A bigger aperture for astro imaging worth it ?

[red dwalf]

Is there any advantage of increasing the aperture for astro imaging ? 

My thinking is a bigger aperture collects more photons and the light grasp is also increased leading to a bigger aperture scope being able to capture fainter parts of an object for the same given exposure time, does this make sense or is my logic flawed ?

I'm currently using an William optics gt 81 mm triplet in a fairly light polluted area and think of a bigger aperture scope for the reasons above.

All advice gratefully received.

[vlaiv]

Yes if paired up with appropriate camera.

For larger aperture, and of course larger focal length that comes with it - for same pixel size of camera you will get bigger resolution - less arc seconds per pixel. But if you choose the camera with larger pixels then you will benefit from larger aperture.

So, if you plan to switch to bigger aperture, plan to get appropriate camera as well in order to get real gains in exposure length. Probably the best way to think of it is similar to visual. Using the same eyepiece on two different scopes will yield different magnifications and if you go overboard with magnification - image will get darker - light is spread over bigger surface. Same thing happens with camera. If you choose for example 8" scope and camera that gives you 1"/pixel, and 4" scope and different camera that also gives 1"/pixel - its pretty easy to see that in first case you'll have 8" worth of light at 1"/pixel while in later case 4 times less than that.

This does not translate in 4 times less imaging time with larger aperture. It would if only thing you had to worry about was light coming from target. But there are all sorts of noise sources that one must deal with - notably LP in this case must be taken into account - same thing that applies to light from source applies to LP light - bigger aperture - more LP light collected. So real gains are sure to be less then aperture difference, but can be significant in any case.

[alacant]

In my experience if you can get bigger and faster so much the better. Here is 29x120s with a 150mm f5 compared to 18x60s with a 208mm f3.9. I should also mention that I sometimes use an old 150mm f8 refractor for galaxies. I suppose it depends what you want to snap. HTH.

[MartinB]

For a given focal length the wider the aperture the more light will fall onto the camera chip and that generally speaking is a very good thing!  

Where things get murky is when you start to compare scopes of different focal lengths.  Will an 80mm F5 scope get more light onto the chip than a 250mm F10? No!  The larger aperture will always take in more light from a given piece of sky than a narrower aperture scope but it is actually taking in light from a much smaller area of sky.   If you imaged M51 with these two scopes for the same period of time M51 itself would have a much better signal to noise with the F10 scope than the F5.   However, the image from the 80mm scope will be surrounded by a nice star field.  

Overall, if you increase the aperture but keep the same F ratio you end up with a more magnified image with a similar signal to noise ratio.  

[Xplode]

1 hour ago, MartinB said:

For a given focal length the wider the aperture the more light will fall onto the camera chip and that generally speaking is a very good thing!  

Where things get murky is when you start to compare scopes of different focal lengths.  Will an 80mm F5 scope get more light onto the chip than a 250mm F10? No!  The larger aperture will always take in more light from a given piece of sky than a narrower aperture scope but it is actually taking in light from a much smaller area of sky.   If you imaged M51 with these two scopes for the same period of time M51 itself would have a much better signal to noise with the F10 scope than the F5.   However, the image from the 80mm scope will be surrounded by a nice star field.  

Overall, if you increase the aperture but keep the same F ratio you end up with a more magnified image with a similar signal to noise ratio.  

I don't agree with what you're saying here, why? Focal length and size of the scope doesn't really matter, it's all about the f value because that's what in the end decides how many photons fall on each pixel.

80 mm f5 vs 250mm f10 would mean the f5 scope has 4x the signal in a given time, but of course much lower theoretical detail.

[MartinB]

27 minutes ago, Xplode said:

I don't agree with what you're saying here, why? Focal length and size of the scope doesn't really matter, it's all about the f value because that's what in the end decides how many photons fall on each pixel.

80 mm f5 vs 250mm f10 would mean the f5 scope has 4x the signal in a given time, but of course much lower theoretical detail.

A scope with  250mm of clear aperture aperture will gather nearly 10 times as may photons as an 80mm scope for a given area of sky.  What you are suggesting is known as the "focal ratio myth"  https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjpp7yA1-rRAhVnDcAKHf5uAmcQFggdMAA&url=http%3A%2F%2Fwww.stanmooreastro.com%2Ff_ratio_myth.htm&usg=AFQjCNFzwnzw8_KvfqAe15cJ7odWsWLjXg&sig2=smFJ8GQbFidwrS6SVHbOdA  and more here https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwjpp7yA1-rRAhVnDcAKHf5uAmcQFggpMAI&url=http%3A%2F%2Fwww.stark-labs.com%2Fhelp%2Fblog%2Ffiles%2FFratioAperture.php&usg=AFQjCNGGzkawFTAs77IZqSt7GmWh0OjQ3w&sig2=ld579LFMqFZAeXDsljz8Kw

[ollypenrice]

I entirely agree with Martin. F ratio is a 'derived variable' and can be very misleading. The numbers which have absolute value are aperture and focal length.

Think in terms of object photos - the photons arriving on your chip from the object. For a given aperture you can change your focal length (using a reducer) but doing so will have precisely no effect whatever on the number of photons coming from M33. There is only one way to get more photons from M33 in a given time and that is to have more aperture.  When photography discovered the F ratio rule (exposure time going as the square of the F ratio) the focal length was never a variable. Only aperture was variable. If you stick with that then the rule works. When you make the focal length the variable it does not work, just as Martin said. You have discovered the dreaded F ratio myth! 

Olly

[alacant]

5 hours ago, red dwalf said:

Is there any advantage of increasing the aperture for astro imaging ? 

Mmm. Just trying to help answer the OP's question. My example shows the same area of sky from two similar focal length telescopes. I think the larger aperture is better, so it's a yes from me. HTH.

 

[Xplode]

Read what Craig Stark writes on that 2nd, he doesn't agree with either what i'm saying or what you are saying, he is somewhere in between what we're saying.

There is no clear rule concerning this because there's so many factors involved so saying that f ratio doesn't matter isn't right...just as it's not right what i'm saying that only f ratio matters.

 

So what i can say from what Craig Stark writes is this: untill the signal is above the read noise f ratio rules, after it's above the read noise f ratio isn't as important as people think it is.

[Alien 13]

I do wonder how much light from the collected photons gets onto a given chip size probably less than 10% in a lot of cases or maybe even lower at 1%, you only have to focus a 50 mm bino lens on a daytime object while projecting the image in a dark room to see that the projected image can be a meter across.

Alan

[SilverAstro]

3 hours ago, Alien 13 said:

I do wonder how much light from the collected photons gets onto a given chip size probably less than 10% in a lot of cases or maybe even lower at 1%, you only have to focus a 50 mm bino lens on a daytime object while projecting the image in a dark room to see that the projected image can be a meter across.

Oh ho ! that is eyepiece projection, a whole other kettle of fish, lets not go there,, please !!

Else the camera could become even more obscura   

PS ever tried a 1/4in. pinhole projected down a dark 30yd corridor ? you can see sunspots !

[ollypenrice]

11 hours ago, Xplode said:

Read what Craig Stark writes on that 2nd, he doesn't agree with either what i'm saying or what you are saying, he is somewhere in between what we're saying.

There is no clear rule concerning this because there's so many factors involved so saying that f ratio doesn't matter isn't right...just as it's not right what i'm saying that only f ratio matters.

 

So what i can say from what Craig Stark writes is this: untill the signal is above the read noise f ratio rules, after it's above the read noise f ratio isn't as important as people think it is.

Craig Stark is right to add a caveat to the discussion. If the signal per pixel is too weak to get above the noise floor then that signal is lost. I don't think that he is taking a midway position but simply adding this proviso. I agree with it and didn't mention it before because it introduced another complexity. In a video lecture that I saw he endorsed Fred Espenak's stand on the F ratio myth but pointed out the need to get above the noise floor for it to apply.

Nobody is saying F ratio doesn't matter. What I, at least, am trying to do is to point out that it is not possible to use F ratio alone to define signal strength from a particular object. So...

1) If the FL is fixed then more aperture brings in more light and exposure times go down. The focal ratio has also gone down but it is the extra aperture which is the active agent.

2) If you keep aperture constant and shorten the FL do you reduce exposure time? Yes or no, depending on target. Yes if you want everything in the wider FOV. Light from a wider area is concentrated onto the chip so there is more 'wanted' light and exposure times go down.  No if you are imaging a small object surrounded by sky that you don't particularly want and will possibly crop out later. (And also yes if, at the original FL, your faint signal was not getting above the noise floor. But you could increase sub exposure times, usually, to see that it did.)

Olly

[red dwalf]

Just now, ollypenrice said:

If the FL is fixed then more aperture brings in more light and exposure times go down

this is one of the ideas that got me asking the question,

if i can get a scope at a bigger aperture and around the same f5 ratio, ( although i`m a believer in the f ratio myth to some extent, i see the shorter f ratio meaning that a bigger field of view is achievable when paired with the same size camera chip), usually with a bigger aperture the focal length increases but if the slight increase still allows me to frame most of the targets well in a moderate to poor light polluted area my greater light grasp and shorter exposures would therefore help with the reduction of the seeing conditions and intimately the gradients i pick up in my images, especially when LRGB imaging.

[MartinB]

2 hours ago, red dwalf said:

 

if i can get a scope at a bigger aperture and around the same f5 ratio, ( although i`m a believer in the f ratio myth to some extent, i see the shorter f ratio meaning that a bigger field of view is achievable when paired with the same size camera chip), usually with a bigger aperture the focal length increases but if the slight increase still allows me to frame most of the targets well in a moderate to poor light polluted area my greater light grasp and shorter exposures would therefore help with the reduction of the seeing conditions and intimately the gradients i pick up in my images, especially when LRGB imaging.

"Seeing" refers to the stability of the light signal coming through the Earths atmosphere.  Instablility or seeing becomes more of a problem with increases in focal length.  Although, with greater light grasp, you have the potential to reduce exposure times the light pollution will still be having the same effect.  Also wider field of views/ low F ratio scopes tend to aggravate gradient problems.  

 

[ollypenrice]

3 hours ago, red dwalf said:

this is one of the ideas that got me asking the question,

if i can get a scope at a bigger aperture and around the same f5 ratio, ( although i`m a believer in the f ratio myth to some extent, i see the shorter f ratio meaning that a bigger field of view is achievable when paired with the same size camera chip), usually with a bigger aperture the focal length increases but if the slight increase still allows me to frame most of the targets well in a moderate to poor light polluted area my greater light grasp and shorter exposures would therefore help with the reduction of the seeing conditions and intimately the gradients i pick up in my images, especially when LRGB imaging.

I believe that it really is best to use the terms with strict correctness or it is easy to come to the wrong conclusion.

Field of view is determined by chip size and focal length. Nothing else. Focal ratio, as a term, is entirely unconnected with FOV. (Changing the F stop on a camera lens does not alter the FOV, though it does alter the depth of field.)

Focal ratio, at a fixed focal length, is synonimous with aperture so the more aperture the faster the exposure. The focal ratio does accurately reflect this when the FL is fixed. There is no myth here, it is a matter of hard fact. 

If you compare two F 5 scopes of different sizes, the big one has more aperture and more focal length, by definition. Will it be faster to image object x in the big one? It depends on the object. If the big scope has insufficient FOV to cover object x you will need to do a mosaic taking twice as long. If object x fits on the chip of the big scope then the big scope will give more resolution in equivalent time OR you could shoot the object in less time, resample the image down to the size of the image from the smaller scope, and get the smaller scope's resolution and S/N ratio in less time.

Olly

[red dwalf]

i`ve done a little more reading up on ideal scopes and camera combinations and it seems to be that the ideal arc sec / pixel rate is somewhere between 1 and 2, this being the case i have attempted a little maths and with my scope and camera combination i get to 2.45 arc sec / pixel.

William optics gt 81 focal length 478 mm, with reducer/ flattener = 382.4mm 

Atik 460 mono pixel size 4.54um gives a arc sec / pixel of  2.45, and 1.96 without the reducer / flattener, 1.96 arc sec / pixel seems to good for my area with moderate to awful light pollution but 2.45 looks to be over sampling.

guide scope focal length 225mm with a guide camera pixel size of 3.75um gives a number of 1.68 pixel / second, i think if i`ve done this calculation correctly.

would anyone be kind enough to check my maths are give any advice on imaging at 2.45 arc sec / pixel, would i be better trying to reduce this, i know a lot depends on my conditions when imaging.

[Robp]

44 minutes ago, red dwalf said:

i`ve done a little more reading up on ideal scopes and camera combinations and it seems to be that the ideal arc sec / pixel rate is somewhere between 1 and 2, this being the case i have attempted a little maths and with my scope and camera combination i get to 2.45 arc sec / pixel.

William optics gt 81 focal length 478 mm, with reducer/ flattener = 382.4mm 

Atik 460 mono pixel size 4.54um gives a arc sec / pixel of  2.45, and 1.96 without the reducer / flattener, 1.96 arc sec / pixel seems to good for my area with moderate to awful light pollution but 2.45 looks to be over sampling.

guide scope focal length 225mm with a guide camera pixel size of 3.75um gives a number of 1.68 pixel / second, i think if i`ve done this calculation correctly.

would anyone be kind enough to check my maths are give any advice on imaging at 2.45 arc sec / pixel, would i be better trying to reduce this, i know a lot depends on my conditions when imaging.

This may be of assistance to you:

http://astronomy.tools/calculators/field_of_view/

Select imaging mode and then enter a target, scope, camera and the reducer and it will calculate it all automatically and also show you the FOV on the selected target.

[ollypenrice]

Your values are correct on my calculator. I don't believe there is any reason at all to worry about getting below 2.4"PP. This is not a focal length for galaxxy shooting (once you've had a look at M31, M33 and M101) so going for the finest resolution would not be a priority for me. I would say the combination was good. However, while you would suffer some loss in resolution if you went for an 8300 chipped camera you would have an enormous gain in field of view. The 8300 chip is much larger. My feeling would be that, with the targets available to short focal lengths, more field of view would be very, very tempting. You'd be working at 2.9"PP. For reference our twin Tak is working at 3.5"PP. Sara Wager, shooting with an FL of 328mm (so shorter than yours) takes excellent pictures with the 8300. In fact she had her Sony chip replaced by the larger one by QSI and greatly preferred the increase in FOV. http://www.swagastro.com/  I can't honestly say that her Baby Q pictures look marred by under sampling!

Even our efforts at 3.5"PP look OK to me, though we'd have smaller pixels at this FOV if we could buy them.

The Sony-Kodak debate also involves the greater sensitivity and lower noise of the Sony. Given the fact that you'll use calibration anyway I think you might find the difference underwhelming.

BTW, I'm dithering over this myself because I want a smaller pixel camera to do galaxy imaging in the TEC at just over a metre in FL. In my case the targets will be small so the FOV doesn't matter but the resolution does matter because galaxies, unlike nebulae, are packed with tiny details.

Olly

 

[red dwalf]

Excellent help, thank you so much for everyone's help on this, one thing I have gained from it all is that one scope, and camera, will not do it all, which is frustrating, I'd like to do more LRGB on galaxies but the last year or so I've been concentrating on narrow band stuff.