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RE: [Utah-astronomy] Mirrors
by David Dunn 23 Feb '03

23 Feb '03

I hadn't thought of the area of the mirror. I just ran the numbers. I could get my eyepiece down to about 60" using a 7.3" diagonal. That would leave 93.1% of the area of my mirror unobstructed. My secondary would be 132" from the primary. That will be real tempting since I use curved spiders anyway. I just didn't want to block all the light. Dave -----Original Message----- From: Chuck Hards [mailto:chuckhards@yahoo.com] Sent: Friday, February 21, 2003 1:58 PM To: utah-astronomy(a)mailman.xmission.com Subject: RE: [Utah-astronomy] Mirrors Dave, consider a modified Newtonian approach, where the diagonal is set at a much lower angle than 45-degrees. The light beam is shot back down the tube and exits at a grazing angle, basically as low on the tube as you want. A third mirror can then turn beam back out at a right angle to the tube. Once you get above 16" inches or so, the light loss of a secondary mirror becomes minimal in proportion to total area. The major advantage of an unobstructed system is a lack of diffraction artifacts, but unless you get the correction JUST RIGHT, the lack of diffraction spikes & better energy distribution in the diffraction rings won't offset the astigmatic images. And even the best unobstructed systems I've seen don't produce textbook-perfect spot diagrams. Also, typically the larger the unobstructed system, the more difficult the optical work, -AND- an absoulutely rigid tube assembly is mandatory. My 6 cents. C. --- David Dunn <david.dunn(a)albertsons.com> wrote: > I think that next year I will try making a 20" f/10. > I plan to fold the > optics and view from the ground. I am researching > this year to see how I can > correct for Astigmatism and Coma because I would > like to put the secondary > off axis. So far I think I will have the 20" ground > to f/10. I will use a > 10" secondary and f/8 and bring the light back down > to a diagonal to put the > light in my eyepiece. > > Dave > > -----Original Message----- > From: Brent Watson [mailto:brentjwatson@yahoo.com] > Sent: Friday, February 21, 2003 11:06 AM > To: utah-astronomy(a)mailman.xmission.com > Subject: Re: [Utah-astronomy] Mirrors > > > My 22 inch is f6.3. Only a couple of folks have > required oxygen. > > Brent > > --- Greg Taylor <astronomus_maximus(a)yahoo.com> > wrote: > > Imagine a 30" f/7... > > > > "Pass me that pressure suit willya?" > > > > "Sure thing; oh, and watch out for those > satellites, > > they come a little close. By the way, did you > > remember > > your parachute?" > > > > "Naw, I'd just burn up if I fell anyhow..." > > > > Cave made some sweet mirrors, so I've been told. > > You > > can probably get some nice high power views with > it > > too. > > > > Agreeing with what Chuck said, my 10 f/5.6 does a > > better job than it would if it were f/4.5, being > > mass > > produced and all. On a rare night of perfect > seeing > > last summer (very far from here), I was able to > push > > it to 580x without image degradation. I saw detail > > on > > Uranus! (Boy that dob driver comes in handy at > those > > high magnifications) > > > > > > --- David L Bennett <dlbennett(a)mac.com> wrote: > > > Don't forget considerations of field of view! > My > > > 10" f/5 Cave with a > > > 40mm Tele Vue Wide Field gives some very > > impressive > > > Milky Way shots ;) > > > > > > On Friday, February 21, 2003, at 09:58 AM, > Chuck > > > Hards wrote: > > > > > > > Everyone should, unless there is a specific > > reason > > > for > > > > going "short". Of course, legitimate reasons > > > include > > > > transportation and ergonomics. > > > > > > > > Brent's always been right-on in that > department! > > > :) > > > > > > > > C. > > > > > > > > > _______________________________________________ > > > Utah-Astronomy mailing list > > > Utah-Astronomy(a)mailman.xmission.com > > > > > > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > > > > __________________________________________________ > > Do you Yahoo!? > > Yahoo! Tax Center - forms, calculators, tips, more > > http://taxes.yahoo.com/ > > > > _______________________________________________ > > Utah-Astronomy mailing list > > Utah-Astronomy(a)mailman.xmission.com > > > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > > > __________________________________________________ > Do you Yahoo!? > Yahoo! Tax Center - forms, calculators, tips, more > http://taxes.yahoo.com/ > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/ _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy(a)mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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I knew it, I knew it. Ha!
by Greg Taylor 23 Feb '03

23 Feb '03

Does Sharp Image Of Distant Galaxy Shred The Fabric Of Space And Time? The sharp image of a galaxy halfway across the universe might shred modern theories about the structures of time and space, and change the way astrophysicists view the "Big Bang." http://www.uah.edu/News/2003news/sharp_image.html I always knew that quantum time and gravity were bunk! Ha! proof at last! Greg __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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Best Focal Ratio (long)
by Brent Watson 22 Feb '03

22 Feb '03

Please excuse my yelling on the forum. I am trying to place emphasis in certain areas and I can't figure out how to underline nor use italics. First let me state the following: THE BEST TELESCOPE IS THE ONE THAT GETS USED. A telescope may have superior optics, light gathering, image and everything else, but if it is nothing more than a museum piece, or a coat hangar, it is useless. You should make telescopes that you feel like getting out at a moments notice and using. Now, in answer to your question, there are reasons for making telescopes with different focal lengths. Here are some of them: 1. Portability. Generally, a shorter focal ratio telescope is a more portable telescope. Like everything else, here is one of the points that you need to weigh against others. Folding an optical path will introduce optical errors as easily as making a short focal ratio telescope. See my opening comment. 2. Image quality. Generally, a longer focal ratio will yield a better AND more pleasing image. There will not be as many optical errors in it, and it is easier to fabricate. 3. Photography. As discussed before, a short focal ratio will give you a shorter exposure time FOR EXTENDED OBJECTS. 4. Optical Design. Some optical designs require longer focal ratios. For example, a doublet refractor made with standard optical glass needs to have a long focal ratio to reduce secondary color. (The blue haze around an image) Folded designs many times are long focus because the image plane and eyepiece are a long ways away from the secondary. This requires the secondary to also be curved. 5. Image Size. Many observing requirements dictate a large image size. This forces a long focal length. Solar telescopes come to mind here. One of the most pleasing solar telescopes I have seen consists of nothing more than a 4 f150 lens. This gives an image size of 5-1/4. The advantage is that there is never a concentration of the suns energy, and everything runs very cool. 6. Special Requirements. Some optical devices, such as h-alpha and other interference filters, will not accept large light cones. In this case, a long focal ratio is required for the filter to work properly. If the light cone is too steep (too large of an angle) the filter will not have the narrow bandpass needed to see only one wavelength. 7. Appearance. Some people just like the looks of a long refractor. It looks more like what people have seen all their lives, and has an antique look to it. A telescope sticking way up in the air will attract a bunch of attention at a public star party. Well, there is a long answer to your question. It really depends on what you are going to do with the scope. Some applications dictate long focal ratios and some need short ones. Brent __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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RE: [Utah-astronomy] Columbia attachment test
by Kim Hyatt 22 Feb '03

22 Feb '03

Thanks - I obviously didn't look that far. -----Original Message----- From: Dan Hanks [mailto:hanksdc@plug.org] Sent: Tuesday, February 18, 2003 9:26 AM To: 'utah-astronomy(a)mailman.xmission.com' Subject: RE: [Utah-astronomy] Columbia attachment test I don't think it would be a hoax, as you can find the PDF from NASA's site: http://www.nasa.gov/columbia/COL_landgear_email_030212.html Click on the link 'Read text of email' -- Dan Hanks On Tue, 18 Feb 2003, Kim Hyatt wrote: > Brent, do we know if this is genuine? Could this be an elaborate hoax? > > -----Original Message----- > From: David L Bennett [mailto:dlbennett@mac.com] > Sent: Tuesday, February 18, 2003 9:20 AM > To: utah-astronomy(a)mailman.xmission.com > Subject: Re: [Utah-astronomy] Columbia attachment test > > > Thanks Brent for posting the pdf about the potential gear breach > concerns. Very very interesting and frightening, especially the > forwarded statement; "looks like they believe all has been addressed..." > > Dave > > On Monday, February 17, 2003, at 09:20 PM, Brent Watson wrote: > > > I've been accumulating some good info on the Columbia > > disaster, and thought I'd share it. This is a test. > > Here is an attachment you can look at. It is > > purported to be an internal NASA memo written the day > > before the re-entry. Let's see if it makes it through > > the server. If so, I'll send a couple more items. > > > > Brent > > > > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy(a)mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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Re: [Utah-astronomy] Mirrors
by Chuck Hards 22 Feb '03

22 Feb '03

Hi Jim: To the best of my knowledge, there is no optimum focal length. Focal length determines magnification, so it should be chosen to suit the application. Magnification should never exceed the diffraction limit for a given aperture. Telescopes are like airplanes in that if you design one that does "everything", it will do nothing well. Some of the large observatories use auxilliary foci with some whopping final f/ratios, sometimes at or over the f/100 you speak of. Of course, they have apertures larger than our houses, so there is plenty of light to support these image scales. Even so, long exposures (or integration times, or many short exposures, stacked) are sometimes needed to record the data. C. --- Jim Gibson <xajax99(a)yahoo.com> wrote: > I was wondering since we explored the bottom end of > the spectrum, and discounting the physical length > for a moment, ( I hope this isnt too dumb of a > question) is there an optimum focal length? For > example is an f15 all that much better than an f10. > Is an f100 really 10 times better optically than an > f10? Is there and upper end for focal length? I was > thinking that if there is a diminishing return and > an f10 is only slightly less optically than an f15 > then when physical restraints are considered I could > deal with it. But if the optimum focal length is > really up there, then I will be back to where > everyone else is the maximum minima or what will > fit in my car. Or, like David what is the best way > to fold the optics. > > It seems to me that folding the light is like > running a long cable. You are going to have some > line loss. Of course the light loss will probably > only occur at each juncture or device its run > through. __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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Columbia final
by Patrick Wiggins 21 Feb '03

21 Feb '03

Hi, Well, tomorrow will be two weeks since Columbia was lost and the mainstream media seems to be doing a pretty good job of covering what news there is. So, if there are no objections, I'm going to take the Columbia stuff off my web site this weekend and stop posting day-to-day Columbia stuff here. Here's looking forward to the shuttle flying again soon. Patrick

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RE: [Utah-astronomy] Mirrors
by David Dunn 21 Feb '03

21 Feb '03

I think that next year I will try making a 20" f/10. I plan to fold the optics and view from the ground. I am researching this year to see how I can correct for Astigmatism and Coma because I would like to put the secondary off axis. So far I think I will have the 20" ground to f/10. I will use a 10" secondary and f/8 and bring the light back down to a diagonal to put the light in my eyepiece. Dave -----Original Message----- From: Brent Watson [mailto:brentjwatson@yahoo.com] Sent: Friday, February 21, 2003 11:06 AM To: utah-astronomy(a)mailman.xmission.com Subject: Re: [Utah-astronomy] Mirrors My 22 inch is f6.3. Only a couple of folks have required oxygen. Brent --- Greg Taylor <astronomus_maximus(a)yahoo.com> wrote: > Imagine a 30" f/7... > > "Pass me that pressure suit willya?" > > "Sure thing; oh, and watch out for those satellites, > they come a little close. By the way, did you > remember > your parachute?" > > "Naw, I'd just burn up if I fell anyhow..." > > Cave made some sweet mirrors, so I've been told. > You > can probably get some nice high power views with it > too. > > Agreeing with what Chuck said, my 10 f/5.6 does a > better job than it would if it were f/4.5, being > mass > produced and all. On a rare night of perfect seeing > last summer (very far from here), I was able to push > it to 580x without image degradation. I saw detail > on > Uranus! (Boy that dob driver comes in handy at those > high magnifications) > > > --- David L Bennett <dlbennett(a)mac.com> wrote: > > Don't forget considerations of field of view! My > > 10" f/5 Cave with a > > 40mm Tele Vue Wide Field gives some very > impressive > > Milky Way shots ;) > > > > On Friday, February 21, 2003, at 09:58 AM, Chuck > > Hards wrote: > > > > > Everyone should, unless there is a specific > reason > > for > > > going "short". Of course, legitimate reasons > > include > > > transportation and ergonomics. > > > > > > Brent's always been right-on in that department! > > :) > > > > > > C. > > > > > > _______________________________________________ > > Utah-Astronomy mailing list > > Utah-Astronomy(a)mailman.xmission.com > > > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > > __________________________________________________ > Do you Yahoo!? > Yahoo! Tax Center - forms, calculators, tips, more > http://taxes.yahoo.com/ > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/ _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy(a)mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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RE: [Utah-astronomy] Mirrors
by David Dunn 21 Feb '03

21 Feb '03

This isn't pestering. This is allowing a bunch of people to learn with you. I am sure that most of the people on the list don't understand these concepts and appreciate Chuck, Brent and others that share their knowledge. -----Original Message----- From: Richard Tenney [mailto:retenney@yahoo.com] Sent: Friday, February 21, 2003 3:01 PM To: utah-astronomy(a)mailman.xmission.com Subject: Re: [Utah-astronomy] Mirrors My question obviously does belie my complete ignorance of astrophotography and the various methods used to image deep sky objects. When I read that a short focal length scope can photograph a given object X in "half" the time as a longer focal length scope of the same aperture, I see now what they didn't tell me is that the resulting image on the film is also 1/4 the size -- that's a big "duh". So I still fail to see the photographic advantage of a fast scope, unless what is imaged is huge (e.g., the veil nebula) and image size isn't all that important, right? Am I thinking straight here? At any rate, I'll quit pestering the list with my ignorance and go check out a few books on astrophotography and get a clue how this all works. -Rich --- Chuck Hards <chuckhards(a)yahoo.com> wrote: > It's not different. It's identical, given identical > apertures. > You're confused because you're using eyepiece > projection and making the image identical. > > You get a shorter exposure in an f/4 scope than an > f/8 > scope of identical aperture because the image is > only > 1/4 as large at the focal plane. If you make the > images of identical size through eyepiece > projection, > they are of the same brightness because they are now > the same size (cover the same area). You are now no > longer comparing an f/4 to an f/8. You've made them > identical with the eyepiece, as far as brightness > goes. > > C. __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/ _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy(a)mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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RE: [Utah-astronomy] Mirrors
by Dale Hooper 21 Feb '03

21 Feb '03

Personally, I'd like to see Dave build a nice Ritchey-Chrétien (hyperbolic primary, convex hyperbolic secondary) - so he can teach me how to build and test one. <g> After all, that's what all the big-boys (HST, Mauna Kea, MMT, etc. etc.) use. That way I can build by dream 30" RC ten years from now. Clear and dark skies, Dale. > -----Original Message----- > From: Chuck Hards [mailto:chuckhards@yahoo.com] > Sent: Friday, February 21, 2003 1:58 PM > To: utah-astronomy(a)mailman.xmission.com > Subject: RE: [Utah-astronomy] Mirrors > > > Dave, consider a modified Newtonian approach, where > the diagonal is set at a much lower angle than > 45-degrees. The light beam is shot back down the tube > and exits at a grazing angle, basically as low on the > tube as you want. A third mirror can then turn beam > back out at a right angle to the tube. > > Once you get above 16" inches or so, the light loss of > a secondary mirror becomes minimal in proportion to > total area. The major advantage of an unobstructed > system is a lack of diffraction artifacts, but unless > you get the correction JUST RIGHT, the lack of > diffraction spikes & better energy distribution in the > diffraction rings won't offset the astigmatic images. > And even the best unobstructed systems I've seen don't > produce textbook-perfect spot diagrams. > > Also, typically the larger the unobstructed system, > the more difficult the optical work, -AND- an > absoulutely rigid tube assembly is mandatory. > > My 6 cents. > > C. > --- David Dunn <david.dunn(a)albertsons.com> wrote: > > I think that next year I will try making a 20" f/10. > > I plan to fold the > > optics and view from the ground. I am researching > > this year to see how I can > > correct for Astigmatism and Coma because I would > > like to put the secondary > > off axis. So far I think I will have the 20" ground > > to f/10. I will use a > > 10" secondary and f/8 and bring the light back down > > to a diagonal to put the > > light in my eyepiece. > > > > Dave > > >

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RE: [Utah-astronomy] Today's NASA Briefing
by Dale Hooper 21 Feb '03

21 Feb '03

There is something that I have been really curious about that I haven't seen mentioned (please forgive me if I somehow just missed it). We know that the shuttle was following an atypical reentry track - because we were able to observe it over southern Utah. I'm wondering if this reentry profile presents times when there are greater stresses on the orbiter than a typical reentry profile? I.e., more severe banking that might overly stress an already somewhat damaged wing; which wouldn't have caused a failure in a more typical reentry profile. Certainly, this wouldn't be a root cause - but might be a contributing factor. Obviously, I've entered into second tier speculation. <g>

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