Deep NGC3199 Nebula in 3nm Ha, 28hrs.

[Bassnut]

Hi Guys

A pushed NGC3199 in Ha before I add colour and probably curve it down a touch.
It’s a bit brighter than recent efforts, so allowed more stretching and sharpening. Its pretty deep I think, cant find many stretched examples.

Click here (http://fredsastro.smugmug.com/Photography/Astrophotography-1/i-V3MpVsG/A) for big.

Taken on an RCOS 10" RC Scope at f9, SBIG STXL6303E Camera and PME mount at itelescopes Siding Spring Observatory.
Processed with CCDstack, Star Tools and Photoshop. 28 hrs Total.
42 off 3nm Ha subs 40min each bin1.

[RickS]

Yep, looks very deep, Fred. I have about 12 hours of Ha from last year (and 9 hours each of OIII and SII.) You've definitely got some faint stuff in the Ha that I didn't catch. Well done!

I struggled to get colours I liked with this object when I tried to process it last year. Since then I've learned a bunch of new tricks and built a handy PI colour masking script so I reckon I might be able to do it justice now. Must put it back in my processing queue :)

Cheers,
Rick.

[Stevec35]

Pretty deep all right Fred - nice one.

Cheers

Steve

[h0ughy]

need a snorkel and tank - very deep Fred - impressive

[strongmanmike]

Gee! in normal images of this object it is more of a Semi circle or cresent, you have essentially an entire circle or bubble here, almost unrecognisable, very cool Fred :thumbsup:

Mike

[multiweb]

Very cool details. Deep as. This one is very faint. :thumbsup:

[Paul Haese]

Huge scale Fred and that has meant lots of detail. I spent a few minutes looking at the detail. A very deep image. I remember doing this object myself, I think it is worth getting the data for the colour. I reckon this will be a cracker once you have it done.

[Placidus]

Not just deep but amazingly sharp. Good processing. I think Hershel's Ring is surprisingly very difficult, perhaps because the finest detail is also very faint. You've got it.

[Bassnut]

Thanks Rick. Yes, the colour data I have doesnt look flash. I havent tried it yet, suspected it might be a problem hence this post 1st. I will try it though.



Thanks Steve, if you recon its deep, it is :-).



Thanks Dave.



Thanks for the detailed reply Mike. I dont know why others werent the full bubble, I didnt find it that faint even stretched the noise wasnt that bad.



Thanks Marc. Faint, mmm, do you know what the mag is perchance?.



Thanks Paul. I had a look at yours before actually, one of the few I could find. I think your colour was pretty good, hope to do as well.



Thanks Mike. I did spend a fair bit of time processing, got to multiple masked layers in PS which I dont do so much these days, but cant say it was all difficult. Id be interested later to see how you went with it and what you found difficult.

[Paul Haese]

Fred, I reckon Ken Crawfords looks pretty good for colour and detail. Yours should be one step beyond that.

[Bassnut]

Yes, I did find Kens before, he got an APOD for that. I think its pretty ordinary generally actually for depth and detail (colour isnt bad), but it was taken in 2008 !, I think he would cream that these days. His new web site is a bit corny IMO :P

[Paul Haese]

Fair enough. Yes not real keen on those zoomable sites myself.

[clive milne]

Fred, I would be interested to know the relative contribution to the noise floor from the background signal, dark current and read out.

regards,
c

[Bassnut]

OK, well, I dont know how to seperate those factors out really, perhaps you can tell me?.
The cam is a 6303 NABG fairly high QE at -35deg. When I apply darks, the noise doesnt change much. The subs were taken in pretty dark 1.5-2.5 arc sec skies, but the moon was up most of the time in varying degrees.
A typical 40min sub had 1000ADU in the darkest background, and 1600ADU in the brightest nebula.

[RickS]

What's your average ADU value in a master bias, Fred? Individual cameras and sensors vary but it should be possible to get a rough idea of read noise and dark noise from camera specs.

[Paul Haese]

You should find some of that info in your Fit header if you use Maxim for capture. Not sure about other programmes.

[clive milne]

Fred, the noise calculation is actually fairly simple.
In the frames when there was no interference from the moon, I take it that the background was on average 1000 adu?
What was the dark current?
(Assuming the same exposure time and temperature)

[Ross G]

Another new object for me Fred.

Great detail with nice tones and very sharp.

Ross.

[clive milne]

Hi Fred, my guestimate (from the manufacturers specs) is as follows. :

Read noise= 11e
Dark current @ 0C = 0.3e/s
Dark current doubling temperature = 6C
Therefore, @ -35C dark current= 0.00526 e/s
And for a 40m exposure, dark current= 12.6e
Dark noise is therefore 12.6^(0.5) = 3.55e (this is very good)

If the 40 min subs have a background level of 1000adu, and the camera gain is 1.47, the charge depth per pixel is 680e,
Therefore the shot noise is (680)^0.5= 26.1e

Total noise for the background per sub is therefore: (26.1^2 + 3.55^2 + 11^2)^0.5 = 28.5e

If you are using maxim, you will need to subtract 100adu from your baseline, in which case the shot noise value 24.1e and the total background noise will be 26.7e RMS per pixel.

Ergo, the sky background brightness is by far the dominant noise source in your system. That is mildly surprising considering you are using a 3nm bandpass filter @ f9.

fwiw) The following link is a good read:

http://www.starrywonders.com/snr.html

best,
c

[Bassnut]

Thanks Clive, I was actually collecting all what you said above when I noticed your reply. Thanks for those calculations.
I calibrated a sub with 1000ADU back ground and now the background ADUs are 20-40. A bias sub has 800ADUs.

Did you assume dark subtracted background of 1000adu?. Sorry for the confusion.

[clive milne]

Hi Fred,
If the camera performs as per specifications, the dark current will amount to only 20adu for a 40 minute sub.

There is a spreadsheet you can access on the web page I posted a link to (above) where you can plug all your numbers into. It will calculate your optimum sub exposure times.

My interest in your system is that I am curious to see how the 6303e performs using 3nm filters. In theory, this is one application where you want an ultra low noise camera (like an fli 694) Other ways to minimise the ratio of camera noise to shot noise would be to bin 2x2 and use a faster focal ratio.


Incidentally, I'm writing this on my phone so I can't use the spreadsheet... (lightning from the recent storms has fried my modem)

regards
c

[Paul Haese]

Interesting. I used his calculator to determine that I can do subs in Lum on the STXL at 24.01 minutes for the RC. I am doing those at 30 minutes now so it is close, but perhaps an extra 15 minutes per hour would be better served getting another sub.

Do you think an 11002 sensor has better characteristics than the 11000? From what I have seen it is far noisier. My subs from the STL while cooled less were always cleaner than on the 11002. And; this has been a common comment from other users.

In any event it looks like long subs do work in a dark sky environment and negate the need to go shorter. Am I reading this right? Since his figure of 0.9 for F is arbitrary but looks reasonable. Thoughts?

[clive milne]

I think the spreadsheet works on the premise that for a given total exposure time the signal to noise minima is achieved with sub exposure lengths which reach the point of diminishing returns.
I take that to mean that it is better to have lots of good frames, rather than a few (slightly) better ones. The spreadsheet is an attempt to derive the best compromise, statistically. Obviously, anything that causes a variation in the ratio of signal electrons to readout noise electrons will have an influence on the optimum sub exposure length.
Implicitly, this includes filters and moon phase.

I'm not sure what happens in that spreadsheet calculation when you specify an infinite total integration time. (My computer is offline) Intuitively, I would assume it would be best to set sub exposure length to limit saturation.

[clive milne]

I'm not sure....

In my discussion on the subject with Peter, iirc) he indicated that the STXL has much quiter electronics so the sins of the chip become obvious, where previously they were buried. I have never held one in my hands so can only defer to his judgement, or your judgement for that matter, being as you have a long history with both cameras.

[clive milne]

Yes, that is consistent with my understanding of the maths at least. Basically, you just need to expose long enough for the shot noise to be dominant.

[RickS]

Fred: when you say you have a 1000 ADU reading in the dark background areas are you talking about a sub which is raw, direct from the camera or one that has been calibrated? If you are talking about an uncalibrated sub what's the rough average ADU value in a bias frame from your camera?

I'd be really surprised if your 40 minute narrowband subs are shot noise limited. Mine certainly aren't with a KAF-16803 and 12" of aperture at f/9 under much brighter skies than yours.

Cheers,
Rick.

[Bassnut]

Rick. As per post 20.
I calibrated a sub with 1000ADU back ground and now the background ADUs are 20-40. A bias sub has 800ADUs, roughly, it bounches round a lot over the view.
The moon was up, but far from the object.

[RickS]

Sorry Fred, I missed that second time through the thread :)



That's a little odd. I would have expected background ADUs after calibration to be around 180 ADU (1000 ADU less 800 ADU bias and 18.5 ADU dark current.) I'm not sure exactly how the Maxim pedestal works. Maybe that explains part of the discrepancy?



Clive: I think the charge depth per pixel above is way too big given Fred's additional info. If it is actually around 20-40 ADU (as per the calibrated sub) the the background shot noise is much smaller and read noise is dominant. Do you agree?

Cheers,
Rick.

[clive milne]

Hi Rick,
Yes i agree, my calculation is in error. I'm a little confused though by the calibrated frame background level being 20 - 40. I would have thought (as you have done) that with a bias of 800, and subs of 1000, the background should be 200 minus dark current = 180 (or 80 if the 100 adu maxim pedestal is present)

Anyway, irrespective of which value we take as being representative of the sky background (20, 40, 80 or 180) the camera noise is dominant by a large margin.
In some respects, this is good news.



regards

c

[RickS]

Thanks, Clive.

I'm in exactly the same situation with even one hour narrowband subs with my 12" f/9 scope and KAF-16803 camera. An ICX-694 with much lower read noise would give me better SNR but I'd find it hard to give up the large FOV.

Sorry to stomp all over your thread, Fred :)

[Bassnut]

Sorry guys, I measured a bin 2 bias. Your right, a bin 1 bias is 1050 odd.
BTW, here is a more stretched (not processed much) version I didnt bother with, looked a bit messy.

[clive milne]

Fred, that looks impressive, actually better than the original. .. the caveat being my internet access is reduced to my galaxy s4.

I'm a little confused, however, with your bin1 bias value. Perhaps it is a result of a typo or I am interpreting it incorrectly, but it seems inconceivable that a bias frame could have a mean background value greater than a 40min sub?

best

C

[Bassnut]

Yeah, I thought that was odd. I measured the master bias now, same, 1050 odd. I looked again at a sub again and its background 1000 to 1030 odd and 20-60 calibrated. I dont use flats. The dark master is 1060 odd. Im not very bright, im just telling you what I measure. CCD stack scales cal with temp, I dont know if that makes a difference. Temp is always very close to -35deg though.

Ive wondered, CCD sensors "deteriorate" over time apparently. Im guessing cycling -35 to ambient say 20 deg average every day would be stressfull, 55deg delta at least. My cam for the last 2 years has always been below 0, 24/7 . -35deg imaging and -10 daytime idle. I wonder if that affects the CCD spec, long term?.

[RickS]

I have a significant amount of variation in my bias values over time, maybe 30 or 40 ADU. That's what lead me to do overscan calibration which makes everything wonderful :D

Cheers,
Rick.

[clive milne]

With Fred's indulgence, I would like to submit two prescriptions optimised for narrow band imaging for your consideration.

If we stipulated a 46 arc minute fov (this is what you get with a 16803 and a 12" f9)
you could achieve this using an icx694 chip on a 12" F4.28 primary mirror with an ASA 0.73 reducer corrector.
Because each pixel in the Sony chip covers 1.5x1.5x the area of the sky (compared to the 16803 scenario) and has a 40% qe advantage, the shot level will be a little more than 3x greater (all be it at 0.667x the resolution) If you were to house the 694 in a microline body, the camera noise would be reduced to 1/3 that of a typical 16803.
The signal to noise ratio for narrow band imaging would be a factor of 10 better in the system using the Sony chip. It then follows that to achieve the same signal to noise ratio as a 100 hour exposure using a 16803, would require just 1 hour with a microline 694. The caveat being the resolution is reduced and this is only valid for 3nm bandpass imaging.

Prescription two is probably dream-ware for all but a handfull of people but is still do-able.

A 32" primary mirror is imho) the point of diminishing returns for a number of reasons that I don't need to go in to here.
If we specified an f4.5 primary, the Keller corrector would bring the focal length down to 105 inches (within a few percent of a 12"f9) so the native resolution would be equivalent If we binned the icx694 2x2.

The 32" /694 combination would be accumulating signal more than 10x faster than the 12" / 16803.

Now if you used a Raptor Photonics Kingfisher V icx694 camera which has cooling capacity of 110 degrees below ambient (0.000,001e- s) and a readout noise below 1.5,
the signal to noise ratio is 60 times better than the 16803.

What you could do in one night with this rig would take you 20,000 hours with your current equipment. Basically a lifetime.

This assumes that the background level for a 1 hour exposure at f9 through a 3nm bandpass filter is around 20 adu. If it is higher, then the shot noise will limit the s/r advantage of the second system.

You can perhaps see why I was interested in the sky background levels you are getting.

best
c

[RickS]

Interesting thoughts, Clive. I wanted to run the numbers before I replied so it took me a little while.

If I was only interested in narrowband imaging it would certainly be worth investing in a camera with lower noise. I can run my scope at f/4.9 as well as f/9, so with a ICX694 that would be a very fast system.

The 32" system sounds very cool and would be fantastic for digging out faint planetary halos. That might be a project for my retirement. Hopefully EMCCDs will be affordable by then too :)

Cheers,
Rick.

[BeanerSA]

Noob question: What is "Ha"?

[RickS]

http://en.wikipedia.org/wiki/H-alpha

[clive milne]

Hey mate,
I have been giving this a bit more thought over the last week or so and have come up with an extreme astrograph for the 694 chip which you could easily put together with off the shelf components.
Let's start with something like 25" to 32" F4.5 newtonian primary.
You would employ a field corrector that gives you a 40mm flat field with minimal vignetting (3" 1x Wynne would do)
Immediately after the Wynne, you would have your off axis guider + filter carousel. All pretty straightforward so far.
Rather than place your camera at the focal plane, let the light cone go through focus and start to diverge again.
The next element in line is a high quality 85mm camera lens. You would orient the lens essentially back to front, with the nominal focal plane of the lens coincident with that of the Wynne (does this need further clarification?)
Each light bundle exiting the 85mm lens will now be focused to infinity and be parallel. (This is an important enabler... but more on this later)
The parallel light bundles are then brought back to focus by the next element which is a 28mm f1.4 high quality lens.
Obviously the 85mm & 28mm lenses need to be oriented with their field lenses facing each other.
If the primary mirror chosen is a 32" f4.5, the resulting astrograph using these relay lenses will have a 1200mm focal length and an f ratio of f1.5
This system has a number of important advantages over conventional astrographs...

* Collimation is no more difficult than a typical large newtonian.

* If the camera is fitted with the appropriate bayonet fitting for the 28mm lens it should be orthoganal, so you avoid focal plane tilt issues typical of fast systems.

* Tilt and de-centre of the second relay lens with respect to the first does not degrade the psf at the final focal plane (within sensible limits)

* Because the filter carousel is at a location where the native focal ratio is f4.5, you don't suffer band pass shift with 3nm filters.

* This system gives you the opportunity of placing an ultra-narrowband filter (between the relay lenses) ... maybe an etalon? so you can bring the filter bandwidth down to 0.5nm This (theoretically) results in a reduction in the snr be a factor of 6x .... ie) it is 36x faster than a shot noise limited 3nm system (which doesn't actually exist)

.... There is more I could write but I am out of time so I'll finish with this comment;

I cannot even imagine what could be done with such an instrument. However, I do know that you would be breaking new ground with every exposure.

best
c

[clive milne]

An update on the above prescription:

I just did a rough estimate of the beam angles in the afocal zone (between the relay lenses) and realised the system needed tweaking in order to make it suitable for an etalon in this location.

Relay lens 1 needs to have a 300mm focal length and a speed greater than f4.5 This brings the angle of incidence of the beams at the field edge down to 2 degrees and the transmission of the emission line should stay above 90%
The focal length of relay lens 2 is negotiable, with 85mm being the sensible bottom limit. This lens would work well: https://www.canon.com.au/en-AU/Personal/Products/Cameras-and-Accessories/Camera-Lenses/EF85mm-f12L-II-USM-Lens

Final focal length for the 32" f4.5 system employing the 300mm/85mm relay lens arrangement would be 1020mm @ f1.275

The diameter of the etalon would be defined by the beam width at the afocal position, which is around 70mm.

The consistency of illumination accross the 694 chip would be excellent and unless you chose a really poor quality 300mm lens, the factor limiting the final resolution would be the sharpness of the 85mm lens. If you chose the Cannon 85/1.2, it would be pinpoint edge to edge with negligible lateral colour and vignetting.

One side benefit is that you could detune the etalon away from the Ha emission line and take 0.7nm continuum sub frames for later subtraction, or even pick up the N1 & maybe N2 line if you're lucky. (your pre-filter would need to be chosen to suit though)

[RickS]

Clive,

Just add an EMCCD camera and voila... lucky imaging of DSOs in narrowband.

Cheers,
Rick.

[clive milne]

Yep, they're an interesting idea and key enabler for high frame rate astronomy of low surface brightness objects.

I still like the Kingfisher V with the icx 694 for deep exposures though.
The readout noise approaches emccd's, but for dark noise the sony chip is in a class of it's own. In the kingfisher, it equates to 1 electron per fortnight!

[RickS]

That's very impressive. I wonder how long they take to cool down. Have you ever priced one?

[Mighty_oz]

Interesting discussion guys 😃
Clive would your system still be workable with a 25" F4.9 system ? Anything major u would have to do with it ? I've not heard any mention of a rotator for the newts, so the camera is video , yes ? ( pardon my ignorance, still learning like the rest, hehe )

[clive milne]

Marcus,
The system could be made to work with a 25" f4.9 Newtonian. The optical design is easy. The (mechanical) engineering challenges however, would not be trivial nor inexpensive.
The ccd that I would recommend is not what I would call a video camera as such, even though it can take video. There are budget options (for the camera) a second QHY22 will set you back $3k. The pick of them (Raptor Photonics) will be around the $10k mark.

[clive milne]

Rick,
I'm not sure about the cooling down time on the Kingfisher.
What I can tell you is that it requires 0.5L/min of cooling water, and the system needs to dump 100W of heat (about the same as a human body at idle)
As for frame grabbing software.. I don't know.
To my understanding, one of the professional observatories on the east coast has invested in one of these cameras so it is obviously possible.

best
c

[RickS]

Something to think about if/when I have time to tinker in future, thanks Clive.

[clive milne]

Your welcome.

fwiw) I have been chewing on this idea a little more and come to the conclusion that inserting a Fabry Perot etalon into the collimated beam (between the relay lens pair) isn't really viable because the divergence tolerance is so small (equivalent to f30)
You can achieve a better s/nr by sticking to an ultra narrow band filter (1nm for example) The reason is that the interference filter gives you much greater flexibility with the choice of final focal ratio after the relay lenses with large apertures and enough fov to cover an icx694 chip.
At the end of the day the purpose of the exercise was to configure a system where shot noise was the limiting factor.

best
c