20 x 30secs vs 5 x 2mins

[sejanus]

So,

After stacking, would 20 x 30sec exposures give an identical results to 5 x 2 min exposures?

Or is it better to run multiple longer exposures rather than multiple shorter?

I'm thinking in terms of tracking/alignment the shorter exposures would be more easy to swallow, and would probably be better for the camera sensor heat as well.

I think I'm a little lazy and am trying to put off learning drift alignment from all the horror stories I have seen of people spending hours at it :lol:

hope this makes sense!

[sheeny]

G'Day Sejanus,

Reading between the lines in your post, I suspect you almost know the answer;) ...

More short exposures is easier to manage, but the same total time in longer exposures will usually give a better result in terms of revealing faint detail. So it is a trade off - ease of guiding and alignment versus detail revealled.

Al.

[Striker]

Yes 5 x 2m will give a better result because 30 seconds gives you a high level signal to noise ratio....the more exposures stacked will improve this.

But you can only image as long as your mount can handle so if your not prepared to do a proper drift align/polar aligh you will benifit from a lot more shorter exposures then trying to push longer exposure with star and drift errors ruining your image.

If this is your permamant method it would be a good idea to have a fast scope where 30 seconds exposures can still give a great result at say F4 or even quicker.

[Dennis]

I suspect you may know the asnwer to your question, so I'll write for others who may not yet be into CCD imaging.

At a dark sky site, I think the combined 5x2min image would be better than the 20x30sec image. Let’s use the image of a galaxy as an example.

We would like to maximise the collection of (wanted) galaxy photons = Signal.
We would like to minimise the collection of (unwanted) non-galaxy photons = Noise.

What is the source of this (unwanted) Noise? Some examples are:

Random noise in the CCD chip.
Background sky noise.
Variations in sensitivity between pixels.
Read out noise from the electrical circuitry.
Cosmic ray strikes.
Thermal noise.

Some of this noise is system noise that is predictable and so it can be “removed” through Dark Frame subtraction and Flat Field dividing.

Looking at the Signal-to-Noise ration (S/N) by plugging some imaginary values into an example, things may become a little clearer. If you have a galaxy arm in an image represented by say, 1000 Signal photons and 100 Noise photons, the S/N Ratio is 1000/100 = 10.

If you now expose for longer, and collect say, 5000 Signal photons with only a slight increase of Noise to say 200 photons, we have a S/N ratio of 5000/200 = 25 so proportionally, the noise is far less and therefore the galaxy arm will be able to cope with more processing to reveal “real” fainter details, whilst minimising the amplification of any “unreal” noise artefacts.

Cheers

Dennis

[sejanus]

thanks guys

Do the lucky ones with cooled CCD's still stack or do they run really long exposures?

[Dennis]

With my cooled SBIG ST7E, I still have to stack. Also, a 20x3 minute exposure looks better than a 60x1 min exposure. By better, I mean that the 20x3min final product looks more like a nicely exposed, smooth, richly toned photograph whereas the 60x1min looks more like a ccd image, if you get my drift.

When I take images from a dark site, the final stacked and processed image has that smooth, richer look to it, compared to the image captured from Brisbane for the same duration.

If you capture many more sub-frames from a moderately lightly polluted site, then eventually the signal will improve faster than the noise worsens, so the final image will look nicer. This of course takes time, especially if you are doing RGB work with a filter wheel.

Cheers

Dennis

[jjjnettie]

I was pondering the same thing this evening. (Not that I can do this sort of imaging) Like Tony said, alot would depend on your mount.
Any chance of two comparison images?

[sejanus]

thanks dennis

you should write a book :)

[Gama]

Read here the pros and cons of both methods.

http://www.optcorp.com/product.aspx?pid=426-428-432-3253

[avandonk]

If you have sixty frames corrected for sensor noise (dark frame subtracted) and for corrected for flats taken for 30sec each say. you could median combine the odd and even frames andd then SUM these two frames. This is now the equivalent of 30x1min frames.
Or take each fourth frame (4x15) and median combine each set of 15 frames to end up with four frames and then SUM these. This is the equivalent 15x2min frames.
This works best if there is slight drift while taking frames as the sensor noise does not add. The residual noise disappears by MEDIAN combining.

See this thread
http://www.iceinspace.com.au/forum/showthread.php?t=15251

Bert

[Gama]

Thats right you wouldnt need any drift, as Noise is random and never has the same noise pattern on any following exposures.