Many short frames or a few long ones?

[ryandsimmons]

I was just wondering how people decide on the above?

Tonight for instance I am planning on trying for the pinwheel galaxy.  I am in medium / low light polluted skies and I was thinking about how carefully to set up my scope.  I could do a reasonably quick set up to get easy 30-45 second exposures and take a couple of hours worth.  Or I could set up my scope extra carefully (as I did once) and got for 3 minute exposures.

How much actual benefit is there in having your 30 minute exposure made up of 10 3 minutes as opposed to 60 30 second pictures?

[nmoushon]

You will get WAY more out of the 3minutes exposure than you would from 30sec exposures....no matter how many short ones you add up.

Its all about signal-to-noise ratio. You will get much more signal in your 3min exposure than you would in a 30sec exposure while your noise will go up but not as much as your signal. Plus adding darks will cut it even more. The longer the exposure the deeper the image. This will allow you to pick up for of the fainter objects and parts as well.

I did 8 hours of 2min sub on M42 and was juuuuust able to pick up the faint outer dust that surrounds the main mass of M42. Even then could not pull it out and make it look good without adding a tramendous amount of noise and ruining the rest of the image. I've not tried it with 5min subs since my laptop has died and havent been able to go out and image in over 6months but I've seen the same outer dust being picked up in as little as a hour or two with 5 min subs.

The only thing that goes against this is your sky limitations. Depending on you local LP long can actually be worse but from what you've described you shouldn't be any where near your skies limits.

[lensman57]

I was just wondering how people decide on the above?

Tonight for instance I am planning on trying for the pinwheel galaxy.  I am in medium / low light polluted skies and I was thinking about how carefully to set up my scope.  I could do a reasonably quick set up to get easy 30-45 second exposures and take a couple of hours worth.  Or I could set up my scope extra carefully (as I did once) and got for 3 minute exposures.

How much actual benefit is there in having your 30 minute exposure made up of 10 3 minutes as opposed to 60 30 second pictures?

The more subs you take the higher the SNR but get to a high signal you need to have as long an exposure per sub before the sky back ground level ruins the day. Try to go for 30~35 subs of 3 minutes if oyu have a good LP filter and as many darks, Bias and flats then the stack will sing for you.

A.G

[LukeSkywatcher]

Every location is different. In heavily light polluted areas its best to take many short exposures and stack them. If the LP isnt too bad then you can take fewer and longer exposures.

When i lived about 20Km from Dublin city (in an area which i didnt think was very LP'd), I could take exposures of about 20s each and then stack them with good results. Now im about 30Km inland from where i was in a pretty rural location (visually the skies appear much darker), im sure when i image, i can go for longer exposures and less of them.

It really is all trial and error and experimenting with different ISO levels.

[wxsatuser]

I was just wondering how people decide on the above?

Tonight for instance I am planning on trying for the pinwheel galaxy.  I am in medium / low light polluted skies and I was thinking about how carefully to set up my scope.  I could do a reasonably quick set up to get easy 30-45 second exposures and take a couple of hours worth.  Or I could set up my scope extra carefully (as I did once) and got for 3 minute exposures.

How much actual benefit is there in having your 30 minute exposure made up of 10 3 minutes as opposed to 60 30 second pictures?

The most important thing with exposure length is to get the signal out of the read noise.

In your example you would need a lot more of the shorter subs to equal the the 3minute subs.

[johnrt]

Indeed as stated above it's all about signal v's noise. Noise is everything in your image that you don't want, camera read out noise, thermal noise from the chip, light pollution, atmospheric seeing conditions etc.

You need to expose long enough so the signal you *do* want has a chance to exceed and swamp the signal you *don't* want. The longer the exposure the more signal you will detect from the DSO. But there comes a point when limiting conditions of light pollution etc. will begin to swamp to the signal at the other end of the scale, which is why a couple of 4 hour subs isn't the best idea.

The long and short of it is that in a 3 min sub you will detect and retain much more data from the DSO than in a 30 sec sub, so it doesn't matter how many 30 sec subs you take they will never contain the information you have captured in your 3 minute sub.

[LukeSkywatcher]

 so it doesn't matter how many 30 sec subs you take they will never contain the information you have captured in your 3 minute sub.

Learn something new every day. I was convinced that 6x30s subs when stacked would have the same data as a single 3 minute sub.

Can you tell that i am really NOT an imager!!!

[nmoushon]

 so it doesn't matter how many 30 sec subs you take they will never contain the information you have captured in your 3 minute sub.

Learn something new every day. I was convinced that 6x30s subs when stacked would have the same data as a single 3 minute sub.

Can you tell that i am really NOT an imager!!!

 Now this isnt neccessarily true. Stacking 6x30 could produce something very similar to 1x180 but once you start trying to gain a total  of a lot more,even just 30min, you can tell a difference. Again its all about the SNR. So you can stack a bunch of short sub to get something similar to the amount of 1 long sub. But once you start adding a bunch of long single subs together they start adding up real fast and you'll soon find that the SNR is much better. Plus on top of that you can go a lot deeper and get a lot more fainter stuff in your data.

[johnrt]

 Now this isnt neccessarily true. Stacking 6x30 could produce something very similar to 1x180 but once you start trying to gain a total  of a lot more,even just 30min, you can tell a difference. Again its all about the SNR. So you can stack a bunch of short sub to get something similar to the amount of 1 long sub. But once you start adding a bunch of long single subs together they start adding up real fast and you'll soon find that the SNR is much better. Plus on top of that you can go a lot deeper and get a lot more fainter stuff in your data.

Ah well, that all depends on the noise readout of the camera and the quantum efficiency of the chip not to mention the conditions and seeing. You will not have the information in a stack of 100 or 1000 30 sec frames as a single 3 minute (x6 longer) sub if the signal from the DSO in the 30 sec sub does not exceed the sources of noise we've all mentioned above.

We are both hypothetically correct, because it all depends on lots of factors. But one thing we do agree on is the OP should shoot the 3 minuts subs!!

[nmoushon]

Here are some examples. The first one is my image of M42 shot with 8hrs of 2min exposures. I was just able to bring out the dark dust lanes in between the two nebulas. Now the second link shows M42 with only 3hrs 20mins of 10min exposures and it clear shows the same dark dust lanes and more. Plus not to mention much better noise control. Now a good portion of that noise control is due to it being captured with a CCD vs my DSLR but the length of the sub plays a good part of that too. Mainly because of the SNR.

http://stargazerslounge.com/topic/177259-8hrs-m31-m42/ -My M42

http://www.astrobin.com/66317/ -Someone else M42 I found on astrobin

[nmoushon]

Ah well, that all depends on the noise readout of the camera and the quantum efficiency of the chip not to mention the conditions and seeing. You will not have the information in a stack of 100 or 1000 30 sec frames as a single 3 minute (x6 longer) sub if the signal from the DSO does not exceed the sources of noise we've all mentioned above.

We are both hypothetically correct, because it all depends on lots of factors. But one thing we do agree on is the OP should shoot the 3 minuts subs!!

yes you are very much correct. There is a lot more that factors into it that than simply stack short vs long frames but I was trying to keep it simple.  And incase the OP can't tell we are all supporting him to shoot 3 min subs

[LukeSkywatcher]

Ah well, that all depends on the noise readout of the camera and the quantum efficiency of the chip not to mention the conditions and seeing. You will not have the information in a stack of 100 or 1000 30 sec frames as a single 3 minute (x6 longer) sub if the signal from the DSO in the 30 sec sub does not exceed the sources of noise we've all mentioned above.

We are both hypothetically correct, because it all depends on lots of factors. But one thing we do agree on is the OP should shoot the 3 minuts subs!!

Hypothetically, 6x30s should equal a single 3 min exposure, but there are other limiting factors.

[johnrt]

Hypothetically, 6x30s should equal a single 3 min exposure, but there are other limiting factors.

In a perfect set-up with a perfectly efficient camera yes, but unfortunaltey the reality is far from perfect.

[ryandsimmons]

Cheers for all the info guys. Currently 2 hours in and taking 100 or so 1 minute 30 secs subs. Just could not get the 3 minutes to get round stars tonight.

The longest I have done before is 30 subs of 30 secs so I am very interested to see the difference.

[ryandsimmons]

Hmmm... rather dissapointed here with my first attempt at Bodes Nebula.  It is nowhere near as clear as my previous attempts at the Whirlpool Galaxy.

41 frames, at 90 seconds, with flats and darks.

On the positive side the flats appear to have worked for once, I cannot see the vignetting that I have had on previous attempts.  Small adjustments every 10 frames or so also reduced the wavy effect that I have previously had.

However all I have is stars, with a bright core.  In order to even see the faintest spiraling I need to saturate the image.  Here are the image straight out of DSS (well, out of DSS then converted to JPG to reduce size.) Then an image that I have quickly adjusted to get at least a vague image.  The question is can I pull anything from this, or should I give up on this image?

Boderaw by Ryan Simmons, on Flickr

Bode by Ryan Simmons, on Flickr

[ollypenrice]

Hmmm... rather dissapointed here with my first attempt at Bodes Nebula.  It is nowhere near as clear as my previous attempts at the Whirlpool Galaxy.

41 frames, at 90 seconds, with flats and darks.

On the positive side the flats appear to have worked for once, I cannot see the vignetting that I have had on previous attempts.  Small adjustments every 10 frames or so also reduced the wavy effect that I have previously had.

However all I have is stars, with a bright core.  In order to even see the faintest spiraling I need to saturate the image.  Here are the image straight out of DSS (well, out of DSS then converted to JPG to reduce size.) Then an image that I have quickly adjusted to get at least a vague image.  The question is can I pull anything from this, or should I give up on this image?

Boderaw by Ryan Simmons, on Flickr

Bode by Ryan Simmons, on Flickr

If you post the linear TIFF or PM me with a Dropbox link I can have a quick look at it.

Olly

[ryandsimmons]

That is an incredibly kind offer thank you.

What sort of size Tiff do you need?  This image straight from DSS is 68meg which seems excessive.

[ollypenrice]

Good advice above. My own imaging history is one of taking longer and longer subs over the years. I now default to 30 minutes luminance at F5 and use shorter only with good reason, like avoiding saturation. Of course, I have a dark site. You need to find your sky fog limit and expose till you are just short of it.

The outer halo of M31 is reckoned to be faint. I found more of it in 7x30 minutes than in 44x15. This used the 7x30. http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-xbvjFDF/0/O/M31%20Outer%20HaloLHE.jpg

Olly

[ryandsimmons]

That is an astounding picture!  The colours especially are phenomenal.

Going by my last pics, being just 1 mile from Gatwick airport I think 90 seconds may be my limit. 

Once I have more practice though I will start driving out to dark sky areas.

[dph1nm]

There is a lot of misunderstanding around about this question. Firstly, you can get the same quality of image from short subs as from long subs - you do not lose data just because you have a short sub. However, you will need more total exposure with the short subs because of the increased read-noise. How much longer will depend upon your exact circumstances, but, roughly speaking, if you have a lot of light pollution you will hardly need any more time, whereas if you are at a dark site it could be several times the exposure.

NigelM