the images in PI. With some histogram stretching, denoise and contrast enhancement I’ve ended up with possibly the best astroimage I have ever taken.

Horsehead Nebula Region

Be sure to click through to Flickr and see the full resolution version.

I’m now a total PI convert, it’s expensive and complicated but now I’ve got a hang of the basics there’s no stopping me now!

 

(357439) 2004 BL86

There are currently (as of 11th Feb 2015) 1544 Potentially Hazardous Asteroids (PHAs). These are small bodies that have the potential to make threatening close approaches to the Earth.

Most are very small, from a few metres to a few tens of metres across, but there are quite few larger ones a few kilometres across. Clearly if one of these larger bodies hit the Earth it would make a mess. A big mess. Fortunately no PHAs are known to be on a collision course with the Earth.

2004 BL86 is one such PHA which was thought to be around 600–700m across. It’s orbit is known well enough that it has been given a numeric designation (it’s name means it was discovered in 2004 and BL86 is a sequential tag indicating it’s order of discovery in that year).

On 26/27 January 2015 it made a close approach to Earth, about 3.1 times the Earth-Moon distance away at closest. Small asteroids come close to Earth all the time, often closer than this, but what made this one unusual was the size of the object. This large size meant it would be bright enough to be readily visible by amateur astronomers.

As it approached Earth professional observatories started observing the asteroid using radar and it was discovered that it was an almost spherical object (unusual for small bodies) around 325m across. They also discovered it has a small moon (this is not unusual for PHAs).

After observing the asteroid visually through my telescope (likely to be the highest numbered asteroid I will ever view visually) I took a couple of videos using my asteroid occultation setup and a sequence of 20s images taken 20s apart. Once aligned and stacked these images show the asteroid’s movement as a dashed line.

(357439) 2004 BL86
The asteroid appeared about as bright as a 9th magnitude star (typically stars down to 6th magnitude are visible naked-eye from a very dark site).

Horsehead Nebula

B33 – The Horsehead Nebula

Imaging opportunites have been few and far between this autumn. Whenever it has been clear there has been a bright Moon, it has been too windy or I’ve been busy. But at last we had a clear night with no Moon and I could finally image something!

My choice was the Horsehead Nebula in Orion. An object I’ve not imaged before. The Horsehead is a famous dark nebula (Barnard 33) shaped like a horse’s head which is silhouetted in front of a huge bright emission nebula (IC 434 or Sh2-277) known as the Flame Nebula. The whole complex surrounds Alnitak which is the lefthand star in the distinctive line of three stars that make up the Belt of Orion. The Horsehead is found just below Alnitak.

I imaged using my QHY22 camera and a H-alpha filter. This filter only transmits light from a specific deep-red visible spectral line. This light is emitted when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is interesting to amateur astronomers as it’s emitted by emission nebula and local light pollution (even moonlight) won’t interfere with the imaging. By imaging using a H-alpha filter you can get great images of nebulae with high contrast. The payoff is that you need long exposures (typically 5-15 minutes each) and for that you need really good auto-guiding.

Auto-guiding involves using a second telescope mounted in parallel to the imaging telescope with a second camera taking continuous short exposures (typically 1-2s long). Using a clever bit of software the position of a ‘guiding star’ on each exposure is compared to the previous and if it has moved due to errors in the telescope tracking the rotation of the Earth the software automatically applies a correction to the position of the telescope so that the star remains exactly in the same place whilst you are imaging. Without guiding you are limited to shorter exposures of around one minute each as the errors in tracking add up and the stars will start to trail slightly.

My guiding setup has been working really well since I set up the Starshed Enterprise and I can usually get five minute exposures with no visible star trailing. To reduce noise in the resulting image you need to stack as many exposures as you can get and I typically aim for a minimum of 20 exposures. Twenty exposures of five minutes each is around two hours of imaging including taking dark frames for calibration.

It’s preferable to get all of the imaging done before the object crosses the meridian (due South) as although you can continue imaging for a while afterwards eventually the telescope tube will hit the mount and you have to perform what is called a ‘Meridian Flip’. This involves re-pointing the telescope from the eastern hemisphere to the western hemisphere and is a bit of a faff. On this evening I started early enough and the Horsehead was far enough east that I didn’t need to perform a flip for two hours worth of imaging.

In the end I managed to get 19x300s exposures. I had to stop as the secondary mirror was completely dewed up (a bit like how a bathroom mirror steams up after a shower). I could have cleared it with a quick blast from a 12V hairdryer but the whole observatory was dripping with dew and it was close to flip time so I packed up. I need to experiment with heaters or fans to prevent dewing up of the secondary as it has been a bit of a pain.

Capturing, stacking and processing was all done in Nebulosity 3. The resultant image is still a little bit noisy (grainy) but I’m pretty pleased with the result. I plan to capture some more exposures at some point to improve the image further. I might even be able to take some of the surrounding area and create a mosaic image as the nebulosity stretches way beyond the field of view of my setup.

M51 – Whirpool Galaxy

I’ve been taking some images from the Starshed Enterprise when it has been clear over the past few weeks. Experimenting with different settings etc. I’ve now got a nice system going, autoguiding is working well and I’m starting to get some nice images.

Messier 51 (Whirpool Galaxy) was the first galaxy shown to have a spiral structure when in 1845 Lord Rosse observed it with his giant 72″ telescope in Ireland. It is found in Canes Venatici below the tail of Ursa Major (the handle of the Big Dipper) so is well placed for observing at this time of year as it’s almost directly overhead. It has a smaller companion which it is interacting with (NGC 5195) seen as the compact galaxy at the end of a dark dust lane in one of the spiral arms. Tongues of material are being thrown out from the system as they interact, three distinct fingers can be seen stretching upwards in my image.

M51 - Whirlpool Galaxy

Supernova 2014J

M82 - Supernova 2014J by jochta
M82 – Supernova 2014J, a photo by jochta on Flickr.

A bright supernova in M82 was discovered on January 21st, the closest Type 1a supernova for 40 years. Here’s my image of it on January 25th when it was about magnitude 11.0. I have the collimation of my new 12″ telescope sorted now and this is a stack of 15x60s images captured in Nebulosity using an Atik 16IC-S camera. The supernova is below-right of the centre of the galaxy in the centre of the image.

NGC 1662

NGC 1662 by jochta
NGC 1662, a photo by jochta on Flickr.

First image from my observatory. A fairly random, non-descript open cluster in Orion which was small enough and well-positioned to test the new mount and EQMod settings.This is a stack of 12 unguided 60s exposures through my smaller 200mm Newtonian telescope. No filters, flats or darks.

Some say this cluster looks like a little boat and it kind of does. The bottom of the boat is the line of stars running diagonally down to the bottom left of the image, it has a prow and a stern. The cluster of multiple stars in the centre is the mast.

Observatory Build – Part 7

I think this will be the last post entitled “Observatory Build” as it is now built! I had three friends around at the weekend and we just about managed to get the roof on between us. I borrowed a hefty trolley from work which made getting down the side of the house and to the end of the garden doable. I think it would have been impossible without. The roof went around the corner easily, I had been very worried that it wouldn’t make it.

So here it is with the roof on, guttering fitted etc. The previous owner had a water butt on the other side hence the truncated downpipe. I’m going to deck the area under the runoff in the spring and I will have a water butt too. I will also replace the vertical posts with longer ones and brace them more attractively.

Observatory built
Observatory built

Since the weekend I’ve finished off the walls inside, temporarily set up the cupboards and started wiring up the electrics. I’m going to order the new telescope and mount soon so I can buy the right height pier and get that installed.