Longer or shorter exposures with these warm summer nights?

[nmoushon]

I'm going camping for three days and two nights and will be in a nice dark sky location so I'm taking my kit with me to do some imaging (Praying that the cloud gods will be nice to me). Unfortunately it's going to be rather warm, even at night, and need some suggestions. I've just started guiding and have only 2 nights of imaging under my belt so far. So with these warm summer nights, anywhere from 50 - 70°F at midnight, what sub lengths should I go for? I can achieve 2min unguided or 5min guided. I'm using a 450D so noise is already an issue but the warmth of the summer I'm not sure if I should stick with 2min subs and keep the camera temp and noise down but loose some depth to my data or go for 5min subs for depth of data but risk high camera temps and noise?

Also what kind of times intervals do you have in between subs? I normal have 10sec between shots in the winter but I'm thinking I might need to bump that up to 20 or 30 sec to help with camera temp.

[Gina]

I'm afraid you just won't be able to do long exposures with an uncooled DSLR at those temperatures. You might get away with lots of short ones. Remember to take darks at the same temperature.

[nmoushon]

I was afraid of that. Oh well. I'm going for M20 so it's pretty bright. Thanks Gina.

[Gina]

It's always worth a try

[Tim]

Trial and error, but with more shorter exposures you can at least get the noise down, and you might be able to get a few longer ones and use them as a blend to get some detail in the important areas.

Remember to check focus regularly though, often the change in temperatures can be more dramatic in warmer months.

Enjoy the break

[astrosathya]

Hi Mate,

As you see mu location is Bangalore, India. We have a daytime temperature of about 92 F and peak in summer is 97F. I have a Canon 500D like many of my friends. We take upto five minutes at ISO 800 and we are absolutely fine. My camera was purchased from Currys in Arndale centre, Manchester. It works absolutely fine for me. Before you take a shot, just let it acclamatise to the outside temperature.

Regards

[nmoushon]

Hi Mate,

As you see mu location is Bangalore, India. We have a daytime temperature of about 92 F and peak in summer is 97F. I have a Canon 500D like many of my friends. We take upto five minutes at ISO 800 and we are absolutely fine. My camera was purchased from Currys in Arndale centre, Manchester. It works absolutely fine for me. Before you take a shot, just let it acclamatise to the outside temperature.

Regards

Thanks for the tip. I should probably drop the ISO to 800 instead of 1600. But if I shoot a 2.5min at ISO1600 wouldn't that be the same as 5min at ISO800? Especially in regards to fainter nebula. Whats your dark to light ratio during the summer? My peak summer temps are very close to yours so would love to see what you do. This is my first venture into real long exposures since last summer I was capped at 2min nonguided .

[Physicist13]

2 mins is fine! Its a v common misconception that you need to go longer! Just take lots and stack and the results will be just as good!

[nmoushon]

2 mins is fine! Its a v common misconception that you need to go longer! Just take lots and stack and the results will be just as good!

Oh I have used 2min subs for over a year now as that was my max unguided. I now you can get great images with 2min subs. I took 8hrs of 2min subs for M31 and M42 this last winter. But even at 8hrs you can just barely tease out the fainter nebula around M42. Thats where 5min subs come in. Not that it will greatly improve noise or quality but will greatly improve depth and allow you to get fainter objects/nebula with less over all time. Which is what I'm trying to achieve.

[IanL]

Oh I have used 2min subs for over a year now as that was my max unguided. I now you can get great images with 2min subs. I took 8hrs of 2min subs for M31 and M42 this last winter. But even at 8hrs you can just barely tease out the fainter nebula around M42. Thats where 5min subs come in. Not that it will greatly improve noise or quality but will greatly improve depth and allow you to get fainter objects/nebula with less over all time. Which is what I'm trying to achieve.

In a nutshell, "no". ISO (or gain) does not affect the amount of signal collected by you camera. The signal depends on:

- The number of photons received from the target per unit of time (its brightness).

- The light collecting power of your scope/lens. For point sources (stars) that increases with aperture, and for extended sources (nebulae, galaxies, etc.) it increases as focal ratio gets lower (i.e. larger aperture or shorter focal length).

- The percentage of photons your camera's sensor converts in to measurable electrons, i.e. the camera's Quantum Efficiency (which will be a lot less than 100% but is pretty much fixed for each pixel).

Changing the ISO (or gain, same thing) has no effect on any of the foregoing. Put it another way, when you change from ISO800 to ISO1600 does:

- The object in the sky get brighter? No.

- The telescope aperture increase or the focal length decrease? No.

- The quantum efficiency of the sensor increase? No.

For a given target and a given scope/camera combination, the only factor that can increase the signal in your image matters is total exposure length. That's it, nothing else, honestly.

The image 'out of the box' looks brighter at higher ISOs because the camera has already multiplied the pixel values before writing them to the file. But if you shoot at (say) ISO400, you can make an image look just as bright as one shot at ISO1600 in post processing by using a histogram transformation. There is no more 'information' in the ISO1600 image than the ISO400 image, and indeed there may be less in some cases (see below).

Not to say that ISO/gain is irrelevant though:

- If your ISO/gain is too high, you lose dynamic range. Basically the raw electrons in the camera are converted to numbers in the image file, and at high ISOs you can reach the maximum brightness level in the image file from fewer electrons in the sensor. The upshot is that at high ISOs you can only expose for a shorter time before the highlights in your image becomes burned out never to be recovered. In practical terms it probably doesn't matter for 2 or 5 minute exposures unless you are shooting something really bright like M42 or have a bright star in frame, but it is easy to avoid the problem in the first place.

- On the other hand tests have shown that readout noise for DSLRs can be worse at low ISOs, so you probably want to avoid ISO100 and ISO200 in most cases. It varies by camera but personally I only shoot at ISO400 with my 500D. Not saying that you can't shoot at 800 or 1600 provided you avoid the dynamic range problems, but I haven't seen any benefit.

Dark current and more importantly dark current noise are the bane of DSLR imagers; as you realise it increases with environmental temperature and exposure length. In theory it is easy enough to remove using dark frames shot at the same temperature and for the same exposure length, but in practice it is hard to get right on a camera that lacks set-point cooling. Also Canon cameras have been shown to reduce dark current on-chip for shorter exposures before creating the RAW files (but not the dark current noise), which makes it even more complex.

On the plus side in the UK, it is generally too bright at night to do much imaging in the warmer months, but I do sympathise with those of you in warm climates at lower latitudes!

[nmoushon]

In a nutshell, "no". ISO (or gain) does not affect the amount of signal collected by you camera. The signal depends on:

- The number of photons received from the target per unit of time (its brightness).

- The light collecting power of your scope/lens. For point sources (stars) that increases with aperture, and for extended sources (nebulae, galaxies, etc.) it increases as focal ratio gets lower (i.e. larger aperture or shorter focal length).

- The percentage of photons your camera's sensor converts in to measurable electrons, i.e. the camera's Quantum Efficiency (which will be a lot less than 100% but is pretty much fixed for each pixel).

Changing the ISO (or gain, same thing) has no effect on any of the foregoing. Put it another way, when you change from ISO800 to ISO1600 does:

- The object in the sky get brighter? No.

- The telescope aperture increase or the focal length decrease? No.

- The quantum efficiency of the sensor increase? No.

For a given target and a given scope/camera combination, the only factor that can increase the signal in your image matters is total exposure length. That's it, nothing else, honestly.

The image 'out of the box' looks brighter at higher ISOs because the camera has already multiplied the pixel values before writing them to the file. But if you shoot at (say) ISO400, you can make an image look just as bright as one shot at ISO1600 in post processing by using a histogram transformation. There is no more 'information' in the ISO1600 image than the ISO400 image, and indeed there may be less in some cases (see below).

Not to say that ISO/gain is irrelevant though:

- If your ISO/gain is too high, you lose dynamic range. Basically the raw electrons in the camera are converted to numbers in the image file, and at high ISOs you can reach the maximum brightness level in the image file from fewer electrons in the sensor. The upshot is that at high ISOs you can only expose for a shorter time before the highlights in your image becomes burned out never to be recovered. In practical terms it probably doesn't matter for 2 or 5 minute exposures unless you are shooting something really bright like M42 or have a bright star in frame, but it is easy to avoid the problem in the first place.

- On the other hand tests have shown that readout noise for DSLRs can be worse at low ISOs, so you probably want to avoid ISO100 and ISO200 in most cases. It varies by camera but personally I only shoot at ISO400 with my 500D. Not saying that you can't shoot at 800 or 1600 provided you avoid the dynamic range problems, but I haven't seen any benefit.

Dark current and more importantly dark current noise are the bane of DSLR imagers; as you realise it increases with environmental temperature and exposure length. In theory it is easy enough to remove using dark frames shot at the same temperature and for the same exposure length, but in practice it is hard to get right on a camera that lacks set-point cooling. Also Canon cameras have been shown to reduce dark current on-chip for shorter exposures before creating the RAW files (but not the dark current noise), which makes it even more complex.

On the plus side in the UK, it is generally too bright at night to do much imaging in the warmer months, but I do sympathise with those of you in warm climates at lower latitudes!

Over the last 18 months or so that I've been on here and the countless time that ISO has been brought up I think this is the most straight up and simplest answer I've read on the subject. Well done and thank you for a great explanation. And I do holey agree with you.

[Physicist13]

ISO does matter as it introduces addition amplifier noise which increases more than the signal gain. the optimum is usually around ISO800 but will depend on each camera slightly.

2mins exp subs if fine in ALL cases! Its a fallacy that you need 5mins or longer. the signal you want is still there just that the noise is larger so you need more subs. Its total exp time that matters. Technically, once you are above the "read noise" the other random noise will averge away.

The dynamic range arg above is valid, but very often its tolerated to saturate the stars in the image as its usually the DSO you want to see.

[ronin]

Does the Canon not have a noise reduction feature in it somewhere ?

Basically you take a 5 minute exposure then it takes another 5 minute exposure as a dark, then it subtracts the dark from your exposure and spits out a noise reduced exposure. Only heard of this recently, but had the idea it was not just on the newer models but went back some time.

Not sure if it is part of the camera software itself or part of the supplied software that comes with the camera - wouild means you need the software on a laptop and connected to the camera. Also doubles the time to get a completed shot.

Time to read the instructions.

[ronin]

Found this short bit of information about it:

The camera uses a "dark frame subtraction" method for Long Exposure NR -- it takes a "shot" with the shutter closed that is equal to the time of your long exposure, then calculates the image noise in the dark frame and removes it from your exposure.

Seems to be under the name Long Exposure Noise Reduction or LENR, most mention is with RAW files but one items says applied to JPEG's as well if requested.

Anyway one option to reduce the noise using the supplied features.

[dph1nm]

ISO does matter as it introduces addition amplifier noise which increases more than the signal gain.

Depends on the camera electronics. Canons actually have lower noise at higher ISO, whereas I believe for Nikons it is much the same at all ISO.

NigelM

[ollypenrice]

2 mins is fine! Its a v common misconception that you need to go longer! Just take lots and stack and the results will be just as good!

Sorry but this is not true. It is so far from being true that I hardly know where to start but Yves and I don't take two minute subs.

We rather prefer thirty minute subs unless that is going to kill star colour. Two minute subs at a dark site are a dead loss.

Olly

[nmoushon]

Found this short bit of information about it:

The camera uses a "dark frame subtraction" method for Long Exposure NR -- it takes a "shot" with the shutter closed that is equal to the time of your long exposure, then calculates the image noise in the dark frame and removes it from your exposure.

Seems to be under the name Long Exposure Noise Reduction or LENR, most mention is with RAW files but one items says applied to JPEG's as well if requested.

Anyway one option to reduce the noise using the supplied features.

Yes you can use this option but it not the best. You loose valuable clear sky time taking darks like this. I also remember something that this isnt the best way to take darks either besides wasting clear skies but can't remember off the top of my head.

Sorry but this is not true. It is so far from being true that I hardly know where to start but Yves and I don't take two minute subs.

We rather prefer thirty minute subs unless that is going to kill star colour. Two minute subs at a dark site are a dead loss.

Olly

Olly, I think you are mostly correct here. Yes Physicist13 is wrong that 2min will get you just as good of shots as longer subs you just need to take more. I've proven that here: http://stargazerslounge.com/topic/177259-8hrs-m31-m42/page__hl__%2Bm42+%2Bhrs ​But Physicist13 is partially correct in the fact that you can get good images with 2min subs you just need more of them which I also proved in my write up. This especially comes into play when you are using a DSLR vs a CCD camera which your pictures where taken with. And even to compare a DSLR to a modded or cooled DSLR even.

Also need to consider where your located at and your equipment. But you will get better images with longer subs hands down. Just don't want to discourage people that don't have to equipment to get even 5min subs let alone 30mins that they can't get good quality images. Obviously you get better quality the longer your subs are.

[swag72]

2 mins is fine! Its a v common misconception that you need to go longer! Just take lots and stack and the results will be just as good!

The original quoate as above, says clearly that the results will be 'just as good'

.......Physicist13 is partially correct in the fact that you can get good images with 2min subs you just need more of them which I also proved in my write up........

You are saying here that Physicist13 said that you can get good images with 2 minute subs .... I've re read the thread many times and no where does he say this, he says that the results will be JUST as good. I know this may sound like I am splitting hairs, but the two quotes above are very different indeed. In fact, he never even said the second!!

I don't doubt that you can get good images with 2 minute subs, there are images out there to prove it - Yes they are good. But masses of 2 minute subs will not be as good as fewer long ones with the same overall exposure time.

[Jessun]

Another interesting thread! Side by side with any discussion about f-ratio, sub length is right up there in the confusion league.

I say confusion, because in this day and age, with so many imagers around having done it since the dawn of amateur equipment, there shouldn't be a discussion about it.

But it's still on going, so that must mean something. It would be great if a CCD open to the dark sky for x amount of time will show the exact same image regardless of how many times it's briefly interrupted to read the data, but some difficult concepts kick in; read noise, linearity, well depth, residual image etc... So we can't do a simple paper exercise to sum up our exposure. Things go on in that CCD that I sure don't fully understand.

What does play a big part though is sky quality - all its variables, and this will introduce noise to a sub. Not by classic tech talk definition, but simply stuff that shouldn't be there - light pollution, moisture, particles, a turbulent layer somewhere - the list goes on, and this is for most of us the main enemy in the fight for best signal to noise ratio. The longest reasonable sub length is often determined by these factors - excluding mount and guiding performance - and it will vary night from night.

My basic understanding of this all is that it's beneficial to go for as long a sub length as you can comfortably do given sky conditions - to use as much of the dynamic range of the CCD in each sub. And then do as many as you can to benefit from the noise reduction that follows the familiar S/N graph, where most of the reduction is gained in the first 25 or so subs.

This sounds obvious perhaps, like saying go for the longest FL and the lowest f-ratio at the same time, but in the case of sub length the fact is that it can change due to sky conditions - for the same imager. To dither between sub length is sometimes a necessary evil and practice can hit theory on the head.

Olly, interestingly enough, I believe you are bolting together the definite argument killer with those TAKs and Atiks! An hour long comparison would be great to see. One camera could run 1 minute subs, and the other 6 perhaps - no filters. So it's 60 against 10 subs. (10 so as to give that stack a sporting noise reduction chance). What target? Well, I don't know but something like the lovely M51 - with the tidal waves in mind, or perhaps a nice wide nebula. One TAK hour is no ordinary hour so I'm sure data would be abundant for a comparison.

/Jesper

[ollypenrice]

Actually my post was made in error. I was unceremoniously cut off from the net and when I got going again I posted in the wrong thread!

But since I'm here, it's worth noting what Tim has said about ultra long exposures in NB from a far from perfect site. He chases the fearully faint stuff with the longest subs ever seen on here.

I feel that you go till the skyglow won't let you carry on or until the stars begin to suffer. In my case the sky holds out for ever but the stars do begin to swell.

Olly

[nmoushon]

You are saying here that Physicist13 said that you can get good images with 2 minute subs .... I've re read the thread many times and no where does he say this, he says that the results will be JUST as good. I know this may sound like I am splitting hairs, but the two quotes above are very different indeed. In fact, he never even said the second!!

I don't doubt that you can get good images with 2 minute subs, there are images out there to prove it - Yes they are good. But masses of 2 minute subs will not be as good as fewer long ones with the same overall exposure time.

You are correct about what he said and didnt say...I was giving him the benefit of the doubt here but maybe I was in the wrong place to do so. But who knows. Anyways, yes, 2min subs will never achieve what longer subs can no matter how many you collect. I took 8hrs of 2min subs and still was barely barely barely able to tease out the fainter nebula around M42 but not without completely and utterly over stretching the image. When winter comes around again I will be back on M42 but now I have guiding and will be doing much longer subs so add to my data as well as to show what longer subs can achieve over shorter.

Another interesting thread! Side by side with any discussion about f-ratio, sub length is right up there in the confusion league.

I say confusion, because in this day and age, with so many imagers around having done it since the dawn of amateur equipment, there shouldn't be a discussion about it.

But it's still on going, so that must mean something. It would be great if a CCD open to the dark sky for x amount of time will show the exact same image regardless of how many times it's briefly interrupted to read the data, but some difficult concepts kick in; read noise, linearity, well depth, residual image etc... So we can't do a simple paper exercise to sum up our exposure. Things go on in that CCD that I sure don't fully understand.

What does play a big part though is sky quality - all its variables, and this will introduce noise to a sub. Not by classic tech talk definition, but simply stuff that shouldn't be there - light pollution, moisture, particles, a turbulent layer somewhere - the list goes on, and this is for most of us the main enemy in the fight for best signal to noise ratio. The longest reasonable sub length is often determined by these factors - excluding mount and guiding performance - and it will vary night from night.

My basic understanding of this all is that it's beneficial to go for as long a sub length as you can comfortably do given sky conditions - to use as much of the dynamic range of the CCD in each sub. And then do as many as you can to benefit from the noise reduction that follows the familiar S/N graph, where most of the reduction is gained in the first 25 or so subs.

This sounds obvious perhaps, like saying go for the longest FL and the lowest f-ratio at the same time, but in the case of sub length the fact is that it can change due to sky conditions - for the same imager. To dither between sub length is sometimes a necessary evil and practice can hit theory on the head.

Olly, interestingly enough, I believe you are bolting together the definite argument killer with those TAKs and Atiks! An hour long comparison would be great to see. One camera could run 1 minute subs, and the other 6 perhaps - no filters. So it's 60 against 10 subs. (10 so as to give that stack a sporting noise reduction chance). What target? Well, I don't know but something like the lovely M51 - with the tidal waves in mind, or perhaps a nice wide nebula. One TAK hour is no ordinary hour so I'm sure data would be abundant for a comparison.

/Jesper

I like your points about sky conditions and such and I totally agree. But what I think is even more important to take into consideration is what camera you have. DSLR vs modded/cooled DSLR vs cooled CCD. Even if I could achieve 30min subs with my DSLR I dont know if it would be worth it because of all the heat gain and no cooling. Even during the winter. I think putting DSLR in the same class as CCD is a bit unfair. But thats probably a different thread.

[ollypenrice]

Yes, you have to be able to cool your chip in order to benefit from long subs. No doubt about that. However, there is not a snowflake in hell's chance that lots of short subs match fewer long ones at a dark site with a cooled camera and the same total imaging time. You have to get over the read noise to catch the faint stuff, which you can't do in short subs. I've had years of messing about with this, honestly, and it is incredibly obvious in practice. You want the IFN? Loooooonnnnngggg subs. (Hundreds of 'em!!! )

Olly

[Physicist13]

Yes, you have to be able to cool your chip in order to benefit from long subs. No doubt about that. However, there is not a snowflake in hell's chance that lots of short subs match fewer long ones at a dark site with a cooled camera and the same total imaging time. You have to get over the read noise to catch the faint stuff, which you can't do in short subs. I've had years of messing about with this, honestly, and it is incredibly obvious in practice. You want the IFN? Loooooonnnnngggg subs. (Hundreds of 'em!!! )

Olly

NOt a true statement. You have to do the Maths to determine. The same SNR can be reached.

[ollypenrice]

NOt a true statement. You have to do the Maths to determine. The same SNR can be reached.

Sorry but I have to say, do more imaging. There are all sorts of mathematical assertions made about imaging and not all of them are correct. One much discussed at the moment is the Nyquvist theorem. Praticising imagers with the right kit and the right experience are satisfied that they are disproving it and the pictures they are posting in support of that idea convince me. In the end I'm more interested in the pictures than the theorem but this might not describe everybody.

Are you seriously telling me to take 120 x 1 minute images in the hope of matching 4 x 30 minutes? Have you taken any thirty minute subs and processed them? I really can't believe that you have because there is absolutely no comparison.

Olly

[swag72]

NOt a true statement. You have to do the Maths to determine. The same SNR can be reached.

Can a maths theory take into account all of the multitude of differences that is applicable to AP? I don't doubt that on a piece of paper a mathematical formula can be drawn up that will explain SNR and all sorts of stuff pertaining to exposure length etc. But in the real world we have to contend with so many things that can affect this formula (such as seeing, LP, camera's, scopes) that the theory surely becomes of limited use.

I don't profess to have much of a mathematical knowledge, but I do know that something as simple as transparency can greatly affect data from one night to the next - How do you account for that in a formula? For something like that to work you surely have to assume that the data is not affected by anything - Just not true in real life.

I use 30 minute subs for all of my narrowband imaging. I wouldn't dream of doing any less. As an example, I have taken a load of faint objects of late and all at 30 minutes. The amount of signal from them and noise is massively different to the brighter objects that I'm on at the minute - How is that taken into account in mathematical theories in a classroom?

I'm not being argumentative, just reading this with interest.