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#21
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RichA wrote in message
... On Wed, 2 Feb 2005 16:39:48 -0500, "Dan McShane" Un, no. Any obstructed scope with an obstruction under about 15% performs almost like it had no obstruction but, NO reflector I've ever seen produces images like the best apos, inch per inch, unobstructed or not. The use of a "sector" of a larger paraboloid is interesting, but it not a perfect sub for an apo, despite the high price. -Rich Rich, The reality is that many of my customers *do* and have ordered these scopes because they *are* a good sub for an APO and against all but the most top shelf APOs are a tad better. And why shouldn`t they? They do something no APO can in bringing the entire visible spectrum to the same focal point, while also doing something no conventional newt can by doing so with zero effects from CO. Dan McShane That would be fine, if they actually provided something unique, they don't. Huh????? "While the 6" found all of the galaxies, the contrast on the DGM was in another league. It's as if someone "erased" half of the moon. The gains in contrast meant that I could pick out smaller galaxies in the Markarian's Chain region that were barely visible in the Meade. Indeed, all objects were brighter in the 5.5" than in the 6". http://www.scopereviews.com/page1f.html#3 "We looked at the double-double at 275X and all four components were still tight little round balls. The contrast was impressive, too. You could have easily fooled me into thinking I was looking through a world- class 4" apochromatic refractor. I did some quick comparisons with my Traveler. While the AP threw up a slightly brighter image, there was little to choose between the scopes." http://www.scopereviews.com/page1m.html#3 "First, the high level of contrast available from the OA-4 allows me to consistently pick out features on Jupiter that are hard to find when seeing is less than perfect, thereby acting against the larger apertures. Jupiter's 4 main moons are consistently resolved into disks under very good seeing. I have observed numerous transit shadows, and even the transits themselves. Seeing the giant planet's disk so starkly resolved against a black sky is a sight to behold. Lesser telescopes smear the edges, and suffer from glare, false color and, some fuzziness. I remember comparing the OA-4 to a C5 recently. The views of Jupiter and Saturn were worse in every way through the Cat. In other words, the Celestron rendered the images duller, less resolved, and to my surprise, dimmer. When I first got the telescope, I was able to compare it with an older Televue Genesis. It was difficult to compare the planets precisely because of the vastly different focal lengths, but a view of the limb of the Moon told the story the Genesis displayed some color, while the OA-4 was flawless." http://www.cloudynights.com/reviews/oa4.htm "Compared to a conventional 4 inch Chinese achromatic refractor, the difference is laughable. The OA-4.0 exhibits contrast and resolution that is nothing short of extraordinary, giving the lunar features a somewhat surreal three-dimensional quality. The surrounding sky is absolutely black in stark contrast to the lunar surface." http://www.cloudynights.com/reviews/oa42.htm "Recently I've been running comparisons between my old 4.25-inch and the OA. Based on aperture, I expected the OA images to be 2.45 times brighter - about one magnitude - and have 1.53 times the resolution - visibly sharper, but not astoundingly so. But the differences are astounding, especially in dark conditions with good seeing. The Sgr-4 shows Jupiter's two main bands, while the OA at the same magnification shows two more, one to the North and one to the South. With the OA you can see that Io is orange. When the Sgr-4 shows Saturn's "A" and "B" rings, and the Cassini division in the ansae, the OA shows the "Crepe" ring as well, and the Cassini division is visible over the whole ring." http://www.cloudynights.com/reviews4/OTA6.htm "At first glance, *$750* (mounted) seems like too much for a 4" Newt. Look through one, and you won't feel that way, especially when you pit the price/performance against the cost of an apochromat. I can offer no negatives on the 'scope, and can recommend it without hesitation, though I'd have to say I think the OA-5.5 or 7 would be a superlative purchase if finances permit!" *(2001 prices, currently $825)* http://www.excelsis.com/1.0/entryvot...12&entryid=369 "This is my first newtonian reflector scope, and I've been into astronomy after nearly 30 years of absence, so I at first expected I would have some difficulty in collimating it. Fortunately, the expectation has turned out to be the opposite. Reading the "Use and Care Manual" on his web page, the assembly and collimation took me just half an hour. With a 4mm Ortho(yealding 255X), I noticed color differences inside the NEB and SEB on Jupiter. Even though Saturn was relatively low in the west I was able to see the Cassini Division. I'm amazed that even at this magnitude, which is claimed as the maximum on their web site, the image shows no breakdown at all. Lunar work is beyond description. even my wife, who usually shows little interest in stars, is amazed and lost for words." http://www.excelsis.com/1.0/entryvot...12&entryid=369 Nothin unique there.... I guess these folks just need to read SAA alot more often, and get their eyes examined ASAP! :-) Dan McShane |
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Bill,
Your welcome. I am usually at most of the summer SP`s here on the east coast, Stellafane, Rockland SP, Connecticut, Arunah Hill Days, The Junction, etc. clear skies! Dan McShane Jb2269 wrote in message ... Dan, Thanks for the historical notes. I hear the design you manufacture gives high quality images and hope to check it out at a star party one day. Bill Bambrick 41 N, 73 W, 95 ASL |
#23
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Dan McShane wrote:
They have more coma than a same aperture/F/# conventional newt, but no where near what the "parent" 10" F/4 has. The OA-4 (98mm F10.4) has about the same coma as a F/6.5 conventional newt. It is fairly easy to figure out blur size for an off-axis segmant (OAS) mirror with the parent mirror specs. known. Approximately, linear blur length is reduced by the ratio of OAS mirror diameter to the parent mirror radius (in fact, there is little difference here, since an effective blur length in the parent mirror is truncated by c.obstruction), while the blur width is approximately reduced by the ratio of the diameters (OAS vs. parent mirror). In the above example, the OA blur would have ~80% of the original comatic blur length-wise, and ~40% of its width. Since the length/width ratio of the comatic blur is 1.5:1, this means that the OA blur would have it approx. 3:1. Although the OAS aberration has identical origins to that of coma, it is now different blur with different energy re-distribution, and should be called differently. It can't be compared to the parent-mirror coma directly, just as it can't be done with any other type of aberration. The factor that has much more influence on an effective size of aberration than reduction in the linear blur size is larger Airy disc of the OAS. In this example, the Airy disc is 2.6 times larger than that of the parent mirror, making relative size of the blur in the OA as much smaller. Considering different nature of the aberration in the OAS, its exact effect can't be determined by direct comparison of blur sizes. However, for a quick approximation, the increase in Airy disc size should be a good enough basis. In this example, with the Airy disc being 2.6 times larger than for the f/4 parent mirror, the field quality should be similar to that of an f/4*2.6^0.33 or f/5.5 parabola. Vlad |
#24
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"Dan McShane" wrote in message ... RichA wrote in message cShane That would be fine, if they actually provided something unique, they don't. Huh????? Dan, Does the OA 3.6" provide better views than the 10" newt with an OA mask? How do the views compare to the 80ED or 100ED? peace, jon |
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You can have a =ADNewtonian made with a long focal ratio
too. The obstruction (if the scope is for visual use) will be miniscule and the only thing you'll see are diffraction spikes. Well of course, but the lack of obstruction and diffraction spikes are in fact the very reasons that one would consider an OA Newt over a standard Newt. I think there's little dispute that the standard Newtonian far and away offers the most brightness and resolution per unit dollar of any design. But those spikes really *can* hurt in certain situations, such as when observing close companions to bright stars. I'm sure that observing Sirius B would be much easier in an OA Newt than a standard one, despite the poorer resolution. Regarding central obstruction, many on this newsgroup and even Harold Suiter state that the impact is negligible when it's smaller than 20% of the primary's diameter. Then why do APOs consistently outperform Newts inch-for-inch? There are several reasons: - refractive surfaces vs. reflective (less light loss) - no tube currents - not a folded light path Cheers, Ritesh |
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On 3 Feb 2005 12:36:48 -0800, "Axel" wrote:
You can have a *Newtonian made with a long focal ratio too. The obstruction (if the scope is for visual use) will be miniscule and the only thing you'll see are diffraction spikes. Well of course, but the lack of obstruction and diffraction spikes are in fact the very reasons that one would consider an OA Newt over a standard Newt. I think there's little dispute that the standard Newtonian far and away offers the most brightness and resolution per unit dollar of any design. But those spikes really *can* hurt in certain situations, such as when observing close companions to bright stars. I'm sure that observing Sirius B would be much easier in an OA Newt than a standard one, despite the poorer resolution. One of the early Sky and Tel reports of splitting Sirius was accomplished with a C14 using a hexagonal mask that produced diffraction spikes and made it possible (by orientating the mask) to see the companion between the spikes. Cute trick. -Rich |
#27
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Jax wrote in message ... "Dan McShane" wrote in message ... RichA wrote in message cShane That would be fine, if they actually provided something unique, they don't. Huh????? Dan, Does the OA 3.6" provide better views than the 10" newt with an OA mask? How do the views compare to the 80ED or 100ED? peace, jon Hi Jon, As far as the OA mask, provided the mirror has an excellent figure and stays about .25" from the edge of the primary you would likely get a very similar view. Although the caveat here is that a stand alone OA-4 doesn`t take as long to cool and will adjust to falling temps better than the big scope. Also most 10" conventional newts are in the F/5-6 range, not F/4, so the masked "OANs" would be around f/13.5, as are the Orion scopes. Haven`t been able to compare the OA-3.6/4.0 vs. the 100ED, but can comment on the 80ED in comparison to my 90mm OAN. Essentially the OAN was superior in every type of viewing, producing sharper images, higher contrast, and of course perfect color rendition. Wide field views were the only situation where the short FL refractor had an edge. However the ATS configuration (OA-3.6) is no slouch for wide fields despite it`s longer FL. I have one of the budget 30mm 80 degree 2" EPs which performs very well with the scope producing a 2.35 degree FOV. And then on the flip side the longer FL allows for higher mag. without barlows, or absurdly short FL EP`s. On pure speculation I would suspect that the 100ED wouldn`t fair much better assuming the same level of optics quality. Dan McShane |
#28
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Bill Ryan wrote:
"Tim Killian" wrote in message ... It's an off-axis Newtonian design that's been available from several companies. In theory, you get the performance of an APO refractor because there is no obstruction from the secondary mirror. But you also get the coma inherent with any newtonian design and the focal ratio is higher than most APOs these days. This makes it primarily a visual instrument. That's unfortunate. I guess then it would make better sense to spend $350 more for their 100mm apo refractor ota. Bill Depends on two things: 1) The quality of each optically... The mirror in the OA "might" be better figured than the lenses in the ED. Certainly the OA4 from DGM Optics is very, very good (not that Orion is selling the DGM model). 2) Whether you have the $350... If you don't, and you want a high contrast, color free telescope that performs similarly to a 100ED/F9... well there ya go. Realistically, it seems that at least some folks buy such scopes with too high an expectation out of a 4" aperture (I've come full circle). My 102ED/F9 Vixen refractor is a very, very nice scope (and light weight too, at just 8 lbs.), but it is still only a 4" aperture. The advantages and disadvantages of small aperture, long focal length scopes need to be experienced, and I've come to expect that the difference between the OA and the 100ED are very minimal. The reason I've come full circle, is that I started with a 4" F13 Mak that was of questionable quality, and wasn't satisfied with it as an entry level amateur astromer. I then went through a number of other options, as well as experienced what others had purchased, only to come back to a 4" refractor as a primary scope. I still have a 12.5" Dobsonian when needed, but the Vixen 102ED on GP with drives is great for planets, moon, and open cluster observing. So too would the Orion 100ED be, if equal in quality (which it is probably not). The Vixen is easily capable of 70x per inch under excellent seeing. But then, my 12.5" Dob cost me half the price of the Vixen setup, and it can do 280x on planets at a meager 22x per inch (plus a whole lot more). But, in the cold months, it takes hours before it's ready to go, and the temperature drops around here are capable of out pacing the mirror. Nothing like a lightweight, fast cooling solution. Of which, both the OA and the refractor are capable. If the bottom line is price/performance, the OA is likely the winner, but a refractor is generally more trouble free. If the two perform equally. Unfortunately, there's no way to know that (equal quality), without doing a direct comparison. Order them both, keep the better one. You have 30 days (last I checked). -Stephen Paul |
#29
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Stephen Paul wrote in message ... Bill Ryan wrote: "Tim Killian" wrote in message ... It's an off-axis Newtonian design that's been available from several companies. In theory, you get the performance of an APO refractor because there is no obstruction from the secondary mirror. But you also get the coma inherent with any newtonian design and the focal ratio is higher than most APOs these days. This makes it primarily a visual instrument. That's unfortunate. I guess then it would make better sense to spend $350 more for their 100mm apo refractor ota. Bill Depends on two things: 1) The quality of each optically... The mirror in the OA "might" be better figured than the lenses in the ED. Certainly the OA4 from DGM Optics is very, very good (not that Orion is selling the DGM model). 2) Whether you have the $350... If you don't, and you want a high contrast, color free telescope that performs similarly to a 100ED/F9... well there ya go. Realistically, it seems that at least some folks buy such scopes with too high an expectation out of a 4" aperture (I've come full circle). My 102ED/F9 Vixen refractor is a very, very nice scope (and light weight too, at just 8 lbs.), but it is still only a 4" aperture. The advantages and disadvantages of small aperture, long focal length scopes need to be experienced, and I've come to expect that the difference between the OA and the 100ED are very minimal. The reason I've come full circle, is that I started with a 4" F13 Mak that was of questionable quality, and wasn't satisfied with it as an entry level amateur astromer. I then went through a number of other options, as well as experienced what others had purchased, only to come back to a 4" refractor as a primary scope. I still have a 12.5" Dobsonian when needed, but the Vixen 102ED on GP with drives is great for planets, moon, and open cluster observing. So too would the Orion 100ED be, if equal in quality (which it is probably not). The Vixen is easily capable of 70x per inch under excellent seeing. But then, my 12.5" Dob cost me half the price of the Vixen setup, and it can do 280x on planets at a meager 22x per inch (plus a whole lot more). But, in the cold months, it takes hours before it's ready to go, and the temperature drops around here are capable of out pacing the mirror. Nothing like a lightweight, fast cooling solution. Of which, both the OA and the refractor are capable. If the bottom line is price/performance, the OA is likely the winner, but a refractor is generally more trouble free. If the two perform equally. Unfortunately, there's no way to know that (equal quality), without doing a direct comparison. Order them both, keep the better one. You have 30 days (last I checked). -Stephen Paul Stephen, I would fortify the opinion that only a top shelf 4" APO will give an OA-4/3.6 a run the money. I think we saw this when comparing my 90mm ATS vs Paul`s TV(101?), where the consensus seemed to be that the 90mm had a slight edge in sharpness. I haven`t viewed the Orion 100ED, but have with the 80ED, at last years Conjunction and as mentioned previously in this thread, the 90mm ATS was clearly superior, not just brightness, but sharpness and contrast. The other aspect of the 80ED that surprised me was it had a fair bit of off-axis aberrations. Not sure if it was EP related, or just inherent of the F/#. On the other hand it shouldn`t be surprising as many of the fast APO`s must employ some sort of in line corrector/field flattener to produce a wide flat FOV. Dan |
#30
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Dan McShane wrote:
Stephen Paul wrote in message Nothing like a lightweight, fast cooling solution. Of which, both the OA and the refractor are capable. If the bottom line is price/performance, the OA is likely the winner, but a refractor is generally more trouble free. If the two perform equally. Unfortunately, there's no way to know that (equal quality), without doing a direct comparison. Order them both, keep the better one. You have 30 days (last I checked). Stephen, I would fortify the opinion that only a top shelf 4" APO will give an OA-4/3.6 a run the money. Right, and so the question remains whether the Orion OA is better at the eyepiece than the Orion 100ED, and whether the buyer is one who cares more about the optical differences than the differences in design (eyepiece position, maintenance, and mount requirements come to mind). I hope that there's no one reading this who is interpreting my comments as negatively or positively favoring one design over the other as a matter of optical quality. While I do prefer the refractor for the design, I think I'm on record as backing the OA 100% as an optically excellent alternative to a long focus APO of equal aperture. The OA Pros: Excellent optical performance if will implemented. The Cons: Length of OTA and resultant eyepiece position. (Long focus Newts just don't scale well, imposing aperture limits.) -Stephen |
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