Exposure time

[Guest]

I'm using a canon 450D with my SW 200PDS and getting some fairly decent pictures but I was wondering how do I know if the exposure times I'm using are right?  How do I decide what exposure time to use? You can get something over quite a range of exposure times. If it's too long then the background sky becomes grey but you can fix that in the processing.  In fact you can do quite a lot in the processing which complicates the issue for me.

Also, suppose I did 50 x 30 second exposures. Is that the same as doing 25 one minute exposures?

At the moment I just judge it by eye but it seems a bit vague.

Cheers

Steve

 

[vlaiv]

25x1minute is always better than 50x30s - as long as camera has any read noise.

If camera has no read noise - then the two are equal. In fact - stack of any exposure length totaling to same time will produce same result (yes indeed, even 15,000x0.1s).

Just how much difference there is between 25x1 minute and 50x30 seconds, depends on how large read noise is to other noise sources. Read noise is only kind of noise that is related to number of subs - all other noise sources are related to exposure time and add up accordingly.

You can either measure read noise and other noise sources and thus determine adequate exposure length, or just do what would be sensible and not worry too much about it. At some point light pollution just becomes dominant thing and differences become too small. You don't really have to measure it to know when your subs start turning grey - that is enough in terms of exposure length and going longer won't make much difference.

Another important thing is - longer subs run a risk of loosing more data in single chunk - either due to poor guiding or wind gust, or earthquake or for whatever reason (yes, earthquakes do happen and they do ruin subs ), so even if for example 5 minutes would make slightly less noisy image - you can comfortably settle for 2 minutes if subs turn out good and you get to use them all.

 

[Guest]

Thanks, At the moment I'm not guiding so I try to keep the exposure times short but I understand what you say.

 

[Aramcheck]

There's a good talk about various noise sources & optimising exposure time on youtube by Dr. Robin Glover from Sharpcap: https://youtu.be/3RH93UvP358?t=61

Cheers
Ivor

[Chefgage]

5 hours ago, woodblock said:

I'm using a canon 450D with my SW 200PDS and getting some fairly decent pictures but I was wondering how do I know if the exposure times I'm using are right?  How do I decide what exposure time to use? You can get something over quite a range of exposure times. If it's too long then the background sky becomes grey but you can fix that in the processing.  In fact you can do quite a lot in the processing which complicates the issue for me.

Also, suppose I did 50 x 30 second exposures. Is that the same as doing 25 one minute exposures?

At the moment I just judge it by eye but it seems a bit vague.

Cheers

Steve

 

Do you check the histogram on the camera?

[Guest]

No I don't, what should I look for?

 

[Adam1234]

On 23/04/2020 at 17:59, woodblock said:

No I don't, what should I look for?

 

I've been wondering about the best exposure times too and getting the balance between the object and background sky.

Anyway to your question on the histogram, at a minimum, you want to expose for long enough that the histogram is clear of the left side, but make sure that it doesn't clip into the far right. 

[Guest]

Thanks Adam,

I've been looking at the histograms of the ones I took of M82 the other night.  I suppose the way it works is that if the left hand end is clipped then you're losing detail in the darks and if it's clipped on the right hand end them you're losing detail in the lights. If you were taking a picture of a star field then you'd have a peak on the left for the darks and a much lower peak on the right for the stars with not much in between since the picture is mostly dark. It looks a bit clearer if you look at the log plot but even so the peak representing the lights (stars) is pretty small.

Cheers

Steve

 

 

[Adam1234]

2 hours ago, woodblock said:

Thanks Adam,

I've been looking at the histograms of the ones I took of M82 the other night.  I suppose the way it works is that if the left hand end is clipped then you're losing detail in the darks and if it's clipped on the right hand end them you're losing detail in the lights. If you were taking a picture of a star field then you'd have a peak on the left for the darks and a much lower peak on the right for the stars with not much in between since the picture is mostly dark. It looks a bit clearer if you look at the log plot but even so the peak representing the lights (stars) is pretty small.

Cheers

Steve

 

 

Yeah, if the histogram is clipped on the left you've under exposed and it will be too dark. clipped on the right and it will just be white. It takes a lot to clip the right though I think, I've been getting away with 10-minute subs with a canon dslr and I've got 4 street lights outside my garden. 

My advice is experiment with different exposures and see what looks good to you.

Edited April 24, 2020 by Adam1234

[rickwayne]

The histogram advice is good. If must err, think about what you're imaging and optimize for that. Want deeply saturated star colors? Stay away from the right edge of the graph. Want to bring up faint nebulosity? Beware the left. Can get all your data in between? Good on ya!

[wxsatuser]

About 20>25% on the Canon luminance histogram should do it.

Remember when you look at the RAW histogram it will still be hard left unlike the back of the camera histogram.

Edited April 25, 2020 by wxsatuser

[vlaiv]

I'm afraid that histogram advice is not very useful.

- It depends on range displayed (full range / min-max pixel value range)

- If you have clipping to the left - don't use that camera at all - you won't be able to calibrate your data properly. On a normal camera that should never happen. Bias/offest signal makes sure of that. With dedicated CMOS cameras you sometimes have offset setting to control that it does not happen.

- If you have clipping to the right - you will loose information in some areas (clipped pixels) - but that can be dealt with very easily - make a few shorter exposures that you will use to fill in those clipped pixels of long exposures. You don't need much of these filler subs - just a few as they will only be used where signal is strong (so strong that it clips in long exposure) so SNR will be good.

- histogram does not tell you anything about noise distribution that is only relevant in choosing optimal sub duration.