Re: Re: [Utah-astronomy] OAS excursion to St. George
Joe, The OIII filter is one nebula filter. The other one I have is a UHC nebula filter. You definitely want the OIII for planetary nebulae. You saw how it picked up the faint nebulosity in Abell 21. If I would of had more time I could have showed you the faint Jones1 in Pegasus before it moved west into the light pollution. Debbie
From: Joe Bauman <bau@desnews.com> Date: 2005/02/08 Tue PM 12:39:48 MST To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] OAS excursion to St. George
Hi Deb, it certainly was fascinating. You mentioned an OIII filter -- is that the same as a nebula filter? I have a nebula filter and probably should have used it! Best wishes, Joe
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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 +++++
On Feb 9, 2005, at 9:38 AM, Joe Bauman wrote:
[...] 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
This question I can at least partially answer. I wear some pretty strong glasses (4.5 diopters). Sometimes I view through the telescope with glasses removed. The focus adjustment allows me to make this work (well, the astigmatism is still frustrating...). Jim
Thanks, Jim, that's one reason! But while you have the view focused without wearing your glasses, why isn't that setting good enough for everything in the heavens above? Why do you have to refocus when you look at something else, or when you take a look another day? The only reason I can come up with, and it doesn't seem like a complete answer, is that the atmosphere might act like a lens and distort the view depending on the angle of the telescope to the curvature of the atmosphere. Is that the reason? Does Hubble need to focus when it points to a different target? -- Joe
This question I can at least partially answer. I wear some pretty strong glasses (4.5 diopters). Sometimes I view through the telescope with glasses removed. The focus adjustment allows me to make this work (well, the astigmatism is still frustrating...).
Jim
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Hi Joe, Based on the way I understand it, you are correct. The Earth's atmosphere and the curvature of same, is the main reason the focus changes over long periods of viewing. The focus of an object changes as it climbs higher into the night sky where the atmospere is preceived to be thinner. I know when I was at Bryce this past fall, I noticed I didn't have to play around with the focus near as much as I do here in the valley. Temperature plays a part in focus as well. As the OTA heats and cools, it, like everything else, will shift creating focus problems. I'm not sure about the Hubble, but I believe that once outside of the influence of Earths atmospere, where the temperatures are stable, focus problems are less of an issue. But I may be wrong... Guy Quoting Joe Bauman <bau@desnews.com>:
Thanks, Jim, that's one reason! But while you have the view focused without wearing your glasses, why isn't that setting good enough for everything in the heavens above? Why do you have to refocus when you look at something else, or when you take a look another day? The only reason I can come up with, and it doesn't seem like a complete answer, is that the atmosphere might act like a lens and distort the view depending on the angle of the telescope to the curvature of the atmosphere. Is that the reason? Does Hubble need to focus when it points to a different target? -- Joe
This question I can at least partially answer. I wear some pretty strong glasses (4.5 diopters). Sometimes I view through the telescope with glasses removed. The focus adjustment allows me to make this work (well, the astigmatism is still frustrating...).
Jim
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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.
Joe, I took a college astronomy class believe it or not, and I didn't learn anything. I was too busy throwing spit wads in class... ;) from my corner perch, I do remember hearing a discussion which may have a bearing on your question. The theory regarding the curvature of space and time was proved, was proven, arrrggghh, was 'confirmed', during a solar eclipse, where a visible star's position had changed when it was near the edge of the eclipsed sun. This curvature of space around objects may explain why one object, Jupiter, is in focus and one, the moon, is slightly blurred. I know that if I hold my thumb up in front of my eye and move my thumb towards this letter "A", the view of the letter "A" becomes real sharp as I near the edge. Perhaps this is similar to what is happening with the moon and Jupiter, but on a smaller scale. You know if I were to use both thumbs and both eyes, hey, I would have perfect vision. Of course I would look pretty silly trying to operate a vehicle using my elbows... ;) Guy
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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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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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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Quoting Brent Watson <brentjwatson@yahoo.com>:
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.
But positional errors are the result of distortion which I would think affects focus.
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.
That sounds about right. Thanks Brent.
participants (7)
-
astrodeb＠charter.net -
Brent Watson -
diveboss＠xmission.com -
Jim Cobb -
Joe Bauman -
Siegfried Jachmann -
ziggy943＠xmission.com