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jimb1001
sage

Reged: 11/14/09

Loc: Florida
Mallincam and Light Gathering
#5123814 - 03/15/12 01:13 PM

I know little of the mathematics of astronomy but I would like to know more about light gathering and the Mallincam.

Specifically, how gain and integration relate to scope size.

For example, with an eye to the eyepiece a certain amount of light hits the eye for image forming in the brain.
For arguments sake, lets say the telescope is a Celestron 9.25 SCT and the eyepiece is an 8mm 55 degree plossel. Aperture will determine the brightness (visibility) of the image.

Replacing the eyepiece with a Mallincam and setting the gain at 4, how much light will reach the sensor with 1 sec of integration, 2 seconds, and so forth?

Approaching from the opposite, gain at 4, integration at 10 seconds, how much light reaches the sensor with a 9.25, how much with an 8" SCT, a 6" SCT.

Does it matter what size telescope is used with a Mallincam or can aperature be compensated for by integration or gain?

I realize that dark sky and seeing conditions will have as big an effect on the image as anything else but they would be constants in this equation or would they effect different scopes in different ways even with a Mallincam attached?

Ideas?

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: jimb1001]
#5123903 - 03/15/12 01:55 PM

To add on to this. What exactly does "integration" mean for 1 second, 2 second, ...etc for the Mallincam? Does that mean that it takes a single 1 second, 2 second ....etc exposure. Or does that mean that it takes smaller exposures and stacks them like stacking software does?

Does the integration of the Mallincam increase the exposure without increasing the shutter duration?

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mclewis1

Reged: 02/25/06

Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5124103 - 03/15/12 03:56 PM

Integration on the Mallincam means exposure not stacking time. You also have to remember that while the Mallincam integrates for X seconds, it stores that image in a buffer and outputs it as a continuous NTSC (29.97 frame/sec) video signal and not as a one shot dump.

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: mclewis1]
#5124123 - 03/15/12 04:12 PM

Quote:

Integration on the Mallincam means exposure not stacking time. You also have to remember that while the Mallincam integrates for X seconds, it stores that image in a buffer and outputs it as a continuous NTSC (29.97 frame/sec) video signal and not as a one shot dump.

Thanks for that info. I guess that is one form of stacking. It is really continuous stacking though.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5124789 - 03/16/12 01:24 AM

I've never bought into the notion that a camera equates to some particular gain in aperture. For the single reason that performance depends on subject brightness.

Take the Moon. The camera cannot reveal more than I can see directly by eye. The same goes for the planets. (Note that we're neglecting in this discussion such 'tricks' as the stacking of multiple images and other processing.)

Now consider a moderately bright object such as M42, the brighter parts of which are about 14 mag/arcsec^2. If I set my camera to an exposure about equivalent to the 'intrgration time' of my eye, which if I'm charitable about it, I might define as 1/15 second, again, my eye reveals more nebulosity.

It's only when we get down into the surface brightness regime of dimmer than about 18 mag/arcsec^2 does the camera come into its own. For then the longer integration, in conjunction with the not insignificant contrast gain due to compression of dynamic range, reveals lower surface brightness than the eye can detect.

And this is the key. If the camera can record very low surface brightness, low contrast stuff beyond visual detection in ANY telescope, one could say the camera's equivalent aperture approaches infinity. But for bright targets the camera is only as good as the scope it's attached to.

Perhaps the only target for which the concept of an aperture equivalent could reasonably apply is stars. The difference in their visibility limit both visually and in an image under given sky brightness will be pretty closely correlated. But I've never bothered to assess this, as so far my interest has mainly centered on nebulosity.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Mallincam and Light Gathering [Re: GlennLeDrew]
#5124811 - 03/16/12 01:44 AM

I forgot to add that for faint objects it might appear that the camera is revealing more detail. Not so. Rather, it is the eye which is failing to reveal the detail which is there. Of course it's natural to compare the small scope image of some nebula or galaxy with the visual aspect delivered by a bigger scope, pondering upon what it would take to make an equivalent view.

If all 'faint fuzzies' had the same and nearly uniform surface brightness, this would be valid. But there is a quite large range in surface brightness from 14 to fainter than 25 mag/arcsec^2 (to which the eye is sensitive). This 11+ range in magnitudes is a brightness ratio of at least 25,000!!!

This alone points to the incredible dynamic range of our visual system.

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jimb1001
sage

Reged: 11/14/09

Loc: Florida
Re: Mallincam and Light Gathering [Re: GlennLeDrew]
#5124834 - 03/16/12 02:24 AM

Quote:

I've never bought into the notion that a camera equates to some particular gain in aperture. For the single reason that performance depends on subject brightness.

Take the Moon. The camera cannot reveal more than I can see directly by eye. The same goes for the planets. (Note that we're neglecting in this discussion such 'tricks' as the stacking of multiple images and other processing.)

Now consider a moderately bright object such as M42, the brighter parts of which are about 14 mag/arcsec^2. If I set my camera to an exposure about equivalent to the 'intrgration time' of my eye, which if I'm charitable about it, I might define as 1/15 second, again, my eye reveals more nebulosity.

It's only when we get down into the surface brightness regime of dimmer than about 18 mag/arcsec^2 does the camera come into its own. For then the longer integration, in conjunction with the not insignificant contrast gain due to compression of dynamic range, reveals lower surface brightness than the eye can detect.

And this is the key. If the camera can record very low surface brightness, low contrast stuff beyond visual detection in ANY telescope, one could say the camera's equivalent aperture approaches infinity. But for bright targets the camera is only as good as the scope it's attached to.

Perhaps the only target for which the concept of an aperture equivalent could reasonably apply is stars. The difference in their visibility limit both visually and in an image under given sky brightness will be pretty closely correlated. But I've never bothered to assess this, as so far my interest has mainly centered on nebulosity.

For the brighter objects, the moon for example, the question is moot as the light available to the eye is sufficient to reveal all pertinent detail, given good conditions.

For the planets, the camera has a huge advantage, again, given the sensor recieving the light. Clearly still cameras, given enough exposure time, will significantly outperform the human eye, on a telescope by telescope basis. Why else would astro photographers be satisfied with smallish refractors? Excepting of course the obvious issues of mount stability. The issue of course really one of increased exposure substituting for aperture in the procurement of light.

Clearly the relationship has more to do with light gathering than surface brightness as surface brightness is a constant whether using the naked eye, the eyepiece, the still camera or the video camera. Yet only the telescope/eyepiece combination limits the image visibility by the aperture of the telescope.

Again, limiting the apterture equivalence to only stars would seem to say that dso magnitudes are different from star magnitudes, and yet, a magnitude is a magnitude.

The reality, of course, is that larger apertures shorten integration times and while half a minute or a minute may not matter to a good mount, a 20 inch dob can shorten the integration time needed compared to a 3" refractor. Thus limiting the opportunity for error and allowing the camera to see "deeper" into the cosmos.

So again, it would seem that there must be a relationship between the sensor and the aperture of the telescope where the magnitude of the object is a constant, the gain is a constant, and the aperture of the telescope and the integration time relate to one another.

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jimb1001
sage

Reged: 11/14/09

Loc: Florida
Re: Mallincam and Light Gathering [Re: GlennLeDrew]
#5124840 - 03/16/12 02:35 AM

Quote:

I forgot to add that for faint objects it might appear that the camera is revealing more detail. Not so. Rather, it is the eye which is failing to reveal the detail which is there. Of course it's natural to compare the small scope image of some nebula or galaxy with the visual aspect delivered by a bigger scope, pondering upon what it would take to make an equivalent view.

If all 'faint fuzzies' had the same and nearly uniform surface brightness, this would be valid. But there is a quite large range in surface brightness from 14 to fainter than 25 mag/arcsec^2 (to which the eye is sensitive). This 11+ range in magnitudes is a brightness ratio of at least 25,000!!!

This alone points to the incredible dynamic range of our visual system.

The camera is revealing more or the eye fails to reveal what's there? Of course the eye is still viewing the image generated by the camera. If it sees more detail, there is more detail there to be seen.

Surface brightness as expressed by the magnitude table is, would think, a constant. While there is a wide range of magnitudes, all mag 10 objects, for example, will be about as easily seen using the same equipment. All mag 10 objects will present the same image brightness when seen through a 10" telescope, or, alternately, through a 10" telescope with a mallincam attached and set to 10 seconds of integration.

The question remains, and this is true of still camera or video camera, if I have a 4" telescope that will, on a given night, provide a pleasing view of the Dumbell Nebula with 10 seconds of integration, how many seconds of integration will a 10" scope require to achieve equivalent brightness?

If it comes down to it I suppose I can switch my 9.25, 6" and 3.5" some night but I suspect there is a formula that will save me the trouble.

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nytecam
Postmaster

Reged: 08/20/05

Loc: London UK
Re: Mallincam and Light Gathering [Re: jimb1001]
#5124900 - 03/16/12 06:09 AM

my brief exposure tests on M27 and m57 thus...

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: nytecam]
#5125093 - 03/16/12 09:30 AM

This really is simpler than some people make it sound. The truth is that all of these situations are simply a camera attached to a lens. Yes I said it. A telescope is just another form of the traditional lens. Our eyes are just a camera and a lens(More accurately a sensor and a lens. Our brain is the processor).

All of the same principles of non-macro terrestrial photography apply. Focal Ratio(F-number), Shutter duration, and Exposure gain are the only parameters involved in the RAW image capture whether you are using a telescope or a traditional lens, or your eye and a telescope for that matter.

Note that the absolute aperture of the telescopes front opening is not a factor in determining the final exposure of a picture or image in our brain. That is because that measurement alone does not determine the amplitude of light that reaches the sensor or our eye. The focal ratio of the entire system determines how much light reaches the sensor or our eye.

The problem with eye-piece viewing is that your eye cannot vary the shutter duration. It can only vary the aperture of your eye. This means that the only way to increase the incoming light is to decrease the F-number of the system as a whole.

There are two ways to do that. You can increase the opening of the telescopes aperture and keep the focal length fixed and it will definitely decrease the F-number. However, telescope manufactures tend to increase the focal length proportionally as they increase the front aperture in order to keep the focal ratio the same.

They do this because they know that they can just add additional optics to change the focal ratio(ie: different eye pieces, barlows, and reducers).

Therefore, adding additional optics is the second option and the most commonly used. Even for eye-piece viewing if you step up to a much bigger aperture scope you are actually increasing the incoming light to your eye by using a different eye-piece/reducer and not just by increasing the scopes aperture if the scope’s focal ratio is held constant.

If the focal ratio of the entire system(Telescope/Lens and eye-piece/reducer) is F10 then the same amount of light will hit the camera as would hit your eye. The final exposure of the picture/(image in your brain) will be determined by the shutter speed and exposure gain of the camera and the aperture of your eye.

However, there is one big difference about your eye vs. the camera sensor. Your eye has a much bigger dynamic range than any consumer camera I have come across. For high dynamic range objects that will affect the final image greatly. HDR photography can get those two pretty close though.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5125237 - 03/16/12 10:55 AM

One persistent myth has it that for extended objects, increasing aperture means shorter exposures. Aperture by itself is not the factor. F/ratio is.

A 2"' f/5 and a 10" f/5 will yield identical image surface brightness. The same integration times work equally well for both on any given object.

A 2" f/5 will require shorter integrations than would a 10" f/7. The smaller scope's faster focal ratio delivers to the camera a brighter image.

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: GlennLeDrew]
#5125242 - 03/16/12 10:59 AM

Quote:

One persistent myth has it that for extended objects, increasing aperture means shorter exposures. Aperture by itself is not the factor. F/ratio is.

A 2"' f/5 and a 10" f/5 will yield identical image surface brightness. The same integration times work equally well for both on any given object.

A 2" f/5 will require shorter integrations than would a 10" f/7. The smaller scope's faster focal ratio delivers to the camera a brighter image.

Precisely, and very succinctly put. I have seen this concept mixed up too many times.

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jimb1001
sage

Reged: 11/14/09

Loc: Florida
Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5125372 - 03/16/12 12:07 PM

Quote:

This really is simpler than some people make it sound. The truth is that all of these situations are simply a camera attached to a lens. Yes I said it. A telescope is just another form of the traditional lens. Our eyes are just a camera and a lens(More accurately a sensor and a lens. Our brain is the processor).

All of the same principles of non-macro terrestrial photography apply. Focal Ratio(F-number), Shutter duration, and Exposure gain are the only parameters involved in the RAW image capture whether you are using a telescope or a traditional lens, or your eye and a telescope for that matter.

Note that the absolute aperture of the telescopes front opening is not a factor in determining the final exposure of a picture or image in our brain. That is because that measurement alone does not determine the amplitude of light that reaches the sensor or our eye. The focal ratio of the entire system determines how much light reaches the sensor or our eye.

The problem with eye-piece viewing is that your eye cannot vary the shutter duration. It can only vary the aperture of your eye. This means that the only way to increase the incoming light is to decrease the F-number of the system as a whole.

There are two ways to do that. You can increase the opening of the telescopes aperture and keep the focal length fixed and it will definitely decrease the F-number. However, telescope manufactures tend to increase the focal length proportionally as they increase the front aperture in order to keep the focal ratio the same.

They do this because they know that they can just add additional optics to change the focal ratio(ie: different eye pieces, barlows, and reducers).

Therefore, adding additional optics is the second option and the most commonly used. Even for eye-piece viewing if you step up to a much bigger aperture scope you are actually increasing the incoming light to your eye by using a different eye-piece/reducer and not just by increasing the scopes aperture if the scope’s focal ratio is held constant.

If the focal ratio of the entire system(Telescope/Lens and eye-piece/reducer) is F10 then the same amount of light will hit the camera as would hit your eye. The final exposure of the picture/(image in your brain) will be determined by the shutter speed and exposure gain of the camera and the aperture of your eye.

However, there is one big difference about your eye vs. the camera sensor. Your eye has a much bigger dynamic range than any consumer camera I have come across. For high dynamic range objects that will affect the final image greatly. HDR photography can get those two pretty close though.

Absolutely true.

But of course aperture still rules. While reduced focal ratio concentrates the gathered light into a smaller image, the amount of light is determined by the aperture.

However, the issue isn't with final focal ratio its with exposure time with a still camera or integration time with a video camera.

As the photos above show, integration time has the potential to render focal ratio insignificant.

Reduce the focal ratio as low as you can, you will not see what you can see with 30 seconds of integration on dsos. The circuitry in the Mallincam accumulates much more light into the image than the eye can.

But, again, with say an F10 focal ratio I can increase integration time to create a larger, brighter image than I can get by reducing the focal ratio on my 10" SCT as low as possible.

The question is, what is the mathematical ratio between integration time and focal ratio. A 10" Meade sct with a .5 reducer results in a produces an image of a certain size and brightness. How much integration is required to to produce an image of the same brightness with a 4" refractor with the same focal ratio?

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jimb1001
sage

Reged: 11/14/09

Loc: Florida
Re: Mallincam and Light Gathering [Re: nytecam]
#5125381 - 03/16/12 12:10 PM

Quote:

my brief exposure tests on M27 and m57 thus...

Thanks so much for illustrating the point. You've perfectly demonstrated the benefits of integration.

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psu_13
sage

Reged: 05/30/10

Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5125537 - 03/16/12 01:42 PM

I think the whole f-ratio vs. aperture question is more complicated.

Typically the "fast f-ratio means short exposures" idea comes from an analogy to traditional cameras, where shutter speed and aperture are locked together. But that analogy is misleading because when you change the f-stop on a camera lens you are actually opening or closing an little hole, thus increasing or decreasing the physical aperture.

When you change the f-ratio of a telescope you are changing the effective focal length and therefore the image scale. With extended objects this can change the amount of light from the object that hits the chip and thus make the object appear "brighter" ... thus you get shorter exposures.

On the other hand, my bet is that if you put two telescopes side by side and equalized the image scale and exposure time the scope with the larger aperture would make the brighter pictures because ... it has more aperture.

So the truth is, it seems to me that both f-ratio *and* aperture matter. But for different reasons.

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: psu_13]
#5125628 - 03/16/12 02:45 PM

Quote:

When you change the f-ratio of a telescope you are changing the effective focal length and therefore the image scale.

I am not sure what your statement is referring to. If “change the f-ratio” means make the aperture of the front opening bigger then it may change the F-number of the scope but that doesn’t mean that the focal length has to change.

They make an 8” 800mm F4.0 and they could make a 10” 800mm F3.15 scope. That changes the F-ratio but keeps the focal length and field of view the same. Both of these scopes will yield different brightness levels for a camera. However, they could also make a 10” 1000mm F4.0 scope that would have a narrower field of view but yield the exact same brightness for the portion of the object that is within its field of view.

Really it is as simple as I described. A telescope is a lens just as any other camera lens. It still has to follow the laws of physics. Focal Ratio determines the exposure of the image not the front aperture of the lens/telescope.

Edited by mpgxsvcd (03/16/12 03:27 PM)

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: mclewis1]
#5125693 - 03/16/12 03:26 PM

Quote:

Integration on the Mallincam means exposure not stacking time. You also have to remember that while the Mallincam integrates for X seconds, it stores that image in a buffer and outputs it as a continuous NTSC (29.97 frame/sec) video signal and not as a one shot dump.

Does the Mallincam update the screen with a buffer image every 29.97 times a second? How often does the image change when integrating with the Mallincam?

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mpgxsvcd
Pooh-Bah

Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5125737 - 03/16/12 03:51 PM

You need to think of this all as “Stops of Exposure”. It is a unitless measurement that refers to multiples of exposure. A 1 stop gain in exposure is a 2x increase of exposure. Changing the shutter speed from 1 second to 2 seconds gains you 1 stop and changing the aperture from F4.0 to F2.0 will gain you 2 stops(4x the exposure).

Let’s just say that your mount can offer you a usable shutter duration of 1 second to 128 seconds. That is 7 stops of exposure. That is a lot of stops when you compare it to the fact that going from an F16 to an F2.0 scope only gives you 6 stops.

You are far better off trying to get a good mount and camera that can handle longer exposure times than you are trying to get an ultra low focal ratio telescope. And you are wasting your time and money trying to get a bigger scope if you are already using an rock bottom focal ratio(ie: F2.0-F4.0) and your focal length is already acceptable at that focal ratio.

If you increase the telescope aperture and keep the entire system focal ratio the same the exposure of the image/picture will stay the same for a given exposure time.

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mclewis1

Reged: 02/25/06

Re: Mallincam and Light Gathering [Re: mpgxsvcd]
#5125958 - 03/16/12 06:13 PM

Quote:

Quote:

Integration on the Mallincam means exposure not stacking time. You also have to remember that while the Mallincam integrates for X seconds, it stores that image in a buffer and outputs it as a continuous NTSC (29.97 frame/sec) video signal and not as a one shot dump.

Does the Mallincam update the screen with a buffer image every 29.97 times a second? How often does the image change when integrating with the Mallincam?

Yes the output is 29.97 frames a second (NTSC version). The images changes at the end of each integration which can be 1/12,000 of a second to I believe 999 seconds (CCD mode on the Mallincam Xtreme). The normal or more commonly used range is 1/12,000 to a minute or two.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Mallincam and Light Gathering [Re: mclewis1]
#5126158 - 03/16/12 08:23 PM

If you take a 200mm aperture f/4 scope and place a mask in front to stop it down to 100mm aperture, you will have a 100mm f/8 scope.

The image scale is of course unchanged.

The image surface brightness is 1/4. Therefore the integration time would have to be quadrupled in order to obtain equal image density.

Take the same 200mm f/4 scope and install a 0.5X reducer.

The focal length is now effectively halved to 400mm, and so image scale is reduced to half.

The f/ratio is now f/2, and image surface brightness is 4 times brighter. Integration time can now be reduced to 1/4.

Take the same 200mm f/4 scope and install a 2X Barlow.

Focal length is doubled to 1600mm, and so image scale has also doubled.

The scope now works at f/8 ( just as when masked down earlier), and so image brightness is 1/4, and integration time must be made 4X longer.

To compare the masked and Barlowed situation, note that both configurations are delivering identical surface brightness and hence require identical exposure, but the Barlowed case has twice the image scale. This is the power of a larger aperture; more detail at given f/ratio.

And I must agree with the earlier analogy between scopes and camera lenses. From the imaging standpoint they behave exactly the same. It matters not whether one has a fixed aperture and the other has a variable diaphragm. Focal ratio always is the determinant of surface brightness at the sensor.

Do not fall into the common trap of comparing afocal configurations (where an eyepiece is used for visual viewing) with the imaging application.

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