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Mike I. Jones
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5543923 - 11/28/12 01:44 PM

Notice that Ed synthesized a lens by using a plano-CX and plano-CC of different radii, plano sides together, and made from the same glass (BK7). He's clever that way!
Mike


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5544047 - 11/28/12 03:09 PM

Quote:

Notice that Ed synthesized a lens by using a plano-CX and plano-CC of different radii, plano sides together, and made from the same glass (BK7). He's clever that way!
Mike



Yes, that is very clever. With off-the-self lenses he had no choice but to combine them.

Mladen


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: cjc]
      #5545177 - 11/29/12 09:42 AM Attachment (31 downloads)

Chris, anyone who needs a 3-inch focuser can just as well custom order the optics because chances are such a person will most likely have a camera with the processor to match, and the sky is the limit (no pun intended).

For an average Joe ATM, even if skills and tooling are no obstacle, it simply doesn't pay to try to make correctors lenses and hyperboloidal mirrors if one can buy a corrector for an existing paraboloid for a price between $450 and $600.

The only possible exception to this fact is Ed Jones' Rosin with commercially available lenses. But you get what you pay for.

That being usually true, let me say that Ed's configuration is darn good given that the corrector is made up of off-the-shelf lenses, but don't expect to get the performance of Mike Jones' optimized Rosin design.

Even if you can get such lenses at bargain prices, and even if they meet the tolerance requirements (which we still don't have), you still have to make, or have someone make for you, a precision corrector cell, anodize it, etc.

If scaled to a 10-inch f/4, Ed' design has a diffraction-limited field four times smaller than Mike's 10-inch f/3.5 configuration, mainly due to astrigmatism.

If you're willing to settle for the outdated 20-micron photograpahic resolution, then Ed's configuration can cover a field of up to 24 mm across photographically, but at a considerable loss in limiting stellar magnitude.

However, plopping a Paracorr into your focuser of your already existing Newotnian is simply unmatched in convenience, and in terms of saving time and money - and a large corrected fov.

Ed Jones, do you have any idea how much your off-the-shelf lenses would cost? Thanks.

Mladen

Edited by MKV (11/29/12 09:56 AM)


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cjc
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Reged: 10/15/10

Loc: Derbyshire, England
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5545448 - 11/29/12 12:21 PM

Quote:

However, plopping a Paracorr into your focuser of your already existing Newotnian is simply unmatched in convenience, and in terms of saving time and money - and a large corrected fov.




I agree entirely and parabolic mirrors are more easily made or bought from a wide range of sources. I apologise if I have highjacked the thread which is after all in the ATM forum. I was really only wanting draw attention to off the shelf options, and not only the Paracorr. At a higher cost there are the ASA devices, while for less there is for example the Astro-Tech/ GSO corrector designed by Roger Ceragioli which is currently on offer at $120 and apparently in stock at Agena!


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: cjc]
      #5545518 - 11/29/12 01:06 PM

Chris, you may answer via PM if you feel this is too much off topic, but I think some people may be interested. The Astro-Tech (GSO) comma corrector is indeed cheap but not without problems (and there's plenty on the web about that). Also, pictures are worth a thousand words, and I don't see any spot, MTF, PSF, interferometric, or imaging evidence of the product's performance.

I am not promoting Paracorr 2. I am simply establshing that, so far, the makret has one fairly sucessful and affordable product by that name and that making one's own corrector and precision lens cell (not to mention hyperbolic primary) is not practical or possible for most amateurs.

To the Admin: please consider creating a theoretical design forum where one can post theoretical designs and discuss optical theory at liberty.

Mladen

Edited by MKV (11/29/12 01:11 PM)


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kfrederick
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5545733 - 11/29/12 03:45 PM

Only 63.4% of the light reaches the focus .About the same as a 6 inch unobstructed . Unless I am reading Oslo wrong

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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: kfrederick]
      #5545780 - 11/29/12 04:08 PM

Guys - Don't sweat it, I'm not worried about this particular thread going off topic. I kind of started it to re-seed these kinds of discussions anyway.

Hijack away!
Mike


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5545811 - 11/29/12 04:32 PM

Quote:

Guys - Don't sweat it, I'm not worried about this particular thread going off topic. I kind of started it to re-seed these kinds of discussions anyway.

Hijack away!
Mike



Thanks, Mike!


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: kfrederick]
      #5545814 - 11/29/12 04:34 PM

Quote:

Only 63.4% of the light reaches the focus .About the same as a 6 inch unobstructed . Unless I am reading Oslo wrong



Which one are you talking about, K?


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kfrederick
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5545824 - 11/29/12 04:40 PM

The Oslo file says 63.4% of the rays reach the focus in Mikes 10 inch rosin . So it it is like a 6.34inch unobstructed in light .

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Mark Harry
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: kfrederick]
      #5545935 - 11/29/12 06:09 PM

More like a 7.94"???
M.


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Ed Jones
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5546253 - 11/29/12 09:08 PM

Mladen,
I think lenses this size will cost about $100 or more each so as I said you can buy a Paracorr for almost the same money. This was just my best try at using off-the-shelf lenses to make a photographic scope.


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Ed Jones]
      #5546353 - 11/29/12 10:10 PM

Quote:

Mladen,
I think lenses this size will cost about $100 or more each so as I said you can buy a Paracorr for almost the same money. This was just my best try at using off-the-shelf lenses to make a photographic scope.



Thanks Ed. I think you did a fantastic job!

Mladen


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: kfrederick]
      #5546421 - 11/29/12 11:04 PM

Quote:

The Oslo file says 63.4% of the rays reach the focus in Mikes 10 inch rosin . So it it is like a 6.34inch unobstructed in light .



K, the "unobstructed" aperture residual is simply the difference in areas between the full aperture and the central obstruction.

The full aperture radius is 5 inches, so the full aperture area A1 = Pi*r^2 or Pi*25 = 78.5398 sq. inches. The radius of the central obstruction is 1.894 inches, and the CO area A2 = Pi*1.894^2 = 3.14159 sq. inches. Subtracting A2 from A1 gives A3 or the area of the unobstructed residual, which is A3 = 67.27 sq. inches. This area is then A3 = Pi*r3^2, and its radius r3 = (A3/Pi)^0.5.

And since the diameter is 2*radius, the equivalent unobstructed aperture diameter D3 = 2*(A3/Pi)^0.5 = 9.25 inches, which gives a residual focal ratio of f/3.8, instead of the geometric f/3.5.

Mladen


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Mike I. Jones
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5546511 - 11/30/12 12:27 AM

Don't get T-number (light gathering power) and F-number (resolution) mixed up. The system aperture is still 10" and focal length is 35.468", so the focal ratio (F-number) is still f/3.5468 (f/3.5 close enough). The visual resolution follows the focal ratio per the obscured system diffraction disk formula. The T-number relates to the system's actual light gathering power, meaning the optical flux impinging on a camera focal plane (thus driving exposure times) and the eye (thus driving limiting magnitude). In this system focal plane or retinal irradiance (in watts/cm2) appears to come from an f/3.8 system, not an f/3.5 system. If transmission losses in the system are also accounted for, the T-number might even be more like f/3.85 to f/3.9.

The system resolution is set by the focal ratio. The limiting magnitude and photographic speed is set by the T-number.

Mike


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5546728 - 11/30/12 07:23 AM

Thanks for clarifying the terminology and the concepts, Mike.

I would only like to add that, commonly, the terms f-number and t-number are used interchanably, such as, for example, here, and here, and always as simultaenous change of aperture diameter with aperture area.

In reflecting optics, with few exceptions, this is not the case because the central obstructioon reduces the area but not the physical aperture, hence the need to use separate terms and concepts, as Mike explained.

Personally, I like to distinguish the concepts as geometric or absolute aperture (f-number) and relative aperture (t-number), so as to avoid as much as possible the f-number and t-number terms, given that they are frequently used interchangeably and can lead to confusion.

Mladen


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wh48gs
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5548349 - 12/01/12 07:02 AM

Quote:

I like to distinguish the concepts as geometric or absolute aperture (f-number) and relative aperture (t-number)




They are both relative; f-number is routinely referred to as relative aperture alone, which is O.K. as long as we use effective relative aperture for T-stop, based on the actually transmitted light (which means it often isn't the same on and off-axis). Still, it doesn't hurt to make sure which one it is, and add to the former geometric, or nominal.

The actual relative aperture at the detector is also affected by aberrations, specifics depending on the properties of detector (e.g. pixel size w/CCD).

Vla


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: wh48gs]
      #5548441 - 12/01/12 09:03 AM

Quote:

Quote:

I like to distinguish the concepts as geometric or absolute aperture (f-number) and relative aperture (t-number)




They are both relative; f-number is routinely referred to as relative aperture alone, which is O.K. as long as we use effective relative aperture for T-stop, based on the actually transmitted light (which means it often isn't the same on and off-axis). Still, it doesn't hurt to make sure which one it is, and add to the former geometric, or nominal.

The actual relative aperture at the detector is also affected by aberrations, specifics depending on the properties of detector (e.g. pixel size w/CCD).

Vla



Thanks Vla. I was reluctant to use "absolute" when referring to the f-number (defined as focal length divided by the diamter of the objective, or F/D), but in a system with a fixed objective diameter and focal length it seems applicable and justifiable. This is why I also prefer the term "geometric".

In reflective systems, you also have fixed parameters. The F/D doesn't change with the size of the central obstruction. So, while their absolute or geomtric f-ratio remains nominally the same regardless of the size of the central obstruction, the "effective" (which is another good term, as you noted) or relative light gathering area of such systems is. This results in a smaller light gathering area or throughput of light energy which is equivalent to a smaller unobstructed aperture.

Mike prefers to use t-term for this, but as I noted, this term is elsewhere used interchangeably with f-number, which can lead to a conflation of both, and in unobstructed configurations it actually is one and the same.

Whatever term is used, it probably doesn't hurt to define it prior to using it. In Mike's example, the 10-inch light-gathering area of the mirror is effectively reduced by the central obstruction to that equivalent of a 9.25-inch unobstructed aperture, and represents the difference in the area of the mirror and the central obstruction.

Mladen


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Mike I. Jones
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5548565 - 12/01/12 10:44 AM

Just to continue, the unobscured pupil area in cm2 is multiplied by the irradiance (power density) at the entrance pupil in watts/cm2 from a given star, then by the system transmission throughput, to give the irradiance at the focal plane, also being power density in watts/cm2. This is the total irradiance contained in the system diffraction pattern; point spread function plots then give the distribution of irradiance power density in the diffraction pattern, and encircled or ensquared energy plots are used to give the accumulated power density as a function of PSF radius. Multiplying the exposure time by the irradiance gives the energy density in Joules/cm2 available to a photographic sensor. Visual limiting magnitude depends on the obscured pupil equivalent aperture, not the full entrance pupil diameter.
Mike


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MKV
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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5548600 - 12/01/12 11:14 AM

Mike, wouldn't the brightness of a star appear the same in, for example, a 10-inch telescope regardless of the focal ratio?

Mladen


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