imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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How can you determine your effective magnification when using a CCD for prime-focus photography? Is there a formula that would for example use your scope's FL and perhaps the dimensions of the CCD to determine the magnification?
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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Richard Scott
professor emeritus
Reged: 07/25/06
Posts: 708
Loc: Tampa FL
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My understanding is that prime focus is roughly the equivalent of a 4mm eyepiece.
4mm divided by your focal length in millimeters ought to give you a rough estimation.
Richard
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11” Celestron XLT SCT
Orion ED80 APO
CG-5 GEM
JMI NGF-CM Focuser
Orion SSDSI
Meade DSI Pro
an assortment of 1.25” & 2" stuff
all tucked inside the Blue Moon Observatory
http://tinyurl.com/29b9wn
Basically more scope than my abilities warrant
"I know I can never look upon the stars without wondering why the whole world does not become astronomers."
Thomas Wright
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Richard, I just realized it was you. Missed you out at Farr's a couple of weekends ago. Despite the forecast the weather was great.
Anyway, about 4mm huh? So that says then that all CCD's, regardless of their size, need to be about the same distance from the vertex of the light cone to focus?
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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Chris_H
Carpal Tunnel
Reged: 11/24/03
Posts: 2858
Loc: Norway
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If you mean the toucam, its around 6mm
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Chris
"Big Papa Smurf" (254mm f/4.7 Sky-Watcher Newt)
Custom Orion Optics 150mm f/8 reflektor "Planet Killer" (On its way!!!)
SPC900
Canon Powershot A610 (CHDK modded)
Canon 10D (modded)
Canon EF 50mm f/1.8 II
Canon EF 70-200mm f/4 USM L
My Website
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Chris, you are saying that the TouCam comes to focus at a distance of 6mm from the vertex?
Here is my logic with that said. Hopefully someone can comment on this:
1) If you know the distance at which your chip comes to focus (in this case, theoretically 6mm) then you should be able to use that value in place of the EP focal length?
2) If #1 is true, then the size of the chip merely determines how large of an area of the FOV is sampled. This would still have a perceived effect on magnification though. For example, a smaller chip that focuses at 6mm will cover a smaller area of the FOV than a larger chip that focuses at the exact same distance. Therefore, the smaller chip is "zoomed-in" and resolves a smaller area than the larger chip.
3) If #2 is true, then a smaller chip should yield greater detail on a given target, but without altering the real magnification that is determined by the dividing the distance from the vertex at which the chip is focuses by the FL of the scope.
Apologies if that doesn't make sense typed out.
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
Edited by imhotep (04/24/07 05:03 PM)
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Lou
member
Reged: 03/27/05
Posts: 93
Loc: Chester County, PA
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I divide my telescope's focal length by 50mm to get the 'magnification.' Since a 50mm lens is a 'normal' lens for 35mm film-sized sensors, i.e. it shows a field of view roughly equivalent to normal vision, then my telescope (1200mm) at prime focus 'magnifies' 24x.
The dimensions of your CCD do not determine 'magnification.' It only determines what part of the field of view you can see versus what the lens' image circle is.
"All lenses of the same focal length give images of the same size at any given subject distance." -- Ansel Adams
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Regards,
Louis Marchesi
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Dr Nick
scholastic sledgehammer
Reged: 01/27/06
Posts: 798
Loc: Glen Innes, NSW, Australia
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Hmm, I'm personally not sure, but I like the explanation that imhotep gave, it seems reasonable exept I dont see how #3 could be true, however, I do beleive #1 and #2 are plausible.
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Clear Skies, Nick
http://www.NicksAstronomy.com
Sky Watcher 8" f/5 Newtonian(which I got when I was 10yrs old and 5yrs later I'm still adding to) w/JMI motofocus/standard, axis guider.
ToUcam Pro w/Phillips vlounge/registax/and Virtual Dub,
DSI Pro II w/envisage/photoshop cs2/,
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gavinm
professor emeritus
Reged: 08/26/05
Posts: 559
Loc: Auckland New Zealand
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To talk about magnification with digital images is unrealistic. You can magnify them as much as you want depending on resolution, so Lou's statement (from Adams) is the most correct.
And #3 would only be true if resolution was infinite ( or large enough)
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Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R w/ UHTC
SBIG ST7-XME + CFW10
Skywatcher Equinox ED80 Pro (ADM dovetail)
+ other stuff
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russ_watters
Pooh-Bah
Reged: 11/24/04
Posts: 1227
Loc: Trappe, PA
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Trying to apply the eyepiece calculation to a CCD is utterly useless since the final image is viewed on a monitor or printed out on paper and you won't necessarily know how far apart the pixels actually are on either. The only way to get the actual magnification of a printed image is to physically measure the size of the object and pick an arbitrrary viewing distance to calculate the angular diameter.
When I posted a Saturn image here, everyone said it was "big" (read: too big, making it blurry) because most people run their monitors at something like 75dpi whereas my laptop is 13 inches wide and 1680 pixels = 129dpi.
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Equipment: Orion Atlas 11, ED80, DSI-C, DSI II Pro, Dell Inspiron Laptop.
www.russsscope.net
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Lou, you're calculations for using the 50mm are correct. However, I'm talking about prime-focus astrophotography. The telescope serves as the lens, focusing an image directly onto the image plane of the camera. You quoted Ansel Adams so I'm assuming you're pretty familiar with photography in general. I'd like to illustrate my point in #3 using the crop factor associated with DSLRs as an example. DSLRs give the user a greater effective FL when using standard 35mm lenses. As you probably already know, a typical crop factor is about 1.5 or 1.6. This is because the chip is smaller in size than a 35mm slide, and therefore crops the image circle that is cast by 35 mm lenses. The result is that a lens that would yield 50mm FL on a 35mm camera becomes a 75mm lens on a DSLR. Roughly the same thing is going on with prime focus photography. If you have two censors that come to focus at the SAME distance, and the pixel ratios are also equal (meaning the smaller chip has a denser pixel array), then the smaller chip
should produce an image that has the effect of being magnified more so than the image produced by the larger chip.
Gavin & Russ: I think I need to explain myself a little better. What you've both said is totally correct. I realize that trying to determing magnification based upon a digital sensor is somewhat unrealistic. My goal here is to determine the limits of my scope with regard to using barlows with a CCD. Let me explain.
For simple visual work, I usually go by the rule of thumb - a maximum of 50x per inch of aperture. On my 4.25" scope this comes out to 212.5x. So for visual observing, my 7mm ortho yields 157x, but with a 2x barlow it is at 314x which obviously isn't practical for my small aperture.
With that said, all of my images taken with the SPC900NC coupled to the same 2x barlow have been fine. I would like to figure out how to determine the same magnificaiton limits for a given optics/ccd combination. I've been considering a 3x purchase and thought it would be beneficial to know if doing so would be pushing the optics past their practical limits before spending the money.
I hope that served as a better explanation and I'm interested in more opinions and feedback from all.
Edited by imhotep (04/25/07 09:24 AM)
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Do not confuse linear magnification with angular magnification.
Linear magnification does not change with format, but it cannot be used at infinity - magnification becomes infinitly small at that point. Linear magnification is the difference between object and image size. (Obviously, this is not used in observing as 1x would mean you could see rocks on the moon life-size if it were linear magnification.)
Angular magnification is used at infinity (or undetermined object distances) and that is what is used in observing. Angular magnification describes the difference in viewing angle based on a norm - approximately 52 degrees if I remember correctly. To calculate angular magnification simply divide the effective focal length of the optical system by the diagonal size of the film/sensor - or the area used if you are cropping. Simple.
As far as displaying the image, linear magnification does change with display size as it is changing image size. Angular magnification does not because the area that is viewed is not changing.
Edited by wilash (04/25/07 03:48 PM)
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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BTW, do not confuse the previous post with another term applied to zoom lenses. A 4x zoom lens means the maximum focal length is four times the minimum focal length of the zoom range. 10-40, 25-100, 100-400 are all 4x zoom lenses.
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Chuck Anstey
scholastic sledgehammer
Reged: 01/17/05
Posts: 834
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To add to this because I haven't seen it mentioned, the two most important values related to imaging are image scale measured in arc-seconds / pixel and true FOV. You need to know the arc-seconds per pixel to know if you are over-sampled, under-sampled, or just right for your given optics, seeing conditions, and ability to guide. The true FOV is necessary so you know if the object will fit in the frame. No point in trying to image the North America Nebula with a DSI on a C11 unless you are going to do a mosaic. I do not remember the formulas for calculating these values off hand.
Just remember, visual uses magnification as a measure but imaging uses arc-seconds / pixel. Both use true FOV.
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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One more thought about angular magnification. The method given in my first post is in reference to field size in imaging. If you apply this to observing with EPs with the same focal length and different AFOV in observing, then the angular magnification is the same, but field size is different between EPs. This is correct in observing because the eye's angle of view is constant. So while it seems a contradiction between imaging and observing, it is not.
I hope that made sense. 
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Don't mind me. I just enjoy posting to myself.
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Will, thank you for the explaination. Now I'm left with another quandry. It should be simple as you say to just divide the diagonal dimension of my sensor by the scope's FL. I'm wondering if doing so is still realistically linked to the 50x per inch rule though.
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Since I am on the subject, with a 35mm frame, you need to divide the focal length of the optics by 43mm, not 50mm.
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Chuck, thank you for sharing your knowledge. I would be very interested to learn the formulas that are used to calculate FOV and arcseconds/pixel if you have some sources. Again, thank you.
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Quote:
Will, thank you for the explaination. Now I'm left with another quandry. It should be simple as you say to just divide the diagonal dimension of my sensor by the scope's FL. I'm wondering if doing so is still realistically linked to the 50x per inch rule though.
What has any of this got to do with the 50x per inch rule? That rule is a guideline for observers. Nothing to do with imaging.
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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OK. Your magnification limit in an imaging system is very complex - how good are you with Fourier Transforms? Basically, you need to think about sensor size and diffraction of the optical system. If you push the magnification and end up with a slow focal ratio, you are also losing to diffraction. Depending on format, you could be limited to a f/11 system, you could also get away with something higher. Naturally, it depends what you do with the data and how "good" you think the result is.
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