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Old 21-10-2013, 12:45 PM
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Shiraz (Ray)
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How many hours is "long enough"

There is a clear trend around here to take longer exposure images and some that have been recently posted demonstrate how effective long exposures can be. But then the question is, how long is "long enough".

Well, it seems that it depends ....

To demonstrate some of the issues, take 2 hypothetical but quite practical broadband systems that may be used by amateurs:
system 1: 12 inch f3.8 with a KAF3200ME based camera used under dark sky
system 2: 4 inch f7 with a small pixel (4.5micron) DSLR used under suburban sky (assume sky is about 2 mags brighter)

- system 1 has at least 3x the broadband quantum efficiency of system 2 by virtue of the Bayer filters,
- system 1 has ~8x the aperture area of system 2, assuming some central obstruction in system 1,
- both have about the same pixel scale, so there is no major sensitivity or resolution difference due to sampling,
- system 1 sky produces <1/2 the noise of system 2 sky

Combining all of these factors and assuming that system 1 will require additional time to gather colour data, yields the conclusion that system 1 will have more than 10x the signal level and less than 1/2 the noise of system 2 for a given exposure time. If system 1 needed 2 hours to image NGC 253 in colour, system 2 would need almost a week spent under the stars to produce a result with the same SNR - and this is for two quite practical systems.

two messages:
1. design your system very carefully if you want to minimise imaging time - system choices can greatly affect sensitivity and imaging time
2. different systems may require vastly different imaging times to reach the same signal to noise ratio on a given object, so there is no universally applicable yardstick as to how long is "long enough".

thanks for reading - appreciate any discussion. Regards ray

Last edited by Shiraz; 21-10-2013 at 01:00 PM.
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Old 21-10-2013, 01:06 PM
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Ray,

An interesting discussion...

Another important, but less technical, parameter is the quality level desired by the imager. Three years ago I was happy with an hour of data from an OSC camera producing a recognizable image. Now I'm chasing features that need 30 minute subs with a 12" scope just to become faintly visible.

Cheers,
Rick.
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Old 21-10-2013, 02:47 PM
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Also a key factor is patience. Its easy to suffer from object-itis where you feel the need to image several different objects in a small amount of time!

Also sometimes you can invest time in mega hours on an object and the resulting image is really not worth the effort. Not all objects image well for your system, location, time of year etc etc.

But generally speaking the top images are always long exposure times.
The main difficulty is noise reduces by a much larger amount of exposure time.

To reduce noise by half and therefore double signal/noise requries something like 4X exposure time so its diminishing returns at some point.

Some objects are worth it though as Rolf's recent and Mike Sids earlier Cent A shows. Some of those shell structures and jets simply do not show up in under 15 -20 hours of imaging.

Same with Helix with its 2nd outer shell.

Greg.
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Old 21-10-2013, 03:25 PM
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If you only get to use your gear one weekend a month, one has to take what one can get. It's a smorgasbord of delights up there and if all you can get is a nibble on each one, so be it.
There are only a few of us able to image the same object night after night to achieve the hours necessary to get a deep image.
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Old 21-10-2013, 03:34 PM
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Originally Posted by jjjnettie View Post
If you only get to use your gear one weekend a month, one has to take what one can get. It's a smorgasbord of delights up there and if all you can get is a nibble on each one, so be it.
There are only a few of us able to image the same object night after night to achieve the hours necessary to get a deep image.
+1 . What she said You observatory yuppies forgot the smell of grass.
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Old 21-10-2013, 03:42 PM
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+1 . What she said You observatory yuppies forgot the smell of grass.
... and the automated observatory yuppies forget the bone chilling fear induced by hearing the pitter-patter of rain on the roof when you're warm in bed at 2:30 am thinking, "hmmm, is my telescope covered up?"
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Old 21-10-2013, 04:04 PM
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... and the automated observatory yuppies forget the bone chilling fear induced by hearing the pitter-patter of rain on the roof when you're warm in bed at 2:30 am thinking, "hmmm, is my telescope covered up?"
Got plastic bags for that mate. ASCOM compliant. Better than max dome.
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Old 21-10-2013, 05:42 PM
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System 3... Any telescope with unmodified DSLR trying to image in summer tropics between clouds with night temps @ 30C.

I've discovered it doesn't matter how long I image for under these circumstances. The DSLR just turns to a seething pile of noise at these temps. Pretty much limited to the Sun and Moon in summer.
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Old 21-10-2013, 06:16 PM
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System 3... Any telescope with unmodified DSLR trying to image in summer tropics between clouds with night temps @ 30C.

I've discovered it doesn't matter how long I image for under these circumstances. The DSLR just turns to a seething pile of noise at these temps. Pretty much limited to the Sun and Moon in summer.
I don't even bother trying....if it's not raining, the sky is washed out and hazy.
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Old 21-10-2013, 08:23 PM
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After years gone by hunting things visually, I still get a real kick out a seeing single 5min sub showing more detail than I could EVER hope to see with a remotely affordable visual rig.

There's definitely no harm in running the sums on focal ratio, spectrum sensitivity, FOV etc. I suspect an ED80 and DSLR will be about 10 times cheaper than the 12" F3.8 CCD rig too though!?
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Old 21-10-2013, 08:44 PM
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It all comes down to "is it worth it?"

If there is something faint you think you can reveal with a longer exposure, or as Fred shows us you want to squeak the most out of your processing/sharpening, then long exposures is certainly the go, no question, sure it is harder the less automated and/or smaller your equipment ()...but thinking that long exposures should be the norm and if you aren't doing them then somehow you aren't producing something good...is quite frankly, missing the point of our hobby and as Ray has shown, plucking arbitrary exposure times of 20 or 30hrs out of thin air is kinda meaningless really in the scheme of things...that's my take anyway

Mike
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Old 21-10-2013, 09:26 PM
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thanks for the thoughtful posts and interesting discussion.

Looking back it seems that the original post was bit naive to ignore user requirements and expectations, as well as weather limitations and cost. However, the general thrust still seems to be backed up by the later posts - there really can be no prescriptive requirement on the length of exposures, for a whole gaggle of reasons. I wasn't trying to knock any systems, just to point out how radically different they could be in sensitivity.

Must say that I personally enjoy watching subs as they come in - nice to be part of the acquisition process. I can see how automation would allow many more/dimmer targets to be imaged though - I guess it all depends on what each of us wants from the hobby .

Last edited by Shiraz; 22-10-2013 at 06:35 AM.
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Old 21-10-2013, 10:16 PM
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Must say that I personally enjoy watching subs as they come in - nice to be part of the acquisition process. .
Yeah I enjoy that too, although I do leave the scope for extended periods throughout the night...but even more, when the seeing is behaving, in Astroart I looove watching the guide star centroid history plot in real time make a beautiful tight round cluster of dots for hours on end ...something about that really turns me on

Mike
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Old 22-10-2013, 07:05 AM
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Starizona have a quick guide on optimum exposure for any CCD system.

Here

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

They have an ideal exposure calculator here

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

All you need to input is your camera and sky background for a given exposure.

It is also worthwhile if you want to get into the mathematics to have a read of the info on Stan Moore's and John Smith's sites.

Stan Moore here
http://www.stanmooreastro.com/

John Smith here

http://www.hiddenloft.com/notes.htm

and here

http://www.hiddenloft.com/notes/acq.htm


The basic take home message is to take 2N+1 exposures that are half or a third of the exposure needed for sky background to overwhelm read noise.

With 3nm narrow band this of course allows for very long exposures. Then the inherent camera thermal and random noise become a problem. That is why a faster optic is preferable.

Bert

Last edited by avandonk; 22-10-2013 at 07:27 AM.
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Old 22-10-2013, 07:09 AM
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these are some of the best references out there Bert - thanks.

the ideal exposure calculators provide "optimum" sub exposure time (read noise = 5% total), but not the overall exposure needed for a given object. Do you have any ideas on what overall SNR is required to get a "good" image?
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Old 22-10-2013, 07:19 AM
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That's interesting information. Is there a way to work out the details for DSLR cameras as well (it seemed CCD centric for the Starizona articles)?
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Old 22-10-2013, 07:30 AM
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An alternative view on subframe lengths, optimal number of subframes, etc. (Warning: lots of mathematics):
http://www.cloudynights.com/item.php?item_id=1622
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Old 22-10-2013, 07:37 AM
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That's interesting information. Is there a way to work out the details for DSLR cameras as well (it seemed CCD centric for the Starizona articles)?
It is almost impossible to have a guide for DSLR's as the thermal noise varies so much with ambient temperature.

I used to have a Canon 5DH and a 300mm lens at F3.6 which I cooled with a Peltier fridge to -12C. This reduced the thermal noise considerably but not quite enough to image dim objects such as the Vela Super Nova remnant or the dim nebulosity in the LMC and SMC in NB as clearly as I wanted in a reasonable time. The Bayer filter array is the major reason for this.

Bert
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Old 22-10-2013, 07:58 AM
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Originally Posted by Shiraz View Post
these are some of the best references out there Bert - thanks.

the ideal exposure calculators provide "optimum" sub exposure time (read noise = 5% total), but not the overall exposure needed for a given object. Do you have any ideas on what overall SNR is required to get a "good" image?

It purely depends on how 'bright' the target is. A low signal to noise will record bright stars in seconds. As objects get dimmer the signal to noise needs to be far higher. This is so that the dim signal is larger than the noise. If it is not, it will never be recorded!

If the signal is just above the noise then the integrated sampling time needs to be very long to improve the ratio of S/N.

As in all of science it is the signal to noise ratio that is the only dogma for evaluating data.

We must never forget though that one human's noise can be another human's signal. Even then the measuring system must differentiate between the two. I changed man to human for Jocelyn Bell who really was the 'man' to discover pulsars and was ignored in the Nobel Prize!

Bert

Last edited by avandonk; 22-10-2013 at 08:12 AM.
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Old 22-10-2013, 08:18 AM
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nice read and some very useful info. I need to point out though, a while back I read somewhere if one can't get long exposures then aim for multiple short ones. Not sure how effective this would be....
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