escaner
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Reged: 09/25/07
Posts: 22
Loc: Barcelona, Spain
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I know about the tendency of Plossl eyepieces to vignette when barlowed, but I have read that it is mainly for long focal lengths.
Do you know whether the Tele Vue Plossl 8mm would vignette with a TV 2x barlow?
Thank you very much.
Oscar
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Lawrence Sayre
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Loc: N.E. Ohio
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I can't tell you if it will vignette to any significant degree, but I can calculate that the result of this pairing is an effective 3.85 mm FL.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Loren Toole
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Reged: 03/23/04
Posts: 97
Loc: New Mexico USA
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Oscar
I routinely use my TV 8mm with a 2.8X Klee shorty barlow. Admittedly this is somewhat different from the "long tube" barlows that TV sells, however the Klee is notorious for vignetting. In short, I've noticed no vignetting. The 8mm is used nearly exclusively for planets, so field of view or edge illumination isn't really a concern. I often view farther out from the exit pupil than needed to see the full field so I may not notice anyway.
I predict if you see ANY vignetting, it will be so slight as to be of no consequence.
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My binos: Nikon 7x35/8x40, Fuji 10x70, Obie 15x60, Barska 20x80
My scopes: 5"f5 newt, 4"f5 TV Genesis, 3"f9, 3"f5
ARCO OBSERVATORY 7400'
Edited by Loren Toole (08/05/08 09:09 PM)
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RogerRZ
Whatta you lookin' at?
Reged: 01/09/06
Posts: 2130
Loc: West Collette, NB, Canada
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I have not detected any vignetting with my 8mm Plossl, and 1.8x TV Barlow. Eye relief was short, though...
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-Roger Pitre-
1 X 7 binocular
Meade 2080 LX10
5" Celestron SCT
Skywatcher Equinox 66
Starblast OTA
EQ6 Pro mount (spiffed up with ADM stuff)
8X42 Bushnell binos (pretty decent for 25$)
Canon Rebel XT (AKA 350D)
Starshoot autoguider
Tolerant Spousal Unit (AKA The Lovely Mrs. RogerRZ)
----------AJP Observatory----------
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Mike Hosea
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Reged: 09/24/03
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Loc: "Metrowest" Boston
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Quote:
Do you know whether the Tele Vue Plossl 8mm would vignette with a TV 2x barlow?
I've tried it. I don't recall any vignetting with the 8mm and the 1.25" TV 2x Barlow. The 15mm vignettes with that Barlow, however.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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escaner
member
Reged: 09/25/07
Posts: 22
Loc: Barcelona, Spain
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Thank you, guys!
Lawrence, interesting figure. It means that the eyepiece is actually 7.7mm or that the barlow is a bit more than 2x? I was considering an Ultrascopic 7.5mm too, but preferred the slightly longer focal lengh of the Tele Vue.
About the e.r. of this ep, do you think it would be any worse than in a Kokusai Kohki/UO volcano Or.6?
Thank's again.
Oscar
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Lawrence Sayre
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The Barlows working length from principle plane to eyepiece bearing shoulder is 86.5 mm, but its focal length is only -74.4 mm, so it is a 2.163X Barlow. However, the eyepieces focal plane (field stop) is 6.35 mm below its focuser shoulder, so it negates some of the "extra" Barlow length vs. Barlow focal length (and therefore some of the magnification factor) discrepancy, and therefore for this eyepiece the Barlow acts as a 2.0773X.
Barlow Mag = 1 - (86.5/-74.4) = 2.163X
Barlow Mag with eyepiece = 1 - [(86.5 - 6.35)/-74.4] = 2.0773X
8 mm eyepiece/2.0773 = 3.851 mm effective focal length
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
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Posts: 3226
Loc: "Metrowest" Boston
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I might add here that my direct measurements of magnification effect using drift timing with my TV 2x Barlow and eyepieces with known effective focal plane locations have not confirmed Lawrence's figures on the TV 2x. In fact we don't know where the principle plane of the negative achromat used in this Barlow is exactly, and IIRC, that could explain the discrepancy.
To distance yourself from this question, I would suggest drift-timing using the eyepiece with and without the Barlow. As long as you can see the actual eyepiece field stop either way, the ratio of the longer time to the shorter one is the magnification effect. When there is vignetting to the point that there is opaquing of the view in the neighborhood of the field stop, this procedure will overstate the magnification factor. Done carefully, however, it will not understate it.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Lawrence Sayre
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Mike, my 1.25" 2X TeleVue Barlow magnifies appreciably more than does my GSO 2" 2X ED with the same eyepieces. I've checked this on the moon a number of times, gaging the span from crater to crater as my field width guide. Either the TV is greater than 2X, or the GSO is less than 2X, or both. 2.163X represents my adherence to the mid point assumption.
Since the GSO requires that I move the eyepieces a bit farther out due to the need for the 1.25" to 2" adapter, if all else was equal, the GSO should magnify the most. Yet in all cases the field is much less wide in the TV Barlow, indicating that it magnifies far more.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Lawrence Sayre
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Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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And looking at it another way, even if my "mid point of the achromat" assumption for the position of the Barlows principle plane is incorrect, provided that the principle plane resides somewhere within the achromat itself, the TV Barlow still must be a greater than 2X Barlow.
The achromat is roughly 7 mm thick. If I was off in either direction from the mid point as regards the position for the principle plane by 100% (or 3.5mm) then the result would be that the 1.25" 2X TV Barlow is either 2.12X or 2.21X, with both being greater than 2X. The mid point is where I have heard that the principle plane should be calculated from if more specific info is not available. The real answer of course will reside somewhere between 2.12X and 2.21X. 2.163X represents strict adherence to the mid point assumption. These figures are all for the case where an eyepieces focal plane (field stop) is located right at its focuser shoulder.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
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Loc: "Metrowest" Boston
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Interesting, but there are too many unknown variables there to make me doubt my repeated direct measurements. Why don't you just take an eyepiece for which you know the location of the effective focal plane and do the drift test? I'll be very interested to see what numbers you get.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Lawrence Sayre
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Reged: 10/16/04
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Loc: N.E. Ohio
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I find drift tests with a Dob to be nigh on impossible to do correctly (I.E. accurately).
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
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Posts: 3226
Loc: "Metrowest" Boston
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It may be slightly more than 2.0x. I don't remember the numbers. I think the Barlow is set up to be 2.0x (i.e. between 1.95x and 2.05x) when the eyepiece effective focal plane is 0.25" below the barrel flange, e.g. with an 8mm TV Plossl. I agree it's probably closer to 2.05x than 1.95x.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Mike Hosea
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Loc: "Metrowest" Boston
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Quote:
I find drift tests with a Dob to be nigh on impossible to do correctly (I.E. accurately).
It is difficult, but it beats putting the principal plane at the center of the achromat. Talk about rough approximations!
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Lawrence Sayre
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Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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This (1.95X to 2.05X) would require that the principle plane of the achromat is hovering in space, and not to be found within the confines of the achromat.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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KaStern
professor emeritus
Reged: 04/18/06
Posts: 547
Loc: Dortmund
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Hello Oscar,
not exactly what you asked for, but maxbe interesting for other stargazers:
The 11mm Televue Plössl does not vignette with the 3x Televue Barlow.
And it does not vignette (as far as I can see) with the 2.2x Klee Barlow,
nor with the 2.8x Klee Barlow.
I second Mike Hosea and prefer to measure the drift time without barlow
devided by drift time with barlow to conclude the barlow factor.
Clear skies, Karsten
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200/1200mm ATM Dobson
150/1800mm Yolo
114/500mm Newt
80/1470mm Achromat
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Mike Hosea
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Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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Quote:
This (1.95X to 2.05X) would require that the principle plane of the achromat is hovering in space, and not to be found within the confines of the achromat.
This happens with meniscus lenses, but I wouldn't have thought it so here. What is the precise distance from plane of the top of the eyepiece holder to the axial point on the inside surface of the achromat again?
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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Lawrence Sayre
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Reged: 10/16/04
Posts: 3677
Loc: N.E. Ohio
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~83 mm to the front, ~90 mm to the back, and ~86.5 mm to the center. 83 mm being the answer you seek.
PS: I just posted a thread in the "ATM, Optics and DIY" forum, asking how the position of the principle plane is determined for a negative achromat.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3226
Loc: "Metrowest" Boston
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Thanks.
Well, 1 + (83 - 6.35)/74.4 = 2.03x. That's if the principal plane intersects the center point on the inside lens surface. Furthermore, 1 + 83/74.4 = 2.12x, which is not far from my measured 2.10x with the Pentax 12XF. Also note that some catalogs I have reviewed spec'd their negative achromat focal lengths at +/-2%. I have no idea what the variability would actually be here, hopefully less, but just to illustrate the effect, +/-2% on -74.4 would put the nominal magnification (i.e. with TV's usual 1.25" eyepiece "F" value of 0.25") between 2.01x and 2.05x, depending on the sample, and for Pentax's focal point to bounds of 2.09x and 2.14x.
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Mike
- 7" f/6.7 home-built planetary Newt
- 35mm Panoptic
- 13mm Ethos
- 5mm Tak LE
- 2x TV Barlow
- Canon 10x30IS Binoculars
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