Astronomy/telescopes

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Expert: - 6/7/2011


Question
I have a 12inch telescope with a limited resolution of 1 arcsecond. How far can a planet be seen before it begins to appear like a star? The planet in particular I want to see has a diameter of 10^8m.

Answer
Hi Victor,
Sorry, but this is a homework math problem... how do I know?  No one gives a planetary diameter in meters, but rather either in Km (100,000 in this case) or 62,150 miles. And since there is no Solar System planet with that diameter, it's not only a homework problem, it's also a bad,  hypothetical homework problem at that.
And sorry, I'm not real good at that kind of math.  However, I feel that there is a piece of information missing, so the problem is not solvable as stated (truth be known), you also need... AT WHAT POWER eyepiece being used???
Saturn, at close to that value (not really, Saturn is about 75,000 miles in diameter) looks rather stellar in a 1 inch glass at a low 5 power, but can be a very small disk of about 20 arc-seconds at its upper limit of 50 power.  And the same would be true of a 12 inch scope if Saturn happened to be out by double Pluto's orbit (say some 7 billion miles distant)... it would appear very stellar-like at a low 20 power, but a small disk of about 10 arc-seconds at a higher, say 500 power. (All scopes, the maximum effective power is 50 X per inch of aperture, assuming a very high quality figure that you've paid big bucks for.)
So your math problem has to state a power being used by the given telescope to determine if and when a planet being viewed "appears stellar-like" in nature.
In practicable terms, a lot depends on the condition of the atmosphere.  With a 30 inch scope, theoretically I can split a double star down at far less than 1 arc-second, but in actuality, the stability of the atmosphere determines my resolution limits on any given night. (And the same is true for a 12 inch glass also). Typically, an atmospheric 1 arc-second resolution is a pretty good steady night sky out there.
Sorry, but that's the best I can do for you.
Clear Skies,
Tom Whiting
Erie, PA  

Astronomy

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Tom Whiting

Expertise

Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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