how do you know how long to expose an object for

[iwols]

just wondered how people decide how long to expose objects for 1min ,5min,10min? whats the recipe please thanks

[Freddie]

Assuming you have no issues with guiding and can always produce tight round stars, take test exposures and expose for as long as you can without saturating anything or having the background overwhelm the data if you have light pollution.

[Tim]

20 minutes ago, iwols said:

just wondered how people decide how long to expose objects for 1min ,5min,10min? whats the recipe please thanks

There isn't a straightforward answer to this one, especially in the UK, apart from "Trial and Error".

Start with the capability of your mount to track accurately, this will be the biggest limiting factor at first.

Next, find out how long is a useful maximum exposure with your particular camera. This will be a variable factor.

Then the other variables come in to play, sky conditions, light pollution, moonlight, wind, gremlins, and individual targets.

Especially with DSLR or OSC cameras you have lots to think about each and every night to get the best.

For the sake of matching up with other image libraries I have collected, I normally try and stick to a set list of exposure times, 100 seconds, 300, 600, and so on.

Very rarely do two UK nights produce the same FWHM and sky background results.

[kens]

I aim for the sky background to be between 3x and 10x read-noise squared for a given camera at a given gain level. At this level read noise is not the dominant contributor in SNR. 

[michael.h.f.wilkinson]

When it comes to planets, lunar, and the sun, things are simpler. most planetary imaging software can show the live histogram of the image. This can be used to select the maximum exposure (and gain, if need be) that doesn't saturate the sensor. I aim to keep the the maximum value at about 90-95% of maximum capacity.

[BlueAstra]

1 hour ago, kens said:

I aim for the sky background to be between 3x and 10x read-noise squared for a given camera at a given gain level. At this level read noise is not the dominant contributor in SNR. 

So for a practical example I just looked at a 200s luminance exposure from my location. MaximDL histogram says the brightness range is 2188 - 41654 (adu?). My sensor has a read noise of 7-10 electrons, say 10. So I should aim for 300 - 1000 (adu?) in the background? In which case the sub was over exposed, maybe by a factor of 2?

[kens]

You need to convert the read noise from electrons to ADU. For that you need to know the gain in e-/ADU and the bias offset of your camera. If not published there are ways to work them out.

So lets say your camera has 10e- read noise and you are aiming for 3xRN^2. You'd take 3x10x10 = 300 and divide by the gain in e-/ADU then add the bias offset. That gives the target ADU for the median of the histogram - typically the peak. So if your gain is, say, 0.5e-/ADU and offset is 100 then the target ADU would be 700 to 2100.

Being higher does not mean you are overexposed as such. That depends entirely on your target. What it does mean is that read noise is not dominant in your subs so going higher means that your subs may be longer than necessary - and therefore subject to issues like satellites, aircraft, vibrations etc ruining your subs, and reducing your dynamic range. Its the dynamic range that could cause brighter parts of the image to be saturated (overexposed). Like all things its a balancing act which is why a range of values is suggested.

 

 

[BlueAstra]

With my SXV-H16 the read noise is 10e, gain 0.6e/ADU, however I cannot find an offset value anywhere. So my calculated median range with be (500 - 1667) + Offset. My example image statistics say range 2031 - 50440 adu, median 2487 adu. With your example offset value of 100 my target would be 600 - 1767 adu, so it looks like over exposure. However, the offset value is only a guess. Is a value of 100 typical? How can you measure offset?

[ollypenrice]

Like Tim, I do everything by trial and error. I've engaged in endless theoretical discussions and been assured that this or that would be the longest useful exposure time. These assertions do not agree with my findings and I routinely double them - from my site with my camera. If eveyone follows an agreed orthodoxy there will be no progress. The night I tried the 30 minute luminance sub was a great night! (Be aware, I live at a seriously dark site.)

Olly

[Demonperformer]

There is a link to an excellent article on www.stargazerslounge.com/topic/313785-converting-sqm-mag-to-number-of-photons/

[iwols]

12 minutes ago, Demonperformer said:

There is a link to an excellent article on www.stargazerslounge.com/topic/31785-converting-sqm-mag-to-number-of-photons/

hi thanks but link not working for me

[Demonperformer]

Sorry - typing it in on my phone and missed a digit. (Haven't worked out how to do a copy/paste on it yet!) If it still does not work try a search for 'SQM mag'. 

[kens]

4 hours ago, BlueAstra said:

With my SXV-H16 the read noise is 10e, gain 0.6e/ADU, however I cannot find an offset value anywhere. So my calculated median range with be (500 - 1667) + Offset. My example image statistics say range 2031 - 50440 adu, median 2487 adu. With your example offset value of 100 my target would be 600 - 1767 adu, so it looks like over exposure. However, the offset value is only a guess. Is a value of 100 typical? How can you measure offset?

You can work out the offset from the median value of a bias frame. And as I said, you are not overexposed if you are above 10xRN^2. You will still be getting a SNR benefit but you could also get that by stacking more subs.

Even if you were below 3xRN^2  you would not really be under exposed but the fainter details in the image would be harder to tease out from the read noise.

I agree with Olly that you need to experiment to find what works best for you. These and other formulae are just starting points and tend to be one dimensional. The one I gave tells you the minimum exposure to avoid being read noise limited. But even with that there is a range. 10x is great but you might not be able to reach that in dark skies with narrow band filters. But 3x is probably ok in that case. In light polluted skies you might find that even at 10x your exposures are just seconds long, any you need thousands to make up the integration time but don’t have the disk space nor processing power for that.

 

[wimvb]

11 hours ago, ollypenrice said:

The night I tried the 30 minute luminance sub was a great night! (Be aware, I live at a seriously dark site.)

If I would try that, I'd end up with a nice image of the White Nebula (rather featureless).

I understand that with dark skies, the background can take long exposures. But what about the stars? Don't you get bloated stars, doing a 30 minutes unfiltered (luminance) exposure?

[ollypenrice]

1 hour ago, wimvb said:

If I would try that, I'd end up with a nice image of the White Nebula (rather featureless).

I understand that with dark skies, the background can take long exposures. But what about the stars? Don't you get bloated stars, doing a 30 minutes unfiltered (luminance) exposure?

Well, I post my images here. My stars, for better or worse, are as you see them. However, I won't necessarily apply the luminance, or all the lmuminance, to the stars. It depends on the target but when stars are just set on background sky it's easy just to use the RGB layer for them. You can't do this for stars seen on nebulosity so that requires more thought. I never worry about exceeding 65000 ADU on stellar cores, though. They can be rescued in processing. I'm far more concerned to get enough signal on the faint stuff.

I should add that the 30 min subs are taken with the big Kodak chipped cameras with deep wells. I haven't gone beyond 20 mins on the Sony.

Olly

PS I use a Baader L filter so the luminance is cut at IR and UV. It isn't really unfiltered in the strict sense.