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iceman
Post Laureate
Reged: 03/07/04
Posts: 4353
Loc: Gosford, Australia
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Hi.
Is there any difference between (say) using a 32mm EP with a 2x Barlow, compared to a 15/16mm EP?
Theoretically, they should produce the same magnification, right?
I was just wondering if there's any difference in what you see, based on the extra pieces of glass it has to go through with the barlow?
Thanks for any replies.
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Mike
. mikesalway.com.au - Astronomy and Photography by Mike Salway
. IceInSpace - The Australian Amateur Astronomy Community
. My Bio | My Jupiter 2007 Gallery | My Image Gallery
Edited by iceman (07/11/04 04:38 PM)
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3229
Loc: "Metrowest" Boston
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If the Barlow is a good one with modern multi-coatings, there is virtually no difference in what you see in the middle of the field. However, most (not all) 32mm eyepieces will vignette with a 1.25" Barlow. This means that the apparent size of the field shrinks. So what was, say, a 32mm eyepiece with a 50 degree apparent field may become an 16mm eyepiece with a 40 degree apparent field when combined with a 2x Barlow. Also, the eye relief is increased somewhat, which is probably not a good thing with most 32mm eyepieces.
The vignetting and increased eye relief are not usually problems when combining medium and short focal length eyepieces with Barlows, so my advice would be to actually buy medium focal length eyepieces that you want and use the Barlow to achieve medium-high and high magnifications with your medium focal length eyepieces. You might relegate a combination of a long focal length eyepiece with a Barlow to filling a gap, you know, just in case that might be the magnification you really want to have once in awhile. I think you need to experience the effects of magnification yourself and come to your own conclusions, but if a DSO is much to look at with ANY magnification, I don't think there is just one narrow range of magnifications that will work on it, so I don't worry very much about gaps at medium and low magnifications. But that's me.
One good thing Barlows do in modern fast scopes is slow down the light cone for simpler eyepieces like Plossls and Abbe orthos. This means less astigmatism at the edge of field, which means that if you're letting a planet drift across the field at high power in your Dob, the image is better at the edges with the Barlow than with an equivalent focal length eyepiece of the same simple design without a Barlow. That's at the edge of field. In the middle, again, there's not much difference, a few percentage points of transmission and a little bit more scatter. Most people could not tell the difference if their life depended on it (and I'm confident only a small fraction of those who think they do could actually prove it in a controlled experiment).
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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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iceman
Post Laureate
Reged: 03/07/04
Posts: 4353
Loc: Gosford, Australia
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Thanks for your in-depth reply, much appreciated.
I'm getting a 9, 15, 25 and 32mm EP with a 2x barlow, so that should cover all of the magnifications that are worth using I guess.
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Mike
. mikesalway.com.au - Astronomy and Photography by Mike Salway
. IceInSpace - The Australian Amateur Astronomy Community
. My Bio | My Jupiter 2007 Gallery | My Image Gallery
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Quote:
One good thing Barlows do in modern fast scopes is slow down the light cone for simpler eyepieces like Plossls and Abbe orthos. This means less astigmatism at the edge of field, which means that if you're letting a planet drift across the field at high power in your Dob, the image is better at the edges with the Barlow than with an equivalent focal length eyepiece of the same simple design without a Barlow.
Mike H.,
I've been reading your eyepiece posts and learning something from you for years on various different internet venues. It used to be that I'd learn something new almost everytime I read one of your posts. Now, I've finally got enough of this stuff into my head to where I'm only learning something new, .... oh maybe every third post.  Keep up the good work. Your posts cut through a lot of the mystique of eyepiece selection and construction.
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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Tim2723
The Moon Guy
Reged: 02/19/04
Posts: 5121
Loc: Northern New Jersey
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Hi Mike,
I have to agree with Mike and Mike, Mike, you're knowledge of EP's is remarkable. I have another question for you.
Given that eyepieces, barlows, diagonals, extension tubes, or anything else that fits together in the optical path has to be made with a machined slip-fit, what is the effect of cumulative position error on the final result? For example:
Suppose we have a refractor with a 2" focuser. Into that we place a 1.25" adaptor, into which is placed an extension tube, then a star diagonal, then a barlow, finally an eyepiece. That would mean there are five thumb screws holding everything together. If each device can move say 0.005" to the left or right and all the thumb screws are on the same side, is the center of the EP lens 0.025" off center from the optical axis, and is so, what is the result on the image?
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The crwth will set you free!
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3229
Loc: "Metrowest" Boston
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Quote:
Suppose we have a refractor with a 2" focuser. Into that we place a 1.25" adaptor, into which is placed an extension tube, then a star diagonal, then a barlow, finally an eyepiece. That would mean there are five thumb screws holding everything together. If each device can move say 0.005" to the left or right and all the thumb screws are on the same side, is the center of the EP lens 0.025" off center from the optical axis, and is so, what is the result on the image?
It would seems so, but 0.025" is just a little over a half a mm. This would be like a small collimation error. For example, the same total error would arise in a mirror diagonal if the face of the mirror were at exactly 45 degrees but at a perpendicular distance of 0.45mm from where it should be. I'll bet this is less than the typical error in inexpensive mirror diagonals. So I expect it would be an easily tolerable error (slight amount of astigmatism on-axis in the eyepiece and whatever off-axis aberrations coming from the telescope). But then again, in the spirit of trying to get everything working as well is it can work, I do usually recommend paying extra to buy mirror diagonals with machined aluminum casings rather than plastic or cast metal in order to minimize this source of collimation error. With the given setup, I'd also worry about flexure increasing the error, so I personally don't like doing such things, and yet I really do expect very little in the way of practical problems with it.
As for helping out, thanks all for the kind words. That's what I'm here for. Other people who know this stuff also know that it can be found in the writings of Al Nagler, Thomas Back, Rutten and van Venrooij's Telescope Optics book, Suiter's Star Testing book, etc. I just apply what these people have taught me through their writings and try to pass it along.
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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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Anonymous
Unregistered
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I have to say that Mike is very knowledgeable indeed. Have you considered writing a small book or pamphlet that explainw alot of this and woudl use multiple scopes as examples. You would have a best seller, as far as astronomy books go. By the way, what is your real profession?
Edited by AstroPro (07/11/04 11:15 PM)
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3229
Loc: "Metrowest" Boston
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I've thought about a book. I will think some more about it. BTW, I read your review on the Zeiss Lens Cloths and bought some as a result. Very nice. I haven't tried them on an eyepiece, but I got good results with my multicoated eyeglasses.
It's difficult to answer the what is my profession question--always has been. I've been called an actuarial assistant, a programmer, an industrial engineer, a university professor, and a mathematical software designer and algorithm developer. I expect it's the university professor part of me that sort of comes through when I'm answering questions about eyepieces and such. Actually, right now I am floating, aka "was RIF'd", "was downsided", "got laid off" (I like that one because it sounds like a good thing), "am between jobs", so stay tuned to see what some company (or maybe the US Government) will call me next.
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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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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10954
Loc: Los Angeles
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Mike,
I sympathize. After 27 years at my firm, they moved the corporate offices across the country to the other coast and laid off all the CA staff.
I'm sure I'll end up somewhere, or doing consulting work (something you can do until you're too old to stand up), but I'm in the same situation as you right now.
My experience has been with > 300 eyepieces and 11 different scopes owned over the years. And, I've had a chance to look through and use many other scopes because of the popularity of the site where I observe. It also doesn't hurt to have a background in chemistry and physics.
But even I learn from your posts. You are gifted in being able to explain things well. I am still learning in that area. For years I lectured to 3rd to 5th graders on Astronomy (as a guest lecturer), and now I always set aside a couple hours to talk to the college students that inevitably show up at Mt. Pinos every month (they think they're getting extra credits, but what they're really getting is a cerebral injection of astronomical obsession). There has to be a "next generation" of observers to carry on the torch (for dark skies, among other things).
My challenge for all the readers of the Cloudy Nights Forums is to make a point to invite friends (and even their children) over to observe the sky the next time you observe the stars. My next door neighbor is a movie actor, and has little time for hobbies, and so he had a 60mm dime-store refractor he used to look at the moon (but that was all). After 3-4 nights of Saturn, Jupiter, the moon, and double stars through a decent scope, he's now going to take the time to visit Mt. Pinos on New Moon to see truly dark skies.
You just never know when the thrill of being under dark skies will awaken some inner longing for the peace and serenity of the night.
Let's keep up the good work.
Starry Nights!
Don Pensack
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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I sort of base it on the focal ratio of the scope...
F/10 or slower, I'd opt for the eye piece.
F/6 to f/10, It is a toss up
F/5 or below, the barlow all the way, especially for observing planets / double stars.
I find I use my TV big barlow with my 2" eye pieces for everything these days. But I use mostly ultra fast scopes.
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