Rich, That's it. You can make the image larger, but you only have a limited budget of light. You need to balance that budget against film/CCD resolution. Film graininess will place a lower limit on the image size as will CCD element spacing. The balance is struck by choosing a resolution at the image plane that is within the diffraction limits of your scope and fit the image sensor, while giving you an acceptable exposure time. The sensor can be either film or solid state. The final balance usually requires pushing the sensor to the limit AND using darkroom techniques. Even after all this, many other things such as guiding error, processing error, dust, etc., etc., etc., make it so you only get a good shot on rare occasions. I have done a liimited amount of astrophotography and ended up wasting many yards of film. I have two or three good shots. John Dobson spoke well when he asked why an amateur would want to do this. The good photos have already been taken by the pros. The astrophotos I cherish the most are those locked up in my own memory, and refreshed regularly by observation and drawing. Once more, each person is different, but I believe most people share my experience. Maybe because they are mine! Brent --- Richard Tenney <retenney@yahoo.com> wrote:
Brent,
Let's see if I got this right now: Taking photos at the focal plane of a given object will be smaller and brighter in faster scopes, larger and dimmer in slower scopes for the same exposure times. Got it.
Therefore, photos taken at the focal plane of very small deep sky objects (e.g., a very small planetary nebula) still have to somehow be magnified to properly see the details, achieved either using some sort of lens elements beyond the focal plane (barlow, eyepiece projection or whatever, I don't know how this works yet exactly) to create that larger image on the film itself, or later in the darkroom by enlarging the very small negative on the film to make the final print. Am I getting close? Assuming I've got it right, what works better, making the negative image larger in the first place, or the print image large from a small negative image later on?
-Rich
--- Brent Watson <brentjwatson@yahoo.com> wrote:
Rich,
Let me take a crack at this. I think the key is to remember that in a linear optical system, angles are preserved. This means that if an object subtends one degree in the sky, it will subtend one degree in a telescope.
Here's an example. The moon subtends about 1/2 degree in the sky. For an 8" f7 scope (focal length 56 inches) a 1/2 degree image will be .49 inches tall. The image height for an 8" f4.5 (36 inch focal length) will be .31 inches.
An 8" scope gathers 8 inches worth of light. You can spend the light any way you want to. FOR EXTENDED OBJECTS, all eight inches worth of that light is in the image. If, however, you form a smaller image because of a shorter focal length, that image will still contain all the light, but being smaller, it will be brighter.
This is not true for stars. They are point sources, and thorough good optical systems of any f ratio their size is determined by aperture. A star in an 8" f7 is the same size as a star in an 8" f4.5. It is determined by diffraction limits.
What's the result of this? Extended objects AT THE FOCAL PLANE will be brighter with faster scopes. Stars will be the same brightness independent of f ratio. An 8" scope will show the same magnitude star no matter what the f ratio is.
If you run 50 power, the image is the same brightness in the f7 scope as it is in the f4.5 scope. You have magnified the image fifty times OVER WHAT THE EYE SEES in both cases, and you have done it with an 8" scope. The brightness will be the same in both. It is only at the focal plane where the brightness difference exists between the two focal ratios. Remember that power per inch of aperture determines visual brightness.
This is th basic principle of what is happening. Other things will come into play (like magnification and non-linearity of the eye) that modify this, but the basic rule remains the same.
Still clear as mud?
Brent
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