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主题:【文摘】继续天文摄影的话题(一) -- 闲看蚂蚁上树

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家园 【文摘】继续天文摄影的话题(一)

天文摄影的内外在要求

  对入门的天文同好而言,天文摄影不是一件容易的事,分为两部份:

 一、天文摄影的内在要求:

  1.身体健康:天文摄影都在夜间进行(太阳除外),而且要长时间熬夜,身体不好可会受不了的。

  2.满腔热血:天文摄影是一种长时间的等待,成功比率又不高,所以没有满腔热血可提不起劲来。

 二、天文摄影的外在要求:

  1.懂得地理:天文摄影是很挑环境的,所以要能够知道怎样的环境才是适合天文摄影的。

  2.会看风水:除了环境之外,天文摄影也很挑天气。要学会判断大气的稳定度及透明度。

  3.钱包鼓鼓:天文摄影的器材都不便宜,『囊中羞涩』的话就玩不起来了。

 

天文摄影的种类与器材

  对业余天文同好而言,天文摄影可以分成:固定摄影、追踪摄影及放大摄影三种。现将这三种摄影法所需的器材表列如下:

  固定摄影:单反相机,可交换的镜头,三脚架,快门线.

  追踪摄影:赤道仪,望远镜或相机镜头,单反相机,快门线,导星装置.

  放大摄影:赤道仪,望远镜,放大摄影装置,放大摄影目镜,相机接环,单反相机,快门线.

  基本上,这些器材并不限厂牌或等级,但还是以高级型比较好。另外如前所述,天文摄影都是在夜间进行,所以有些个人装备是很必需的:

  个人装备:羽毛衣,羽毛裤,防寒头套、手套、鞋袜,头灯.

  望远镜装备:电源(电池),纪录纸,怀炉或除雾电热线,定时闹钟.

  最后需要的是一部车。天文摄影的设备都是精密机器,都需要小心呵护,而且赤道仪不但贵,而且又重,如果是利用公共运输系统来搬运,实在非常不方便,久了也怕会对这些「贵重」仪器产生不良后果。

(注:固定摄影就不要车)


本帖一共被 2 帖 引用 (帖内工具实现)
家园 【文摘】二, 固定摄影

拍摄前的准备工作

  拍摄前首先要做的是把相机架在三角架上,镜头对着天空并按下快门。这是最简单的拍摄方法,对第一次天文拍摄人我们强烈建议采用。

(数码)单反

  对于天文摄影,ISO感光度最好设置在400。当曝光时间长于30秒的时候,把快门设定为B门。

广角镜和标镜

  固定拍摄的时候,35mm广角镜头(用在EOS 300D数码单反上等效焦距变为56mm)是十分实用的。配上全开光圈下有出色成像的镜头就更加理想了。

遮光罩

  为了避免眩光和减轻晚间的露水,最好为镜头配上遮光罩。

快门线

  长时间曝光容易造成的机身震动,建议使用快门线控制快门开合。

三脚架

  长时间曝光时,郊外空旷风较大,容易造成三脚架的轻微移动,想拍出理想的作品就比较困难了。虽然是重了点,但还是建议使用一支坚固的三脚架。

手电筒

  因为是在昏暗的环境下拍摄,手电筒也是必须装备之一。最好准备两支,一支强光,用于前期铺设器材;一支弱光,用于设置相机和记录数据之用。

便携取暖器

  为了避免夜间露水打湿镜头,拍摄之前最好到户外用品店购买一种便携口袋取暖器,拍摄时捆绑在镜头上。

点看全图

星座图

  知道拍摄时天上会出现什么星座是十分有用的,在去之前最好计划以下拍摄安排。

固定摄影的方法

  固定摄影简单(架上三角架就可以),但这么简单并不保证拍出来的会张张精采。首先我们要知道东西南北四个方向的星星轨迹是不一样的。向北边拍,会拍到以北极星为中心的同心圆轨迹;东边则是拍出从画面左下到右上的直线轨迹;南边也是同心圆,但因为看不到南极星,所以轨迹是半圆形的;西边则刚好跟东边相反,是从画面左上到右下的直线轨迹。知道了星星轨迹的方向,就可以事先想好背景星空的样子,然后找出自己要的景物和方向,拍出一张自己满意的作品。

  第二个要知道的是曝光时间及光圈对星迹的关系。地球每24小时自转一圈,所以曝光时间愈久,地球转得愈多,相对星迹就愈长了。另外,"相同的曝光时间"对不同焦距的镜头也有不一样的星迹长度:长焦距的星迹就较长;短焦距的星迹就较短。

  光圈值跟星迹长短没有关系,只跟星迹粗细有关。光圈愈小(光圈的数字愈大),星迹就愈细,反之则愈粗。但是光圈太小,很多暗星拍不出来;光圈太大,星迹太粗又不好看,所以一般是把光圈开到F5.6(400°底片),然后再根据实际情况做调整。

家园 【文摘】三, 直焦点摄影

  天文摄影中,最吸引人、最迷人、最累人也最花钱的是"直接焦点摄影"。所谓直接焦点摄影(以下称直焦点摄影),就是把相机直接接上望远镜(注:可以用一个piggy back),在望远镜及相机之间不加任何光学对象,直接使用望远镜来拍摄的一种方法。虽是不加任何光学对象,不过,如果是使用降焦镜或延焦镜,仍可算是直焦点摄影。所以其实用相机镜头来拍也是一种直焦点摄影,只是我们直接用望远镜来当镜头罢了。

  另外,所谓的直焦点摄影,是指追踪摄影而言,"没有人会把相机装上望远镜,然后拿来拍固定摄影"(希望您不要做傻事),所以以下所称的直焦点摄影都是使用赤道仪的追踪摄影。

直焦点摄影的装备

  通常望远镜的焦长都不短,焦比也不亮,而且天体都是暗淡的,所以都需要长时间曝光才能把天体拍摄下来。可是地球会自转、天体会移动,如果长时间曝光的话,那目标天体不就跑掉了吗?为了克服这个问题,直焦点摄影需要很多辅助设备,我们来看看:

  相机部份:单反相机(最好是B快门不用电力的机械式相机),快门线.

  望远镜部份:任何一型望远镜(以品质高、口径大、焦比小者为佳).

  赤道仪(坚固稳定为佳).

  其余相关配件:导星望远镜、目镜导星监视器、星表星图、纪录用具、怀炉、手电筒.

 

  直焦点摄影是很挑仪器的,尤其是望远镜和赤道仪(注:从后文可以看出 polar alignment 是非常重要的)。这是因为直焦点摄影的对象都是又小又暗的,所以曝光常常需要数十分钟,甚至一、二个小时。在这么长的曝光时间中,赤道仪优劣与否对照片品质有极大的影响,也对摄影者的体力有直接影响(因为您可以不用导星导得那么累)。虽说赤道仪很重要,但直接影响成像品质的,还是望远镜的品质。一支高级的望远镜拍出来的星像照片,绝对会比一般的望远镜来的好,所以望远镜的钱也是不能省的。

  一般来说,折射镜拍摄的效果会比反射镜好,尤其对散光星云特别明显。但折射镜的价钱比反射镜高很多,所以到底要用何种望远镜,实在是一个见仁见智的问题。笔者建议选择望远镜的原则是:折射式-口径10公分以上,焦比不要超过8;反射式-口径20公分以上,焦比不要超过6,品牌上挑比较老牌、有口碑的厂商。如果经费上不允许做大投资,那么折射式10公分、反射式15公分是下限,再小的话,分解能、集光力都不够了。

  直焦点摄影除了望远镜、赤道仪要花钱外,另外还有一些要花钱的,就是导星的外围装备。导星装备包含一个导星云台、一支导星望远镜、一个导星监视器和二颗目镜(高、低倍各一个)。这些东西价钱可高可低,但绝对是另一笔不小的开销。

 

直焦点摄影的对象

  因为望远镜的焦距都不短,所以直焦点摄影只能拍到天空上一个很小的范围,不能拍大面积的星座。在这个小范围内可能有星云、星团或星系。也就是说,直焦点摄影的主要对象是星云、星团和星系。至于行星,因为它太小了,所以不能用直焦点摄影,而要用扩大摄影法,太阳系内能用直焦点拍摄的只有太阳、月球和彗星。

 

直焦点摄影的方法

  直焦点摄影的第一个步骤是架好您的望远镜。这个动作包含:把望远镜、相机、导星装备等架好、锁紧在赤道仪上,并做好赤经、赤纬轴的平衡。平衡这个动作极为重要,它是决定您今晚拍照成功与否的极大关键。再来是校正极轴。极轴就等于是地球的自转轴,如果能非常准确的对准正北极点,那要拍坏了还真是难。

  第三个步骤是导星。直焦点摄影最麻烦的部份就是导星。当我们到达观测地,把赤道仪和望远镜架好、相机接好、对好极轴后,并不是就可以按下快门了。在前面说过:由于地球会自转,所以必须要有赤道仪。但赤道仪不是地球,在马达的运转和极轴的校正上,不可能完全等于地球的自转,所以会产生误差。这个误差会在照片上显现出来,所以我们要用另一支望远镜-导星镜来观看并修正这个误差(因为单眼相机按下快门后,窗口内看不见影像,所以不能用拍摄中的望远镜来做这件事)。也就是说,当我们架好仪器后,就得用导星镜在拍摄天体附近找一颗够亮的星,放入导星监视器的视野内,然后用高倍目镜,从曝光开始到曝光结束,盯着这颗星看并修正您所看到的偏移(也就是赤道仪的误差)。每一张都得如此,焦点愈长愈要仔细、小心。您想想,在寒冷的夜里,您必须盯着导星监视器一连看上好几个小时,那儿也不能去、什么事都不能做、还得跟瞌睡虫抗战!这实在不是人该做的事。

 

直焦点摄影的问题

  会让您一个晚上的辛苦白费的几个问题是:赤道仪的平衡、极轴的校准、目标找不到(或找错)、震动、灯光及选错底片等。

  平衡及极轴的影响前面已经叙述,所以就不再说了。找不到要拍的天体,这是新手常常会有的问题。因为大部份的天体都很暗,如果望远镜口径太小,在相机窗口内根本看不到要拍的天体,所以得利用坐标来帮我们确定位置。每一台赤道仪上都有坐标环,先在望远镜视野内找到一颗已知坐标的恒星,然后把坐标环对好该星的坐标(这个动作叫归零),再来就可以用这个已经正确的坐标环和星表上的坐标来找出要拍的天体了。当然我们也可以利用寻星镜,看寻星镜内的星空,再比对星图,也是可以正确地找到要拍的天体。不过,建议寻星镜最好口径有5公分以上,以免力有未殆。

  震动和灯光是小细节,但仍会影响拍摄结果,不能不小心。选错底片是一个不会让您全军覆没、但会让您再拍一次的问题。天体很暗,我们得选用高感度的底片,但太高感度的底片粒子很粗,解像力不好,所以要考虑望远镜的焦长、大气的稳定度和拍摄的天体这三个变量,来选择底片。通常用的底片是四百度~八百度的底片。

点看全图

直接投影法

点看全图

加倍镜投影法


本帖一共被 1 帖 引用 (帖内工具实现)
家园 【文摘】四,放大摄影

最后一种天文摄影-那就是放大摄影。

  放大摄影跟直焦点摄影一样,都是属于追踪摄影,只是曝光时间比较短罢了。但是放大摄影受制于外在影响很大,是所有天文摄影中最不好拍、最不容易拍到好作品的一种摄影法。

 

放大摄影的装备

  放大摄影的装备与直焦点摄影的装备重复性蛮大的。不过放大摄影不用导星,可以省下一笔导星装备的费用,另外再增购放大摄影用的接筒即可。

  相机部份:单反相机(最好是有内藏测光表的电子相机),快门线.

  望远镜部份:任何一型望远镜(以大口径、长焦距为佳),高精度赤道仪.

  其余相关配件:放大摄影筒,放大投影用目镜,记录用具.

  各位可能心里已在怀疑笔者有没有打错字了,怎么在前几期讲到的天文摄影中,都推荐使用机械式单反相机,在放大摄影时就变成"电子相机"了呢?

  没错!笔者就是推荐使用电子相机!理由是:一、放大摄影的曝光时间都不长,最多不过十数秒而已,所以长时间曝光会耗电的问题可以说是不存在的。二、行星的亮度一般都还算够亮(月亮就更不用提了),所以放大影像后,在某些高级的电子相机上还能测光,这对判断曝光时间的帮助很大,可以减少试误法所浪费的底片。

  望远镜当然是愈大愈好,这是真理,而且焦距长的比较好,这是因为焦距长比较容易提高放大倍数。另外,放大摄影需要利用目镜来把天体的影像放大,而放大摄影用的目镜与一般目视的目镜不同,用放大投影专用的目镜拍出来的品质会比较好,但相对的,目镜的价钱也比较高。

 

放大摄影的对象

  放大摄影主要是拍摄行星及太阳、月球表面上的局部,例如太阳黑子或是月面上的某个火山坑。这是因为行星的视直径很小,如果只用望远镜的焦距来拍,相片上的行星会小到只是一个点,无法看出行星的表面,所以必须要用目镜来把行星的影像放大,才能看出行星表面的模样或变化。不过也曾经有人用低倍目镜放大来拍行星状星云,会这样做主要是鉴于望远镜焦距不足,拍出来的星云太小。这种拍法相当困难,没有相当经验是很难拍好的。

 

放大摄影的方法

  放大摄影的先前准备动作与直焦点摄影是一样的,都是先架好仪器(可以不架导星装备),做好平衡,对正极轴,然后找到目标,取景测光完毕后就可以按下快门了。唯一跟直焦点摄影不同的是相机的装法,相机并不是直接装上望远镜的。在望远镜与相机之间必需要有一个能衔接这二者,并且能装上目镜的一个接筒,笔者称这个东西叫放大摄影接筒。在望远镜之后先接上放大摄影接筒,再接上相机。接筒内先放入低倍目镜,找到要拍的天体后,再换上高倍目镜,然后取景对焦测光按快门。很简单,对不对?

 

放大摄影的问题

  如果这么简单就好了。在地球上,我们需要空气才能生存,但就是这个大气层,严重干扰了天文观测和摄影。我们先知道二件事:一、空气的扰动会使得星点摇晃,造成画面的不清晰。二、焦距短的看的范围广,焦距长的看的范围窄。放大摄影之所以要用目镜来放大,就是因为望远镜的焦距不够长,必须利用目镜的放大功能来等效于焦距的延长。那焦距一长,看的范围就很小了,所以只要有一点点的空气扰动,在望远镜内就会很明显。因此,要拍到一张好的放大摄影作品,除了实力外,还得会察老天爷的脸色。

  放大摄影的第二个问题是赤道仪的追踪精度。如上面提的,行星太小,所以要用目镜来放大(也就是延长焦距)。当我们放大的倍数极大时,延长的焦距大约都有数万mm,这么长的焦距,如果赤道仪的极轴对的不准或赤道仪追踪精准度不够,不要说曝光只有几秒,就算短到一秒内都会影响到拍摄天体的清晰度(也就是解像力)。再加上适合拍放大摄影的望远镜口径都不小,所以一台"够份量"的赤道仪是极必需的。

  第三个问题比较怪,是震动的问题-不是地面或望远镜的震动而是相机反光镜的震动。单反反光式相机机内都有一面反光镜,在按下快门的那一瞬间反光镜会弹起,这个动作会造成相机的震动(或说是望远镜的震动),愈暗的天体对这个动作愈不敏感,但对行星或月面的影响就不小了,所以如果相机的反光镜能弹上并锁住然后再曝光,这种相机才比较适合放大摄影用。

  第四个问题是对焦。当我们用高倍目镜把行星放大后,行星会变得很暗,非常难对焦。除了使用比较亮的对焦屏之外,就只能多拍来累积经验了。

  以上的问题现在有一个解决的方法(当然不是百分之百解决),这要拜科技进步之赐,那就是用高感度底片。现在世界各大胶卷制造商所生产的高感度胶卷,粒子已不比以前的一百度胶卷的粒子粗了,所以我们可以用四百度甚至八百度的胶卷来拍行星或月面,仍然可以得到很好的效果,并且可以利用高感度来缩短曝光时间,克服大气扰动及追踪误差的问题,可以说是一举数得。

 

-------------------------------------------------------

如图:目镜法

目镜法也是将望远镜的射出光线调成平行,相机的镜头调在无限远即可,目镜与相机距离随意,当然越近越好,比较好安装。

此法的合成焦距多少?假设原有物镜焦距F1,目镜焦距为F2,相机的镜头焦距为F3,合成焦距为F4。

则:F4=(F1/F2)×F3

也就是等于前端原来望远镜的倍数乘上相机焦距。

点看全图

点看全图

暗角产生的原因

家园 【文摘】五,进阶天文摄影

一.天文摄影的特性:

  天文摄影的对象是暗淡的天体,需要消除的是地球的自转,所以必须使用特别的器材,做极长时间的曝光。 二.天文摄影失败的原因:

 (一)极轴不准:蒙气差

  虽然使用极轴望远镜来对正极轴的精度相当高,对于短焦距的望远镜追踪摄影及观测绰绰有余。但对长焦距的摄影,例如:1000mm的焦距,要求的对极轴精确度就要很高了!这就不是光靠极轴望远镜对极轴就够的。当赤道仪的极轴望远镜使用于低纬度的地区时,大气折射所产生的影响使得要精确地对正极轴几乎是不可能的。大气折射对星星确实位置的影响如下:

星星高度 蒙气差(大气折射量)   星星高度  蒙气差(大气折射量)

 10°    0°5' 17"         20°    0°2' 38"

 30°    0°1' 40"         40°    0°1' 09"

  因大气折射,实际星星的位置会比观测的位置低,在台湾北极星的高度约25度,所受蒙气差的影响约2分角,也就是说就算你把北极星对到同心圆漂亮地绕着望远镜极轴中心转的程度,实际上仍有约2分角的误差。那么以极轴望远镜中心偏上2分角位置为正确赤道仪中心来对准,应该可以校正这2分的误差。可是赤道仪出厂时,极望与机械轴偏心仍有误差量,也就是说改善是有限度的。

  精确的对好极轴是不需要极轴望远镜的,但是有极轴望远镜可以先把极轴对得差不多,更方便我们用这一方法。这法子其实是很古老的,基本而有效,可以精确到令人满意的程度。

  △精确对极轴的方法

  固定式观测站,天文台的望远镜或是要求长焦距的天文摄影时,需要极精确地对准极轴,上述的方法是不够的。下面是一个精确对极轴的方法(drift method):

  1.先以上述方法对好极轴。

  2.drift-漂移法。

  a.使用巴洛镜及有视野照明的十字线目镜,尽量提高望远镜倍率,倍率越高,极轴可以对得越准确。

  b.将一颗在天顶子午线附近离天球赤道5°以内的亮星导入望远镜视野内,天球赤道的赤纬值是0°。

  c.将星星置入十字线交点中心,切换赤纬马达至「高速」运转模式,驱动赤纬马达使星星移动,调整目镜使星星沿着十字线中的一条重合运动,此方向即为赤纬方向;另一条线与星星移动方向垂直,即为赤经方向。

  d.监视亮星在赤纬方向上的漂移,调整「水平方位微调钮」使亮星回到赤纬线上,直到亮星一直保持在赤纬线上,没有赤纬方向上的漂移运动为止。请忽略任何赤经方向上的漂移量。

  e.重复同样的程序,将一颗在东方高度20°以上附近离天球赤道5°以内的亮星导入望远镜视野内,重复步骤c,只监视亮星在赤纬方向上的漂移,调整「倾斜角微调钮」使亮星回到赤纬线上,直到亮星一直保持在赤纬线上,没有赤纬方向上的漂移运动为止,请忽略任何赤经方向上的漂移量。这样一来,在任何观测及摄影的场合下,赤纬几乎不会有任何的漂移;可以完全地忽略赤纬的修正,所有的可能误差来自于赤经轴蜗杆蜗轮的周期性运动及大气折射的效应,只需要考虑赤经方向上的修正。这也适用于没有赤纬马达电动修正的赤道仪欲从事长焦距天文摄影的时候使用。

 (二)导星不精确:

  在准确对正极轴后,仍然会有摄影失败的情形发生,这时大部份的原因出现在导星精度不够的问题上。以vixen生产的GA-4导星监视器来说,如果被导星保持在最内圈的范围内移动,并且要求星点在底片上的移动范围大小在20μ以内,这种条件下,主镜焦距上限是导星镜焦距的0.8倍。可是大部份的导星镜焦距都比主镜短,所以必须要提高导星的精度,才能弥补导星镜焦距之不足。适当的做法是:让被导星保持在最内圈的 1/2或1/4内移动,也可以把被导星放在垂直线相交处,利用缩小范围的方式来提高导星的精度,精度最高可让主镜焦距是导星镜的1.5倍。

 (三)整体强度不足:

  当极轴对得正,导星导得准,在经过一个小时的曝光后,星点仍然莫名其妙的拖迹,这是会让人捉狂的。仔细检查底片,这种拖迹方向通当不是赤经或赤纬方向,这种追踪失败是因为主镜、导星镜或云台板的强度不足。强度不足最常出现的地方是:主镜对焦座、导星镜对焦座、导星镜目镜座、接环及云台板上。经过长时间的曝光后,主镜、导星镜等都会有极轻量的变形,而且曝光愈久愈会有这个问题,这些变形量总合后,对长焦距摄影是一个不能忽视的问题。

 (四)选错底片:(注:数码相机我感觉可能要用专门为天文摄影设计的,如eos 20a)

  恒星、星团、星系及反射型散光星云(如M45)的光属于连续光谱,而发射型星云--红色星云及行星状星云来的光,则主要集中在Hα(6563A)及〔N II 〕(6548A、6584A)、其次有Hβ(4861A)、〔O III 〕(4959A、5007A)及〔O II 〕(3726A、3727A)等。Hα及〔N II 〕的是红色光,这也就是这些星云呈红色的原因。对这些色光来说,大部份的黑白底片都不能感光到6500A的红色光,只有TP底片例外。这也就是说,如果拿T-MAX底片来拍北美洲星云,曝光再久都不能拍出充足的影像。对彩色底片来说,也有这个问题。各家厂商的彩色底片在低照度倒数率失效下,特性都不太一样,所以会有同样拍北美洲星云,同样仪器及曝光条件下,甲牌底片及乙牌底片会有不同的表现。自己必须了解自己常用底片的特性。

 (五)曝光不足:

  天文摄影是在拍摄极暗淡的光源,对这么暗的光来说,底片的倒数率失效是很严重的。如果我们根据某作品的曝光时间,单纯地转成自己光学系统的曝光时间,这通常是不行的。例如:F2.8的光学系统曝光20分钟,则F5.6的光学系统曝光就不能是20分X4=80分,用F5.6/80分钟是拍不出与F2.8/20分钟同样的影像浓度的。这是因为底片的倒数率失效让底片的感度大大地下降了。为了影像品质而选择低感度的底片,但却要付出极长时间曝光的代价,通常是不会成功的。

  解决的方法有:氢气增感、冷冻相机、负片重迭及增感显影等。

  氢气增感:将底片乳剂中的O2、H2O等气体抽离,以抑制倒数率失效,再利用氢气让底片产生轻微化学感光,可以有效提升底片感度3~10之多。

  冷冻相机:利用干冰将底片温度降到零下70度以下,以抑制倒数率失效,可以有3~4倍的增感效果。但在实用上,要冷却底片不难,要防止底片结霜才是大困难。

  负片重迭:将二张同内容、曝光较短的负片重迭洗相,会得到比单一张但二倍曝光时间的相片更强的影像。这是利用二张负片重迭后,影像反差大增的现像来弥补曝光时间的缩短。

  增感显影:这是指拍摄完毕后,在底片显影时延长显影时间以提高底片影像浓度的方法。适用于TP、T-MAX等黑白底片及彩色正片,负片则效果有限。

家园 【文摘】最后附一篇英文deep sky 摄影的个人操作经验

(Deep Sky) Astrophotography for Amateurs

by Olivier THIZY

I have seen several questions on astrophotography for amateurs, and I was thinking that I could share my experience here. Of course, it will not replace books that one can find on the subject. I strongly recommend "Astrophotography for the Amateur" (Michael Covington) as the book to own if you want to start in this field.

Everything described here is from my own experience, and covers only the basics of astrophotography. I am still far away from Fletcher and Hallas, but I enjoy what I do. I tried to explain my own evolution, from simple picture of the sky to prime focus one. It is not the only way, but it helps to practice the basics before going to long focal deepsky astrophotography.

1) GET STARTED: PHOTO ON TRIPOD

Before even starting with photography with a telescope, I recommend to practice photography using a simple tripod. There are many things to do with a simple tripod:

Moon and Planets at dawn or dusk: choose a nice foreground landscape (trees, mountains...). Evening just after the New Moon are very nice for that. Full Moon set above cities are very nice too. I usualy use a 100-400ASA slide film.

Constellations: Using a fast film (I like the Kodak Ektar 1000) and short exposures (30"-45" for a 50mm lens), one can record a lot of details on constellations. I got some nice results on the Milky way, especialy around Sagittarius. Those pictures are also a perfect test for the photo shop: is the sky coming black from their lab?

Star trails: Long exposure will record star movement over the sky. Combined with a nice foreground, you can get some pretty nice results. I like to use a flash light or a Flash to show some trees or my telescope on the pictures. And if get lucky, you may record a shooting star! I usualy use a 400ASA film for those pictures, except when I want to do a very long rotation around the north pole for which I use a 100ASA film.

Other: There are several other pictures to take with a tripod: bright comet, aurora, eclipses (lunar or solar)...

To do astrophotography on a tripod, you need a very minimal material:

A camera: If possible, use a one that doesn't run off battery for long exposure. Some exposures like star trails need very long exposures, and a camera that runs off battery will die during the night, usualy in the middle of an exposure! There were several discussions on what is the best camera for astrophotography. If you want to invest at this point on a camera, I strongly recommend an Olympus OM1. It is easily available on the second market for $150-$200 with a simple lens, and it makes nice outdoor pictures too.

A lens: At this point, a short focal is recommended (35mm-50mm). More open is the lens, better it is (f-ratio around 1.5).

A cable-release: It allows to keep the camera open during the exposure. One can find it in any photo shop. Choose one that has an automatic lock system, it is easier than the one with a screw.

A tripod: choose a light one, that you can take with you during your observing trip, hiking...

I am still doing those kind of pictures. I really enjoy the search of the perfect foreground, the perfect composition. And nothing can be more enjoyable than taking the crescent moon in the morning after a long observing (or photographing!) night...

2) TRACKING?

Tracking is essential to do long exposure on the sky and having round stars. With tracking, you can use slower film with less grain, or use longer focal to show more details on the sky.

A very simple way to track is to use three pieces of wood and a screw! It is both cheap and fun to build, and you can have very nice results.

If you have a simple telescope, you can fix your camera on top of it (piggy backed). I built a camera stand for my old 4.5" using a piece of wood and some rope (I also replaced those flexible cable with electronic buttons that created much less vibration)! You can buy a piggy back, which is very expensive for what it is but it works fine. You can also buy a rotative head for fixing your camera from a local photo shop or from Orion for example; it will help orienting the camera to get the field of view you want.

A lot of people with an equatorial telescope are wondering how to start in deep sky photography. I really think that the best is to do piggy back. You will need an equatorial mount for long exposures, as Alt-Az mount will have field rotation.

With short focal lengths (50-150mm), you won't need a very good polar alignment. If you want to do long exposure (1hr for example) or use a long focal lens (200mm-500mm), you will certainly need a good polar alignement.

I personaly use the drift method to align my telescope (a SCT 8"). It takes approximatly 30' for a good alignement for a 500mm focal lens. First, level your tripod and orientate it to Polaris (not necessary, it just helps). If you have a Polar alignement circle, it can also help to use them.

Look at a star near the Equator, South. Track the star in RA only, and look if the star goes up or down in your eyepiece (supposing your are looking straight). Rotate the base of your telescope to adjust it until the star is not moving anymore.

Then, move to a star at East (West works also). Do the same, but adjust the latitude this time (the angle between your telescope axe and the horizon plan). By switching several times from South to East (West), you should be able to adjust your polar alignement quite quickly. Of course, the first time you will spend a lot of time; take notes of what you are doing, and it will be much quicker after.

To practice polar alignement is very important if you want to move to prime focus deepsky photography. That's why I always recommend to start with piggy back first: it is much easier to do!

Piggy Back photography offers a wide range of new projects:

Constellation: you will get much more details than exposures on tripod, and you can use finer grain films. And why not your own photographic atlas? Great as finding chart later!

Milky way: why not a mosaic of the milky way? In two hours, I did the milky way in August using a 50mm lens and Ektar 1000 film, 3 minutes each pictures. Great results!

Comet or asteroids: bright one are easily accessible with a 50mm-150mm focale lens. 10-20 minutes with Ektar 1000 and a 135mm (F/2.4) gives nice results.

Movement of planets over the sky: show how a planet retrograds over a constellation.

Shooting stars: Add a friend at several miles from you, and you can calculate the position of the meteors; Add a rotational screen front of your lens and you can calculate the speed!

3) PRIME FOCUS

Prime focus is certainly a goal for a lot of people: be able to record faint objects, show spiral arms in galaxies, record the central star of a planetary nebula... But there are certainly a lot of traps not to fall in!

First of all, material is very important for deep sky astrophotography:

Telescope: Large aperture is a plus, especialy if you have a fix observatory. If you need a portable solution like me, SCT have great advantages. I personaly choosed Celestron Ultima 8, which is very good for astrophotography, at least a good price/performance ratio. If you have a location where you like to observe, it is nice to build a fixed tripod. Then, you will just have to come and install your telescope, the polar alignement will be almost already done. Of course, your telescope should have a drive corrector to track on RA and allow correction. A DEC motor is also useful, even if one can still take pictures with the Ultima 8 without DEC motor; it just makes life easier.

PEC: Periodic Error Correction is a great invention. You spend some time (usualy ten minutes) to setup the PEC, but guiding is much easier after.

Reducer/Corrector: A short f-ratio is nice for deepsky astrophotography, it allows shorter exposures. For SCT, there are reducer that also correct the field of view. It allows also to have a full image on the film. I heard that one can stack reducer/corrector but I never tried. Even if it is optional, I strongly recommend this accessory. Celestron has a very nice one for SCTs.

Off-Axis Guider: Guiding is critical for nice round star on long exposures. Lumicon has an excellent one, I use the Orion Ultra Guider which is nice too.

Finding a guiding star is not always easy. Usualy, I use a low power eyepiece to find the star, then switch to a higher reticuled eyepiece. A 9mm is nice for a SCT 8" f/10 or f/6.3; a 12.5mm eyepiece is not enough power. If I can't find a guiding star, I just change target!

Front Cover: I usualy use a plain carton. I cover the telescope with it, then open the camera and wait for the vibration beeing stabilized, then I remove the cover. Most of the time, the guiding star is still centered. Very useful... And while you have the carton, save a piece to make a dew protector!

Deep Sky filter: If you live in light polluted area, a Deep Sky filter can help you to limit the sky fog effect. It is an optional accessories; but after several pictures taken, it will become necessary.

We already talked about polar alignement. Of course, alignement for prime focus is much more important than for piggy back; I usualy spend two hours to do it, and I can do up to 60' exposures without field rotation. During summer Star Parties, I like to walk around while the night is coming talking with other people on what they are planning. Two hours can pass very quickly...

Focusing is also very important for prime focus astrophotography. Do not expect to do proper focus just by looking through the camera, it is not precise. I always use a razor edge to cut a star beam (well, also my finger sometimes, so becareful).

Principle of this 'foucault' method is easy. Point the telescope to a star, and do an approximative focusing through the camera. Then, open the camera body (without a film!), and open the shutter in Bexposure. You should see the star pattern, as a donut shape through a SCT telescope. Take a razor blade, and tape it on the camera, exactly where the film should be.

By moving the telescope (I usualy tape the razor blade in order to be able to cut the beam just by moving in DEC), the razor edge will cut the star beam. If the razor is out of focus, the star pattern will disapear slowly from one side or the other depending if your are inside or outside of focus. At the focus point, the star pattern disapears suddendly, and it is hard to tell from which side.

This is an easy method, and it takes around 5'-10' to do it. It is very precise, and pictures will gain in sharpness.

Choosing a film for deepsky astrophotography is not easy. There are hundreds of articles on this subject, and it is changing all the time as new film are coming out.

Basicaly, Hyper film are the best; I used a few times TP2415 Hyper and Ektar 1000 Hyper, and I got the best results. Lumicon sell Hyper films, it is a good alternative if you don't have your own Hyper system.

Otherwise, Ektar 1000 is a nice film for bright objects. I heard about Kodachrome 1600P, but never had a chance to try it. TMAX 3200 is a very good Black & White film to practice the technic, including the photo lab technic. I would use those film to try and improve the technic, and then buy Hyper film for better results.

Now, the most difficult may be to find the object and to center it! Long hours practicing visual observation will certainly help. I am a Star Hopping fan, I enjoy following star patterns in my finderscope... and having the object right in the middle! Several computer program can help to create perfect charts for the search; I use Guide CD-ROM or Megastar. The SKY, a little bit expensive, looks nice too. Of course, digital circles can also help.

I spent several hours to calibrate my telescope. I draw pictures to show where the camera field of view was in the finder depending on the camera orientation. And I calibrated my finder with Guide charts too. Now, I am able to imagine the object position inside the finderscope (especialy when there is a group of galaxies and you want all of them in the field).

Also, I know where is the guiding eyepiece field. Sometimes, I use a low power eyepiece and try to locate the object within the guiding field (through the prism in the off-axis guider). It works pretty well for 12-13 magnitude galaxies. then, I just move the telescope to center the object, and try to find a guiding star.

There are hundreds of projects to do with prime focus astrophotography, beyond the simple art of photographying what you see. Let go your imagination!

4) STORING

Astrophotography overall is fun, and it is not so hard to do. Buy a good book, try easy shots first, then move to piggy back and prime focus. You will get nice results very quickly and you will have to store them.

First, always note what you are taking (subject, film, lens, fratio, camera, exposure, date/time). I used to take notes on paper, but I always lost them! Now, I have one record-book for all my visual observations as well as my astrophotos; it works great.

Then, protect your films; I use binders with film protector sheets, sorted by Date/Time.

I started to move to Photo CD (Kodak) format; it is way too expensive, but it helps to keep all my best pictures, and to process them using a computer. The only problem: you can't record them after processing, except if you have GB of disc!

Last but not least, it is always nice to have an album with your best pictures. I use to sort them my Date/Time, it is easier and less maintenance. But a sort by subject would be better.

And if you take nice pictures, scan them and post them on the net!

Olivier THIZY

家园 没说的,献花。第六朵给[闲看蚂蚁上树]

天文说不上爱好,只能说从小就好奇着。

工作后也曾有过元旦时,在京西山上看星星的日子。

原谅我,话只能说到此为止。(理由见拙作《你泻我觅》

有些小故事日后或许能在龙行天下的话题中露出一些的(这坑可是透明的说)

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