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Live video focal ratio vs. aperture

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#1 Pharquart

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Posted 07 May 2013 - 08:24 PM

I've asked a similar question to this in the past, but now I'm approaching from a different direction.

I have a Samsung SCB-2000 camera that I use to feed live video to a monitor. I'm not interested in imaging, retouching, stacking, or other permanent records. I just want to show people stuff in the sky for outreach.

The Samsung seems to work very well for planets (1 night out so far). DSOs were another story. I wasn't in a good place for general light pollution (red zone, close to white) or for local light pollution (near a street light). However, M51 just had the core visible, and that at the highest integration time with lots of hot pixels mixed in. That was in an 8" SCT reduced down to F/5.

So here's my question for understanding: is focal ratio the only thing to worry about, or do I also have to consider aperture for DSOs? Is an 8" F/5 the same for video as a 80mm F/5? If I cut the focal ratio in half, I get 4x the light on the chip (right?) If I cut the aperture in half, I get 1/4 the light gathering power. Do those offset?

I'm considering buying or building a shorter focal ratio scope that I can use to better show DSOs with my camera. Smaller aperture is cheaper/lighter, but do I sacrifice light grasp at my camera?

Brian

#2 GlennLeDrew

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Posted 07 May 2013 - 08:53 PM

Image surface brightness is controlled by the f/ratio. A 2" f/5, 8" f/5 and 30" f/5 will all deliver equally bright images. The larger the aperture, the more detail at given f/ratio.

Your camera, with its limited exposure duration, likes to have the brightest possible image where extended objects are concerned. With a Meade f/3.3 reducer you will enjoy an image brighter than that at f/5 by a factor of 2.78. That will help considerably.

#3 jchaller

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Posted 07 May 2013 - 09:31 PM

Hello Brian.
I have an 11" scope and a Samsung (SDC-435) and it took me awhile to figure out the best camera settings (as least for me) to use. The following image of M51 is what I was able to get in a Red zone at f6.3 - the scope was pointed away from the worst light pollution.

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#4 Pharquart

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Posted 07 May 2013 - 09:59 PM

Thanks for the responses. I still haven't got my mind completely wrapped around F ratio being the only thing affecting brightness. I'm getting there, but I'll have to chew on it for a while. I'll doodle some diagrams and I'll get there.

So for Saturn, for example, is it correct to say that the image brightness will be the same in a 2" F/5 as a 4" F/5, but the size of the image will be 2x in the 4" (because of 2x increase in focal length)? Is that what you mean by "more detail"?

Thanks for the sample image, Jim. I saw something similar: 2 brighter areas. In fact, I thought the cores looked more like globular clusters, and that's what I see in your pic, though a little denser than what I saw. I see some of the spiral arms of M51 in your pic, something I didn't see in mine. I had to give up when my battery tank started to get low and I lost good tracking, otherwise I may have played around more with camera settings.

Brian

#5 nytecam

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Posted 08 May 2013 - 04:11 AM

Thanks for the responses. I still haven't got my mind completely wrapped around F ratio being the only thing affecting brightness. I'm getting there, but I'll have to chew on it for a while. I'll doodle some diagrams and I'll get there. So for Saturn, for example, is it correct to say that the image brightness will be the same in a 2" F/5 as a 4" F/5, but the size of the image will be 2x in the 4" (because of 2x increase in focal length)? Is that what you mean by "more detail"? Thanks for the sample image, Jim. I saw something similar: 2 brighter areas. In fact, I thought the cores looked more like globular clusters, and that's what I see in your pic, though a little denser than what I saw. I see some of the spiral arms of M51 in your pic, something I didn't see in mine. I had to give up when my battery tank started to get low and I lost good tracking, otherwise I may have played around more with camera settings. Brian

Solar system stuff [sun with safe filter, moon and bright planets] and faint fuzzies are totally different ballgames - the latter under light pollution are very challenging with the limited Sammy max exposure but possible.

For SS you can probably use the scope's native fl or even Barlows to increase image scale and detail using the max Sammy exposure or maybe less to avoid cam shake.

DSOs are inordinately dim by comparison and need a 'fast' f/ratios via focal reducers [I use f/3.3 FR] to catch every photon again with the max Sammy exposure. Detail and image scale using these shorter focal lengths is compremised so tweak monitor and cam controls for best results may be necessary. Good luck :grin:

#6 ccs_hello

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Posted 08 May 2013 - 06:33 AM

Brian,

Your focal ratio question usually would have generated tons of replies in an imaging forum, so let me try if I can reduce the traffic a little bit.

1. The answer will be the same, be it using a (long exposure) videocam in near realtime mode, an astro CCD imager, or a DSLR.

2. The Answer will be different depend on if the study subject is a point light source (a star) or an extended object (e.g., a galaxy, such as M51.)

3. This one is the one that usually triggers the apples vs. orange debate:

Assuming the same imaging device is used:

3a. (more degrees of freedom:) choice of two OTAs, one is 320mm f/5 (fl=1600mm) and the other is 80mm f/5 (fl=400mm). Note that not only the aperture is different (can collect more photon flux) but the sky coverage is different (larger fl sees less sky coverage -- FoV.)
This comparison assumes both are perfect APO refractors to avoid OTA design type debate/tradeoffs.

3b. (one degree of freedom) only one optical system (e.g., a camera lens), manually adjust lens's aperture (f-number ring in camera lens or change aperture mask in an OTA), and compare. Note that in this case, fl never changes. Reducing the f-ratio effectively reduces the aperture size proportionally.

3c. (one degree of freedom:) only one OTA is used but this time use auxiliary optics to adjust formula. Adding a focal reducer to reduce the f-ratio (now fl is lowered but aperture is still the same). Adding a Barlow to increase the f-ratio (now fl is higher but aperture value stays the same.)

Hope this set up the reference framework properly.

Clear Skies!

ccs_hello

#7 mpgxsvcd

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Posted 08 May 2013 - 11:01 AM

Brain,

Your focal ratio question usually would have generated tons of replies in an imaging forum, so let me try if I can reduce the traffic a little bit.

1. The answer will be the same, be it using a (long exposure) videocam in near realtime mode, an astro CCD imager, or a DSLR.

2. The Answer will be different depend on if the study subject is a point light source (a star) or an extended object (e.g., a galaxy, such as M51.)

3. This one is the one that usually triggers the apples vs. orange debate:

Assuming the same imaging device is used:

3a. (more degrees of freedom:) choice of two OTAs, one is 320mm f/5 (fl=1600mm) and the other is 80mm f/5 (fl=400mm). Note that not only the aperture is different (can collect more photon flux) but the sky coverage is different (larger fl sees less sky coverage -- FoV.)
This comparison assumes both are perfect APO refractors to avoid OTA design type debate/tradeoffs.

3b. (one degree of freedom) only one optical system (e.g., a camera lens), manually adjust lens's aperture (f-number ring in camera lens or change aperture mask in an OTA), and compare. Note that in this case, fl never changes. Reducing the f-ratio effectively reduces the aperture size proportionally.

3c. (one degree of freedom:) only one OTA is used but this time use auxiliary optics to adjust formula. Adding a focal reducer to reduce the f-ratio (now fl is lowered but aperture is still the same). Adding a Barlow to increase the f-ratio (now fl is higher but aperture value stays the same.)

Hope this set up the reference framework properly.

Clear Skies!

ccs_hello


Have there ever been any definitive tests that show that front Aperture is the only thing that affects a point source’s brightness?

The focal ratio vs. exposure relationship is a fundamental aspect of optics and physics. That doesn’t disappear because of it being a point source.

As the image size decreases the effects of lengthening the focal length decrease. When the object is a point source it won’t matter how long your focal length is the image scale of the object won’t change. That part I agree with.

However, the brightness of a star will decrease if you increase the focal ratio towards infinity and leave the front aperture alone. A simple barlow will demonstrate that if you decrease the point source’s exposure below the point of clipping.

The issue that comes into play is that stars are almost always completely clipped exposures. Once you pass the point of clipping how the sensor handles the clipped light becomes a bigger factor than the optics.

If you decrease the focal ratio you won’t always increase the brightness of a point source because of clipping. However, if you have an unclipped point source then the focal ratio is what will determine the exposure just like a non-point source will.

Focal length is the only thing that does not have any direct impact on the image of a point source.

#8 mpgxsvcd

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Posted 08 May 2013 - 12:29 PM

Brain,


Freudian Slip?

#9 GlennLeDrew

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Posted 08 May 2013 - 01:25 PM

To understand how f/ratio works to control image brightness, imagine the following.

You're inside an unlit box whose roof is at 10 feet. You have a sheet of paper on the floor and a light meter to measure its brightness.

You cut a hole 1 foot across in the roof, through which the light of the (uniformly bright) daytime sky floods. From the paper's perspective, this is an f/10 aperture. You take a measure of the illumination on the paper, and get, say, 10 units.

Now you enlarge the hole to 2 feet. For the paper at floor level, this is equivalent to f/5. Your meter will now show the paper as being illuminated to 40 units. This is 4X more than at 'f/10' because a 2 foot hole lets in 4X more light than a 1 foot hole.

A telescope aperture is exactly like this example of a skylight. To any point on the detector, the aperture is an opening through which light pours; the larger the angular extent (more specifically, the solid angle), the higher the photon flux.

Many get confused by trying to picture the 'concentration' of light via the focusing action of the objective. This is unnecessary. Simply imagine the way a floor gets brighter when a skylight is enlarged. Even though in the latter case there is no focusing action, the result is the same for extended sources.

#10 Spacetravelerx

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Posted 08 May 2013 - 01:44 PM

Glenn,

Excellent example!

#11 mpgxsvcd

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Posted 08 May 2013 - 01:44 PM

@GlennLeDrew

+1

#12 Rich Brady

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Posted 09 May 2013 - 03:35 PM

Excellent explanation. That helps me out as well.

:-)

#13 Jeff Morgan

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Posted 11 May 2013 - 03:19 PM

So using your example Glenn, would suspending the paper 5 feet off the ground while maintaining the 1 square foot hole also result in f/5 illumination?

#14 GlennLeDrew

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Posted 11 May 2013 - 04:10 PM

Yes, Jeff. One could liken the decreased distance (from 10 feet to 5) between the 1 foot hole and paper to the action of a focal reducer (which reduces the effective focal length, not increasing the aperture.)






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