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Tuesday
Dec042012

Discovering Asteroids at itelescope.net: Part 5-Artifacts

Discovering Asteroids

Go to Part  1 - 2 - 3 - 4 - 5 - 6 - 7 - 8

Why Can’t We Image Yet?

If you have read the previous four articles, you will be familiar with the asteroid discovery rules, the major surveys, how to select an itelescope.net scope and where to point it. At first sight there is nothing to stop us imaging the night sky, finding asteroids, measuring their positions and reporting the results to the Minor Planet Center (MPC). 

Unfortunately there is a very real problem and it concerns the “finding asteroids” part of the process. Asteroids appear as moving objects relative to a fixed background of stars but as you will see although all asteroids are moving objects not all moving objects are asteroids. 

Some of the non-asteroidal moving objects, or artifacts, are so different in appearance from the true thing that we can immediately see them for what they are. However others look like asteroids, appear to move like asteroids and indeed have motions so similar to those of true asteroids that if we report their positions to the MPC, the MPC will believe us and add the artifact to their database. 

If that is not bad enough some artifacts can mimic the motion of Near Earth Asteroids (NEO’s) and if we report one and the MPC believe us, they will add it to their NEO Confirmation Page

This lists all the suspect NEO’s that have been reported in the last few days and is used by the major surveys when selecting objects for follow-up measurements. We are now in the situation where we are responsible for professional observatories wasting time looking for an object which does not actually exist.

Clearly this is something we really do not want to do and this is why I will show you how to avoid it happening.

Before I go into details, let me define some terms and introduce you to some software we will be using:- 

Pointing

When you use one of the itelescope.net scopes, you tell it the centre point of the field of view that you want it to image and you do this in terms of RA and Dec coordinates. The telescope then slews to that general area, takes a location image, compares it with the star map stored in its memory, calculates exactly where it is pointing and how it needs to move to point to the position you have requested. 

Tracking

Once the telescope has been pointed correctly it has to be moved continuously and precisely in a way that compensates for the rotation of the Earth. This is the job of the telescope mount. In the case of some mounts, for example the one used on T11, the motors and gearing are so well constructed and programmed that once the scope has been pointed it will remain on target for up to 10 minutes without any further input being required. This process of correcting for the Earth’s rotation is termed tracking. 

Other mounts are not so accurate and require some additional information input in order to remain on target. This information input and position correction process is termed guiding. itelescope.net provides information for each scope regarding the maximum time that you can image without guiding. 

Guiding

The guiding process involves the use of software which selects a suitable star as close as convenient to the target centre then measures its position at the start of the exposure and every few seconds during the exposure period. If the tracking is inaccurate and the star appears to drift out of position, the computer will calculate the distance and the direction that the scope will have to move in order to correct for this drift. This information is then fed to the mount which carries out the necessary correction. 

Reducing

This process involves the use of software that can select a number of stars in an image and then obtain their RA-Dec coordinates using a suitable stellar library. Using the positions of these reference stars it is possible to calculate the RA Dec coordinates of any point within the image. 

Aligning

This software operation is a process where a group of reduced images with the same field of view are shifted laterally and rotated until each star in each image is lined up with the corresponding star in all the other images. For example if we had a set of such images on clear acetate sheets we could hold the entire set up to the light and manipulate each sheet until we got a perfect alignment.

The alignment process is a necessary first step prior to both stacking and blinking. 

Stacking

Discovering asteroids inevitable requires us to detect faint moving objects. In order to do this we need to collect as many photons as possible. For non-moving objects like stars the longer the exposure time the more photons we collect. The problem with moving objects is that if we use too long an exposure period the image start to trail, in other words it will change from an essentially circular spot to a more elliptical shape and then into a line or trail which marks the motion of the object across the sky. 

One way around this problem is to take numerous shorter-exposure images in each of which the moving object appears as a circular spot in a slightly different position. We then use suitable software to superimpose these images one on top of the other. If we superimpose them exactly we would end up with circular stars and a trailed image of the moving object. However by some cunning electronic trickery we can displace each image slightly and end up with trailed stars and a circular image of the moving object. The point of doing is that effectively we produce a circular image of the moving object over a longer exposure period. In this way we concentrate the maximum number of photons on the minimum number of pixels and get a brighter image which is easier to detect. 

Blinking

I normally take 15 images each with an exposure time of five minutes and then stack them as 3 sets of 5 images. Each set therefore has an exposure time of 25 minutes. Using suitable software I then display each set one after the other repeatedly and look for objects that appear to move. This process is termed blinking and is equally applicable to individual unstacked images. 

Now that we have defined terms we need to consider the software required to carry out each task. The good news is that we don’t have to worry about pointing, tracking, and guiding as these are all carried out by itelescope.net as part of the service. The remaining tasks are our responsibility and for these I recommend Astrometrica. 

You can get detailsof this software here: http://www.astrometrica.at/ There is also a user group

As you will see this is an extremely versatile tool and in addition to reduction, alignment, stacking and blinking it also enables us to measure the position of asteroids and to email the results to the MPC in their required format. 

It’s the Real Thing

Before we start looking at artifacts pretending to be asteroids here are an images of a real one:- 

This is asteroid 34997 at magnitude 17.9 imaged from Nerpio using T17. The animation is produced by blinking three stacked images in sequence repeatedly. On of the many useful features of Astrometrica is the option to display a labelled location box around all asteroids for which the MPC has calculated an orbit. 

How about This? 

At first sight it looks believable: a moving object with magnitude 18.5, travelling in more or less a straight line and at a speed typical of asteroids in this region. The absence of a location box means that the MPC has not calculated an orbit. If we searched using the MPC Minor Planet Checker for objects that MPC knows about but has not yet calculated an orbit we would draw a blank. At this point we might be thinking in terms of a potential new discovery but before we report it to MPC let’s just stop and check. 

One good initial check we can carry out involves the use of Find_Orb.

Find_Orb is a piece of software which can process the position measurements that we make using Astrometrica and find the best orbit that fits the data. Like Astrometrica there is a user group

As well as generating an orbit, Find_Orb will also give us an indication of how accurate our measurements are. If we take asteroid 34997 as an example we have used three images to produce the animation. If we measure the RA and Dec position of the asteroid in each image and process this data using Find_Orb it will calculate the residuals for each of the six data points. It is rather like drawing the best straight line through a set of points on a graph and then measuring how far away each point is from the line. The residuals are expressed in arcseconds and as a general guide we should be able to produce measurements with residuals less than 1.00 arcseconds and ideally less than 0.50. If any of the residuals are greater than or equal to 1.5 arcseconds the MPC will not regard them as good enough for use when they try to link observations made on different nights. 

Processing the coordinates from 34997 gave the following results: 

Image Number

RA Residual

(arcseconds)

Dec Residual

(arcseconds)

1

0.04

0.04

2

0.07

0.07

3

0.03

0.03

These small residuals show that our measurements were pretty accurate although to be fair it is a bright object which makes it easier to get good results. 

Now let’s check the residuals from nfxx2: 

Image Number

RA Residual

(arcseconds)

Dec Residual

(arcseconds)

1

0.13

0.07

2

0.25

0.14

3

0.13

0.07

 

Admittedly the residuals are not as good as those from 34997 but I am reasonably certain that if we reported them they would be accepted by the MPC. 

In fact nfxx2 is not an asteroid; it is a small cluster of defective pixels (hot pixels) which act as if they are detecting photons even when they are not illuminated. The result is that each image taken with this camera will show this small spot of light in the same position. 

At first sight it is not easy to see why a defect at a fixed location on the camera chip should give rise to an apparently moving image. The answer lies in the image alignment process. 

The Alignment Illusion

This illusion can be created using either stacked or unstacked images and is a result of tiny errors in pointing between successive images. I have deliberately used unstacked images in the following animations to prove to you that the illusion is created by alignment prior to blinking. Had I used stacked images they would have also shown the illusion but it would have been created by the alignment prior to blinking process and not simply by stacking. 

The image is an animated screen print of what you see when you blink three images without first aligning them. In this mode the CCD frame appears virtually motionless. During the 100-minute imaging session the scope has drifted slightly off-target both downwards and to the left and as a consequence we see stars appearing to move upwards and to the right In the second image only we see a line of illuminated pixels appear close to the left-hand edge of the image. This artifact is a cosmic ray strike. 

The object labelled nfxx3 is a hot pixel which appears in all three images. It occupies a fixed position on the CCD chip and like the CCD frame appears virtually motionless. 

It is possible, using Astrometrica, to align these three images so that the stars are superimposed on each other. This animation shows the result. 

The alignment process involves moving the images to compensate for the off-target drift. This results in the apparent motion of the CCD frame and of course the nfxx3 hot pixel. In this animation the illusion is revealed because we can see the hot pixel moving with the frame but if the frame was not in view then all we would see is nfxx3 appearing the move relative to the stars just like an asteroid. 

Aren’t Hot Pixels Easily Recognised?

I sense that the more experienced among you will feel that nfxx3 looks nothing like an asteroid and that you would never mistake it for one. That is a valid point and I chose it for demonstration purposes simply because it happened to be conveniently located at the corner of the image where you can see both the object and the frame. During the past decade I’ve seen my fair share of hot pixels and although the majority of them can be recognised by their small size, abnormal brightness and generally blocky appearance there are some that are quite good asteroid mimics. 

Object nfxx4 is more difficult to distinguish from an asteroid. It is in fact a larger than average cluster of hot pixels and in the bottom right-hand corner of the image you can see a more recognisable hot pixel moving in step with it. 

One way to avoid confusing artifacts with real objects is a process known as dithering. 

Start Dithering

I use the term dithering to describe multiple re-targeting during an imaging session in order to reduce the possibility that any drift off-target with time can mimic the motion of an asteroid. 

Those of you who have used ACP Observatory Control Software may know that one of the directives is #DITHER. This enables you to shift the target at the beginning of each exposure by a random number of pixels in both RA and Dec. I have tried this but found that there is a disadvantage when using it to make relatively large shifts when a telescope is also being guided. Effectively what you are doing is asking the scope to lock on to a guide star but then at the beginning of each exposure giving the mount a crafty kick. Another disadvantage is the random nature of each shift which brings with it the possibility that the shifted positions may result in a linear drift. 

I find that a better way is to actually re-target at about one-third and two-thirds of the way into the imaging session and to displace the RA by 10 seconds of time and the Declination by 30 seconds of arc. I do this in such a way the image centres cannot form a straight line and consequently any defect on the chip will appear to move in a non-linear fashion. 

This is the pattern I use when taking 15 individual exposures 

Image Numbers

Right Ascension

Declination

1 to 5

Initial Value

15 18 00

Initial Value

04 17 00

6 to 10

Initial Value + 10 seconds

15 18 10

Initial Value + 30 seconds

04 17 30

11 to15

Initial Value + 10 seconds

15 18 10

Initial Value – 30 seconds

04 16 30

 

This image shows the positions of the example coordinates plotted out using planetarium software. As you can see the 1 arcsecond diameter circles do not lie in a straight line and the apparent motion of any hot pixels when blinking the three images will be correspondingly non-linear. 

Just Checking

If you do discover what you believe to be a Near Earth Object and if you do report it to MPC, you may get an email from them asking if you are absolutely certain that your measurements have been made on a real object. I speak from experience when I say that then is not the time to start checking. 

In such a situation before I report anything, I first locate some hot pixels and check that their motion is non-linear and/or distinctly different from that of my NEO suspect. If I am still in doubt I then re-measure the position using un-stacked single images. I have found that although three stacked images can (as we saw with nfxx2 and nfxx4) give rise to apparent linear motion, a check involving 15 images is very likely to reveal signs of irregular motion with an artifact. 

If after all these checks I cannot be completely certain that I am looking at a real object then I do not report it. I either call it a day or I use the measures I have obtained to predict its position the next night and then try and image it again. If when I check where it should be I find myself looking at a patch of empty sky then I know it was an artifact. However if I find it and the motion and the magnitude match my prediction then I know it is real. I can then report two nights and wait, fully prepared, for any MPC reality check. 

Other Artifacts

1. Random Noise 

If I detect what appears to be a very faint moving object i.e. one that is only just within the limit of detection, I have to consider the possibility that it could be a chance alignment of random noise. This animation shows an example of such a case. 

As you can see the pixels representing the star-free sky vary in brightness from image to image. This variation is due partly to variations in the very low levels at light pollution at Mayhill and partly due to electronic effects which cause individual pixels to produce photoelectrons even when they are not impacted by photons. 

These random effects can give rise to small clusters of pixels that are brighter than those that surround them. If for example you blink three images, you can get the situation where each image has such a cluster and where the clusters lie more or less on a straight line. 

In this particular example when I checked the residuals using Find_Orb as described above they were not particularly good and when I re-stacked the images using a different speed and angle the “object” was no longer detectable. 

This re-stacking test is a good way of distinguishing between real faint moving objects and random noise clusters. In the case of single images I find that although you may see chance alignment of random noise when you blink three images, the illusion is very unlikely to persist once you increase the number of images that you blink. 

2. High Energy Radiation 

The cosmic ray strike that I pointed out earlier in this article is but one of many examples of how CCDs can detect things other than visible light photons. 

This publication http://snap.lbl.gov/ccdweb/ccdrad_talk_spie02.pdf gives further details. In my experience you see examples of these in most images but the number increases significantly at time of high solar activity. The only ones that resemble asteroids take the form of circular spots that appear in individual images. Fortunately these spots do not occur frequently enough to give any realistic possibility of a chance alignment being mistaken for a real object. 

What Next?

In my next article I plan to detail the process of astrometry using unstacked images.

 

Tuesday
Dec042012

Discovering Asteroids: Part 6 - Astrometry Using Unstacked Images

Discovering Asteroids

Go to Part  1 - 2 - 3 - 4 - 5 - 6 - 7 - 8

To Stack or not to Stack?

In Part 5 of this series I described how we can use image stacking to enable us to produce long-exposure images of moving objects. The advantage of image stacking is that it allows us to see objects that would be too faint to detect using unstacked images. One disadvantage is that astrometry using stacked images is more complex than working with unstacked images. It is for this reason that I intend to deal with stacked image astrometry in a later article. In this article I will describe how to use unstacked images to measure the position of relatively bright asteroids and report our findings to the Minor Planet Center (MPC).

 

If I have a choice I always use unstacked images the reasons being:-

  1. I have more images to choose from so I can reject those where the object is close to a star, where the seeing degrades or where a cloud moves across the field of view.

  2. All image processing brings with it the risk of degrading the accuracy of the measurements so I only use image stacking if it is really necessary.

  3. Stacking images reduces the observation arc i.e. time interval between the first and the last image. As a general rule, the longer the observation arc, the more useful it is in predicting the position of the object at some future date.

How Good do our Measurements Need to be?

 

The iTelescope observatories have all been assigned observatory codes by the MPC and this means that we can log on to a telescope, measure the position of an asteroid and report its position to them.

 

The MPC accepts measurements from both professionals and amateurs but their requirements for both are exactly the same. They emphasise the importance of both accuracy and consistency. In the case of accuracy they are expecting us to produce measurements that are accurate to within 1 arcsecond.

 

If either of the residuals in a RA-Dec measurement are greater than or equal to 1.5 arcseconds then the MPC will not include that particular data point when calculating an orbit. For any given night the MPC require at least two RA-Dec data points on an object so if I were to submit three data points and two of them had a residual greater than or equal to 1.5 arcseconds then all three data points would be excluded from the orbit solution. This is significant because the lost night’s observations might have included a discovery observation.

 

In the case of consistency the MPC would like our measurements to vary by no more than a few tenths of an arcsecond.

 

The difference between accuracy and consistency can be shown by the arcsecond residuals obtained when four different observers each make six different measurements:-

 

A: 0.0, 0.2, 1.6, 0.9, 0.1, 1.3

 

B: 0.0, 0.4, 0.1, 0.5, 0.1, 0.9

 

C: 0.6, 0.6, 0.8, 0.7, 0.7, 0.6

 

D. 0.1, 0.3 0.1, 0.2, 0.3, 0.0

 

Observer A is neither accurate nor consistent. Their residuals are basically all over the place ranging from 0.0 (spot on) to 1.6 (not good enough for linking).

 

Observer B is accurate insomuch as all the residuals are less than 1 but not very consistent since their residuals range between 0.0 and 0.9.

 

Observer C is less accurate that Observer B since all of their residuals are greater than 0.5 but they are much more consistent with the spread of residuals only ranging from 0.6 to 0.8.

 

Observer D is up to professional standard with results that are both accurate and consistent.

 

The MPC value night-to-night consistency and would probably prefer the consistent observations of C to those of the slightly more accurate but less consistent values supplied by B.

 

What I aim now is to show you what needs to be done in order to produce results which meet the MPC accuracy and consistency requirements. I will be using various items of software but I will not be going into their operational details. The reason for this omission is partly because to do otherwise would make this a very long article but mainly because the authors of the software do a really good job in explaining how to use their products.

 

Asteroid Selection

 

In order to establish how good my measurements were I needed to choose an asteroid whose position is known very accurately and which is within an acceptable magnitude range. The MPC advise that asteroids with numbers between 400 and 40,000 meet these requirements.

 

In Part 3 of this series, I dealt with telescope selection and had my observations been limited to an accuracy check using a relatively bright asteroid, I could have used any of the seven telescopes suitable for asteroid observation in the Northern Hemisphere (Siding Spring observatory was still under construction at this time). However, I wanted to combine the check with a follow-up observation on one of my potential discoveries at magnitude 21.7 and this prompted me to choose T11.

 

I carried out the observation in March 2012 and, using SkyMap, I found that the earliest date when the weather conditions were acceptable and the telescope was available was March 30th. I displayed the first 40,000 numbered asteroids in my target area at midnight on that day and selected a number of possible candidates.

 

I then used the MPC Ephemeris page to eliminate those that were moving at more than 1 arcseconds per minute. Anything faster than that would have been more difficult to measure accurately. My final choice was 34997 1978 OP. On March 30th 2012, the asteroid was at magnitude 18.3 and was moving at 0.1 arcseconds per minute.

 

The timing of the observation was a compromise between avoiding lunar interference while keeping the asteroid as high in the sky as possible. I chose a start time of 00 30 hrs Mountain Daylight Time which meant that the asteroid’s elevation ranged from 55 to 33 degrees.

 

Time is of the Essence

 

Asteroid observation means having to work with up to three different time zones:-

 

1. Observatory Local Time which is used for planning the observation and reserving the telescope.

 

2. Universal Time (UT) which is used when reporting results to the MPC.

 

3. Observer Local Time which you need to know if you want to be at your computer watching while the observations take place.

 

Intercontinental time conversions are complicated by the fact that different time zones may have daylight saving times that change on different days (and at different times of day). In addition the conversion may result in a date change.

 

I find this time zone converter useful:-

 

http://www.timeanddate.com/worldclock/converter.html

 

and using it I found that 00 30 hours Mountain Daylight Time in New Mexico converted to 07 30 hours British Summer Time in England and to 06 30 hours UT. The date remained March 30th in each case.

 

Targeting

 

I used the MPC Ephemeris page to obtain the exact position of 34997 at the start of observation but in order to do this I had to express the UT date and time in a decimal form.

 

The calculation of decimal UT involves converting UT to seconds and then dividing it by the 86,400 seconds in a day. In this case, 06 30 hours UT on March 30th converts to 30.27083 decimal UT.

The coordinates of the asteroid at this time were: - R A 09 16 47.7, Dec +28 40 32

 

Choosing the Binning

 

Reference to the T11 information page showed that for optimum results I had a choice between 1 x 1 and 2 x 2 binning and a check of the calibration images currently available revealed that I could use either of these options. I chose 2 x 2 binning which as we saw in Part 3 gives higher sensitivity at the expense of reduced accuracy. Had I not had also been trying to follow up a magnitude 21.7 asteroid I would have gone for the more accurate option.

 

Choosing the Exposure Time

 

The points I needed to take into account were that the exposure time needed to be:-

  1. long enough to give a detectable image of the asteroid.

  2. short enough to prevent the image trailing

  3. a time interval for which iTelescope produced flats and darks calibration images.

 

The 2 x 2 binning level gives a resolution of 1.62 arcseconds per pixel and if we want to avoid trailing we need an exposure time short enough to prevent the asteroid moving more than 1 pixel i.e. more than 1.62 arcseconds. As we saw earlier, 34997 on the planned observation day would be moving at 0.1 arcseconds per minute so the longest exposure that keeps the motion to less than 1.62 arcseconds can be calculated by dividing the resolution by the speed i.e. 1.62 / 0.1 = 16.2 minutes which is 972 seconds.

 

If we log on to T11 and navigate to the telescope information page we see that exposure times greater than 600 seconds require guiding. I decided to avoid the need to guide by selecting a shorter exposure time and reference to the general T11 information page showed that the longest guide-free time recommended was 300 seconds. On this basis I opted for a 300 second exposure.

 

Choosing the Number of Exposures

 

The MPC recommend that the observation arc should be at least 30 minutes and preferably between one and two hours. They ask that the arc should be made up of between three and five positions. Single positions are rejected while more than five on a single night do not add significantly to the accuracy of the arc.

 

With this in mind I decide to obtain 15 images each with a 300 second exposure. I know from experience that, allowing for the download time of each image, this will give me an unstacked observation arc of about 1 hour 45 minutes. If I stacked the images as 3 sets of five, the arc reduces to about one hour.

 

Choosing the Filter

 

Depending on which telescope you select you will find that it is fitted with either a clear or a luminance filter. T11 comes with a luminance filter and this is the one I selected.

 

Preparing the Observation Plan

 

I used the ACP Planner to produce the plan that would control the telescope during the imaging session. iTelescope use a customised version of ACP Observatory Control Software and this is described here:- https://go.itelescope.net/downloads/itn-acp-directives.pdf

 

In Part 5 of this series, I recommended the use of dithering in order to prevent confusing artifacts with real asteroids and my plan incorporates the multiple re-targeting that I detailed therein.

 

This is the plan I produced:-

 

;

; --------------------------------------------

; This plan was generated by ACP Planner 3.2.2

; --------------------------------------------

;

; For: Norman

; Targets: 3

;

; NOTE: Timing features are disabled

;

; Autofocus at start of run.

;

; ---------------------------------------------

;

#autofocus

;

; === Target 34997a ===

;

#count 5

#filter Luminance

#interval 300

#binning 2

34997a 09:16:50.00 28° 40' 30.0"

;

; === Target 34997b ===

;

#count 5

#filter Luminance

#interval 300

#binning 2

34997b 09:17:00.00 28° 41' 00.0"

;

; === Target 34997c ===

;

#count 5

#filter Luminance

#interval 300

#binning 2

34997c 09:17:00.00 28° 40' 00.0"

;

#shutdown ; Shut down the observatory

;

; -----------

; END OF PLAN

; -----------

;

 

After the initial focussing command, there are instructions to collect three sets each of five images. The exposure time, filter and binning are the same throughout but the target coordinates are dithered. 

 

Setting up for Image Processing

 

I use Astrometrica and I suggest that if you are unfamiliar with this software you start by reading through the tutorials in the Help section. The next step is to set up the six screens that control the way in which the program processes the data and reports the results. You will find details of how to do this in the Help section under Dialog Boxes – Program Settings. 

 

Evaluating the Images

 

The sky conditions were not ideal when I obtained my 15 images. The cloud meter read minus 20 and the seeing ranges averaged around 2.8 arcseconds. After downloading I opened each image in turn to check that the tracking was good and faint stars were detectable. In fact all the images looked OK so the next step was to select one of them and carry out a reduction.

 

If you have no difficulty in reducing images you can skip the following section but when I first used Astrometrica I did experience problems with images which stubbornly resisted reduction. This is the method that I used to overcome the problem.

 

Reduction

 

I refer to T11 throughout but the method is the same for any of the iTelescope scopes.

The first thing I do is to display the target area using SkyMap with a CCD frame set to show the T11 field of view. I do this so that North is up and West is to the right. I then compare this with my image. Hopefully the pattern of stars in SkyMap and the image is identical but if not my next step is open the Program Setting-CCD window and change the settings of the Flip Horizontal and Flip Vertical tick boxes (if they are both ticked, un-tick them and vice versa). Closing and reopening the image should display it inverted both laterally and vertically.

 

If the images still does not match SkyMap it is possible that there is a targeting error or that the CCD is not mounted in a North-South vertical: East-West horizontal orientation.

I can generally spot any targeting error by adjusting SkyMap to give a wider field of view and then trying to match the image star pattern both before and after inversion. If this does not work I log on to T11 and open up the appropriate log file.

 

In order to locate the target area, the telescope software will have carried out an image reduction and this information is recorded in the log. If you scroll down to the point where the first image is recorded and then look a few lines above you will find the following lines:-

 

True focal length is ……

Imager sky position angle is ….

 

In this case my log showed the focal length as 226.0 cm and the sky position angle as 192.5 degrees.

I then enter these values into the Astrometrica CCD window. Astrometrica allows you to choose between Automatic and Manual reduction. I favour Manual because it allows me to see what is going on. I can set Astrometrica to Manual by opening the Astrometrica Program window and setting Reference Star Matching-Number of Stars to zero. This image shows the result of my attempted reduction. I have displayed it in negative form for clarity

 

 

 

As you can see there is a problem. The red circles show where Astrometrica thinks the stars should be and black dots show where they actually are. In fact what has happened is that the 192.5 degree angle recorded in the log refers to a reduction carried out on an inverted image. All we have to do is to subtract (or add) 180 degrees which gives us an angle of 12.5 degrees.

This image shows the result of a reduction carried out after re-setting the sky position angle to 12.5 degrees, Although we still do not have an alignment you can see that if we were to shift the red circles slightly to the left and very slightly upwards the stars and the circles would align. In practice I find it helps if I reduce the magnitude until only the circles representing the brightest stars are displayed.

This image shows the circles and stars aligned and if I now click on OK, the image will reduce.

The last refinement you can carry out is open the Astrometrica log file and note the focal length and rotation. I do this and feed the values back into the CCD window. Another piece of useful information in the log is the RA-Dec Center Coordinates i.e. the true image centre. If there is a significant pointing error (which can happen when the sky conditions are poor) then it can save time to enter these values just before carrying out the reduction. If you do this it minimises the time you have to spend aligning the circles and stars.

When you reduce an image, Astrometrica displays a Data Reduction Results window and this includes the dRa and dDec residuals. I find that with a good reduction these values are less than 0.3 arcseconds.

Evaluating the Images

I find a good way of evaluating image quality is to reduce the image and then check both the residuals and the total number of stars detected. If the sky condition worsens during an imaging session the number of stars detected is reduced and the residuals are increased.

I reduced all 15 images at the same time and discarded one with poor residuals and a low star count. I then used Known Object Overlay to identify the asteroid and blinked the images in order to check if there was any stellar interference. Finally I select five of the images which were of good quality and gave the longest observation arc.

Position Measurement

I loaded the five selected images, reduced then and then added the know object overlay. I then blinked the images and centered 34997 in the field of view at maximum magnification.

I measured each image in turn by stopping the blink option and then positioning the cursor as close as possible to the centre of the asteroid and then clicking. In the Help section of Astrometrica there are details of other measurement options involving the use of the Control and Shift keys which you may find useful.

Whichever method I use, I always check (using the Object Verification Window) that the asteroid is centred correctly within the locating circle. The diameter of this circle is controlled by the Aperture Radius setting which is located in Program Setting window. I sometimes have problems centering an object within the locating circle and I find that making the Aperture Radius larger or smaller usually overcomes this problem.

Having centered the asteroid correctly I then gave it what the MPC term an Observer Assigned Temporary Designation which is basically my reference number. The MPC favour the method followed by the professionals whereby each object observed on any given night is assigned its own unique Observer Assigned Temporary Designation. The designation can be a maximum of six characters but should not be in a form that could be confused with the designations formats used by MPC i.e. numbers only (34997), unpacked provisional designation, (1978 OP) and packed provisional designation (currently a letter followed by four digits). I suggest that you adopt a system based on your initials (case dependent) followed by a number. I called this object nf458.

I used the same method and designation for the remaining four images.

Reporting to MPC

Astrometrica prepares a report in the approved MPC format so all I had to do was to check that I had five readings all with the same designation and then click the send button.

MPC has a well-automated system for dealing with reported positions and the first email I received was their standard autoack message which said:-

The receipt of a message (probably containing observations) is hereby
acknowledged.

The formatting code returned the following statistics:

Number of header lines read =  8

Number of observation lines read =  5

I normally get one of these within a few minutes of sending a report but at busy times it can take longer. If you don’t get one within 24 hours the send it again but include RESEND in the subject line.

The next message I usually get is information regarding their designation of the object I have reported. This autodes message is very brief and in this case of 34997 simply read nf458 (34997.

It means “we have identified to object you call nf458 as the numbered asteroid 34997”. The bracket is highly significant to those hoping to discover new asteroids because if it is absent it means that the MPC has not been able to identify it with a known object.

How Good were my Measurements?

In order to check the accuracy of my results all I had to do was to use the MPC Ephemeris page to display the residuals for 34997

If you have reported an object for which the MPC has rated observations as Desirable or Highly Desirable then they normally report your residuals within a day or so. However well observed-objects like 34997 are rated as No Observations Needed at this Time. What this means in practice is that for MPC this is a low priority matter and they normally wait until the end of the lunar month before including the results in the residuals section.

The residuals for the five positions I reported were:

RA Dec

0.2- 0.0

0.1+ 0.2+

0.1+ 0.1-

0.2+ 0.0

0.0 0.1-

There is a lot credit due here to iTelescope for enabling me to produce residuals of this accuracy bearing in mind that the sky conditions were not ideal. In order to achieve this accuracy you need perfectly aligned optics, spot-on tracking and a clock which is constantly synchronised with Universal Time.

What Next?

Now that we have demonstrated that the equipment can produce accurate results, we are ready for our first discovery mission. This will be the subject of my next article.