{"id":1618,"date":"2019-02-11T10:41:18","date_gmt":"2019-02-11T08:41:18","guid":{"rendered":"https:\/\/www.e-astronomer.com\/?page_id=1618"},"modified":"2019-02-12T09:53:43","modified_gmt":"2019-02-12T07:53:43","slug":"hd-planetary-imaging","status":"publish","type":"page","link":"https:\/\/www.e-astronomer.com\/?page_id=1618","title":{"rendered":"HD Planetary imaging"},"content":{"rendered":"\n

Much has changed in the last few years regarding the way in which planetary images are taken.\u00a0 Planetary imaging used to be one of the most challenging aspects of astronomical imaging, but new technology has now made capturing high-resolution pictures easy and inexpensive.<\/p>\n

The Keys to Good Planetary Imaging<\/b><\/p>\n

The most important factor<\/i> in planetary imaging is atmospheric stability, or good seeing conditions.\u00a0 If the atmosphere is turbulent, no amount of equipment can compensate for it.\u00a0 The planets are small and require very high magnifications in order to obtain high resolution images.\u00a0 Unsteady air will smear the image of a planet, even during a brief exposure, blurring any fine details.<\/p>\n

Even on nights of good seeing there will be moments when the conditions are better than average.\u00a0 Ideally, you want to capture an image during the moment of greatest clarity.<\/p>\n

The reason this is normally difficult \u2014 and was almost impossible in the days of film photography \u2014 is that you never know when this moment will come.\u00a0 With film, exposures are longer than with CCDs, so there is more likelihood of the image blurring during the course of the exposure.\u00a0 Another drawback is that to capture the proper moment of good seeing, you have to continue taking picture after picture hoping to grab one during a steady instant of seeing.\u00a0 With film, this was a problem because you only got 24 or 36 shots in a roll of film.\u00a0 Then you were out $6 plus processing (and scanning if you wanted digital pictures).\u00a0 And out of 36 shots you may not have gotten more than 1 or 2 good ones, if that.<\/p>\n

Even with a CCD camera, the situation is little improved.\u00a0 The increased sensitivity of a CCD chip allows shorter exposures, resulting in more good images for a given amount taken, but taking these images still consumes a lot of time and effort.\u00a0 Ideally, hundreds or even thousands of images will be captured, because stacking multiple good images results in a better final image.<\/p>\n

Webcams are ideal for planetary imaging because they allows those hundreds or thousands of pictures to be captured in a matter of seconds at a very high frame rate (usually around 10-20 frames per second).\u00a0 The images are captured as a video file which can then be broken down into individual component frames.\u00a0 The latest software packages, such as Registax, allow the individual frames to be analyzed and sorted for sharpness.\u00a0 The worst images are rejected, while the best files are combined for processing.\u00a0 This is a completely automated procedure.\u00a0 In a matter of minutes, planetary images can be captured with more resolution than anything ever taken with even the best CCD cameras!<\/p>\n

Note:\u00a0 <\/b>More specific imaging techniques for webcams are covered on the<\/i> Webcam <\/b>page, but most of the details below apply equally well to imaging with CCDs or webcams.<\/i><\/p>\n

Imaging the Planets<\/b><\/p>\n

In addition to a telescope and CCD camera or webcam, it is necessary to have some means for magnifying the image on the imaging sensor.\u00a0 This is needed due to the very small apparent sizes of the planets.\u00a0 To capture sufficient detail, the disk of a planet must cover quite a few pixels on the chip.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
\n

Planet<\/b><\/p>\n<\/td>\n

\n

Apparent Size at Best Visibility*<\/b><\/p>\n<\/td>\n<\/tr>\n

\n

Mercury<\/p>\n<\/td>\n

\n

7.5\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Venus<\/p>\n<\/td>\n

\n

25\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Mars<\/p>\n<\/td>\n

\n

25\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Jupiter<\/p>\n<\/td>\n

\n

45\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Saturn<\/p>\n<\/td>\n

\n

21\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Uranus<\/p>\n<\/td>\n

\n

3.7\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Neptune<\/p>\n<\/td>\n

\n

2.3\u2033<\/p>\n<\/td>\n<\/tr>\n

\n

Pluto<\/p>\n<\/td>\n

\n

0.1\u2033<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

* Best visibility is opposition for planets outside Earth\u2019s orbit and greatest elongation for planets inside Earth\u2019s orbit.<\/i><\/p>\n

The most common accessories for amplifying the image are Barlows and eyepiece-projection adapters.\u00a0 If your telescope has a fairly long focal length (2000mm or more) a Barlow will probably be sufficient.\u00a0 The exceptions to this might be if you have a CCD with very large pixels (16-microns or more), or if you are seeking the most detail possible and have very good seeing conditions. For smaller (shorter-focal-length) telescopes, or for large-pixel CCDs, the usual method of magnification is eyepiece projection.\u00a0 By shooting through an eyepiece, more magnification is provided than a Barlow can give.<\/p>\n

Note:<\/b>\u00a0 Most Barlows provide 2x magnification, but there are some high-quality 3x Barlows available as well.\u00a0 Also, products such as TeleVue\u2019s Powermates and Meade\u2019s TeleExtenders provide up to 5x magnification.\u00a0 These are also ideal for planetary imaging.<\/i><\/p>\n

How Much Power?<\/b><\/p>\n

What amount of magnification should be used when imaging the planets?\u00a0 Often, an image scale of about 0.25 arcseconds\/pixel has been recommended.\u00a0 This should reveal the most detail possible under good seeing conditions without being too much magnification.\u00a0 However, in excellent seeing conditions, much higher image scales have been used with much success.\u00a0 A scale of 0.1 arcseconds\/pixel might be a better recommendation for exceptional atmospheric conditions.\u00a0 The required focal length depends on the size of the pixels in your CCD camera.\u00a0 Below is a chart showing the necessary focal length to achieve a 0.25\u2033\/pixel scale and 0.1\u2033\/pixel scale with various common pixel sizes.\u00a0 Formulas are given as well.<\/p>\n

0.25\u2033\/pixel:\u00a0 Focal Length = Pixel Size * 825<\/b><\/p>\n

0.1\u2033\/pixel:\u00a0 Focal Length = Pixel Size * 2060<\/b><\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
\n

Pixel Size<\/b><\/p>\n<\/td>\n

\n

Focal Length (0.25\u2033)<\/b><\/p>\n<\/td>\n

\n

Focal Length (0.1\u2033)<\/b><\/p>\n<\/td>\n<\/tr>\n

\n

3.5 microns<\/p>\n<\/td>\n

\n

2900mm<\/p>\n<\/td>\n

\n

7200mm<\/p>\n<\/td>\n<\/tr>\n

\n

5.6 microns (typical webcam)<\/p>\n<\/td>\n

\n

4600mm<\/p>\n<\/td>\n

\n

11,500mm<\/p>\n<\/td>\n<\/tr>\n

\n

6.8 microns<\/p>\n<\/td>\n

\n

5600mm<\/p>\n<\/td>\n

\n

14,000mm<\/p>\n<\/td>\n<\/tr>\n

\n

7.4 microns<\/p>\n<\/td>\n

\n

6100mm<\/p>\n<\/td>\n

\n

15,200mm<\/p>\n<\/td>\n<\/tr>\n

\n

9 microns<\/p>\n<\/td>\n

\n

7400mm<\/p>\n<\/td>\n

\n

18,500mm<\/p>\n<\/td>\n<\/tr>\n

\n

13 microns<\/p>\n<\/td>\n

\n

10,800mm<\/p>\n<\/td>\n

\n

26,800mm<\/p>\n<\/td>\n<\/tr>\n

\n

16 microns<\/p>\n<\/td>\n

\n

13,200mm<\/p>\n<\/td>\n

\n

33,000mm<\/p>\n<\/td>\n<\/tr>\n

\n

18 microns<\/p>\n<\/td>\n

\n

14,800mm<\/p>\n<\/td>\n

\n

37,100mm<\/p>\n<\/td>\n<\/tr>\n

\n

20 microns<\/p>\n<\/td>\n

\n

16,500mm<\/p>\n<\/td>\n

\n

41,200mm<\/p>\n<\/td>\n<\/tr>\n

\n

24 microns<\/p>\n<\/td>\n

\n

19,800mm<\/p>\n<\/td>\n

\n

49,400mm<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

It should be clear that the best combination for planetary imaging would be a camera will small pixels (such as a webcam) and a telescope with a long inherent focal length and large aperture.\u00a0 The reason for having a long inherent focal length is to make the task of increasing the focal length easier.\u00a0 For example, achieving a focal length of 11,500mm with a scope having an inherent focal length of 2000mm requires a 5.75x magnification factor, greater than most Barlow lenses.\u00a0 However, a scope with a 4000mm focal length will require only a 3x Barlow, a standard accessory.\u00a0 Also, the larger the aperture, the faster the focal ratio.\u00a0 While a fast focal ratio is more important for deep sky imaging, it is still important for planetary shots since the shorter the exposure time, the less problems you will have with atmospheric turbulence.\u00a0 So, an 8\u2033 SCT at 11,500mm focal length has a focal ratio of f\/58, while a 14\u2033 SCT at the same focal length has a focal ratio of f\/32.\u00a0 Equivalent exposures are 3.3x shorter with the 14\u2033 scope.\u00a0 (Of course, there\u2019s no limit to this.\u00a0 A 45\u2033 f\/10 scope has a native focal length of 11,500mm and requires no extra magnification, but good luck toting such a scope out to your favorite observing site.)<\/p>\n

If the seeing is less than perfect, or if there are other factors preventing you from realistically using such a high pixel scale, shorter focal lengths are still quite suitable for planetary imaging.\u00a0 The above numbers are starting points, and greater magnification is not always better.\u00a0 The observing conditions must always be taken into account.\u00a0 The image of Jupiter at the top of this page was capturing using a 10\u2033 f\/9 scope and 2x Barlow, yielding a focal length of 4600mm, or an image scale of 0.25\u2033\/pixel.\u00a0 While the scope being used was certainly capable of greater magnification, the average seeing conditions limited the useful power.\u00a0 The resulting image is still quite nice.<\/p>\n

From the above chart and the one at the top of the page, it can be seen that even at these long focal lengths the planets cover very little of the CCD chip.\u00a0 Venus, Mars, and Saturn cover only about 100 pixels, and even giant Jupiter spans only 180 pixels at this scale.\u00a0 Making Pluto even 2 pixels wide requires a focal length of 14,000mm even when using a tiny pixel size.\u00a0 (It also requires seeing conditions about 10 times better than that at the best mountaintop observatories, plus it is 2 million times fainter than Jupiter, but if you like a challenge\u2026.)<\/p>\n

Note:<\/b>\u00a0 Be sure to also visit the <\/i>Planetary Imaging Equipment page for more info and a JavaScript magnification calculator.<\/i><\/p>\n

Exposure Time<\/b><\/p>\n

As with all imaging, exposure time depends on the CCD camera being used, which (if any) filters are in place, the focal ratio of the telescope plus Barlow or eyepiece, and the planet being imaged.\u00a0 In many cases, the necessary exposure time is actually shorter than the camera is possible of taking.\u00a0 Many CCDs will only expose as briefly as 0.1 second.\u00a0 For bright planets like Venus or Jupiter, or when imaging the Moon, this may be too long.\u00a0 The use of a neutral density filter or polarizing filter is recommended with CCD cameras.\u00a0 Webcams can have their frame rate and gain adjusted to alter exposure time.\u00a0 Usually a filter is not required since webcams are capable of shorter exposure times and are less sensitive than CCDs.<\/p>\n

In general, exposure times will be less than 1 second, even for very long-focal-ratio scopes imaging dimmer targets like Saturn.\u00a0 Experimentation is the best procedure.\u00a0 The great thing about CCDs is that you can take a ton of bad images and just throw them away without any worries!<\/p>\n

As with deep-sky objects, a good signal-to-noise ratio is desirable, especially to get the most out of an image during later processing.\u00a0 This requires using a longer exposure time, but not so long that the planet is overexposed (causing a loss of detail in the highlights) or that the atmospheric conditions blur the image.<\/p>\n

Another factor to consider is the rotation of the planet itself.\u00a0 In a single brief exposure, this will not be a problem.\u00a0 This is most noticeable on Jupiter, which rotates once on its axis every 9 hours and 50 minutes.\u00a0 Jupiter is about 45\u2033 in apparent diameter at opposition.\u00a0 This means that a feature 0.25\u2033 wide moves its own width in just 60 seconds.\u00a0 While each individual exposure might be only a few tenths of a second long, by the time a large number of exposures has been taken, the rotation of the planet can blur some of the fine details.\u00a0 This is even more true with CCDs which often have a noticeable delay for download time.<\/p>\n

The necessary readout time for a CCD came be reduced.\u00a0 CCD camera control programs allow you to select a small portion of the frame for downloading.\u00a0 This is ideal for planetary imaging as even a planet as large as Jupiter will not fill the entire frame.\u00a0 Downloading a smaller portion of the frame results in faster downloads, essential for imaging rapidly rotating planets.\u00a0 For example, Jupiter covers, at most, 180 pixels at a scale of 0.25\u2033\/pixel.\u00a0 Taking a 250\u00d7250 pixel subframe (Jupiter plus room to spare) requires reading out only 62,500 pixels.\u00a0 A CCD camera with an array size of 1600\u00d71200, which requires 4.5 seconds to read out a full-frame image, needs less than 0.2 seconds for such a subframe.\u00a0 By taking a sequence of exposures using a subframe, hundreds of images can be captured in just 1 minute.<\/p>\n\n\n\n\n\n\n
\n

Planet<\/b><\/p>\n<\/td>\n

\n

Time Required to Rotate 0.25\u2033 at Opposition<\/b><\/p>\n<\/td>\n<\/tr>\n

\n

Mars<\/p>\n<\/td>\n

\n

280 seconds<\/p>\n<\/td>\n<\/tr>\n

\n

Jupiter<\/p>\n<\/td>\n

\n

60 seconds<\/p>\n<\/td>\n<\/tr>\n

\n

Saturn<\/p>\n<\/td>\n

\n

150 seconds<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Other Considerations<\/b><\/p>\n

Precise polar alignment, a stable mount, and accurate tracking seem more like considerations for deep-sky imaging, but they are equally important for planetary shots.\u00a0 Since the field of view is so small when imaging tiny solar system targets, any tracking errors or drift from polar misalignment can quickly cause the planet to leave the field of view.\u00a0 This is frustrating during the imaging process and can make combining the images later more difficult than necessary.<\/p>\n

Seeing \u2014 the stability of the atmosphere \u2014 is probably the single most important factor is getting good planetary images.\u00a0 Excellent planetary images are routinely obtained from Florida and coastal Texas, low-lying, humid, hazy regions which deep-sky imagers would avoid like the plague.\u00a0 But these conditions make for excellent seeing and very good planetary imaging.\u00a0 Remember that dark skies does not <\/i>necessarily equal steady seeing.\u00a0 In fact, the two are most often mutually exclusive.\u00a0 For example, the mountainous regions of Colorado have very dark skies, the air is often very unstable, making for lousy planetary imaging conditions.\u00a0 On the other hand, the soggy regions of Texas and Florida that are so ideal for planetary imaging are often hazy and unclear.\u00a0 Any region is eventually blessed with a bout of steady skies, so patience is as critical as anything.<\/p>\n

\u00a0<\/p>\n

Always consult a Jet Stream forecast map !!
<\/strong><\/p>\n

\u00a0<\/p>\n

Jet Stream Forecast<\/a><\/p>\n\n\n\n

\"\"<\/figure><\/div>\n","protected":false},"excerpt":{"rendered":"

Much has changed in the last few years regarding the way in which planetary images are taken.\u00a0 Planetary imaging used to be one of the most challenging aspects of astronomical imaging, but new technology has now made capturing high-resolution pictures easy and inexpensive. The Keys to Good Planetary Imaging The most important factor in planetary … <\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"jetpack_post_was_ever_published":false,"footnotes":""},"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/PaIt6s-q6","_links":{"self":[{"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/pages\/1618"}],"collection":[{"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1618"}],"version-history":[{"count":5,"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/pages\/1618\/revisions"}],"predecessor-version":[{"id":1918,"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=\/wp\/v2\/pages\/1618\/revisions\/1918"}],"wp:attachment":[{"href":"https:\/\/www.e-astronomer.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1618"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}