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Space Forum / Amateur Astronomy / November 2004The "It supports 100x per inch" argument
I've often wondered about this. What I've found with decent scopes is that they tend to provide maximum detail at around 40-50x per inch. Additional magnification may not degrade the image much, but it doesn't add any detail. According to Suiter, people who perceive detail gains when going to magnifications above that actually are letting the optical system compensate for a lack of normal visual acuity. That's the first time I've read that. Meade, in one of their latest pieces of literature claims a maximum power of 600x for one of their 8" SCTs. While under perfect conditions this may be usable, I doubt you'd see much beyond 400x with any 8" SCT anytime. -Rich >Subject: The "It supports 100x per inch" argument >From: RichA none@none.com >Date: 11/18/2004 8:57 PM Eastern Standard Time >Message-id: <tgkqp01lkh19irr79m05hl1aha7qjli6mj@4ax.com> >I've often wondered about this. What I've found with >decent scopes is that they tend to provide maximum detail at [quoted text clipped - 9 lines] >see much beyond 400x with any 8" SCT anytime. >-Rich ****************************** I have a old 80's Black C-8 OTA, and it will handle 400x with ease! Even 550x does well in this scope, thats when is seeing very good. Guess i got a hot C-8, i've owned about 10 others, and they dont come close!!! Chas P. > ****************************** > I have a old 80's Black C-8 OTA, and it will handle 400x with ease! Even 550x > does well in this scope, thats when is seeing very good. Guess i got a hot > C-8, i've owned about 10 others, and they dont come close!!! > > Chas P. I have NEVER seen a BLACK C-8. The C-8 from the 80's is the standard off orange or whatever you call it and ALWAYS has been. Unless you or others know something that most don't and have seen other Astronomy Magazines from the "good ol 80's" that others haven't, lay your proof here for all to see. >> I have a old 80's Black C-8 OTA, and it will handle 400x with ease! Even >550x >> does well in this scope, thats when is seeing very good. >> I have a old 80's Black C-8 OTA, and it will handle 400x with ease! Even >550x >> does well in this scope, thats when is seeing very good. > I have NEVER seen a BLACK C-8. The C-8 from the 80's is the standard >off orange or whatever you call it and ALWAYS has been. Unless you or >others know >something that most don't and have seen other Astronomy Magazines from the >"good ol 80's" >that others haven't, lay your proof here for all to see. Love my black Ultima8: http://home.att.net/~telescopebluebook/sct/celestron.htm SSX >Subject: Re: The "It supports 100x per inch" argument >From: saberscorpx@aol.com (SaberScorpX) >Date: 11/19/2004 1:03 AM Eastern Standard Time >Message-id: <20041119010357.16224.00000787@mb-m02.aol.com> >Love my black Ultima8: >http://home.att.net/~telescopebluebook/sct/celestron.htm > >SSX ******************************** Love your web site!!! Chas P. >>> I have a old 80's Black C-8 OTA, and it will handle 400x with ease! >>> Even [quoted text clipped - 12 lines] > > SSX I would have to be desparate for the money to part with mine. I have an MX5-C that mates to it perfectly as an introduction into deep sky CCD imaging, as well as a ToUcam "to beat the seeing" for planets and lunar details. I haven't the tiime or energy to get out there with it all that much, but lack of use doesn't always translate to "useless", if you have a piece of equipment that you enjoy from time to time. The Ultima 8 is a stable enough imaging platform for long exposures, and can be had for pretty cheap compare to the alternatives. As a follow on to that idea, I do wonder how much equipment gets bought and sold, and replaced with something so similar that it wasn't worth the exchange. Exchanges which are ultimately born out of boredom, due to lack of access to decent skies, or the time to enjoy them. As an example, between my (quite) old C5 (sold) and the replacement 4" F5 achromat, the difference in raw performance on most mid to low power objects probably wasn't enough to warrant the exchange, on that basis. When I made that decision, I intentionally gave up color free planets, for wide field's of view. Now that I am experience it, I wish I had a wide field scope that was also capable of color free planets.... turns out I want both. Exchanging one for the other wasn't a good long term solution, and in all the time and money I've wasted going around the block, I could have saved up enough cash for a TV102i, which as a binoview user, seems an ideal solution for a "plop and go" scope. However, I will not sell my Ultima 8, and/or my 12.5" Dob to get one. I just know I would regret it, and end up spending money _all_ over again to replace them. Three scopes are a requirement. One isn't enough, and four aren't necessary if your quick look scope is a fast 4 to 5" apo (not to mention its potential for imaging). But, as I've said elsewhere, I'd trade them all away, if it meant the powers that be, would turn off the lights at night, and give us back a truly daky sky. Telescopes are important for the few who dedicate themselves to the science of astronomy, but the masses desparately need some face time with the depths of the universe, before the human race manages to competely light up the world, and extinguish the soul. -Stephen > Love my black Ultima8: > http://home.att.net/~telescopebluebook/sct/celestron.htm Ok. Now I remember. Thanks > > ****************************** > > I have a old 80's Black C-8 OTA, and it will handle 400x with ease! Even [quoted text clipped - 11 lines] > "good ol 80's" > that others haven't, lay your proof here for all to see. Get glasses and cease using "averted imagination" immediately! Mark >Subject: Re: The "It supports 100x per inch" argument >From: "moT" Mot@nospam.com >Date: 11/19/2004 12:38 AM Eastern Standard Time >Message-id: <BFfnd.320$y72.193@clgrps12> > I have NEVER seen a BLACK C-8. The C-8 from the 80's is the standard >off orange or whatever you call it and ALWAYS has been. Unless you or >others know >something that most don't and have seen other Astronomy Magazines from the >"good ol 80's" >that others haven't, lay your proof here for all to see. ********************************** Just take a look in any Sky& Tele from 1984 and on , and you will see all the black C-8's you would ever want my man!!! Chas P. >>Subject: Re: The "It supports 100x per inch" argument >>From: "moT" Mot@nospam.com [quoted text clipped - 12 lines] > >Chas P. I had a few of them. They all were pretty mediocre, optically. -Rich >Subject: Re: The "It supports 100x per inch" argument >From: RichA none@none.com >Date: 11/19/2004 4:40 PM Eastern Standard Time >Message-id: <q0qsp098sup9ucsgjh1vgkvtlqgebl9s9q@4ax.com> >I had a few of them. They all were pretty mediocre, optically. >-Rich ************************** Seems around 1992 is when Meade and Celestron started making better optics. Now days it would be hard to get a bad OTA... Chas P. << Unless you or others know something that most don't and have seen other Astronomy Magazines from the "good ol 80's" that others haven't, lay your proof here for all to see. >> Most don't? How about most do know. > I have NEVER seen a BLACK C-8. The C-8 from the 80's is the standard > off orange or whatever you call it and ALWAYS has been. Just because you've never seen one doesn't mean they didn't exist, dummy. The Celestron telescopes were indeed black for a while. I own a black C8. Don't know when it was made, but I purchased it only a few years ago so it's likely not from the 80's. -Larry (but ya never know) C. > I have NEVER seen a BLACK C-8. The C-8 from the 80's is the standard > off orange or whatever you call it and ALWAYS has been. Unless you or > others know > something that most don't and have seen other Astronomy Magazines from the > "good ol 80's" > that others haven't, lay your proof here for all to see. I personally find that the average for SCTs from anyone is around 30x/inch, while a great refractor such as one with a Jeagers lens will support 90x/inch on a dead-steady night. It goes back to that central obstruction and how it moves the light distribution in star images from the center out to the rings. And I have to dispute the contention that seeing additional detail on magnifications above 50x/inch has more to do with the mind than the eye, so to speak. From what I understand, if the atmosphere is steady enough and the optics are as close to perfect as they can be, then it's possible to get to 90x/inch and the details there are real, not induced or caused by a compensation mechanism.  SignatureSincerely, --- Dave ---------------------------------------------------------------------- It don't mean a thing unless it has that certain "je ne sais quoi" Duke Ellington ---------------------------------------------------------------------- > I've often wondered about this. What I've found with > decent scopes is that they tend to provide maximum detail at [quoted text clipped - 9 lines] > see much beyond 400x with any 8" SCT anytime. > -Rich > I personally find that the average for SCTs from anyone is around 30x/inch, > while a great refractor such as one with a Jeagers lens will support > 90x/inch on a dead-steady night. It goes back to that central obstruction > and how it moves the light distribution in star images from the center out > to the rings. I think there's a difference, owing to the central obstruction, but even on the big-obstruction C5, it is nowhere near a factor of 3. Maybe a factor of 2, depending on the particular details you're looking at. Remember that in the high-frequency domain, the central obstruction has a much smaller effect than it does at mid-frequencies. > And I have to dispute the contention that seeing additional detail on > magnifications above 50x/inch has more to do with the mind than the eye, so > to speak. From what I understand, if the atmosphere is steady enough and > the optics are as close to perfect as they can be, then it's possible to get > to 90x/inch and the details there are real, not induced or caused by a > compensation mechanism. No, I think Rich is correct on this one. Putting in an eyepiece with a shorter focal length has no impact on what detail is available at the focal plane. The only thing it can do is magnify that detail further so that it is easier to see. The greater an observer's visual acuity, the lower the magnification need be in order to see that available detail. For instance, a 4-inch refractor has an Airy disc of about 1 arcsecond (a little bigger at half-maximum, I think, but let's ignore that for now). If you magnify that at 360x (90x per inch), that sucker is now 6 arcminutes in size. Frankly, I can't imagine that the average observer is going to be able to see more at that point than at 200x (50x per inch), when the Airy disc is about 3.5 arcminutes across. That's about one-eighth the width of the Full Moon. Brian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt >> I personally find that the average for SCTs from anyone is around >> 30x/inch, [quoted text clipped - 10 lines] > Remember that in the high-frequency domain, the central obstruction has > a much smaller effect than it does at mid-frequencies. I'll agree with this, but I also have to say that it seems to me like we're quibbling on the details, but then I like quibbles. Oh! Sorry! I thought I said "tribbles". ^_^ >> And I have to dispute the contention that seeing additional detail on >> magnifications above 50x/inch has more to do with the mind than the eye, [quoted text clipped - 10 lines] > that it is easier to see. The greater an observer's visual acuity, the > lower the magnification need be in order to see that available detail. I thought Rich was saying something different, that the details you think you see are due to some eye-brain effect that "fills in the details", not that these details are really there at the focal plane to see. Rich, can you clarify this? > For instance, a 4-inch refractor has an Airy disc of about 1 arcsecond > (a little bigger at half-maximum, I think, but let's ignore that for now). [quoted text clipped - 4 lines] > Airy disc is about 3.5 arcminutes across. That's about one-eighth the > width of the Full Moon. I'm not sure where you got these numbers, although I agree with general statements that the amount of detail you'll see is dependent on your skill as an observer, and that increased magnification increases the apparent size of the Airy disk. SignatureSincerely, --- Dave ---------------------------------------------------------------------- It don't mean a thing unless it has that certain "je ne sais quoi" Duke Ellington ---------------------------------------------------------------------- > For instance, a 4-inch refractor has an Airy disc of about 1 arcsecond > (a little bigger at half-maximum, I think, but let's ignore that for now). > If you magnify that at 360x (90x per inch), that sucker is now 6 arcminutes > in size. Frankly, I can't imagine that the average observer is going to > be able to see more at that point than at 200x (50x per inch), when the > Airy disc is about 3.5 arcminutes across. Well, I would hate to define an "average observer." But it seems quite indisputable that people sometimes find the optimal magnification to be well beyond the minimum where the Airy disk becomes a palpable circle. I know that I do. One really obvious case -- and the one where ultra-high magnifications are most often used -- is for splitting (or elongating) double stars. When the separation is comparable to the size of the Airy disk, you're essentially viewing fine detail *within* the Airy disk. So it's not surprising that making the Airy disk bigger helps. I've also found ultra-high magnification, up to 80X per inch or so, useful for teasing out some details on the Moon. I can't justify that on theoretical grounds, but I know it's true. Mind you, my eyes are *not* especially good at seeing fine detail -- for instance, splitting the Double-Double naked-eye is unthinkable for me. But yes, in general, Suiter's point is a good one. When people boast about the high magnifications that they use, the most natural way to interpret that is that they're "boasting" about how bad their eyes are. I'm much more impressed when somebody tells me how *low* their magnifications are. As I've said before, the real mystery is why higher magnifications sometimes show you less than lower magnifications. Except in cases where the object simply gets too dim, that's really hard to explain on theoretical grounds. But almost everyone agrees that it happens. As the expression goes, the image "breaks down." One clue is that images break down at lower magnifications for low-contrast subjects like Jupiter than for high-contrast subjects like the Moon. One night I actually found myself using higher magnifications to view M82 than I did to view Saturn. If you think about it, that may not be so odd. M82 is viewed mostly with your rods, which have ultra-low acuity, so the poor seeing that caused high-power images of Saturn to be unviewable was just a minor nuisance on M82. - Tony Flanders > Well, I would hate to define an "average observer." But it seems quite > indisputable that people sometimes find the optimal magnification to > be well beyond the minimum where the Airy disk becomes a palpable circle. > I know that I do. I agree. I just think that 6 arcminutes is well, well beyond that point. To be more accurate, I think it's well beyond the point where it has palpable size. Whether it's a circle or not is a separate issue. > One really obvious case -- and the one where ultra-high magnifications > are most often used -- is for splitting (or elongating) double stars. > When the separation is comparable to the size of the Airy disk, you're > essentially viewing fine detail *within* the Airy disk. So it's not > surprising that making the Airy disk bigger helps. I think here you answer your question about why sometimes high power hurts. It's because the input contrast is too low. The brightness gradient drops in proportion with the increase in power, making it harder to see contrast differences. Brian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt It's an interesting historical note that Herschel, (no i am not that old) observed doubles up to 6000 X " By enlarging the aperture of the telescope we increase the evil that attends magnifying" said he, once upon a time. The book, The History of the Telescope by King goes on to say that he seldom used powers above 200. This was a telescope that was hand cranked by servants. Reading a little further, "He was of the opinion that if the if the object had sufficient light 'as stars undoubtedly have' there was no reason 'why we should limit the powers of our instruments by any theory' (1781) Now we have " improved theories" of why we limit power. Some folks like em big, some folks like em small, I just look at the pocket book. or something like that, (James Brown) dan >>Well, I would hate to define an "average observer." But it seems quite >>indisputable that people sometimes find the optimal magnification to [quoted text clipped - 22 lines] > The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ > My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt "Dan Mckenna" wrote > Some folks like em big, some folks like em small, I just look > at the pocket book. > > or something like that, (James Brown) "Money won't change you" >Reading a little further, > [quoted text clipped - 10 lines] > >dan Dan: Interesting to see King quoted. I just bought The History of the Telescope yesterday at Barnes and Noble, a good read. I am already at page 136 and read your passages on Herschel just a couple of hours ago.. Definitely a good read. Myself, normally I operate at relatively low magnifications, I like bright images most of the time. But for double stars I like playing around with stacking barlows just for the fun of it. With my favorite little 130mm F5 Newtonian I have had it up over 1200X on occasion checking out obvious splits like the double-double. The airy disks are still nice and round.. But for viewing planets, the 250X-300X seems to be the maximum, definitely in the 50X/inch range. jon >>Reading a little further, >> [quoted text clipped - 18 lines] > > Definitely a good read. Some folks get dehydrated when they read it. I found it hard to put down as it answered so many questions about the evolution of the telescope. > Myself, normally I operate at relatively low magnifications, I like bright > images most of the time. [quoted text clipped - 3 lines] > 1200X on occasion checking out obvious splits like the double-double. The airy > disks are still nice and round.. Most of the Pro double star observers I knew... 20 so odd years ago that used an eyepiece micrometer liked as much power as they could get. They talked about getting to know the disk of each telescope they worked on and could pull out separations and position angles from close pairs under not so good seeing. Lots of skill there. dan > But for viewing planets, the 250X-300X seems to be the maximum, definitely in > the 50X/inch range. > > jon >Some folks get dehydrated when they read it. I found it hard to put down >as it answered so many questions about the evolution of the telescope. Dan: A couple of section dragged a bit and I could see how it could be dry for someone not already interested in the topic but for me, its a thriller. jon >>With my favorite little 130mm F5 Newtonian I have had it up over 1200X on occasion checking out obvious splits like the double-double. The airy disks are still nice and round..>> There's no reason why they should not be round at any power, even 10,000x. The Airy disc will simply be bigger, but will have exactly the same shape is it does at 100x. Roland Christen > There's no reason why they should not be round at any power, even 10,000x. The > Airy disc will simply be bigger, but will have exactly the same shape is it > does at 100x. That's true. With my scope they stay round until I get to 10,002x. After that point they are rectangular. SignatureJoe Bergeron http://www.joebergeron.com >>That's true. With my scope they stay round until I get to 10,002x. After that point they are rectangular.>> I assume that means at 10,003x? Rolando >>>That's true. With my scope they stay round until I get to 10,002x. >After that point they are rectangular.>> > >I assume that means at 10,003x? I think something got lost in the translation to 7-bit ASCII, surely he must have meant... 100 squared. Ha ha. Sorry. Tim SignatureFoo. > I assume that means at 10,003x? Actually, I can't verify that because I don't yet have a Nagler Type 6 0.141157652704mm eyepiece. I do of course have a 0.114143542583mm, and star images are definitely rectangular at 10,004x and up. SignatureJoe Bergeron http://www.joebergeron.com > Actually, I can't verify that because I don't yet have a Nagler Type 6 > 0.141157652704mm eyepiece. I do of course have a 0.114143542583mm, and > star images are definitely rectangular at 10,004x and up. Correction! I meant I have a 0.141143542583mm. I hope no one ran out to order the wrong eyepiece because of my error. SignatureJoe Bergeron http://www.joebergeron.com >>>With my favorite little 130mm F5 Newtonian I have had it up over >1200X on occasion checking out obvious splits like the double-double. The airy [quoted text clipped - 5 lines] > >Roland Christen And yet you have "experienced" observers talking about observing double stars at 600x in a 4" scope and the image "not breaking down." I wondered for awhile what the observer thought would happen if the image were breaking down. -Rich >Subject: Re: The "It supports 100x per inch" argument >From: RichA none@none.com >Date: 11/24/2004 7:20 PM Eastern Standard Time >Message-id: <g89aq018lvegauoe80ia9ob3go3saaa7lu@4ax.com> >And yet you have "experienced" observers talking about observing >double stars at 600x in a 4" scope and the image "not breaking down." >I wondered for awhile what the observer thought would happen if >the image were breaking down. >-Rich *********************************** And people think i'm crazy when i use over 1000x on the planets!!! Chas P. >And yet you have "experienced" observers talking about observing >double stars at 600x in a 4" scope and the image "not breaking down." The image never "breaks down" The image is suspended in air just in front of the eyepiece, and it never changes regardless if you examine it with a 25mm, a 2.5mm or a 0.25mm eyepiece. Rolando >Subject: Re: The "It supports 100x per inch" argument >From: chris1011@aol.com (Chris1011) >Date: 11/24/2004 8:06 PM Eastern Standard Time >Message-id: <20041124200652.22033.00001089@mb-m06.aol.com> >The image never "breaks down" The image is suspended in air just in front of >the eyepiece, and it never changes regardless if you examine it with a 25mm, >a >2.5mm or a 0.25mm eyepiece. > >Rolando ******************************** If we could take seeing out of the picture just think what you could see at super high powers... Chas P. >>Subject: Re: The "It supports 100x per inch" argument >>From: chris1011@aol.com (Chris1011) [quoted text clipped - 5 lines] >>a >>2.5mm or a 0.25mm eyepiece. >If we could take seeing out of the picture just think what you could see at >super high powers... You just can't beat a steady view of a big, dim airy disk! Tim SignatureFoo. >Subject: Re: The "It supports 100x per inch" argument >From: Tim Auton tim.auton@uton.groupSexWithoutTheY >Date: 11/24/2004 9:13 PM Eastern Standard Time >Message-id: <4ofaq0t0dvs5oc3vact3hjfr51kbg62cer@4ax.com> >You just can't beat a steady view of a big, dim airy disk! > >Tim ************************** I can care less about a airy disk, just give me a huge image of Jupiter and its moons on a 9+ seeing nite! Nothing like seeing detail on two of Jupiters moons at 1000x + powers. I love to push my scopes to the max... Chas P. >>And yet you have "experienced" observers talking about observing >>double stars at 600x in a 4" scope and the image "not breaking down." [quoted text clipped - 4 lines] > > Rolando To me, "image breakddwon" is a phrase used to describe a situation where a higher magnification shows less detail than a lower one. I use it more to apply to planetary viewing than to double star viewing. I cannot tell you whether the image breakdown, so called, is owing to increasing dimness with increasing magnification or some other cause. If "image breakdown" is not an appropriate term to describe the situation where we see less detail at too high magnifications, it would be very helpful if you could propose a better one. Thanks, Biil Meyers > >>With my favorite little 130mm F5 Newtonian I have had it up over > 1200X on occasion checking out obvious splits like the double-double. The airy [quoted text clipped - 5 lines] > > Roland Christen - when it comes to 1000X the Airy disc getting darker and on its edge you can read the opticalmanufacturer name! (-: Yuri > - when it comes to 1000X the Airy disc getting darker and on its edge > you can read the opticalmanufacturer name! > (-: > Yuri I understand you can see it better if you color the edge with a green felt-tip pen. Brian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt >There's no reason why they should not be round at any power, even 10,000x. >The >Airy disc will simply be bigger, but will have exactly the same shape is it >does at 100x. > >Roland Christen A couple of thoughts: 1. At 100X I cannot see the shape of the Airy Disk. 2. Its nice to have guys like Yuri and yourself comment on this topic, but round airy disks at 1200x in slightly mangled Chinese reflector purchased second hand for $80 are probably less common than in either AP or TEC scopes of similar size. <G> jon >>>With my favorite little 130mm F5 Newtonian I have had it up over > [quoted text clipped - 6 lines] > > Roland Christen .Hi Roland, It seems to me that when I look through a telescope with very good optics, an apo for instance, the stars seem like pinpoints. When I look through a telescope with poor or mediocre optics, some of the SCTs for instance, the images look like tennis balls. When I say that the image of Vega was excellent even at 600x in a particular apo, I mean that to me the image seems "tight" rather than diffuse and blurry. I don't know how to express this perception in terms of the "Airy disk", or other appropriate optical concepts; perhaps you could suggest a proper way to express what I mean. Thanks, Bill Meyers >>>>With my favorite little 130mm F5 Newtonian I have had it up over >> [quoted text clipped - 14 lines] >particular apo, I mean that to me the image seems "tight" rather than >diffuse and blurry. If you have a 4" apo that forms an airy disc, and an 8" SCT that does the same, the stars should not look different when viewed at powers under those which allow you to visibly see the diffraction image as a disk and rings. Provided the seeing will allow both scopes to work at their maximum resolution. People who see "tennis balls" in SCTs are: 1. Looking at stars before the scope has cooled down. 2. Looking at stars during bad seeing. Even with the much brighter diffraction rings caused by the central obstruction of the SCT, the stars will not look "bigger" without cause. -Rich RichA wrote in message ... >>>>>With my favorite little 130mm F5 Newtonian I have had it up over >>> [quoted text clipped - 25 lines] > >-Rich and the answer is : 3- looking at stars with scopes that aren't the falsely advertised Strehl 1.01 , but have 2 waves of spherical aberration ;-) best regards, matt tudor >RichA wrote in message ... >> [quoted text clipped - 37 lines] >best regards, >matt tudor Hopefully, there are no telescopes (except toys) that are afflicted that way today. 1/4 wave is about right for current SCTs and that would have no major effect on the appearance of an in-focus diffraction image or the appearance of stars at low powers. RichA wrote in message ... >>RichA wrote in message ... >>> [quoted text clipped - 44 lines] >in-focus diffraction image or the appearance of stars >at low powers. we were talking about powers that allow you to see the diffraction disk, not low powers . As you noticed, the 2 waves is tongue in cheek to some extent, along the lines of Strehl 1.01 ;-) Anyway, I disagree with you that today's SCT's are consistently better than 1/4 wave instruments. That is wishful thinking . best regards, matt tudor >RichA wrote in message ... >> [quoted text clipped - 56 lines] >best regards, >matt tudor How bad do you think they are? -Rich >>RichA wrote in message ... >>> [quoted text clipped - 59 lines] >How bad do you think they are? >-Rich All over the map, from best case 1/4 wave to worse than 1 wave . Even the best are screwed up by adding a simple accessory like a 2" 90deg diagonal, which adds another 1/4 wave of SA due to pushing the back focus a few inches away from its optimum position. best regards, matt tudor [SCT optics] >All over the map, from best case 1/4 wave to worse than 1 wave . >Even the best are screwed up by adding a simple accessory like a 2" 90deg >diagonal, which adds another 1/4 wave of SA due to pushing the back focus a >few inches away from its optimum position. I had considered there must be an optimum mirror position (focus) for moving-mirror scopes. Is there a way to find out what it is for your own scope (preferably using tape and cardboard rather than a Zygo!)? Tim SignatureFoo. > [SCT optics] > >All over the map, from best case 1/4 wave to worse than 1 wave . [quoted text clipped - 5 lines] > moving-mirror scopes. Is there a way to find out what it is for your > own scope (preferably using tape and cardboard rather than a Zygo!)? A Ronchi tester, will get you very close. The lines show classic curvature, for a scope that is under, or overcorrected for SA. The 'design' distance, should be the point where SA is minimised. Historically, for Celestron scopes, it was the distance for a 35mm film plane, using their standard imaging adapter setup. This is about 4" behind the rear port, and is also close to the focus point, when using a 1.25" diagonal. It is quite 'salutory' at times to look at the lengths of the optical train on some scopes, and realise just how bad SA will be becoming... Best Wishes > [...] > It is quite 'salutory' at times to look at the lengths of the optical > train on some scopes, and realise just how bad SA will be becoming... 'Twould appear to be a good application of the SAFIX. Anyone use one for imaging (vs. just viewing) yet? > > [...] > > It is quite 'salutory' at times to look at the lengths of the optical > > train on some scopes, and realise just how bad SA will be becoming... > > 'Twould appear to be a good application of the SAFIX. Anyone use one for > imaging (vs. just viewing) yet? Unfortunately, the SAFIX system, is not really suited for imaging. It needs a lot better 'hardware' to support the weights involved reliably, and really a 2" version. Many people have asked for this, but it has not (yet) happened.... Best Wishes >I had considered there must be an optimum mirror position (focus) for >moving-mirror scopes. Is there a way to find out what it is for your >own scope (preferably using tape and cardboard rather than a Zygo!)? Trial and error by star test. Put mirror in position X. Test by moving eyepiece, not turning the focusing knob on the back of the telescope. Move the mirror a little. Test again. Repeat until satisfied or tired of the procedure. > People who see "tennis balls" > in SCTs are: > 1. Looking at stars before the scope has cooled down. > 2. Looking at stars during bad seeing. Or #3. They never collimated it. Clear Skies Chuck Taylor Do you observe the moon? Try http://groups.yahoo.com/group/lunar-observing/ Are you interested in optics? Try http://groups.yahoo.com/group/ATM_Optics_Software/ ************************************ >>> People who see "tennis balls" > in SCTs are: > 1. Looking at stars before the scope has cooled down. > 2. Looking at stars during bad seeing. Or #3. They never collimated it.>> If it's out of collimation, you won't see tennis balls. You'll see seagulls. RC >Subject: Re: The "It supports 100x per inch" argument >From: chris1011@aol.com (Chris1011) [quoted text clipped - 11 lines] > >RC ********************************* Or comets!!! Chas P. >>>>People who see "tennis balls" >> [quoted text clipped - 7 lines] > > RC What, you've never seen a squashed tennis ball? ;-) Next time I'll read all of the previous post before replying! Chuck Taylor Do you observe the moon? Try http://groups.yahoo.com/group/lunar-observing/ Are you interested in optics? Try http://groups.yahoo.com/group/ATM_Optics_Software/ ************************************ >>People who see "tennis balls" >>in SCTs are: >>1. Looking at stars before the scope has cooled down. >>2. Looking at stars during bad seeing. > > Or #3. They never collimated it. Or #4: They are using it to watch tennis from far, far away :-) > It seems to me that when I look through a telescope with very good optics, an apo for instance, the stars seem like pinpoints. When I look through a telescope with poor or mediocre optics, some of the SCTs for instance, the images look like tennis balls.> The image that is formed by the telescope will be the same, regardless of how much it is later amplified by the eyepiece. Of course, if the amplification is too low to show the defects, then these will not be visible to your eye, and the telescope will look "perfect". At higher magnifications the image will seem to fall apart. I guess there is no good way to express it except by saying that the image "breaks down". Possibly one could say that the scope shows visible defects at a certain magnification level. Roland Christen >> For instance, a 4-inch refractor has an Airy disc of about 1 arcsecond >> (a little bigger at half-maximum, I think, but let's ignore that for now). [quoted text clipped - 19 lines] >are *not* especially good at seeing fine detail -- for instance, >splitting the Double-Double naked-eye is unthinkable for me. That could mean anything. If the only problem with your eyes is myopea, it's fully corrected by the telescope's focus. If you mean astigmatism, or some other problem that reduces the resolution of your eye, requiring corrective optics external to the telescope, that's another issue. >But yes, in general, Suiter's point is a good one. When people boast >about the high magnifications that they use, the most natural way to >interpret that is that they're "boasting" about how bad their eyes are. >I'm much more impressed when somebody tells me how *low* their >magnifications are. It's odd, but for a time I thought the reason I couldn't detect any additional detail in even the best apos beyond about 40x per inch might have been MY eyes having some shortcoming. Odd that I find it works the other way around. >As I've said before, the real mystery is why higher magnifications >sometimes show you less than lower magnifications. Except in cases [quoted text clipped - 5 lines] >low-contrast subjects like Jupiter than for high-contrast subjects >like the Moon. Colour and tonality also washes out, and being able to see this often means being able to see more features. I hesitate to refer to this as higher resolution because I don't know if that is the proper wording. But it's one reason why a large scope shows more than a small one, the ability to deliver a greater number of hues and shades. -Rich > I personally find that the average for SCTs from anyone is around 30x/inch, I would reduce that to 20 pi. > while a great refractor such as one with a Jeagers lens will support > 90x/inch on a dead-steady night. You have got to be kidding! Great refractors, socalled, were never built around Jeagers achromats. I know you are young but at the time the only reason most people went to Jeagers was because they were cheaper (than Brandt or D&G etc). Jeagers lenses were considered the "surplus junk" of optical lenses. Today theyare cherished? Hardly. The only reason people have gone to them now or acclaim them, is because for every 1 Brandt achromat there were 700+ Jeagers lenses. Take a look at the polish on an average Jeager's lens sometime and get ready for Pit City. Most of them always had pitting haze. M. Then the crowd you're moving around in is not the same as mine, which includes most of the experienced observers in the Los Angeles area. And I've looked through two different 6-inch Jeagers lenses and got the opinion of other observers who looked at the lens and through them. The evidence we saw does not support your statements. Perhaps you saw a couple of bad examples, badly handled through the years? SignatureSincerely, --- Dave ---------------------------------------------------------------------- It don't mean a thing unless it has that certain "je ne sais quoi" Duke Ellington ---------------------------------------------------------------------- >> I personally find that the average for SCTs from anyone is around >> 30x/inch, [quoted text clipped - 16 lines] > > M. >Then the crowd you're moving around in is not the same as mine, which >includes most of the experienced observers in the Los Angeles area. And >I've looked through two different 6-inch Jeagers lenses and got the opinion >of other observers who looked at the lens and through them. The evidence we >saw does not support your statements. Perhaps you saw a couple of bad >examples, badly handled through the years? I remember looking through a 6" f5 Jaegers lens and seeing three Jupiters. One red and one blue flanking a "sort of" white image. -Rich >Subject: Re: The "It supports 100x per inch" argument >From: RichA none@none.com >Date: 11/20/2004 5:21 PM Eastern Standard Time >Message-id: <spgvp09pnvqrfukobtnijdoks6c1kkim0o@4ax.com> >I remember looking through a 6" f5 Jaegers lens and seeing three >Jupiters. One red and one blue flanking a "sort of" white image. >-Rich ***************************** I remember a colorful nite with a 6" f/10 Jaegers, talk about rainbow colored planets! Give me a Newt anyday... Chas P. >>Then the crowd you're moving around in is not the same as mine, which >>includes most of the experienced observers in the Los Angeles area. And [quoted text clipped - 8 lines] > Jupiters. One red and one blue flanking a "sort of" white image. > -Rich Sounds like miscollimation, or atmospheric refraction. The 6" Jaegers was not a wide field design, and the images suffered if the lens was tilted at all. Jeff > Then the crowd you're moving around in is not the same as mine, which > includes most of the experienced observers in the Los Angeles area. And > I've looked through two different 6-inch Jeagers lenses and got the opinion > of other observers who looked at the lens and through them. The evidence we > saw does not support your statements. Perhaps you saw a couple of bad > examples, badly handled through the years? Well I can think of five f/15 6"ers right now that exemplify what Im talking about, all being strored in junk piles because of excess colouration and pittstoplasmosis (pitting). Wannah buy em? $1500 each! Bargain prices. If you think I am kidding just make out a certified check and I "will" gather them up and have them at your door within 30 days, justin time for Xmas. Reply here and I'll do the rest. THIS IS A SERIOUS OFFER> Merry Everything. > -- > Sincerely, [quoted text clipped - 25 lines] > > > > M. Let's see some of the fine photography done with your (or anyone's) Jeagers 6" achromats? Got any? I can show you some, taken 15 days ago. Mook > Then the crowd you're moving around in is not the same as mine, which > includes most of the experienced observers in the Los Angeles area. And [quoted text clipped - 31 lines] > > > > M. Hows the water there? Any riots and mass starvation yet? ~~~~~~~~~~~~ > Then the crowd you're moving around in is not the same as mine, which > includes most of the experienced observers in the Los Angeles area. And [quoted text clipped - 31 lines] > > > > M. >> I personally find that the average for SCTs from anyone is around >> 30x/inch, [quoted text clipped - 16 lines] > > M. I bought two Jaegers lenses, a 4" f/15 in 1972, and a 6" f/15 in the early eighties. I would not sell the four for anything, it is about as perfect a standard achromat as I've ever seen. I've had a lot of pleasurable years using it on some difficult doubles (for a 4") including one of the best views of the companion of antares that I have seen. I have also taken some killer solar eclipse shots with it from as far away as Bolivia. My prize winning shot in Astronomy magazine in 1991 was taken with this lens. The 6" was also a sharp lens visually, but did have significant color; better than some other 6"achromats that I've looked through, worse than others. (all f/15 of course) It also suffered from ghost images on bright objects. Both of these lenses had a smooth polish on all surfaces. The 4" has a wide spacing between the elements, the 6" was a Fraunhofer design with foil spacers between the lens elements. At my work, we have a half dozen Jaegers 6" lenses from f/8 to f/15. We use them for collimators and relay lenses in bench set-ups. I have ronchi tested them, and found them all to have excellent spherical correction, and fairly smooth figures. ( I didn't use these lenses visually, so can't comment on their color correction) Of the nearly dozen Jaegers objectives that I have looked through or tested, I have found them to be of good to excellent quality. That dosn't mean that there aren't poor ones out there, but I havn't seen one. Jeff Schroeder > Meade, in one of their latest pieces of literature claims > a maximum power of 600x for one of their 8" SCTs. That certainly must be a mistake on the part of Meade, they claim nearly 600x for their 2"-3" imported junk scopes, using the same formula their 8" SCT should go up to nearly 2000x. Thanks, Vahe >That certainly must be a mistake on the part of Meade, they claim >nearly 600x for their 2"-3" imported junk scopes, using the same >formula their 8" SCT should go up to nearly 2000x. > >Thanks, The 60mm Meade I just bought at WalMart made no such claims. It also came with two 1.25 inch eyepieces and a barlow that provide 28x, 56x, 78x and 156x, a reasonable range. The box had no color pictures from Hubble, rather they made some quite reasonable claims, the Orion Nebula, Jupiter's moon, the ring struture of Saturn. And, even with the shaky mount, this $50 telescope in my hands has shown far more, including M1 from my light polluted back yard and M76 from a moderately dark site. jon <snip> According to Suiter, people who perceive detail gains > when going to magnifications above that actually are letting > the optical system compensate for a lack of normal visual > acuity. I believe the referenced section appears at pp. 240-241 of Suiter's _Star Testing_: "The human eye is not even close to diffraction-limited. An eye with a 3-mm iris opening . . . can theorectically recolve lines separated by 0.6 arcminutes. . . . . Ironically, some people boast about telescopes that can 'withstand more than 100 power/inch' . . . What they don't realize is that they're not bragging about the telescopes. They are inadvertently admitting the poor quality of their own visual acuities. When using extremely high magnifications beyond 100/inch., the diffraction disk appears bigger than two-thirds the angular diameter of the full Moon." Although the image is inherently blurred by diffractions at extremely high magnifications, the residual constrast in some extended objects can sometimes allow the observer to construct a mental picture of detail seen at lower, but focused, magnifications. Last week, I was walking a friend through the Orion nebula with an f/8 6" Newt going from 25x down to the highest power available with my eyepiece and barlow combinations - around 500x (or around 80x per inch). This was to demonstrate for the newbie how extended objects dim with increasing magnification and how diffraction blurs the image at high powers due to the wave property of light. The newbie's comment at the highest magnifications (80x per inch) the images were less interesting because they were blurry, as compared to the images using the traditional 50x per inch limit. My own impression was that although horribly blurred by diffraction, the 500x (80x per inch) image was still useful to the intermediate or advanced observer. At 80x per inch, dark cloud contrasts remained in the blurred image that helped in constructing a mental picture of nebula structures that could not be interpreted fully at the traditional 50x per inch magnification. The same applied to the Saturn observed on that same night. At the traditional 50x per inch limit, cloud belts could not be seen on Saturn's disk even after applying planetary filters. At 80x per inch, a blurred image of the disk was seen, and with a planetary filter, the resulting contrast was sufficient to image a cloud band, although, due to diffraction dispersion, not detail could be seen. You frame the issue as "Do some inexperienced observers fail to distinguish between poor eyesight and diffraction?" An alternative framing might be: "How can experienced observers make the best use of light considering the diffraction limitations of their individual scopes?" - Canopus >I've often wondered about this. What I've found with >decent scopes is that they tend to provide maximum detail at [quoted text clipped - 8 lines] >under perfect conditions this may be usable, I doubt you'd >see much beyond 400x with any 8" SCT anytime. In my opinion, any single number given by anyone is misleading and probably doesn't mean very much. Even for the same person, "maximum" magnification, even for a perfect scope, will generally vary according to what's being looked at, what the exit pupil size is (impacts brightness of the object), how much the cones are involved, etc., etc. I haven't conducted scientific tests, but I've noticed that different people will prefer different magnifications in the same scope when looking at Saturn or Jupiter, for example, in a search for maximum detail. That this should make sense can be seen in the number of people who buy very large projection TV sets with pictures where the scan lines are clearly visible - a condition that runs me nuts when trying to watch. If there are enough people in the house, it would be interesting to run an experiment like the following: display some various types of images on the computer screen and let the people zoom them to the size where they think the most detail is visible. I'll bet the size will vary by a not-insignificant amount from person to person. This is the case for my limited sample size of two people. Zane |
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