exposure times in relation to scopes?

[Kaptain Klevtsov]

Totally disagree Nigel, the f/5 'scope will be collecting light from 4X the area of sky and delivering it to the camera compared to the f/10 'scope. This means that each pixel in the f/5 case gets the light that would fall on four pixels in the f/10 case. This translates to the pixels saturate (get full up) in 1/4 of the time in the f/5 case when compared to the f/10 case.

Kaptain Klevtsov

[MartinB]

What KK says is as true in practice as it is in theory. Whenever I image I aim to use an exposure time which leaves me with a background sky glow of between 2500 and 3000 ADUs measured in Maxim. Using a 0.67 reducer halves the time taken to get to this background level. Switching between an 80mm F7.5 ED80 to a ZS66 F5.9 also produces the expected reduction in "optimum" exposure time.

[dph1nm]

Totally disagree Nigel, the f/5 'scope will be collecting light from 4X the area of sky and delivering it to the camera compared to the f/10 'scope. This means that each pixel in the f/5 case gets the light that would fall on four pixels in the f/10 case. This translates to the pixels saturate (get full up) in 1/4 of the time in the f/5 case when compared to the f/10 case.

Kaptain Klevtsov

I agree entirely - in fact this is a good reason for imaging at F10 as you can image for 4 times as long (and get twice the s/n per arcsec) before you saturate!

I think there is a general feeling that somehow binning up pixels is cheating. It isn't - with a photon counting device I can just add up the 4 f/10 pixels and get exactly the same result as with the one f/5 pixel. Of course, the CCD at f5 has the advantage of covering a wider area of sky, so it may well be the preferred choice for imaging - if I wanted to cover the same area at f/10 with the same CCD it would take longer, as I would have to do several pointings. However, providing the object(s) I am interested in fit on the chip at f/10 then there is no exposure advantage in going to f/5.

NigelM

[MikeP]

So, exposure time is a function of focal ratio. Yes?

Mike

[cygnusx1]

I think this sums it all up rather neatly

LIGHT-RECORDING POWER OF A SYSTEM: Power = r^2/f^2

(the light-recording power is directly proportional to the square of the radius of the objective and inversely proportional to the square of the f-number)

* where Power is the light-recording power of the system

r is the radius of the objective

f is the f-number (f/) of the system

Example: a 200-mm f/8 system compared with a 100-mm f/5 system

(100^2)/8^2 compared with (50^2)/5^2

156.25 compared with 100, or 1.56 times more light-recording power

taken from > http://www.company7.com/library/astforms.html very handy comprehansive list of astro formulae

[MartinB]

I don't want to win an argument here and wont be posting any more replies. I just don't want people to be misled, especially people like Whiplash who made the original post. I have used deep sky imaging set ups from F2 to F10 and apertures from 66mm to 250mm. I obsess over optimal sub exposure lengths and how much exposure to give a target. Focal ratio is key to expsosure time and focal length to image scale. These are fundamental principles.

The more relevant formula from your link Kevin is this one

EXPOSURE COMPARISON FOR EXTENDED OBJECTS

= (f/S)^2/(f/E)^2 = ((f/S)/(f/E))^2

(the ratio of intensities of illumination is squared according to the inverse square law)

where Exposure Compensation is the exposure compensation to be made to the example system

f/S is the f-number (f/) of the subject system

f/E is the f-number (f/) of the example system

Below is taken from the Starizona website http://starizona.com/acb/ccd/advtheoryexp.aspx

Focal Ratio

Focal ratio is the primary determinant of imaging speed. Deep-sky imagers all know the importance of having a fast scope for reducing exposure times. Focal ratio also affects resolution, assuming a constant aperture (in other words, using a focal reducer on a given telescope) and thus affects SNR in the same way. More importantly, focal ratio determines exposure time necessary to achieve a given sky background flux, the importance of which will be seen in the next section.

A beginners guide to exposure times http://www.darkskyimages.com/gexpose.htm

Another one http://www.astronomynotes.net/2006/10/02/what-is-focal-ratio-and-exposure-factor/

[dph1nm]

Actually, seeing as I clearly can't convince anyone, this sums it up nicely

The F-ratio Myth

To summarise for those who don't want to read it all ...

Example: a 10-minute exposure with a 10” f/10 scope is equivalent to a 5-minute exposure with a 10” f/7 scope.

This is false!

Varying the f-ratio of a constant aperture has little or no affect on real S/N, except in certain limited circumstances. The relationship of exposure-time and f-ratio only holds true for equivalent focal lengths, which means the aperture must be varied to produce a given f-ratio.

and there are couple of images on the site to demonstrate this.

NigelM

[saturn5]

Well as i am off to get a canon dslr today i have been reading this with interest as it is a big jump from my point and click job fo dslr stuff.I have been reading up as much as i can from varying websites to SGL members posts,looking at setttings that they use.

This thread has some saying "black"while others say "white" so it is confusing to someone like me who has no idea about exposure times/focal ratio, etc,having never had the need to use them before.

Someone is right,but i .

[Kaptain Klevtsov]

Nigel, I agree that signal to noise ratio is not, f/ ratio, why would it be?

The link you posted points out nicely why you can't use a standard webcam for DSO imaging as the noise in each frame is too great when compared to the signal. The point is being made for longer subs being better as you get the same amount of signal in fewer frames. As each frame has readout noise you get less noise therefore the S/N ratio is improved.

What it doesn't address is the relationship between getting enough signal in the first place.

Most CCD imaging is done using either guessed or estimated exposure times. These are arrived at by either checking the histogram for a hump somewhere in the middle, or getting the highest value pixel close to, but not too close to, the saturation value of the pixel. This uses most of the camera's dynamic range and so captures the greatest variation in light and dark within the image, which is the object of the exercise.

In the article he points out that the number of photons collected is down to the size of the aperture and that's all there is to it when it comes to catching the light. This is quite true, but it doesn't explain what happens to the light when it hits the detector.

In a long focal length system the size of the image (plate scale) is greater than it is with a short focal length system. This means that the light from the object being imaged is more spread out in the long FL system covering more pixels on the detector (assuming you are using the same detector).

Lets say that you are using an aperture which collects 10,000 photons per second, just to put some numbers in here. If the long FL system forms an image which is 500 pixels square ( the target is a hypothetical big square DSO ) then it follows that these 10,000 photons land on 500*500 pixels, so there are 250000 pixels getting the light. Over 1000 seconds, the light coming in would be 10 million photons (10,000 per second for 1000 seconds) so each detector pixel gets 10,000,000/250000 = 40 photons. This will not saturate the pixels as they typically hold 256 on an 8 bit system or 65536 with a 16 bit system. We have catured an image, albeit a dim one.

Using the same numbers, but with a shorter focal length, the same amount of light is spread over fewer pixels. Lets say that the target shows up as 200 pixels square, so there are now 40,000 pixels getting the light. Each pixel would get 250 photons, which is close to saturated with an 8 bit camera.

An even shorter focal length might use 25,000 pixels, each one getting 400 photons, so we have suceeded in saturating the pixels, the white point is clipped, showing as 256 when they should read 400.

Assuming that we use either of the first two systems where clipping isn't a problem, we get the same number of photons detected per frame, and as its the same camera, the same amount of readout noise, shot noise, pixel noise and all the other kind of noise. This means that the signal to noise ratio is the same. Like the man said, it's not dependant on the focal ratio.

But.

The long focal length version where the brightest pixels show up at a value of 40 needs to be stretched during processing so that the bright bits show up as bright. We would typically scale the brightness of every pixel by a factor of 6 in this case so that the bright bits show up as bright, but not compleetly white. This gives the bright bits a reading of 240. The dark bits, where the noise is, also get stretched by a factor of 6, so the noise is 6X greater than in the raw image.

The short focal length system where the bright bits read 250 doesn't need stretching so the noise stays as it is straight out of the camera, which should give a better image.

Does this make sense?

Kaptain Klevtsov

[cygnusx1]

Well, I for one certainly have learnt a lot from this discussion - even if there seems to be differing views on this.

Everyone is making a point from a different angle.

I was always under the impression, aperture was the only factor in speed of exposure.

Now i realise the issue is more complex than just aperture - f/ratio, s/n , f/l, flux, sky quality, film or dslr or ccd, pixel size, noise all play their part.

What i'm starting to understand is how but more importantly WHY a scopes f/l + f/ratio is so vital in all of this.

Seems to me the old saying "aperture is king" is not the full story!

Thanks for the links Martin, KK and Nige.

[themos]

Look, if f-ratio was not an issue, I could make you a very cheap 3-foot objective with a ridiculously large focal length. There is a reason why such a cheap "King Aperture" telescope is not all that useful.

[dph1nm]

But.

The long focal length version where the brightest pixels show up at a value of 40 needs to be stretched during processing so that the bright bits show up as bright. We would typically scale the brightness of every pixel by a factor of 6 in this case so that the bright bits show up as bright, but not compleetly white. This gives the bright bits a reading of 240. The dark bits, where the noise is, also get stretched by a factor of 6, so the noise is 6X greater than in the raw image.

The short focal length system where the bright bits read 250 doesn't need stretching so the noise stays as it is straight out of the camera, which should give a better image.

But I wouldn't stretch - I would bin up the long focal length version so that the pixels were the same size on the sky as the short focal length version. Then in my 'superpixel' I would have the same signal and the same noise, i.e. the same picture (OK, so in your example, as I can only sensibly bin 2x2 or 3x3, I can't exactly match the pixel sizes, so I either get 160 - lower s/n but higher resolution - or 360 counts - higher s/n but lower resolution).

NigelM

[MartinB]

Stan Moore is nobody's fool so I thought I would consult my guru, Ron Wodaski, on the NewCCDAstro forum. Ron is the author of the CCD imagers bible "The New CCD Astonomy" and is frequented by some of the world's finest imagers.

Here is Ron's response (I hope he doesn't mind me quoting him, it seems more honest than paraphrasing. His book is worth every penny btw!

Yes, there's a myth involved here. But it's not quite what it seems. There

are two ways to look at this: objective, and subjective.

Objectively, the information content of the image doesn't change with focal

ratio. However, the _resolution_ of the image does change! So while the

_total information_ remains the same, what you see changes dramatically. For

most of us, what's important is what we see, not what we compute.

So to an astronomer doing things like photometry, the signal to noise ratio

is the primary concern, and Stan's calculations are spot on and very useful.

To an imager taking pretty pictures, what matters is now cool the image

looks. (to put it bluntly...)

At faster focal ratios, the trade-off of resolution for more extended

nebulosity can be a positive thing, and if so, use that fact.

For example: I run six telescopes remotely for the Tzec Maun foundation.

They range from f/2.8 to f/14. Without question, dim nebulosity looks better

in the faster scopes! (I just took a short image of Centaurus A with an

Epsilon 180 in Australia this morning; the dim portions of the galaxy show

up clearly.)

If you look at Stan's example images at the bottom of the article, note that

the f/2.9 image has been resampled to the same scale as the f/12.4 image.

Note also that Stan knows well enough to take long enough exposures to swamp

read noise with shot noise (a whole topic until itself...), so he has in

effect optimized both exposure durations for that. The result is that Stan

has _removed_ what _can be_ important to the eye: the difference in

resolution.

If you want to optimize; if you want to do metrics; if you want to explore

- then going down this path will make you a smarter imager once you get it

all figured out. Remember that noise is a very non-inutitive concept -

there's not much percentage in understanding uncertainty in the data for a

hunter gatherer!!! But with some application to the math and the concepts,

you can gain understanding of non-intuitive stuff, right? <G>

Ron W

[dph1nm]

Look, if f-ratio was not an issue, I could make you a very cheap 3-foot objective with a ridiculously large focal length. There is a reason why such a cheap "King Aperture" telescope is not all that useful.

I assume you mean it would be cheap to figure - but the reason this would not be useful is that the telecsope would be ridiculously long and you would need a detector with either very large pixels or an awful lot of small ones, both of which would be very expensive - and with the small pixel option you would then have to worry about read noise per pixel.

You might as well say that the current crop of 8metre telescopes are a waste of space, because (money no object) I could just build an incredibly short f-ratio 8inch and image just as deep with that!

NigelM

[themos]

You might as well say that the current crop of 8metre telescopes are a waste of space, because (money no object) I could just build an incredibly short f-ratio 8inch and image just as deep with that!

The Gemini primary mirror is already an f/1.78 (fl=14.402 ap=8.10). I suspect f-ratios don't get much lower than this. My point was that both aperture and f/ratio affect imaging performance.