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From: Better Info on 30 Jun 2010 03:34 On Tue, 29 Jun 2010 22:10:52 -0500, John Turco <jtur(a)concentric.net> wrote: >Better Info wrote: >> >> On Mon, 31 May 2010 05:57:27 -0500, John Turco <jtur(a)concentric.net> wrote: >> >> >Ray Fischer wrote: >> >> >> >> DanP <dan.petre(a)hotmail.com> wrote: >> >> >On 23 May, 18:50, rfisc...(a)sonic.net (Ray Fischer) wrote: >> >> >> DanP <dan.pe...(a)gmail.com> wrote: >> >> >> >On May 23, 3:31 am, rfisc...(a)sonic.net (Ray Fischer) wrote: >> >> >> >> >> >> >> Wrong. Bigger apertures allow higher resolution. That's why big >> >> >> >> telescopes are better than tiny ones. >> >> >> >> >> >> >Telescopes are focused at infinity so that is a different case. >> >> >> >> >> >> ?!? >> >> >> >> >> >> Why is that different? >> >> > >> >> >Because their optics are fixed >> >> >> >> Nope. >> >> >> >> >and you want the biggest lens/mirror>you can get. >> >> >> >> Because bigger means higher resolution. >> > >> > >> >Where astronomical telescopes are concerned, greater light-gathering >> >capability "means higher resolution." >> > >> >Laymen often overstate the importance of magnification, but...if the >> >object in question isn't seen, to begin with, it >can't< be magnified, >> >anyway. >> >> Higher resolution is the result of larger diameters. Greater light >> gathering ability of that larger diameter is just a welcome benefit for >> naked-eye visual observations by humans. Resolution does not depend on the >> amount of light collected. If you put a very strong neutral density filter >> in front of a large 10m dia. telescope, to effectively limit the amount of >> light entering to no more than 1 photon per second, and used a >> high-resolution detector array to measure those photons, in time you will >> still have higher resolution image than could be obtained from a 1m dia. >> telescope with no filter at all. >> >> The Hubble Telescope was trained on a patch of sky where nobody thought >> anything existed, as pure black as the sky can be. Named "The Hubble Deep >> Field" experiment. From a period of 10 days of collecting photons from that >> black region they imaged about 1500 whole galaxies in that empty spot of >> the sky with just as much resolution as they get on bright objects. Image >> brightness has nothing to do with resolution. >> >> http://hubblesite.org/newscenter/archive/releases/1996/01/ > > >All right, then; please, give me additional information. I'd prefer a >telescope with the following capabilities: > > planets appear fairly large, in the eyepiece(s) Most require high magnification so this means large aperture. The larger the physical diameter of the telescope objective the more magnification that can be applied to the image (provided the optics are polished to diffraction-limited precision). > > resolves Saturn's rings distinctly A 16" dia. telescope will easily resolve Cassini's Division, even the Enke gap on a clear night of good seeing. <http://en.wikipedia.org/wiki/Rings_of_Saturn> On a very clear night of good seeing I have seen Saturn look almost as good as this downsized image from HST during a few calm atmospheric moments, appearing about 50% larger than this size through the eyepiece of my 16" diameter reflector. <http://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Saturn_HST_2004-03-22.jpg/260px-Saturn_HST_2004-03-22.jpg> With a 6" diameter off-axis mask, you then turn it into an exceptional planetary performer devoid of any chromatic aberrations and diffraction-spikes from secondary-mirror vanes. This is the preferred method for planetary viewing. Some astronomers will take great pride in a $50,000 apochromatic refractor of 6" diameter for a planetary telescope, but they are still plagued with violet CA. > > has electronic "GO TO" functions Most all do today. > >What's the absolute cheapest that I could obtain such an instrument >(new, used or refurbished)? You'll have to look into a 16" diameter equatorial mounted reflector. They are less expensive than the more compact catadioptric designs, but afford better resolution and image quality. You can go with even less cost by buying (or building) one of the alt-azimuth Dobsonian mounted reflectors. They also have tracking and goto systems designed for those now. If you built your own equatorial relfector of 16" size you're looking at about $1,500-$2,000 if you buy the mirror. Less if you grind and polish your own. If that's still too costly look into a 10" diameter telescope. It might be enough for your needs. I haven't researched telescope prices in over 18 years. My 16" diameter reflector cost about $3800 new that long ago. A mount sturdy enough and with enough precision to move and stabilize that much mass can often cost as much as the telescope's OTA (optical telescope assembly) itself. > >Thanks, for any advice! You'd do better to subscribe to newsgroup sci.astro.amateur and ask your questions there. Even better, read as much of that group as you can as far back as you can retrieve it. Your answers to any further questions you might have have probably already been posted.
From: Martin Brown on 30 Jun 2010 05:33 On 30/06/2010 04:10, John Turco wrote: > Better Info wrote: >> >> On Mon, 31 May 2010 05:57:27 -0500, John Turco<jtur(a)concentric.net> wrote: >> >>> Ray Fischer wrote: >>>> >>>> Because bigger means higher resolution. Only up to a point. There are trick and exceptions where good observables can be obtained from very large interferometer spacings but in general the atmosphere limits the effective resolution for long time exposures from the ground to about what a good 6" scope can do. Very short exposures of bright objects like planest can be lucky and get the moments of good seeing. There are shift and add post processing tricks to make a higher resolution image from the highest contrast frames in a video stream. >>> >>> >>> Where astronomical telescopes are concerned, greater light-gathering >>> capability "means higher resolution." Not quite. Once you get significantly beyond 8" aperture turbulence in the atmosphere dominates the theoretical resolution gain - except during very short periods of "lucky seeing". Google that phrase to see how amateurs using humble webcams can now almost reach professional standards on the planets with modest apertures and a lot of patience! >>> >>> Laymen often overstate the importance of magnification, but...if the >>> object in question isn't seen, to begin with, it>can't< be magnified, >>> anyway. >> >> Higher resolution is the result of larger diameters. Greater light >> gathering ability of that larger diameter is just a welcome benefit for >> naked-eye visual observations by humans. Resolution does not depend on the >> amount of light collected. If you put a very strong neutral density filter >> in front of a large 10m dia. telescope, to effectively limit the amount of >> light entering to no more than 1 photon per second, and used a >> high-resolution detector array to measure those photons, in time you will >> still have higher resolution image than could be obtained from a 1m dia. >> telescope with no filter at all. Actually no - you wouldn't. The atmosphere in all but the very best sites limits the effective resolution for time exposures to around 0.5" arc unless adaptive optics are used to correct the wavefront errors. >> The Hubble Telescope was trained on a patch of sky where nobody thought >> anything existed, as pure black as the sky can be. Named "The Hubble Deep >> Field" experiment. From a period of 10 days of collecting photons from that >> black region they imaged about 1500 whole galaxies in that empty spot of >> the sky with just as much resolution as they get on bright objects. Image >> brightness has nothing to do with resolution. >> >> http://hubblesite.org/newscenter/archive/releases/1996/01/ The HST is in space so does not suffer from atmospheric seeing and yields images consistent with theoretical optical performance for the aperture (at least it did after COSTAR was added to correct its myopia). > > > All right, then; please, give me additional information. I'd prefer a > telescope with the following capabilities: > > planets appear fairly large, in the eyepiece(s) That one is hard to do satisfactorily. You are best off with magnifications upto 20x per inch of aperture (although 40x is sometimes possible to take advantage of moments of good seeing). So for a 10" scope at 200x Jupiter which is between 30-50" arc will only look like a disk 3-5 degrees in diameter or roughly 6-11x bigger than the moon appears with the naked eye. In an eyepiece this still will look "small" compared to the 40 degree apparent field of view. > > resolves Saturn's rings distinctly Almost anything at 4" or above will do this, but not at the moment the rings are almost edge on. See for example: http://www.curtrenz.com/1024q.html > > has electronic "GO TO" functions > > What's the absolute cheapest that I could obtain such an instrument > (new, used or refurbished)? A secondhand ETX 125 unwanted Xmas bought in February after the novelty has worn off. Watch eBay or even you local paper for adverts. The LT series gets you an extra inch or 3 if you will stretch a bit further. A good 4" refractor would be a bit more - physically larger. And given where you are posting you probably want to buy a camera adapter to try your hand at astrophotography and/or wildlife. If you want something that will work as a long telephoto and can live with the donut bokeh on highlights and slight softness then the Russian MTO 1000mm f10 mirror lens is also worth considering. Some specimens are better than others - mine is from the late 1980's. It will just about resolve saturns rings. It makes a compact spotting scope too. There are a few telescope FAQs on the web you could check out too. David Kniseleys (sp?) is as good as any. The vendors are all trying to sell you stuff. GPS on a scope is overkill unless you intend to observe from the back of a moving flatbed truck. > > Thanks, for any advice! NB some (perhaps many) local astro clubs have unused kit (not with goto) lying around with a shortage of keen observers to use it. If you find your local astro club they may have loan kit available for a nominal sum that you can try out and or look through their scopes. If buying new hold out until they offer the free set of eyepieces deal and if buying second hand take someone along from your local astroclub to look the optics over for evidence of using a panscrub to clean them. A bit of dust and grime hardly afects the image at all, but a bunch of parallel scractches will do a lot of damage. Regards, Martin Brown PS All my scopes have been pre-owned.
From: Better Info on 20 Jul 2010 03:24 On Tue, 20 Jul 2010 02:06:53 -0500, John Turco <jtur(a)concentric.net> wrote: > >Besides, "light pollution" has become much worse, in my neighborhood, >these past few years. You might look into "light pollution reduction" (LPR or UHC) filters. These are precision thin-film notch-filters targeted at the spectral frequencies emitted by sodium and mercury vapor lights. The most commonly used lights in urban areas. They're not cheap but could extend the capabilities, both for photography and visual use, by a leap in magnitudes of useful light of objects outside of the polluted atmosphere. Certainly more inexpensive than buying another 10" of aperture. The added benefit being that you can use them on both telescope eyepiece and unaided camera. For a price, greatly dependent on diameter.
From: Martin Brown on 20 Jul 2010 04:47 On 20/07/2010 08:06, John Turco wrote: > Better Info wrote: >> >> On Tue, 29 Jun 2010 22:10:52 -0500, John Turco<jtur(a)concentric.net> wrote: > > <edited for brevity> > >>> All right, then; please, give me additional information. I'd prefer a >>> telescope with the following capabilities: >>> >>> planets appear fairly large, in the eyepiece(s) >> >> Most require high magnification so this means large aperture. The larger >> the physical diameter of the telescope objective the more magnification >> that can be applied to the image (provided the optics are polished to >> diffraction-limited precision). This only holds visually for apertures up to about 6" and photographically for apertures a bit larger using fast video stream based lucky imaging techniques. Atmospheric seeing limits resolution to the roughly size of the coherent wavefront across the scope - usually around 4" apart from very rare ocassions. >> >>> >>> resolves Saturn's rings distinctly >> >> A 16" dia. telescope will easily resolve Cassini's Division, even the Enke >> gap on a clear night of good seeing. >> >> <http://en.wikipedia.org/wiki/Rings_of_Saturn> >> >> On a very clear night of good seeing I have seen Saturn look almost as good >> as this downsized image from HST during a few calm atmospheric moments, >> appearing about 50% larger than this size through the eyepiece of my 16" >> diameter reflector. >> <http://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Saturn_HST_2004-03-22.jpg/260px-Saturn_HST_2004-03-22.jpg> >> With a 6" diameter off-axis mask, you then turn it into an exceptional >> planetary performer devoid of any chromatic aberrations and >> diffraction-spikes from secondary-mirror vanes. This is the preferred >> method for planetary viewing. Some astronomers will take great pride in a >> $50,000 apochromatic refractor of 6" diameter for a planetary telescope, >> but they are still plagued with violet CA. >> >>> >>> has electronic "GO TO" functions >> >> Most all do today. >> >>> >>> What's the absolute cheapest that I could obtain such an instrument >>> (new, used or refurbished)? >> >> You'll have to look into a 16" diameter equatorial mounted reflector. They >> are less expensive than the more compact catadioptric designs, but afford >> better resolution and image quality. You can go with even less cost by >> buying (or building) one of the alt-azimuth Dobsonian mounted reflectors. >> They also have tracking and goto systems designed for those now. >> >> If you built your own equatorial relfector of 16" size you're looking at >> about $1,500-$2,000 if you buy the mirror. Less if you grind and polish >> your own. If that's still too costly look into a 10" diameter telescope. It >> might be enough for your needs. >> >> I haven't researched telescope prices in over 18 years. My 16" diameter >> reflector cost about $3800 new that long ago. A mount sturdy enough and >> with enough precision to move and stabilize that much mass can often cost >> as much as the telescope's OTA (optical telescope assembly) itself. >> >>> >>> Thanks, for any advice! >> >> You'd do better to subscribe to newsgroup sci.astro.amateur and ask your >> questions there. Even better, read as much of that group as you can as far >> back as you can retrieve it. Your answers to any further questions you >> might have have probably already been posted. > > > Okay, thanks, once again. It looks as though my astronomical ambitions > greatly exceed my budget, unfortunately. I'll just have to be content > with my Meade ETX-60AT (60mm "Autostar" refractor) or an incremental > upgrade scope, I guess. A page which discusses how easy the Cassini division is to see in smaller scopes is online at: http://www.cloudynights.com/item.php?item_id=1132 BTW although some of what "Better Info" writes is accurate I suspect he is another variant of the P&S troll. A 6" or 8" would give you some nice views - cheaper way to get them is join an astro club. It is worth asking on s.a.a but it is a shadow of its former self these days and overrun with delusional nutters and cranks :( > > Besides, "light pollution" has become much worse, in my neighborhood, > these past few years. Light pollution doesn't affect looking at the moon or planets. A second hand 8" f10 SCT would be my suggestion if you want a step improvement over what you have now. Regards, Martin Brown
From: Better Info on 20 Jul 2010 20:05
On Tue, 20 Jul 2010 09:47:36 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote: >On 20/07/2010 08:06, John Turco wrote: >> Better Info wrote: >>> >>> On Tue, 29 Jun 2010 22:10:52 -0500, John Turco<jtur(a)concentric.net> wrote: >> >> <edited for brevity> >> >>>> All right, then; please, give me additional information. I'd prefer a >>>> telescope with the following capabilities: >>>> >>>> planets appear fairly large, in the eyepiece(s) >>> >>> Most require high magnification so this means large aperture. The larger >>> the physical diameter of the telescope objective the more magnification >>> that can be applied to the image (provided the optics are polished to >>> diffraction-limited precision). > >This only holds visually for apertures up to about 6" and >photographically for apertures a bit larger using fast video stream >based lucky imaging techniques. Atmospheric seeing limits resolution to >the roughly size of the coherent wavefront across the scope - usually >around 4" apart from very rare ocassions. Not true. You must live or view in an area of really poor seeing conditions. 16" diameter optics are near the limit of seeing in favorable areas. You really shouldn't be handing out advice with your limited viewing experiences. >>> >>>> >>>> resolves Saturn's rings distinctly >>> >>> A 16" dia. telescope will easily resolve Cassini's Division, even the Enke >>> gap on a clear night of good seeing. >>> >>> <http://en.wikipedia.org/wiki/Rings_of_Saturn> >>> >>> On a very clear night of good seeing I have seen Saturn look almost as good >>> as this downsized image from HST during a few calm atmospheric moments, >>> appearing about 50% larger than this size through the eyepiece of my 16" >>> diameter reflector. >>> <http://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Saturn_HST_2004-03-22.jpg/260px-Saturn_HST_2004-03-22.jpg> >>> With a 6" diameter off-axis mask, you then turn it into an exceptional >>> planetary performer devoid of any chromatic aberrations and >>> diffraction-spikes from secondary-mirror vanes. This is the preferred >>> method for planetary viewing. Some astronomers will take great pride in a >>> $50,000 apochromatic refractor of 6" diameter for a planetary telescope, >>> but they are still plagued with violet CA. >>> >>>> >>>> has electronic "GO TO" functions >>> >>> Most all do today. >>> >>>> >>>> What's the absolute cheapest that I could obtain such an instrument >>>> (new, used or refurbished)? >>> >>> You'll have to look into a 16" diameter equatorial mounted reflector. They >>> are less expensive than the more compact catadioptric designs, but afford >>> better resolution and image quality. You can go with even less cost by >>> buying (or building) one of the alt-azimuth Dobsonian mounted reflectors. >>> They also have tracking and goto systems designed for those now. >>> >>> If you built your own equatorial relfector of 16" size you're looking at >>> about $1,500-$2,000 if you buy the mirror. Less if you grind and polish >>> your own. If that's still too costly look into a 10" diameter telescope. It >>> might be enough for your needs. >>> >>> I haven't researched telescope prices in over 18 years. My 16" diameter >>> reflector cost about $3800 new that long ago. A mount sturdy enough and >>> with enough precision to move and stabilize that much mass can often cost >>> as much as the telescope's OTA (optical telescope assembly) itself. >>> >>>> >>>> Thanks, for any advice! >>> >>> You'd do better to subscribe to newsgroup sci.astro.amateur and ask your >>> questions there. Even better, read as much of that group as you can as far >>> back as you can retrieve it. Your answers to any further questions you >>> might have have probably already been posted. >> >> >> Okay, thanks, once again. It looks as though my astronomical ambitions >> greatly exceed my budget, unfortunately. I'll just have to be content >> with my Meade ETX-60AT (60mm "Autostar" refractor) or an incremental >> upgrade scope, I guess. > >A page which discusses how easy the Cassini division is to see in >smaller scopes is online at: > >http://www.cloudynights.com/item.php?item_id=1132 > >BTW although some of what "Better Info" writes is accurate I suspect he >is another variant of the P&S troll. A 6" or 8" would give you some nice >views - cheaper way to get them is join an astro club. > >It is worth asking on s.a.a but it is a shadow of its former self these >days and overrun with delusional nutters and cranks :( >> >> Besides, "light pollution" has become much worse, in my neighborhood, >> these past few years. > >Light pollution doesn't affect looking at the moon or planets. > >A second hand 8" f10 SCT would be my suggestion if you want a step >improvement over what you have now. > >Regards, >Martin Brown |