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#11
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more about Mars magnification
In article , Mick wrote:
At present Mars should be over 1/2 degree at 100 X's, which is a full moon disc. At 200 X's, Mars should appear as the moon would at 100 X's. This just doesnt happen because the moon takes up the whole field in the EP at 100X's So there is some optical trick occuring that eludes me... For Mars to have the same apparent size in the eyepiece at 200x that the Moon has at 100x it would have to be 900 arc seconds wide instead of 20 arc seconds wide. |
#12
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more about Mars magnification
"Mick" wrote in message ...
At 200 X's, Mars should appear as the moon would at 100 X's. This just doesnt happen . . . So there is some optical trick occuring that eludes me... No optical trick - let's review the math: The naked-eye angular diameter of the Moon at its mean distance is 1865 arcseconds, or ~31.1 arcminutes. Magnification = apparent field / true field M = AF_deg / TF_deg See http://www.twcac.org/Tutorials/apare...true_field.htm for a diagram of the apparent and true fields. For photographs illustrating the apparent and true fields, see the bottom of web page: http://www.astro.com.sg/chpt3.htm Apparent field is on the left; true field on the right. If the Moon just fills the eyepiece at 100x, it sounds like you are using Possel eyepiece that has a 52 degree apparent field of view: M = AF_deg / TF_deg ~100x = 52 deg / 31.1 arcminutes = 3120 arcminutes / 31.1 arcminutes = 187200 arcseconds / 1866 arcseconds = ~100x At its closest distance to Earth, Mars will have a naked-eye extended apparent angular size of 25.1 arcseconds, or ~0.42 arcminutes. At what power will Mars fill your Possel eyepiece with a 52 degree apparent field of view? M = 52 degs / 25.1 arcseconds = 187200 arcseconds / 25.1 arcseconds = 7458 power This is considerably higher than observing Mars at 200x, as you have been experimenting with. It is probably impractical to try to construct equal sized images of the Moon and Mars in the apparent field of your eyepiece. Take a 36x eyepiece to look at the Moon - probably one of the lowest powers in your eyepiece rack. In the eyepiece, the Moon would then have a apparent angular size in your 52 degree apparent field-of-view eyepiece of: M = AF_deg / TF_deg 36 = AF / 31.1 arcminutes rearranging - AF = TF x M = 31.1 arcminutes x 20 AF = ~1120 arcminutes = ~18.6 degrees = 18.6 deg / 52 deg = ~36% of the FOV This means that the Moon takes up 18.6 degrees of your 52 degree Possel eyepiece's apparent field of view or about 36% of the eyepiece field of view. With Matt Wier's online telescope simulator you can simulate how the Moon would appear: http://www.stic.net/mattwier/ In the online simulator, move the Eyepiece Focal Length bar in the simulator to its maxmimum (55mm). This will simulate the view of the Moon at 36x. The default apparent field of view in the simulator is 50 degrees. Move the FOV slider bar 52 degrees. To view an equal-sized Mars in the eyepiece - that is to have the same apparent field size (18.6 degrees or 36% of the FOV) in the 52 degree apparent field of your Possel eyepiece - what magnification would you use? M = AF_deg / TF_deg M = 18.6 degrees / 25.1 arcseconds = 66960 arcseconds / 25.1 arcseconds = 2667 power This 2667 power probably exceeds the highest power eyepiece in your rack by 4 to 10 times. The online telescope simulator also contains the Object Data "Mars" to simulate viewing the red planet. (But it cannot be made to simulate 2667 power easily, if at all! - ) Set the simulator to "Mars." Set the magnification to 200x by moving the Eyepiece focal length to 20mm. Viewing Mars at 200x using the 52 degree Possel apparent field of view eyepiece gives: M = AF_deg / TF_deg 200 = AF / 25.1 arcseconds rearranging - AF = TF x M = 25.1 arcseconds x 200 AF = 5020 arcseconds = ~1.4 degrees = 1.4 deg / 52 deg = 2.7% of apparent FOV This is about what is seen of Mars in the simulated (and your real) eyepiece - a small distinct red disk with no detail. You need about 800x to get a reasonably sized image in the Possel's 52 degree apparent field of view: M = AF_deg / TF_deg 800 = AF / 25.1 arcseconds rearranging - AF = TF x M = 25.1 arcseconds x 800 AF = 20080 arcseconds = ~5.6 degrees = 5.6 deg / 52 deg = ~11% of apparent FOV At 800x and 11% apparent FOV, you get a blurry disk with detail begining to resolve. To set the online telescope simulator to 800x, hit the 2x Barlow button. Then move the Eyepiece focal length slider to the left until the magnification field reads about 800x (at about 5mm). You may have to brighten the image up a bit by increasing the simulated telescope's aperture size. In another thread, message ... Peter Lawerence has posted a number of pictures of Mars that his web site states were taken using a Vixen FL-102 f9 refractor, a 20mm eyepiece lens, and a 2x Barlow - all coupled to a digital camera set at a zoom of 10x. See his web site at http://www.pbl33.co.uk/ All put together, this gave Peter about ~920x: fl_obj = 102mm x 9f = 918mm M = fl_obj / fl_eyepiece M = 918 mm / 20 mm = ~46x then add in the Barlow: 2M = 46 x 2 = 92 then the zoom camera 10(2)M = 10 x 2 x 46 = 920 power You can use the simulator at http://www.stic.net/mattwier/ to emulate 920x. (Use the 2x Barlow button. In the Eyepiece focal length box, type in 4.35mm. Brighten the image by increasing the aperture.) The resolution of Mars in the online simulator resembles what Peter was able to accomplish (with additional brightness being added by image stacking instead of a larger aperture). But I always get more of charge out of light coming from the real thing than light received by "internet astronomy." - Hope the above helps in better understanding how light moves through a telescope. I know its confusing. Just stick with it. Regards - Kurt Some web sites for studying field of view: http://www.twcac.org/Tutorials/apare...true_field.htm http://www.spacegazer.com/basics.htm#field http://www.astro.com.sg/chpt3.htm http://www.stic.net/mattwier/ |
#13
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more about Mars magnification
"PrisNo6" wrote in message om... "Mick" wrote in message ... At 200 X's, Mars should appear as the moon would at 100 X's. This just doesnt happen . . . So there is some optical trick occuring that eludes me... No optical trick - let's review the math: The naked-eye angular diameter of the Moon at its mean distance is 1865 arcseconds, or ~31.1 arcminutes. Oh..I do like the simulator however... |
#14
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more about Mars magnification
"Mick" wrote in message ... At 200 X's, Mars should appear as the moon would at 100 X's. This just doesnt happen . . . So there is some optical trick occuring that eludes me... Well, if at 200 X's, Mars really would appear as the moon would at 100 X's, then Mars would, from the Earth, appear half as large as the Moon. And this would imply several things: 1. To the naked eye, Mars would appear as a disk and those with a sharp vision would even be able to distinguish some of Mars' largest surface features -- without a telescope! 2. Mars real diameter would be at least some 160 times the Moon's diameter -- that's some 40 times the Earth's diameter, or some 4 times larger than Jupiter !!!! These things too does not happen. Thus, at 200X, Mars will appear much smaller than the Moon does at 100X. -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://www.stjarnhimlen.se/ http://home.tiscali.se/pausch/ |
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