Optics Question

[carlogambino]

Hi I have a question to ask all the optic expert here... This question is from another school's astro test paper. Here goes:

QX) An f/10 telescope with focal length 3m is used with an eyepiece of 2 mm focal length. What is the magnification of the system? Would the magnification allow all the collected light enter the eye? Use lower limit of magnification, m>=D/8, D is aperture in mm.

Now obviously this question is far from realistic, we all know that the derived magnification of 1500X is ridiculously large for real life etc... (some areas of the q are unrealistic too....e.g ridiculous eye relief) but that's besides the point.

Though the theoritical magnification is easily derived, I'm totally baffled by the second point: would the magnifcation allow ALL the collected light to enter the eye? Calculating the exit pupil of this imaginary "setup" derives 0.2mm. This means that no light is "lost" to the surroudings supposedly. But then again the smallest size of the eye pupil fully contracted is 0.5mm. Does this have any effect on the light collected?

I had thought that the answer would be no (I'm not too sure either), but my friend pointed out that the question was asking if ALL the photons collected by the objective was transferred to the eye. And it was supposedly so. But I'm wondering, with all the unusable magnification and useless theoritical image formed, are ALL the photons actually transferred to the eye? (ignoring optical defects). Hopefully someone can answer, thanx!

[weixing]

Hi,
Hmm... very similar to the question I ask in the thread: Discussion on Wide View Eyepieces
( http://www.singastro.org/viewtopic.php?t=3066 )

Anyway, I'll give my view base on my understanding, but it may not correct.... I'm not an optics expert

Use lower limit of magnification, m>=D/8, D is aperture in mm.

Err.. does this have anything related to your question??

would the magnifcation allow ALL the collected light to enter the eye?

Yes... all the light from the field of view will reach your eye if there is no loss in the optical system.

Calculating the exit pupil of this imaginary "setup" derives 0.2mm. This means that no light is "lost" to the surroudings supposedly. But then again the smallest size of the eye pupil fully contracted is 0.5mm. Does this have any effect on the light collected?

Should not be affected... 0.5mm is still greater than 0.2mm, so all the light in the field of view is still entering the exit pupil.

Anyway, I think your question should be "Is there a minimum limit in the exit pupil??" I think there is no minimum limit in the pupil exit pupil, but the image quality may not be there... so there is no point having such small exit pupil if you can only see a patch of light, right???

By the way, the below is found in TeleVue website:
"Experienced planetary observers use 20x to 30x per inch of aperture to see the most planetary detail. Double star observers go higher, up to 50x per inch (which corresponds to a l/2-mm exit pupil). Beyond this, telescope power and eye limitations degrade the view. "

Have a nice day.