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10" f/3.5 Astrograph with Rosin focal reducer

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#76 MKV

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Posted 30 November 2012 - 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

#77 wh48gs

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Posted 01 December 2012 - 07:02 AM

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

#78 MKV

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Posted 01 December 2012 - 09:03 AM

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

#79 Mike I. Jones

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Posted 01 December 2012 - 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

#80 MKV

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Posted 01 December 2012 - 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

#81 Mike I. Jones

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Posted 01 December 2012 - 04:32 PM

Visually, yes, the system focal ratio does not affect limiting magnitude. The retina is a non-integrating detector, so staring continuously at an object does not make it appear brighter, unless you happen to still be dark adapting at the time.

At lower powers and large eyepiece exit pupils, though, high-brightness light pollution increases the background brightness, and can also interfere with dark adaptation, both of which affect limiting magnitude.

Photographically, the f-number and T-number directly impact photographic exposure times, as cameras are integrating detectors.

Mike

#82 Ed Jones

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Posted 04 December 2012 - 07:37 AM

I deleted my previous post, Zemax found a better design. This is a telecompressor/coma corrector for a F/5 Newt primary. It makes it an f/2.8.

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#83 kfrederick

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Posted 04 December 2012 - 07:45 AM

Great work .Will it work for all size telescope?

#84 Mark Harry

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Posted 04 December 2012 - 08:40 AM

Duel of the Joneseys! I'm eatin' popcorn.....
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#85 MKV

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Posted 04 December 2012 - 12:18 PM

Post deleted by MKV

#86 MKV

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Posted 04 December 2012 - 01:14 PM

Ed, could you show the spots?

Mladen

#87 Ed Jones

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Posted 04 December 2012 - 06:43 PM

Mladen,
Here is the spot diagrams. I optimized it for C,F and D so the deep blue 405 isn't so good but it could be optimized for other ranges. With 5 lenses it there won't be too many ATMs that can do this but it shows what's possible. It works with most focal length mirrors that can have enough BFL space.

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#88 MKV

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Posted 04 December 2012 - 08:02 PM

Mladen,
Here is the spot diagrams. I optimized it for C,F and D so the deep blue 405 isn't so good but it could be optimized for other ranges. With 5 lenses it there won't be too many ATMs that can do this but it shows what's possible. It works with most focal length mirrors that can have enough BFL space.

Thanks Ed. Looks really nice. Obviously this type of corrector is the way to go with bigger mirrors.

For some reason OSLO didn't get the same results with your original configuration. I believe it's the limitation of OSLO.EDU to 10 surfaces, so in order to include the airgap between the two N-PSK57 lenses you'd need 11 surfaces in OSLO.

Also, I don't see why a five-lens corrector would be impossible for an amateur and a four or a three element wouldn't. It wouldn't be cheap, but results are great, and the shapes are not exotic.

Mladen

#89 Ed Jones

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Posted 04 December 2012 - 08:31 PM

Mladen,
I only have 10 surfaces, seems like it should work. I've played around with this design and it can be pushed even better including 405nm.

#90 MKV

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Posted 04 December 2012 - 10:33 PM

Mladen,
I only have 10 surfaces, seems like it should work. I've played around with this design and it can be pushed even better including 405nm.

I just noticed that I was using a wrong glass for the third lens, so disregard any previous attachments.

No doubt the performance can be pushed even better, but what's really attractive and practical about this configuration is that the primary mirror is a relatively slow paraboloid (f/5) and therefore relatively easy to figure to high precision, yet the system operates at f/2.8.

Mladen

#91 ismail

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Posted 19 July 2013 - 03:37 AM

Dear Dr.Mike and Ed ALL Friends here,
I have removed the MEADE 12 inch LX200 Telescope optics.
Primary mirror is 310 mm diameter and Radius of curvature is -1200mm, Sec mirror is 75 mm Diameter and Radius of curvature is 364 mm. Distance between primary and sec is 470 mm. Any option to convert to a cass telescope of this optical Mirrors with 1 deg field or less.

#92 Mike I. Jones

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Posted 19 July 2013 - 09:17 AM

Kind of confused here, ismail. You took the mirrors out of a Meade 12 inch LX200 Cassegrain, measured them, and want to make a Cassegrain out of them, which is what they already were, in? :)
Mike

#93 hellojabil

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Posted 19 July 2013 - 10:25 AM

Thanks Mike. Yes I know it is cass telescope but I would like to know the quality of the image it will have without corrector plate?

#94 Mike I. Jones

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Posted 19 July 2013 - 10:50 AM

Experience is the best teacher in this forum. Set them up and try it for yourself, answer your own question, then let everyone here know what you found out.
Mike

#95 hellojabil

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Posted 20 July 2013 - 03:54 AM

Thanks Yes I am doing that. I have done the primary mirror with ronchi setup it is good sphere. I have tested the sec mirror with ZYGO Interferometer and found one fringe circular. planing for Hindle test for secondary and autocollimation test soon with combination of both mirrors. And planing to enter the values in ZEMAX. hope you can suggest any other things which i can plan for it.






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