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  #21  
Old 22-10-2013, 09:59 AM
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Originally Posted by avandonk View Post
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.
Ah, thanks for that - I thought it might be something like this. Ok, I guess I'll have to keep experimenting for my system.
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  #22  
Old 22-10-2013, 12:58 PM
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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....
This is my understanding but there is still quite a bit of debate on this. Scott Rosen (www.astronomersdoitinthedark.com/) has produced some of the best DSLR images. He tends to do short subs, sometimes only 3 mins, but collects hours of data (10+).
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  #23  
Old 22-10-2013, 01:17 PM
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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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Originally Posted by swannies1983 View Post
This is my understanding but there is still quite a bit of debate on this. Scott Rosen (www.astronomersdoitinthedark.com/) has produced some of the best DSLR images. He tends to do short subs, sometimes only 3 mins, but collects hours of data (10+).
provided the chip temperature does not wander around, the only SNR difference between lots of short subs and a single equivalent long one is that each read of a short sub introduces a burst of read noise. If you have a low read noise camera, short subs are entirely practical, since the overall read noise from lots of them will still end up fairly low. DSLRs will generally have fairly low read noise, so, even though they have low QE, short subs could be applicable for some targets and sky conditions. The trick is to keep the total read noise well below the shot noise and thermal noise. Short subs are not applicable to cameras with high read noise, or if doing narrow band imaging.

In the planetary imaging area, subs as short as 1/100 sec are routinely used on bright targets and an image may be assembled from many thousands of them. There is nothing inherently wrong with stacking huge numbers of short subs - the random nature of noise ensures that the signal will build up out of the noise in exactly the same way as it does with a long single exposure, provided the read noise does not intrude. It is fascinating to stack a hundred plus subs that show nothing but stars and a fairly flat noise background and find that you have a nice nebula standing out clearly in the resulting image.

Last edited by Shiraz; 23-10-2013 at 07:08 AM.
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  #24  
Old 22-10-2013, 06:52 PM
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18 months ago I had a conversation with Martin Pugh about how much integration was needed for any particular image. His opinion has proven right as far as I am concerned. He said nothing under 20 hours. 30 is better. Also image subs longer to go deeper.

I sort of knew this from taking long subs several years ago for many of my deep sky images. For some reason I started following what many do here with 10 minute subs. Now with a narrow aperture this just does not work. It resulted in images with lots of integration but not smooth. I had to stretch the data too much to get the brightness that I wanted and that resulted in lots of uncontrollable noise. Longer subs gives me a better signal to noise and a background ADU over 1000. If you have a wider aperture with a fast f ratio you can use shorter subs to achieve the same result. The point being you need to experiment a little to find what length sub produces low noise with high signal and that will mean it will be different for many objects as each has varying dimness.

However, back to the central issue, is that longer integration time means smoother images. I have found colour data has to have a minimum of 3 hours in each filter, so I don't have to use smoothing. 4 hours works really well. Add the lum or narrow band of 10 hours and you are in the 20 hour mark.

Bottom line for me is the quality I am trying to achieve. Going automated has helped to gather lots of data. So I will stick to what Martin said and aim for that each time and try to be patient.
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  #25  
Old 22-10-2013, 06:59 PM
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I'm largely with you Paul. Definitely longer is better with both total exposure and subs. But of course practicality kicks in and clouds, bad weather, moon, work all conspire to reduce total available time. So that is the goal to take long exposures but sometimes you've got to take what you can get.

Also some bright objects don't seem to gain too much from longer exposure times or with small well cameras a 15/20 minute sub even 30 minutes may blow out highlights.

So its not a rote rule. Some judgement is involved in my opinion.

Greg.
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  #26  
Old 23-10-2013, 09:11 AM
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Both Paul and Greg are quite correct. But first you must evaluate whether collecting mega data is worth it. The only real criteria is that your signal to noise of the dimmest bits of the image you wish to show or record is above at least one or far better, two sigma.

When I was working out what to get for my 'new' system I did not compromise anywhere. I wanted to do very deep NB of dim objects at a moderately wide field. I could most probably spend the rest of my life trying to get the detail in the Vela SNR and the two Magellanic Galaxies and still not get it anywhere near perfect.

If you take as a starting point a very fast optic then the best quality largest 3nm NB filters are a no brainer. All else also follows e.g. a very low noise camera and best focuser available.

With 16 minute exposures with any 3nm NB filter the signal to noise is still improving even after thirty exposures. I would say that double this (60 exposures or 16 hours) is still worth doing.

In a target rich place such as the LMC it is surprising what pops up.

This is about twenty four hours of total exposure. 9MB


http://d1355990.i49.quadrahosting.co...70_NB_HP_N.jpg

Bert
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Last edited by avandonk; 23-10-2013 at 09:22 AM.
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  #27  
Old 23-10-2013, 10:56 AM
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no argument that longer is better (see original post).

I was trying to point out that there is no clear correspondence in imaging time between systems. For example, I imagine that Bert's system will quite comfortably produce an image of the outer parts of Helix in 30 minutes - mine takes a few hours and a DSLR based system could take days - there is no correspondence in imaging time between these systems.

You may decide on exposure times for a given system and for your expectations of quality, but there is no point in someone else with a different system trying to use that as a guide. Even within a given system, you can change the required imaging time by huge amounts - eg, if you decide to 2x2 bin your data, you can reduce your imaging time to 1/4 that required at maximum resolution.

Last edited by Shiraz; 23-10-2013 at 12:36 PM.
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  #28  
Old 23-10-2013, 11:03 AM
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how about a situation with a OSC vs mono filtered /RGB imaging?

Would you say that the OSC, would need say a third of the time, 2/3s? or doesn't it work that way and to get a similar quality image it would take about the same amount of time?

cheers
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  #29  
Old 23-10-2013, 12:30 PM
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Hi Russell
Not knocking OSCs, they certainly have significant advantages. However, they do start out behind the 8 ball in sensitivity, since fewer than 1/3 of the photons falling on the chip actually make it through to the pixels - the rest are absorbed by the Bayer filters. For this reason, you will probably need to image a bit more than 3 times as long with an OSC to get the same luminance result as with an equivalent mono camera (luminance is synthesised in the case of the OSC). To add lower resolution colour data to the mono camera image, you will need to image maybe as long again (in RGB) as for luminance, so overall, the OSC will take about 1.5 times as long as an equivalent mono camera with LRGB filters for roughly the same result.

The situation is exacerbated if the OSC is uncooled, since thermal noise becomes a big issue if the signal levels are inherently low.
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  #30  
Old 23-10-2013, 12:44 PM
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Hi Russell
Not knocking OSCs, they certainly have significant advantages. However, they do start out behind the 8 ball in sensitivity, since fewer than 1/3 of the photons falling on the chip actually make it through to the pixels - the rest are absorbed by the Bayer filters. For this reason, you will probably need to image a bit more than 3 times as long with an OSC to get the same luminance result as with an equivalent mono camera (luminance is synthesised in the case of the OSC). To add lower resolution colour data to the mono camera image, you will need to image maybe as long again (in RGB) as for luminance, so overall, the OSC will take about 1.5 times as long as an equivalent mono camera with LRGB filters for roughly the same result.

The situation is exacerbated if the OSC is uncooled, since thermal noise becomes a big issue if the signal levels are inherently low.
thanks for the info Ray thats very interesting, and something for people to consider in whether they are going OSC or mono w filters.
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  #31  
Old 23-10-2013, 02:21 PM
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Ray I don't think the difference between mono and osc is 3X the time. That's because of interpolation. Where the firmware calculates likely values for a pixel based on its surrounding neighbours. In a sense this is similar to binning.

I would estimate the difference between a mono and a osc CCD to be more like 50% or even less. I have had an STL11 colour and it was quite sensitive. Where it had trouble was in the dim areas or dusty areas. They often were noisy.

Same with DSLRs. Some DSLRs have very high QE compared to CCDs. Nikon D800E Soony Exmor 36.3mp QE is supposed to be 59%. That is the same as my FLI Microline 8300. Having used both that seems about right.

Greg.
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  #32  
Old 23-10-2013, 04:27 PM
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Ray I don't think the difference between mono and osc is 3X the time. That's because of interpolation. Where the firmware calculates likely values for a pixel based on its surrounding neighbours. In a sense this is similar to binning.

I would estimate the difference between a mono and a osc CCD to be more like 50% or even less. I have had an STL11 colour and it was quite sensitive. Where it had trouble was in the dim areas or dusty areas. They often were noisy.

Same with DSLRs. Some DSLRs have very high QE compared to CCDs. Nikon D800E Soony Exmor 36.3mp QE is supposed to be 59%. That is the same as my FLI Microline 8300. Having used both that seems about right.

Greg.
Hi greg

OSC makers normally quote peak QE at the maximum of the green Bayer filter - ie if the chip is illuminated with narrow band green light of just the right wavelength, 59% of the photons will be detected by those pixels that are under green filters. What they do not say is that 0% of that green light will be detected by either the red or blue pixels - it won't even get to them. So the QE claims for OSCs are not remotely apples to apples when comparing with figures from mono chips - QE data will only be comparable at one chosen wavelength and then for only half of the pixels in the OSC.

The Bayer filters do not redirect photons, they absorb them, so if you illuminate the chip with white light, the Bayer filters will stop the red and blue light from getting to the green pixels, the red and green light from the blue pixels and the blue and green light from the red pixels - ie, about 2/3 of the light is absorbed before it gets to any one pixel. If you remove the filters, each pixel will now see the full red+green+blue of the white light, so it will have ~3x as many photons pouring into it - this is a mono chip. The mono chip will be much more sensitive to broadband light (~3x), simply because it sees so many more photons.

The Bayer matrix is limited in fundamental resolution to that of the 2x2 pixel group that make up a Bayer cell - ie Bayer chips have lower resolution than mono chips. The interpolation process attempts to estimate what the missing data might have been to try to recover some of the lost resolution and it is generally very successful in doing so. However, it does not increase sensitivity - that was irrevocably lost when the Bayer filters removed 2/3 of the photons.

OSC chips seem to have higher internal gain than mono chips so that you see the same sorts of signal levels when you switch between mono and OSC. However, the OSC is giving you the same signal from a lot fewer photons by running at higher gain and that will show up as noise in darker regions of the image - which is what you have observed.

rest assured, your 8300 will eat any OSC for breakfast when it comes to broadband SNR. That doesn't mean that OSCs are no good - far from it, they have many advantages, but they are definitely less sensitive than mono chips. regards ray


EDIT: just realised that I have both colour and mono versions of the original QHY5. Its hardly a top line chip, but I can see vastly more guide stars with the mono version - 3x more sensitivity would be about right.

Last edited by Shiraz; 23-10-2013 at 07:00 PM. Reason: addendum
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  #33  
Old 23-10-2013, 06:38 PM
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Good writeup Ray.

I think you are right about the extra gain making up for a lack.

Don't worry I am not planning on OSC. Been there done that. They are great for brighter targets but struggle with dim objects and narrowband.

The Kodak True Sense LRGB colour filter array claims 2X increase in light sensitivity so perhaps 3X is true between mono and colour. I always thought it was less but the gain factor may be what I was confusing with QE. Like high ISO on a DSLR.

Greg.
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  #34  
Old 23-10-2013, 06:57 PM
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Good writeup Ray.

I think you are right about the extra gain making up for a lack.

Don't worry I am not planning on OSC. Been there done that. They are great for brighter targets but struggle with dim objects and narrowband.

The Kodak True Sense LRGB colour filter array claims 2X increase in light sensitivity so perhaps 3X is true between mono and colour. I always thought it was less but the gain factor may be what I was confusing with QE. Like high ISO on a DSLR.

Greg.
Most people would go with an OSC simply because it's a lot more convenient and simpler to run than having to contend with filter wheels, filters etc. Less to setup and still takes good piccies. A mono camera, filter wheel, filters etc etc, is a big investment. Great for a permanent setup, but I feel can be somewhat of an overkill if you want to just haul a light but good setup around, to use visually and also take piccies with. In any case, taking pics through filters reduces the sensitivity and resolution of your mono camera as well.
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  #35  
Old 23-10-2013, 07:16 PM
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Most people would go with an OSC simply because it's a lot more convenient and simpler to run than having to contend with filter wheels, filters etc. Less to setup and still takes good piccies. A mono camera, filter wheel, filters etc etc, is a big investment. Great for a permanent setup, but I feel can be somewhat of an overkill if you want to just haul a light but good setup around, to use visually and also take piccies with. In any case, taking pics through filters reduces the sensitivity and resolution of your mono camera as well.
Taking pics with a mono camera through filters does reduce the sensitivity, but not the resolution. The big advantage is the ability to do luminance frames and make use of all the photons in the visible range.

I am a nomadic imager who started with an OSC and moved to a mono camera very quickly. The better quality is worth the extra time and effort to me. I understand that others may make different choices...
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  #36  
Old 23-10-2013, 08:52 PM
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Thanks Ray for starting this thread and for everyones informative comments, I've learnt a heap

Cheers
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  #37  
Old 23-10-2013, 09:36 PM
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I agree with Ray's original point that total integration times do not directly correlate with the same SnR across different systems and locations. A 12" scope has over twice the aperture (area) than my 8"... so all else being equal, 20 hours for someone else may be similar to 45 hours for me.

However, if you look at the subset of people who actively aim to go really deeeep, produce images that elicits "wow" reactions, gets APODs, and so on, things start to look very similar: mag 21-22 dark skies, 100 - 130 mm refractors for nebulae, 12" reflectors with appropriately matched pixel sizes for galaxies, Astrodon/Astronomik/Baader/etc filters, 11000 / 16803 / 694 chips are all pretty common.

In this case, comparing 1 vs 10 vs 50 vs 100 hours does give a rough idea of what to expect.
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  #38  
Old 24-10-2013, 06:32 AM
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Most people would go with an OSC simply because it's a lot more convenient and simpler to run than having to contend with filter wheels, filters etc. Less to setup
I can relate to this. I always take my OSC with me when I drive out to dark skies once in a blue moon. At home I'm stuck doing NB with a mono. I would never contemplate a filter wheel and doing color on the field. Not enough time for this.
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  #39  
Old 24-10-2013, 07:28 AM
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Thanks Ray for starting this thread and for everyones informative comments, I've learnt a heap
Hi Jo - I agree, there have been some very thoughtful and valuable contributions.

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In this case, comparing 1 vs 10 vs 50 vs 100 hours does give a rough idea of what to expect.
Interesting observation Dave, maybe my original thesis is not applicable within subsets of the astro community - Darwin rules, even in Astro imaging? Even so, there is a 2:1 range of imaging times inherent in the QEs of the chips you mentioned.

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Most people would go with an OSC simply because it's a lot more convenient and simpler to run than having to contend with filter wheels, filters etc..
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Originally Posted by multiweb View Post
I can relate to this. I always take my OSC with me when I drive out to dark skies once in a blue moon. At home I'm stuck doing NB with a mono. I would never contemplate a filter wheel and doing color on the field. Not enough time for this.
and with an OSC, you never get stuck with 3.5 hours of luminance, 21 minutes of red and clouds for the next week - followed by the moon.

Last edited by Shiraz; 24-10-2013 at 08:50 AM.
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  #40  
Old 24-10-2013, 08:12 AM
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Yep there is nothing wrong with DSLR's or OSC cameras. I used to use hypered film in the 1970's and a DSLR would have been far better, heaven in fact!

This is a twelve panel mosaic of a blend of NB and RGB done with my Canon 5DH and 300mm F2.8L lens at f/3.6. The camera was cooled to -12C. 20 MB

http://d1355990.i49.quadrahosting.co...VELASNRMOS.jpg

I did this some years ago. I was pleased at the time but longer or more subs would not improve the image much at all. I had hit the inherent signal to noise wall of the total system.

When Vela rises I will attempt to do the same mosaic with the new system. Here is a thread on just one panel.

http://www.iceinspace.com.au/forum/s...highlight=vela

Bert
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Last edited by avandonk; 24-10-2013 at 08:47 AM.
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