methods eye (visible window) camera – film camera – ccd (digital)

Methods

Eye (visible window)

Camera – film

Camera – CCD (Digital)

Collecting Electromagnetic Information

Eye (visible window)

Advantages:

Everyone wants to see objects directly

Disadvantages:

Light gathering capability – can see without telescope down to magnitude 6

Image cannot be stored

Cannot do a time exposure


Camera – film

Advantages:

* Inexpensive

* Can do a time exposure

Disadvantage:

* Resolution

* Cannot be conveniently manipulated

* Deterioration

* Long Exposure Times


Camera – CCD (Digital)

Advantages:

Resolution

Can do a time exposure

Can store directly to a computer

Can be manipulated

Short Exposure Times

Disadvantages

Expense

Pixilated Image


Collecting Electromagnetic Information – Digital Processing

Raw Image MaskProcessed Image of a comet

Raw Image Processed Image

Comparison – Film vs CCD

The Advantages of CCD Imaging

Professional astronomers have been using CCDs for nearly two decades and have no thoughts about going back to film. The advantages are impressive:

* CCD cameras are up to 100 times more sensitive than film * CCD images are digital, meaning no film-processing, scanning, scratching, or fading * CCD cameras have a much greater dynamic range than film, meaning they can more easily capture both faint and bright detail in a single exposure * CCDs are capable of resolving finer detail than film (this can, however, be limited by the telescope used and the atmospheric conditions)

For the amateur, several other advantages apply as well:

* Taking images with a CCD is generally much easier than with film * Learning goes faster due to the ease-of-use and shorter exposure times * Light pollution is less obtrusive and more easily removed from CCD images, meaning imaging from a backyard site is more likely to be possible with CCD


Efficiency and Exposure

Efficiency

* A CCD can record as much as 90% of the incident photons

* Film will record less than 5% of the incident photons

CCD will record object 10 to 20 times fainter than film with the same exposure time

Exposure Times

* CCD can record the same image with less than one tenth of the exposure time for film

Above: On the left is a CCD image taken from a suburban location. The exposure time is about 30 minutes. On the right is a film image of the same object taken using a similar telescope. The exposure time is over 2 hours and the level of detail is far less than the CCD image. At full resolution the difference is even more impressive.


Exposure

Above: A comparison of the size of a 35mm film negative with a relatively large amateur CCD chip (Kodak's KAF-3202 used in the SBIG ST-10 camera).


Format

The Advantages of Film

The single biggest advantage of film (at present) is format. A 35mm film frame is 35mm x 26mm in size. Amateur CCD chips are typically less than 15mm across. This means that, on a given telescope, more of the sky can be captured at once with film. This is an advantage since many deep-sky objects are very large in apparent size. However, CCD chips as big as (or even bigger than) 35mm film are becoming available to advanced amateurs. It is only a matter of time before these are available at a price within reach of the average amateur.

Figure 1. Replica (via a high quality microscope) of a test pattern recorded on 35mm Fuji Provia film using prime lens optics at f11. The highest resolved grid is indicated by the arrow (at column 1, row 3). It has 75 line pairs per mm, or about 2700 line pairs across the 35mm film. The contrast is about 10% (vs. 100% for pure black to pure white). In the row 0 column 2 pattern the contrast is about 50% or 5 times greater.

Figure 3. Magnification of the central part of the Cool Pix 5000 5.2 megapixel digital camera resolution image. Item col 0, row 3 (i.e. 0/3) is resolved (IMHO, in the same sense as the film doing 1/3). However, note the LACK OF GRAIN NOISE in the digital picture (compare with figure 1). Clean grain-free digital images are an advantage that helps make prints than would normally be associated with film at a given resolution.


Resolution

A CCD camera with the format and resolution of a piece of 35mm film costs much more than a film astrophotography setup. However, most people are willing to sacrifice format for the speed and convenience of a CCD camera. Unless large prints are going to be made, CCDs can otherwise outperform film in almost every aspect. These days most people simply view images on a computer monitor, email images to friends, or post pictures on websites; for these purposes, small images are sufficient.

For the amateur whose goal is to take pretty pictures of the night sky, a high-quality, entry-level CCD offers many advantages at a cost very comparable to film imaging. A quick look at the websites of the most prominent astrophotographers in the world will show that they have invested as much in film as most CCD users have invested in their setups. It will also show that most astrophotographers have begun shooting CCD images as well!


Price

Photometer – An instrument which measure the brightness of an object

Will measure the TOTAL brightness of an object, which might be difficult to interpret. However, when combined with filters, can be used to measure the amount of light produced over a narrow range of frequencies. This can be compared with standard Blackbody radiation curves to determine the temperature of the object

Photometry



Photometry

X

Intensity

Wavelength



Photometry

X

Intensity

Wavelength

Temperature of object is 7000 K

Spectroscopes can be used to analyze the image through the eyepiece. The spectra can then be viewed with the eye, a camera, or a CCE for analysis.

Spectroscopy Revisited

methods eye (visible window) camera – film camera – ccd (digital)

Documents

film image

film frame

ccd images

ccd cameras

amateur ccd chips

single exposure

shorter exposure times

exposure timeexposure