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Anonymous
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This is a continuation of the string on the small equipment forum about ETX and using 2" eyepieces. Here is the link to that string:
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=1,2,3,4,5,8,9,10&Board=small&Number=26450&page=0&view=collapsed&sb=5&o=&fpart=1
One question to start with. Why does the TV Powermate 2" barlows use 17mm clear aperture bottom element. Will these work with their 31- 41mm SWA eyepieces. If you can answer this question we will move on.
Edited by Siebert (12/10/03 04:05 PM)
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DaveC
Pooh-Bah
Reged: 11/27/03
Posts: 1316
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ya know this would be an excellent item to review on cloudy nights. that way actual tests can show actual results 
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Anonymous
Unregistered
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Okay, against my better judgement, I'll bite. It has a 17mm clear aperture because it is a four power powermate that both vastly expands the diameter of the light cone, and vastly reduces its overall brightness. Brightness (or light fall off, which is more appropriate in this case), varies with the square of the area. If (and I don't have the Powermate, so I'll take your word for it) the powermate expands the light cone to the degree necessary to provide a fully-illuminated field of view to the 31-41mm eyepieces, by, say, doubling the diameter of the light cone to 34mm (the minimum, I'd say), the lightcone brightness will fall by a factor of four. And you'd have an unvignetted, albiet very dim, image with an effective focal length of 7.75mm to 10.25mm. So are you claiming that you've developed a 2" 160mm eyepiece coupled to a 4x powermate or barlow to give a wide-field 40mm 2" eyepiece that will take a 1 1/4" lightcone and "expand" it to go through a 2" eyepiece? And you lose how much light doing that? Let's see...  ...1.25^2/2^2=0.39, so your "unvignetted" image is only 40% as bright through your 40mm wide-field as it would be through a 40mm 43 degree AFoV Plossl. You're taking a narrow light cone and spreading it over a larger area of your eyeball, making it dimmer. So I apologize for saying it can't be done without vignetting. It can be done without vignetting, but you lose half the image brightness doing it. And I think that about wraps this up.
Regards.
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Anonymous
Unregistered
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Hi Warpd,
Not exactly wrapped up. But even though the question on the powermates had absolutely nothing to do with the discussion of the 2" adapter, you have demonstrated the capacity for critical thinking by figuring it out. Something that was not apparent before to me. Before you sounded very much to me like an individual who said it was impossible to fly or that splitting the atom would blow up the world. And basing your strong stated conclusions on the simple one test you did was in no way conclusive. You were mistaking the eyepiece's inability to take advantage of the entire illuminated field for a scope's limitation. The line between both of these is far more hazy. And you might quickly dismiss the Excelsis comments which are extremely slow in coming these days (I must have writers block - based on your assertions), when it is far easier to accept a bad review without question. Now the answer:
The 17mm element clear aperture can be effectively doubled or tripled depending on the magnification factor. But the bottom line is when does the appearance of vignetting become objectionable? That is a separate issue from the last question. In order to be objectionable it has to be seen first. And the position of the restricting clear aperture is a major part of this issue. So a restricted clear aperture of 31.75mm's at the field stop would understandably create a perfect dark zone outside with eyepieces from 31mm's on up. But what happens if that restricted clear aperture is sufficiently far away (3" down into the eyepiece tube)to completely mask any appearance of vignetting at all but the very edges. There will be a reduction in light at the outer edges as you gradually get to the edge. But it is perfectly blended into almost the end of the 39-44mm field stop and is impossible to see. This is more important than the fact that it exists. Let's say that your vignetting starts at the outer 20%-30% and falls off by as much as 50% at the last 10% of the viewable fov. This of course is perfectly blended in and gradual and would take the most critical observer to see it. Now why does it only work with my 36mm and 40mm? Because some optical systems deal differently with the incoming light cone. And the particular design of the 36mm and 40mm take full advantage of the entire possible illuminated area. This is particularly important in slower scopes from f/10-f/15 and these eyepieces will provide a wider illuminated area in large SCT's where other eyepieces may show considerable vignetting. So these eyepieces work in conjunction to make up for this limitation of this particular scopes optical design. 
I have developed a number of products that were considered impossible to make. I developed the technology that is now the basis of the OCA/OCS series for binoviewers which was laughed at prior as impossible and the only existing totally 2" binoviewers, to only name two things. I am not trying to brag, but want to make a point. Some people just need to get out of the box. Remember what one famous man said, "Invention is 1% inspiration and 99% perspiration". Math is only the start of the process and gets you nowhere without tons of experimentation and testing.
Harry Siebert
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 911
Loc: NYC
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Hello Harry,
Would you be able to say if this setup would also work in the Orion maks, I have the 102mm? Also, would you know if any other ep brand/design would be able to accomplish the same, with just the adapter? I ask because I'm currently, financially restricted, and hope that one of my existing eps will work for me? Do you need to use a barlow to make this work, and if not, WILL a barlow work to increase magnification as well as add the larger "apparent" FOV?
Thanks for any info you have time to pass on!
Regards,
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Anonymous
Unregistered
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Hi Joe,
This has been done a couple of times for Orion Maks with a diagonal. In this case the original 1.25" eyepiece diagonal was used and modified with a 2" holder. But ideally if a 2" diagonal could be used it would be even better since it pushes the restricted aperture further away from the eyepiece. This would require the visual back of the scope and a 2" diagonal. Then machining a low profile adapter to connect the two. This may be a more expensive since you have to purchase a 2" diagonal. So most opt for sending their 1.25" diagonal for fitting. Very little difference is seen so this may be the better way. You very possibly could use other eyepieces, but I am untirely uncertain which one would use the incoming light as efficiently. The 36mm was specifically designed eliminate blackening out and vignetting and maybe hard to find in another eyepiece. You may ask the posters on the group mentioned who tried the experiment. They may be able to tell you which eyepieces they used. I wish I could be more helpful in this regard.
Harry Siebert
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Anonymous
Unregistered
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Here are some picture illustrations taken on my ETX 90. The eyepieces used are left a Meade 32mm Superplossl 4000 series and on the right Siebert 36mm Observatory. You may also want to pay attention to the extra correction the 36mm has at the edge. You will notice a small amount of brightness dropoff to the end of a well defined fov, but you will notice it on both eyepieces which indicates the camera as accentuating the center edge contrast difference. You will have to excuse the difference in brightness between the two pictures since my digital camera shutter speed cannot be locked and the shutter speeds were different which had a drastic effect on overall brightness. Any brightness reduction at the edge is much less noticeable than is dramatized in the photograph. This goes for both eyepieces. But if you compare the two eyepieces it becomes obvious that one does not appear to vignette more than the other. Although the 36mm has a much wider afov. Any feedback will be appreciated and if need be the photographs can be retaken with an eye to greater detail when controling the camera's variables. But I think the illustration is workable.
Harry Siebert
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Anonymous
Unregistered
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Nice photos, Mr. Siebert.
Two problems: First, as we've already discussed in the previous thread, we both agree the 32mm Plossl vignettes with the ETX. So the fact that it vignettes here is not a surprise. And anybody looking at the picture can see that the 36mm eyepiece vignettes more. Second, the "test" itself is not really valid. Any camera system, 35mm, digital, medium format, will show some vignetting at the edges of the field of view. It is inherent in the design of the lens system. Go to a photo thread or look up lens review articles in the major camera magazines. Don't take my word for it. I believe the generally accepted level of vignetting (light loss) across a camera frame is 1/2 stop, about 50%, from center to corner.
As we are not discussing light-loss in the eyepiece, but rather vignetting caused by the 1 1/4" constriction in the 2" adapter sold by Mr. Siebert, a more appropriate test would be the following: Using a camera without a lens (to reduce the vignetting of the lens/camera system), take a prime focus photo of an evenly illuminated subject (clear blue sky, or, even better, clear blue sky through a sheet of diffusion foam) with a 1 1/4" adapter, and with Mr. Siebert's 2" adapter. Use a 35mm film camera which, conveniently enough, has a 43mm diagonal which roughly equates to the field-stop of a wide-angle, low-power 2" eyepiece. Contact print the film (or scan the negatives or slides) so as to avoid light losses inherent in an enlarger system. Publish those, with markers indicating the image circle of the 1 1/4" adapter (with the 2" adapter being full-frame). We should see vignetting.
Other than the silly little test, above, which I apparently passed, Mr. Siebert has not refuted my claim: Using a 2" wide-field eyepiece in 2" adapter on a 1 1/4" visual back (the ETX) will cause vignetting. In fact, if you read Mr. Siebert's "grading" of my test question, you will see that he agrees with me, hiding this in the "perfectly blended" argument. Yes, it won't be obvious to an inexperienced observer, but it will be there. Mr. Siebert, through meadeguy's reference to the Weasner ETX site, claims 100% illumination. As I've said, that claim is fallacious.
What is my interest in this? None, really, except that I feel a certain, limited obligation to the observing community, and think that manufacturer's have a responsibility to meet their claims. If Mr. Siebert were claiming a useable field of view, with some light loss, I wouldn't have a problem. Then it becomes the buyer's responsibility. Mr. Siebert obviously agrees with what meadeguy excerpted from the Weasner site, or he wouldn't be defending it. That is, 100% illumination with no apparent light loss. So, which is it? 100% illumination, or some light loss not apparent to an inexperienced observer, maske by the perfect blending?
I find this tiresome, really, and will heed the advise of my betters.
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Anonymous
Unregistered
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If the pictures I took are prone toward vignetting because of the camera lenses, as I indicated in my post.
"Any brightness reduction at the edge is much less noticeable than is dramatized in the photograph"
Then the real images will be brighter at the edges, as I indicated.
You can in no way see a reduction in brightness in the eyepiece itself.
But even if it is just as the photos show, this would be a dramatic benefit to the ETX. I think here a picture is worth a thousand words. Your argument states you looked at the pictures but your reasoning seems to exclude what the pictures show. You argument fails to satisfy. We will let the photos speak for themselves
In this context 100% Illumination means the entire FOV is illuminated (No donut effect you described).
This devise may not be for everyone and if knowing that some vignetting is there bothers you despite not being able to see it like Mr. Warped by all means don't bother with it.
But I guarantee you the viewable images through the ETX 90-125 will outmode in terms of brightness and edge correction thoughts of a 80mm refractor. And with a much more comparable true field then possible before.
Harry Siebert
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Anonymous
Unregistered
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Hmm..im thinking about selling my etx-125(fork is extremely low quality) and picking up a orion 102mm mak. That would mean a 2.15 deg. field of view with the 40mm, WOW! The only thing id be concerned about there is the weight of the adapter and that huge 40mm affecting the sturdiness and balance of the 102mm on the eq mount.
Edited by meadeguy (12/13/03 03:20 AM)
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 911
Loc: NYC
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Quote:
.....In this case the original 1.25" eyepiece diagonal was used and modified with a 2" holder. But ideally if a 2" diagonal could be used it would be even better since it pushes the restricted aperture further away from the eyepiece. This would require the visual back of the scope and a 2" diagonal. Then machining a low profile adapter to connect the two. This may be a more expensive since you have to purchase a 2" diagonal......The 36mm was specifically designed eliminate blackening out and vignetting and maybe hard to find in another eyepiece.
Thanks for the response Harry,
Would a 1.25" to 2" adapter, with a 2" diagonal function in the same way? That is the setup the folks in the yahoo group had used. I can't recall what eps they used exactly, one was some 40mm ep that was somewhat obscure, and I don't remember the others. I already have a 2" diagonal for my 102mm f5 refractor. Using your 36mm, is barlowing a workable option for higher mag with wider FOV?
I don't find vignetting of a 40mm plossl in the 102mm mak to be objectionable, so I probably wouldn't find the vignetting in the 2" setup any more offensive. I see in your photos that the 2" setup does cause "more" vignetting,(to my untrained and inexperienced eyes, the vignetting actually looks to be quite proportional in the two FOV's, with the wider FOV probably being more desireable, as well as having, again; to my eyes, a larger "fully illuminated" FOV than the 1.25" shot) but the huge increase in FOV would be useful for framing objects, in an inherently narrow FOV scope, where most of the time the object of interest itself doesn't even fit in the normal FOV. Since I do most of my careful observing/studying in the centeral part of the FOV, slightly dimmer edges aren't a concern.
Sorry for bugging you, and thanks for the help!!
Happy Holidays!!
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Anonymous
Unregistered
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Total weight of adaptor and 36mm eyepiece is 1 LB.
Harry Siebert
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Anonymous
Unregistered
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Hi Joe,
You certainly can barlow and effectively eliminate vignetting altogether since the increase in magnification will eliminate the outer edges and focus in on the completely illuminated area. But one thing needs to be done. The higher the f/ratio of the scope the more drastic the effect a barlow will have on the eye relief and fov of a 2" eyepiece since it increases the focal length even more and can turn a f/10 scope into an f/20 or f/30. So a powermate series barlow must be used to maintain a full field of view and to maintain your eye relief. A Siebert equivilent Ultra Barlow will have a similar effect. Negative achromats are notorous for raising your eye relief and narrowing your apparent fov. It's 4 element design incorporates a pupil correcting large doublet that is equal in diameter to the size of the field stop on the lowest power 2" eyepiece (roughly 42mm's). Powermates will be the same and have a large upper element. A few added fringe benefits to the powermate series type barlows is a telecentric image which reduces image degredation and has a positive impact on photography applications. These are things that simple telenegative barlows can't do. Telenegative meaning 2 element achromat or 3 element apochromatic barlows. In my tests with the ETX 90 40mm 2" eyepieces and x2-x4 telecentric barlows a full field of view was almost always achieved. If this post was not easily understood, I will clarify.
Harry Siebert
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 911
Loc: NYC
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Thanks Harry,
No clarification necessary, I have a 2.5X 1.25" powermate, and now I hardly ever touch my barlow.
Thanks again, and Happy Holidays!!
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3578
Loc: "Metrowest" Boston
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Quote:
Using a 2" wide-field eyepiece in 2" adapter on a 1 1/4" visual back (the ETX) will cause vignetting. [snip] Mr. Siebert, through meadeguy's reference to the Weasner ETX site, claims 100% illumination. As I've said, that claim is fallacious.
Neither the 2" adapter nor a 2" eyepiece is likely to CAUSE vignetting. Rather, vignetting is caused by the ETX itself, which is incapable of providing 100% illumination of even a 27mm field stop. Without doing any measurements I suspect the 100% illuminated field is about what the supplied 26mm Plossl will show, say about 22mm or less. This really has nothing at all to do with the eyepiece holder or visual back design. Rather, it follows from the design of the primary, secondary, and baffles. As I think has been pointed out, this isn't a prohibitive issue. Conventional wisdom in constructing Newts is that a 70% illumination level at the edge of a widest true field is acceptable, but I've found that neither the 50% level at the edge of a widest-true-field 2" eyepiece in my 10" Teleport nor even the 20% level in my 7" f/6.7 Newt prevent me from enjoying the use of an eyepiece that shows the entire available field.
Possibly what is at play here is a correct use of the terms "100% illuminated" and "vignetting" versus colloquial use of the terms. Vignetting, in the most correct usage of the term, is a foregone conclusion with the ETX90 and any field stop larger than about 22mm (or whatever the diameter of the fully-illuminated area size is). However, many people are only familiar with the term in the context of what tends to happen with long focal length Plossls and short Barlows, i.e. when the edge of field is blacked out, effectively reducing the apparent field of view. The term "vignetting" is still used for this phenomenon even though calling that "vignetting" is a like saying that a dead person is "very ill". Anyway, the point is that there clearly is none of that sort of "vignetting" with at least some 2" eyepieces offering wide true fields.
On the subject of what size field stops may be in whole at least partially illuminated by an ETX90 using Siebert's adapter, I have to agree with Harry that both the ETX itself AND the eyepiece design play roles. The ETX secondary acts like a catoptric Barlow that presents a focal plane with divergence, not unlike what happens with a Barlow. Also, to form a larger focal plane past the 1.25" eyepiece holder requires that the focal plane be moved past (above) the eyepiece holder by some distance. If the eyepiece focal plane is too close to the end of its barrel the focal plane will be clipped by the eyepiece holder for sure. As a result of these factors an eyepiece can add to the existing vignetting problem, causing all illumination to be lost near the edge of field where it would otherwise have been possible with another design to present a partially illuminated view.
I suspect the biggest caveat with the adapter is that some 2" eyepieces will work well and some not-so-well with it, and some might require more back focus than is available. Consequently, if I were to buy one, I'd get one of Siebert's eyepieces known to work well with it.
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Mike
- 7" f/6.7 home-built planetary Newt on an Osypowski EQ platform
- 13mm Ethos (5.9mm w/2x TV Barlow, 4.2mm w/2.8x Klee Barlow)
- Canon 15x50IS and 10x30IS Binoculars
- Bogen 501HDV+028B Tripod
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 911
Loc: NYC
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Mike,
Thank you for your unbiased, and impartial explanation of the situation. My level of knowledge, regarding optics, is rudementary, to say the least. Your explanation is so much more eloquent than anything I could ever come up with, and apparently not motivated by any personal issues, and seems to follow suit with what the guys on the yahoo group ,"OSAO", have found/experienced, using the Orion 127mm mak.
They also were unable to put such a thorough scientific explanation behind their findings, but agreed that, contrary to what most folks expected, the findings showed that a wider FOV was possible with such a scope design. I don't believe they were able to definitely prove whether or not there was any "vignetting", or exactly what percentage of loss there was at the edges, but they stated that; the extra field was "there", it was usable, and it was definitely an improvement over the stock setup.
They didn't use any "specially designed" adapter, just a regular old 1.25" to 2", slide-in, celestron adapter, and a few different, 2", 40mm or so eps, I can't remember the ep brands or designs.
Now for a few questions for us optical philestines; I think I understand how the focal plane of the scope needs to be back further, into the ep, so it essentially is a larger circle of light, beyond the focal point(I'm trying to use simpler terms to understand this) am I close on this?
Now assuming I'm close to being right...lol, Ultimately, the size of this focal plane is limited by the opening it has to pass through, meaning back at the "adapter", and/or the scope's visual back, which is essentially the inside of a 1.25" barrel, and so the focal plane, after the focal point, can be no larger than the diameter of the smallest opening it passed through back before the focal point?
Is this focal plane, past the point of focus, the area that's 100% illuminated? Or is the area of 100% illumination somewhere within this this focal plane?
Thanks for now, I'll have more questions after getting some answers to these.
Thanks again, & Happy Newyear!!
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Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3578
Loc: "Metrowest" Boston
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Quote:
Now assuming I'm close to being right...lol, Ultimately, the size of this focal plane is limited by the opening it has to pass through, meaning back at the "adapter", and/or the scope's visual back, which is essentially the inside of a 1.25" barrel, and so the focal plane, after the focal point, can be no larger than the diameter of the smallest opening it passed through back before the focal point?
This question really is at the heart of the matter, and the answer is that the size of the focal plane is NOT limited to the diameter of the smallest constriction in the light path. That's kind of a plumber's insight rather than an optician's. Remember that each point in the view is formed by its own converging cone of light, and each cone has its pointed tip at the focal plane. With a slow scope the shape of these cones is quite "pointy" so that as they near the focal plane, they are each quite narrow in diameter. That is to say, for a given point in the view, ALL the light for that particular point has already been concentrated down to quite a small area at each cross-section anywhere near the focal plane.
Perhaps it will give you some insight to think of holding a 1" length of pipe with an inner diameter of 1.25". Then holding the small section pipe over a piece of paper (some specified short distance above it) think about what size circle you can draw on the paper with a pencil through the pipe. With the paper flush against the end of the pipe, you can only draw a circle as wide as the inner diameter of the pipe, but as you move the pipe a little farther from the paper, you can reach wider and wider points with the pencil. The pencil represents different light cones as you move it around and draw your circle. That's the basic idea, anyway. There's a lot more going on in a Cassegrain-type scope, but Rome wasn't built in a day.
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Mike
- 7" f/6.7 home-built planetary Newt on an Osypowski EQ platform
- 13mm Ethos (5.9mm w/2x TV Barlow, 4.2mm w/2.8x Klee Barlow)
- Canon 15x50IS and 10x30IS Binoculars
- Bogen 501HDV+028B Tripod
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IDONTSEEIT
scholastic sledgehammer
Reged: 05/04/03
Posts: 911
Loc: NYC
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Thanks Mike,
So what you're saying is, since at the point of smallest constriction, there are partial, as well as whole lightcones, for each point on the objective/secondary, coming through and forming the "one" cumulative lightcone, these partial lightcones are what make up the outer, "vignetted" portion of the FOV visible through the ep.
Therefore, if one were to use an ep, that allowed the now diverging lightcone(after the actual focal point of the objective/secondary)to expand enough before processing it into a viewable image, that the resulting image will provide an overall wider FOV, albeit "vignetted" to some extent(which may or may not be noticeable or objectionable), at the outer perimeter? And said ep would be one with a larger fieldstop than the 1.25" constriction, and have a correctly positioned fieldlens to achieve such a feat?
So basically, if I were to get the 1.25" to 2" adapter, and used a properly sized(fieldstop diameter and such), designed(correctly placed fieldlens), and positioned(attained by focusing), 2" ep, as well as a 2" diagonal, I could get what in essence would/may be an acceptable, "larger" FOV out of my 4" mak, albeit with some lightloss at the edges("vignetting"), which may not be noticeable, or at least, not objectionable?
Also, being that to accomplish this "feat", you need to do an unusual amount of focus adjustment of the scope(moving of the primary), how would this affect the overall focal length of the scope, if enough focusing range is even available, and how would that affect the lightcone and the magnification?
Thanks again for your time and patience!!
--------------------
Joe,
C8 OTA on UA Unistar Heavy Deluxe
SV-102V on same UA Unistar Heavy Deluxe(not at the same time)
WO-Megrez 80-II ED on UA Microstar Basic
Orion 80ED on same UA Microstar Basic(also not at the same time)
Miyauchi Bs-60ic "Pleiades" 22X60
Orion UltraView 10X50's
Orion MiniGiants 12X63 & 15X63
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3578
Loc: "Metrowest" Boston
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Yes to the first paragraph. I think yes to the second, but just to clarify, when I talk about moving the focal plane of the ETX90, I'm speaking of using the focus mechanism to move the primary mirror closer to the secondary. This causes the focal plane to be formed farther away from the secondary. In any case you might as well think of an eyepiece as just a magnifier for examining the focal plane. For sure light cones are diverging on the other side of the focal plane, and it's the eyepiece's job to convert them each back into a beam of parallel rays (each light cone corresponds to a beam of parallel rays entering the OTA), but this is another subject, I think.
The reason the focal plane gets wider when it is pushed farther out by focusing is that the light cones are then fatter as they pass through the constriction of the 1.25" eyepiece holder (not to mention the same for every other constriction within the OTA). The part of each light cone that passes within the 1.25" diameter of the eyepiece holder (if any) then can make it through. So some light cones for wide points make it through (partially), whereas before with the focal plane projected somewhere within or very near to the 1.25" eyepiece holder, the corresponding light cones were simply lost in total on the wall of the eyepiece holder (or perhaps somewhere else along the way inside the OTA).
So with the focal plane pushed back past the eyepiece holder, it is indeed wider than the eyepiece holder, but certainly the wider parts of it are not formed using the full aperture of the scope. I don't think this is as obvious as it sounds, and as I noted before, the same is true for many points in the focal plane that are WITHIN the diameter of the eyepiece holder. It's just how the ETX90 and most other Cassegrains are designed.
I don't object to partial illumination at the edges of wide true fields. This is usually par for the course with reflectors that have reasonably sized secondary mirrors. If someone is very sensitive to brightness gradients, they may just have to buy themselves a rich field refractor instead, but for most of us it's okay.
Yes, the focal length of the OTA is longer the more "back focus" is used in SCTs and MCTs that use moving mirror focusing. This does increase the magnification obtained with a given focal length eyepiece. The upshot of this is that a given 1.25" eyepiece will show a little bit less true field due to increased magnification when used in a 1.25" adapter in the 2" adapter than it did in the original 1.25" eyepiece holder straight up.
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Mike
- 7" f/6.7 home-built planetary Newt on an Osypowski EQ platform
- 13mm Ethos (5.9mm w/2x TV Barlow, 4.2mm w/2.8x Klee Barlow)
- Canon 15x50IS and 10x30IS Binoculars
- Bogen 501HDV+028B Tripod
Edited by Mike Hosea (12/30/03 03:10 PM)
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3578
Loc: "Metrowest" Boston
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BTW, Joe, tell your friends on the "OSAO" group that they can do the following to detect vignetting (i.e. partial loss of illumination). Just defocus a bright star in the center of the field so that you can see a good size diffraction pattern. Then pan the star around the view. Where there is any vignetting, the diffraction pattern will be clipped. Try it on either side of focus (because of the effects of focusing that we've just been discussing). It doesn't tell you what is causing the vignetting, but you can see that it is or is not there.
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Mike
- 7" f/6.7 home-built planetary Newt on an Osypowski EQ platform
- 13mm Ethos (5.9mm w/2x TV Barlow, 4.2mm w/2.8x Klee Barlow)
- Canon 15x50IS and 10x30IS Binoculars
- Bogen 501HDV+028B Tripod
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