Some Strategies for Discovering Minor Planets 发现小行星的一些策略

By Andrew Lowe 作者:Andrew Lowe

We live in an exciting time for minor planet research. Advances in hardware and software have allowed us to observe and discover asteroids that were considered too faint for detection only a few years ago. Of course, this ability is not restricted to amateurs. The big professional surveys such as LINEAR, LONEOS, NEAT, Spacewatch, CSS, and others sweep large areas of the sky every night and are very successful at discovering asteroids. 我们生活在一个另人激动的时代。目前,通过一些软件和硬件的投入,业余爱好者们可以观测和发现一些几年前被认为是非常暗弱的小行星。当然,也有很多的专业的天文台,比如 LINEAR, LONEOS, NEAT, Spacewatch, CSS以及更多的巡天系统每晚都在扫描着大片的天空并获得了很多发现。

Although Spacewatch set up an asteroid detection system in the mid-1980s with new CCD technology, the pace of asteroid discoveries accelerated in 1997 when LINEAR commenced operations. The success of the LINEAR concept spurred similar systems: Lowell Observatory’s LONEOS, NASA/JPL’s NEAT, CSS, and others. These surveys submit tens of thousands of observations of known and unknown minor planets to the Minor Planet Center (MPC) every day. Every night, the MPC’s DOU (Daily Orbit Update) contains hundreds of orbits for new discoveries and updated orbits for newly-discovered objects. 
虽然Spacewatch在上世纪80年代最先利用CCD技术开始小行星自动检测系统,但小行星发现步伐的加快还是从1997年LINEAR计划的开始。LINEAR的成功催生了更多的巡天系统:洛威尔天文台的LONEOS,NASA/JPL的NEAT, CSS以及其他的项目。这些巡天系统每天向MPC报告了好几万颗已知或未知的小行星观测数据。每天晚上,DOU都会公布几百个新发现目标的轨道更新数据及新目标数据。

It is perhaps only natural to take the attitude that the big surveys are discovering everything in the sky. After all, LINEAR has been in operation for eight years, and every main-belt asteroid (MBA) has had one favorable opposition during that time. And it’s true that all of the reasonably bright objects have been found over and over again. Nevertheless, I would suggest that it is possible to discover asteroids in spite of LINEAR and the other big surveys. In fact, in this article I’ll talk about strategies for not only maximizing your chances of finding an asteroid, but also using the big surveys to help follow-up your discoveries. 
也许你会想,这些大的巡天系统会发现任何需要发现的东西。毕竟,LINEAR已经工作了8年, 可以这么说,每一个MBA在此期间都已经有一个大冲的机会,事实上,那些比较亮的目标已经被多次发现。然而, 我认为虽然有像LINEAR那样多的大型巡天望远镜的存在,发现小行星还是有可能的。事实上,在本文中,我将不仅论述如何尽大限度的创造发现机会,而且也将介绍如何利用这些大的巡天系统来为自己的发现提供支持数据。

How do I know what I talking about? Well, in my first month using the New Mexico Skies Observatory’s Takahashi Epsilon E250 Astrograph, a 0.25-m f/3.4 telescope, I discovered six asteroids. In my second month, I added twelve more. I’m convinced that with the proper equipment and a focused approach, anyone can do the same. 
我为什么这么认为呢?嗯,我使用”新墨西哥天空“观测站的高桥E-250(口径0.25米,f/3.4)进行搜索,第一个月发现了6颗小行星,第二个月发现了12个还多。我相信,使用合适的设备以及正确的方法,任何人都可以做到。

Here are some points to get us started: 
以下是一些要点:

1. The more fields you shoot, the better the chance of discovering something. While you don’t have to be out every clear night, you do need to put in a concerted effort. And knowing where to shoot will increase your odds of making your own discovery instead of proving up someone else’s. 
1、拍摄的区域越广,获得发现的机会就越多。你不必每个晴夜都出动, 你需要合理的安排。知道拍摄哪里才可以增加自己发现的几率而不是一味地验证别人的发现。

2. You need to be able to image fainter asteroids. If your system doesn’t allow you to image mag. 19 objects, you aren’t going to find much. A 8″ (0.20-m) scope is probably too small. To find anything faint, you’ll spend too much time shooting long exposures. I get good results with the 0.25-m Epsilon. If you have a bigger scope, all the better. 
2、你需要有能力拍摄到暗弱的小行星。如果你的设备不足以拍摄到暗至19等的目标,你将不会有太多发现。8寸(20厘米)的镜子可能算比较小的。为了发现那些暗弱的目标,你将需要花费更多的暴光时间。我使用25厘米的高桥E系列望远镜获得了不错的结果,如果你有更大口径的将会更好。

3. Shoot multiple exposures and stack them at a typical asteroid velocity. The stacking will build up signal and reduce random noise. 
3、多次拍摄并将它们依照典型的小行星运动速度叠加,叠加将有效增加信噪比。

4. Reshoot the same field at least three times over the evening to detect any moving objects among the stars. But don’t reshoot the exact same coordinates. Dither (or slightly shift) the field north and south 10″ so that any defects related to the CCD chip are easily recognized. 
4、对于同一天区拍摄至少三次以利于在繁星中发现移动的目标,但请注意最好不要拍摄完全相同的坐标位置,可以向南或北稍微偏差10″以利于比较容易的在图片中辨认出CCD的固有缺陷。

5. Review your fields as soon as possible, certainly within 24 hours, and get the astrometry submitted to the MPC. Other surveys may be shooting the same area of the sky, and if you delay in sending in your results, you may find that someone else has beaten you to the discovery.
5、一定要尽快查看你拍摄的图片,一定要在24小时内做完天文测量和上报MPC的工作。其他的巡天系统将拍摄相同的区域,如果你延误了上报时间,你将发现发现将会被别人获得。

6. Keep on top of your discoveries. If LINEAR photographed your latest discovery last night, there’s no need to shoot it again tonight. Try to reobserve if a discovery hasn’t been observed in a week. Get a third night for any objects still in Vaisala class (an assumed orbit based on only two nights of astrometry). Remember that the MPC will attempt to link a new asteroid to its database of unknown objects, but if the orbit arc is less than 30 days or so, the orbit won’t be accurate enough to do a proper linkage. 
6、持续跟踪拍摄你的发现目标。如果上一晚LINEAR已经拍摄了你的最近的发现区域,那今晚你就没必要再去拍摄那里了。去尝试重新观测一个在一周内未被观测的目标。试图去获得一个以前仅有两晚观测数据的目标(被称为Vaisala class)的第三晚数据。请机住,MPC总是试图在其未知目标数据库中找到与新发现小行星相匹配的数据,而如果新目标没有多于30天的观测跨度是不能准确建立其这种匹配联系的。

Let’s get down to details. 下面是一些细节。

Everyone knows that the moon plays a big part in sky brightness. While it is possible to find asteroids when the moon is up, it’s a real challenge, and by the time the moon is within a few days of full, basically the sky is so illuminated that it’s impossible to find anything new. After a few days past full, the moon is starting to rise after the end of twilight, and you can shoot a few fields in darkness. These fields are, however, well past opposition, so they are unlikely to contain any new objects. If you have discoveries from the previous month, now is the time to shoot them. They may not have been observed for ten days or more, and they may have strayed some distance from their predicted locations. 
大家知道,月光对天光的影响是巨大的,但是在月光下发现小行星是可能的,当然这也是一个挑战,在满月的那几天,天光太亮,发现新的目标是不大可能的。在满月过后的几天,月亮升起的时间开始越来越晚,你可以有些时间在完全黑暗的环境下拍摄一些区域。然而,这些区域里的目标应该在冲日附近,所以一般没有新天体可供发现。如果你在上个月有一些发现,现在是重新观测它们的时候了。距离上次的观测时间,它们应该超过10天或更久,并且应该比预料的轨道稍微有些偏离。

As the days pass, the moon is rising later and later. This is prime discovery time. Areas just ahead of the opposition point contain asteroids that have been brightening for at least ten days in the glare of the moon. The big surveys are waiting to pounce. If you can get there one or two nights earlier and submit your discoveries to the MPC, the big surveys will follow and confirm your discoveries soon after. 
随着时间一天天的过去,月升的时间越来越晚。现在到了主要的发现时间。冲日点之前的区域应该有一些亮度足够的小行星。之前,这些区域由于月光的干扰至少有10天没有被搜索过。大型巡天系统也在等待机会搜索这里,如果你可以抢在它们之前一晚或两晚发现并上报给MPC,那些巡天系统会很快确认并补充你的发现。

Here’s one of the most common questions I’m asked; “Is it better to submit one-night or two-night discoveries?” To which I respond; “It depends.” If you submit a discovery with two nights of astrometry, it has higher priority compared to a discovery with only one night of astrometry. Let’s say you are shooting in an area where you know LINEAR was active the night before. If you something find new on one night and send it in, all you’ve done is proven up a discovery for LINEAR. If you want to keep this one-night discovery, don’t submit it right away. Wait and get a second night. Submit both at the same time and with a bit of luck, you’ll get the discovery credit and LINEAR provides a bonus third night. 
有一个常见的问题经常被问到:“是上报一晚的观测数据好还是报两晚的数据好呢?”我认为要看实际情况。如果你上报了一个两晚的发现目标数据,那它自然比上报一晚的数据要好得多。如果你在LINEAR前一晚拍摄的地方发现了新目标并以一晚的形式上报,那实际是为LINEAR的观测做了证实和补充。如果你想获得这个发现,你应该不急着上报这个一晚的数据,而是等获得了第二晚的数据后立即上报,如果运气好的话,你将获得该目标的发现权,LINEAR将送给你第三晚的数据。

That’s how it should work in theory. In practice, however, things are always more complicated. Perhaps LONEOS sweeps through the area the same night as your first observation and submits the second night for the LINEAR object. Then you’ll lose it fair and square. 
以上的方法理论上是行得通的,但是,实际操作和处理却显得情况很复杂。也可能恰好LONEOS在第二晚正好扫描了你发现目标的区域,并上报了该目标,那么,你将失去发现权。

In the case when you are shooting as soon as the moon is out of the way, it’s a good idea to send in one-nighters. The big surveys will be passing through in days, and if you’ve swept as much of this area as possible, they will provide the second-night confirmation. 
如果你拍摄搜索的区域在月亮之后,比较好的方式是直接上报一晚的数据,因为那些大型巡天系统将在几天内搜索这些区域,如果你尽力扫描了这个区域的足够范围,那些巡天系统将可能为你提供第二晚的数据。

Now let’s say it’s a week or two later. The moon’s been out of the way for a while. The big surveys have made a few passes across the opposition area, and you find that one of them was in your area a few days ago. What to do now? If you find something new, wait and get a second night. Submit the two nights together. You’ll have priority over the isolated single-nighters, and you’ll be able to use them to improve the orbit. 
现在,让我们说说之后的一到两周,月亮的干扰将逐渐减少,那些大的巡天系统将在冲日点附近区域不断扫描搜索,可能你会发现某个巡天系统已经在几天前搜索过了你搜索的区域,那该怎么办呢?如果你发现一个新目标,先不要上报,而是等待并获得第二晚的数据后再一起上报。这样你将可能优先拥有发现权,并可以利用那些巡天系统的数据近一步确定其轨道。

In a way, it’s easy to know when to shoot — no twilight, and not too close to the time of full moon. But where to shoot? If you are going to spend a good part of the night looking for new asteroids, it makes sense to take a few minutes earlier in the day to get prepared. While no one can tell exactly where a discovery is waiting to be found, you can improve your chances if you know where not to look. Let’s start with that approach first. 
什么时间去拍摄搜索?这个问题似乎很容易回答——除了黎明或黄昏的时候,除了满月的时候应该都可以。但是该拍摄搜索哪个区域呢?如果你打算去发现新的小行星,那么在拍摄搜索之前好好的列个计划是很有用的。虽然很难准确的预言哪个区域将会发现新目标,但是如果你知道什么地方不会有新发现将大大改善你的发现几率。我们可以先试试这种搜索的方法。

Unless you are trying to find an asteroid with an unusual orbit, your best bet is to search along a band within 10?of the ecliptic. Sure, there are some objects further from the ecliptic (my 1991 TJ15 was recovered in Jan. 2005 at a declination of +72?), but if you are trying to find something new, you’ll have better success closer to the ecliptic. Another suggestion: ignore the area past opposition. Asteroids in this area were being picked off in the previous lunation by the big surveys. Even if one did manage to escape detection, it’s already starting to fade out, and you’ll never observe it long enough to determine an accurate orbit. 
除非你打算发现一些不寻常轨道的小行星,一般来说,最好的搜索区域是沿着黄道附近10度的范围内搜索。是的,虽然有一些小行星离黄道比较远(我的1991 TJ15在2005年1月重复发现时离黄道有72度),但是如果你确实想发现一些新目标,请最好在黄道附近搜索。另一个建议是:忽略那些已过冲日点的区域。因为在这些区域的小行星已经在上个月被那些大型巡天系统发现过。即使有个别的小行星可能没有被发现,你也会因为它们的亮度逐渐变暗而无法取得更多晚的数据,这将使你无法准确确定它的轨道。

So that leaves the area ahead of opposition as your prime discovery hunting ground. Where exactly you decide to shoot is up to you. But let me offer a few pointers. If I can get an asteroid’s span-of-observations up to 30 days, then I feel that its orbit is in good shape to be recovered at a subsequent opposition, or to be used by the MPC for precovery efforts (when observations at a previous opposition are shown to link to the asteroid). Now it’s a lot easier to find a faint asteroid 20 days after opposition when it’s already discovered and you know where to look, compared to 20 days before opposition when you are looking for unknown objects. So one strategy I follow with a new discovery field is to shoot about 10 days ahead of opposition, and then hope that any discoveries are bright enough to follow for the next 30 days, to about 20 days past opposition. Of course, the specifics will depend on weather and the phase of the moon, but it’s a good starting point. 
所以,应当重点分片区搜索冲日点之前的区域。更准确的搜索区域还和下面叙述的内容有关。我们的目标是为了使你的发现能够有一个很合适的持续30天跨度的观测数据,从而获得足够精确的轨道用以进行重复发现或前期发现(与以前冲日时间的单晚观测数据联系)。考虑到相比之下,通过已有的一个发现并得到初步轨道的情况下,在冲日点之后20天做重复发现的难度要比在冲日点之前20天(小行星运动速度)发现一个未知目标要容易的多,故一个策略是,我重点搜索冲日点之前10天左右的区域,如果运气好的话,在之后的30天,该目标能够保持一个足够的亮度。当 然这些还取决于天气条件以及月相,但这应该是不错的开始。

So suppose you figure that this is a reasonable plan. How do you find out where this area of the sky is? One approach I use is to type in initial coordinates in the MPC’s MPChecker at http://scully.harvard.edu/~cgi/CheckMP . This service will give you a list of known asteroids in the field. Choose one of them and enter it into the Minor Planet Ephemeris Service (MPES) at http://cfa-www.harvard.edu/iau/MPEph/MPEph.html . The ephemeris for this object will show an upcoming maximum in the elongation from the sun, which roughly corresponds to opposition. Compare that date of maximum elongation to today. Move higher in right ascension if you need a later date, lower in right ascension if you need an earlier date. Eventually, you’ll find a point about 10 days ahead of opposition where you stand a good chance of finding something. If you’ve got one of the many planetarium programs for PCs, you can do the same thing if you’ve got some asteroid elements loaded.
因此,你应该知道将拍摄哪个区域。那么该如何知晓呢。我所应用的一个方法是利用MPC提供的MPChecker(http://scully.harvard.edu/~cgi/CheckMP).在这个网站可以为你提供一个你所指定区域的已知小行星的列表。选择其中一个已知小行星将其输入到小行星星历表服务网站(MPES:http://cfa-www.harvard.edu/iau/MPEph/MPEph.html)。在给出的该小行星的星历表中将显示未来一个月中它与太阳的距角的变化数值及最大值,比较这些距角数值中的最大值,如果你需要冲日点10天之内的坐标就应该选择比该值更大的数值,否则选择就选择比这个最大值小的数值。最终你将找到这个冲日点前10天的位置。如果你有相应的电子星图软件可以载入小行星的轨道根数,你将可以用它们来完成同样的工作。

Unless you have a 0.5-m to 1.0-m scope, there’s another fact of asteroid hunting that you need to consider. In all likelihood, the big surveys have been sampling that same area ahead of opposition for their NEO work, and they’ve been reporting any detections of MBAs. If you want to prove up discoveries that they made days or weeks earlier, that’s fine, but otherwise you’re going to have to avoid areas where they’ve been shooting. There are two resources that will help you here:
除非你有一台口径是在0.5米到1.0米之间的望远镜,用它们搜索小行星将是另一回事。有可能那些大型巡天系统在搜索NEO的时候会拍摄如上段所说的同样区域,如果它们发现了任何MBA也将上报。如果你打算验证这些系统在几天或几周前发现的目标,拍摄这些区域自然是可行的,但是,往往我们事实上是想避免它们拍摄过的区域,以下的两个网址可能对我们有用:

1. http://scully.harvard.edu/~cgi/SkyCoverage.html is the link to the MPC’s Sky Coverage Plots. These plots will show you where the big surveys have been recently shooting. The color coding by date is particularly useful, because it will show you the areas that have been shot on consecutive or nearly-consecutive days. These should be avoided, because discoveries from the first date will be linked to those on the second date, leaving nothing for you to find. If a particular area has been shot on only one night, then it might provide you with an extra night of astrometry if you submit two nights of astrometry for a discovery. 
1、http://scully.harvard.edu/~cgi/SkyCoverage.html,这个链接指向MPC的天空覆盖图网站。这个图显示的是那些大型巡天系统在近期搜索的区域。那些不同的颜色反映了非常有用的信息,你将从中了解到那些区域已经被多次拍摄,这些区域没必要再去搜索,因为被第一晚发现的目标已经能够和后来拍摄的第二晚的数据联系起来,这些区域将不会剩下什么新目标。如果有某个区域仅被拍摄的一次,那你将有可能在拍摄该区域中发现目标,并以两晚的方式上报并获得发现权。

I made my first discovery of 2005 — 2005 AC11 — by choosing a field in a gap in the sky coverage plot. Unfortunately, this strategy was a bit too successful. The only contribution from the professionals was one night of astrometry from CSS, and it duplicated one of the eleven nights of astrometry that I acquired. 2005 BA, a discovery I made while tracking 2005 AC11, was even worse. The big surveys completely missed it, and I ended up getting all of the eleven nights of astrometry. 
我发现的第一颗小行星是2005 AC11,搜索的就是在天空覆盖图中的一个缺口。幸运的是,这个策略是非常成功的。该区域仅有CSS一个专业台曾经拍摄过一晚,并提供了我已经获得的11晚数据中的一晚。我在跟踪2005 AC11的时候又发现了2005 BA,那些大型巡天系统完全没有发现这个目标,我提供了11晚数据。

2. Remember that list of asteroids that popped out of MPChecker when you were trying to find the area of sky 10 days ahead of opposition? Let’s go back to that list. The right-hand column, which is labelled “Further observations?”, will contain a range of dates when additional astrometry would be useful. If the range of dates starts with today’s date, this means that the object hasn’t been observed at this opposition, or hasn’t been observed for over a month. So perhaps a survey hasn’t been through the area in some time. But don’t just focus on one asteroid. Look at the whole list. If it’s too short, choose a bigger “Radius of search” in the selection menu. If all the objects show today’s date in the “Further observations?” column, it means that the area hasn’t been scanned yet by the big surveys. Otherwise, take the list of asteroids and put it into MPES. Turn on the “Show residuals blocks”, and check when the asteroids were last observed. If it was less than ten days ago, chances are that the MPC will link your one-night discovery with any one-nighters from the earlier date. You will need at least a two-night discovery to take priority over any earlier one-nighters. Also note that if the asteroids were observed on more than one night over an interval of less than 10 days, then any potential discoveries have probably already been linked. You can confirm this by checking the “Orbit” column in the MPChecker output. Any object showing a V is classed as Vaisala, which means it has been observed already on two nights during the present opposition. 
2、还记得你为了寻找冲日点前10天的位置时使用的MPChecker网站吗?让我们重新自己看看这个网站中的小行星列表。在最右边写有”Further observations?”那一列,该列描述了该目标需要新的观测数据的时间范围。如果该时间范围是从当天开始的,那表示该目标在本次冲日内还没有被观测到或是至少一个月内没有被观测过。所以,我们可以猜测也许那些大的巡天系统还没有扫描该区域。但请注意不要仅注意某一颗小行星的该数据,你应当查验整个列表,如果小行星太少,你可以在进入之前的”Radius of search”处填写更大的范围。如果在”Further observations?”列里的所有目标都显示范围为从当天开始,这说明那些大的巡天系统还没有拍摄这个区域。另外,请将列表中的小行星放入MPES中查询,勾选”Show residuals blocks”,并注意查看该小行星的最后一次观测时间,如果这个时间短于10天前,你将有机会通过一个连续两晚的观测而获得与其他单晚观测数据的联系。请注意如果在此之前有过间隔少于10天的连续观测,那么任何可能的发现已经被联系起来了。你可以通过看MPChecker输出页面上的”Orbit”列来得到确认。凡是标记有“V”表示已经分类为Vaisala,即表示这个目标已经在本次冲日中观测了两个晚上。

This is a real mountain of information, so let’s condense it down to the essentials. Use MPChecker for the status of a field that you might be interested in shooting. If the asteroids in the listing have today’s date at the start of the “Further Observations?” column, then they haven’t been observed in the last month, and there may be discovery candidates in the field. As a final test, see if there are any objects with V in the “Orbit” column. If not, then there are no asteroids observed on two nights in the field. You can see that MPChecker is a good way of determining if a field may have good potential for discoveries. 
让我们总结一下前面的论述,使用MPChecker来确认你是否打算拍摄某个天区。如果在列表中的小行星的 “Further Observations?”列显示的建议观测时间范围都是从当天开始的,则表示这个区域从上个月以来就没有被观测过,这个天区将可以成为观测的候选区域。作为最终判断,看看”Orbit”列是否有V标记的小行星。如果没有,则表示在此区域没有任何小行星的新发现被连续观测两晚。如此看来,用MPChecker来判断某天区是否可能有新发现是个不错的工具。

Once you have determined where to shoot, you need to get good-quality images. The bigger your scope the better, but there is hope even with a smaller instrument. The Takahashi Epsilon E250 0.25-m (10″) f/3.4 Astrograph at the New Mexico Skies Observatory can easily reach to R=19.5. You need to shoot multiple shots of the same area of the sky, then blink them in order to show any potential movers among the stars. Single exposures don’t give me adequate signal, so I shoot five exposures of 90 seconds each, one after the other. Then I stack them together at the velocity of a typical MBA in the field. I get the velocities from the MPChecker listing. Specify speed and direction, not separate motions in Right Ascension and Declination. Then get a second and third stack at least 15 and 30 minutes later. 
一旦你确定了拍摄区域,下一步就是要获得高质量的图片。望远镜当然是越大越好,但是小望远镜依旧可以做得很好。在New Mexico Skies观测站的Takahashi Epsilon E250 0.25-m (10″) f/3.4望远镜就可以很容易拍摄到19.5等的目标。你需要对同一天区拍摄多张照片并将它们按照时间顺序连续播放,从而可以在不动的恒星中发现潜在移动的目标。单张照片并不足以提供足够的信息,所以我对同一天区连续拍摄5张90秒暴光的图片并将它们按照该天区典型MBA的速度叠加起来作为一次拍摄,我可以从MPChecker那里获得典型MBA的移动速度。指定速度和移动方向,而不是赤经和赤纬的运动。然后在至少间隔15到30分钟后拍摄同一天区的第二和第三批图。

There are many great software packages for processing the data and measuring the positions of asteroids. I’ve used Astrometrica myself for many years, and I’ll refer to it in the processing steps. It’s a great program for the beginner. 
有很多软件可以处理并测量这些图片。我使用Astrometrica已经很多年,下面我将就该软件说说我的拍摄及处理方式,这个软件对于入门者来说非常棒。

I like to book two hours of observing time with the Epsilon, and shoot four discovery fields. Each stack below is a sum of five exposures of 90 seconds each: 
我一般喜欢用Epsilon望远镜在两个小时内观测4个天区。每个天区每次拍摄5张暴光为90秒的图片。

 

  • 1. Shoot field#1 stack#1 拍摄天区1并叠加
  • 2. Shoot field#2 stack#1
  • 3. Shoot field#3 stack#1
  • 4. Shoot field#4 stack#1
  • 5. Shoot field#1 stack#2
  • 6. Shoot field#2 stack#2
  • 7. Shoot field#3 stack#2
  • 8. Shoot field#4 stack#2
  • 9. Shoot field#1 stack#3
  • 10. Shoot field#2 stack#3
  • 11. Shoot field#3 stack#3
  • 12. Shoot field#4 stack#3The multiple aiming-and-shooting processes can get complicated, so it’s great that the Epsilon has a scripting option that allows you to prepare your observing procedure ahead of time in a simple text editor. I copy and paste the commands in the script area, start the script, and let the scope follow the commands for the next two hours. Very convenient! 
    以上的拍摄过程是非常复杂的,好在这台Epsilon望远镜可以通过一个简单的程序脚本编辑器来指定拍摄的计划。我可以很方便的指定下两个小时的观测目标。A critical step when acquiring the data is to dither, or slightly offset in position, the center of the field each time you reshoot. For example, I will shoot stack#1 at the nominal position. When I return for stack#2, I offset the field 10″ to the south. When I finish with stack#3, I offset the field 10″ north from the first stack. Any defects on the CCD chip will jump up and down during blinking, so you can immediately discount them as possible asteroid images. 
    每次拍摄同一天区的中心坐标最好不要完全一致,应当稍微有些不同。比如,我拍摄第一天区第一遍的时候使用正常坐标,当我拍摄第二遍的时候,将坐标向南偏移10″,拍摄第三遍的时候在正常坐标的基础上向北偏移10″。这样任何的CCD缺陷在动画中就会上下跳动,你就可以很容易判断出它们不是小行星。 Let’s jump ahead a few hours. You’ve acquired your data and it’s been transferred to your hard drive, ready for processing. Before you start Astrometrica, download the latest version of MPCORB.dat. I’ve never been able to download from the MPC because of their security settings, but http://mpcorb.klet.org/ always works for me. MPCORB.dat is a listing of all the asteroids that you are likely to encounter on your discovery field. More in a moment. Fire up Astrometrica, and take a look at your “File/Settings…”. Did you save ASTORB.dat to the same path that’s listed in for MPCORB under “Environment”? If not, reload the file, change the path, or transfer the file. Nothing like working with a version that you find out later is a year old. We’ll assume that you’ve reduced FITS files before in Astrometrica, and your numbers for the scope and the CCD are good. What about “Time in File Header”? If you have it set to “Middle of Exposure”, you’ve made your first mistake, and joined the ranks of at least one unnamed ICRAR member who submitted astrometry with incorrect times to the MPC (hint: his initials are A.L.). The keyword DATE-OBS (which actually contains the time and date) in the header refers to the start-of-exposure. So you have to select “Start of Exposure”. 
    让我们再说些别的,当你拍摄了图片并将其保存在硬盘上准备处理,在你运行Astrometrica前,请下载最新版本的MPCORB.dat。也许因为准入设置的问题,我从没有能够从MPC的官方网站下载该文件,好在http://mpcorb.klet.org/这个地址有改文件的镜像。MPCORB.dat记录着所以你拍摄区域的已知的小行星。下载好之后,运行Astrometrica,看看你的设置”File/Settings…”。是否将”Environment”中的路径设置正确(尤其是MPCORB的路径)。软件不处理一年前的图片,这里将假设你已经对FITS图片做了必要的处理,望远镜和CCD设置都是正确的。”Time in File Header”是什么意思?如果你设置成了”Middle of Exposure”,你将犯下第一个错误,并被加入到ICRAR成员名单,代表你上报了错误的时间给MPC(提示:他的开头字母是A.L.)。关键词DATE-OBS中记录的时间和日期是开始暴光的时间,所以在这里请选择”Start of Exposure”。Have you ever shot on one side of the meridian, then on the other side, and then can’t figure out why the Astrometrica parameters for one side don’t work for the other? It happened to me on my first night on the Epsilon. I was shooting some asteroid targets in the east, then Comet C/2004 Q2 (Machholz) in the western half of the sky. After scratching my head for a while, I figured out that for the western views you had to invert them by switching on the “Flip Horizontal” and “Flip Vertical” options. The Epsilon is mounted on a German Equatorial mount, and has to be flipped around when it crosses the meridian. So you are going to need two different configuration files. In the eastern half of the sky I use a file named “Normal start.cfg”, using the default orientation and the start-of-exposure. In the western half of the sky I use “Reverse start.cfg”. 
    你出现过这样的情况吗?在子午线的一边拍摄的图片,在另一边就无法用Astrometrica设置的参数处理图片了?我在用Epsilon望远镜第一晚观测的时候就出现过,我在东边天空拍摄了一些小行星,然后在西边天空拍摄了彗星C/2004 Q2 (Machholz)。经过仔细思索,我终于明白在西边的图片你必须颠倒它们——打开”Flip Horizontal”和”Flip Vertical”选项。Epsilon望远镜使用的是德式赤道仪,在通过子午线的时候必须翻个方向。所以你需要两个设置文件,对于东边的目标,我将设置文件命名为”Normal start.cfg”,包含正常的方位及开始暴光时间。在西面,我将设置文件命名为
    “Reverse start.cfg”。

    You might be thinking at this point that there could be problems if some of the files for the discovery field were shot in the east and some were shot in the west. Well, you are quite correct. If you want to blink the files, they have to be processed using the same configuration, so you can’t blink east files and west files. If you have MaxIm, then you can rotate the western files by 180?and then you are all set. If you don’t have MaxIm, then you will have to plan your evening to have all the files shot on one side of the meridian. Not so easy if you have clouds and you have to abandon your carefully-prepared script and shoot when clear areas open up. 
    你将预见到,如果你拍摄的一组图片即包括东边的图片又包括西边的图片该如何办,是的,这样的图片不可能在Astrometrica播放动画,如果你想让他们仍然能作到播放动画,你可以使用软件MaxIm将拍摄西面的图片旋转180度,如果你没有MaxIm这个软件,你只好在拍摄和做计划的时候避免拍摄子午线附近的天区。这对天气条件不太好的情况下并不容易顾及的到。

    If you’ve followed my shooting procedure, you’ll have fifteen individual FITS files; five for each of the three individual stacks for the discovery field. Load your dark and flat frames. Click on “Track & Stack…” to select the first five exposures for stack#1. When you get to the “Coordinates, Tracking and Stacking” box, you need to add a generic asteroid speed and direction (P.A.) in the “Object Motion” section. These are the numbers you should have written down when you were using MPChecker earlier in the evening to figure out where to shoot. Since I never remember to write them down, I’ll assume you forgot too. So go back and redo that step. Then let the program do the stacking. When the stack appears, click within the field to get the mid-time of the five stacks. Write it down. If you have something to submit later, you’ll need this time as a check. 
    如果你想我这样拍摄图片,你将对于同一天区拍摄1组三套15张图片,每5张图片为一套。载入暗场和平场图片后,点击”Track & Stack…”,选择第一套5张图片进行叠加。在”Coordinates, Tracking and Stacking”对话框中,在”Object Motion”那里你需要填写入一个常规的小行星的运动速度和方向。这里填写的数值就是前面在做计划的时候在MPChecker中查找到的小行星的运动速度和方向,因为我每次都忘记把它们记录下来,我想你也是,所以我们需要在叠加图片前再重新做前面的工作。当你叠加图片的时候应该使用拍摄暴光中点的时间,如果你打算上报一些目标,你需要这个时间区检验你的目标。

    At this point, it’s useful to note that there is a bit of divergence if you are shooting MBAs vs. NEOs. In the case of the fast-movers, you’ll be measuring from the display. Our NEO hunters Robert, Jeffrey, and others have shown that Astrometrica can handle the huge trailing of the reference stars and produce some excellent astrometric results. For MBAs, though, I prefer to save the stack display as a new FITS file. I find that I return often to the field to check for new objects. It’s a bother to have to reload the raw data, darks, flats, recheck the proper stacking velocity again, and then restack. I’ll repeat the whole stacking process for stack#2 with exposures 6 to 10, then stack#3 with exposures 11 to 15. After saving the three stacks, you’ll have three files with names that should tell you something about their processing. Here are the three stacks that contained the discovery images of 2005 CD62 and 2005 CE62: 
    这里,你将注意到在拍摄和搜索MBA及NEO时的不同。对于快速移动的天体,你将可以在显示屏幕中测量。我们的NEO搜索者,Robert, Jeffrey,以及其他人都说,Astrometrica可以对那些参考星拖线处理的非常好。对于MBA,我更喜欢将叠加后的图片另存起来,我发现我经常会为了核对目标而掉入以前的图片,如果每次都那样重新载入暗场,明场,要叠加的图片以及查询叠加的速度和方位角是很烦琐的。我将用暴光6-10的图片进行叠加为第二套,将用暴光11-15的图片进行叠加为第三套。在都另存了之后,文件名将包括了所有的处理过程,下面呈现的就是我发现的2005 CD62 和 2005 CE62所包含的图片。

  • SUM – DISCOVERY4 1 2005-02-14 06-39-12 0.65 295.0 NORMAL.fits
  • SUM – DISCOVERY4 2 2005-02-14 07-13-53 0.65 295.0 NORMAL.fits
  • SUM – DISCOVERY4 3 2005-02-14 07-48-27 0.65 295.0 NORMAL.fitsSUM tells me that they are stacked files. DISCOVERY4 was the last of the four discovery fields. 2005-02-14 is the UTC date. 6:39:12, 7:13:53, and 7:48:27 are the three UTC mid-times. 0.65 295.0 is the speed and direction I used for stacking, and NORMAL reminds me that it was shot in the eastern sky. 
    SUM代表已经经过叠加。DISCOVERY4代表第4个天区。2005-02-14代表拍摄日期,6:39:12, 7:13:53, 和7:48:27代表拍摄中点时刻的世界时。0.65 295.0代表叠加速度和方位。NORMAL代表拍摄的是东边天空。Since these new FITS files are referenced to mid-time, you need to use new configurations in Astrometrica where the “Time in File Header” is set to “Middle of Exposure”. And yes, you need a different set for the eastern sky and the western sky. So you will be working with four configurations, as follows: 
    由于这些FITS图使用的是暴光中点时刻,你需要用新的Astrometrica设置文件,该设置文件中的”Time in File Header”设置成”Middle of Exposure”,当然你也同样需要东边和西边天空的不同设置文件,故我在工作的时候使用以下四个设置文件。
  • Normal start.cfg — raw data for stacking, acquired in the eastern sky
  • Reverse start.cfg — raw data for stacking, acquired in the western sky
  • Normal middle.cfg — stack, eastern sky
  • Reverse middle.cfg — stack, western skyIt’s been a lot of work to get to this point, but your perseverence is about to pay off. Load the three stack FITS files, and confirm that the mid-time for each corresponds to the time you wrote down at the end of the stacking processing. 
    做以上的工作很辛苦,但你的执着将换来回报。载入叠加后的FITS图片,仔细确认你叠加时所写下的暴光的中点时刻。More to come … 更多即将更新……

 

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