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CCD's Versus Traditional Photography - by Ricky Leon Murphy:

Introduction
The Equipment
Olympus OM-1
Phillips ToUcam
CCD Camera
Putting it all together
Summary
References
Image Credits

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Introduction:

Astrophotography is a fun and rewarding hobby. While some consider the merits of this hobby to be scientific in value, it is no secret and no surprise that many photographs take on an artistic appearance. Even more so, artistic skill is almost a requisite for producing some of the more stunning images. As with any artist and their creation, there is no right or wrong piece of equipment – only a right tool for the job. In astrophotography, there is no one magic tool that does it all. Instead, a variety of telescopes and cameras provide the ability to capture a large variety of objects – lunarscapes, planets, nebula and galaxies.

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The Equipment:

I am lucky enough to have what I consider to be a perfect set of tools for astrophotography[1]. The crux of the photographic system lies on my equatorial mounted wedge of my Meade 10” Schmidt-Cassegrain telescope with a Takahashi Sky 90 wide field telescope mounted “piggy-back” on the Meade (meaning it is mounted directly on top of the main telescope). I am also lucky enough to have three different cameras:

  • Olympus OM-1 SLR Camera Body
  • Phillips ToUcam Pro – webcam
  • Santa Barbara Instrument Groups ST-9XE CCD camera

This variety of cameras allows me to photograph anything I want in the night sky – as long as the object is within reach of my optical system.

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Olympus OM-1:

A 35mm SLR camera may seem like an old piece of equipment when comparing to today’s CCD camera capabilities, however there are still some things that are the domain of SLR cameras. While any SLR camera body with a “B” setting will work, the OM-1 is special because it does not use battery power when the shutter is open.

Figure 1

Figure 2

The “B” setting on an SLR camera is called “Bulb.” What this does is allow the shutter of the SLR camera to remain open during an exposure – a requirement when photographing extremely faint objects. There are four major benefits for using an SLR camera for astrophotography.

  1. Star trail photography – a stationary SLR camera is left pointing at an area of sky with the shutter open for at least 15 minutes.
  2. Wide angle photography – while CCD’s have reached the size of a 35mm frame, they are extremely expensive. And SLR is much cheaper and can take some wonderful wide field images.
  3. Color photography – color film can be used without any loss in overall resolution, and the color image is captured in one frame.
  4. The versatility of the SLR – the body of the camera can be easily fixed to any telescope and can also accept a wide variety of SLR lenses, like wide angle “fish-eye” lenses or telephoto lenses. They can also operate without a telescope.

With this camera, my target choices are pretty broad: star trails, wide-angle photography, meteor shower photography, and single shot color imagery through the telescope. One of the best features of this camera is its portability. I do not need to carry computers and power supplies if I want to capture some images. Additionally, if I do want to photograph though the telescope, I can take a color image without having to take three to four different exposures through filters as well as dozens of flat field and dark frames (for image calibration of the CCD camera).

Here are a few samples of what a 35mm SLR camera can do:

Figure 3: Star Trail Image


[1] I do not own any such equipment. For ease of writing the essay, I must pretend. However, I am aware of the merits of the equipment “used” in this essay.


 

Figure 4: A two in one image – the first image is guided “piggy-backed” on an equatorial mounted telescope for several minutes while the second exposure is of the foreground.
 

 

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Phillips ToUcam Pro:

It is difficult to imagine that a webcam can be useful in astrophotography. In fact, specially modified webcams have shown to be very capable with deep-sky photography (Wiley, 2003). Webcams can be modified to include a variety of cooling system and software designed to override the shutter; however, since we already have an SLR camera and a CCD camera there is no need for modifying our webcam.

Figure 5:

The Phillips ToUcam (no longer available) has proven to be the choice for planetary imaging. With its small CCD sensor at 640K by 480K, the size seems “just right” for photographing planets through our Meade 10” telescope. Special adapters can be purchased through places like ScopeTronix to facilitate connection to a telescope.

What makes the webcam special for planetary imaging is its ability to capture up to 30 frames a second. While not a regular snapshot type of image, the individual frames can be “stacked” by special programs like RegiStax or Registar. This image stacking[1] reduces overall noise, and since planets are relatively bright compared to deep sky objects, a short exposure is possible - in addition CCD’s have the ability to be more sensitive than film (Covington, 1999).

The image below shows just how capable a webcam can be for capturing images of the planets.

Figure 6:            

This image of Jupiter was captured by Glen Schaeffer using a ToUcam webcam attached to a 20” Obsession Dobson telescope. Very nice indeed.

While the webcam can excel at planetary imaging, the small CCD size limits any possibility of wide angle photography. Additionally, the shutter cannot be left open without modifications so any other astrophotography is not possible – unless time and effort is made to modify the camera. See Astronomy Magazine, December 2003 issue, page 94 for modification tips.

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The CCD Camera:

Volumes can be written about CCD’s, but the main emphasis here is to determine the place of a CCD camera in an existing astrophotography setup. In reality, a carefully researched product (and enough money) can probably by a CCD that can perform almost all aspects of astrophotography – including wide field photography. However in our case, I happen to have (see footnote number 1) a CCD camera that augments my existing setup.

Figure 7:

 

Santa Barbara Instrument Group’s ST-9XE camera uses a 512K by 512K array CCD of 24 micron pixels. While this may seem like a low resolution for this camera, it is ideal for the long focal length of the Meade 10” telescope. This particular camera does not offer a wide field, but actually falls somewhere in the middle between the 35mm camera and the webcam.

More information on CCD and telescope choice can be found in The New CCD Astronomy by Ron Wodaski. The main concern with focal length and pixel size is something called image scale. The resolution of a star is so small that using a small pixel camera on a long focal length telescope results in over sampling (Wodaski, 2002) – a single star can fall on a group of smaller pixels, but offer no benefit to overall quality. On the other hand, this camera will not work well with my Takahashi Sky 90 because the pixels are too large – several stars can fall on one pixel resulting in an under-sampled image.

Images that have been captured by the ST-9XE camera on a long focal telescope are actually pretty impressive:

Figure 8: M33

Figure 9: NGC 891

My target choices for this CCD camera and my 10” telescope will be:

  • The variety of deep space objects – galaxies, star clusters, planetary nebula
  • Planetary and lunar images
  • High resolution mosaics[2] of larger galaxies and nebula

In addition to all of the deep space objects, I can even do some science with the ST-9XE, including:

  • Searching for supernova
  • Searching for transits of exoplanets

While CCD’s seem magical to use, there is a dark side. To capture a single image, image calibration must also be performed. What image calibration does is negate the flaws within a CCD and calibrate the dark levels. For example, if a CCD has damaged pixels or does not evenly record light levels evenly, image calibration can eliminate these effects. The process of image calibration is as follows:

  • Capture the image of interest
  • Capture a dark frame (a single image with the shutter closed)
  • Capture a bias frame (a single image with the shutter closed for the same duration as the target image)
  • Capture a flat field image (an image with a t-shirt over the telescope)
  • Use image software like CCDSoft or MaxImDL to combine these images to a final product

The interested reader is encouraged to read Ron Wodaski’s The New CCD Astronomy for a more detailed look into the image processing procedures.

These image calibration steps must be performed for each filter if a color image is desired. To capture a color CCD image – since the CCD is monochromatic – the object must be captured through red, green and blue filters then combined with software (Covington, 1999).

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Putting it all together:

To sum up the capabilities and target choices for my three cameras, I have created a nice summary:

 

35mm SLR:

Phillips ToUcam:

SBIG ST-9XE

Accessories:

Can be adapted to any telescope and accept a wide variety of lenses

Cannot accept additional lenses and must be specially adapted to a telescope

Can fit on any telescope and additional components are required for additional lenses

Portability:

No power supplies or telescopes needed. Can operate alone

Must be attached to a computer and a telescope

Can operate away from a telescope with a lenses, but a computer and power supply must follow

Color Photography:

Yes – with color film

Yes – by design

Yes – with special filters and exposures for each filters. A computer is required for processing

Ease of use:

Just aim, focus and expose

Just aim the telescope, focus, and expose, process with computer

Requires numerous calibration images along with the object of interest as well as computer skills

Target Options:

Anything

Planets

Deep sky objects, narrow field photography, and science images

Best choice for:

Star trails, wide angle photography, color photography

Planets

Deep space objects, supernova, science images

Use on Meade 10”:

Yes

Yes

Yes

Use on Takahashi Sky 90:

Yes

No – objects too small and chip too small

No – chip too small and pixel size too large

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Summary:

Astrophotography covers a wide variety of subjects – from wide angle views of constellations, close up views of planets, and images of distant galaxies. With the highlighted setup of cameras and telescopes, I have demonstrated that there is no one perfect instrument. A 35mm SLR film camera has its place for wide angle and color photography and beautiful star trail images. A webcam has been shown to be more than competent in imaging the planets while the CCD camera fills in the blanks. The added benefit of scientific imaging is also possible with the CCD camera. With a balanced toolbox of imaging equipment, the hobby of astrophotography will be complete and rewarding.

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References:

Covington, Michael. Astrophotography for the Amateur. Second Edition. Cambridge University Press, 1999.

Wiley, Keith. “Imaging with Webcams.” Astronomy Magazine, December 2003, pages 94 to 97.

Wodaski, Ron. The New CCD Astronomy. New Astronomy Press. Duvall, WA. 2002.

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Image Credits:

Figure 1: http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/html/m3.htm

Figure 2: http://science.howstuffworks.com/camera7.htm

Figure 3: http://www.mreclipse.com/Astrophoto/SSSP97.html

Figure 4: http://www.mreclipse.com/Astrophoto/sssp97/97SS82.jpg

Figure 5: http://www.neatherd.org/astronomy/Toucam.jpg

Figure 6: http://home.houston.rr.com/gschaeffer/astropix/astropix.html

Figure 7 and 8: http://www.sbig.com

Figure 9: http://users.easystreet.com/daves/ccdimages.html


[1] Imaging stacking is using computer software to lay one image on top of each other.

[2] A mosaic is a combination of smaller images that comprise an overall larger subject

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