Time for Astrophotography exposures?

[batema (Mark)]

When ever I image I usually use exposure times based on images seen here but that is often with different equipment than I have. Is the a mathematical way to determine the best exposure time to take an image? I have a William Optics 110 F7 scope with QHY-9 Mono. ????

Mark

[Octane (Humayun)]

Take a test exposure and check the resulting histogram, but also make sure you don't wash out the sky.

It's really trial and error.

Or, you could use something like Skytools which provides exposure estimates.

H

[rally]

Try here

http://www.ccdware.com/resources/subexposure.cfm

Read the instructions for how and why

[ejd (Erick)]

SimCCD might be useful here. Being without a mono camera I have not had a reason to use it.

http://www.darklights.org/simccd/

[batema (Mark)]

Thankyou all will check them out.

Mark

[gregbradley]

I tend to standardise my exposures as much as possible. It reduces the size of the darks library and simplify processing afterwards.

I think its based on:

1. Darkness of your skies (you can go longer in darker skies). Perhaps more of an issue with DSLRs where they register sky glow faster than dedicated CCDs.

2. Accuracy of your tracking (shorter exposures are easier than longer).

3. Pixel size and resulting well capacity - that may mean shorter exposures on faster scopes otherwise bright stars can be damaged. Not really that relevant in modern CCDs although the trend for Kodaks' latest chips was all very low well capacity. That is why you look at the full well capacity as one of the factors you evaluate when considering a particular chip. The other would be blooming (not very common now) but it would influence exposure time if using a blooming camera with straight LRGB and how many bright stars there are in the image.

4. Amount of cloud cover or wind - if there is cloud around or its windy 5 minute subs are more likely to be OK than a 10 or 15 minute one which could be spoiled by a rogue cloud or gust of wind.

5. Brightness of the object/ noise levels of camera: Ha and O111 S11 need longer exposures to overcome the noise of the camera and lack of signal in dimmer objects. Objects like Omega Centauri would do better with short exposures than 10 minute ones. 20 minutes for Ha is a common one. 30 minutes would be fine if your tracking is spot on.

6. Cooling power of the camera. Colder cameras would allow shorter exposures as the noise floor overall will be somewhat less allowing a signal to show its head above the overall noise.

As a practical observation the most common subexposure for an astro CCD is 10 minutes. Sometimes you see 5 but that really requires quite a low noise camera.

You aim is to get the signal level of the image above the noise floor of the camera. Until you do that there will be nothing to see in your image.
Different cameras have different noise levels, even ones using the same chip!

Greg.

[luigi]

Originally Posted by Octane

Take a test exposure and check the resulting histogram, but c also make sure you don't wash out the sky.

It's really trial and error.

Or, you could use something like Skytools which provides exposure estimates.

H

+1, and I can't understand why people still want to calculate the exposure time if you can just play with it. We are not wasting film any more!

[alistairsam]

Originally Posted by gregbradley

6. Cooling power of the camera. Colder cameras would allow shorter exposures as the noise floor overall will be somewhat less allowing a signal to show its head above the overall noise.

As a practical observation the most common subexposure for an astro CCD is 10 minutes. Sometimes you see 5 but that really requires quite a low noise camera.

Greg.

Hi Greg,
I'm a bit confused by this. doesn't noise increase with length of exposure whether its cooled or uncooled?
eg. a 5 min exposure with an unmodded dslr will show noise but the same with a cooled ccd will show almost none?
or are you referring to sensitivity? so you could get sufficient details with a cooled ccd and that would not have as much noise?
not sure why 5 mins would need a low noise camera and 10 mins won't?

[RickS (Rick)]

Originally Posted by alistairsam

Hi Greg,
I'm a bit confused by this. doesn't noise increase with length of exposure whether its cooled or uncooled?
eg. a 5 min exposure with an unmodded dslr will show noise but the same with a cooled ccd will show almost none?
or are you referring to sensitivity? so you could get sufficient details with a cooled ccd and that would not have as much noise?
not sure why 5 mins would need a low noise camera and 10 mins won't?

There are several sources of noise. One potentially significant source is read noise which is incurred when you amplify the data from the sensor and convert it from analog to digital information. You get the same amount of read noise for a 5 minute or a 10 minute exposure, but you have twice as much signal in the second case.

It's also useful to make a distinction between noise and unwanted signal. When you ask about noise increasing with length of exposure, you're probably thinking about dark current which is actually an unwanted signal that can be removed by dark subtraction. There is also noise associated with the dark current, but this noise will actually decrease over time.

It's difficult to get your head around this stuff. The Stark Labs articles on signal to noise are a good read: http://www.stark-labs.com/craig/articles/articles.html

Cheers,
Rick.

[Iain.k (Iain)]

Originally Posted by luigi

+1, and I can't understand why people still want to calculate the exposure time if you can just play with it. We are not wasting film any more!

I on and off use film depeding on what im taking photos of. It in my option can produce a much clearer image.
But personal preferance.

[luigi]

I don't have a problem with people using or not using film. What I can't understand is to use a digital camera as if it were a film camera.

Originally Posted by Iain.k

I on and off use film depeding on what im taking photos of. It in my option can produce a much clearer image.
But personal preferance.