Thanks for the correction Brent, I was trying to go by memory. Sig ----- Original Message ----- From: "Brent Watson" <brentjwatson@yahoo.com> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Sent: Wednesday, February 09, 2005 7:34 PM Subject: Re: [Utah-astronomy] Question on focus
Siegfried's numbers are essentially correct. The lensmaker's formula tells you the variation in focal distances. It is:
1/f=1/Ff+1/fb
Fb is the rear focal distance (from the mirror to the focal plane). Ff is the front focal distance (from the mirro to the object). f is the infinity focal distance, or what most of us call the focal length. I get a difference of .000000701 inches.
One portion of your need to refocus comes from temperature variations in you optical tube. The gravitational effects of the sun and your thumb are positional errors, not focus errors.
The reason things look sharper when you hold your thumb next to them (ala Guy) is that you are reducing the effective aperture of your eye, and thereby increasing the depth of field.
I also believe the atmospheric effects are positional effects, not focus effects.
One of the main reasons for a focuser on a telescope is to accomodate different eyepieces and imagers, or other instruments. If you only used one eyepiece, and you optics didn't move because of temperature or collimation, you could weld that eyepiece in place and not see a difference.
Brent
--- ziggy943@xmission.com wrote:
Some of this has already been answered. Another answer is our need to check and recheck the focus due to the atmopheric effects.
I don't believe there is a detectible difference in the focus of a telescope between the moon and Jupiter. Both are effectively at optical infinity.
Where F = Infinity focal length, D = distance to object, f = new focal length
f = F/D*F+F
As I recall the formula for Focal length is Focal Length at infinity divided by the distance to the object times the oricginal focal length plus the FL.
The formula may not be exactly right but it's something like that. I can check the formula later.
The change in focal length is a function of both the FL and distance to the object.
Using that formula, for a 100" FL the difference is on the order of 6/10,000,000th of an inch between the Moon and Jupiter. Both essentially at infinity. Jupiter is 4/10,000,000,000 off infinity. Or something like that. At infinity the F/D*F goes to zero and you are left with the focal length.
Between that and 'depth of field," there shouldn't be any difference.
Siegfried
Quoting Joe Bauman <bau@desnews.com>:
Hi all, When I checked out the Sky & Telescope web site below (from Patrick's News), it had a great animation of the moon occulting Jupiter, on the third page. What I can't understand is why Jupiter is in sharp focus while the moonscape is slightly fuzzy. It isn't possible that there could be a real difference in focus between the two, I believe -- granted, Jupiter is thousands of times farther out, but for all practical purposes the focus for both should be at infinity. Or is it because the telescope was tracking on Jupiter, making it relatively stable, but the moon had some lateral motion for each exposure, causing blurring? Just curious. Also, if anyone has a minute, can you tell me why it's necessary to focus a telescope at all? You would think that, like an old Instamatic camera, everything beyond a few miles out would be at the "infinity" focal point. I know that's not true, but I do not understand why.
Thanks, Joe
News, Wednesday, 09 FEB 2005
Celestial Highlights for 2005
Eclipses, conjunctions, and occultations -- as well as plenty of "regular" activities like meteor showers -- will keep observers worldwide busy during the upcoming year.
http://SkyandTelescope.com/observing/highlights/article_1456_1.asp
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