Orion Belt to M42 widefield - update Astrobin

[rcheshire (Rowland)]

Alistair. Me too. I think I have a grip and...

You know these sites

http://starizona.com/acb/ccd/advtheoryexp.aspx

http://www.samirkharusi.net/sub-exposures.html

I get from this... basically speaking

Background flux is less at a dark site and object flux is greater - e/sec
Less integration time, but longer subs to increase background adu - is this correct?

Conversely, background flux is greater with light polluted sky, object flux less.
More integration time required and shorter subs because backround/skyglow flux is greater.

What is the noise if background adu is not adequate? Aside from fixed pattern, thermal noise?

Next installment... read noise, which is considered differently CMOS vs CCD.

[alistairsam]

hi Roland,

I think you're right.
I went back to reading Ray (Shiraz)'s excellent thread on calculating optimum exposure times and this is an excerpt

The read noise is a burst of noise that you get on every read of data from a pixel - you can't do anything to reduce it, except by reducing the number of reads you perform (ie use very few long subs).

The shot noise is determined by random arrival times of photons - it turns out that the signal to noise ratio due to shot noise is just the square root of the signal and the shot noise therefore increases as you increase the total detected signal.

If the sky is really dark and you are using a narrow-band filter, there may not be much sky signal at all and you will need really long subs to get enough signal that the shot noise will overwhelm the read noise.

http://www.iceinspace.com.au/forum/s...1&postcount=17

Thread is here
http://www.iceinspace.com.au/forum/s...=117010&page=2

I think the key is that the shot noise is related to the signal and in a dark site, since the signal will be high, so will the shot noise. so need to expose longer to overwhelm the read noise.

sorry, I'm conscious that this is in the beginners section and I don't want to throw people off with all this info.
but interesting nonetheless.

Strange that I can get enough details in M83 in 45 seconds from a dark site as opposed to 5 mins from a LP site, yet I have to image longer in the dark site!! weird.

Cheers
Alistair

Edit: I think this graph explains it clearly. thanks for the links
http://www.starrywonders.com/comparisons.jpg

[ehgore1978 (Scott)]

You've just inspired me to try widefield great shot!

Scott

[rcheshire (Rowland)]

Great info Alistair and good for beginners too - me!

So next time I'll try a series of shots and work out iso vs exposure time.

Thanks Scott. Hope to see some images from you.

[alistairsam]

Yes, not sure if Ray touched on dslr's there as they use cmos rather than ccd but I'm guessing the concept is the same. So if you get your optimum sky background adu by varying iso and exposure, you should get the best signal. I think..
A plot of signal and background flux with varying iso length and temp would be extremely instructive.

Cheers
Alistair

[rcheshire (Rowland)]

I cant think of another way of doing it. And possibly, the higher iso shots might have saved the day.

[alistairsam]

Hi Rowland,

The histogram method for confirming that 10% to overwhelm read noise as in your original link http://www.samirkharusi.net/sub-exposures.html seems practical. worth pursuing.
I Need to read up more on shot noise.

Cheers
Alistair

[desler]

I really like your final image Rowland! Extra data should really make it pop!

Darren

[rcheshire (Rowland)]

Thanks Darren. It's coming along bit by bit.

Alistair. I have read slightly different explanations of read noise. For CMOS it goes something like this. CMOS LSDs are active and contiually readout with their own amplifier and circuitry, hence microlenses focussing on the fill factor, the light sensitive area of the pixel aside from the circiutry in the pixel generating the noise. CCDs are different, but for now that's another article to read.

[alistairsam]

yes, I thought there'd be a difference. I think the histogram method makes sense as it applies to dslr's without much complication. I had a look at your image's histogram in astrobin and it was about 20% from the left which I think was a good thing. but I guess you'd have to check the histogram of an unprocessed sub to be sure.
I reckon your snr would be better with deeper cooling like the jtw and central ds cooled cams but that's another topic.

Cheers
Alistair

[rcheshire (Rowland)]

I agree concerning the cooling and the system, but I am being conservative with the equipment for longevity. A QHY with an APS-C size sensor with a differential of 45C would be my preference - Sony low noise CCD - for widefield to get those sorts of temperatures.

I might be dead before then, but if they decided to put an APS-C Sony CCD in the open source QHY I would snap it up tomorrow.

[nebulosity. (Jo)]

Great to see your camera is working well Rowland Spectacular shot

Cheers
Jo

[rcheshire (Rowland)]

Thanks Jo.

[rcheshire (Rowland)]

I've managed to resolve the dark calibration issues - most likely a user problem. I also managed to grab a set of iso1600 darks at -5C during the week.

Anyway, here is a link to the new revision at Astrobin. Detail, dynamic range and colour balance seems much improved.

Waiting for the Moon to rise later in the night to get some more data - Orion has almost run its course for another year.

http://www.astrobin.com/82477/F/