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Twisted Lip

Hit the DSLR quality buffer with IC434

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10x900sec lights? I suspect 20x450sec lights would have worked better. Less data would have been lost to planes etc, and random noise would be better reduced by 20 subs than 10.

Edited by Ags
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Will,

I have posted my processed horse here: http://stargazerslounge.com/topic/204703-horse-with-a-flip/

It's still pretty noisy and I had to run Noel's Deep Space Noise Reduction a couple of times. I'm hoping that more data with large dithering will help to reduce the noise further.

Ultimately I think that you are basically right and a DSLR is not the weapon of choice to make astrophotos. However, do you have £3k hanging around to drop on what is just a bit of a fun hobby?!

I'm saving...

I think that looks great Gav, definitely better than mine and with less data too. I suspect you're right, the dithering is helping massively.

Will

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Will,

I have posted my processed horse here: http://stargazerslounge.com/topic/204703-horse-with-a-flip/

It's still pretty noisy and I had to run Noel's Deep Space Noise Reduction a couple of times. I'm hoping that more data with large dithering will help to reduce the noise further.

Ultimately I think that you are basically right and a DSLR is not the weapon of choice to make astrophotos. However, do you have £3k hanging around to drop on what is just a bit of a fun hobby?!

I'm saving...

Great photo Gav.

I know that dark sites make a huge difference to image quality. I wonder what the skies were like when you captured this data? Was it from a dark site, or from a big city? My guess would be it was taken closer to a dark site?

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Thank you both.

I intend to experiment with 'aggressive dithering' tonight!

This was taken in my back garden, which is near the edge of Marlborough, a smallish market town in Wiltshire, recently made famous by Stargazing Live when they had a sun scope set up on the high street for passers by to look through! I also used the IDAS D1 LP filter.

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10x900sec lights? I suspect 20x450sec lights would have worked better. Less data would have been lost to planes etc, and random noise would be better reduced by 20 subs than 10.

Oops, that's the can of worms opened again!!!

A long recent thread concluded that longer subs are definitely better than shorter subs, but also that more subs are better, full stop. So, where does the balance lie?

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Thank you both.

I intend to experiment with 'aggressive dithering' tonight!

This was taken in my back garden, which is near the edge of Marlborough, a smallish market town in Wiltshire, recently made famous by Stargazing Live when they had a sun scope set up on the high street for passers by to look through! I also used the IDAS D1 LP filter.

Looks like you've got fairly dark skies compared to us city imagers!

http://www.avex-asso.org/dossiers/pl/uk/index.html

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Hi Gav,

You are most certainly correct about long subs. A fair few threads recently have highlighted this both from a practical point of view (Olly is a big fan of long subs) and from a more mathematical point of view by myself.

Long subs are good because the imaging time is split up in to just a few subs which means just a few instances of read noise. Short subs are bad because of many reads.

An argument could be made for have enough for a good rejection algorithm.

The random noise would be lower with more shorter subs because there is less signal to create that noise. With double the subs the random noise in a single frame would be lower by a factor of 1.4 but signal has fallen by a factor 2. This is a better sub than the long one?

I reckon there is a trade off, it is between a s few subs as possible (subject to,guiding, LP, saturation etc) to limit the read noise contribution and having just enough for a pixel rejection algorithm. I would have though from 6-10 subs is enough for that. So when you go out next and think how long do I want on this object divide that by something like 6-10 and use that length as your sub. Also good calibration frames and dithering will help too.

Paul

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Oops, that's the can of worms opened again!!!

A long recent thread concluded that longer subs are definitely better than shorter subs, but also that more subs are better, full stop. So, where does the balance lie?

1x9000sec = bad

1000x9sec = bad

Somewhere inbetween that. Hope that narrows it down :-)

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Absolutely, the search for the happy medium continues. I seem to be settling on 900s as my optimum.

Yes, I'm lucky, the light pollution really isn't too bad at all where I am. I'm also slowly lowering all the trees and bushes in the garden, opening up the horizon... Mrs. PhotoGav doesn't seem to be objecting too much... yet!

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Hi Ags,

Can you show that a single 2.5hr sub well calibrated with good flats and darks is bad.

Tim on here has experimented with multi hour subs. Don't think they were bad.

Long subs are simply the best way to a high SNR for a given time.

Cheers

Paul

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Nice images Stuart,

Why do you advise to increase ISO and shorten exposure time? There is a big myth surrounding ISO and that is increasing the ISO somehow gets you more data just because the image looks bright. If you point as scope and open the [removed word] for identical times you will get the same number of photons. That is a fact. I wouldn't advise shortening subs to get more of them. I advise using 15 mins where possible and getting more of them, along with the extra calibration that I was referring to.

Paul

Hi Paul,

We all know that the same amount of photons hit the sensor no matter what the ISO is set to however I believe that there is an issue with quantization when converting the read ADU values and scaling them to fit into 12/14-bit values in a RAW file. This is only my finding in practice, sorry but I don't do pure theory. Personally I like to look at and generate pretty pictures, not pretty graphs and Excel spreadsheets. Don't get me wrong, I understand some of the theory to a point so I am not just dismissing it, I am dismissing the parts that are unclear and everybody dissecting it seems to interpret slightly differently.

I am trying to get out of the habit of recommending settings for people to use, because it always leads to debates. I'll let my and other's images speak for themselves (good or bad). But regarding the OP's original statement that his DSLR's limit has been reached, I believe that is completely incorrect. Perhaps on a very slow scope (thermal) noise will creep in and destroy any additional data but I think there is more mileage in DSLRs than people give them credit for. My recommendation for shortening the exposure time would have probably allowed sigma clipping to do its job better in a like for like session under the same conditions (i.e. duration) even though I am an advocate for longer subs.

However I would d recommend modifying the DSLR, it will make a fairly substantial difference on this and many other targets.

The debate will continue of course...however the theorists will win because the pragmatists don't have enough imaging time (in the UK at least) to disprove them :p

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Hi Ags,

Can you show that a single 2.5hr sub well calibrated with good flats and darks is bad.

Tim on here has experimented with multi hour subs. Don't think they were bad.

Long subs are simply the best way to a high SNR for a given time.

Cheers

Paul

In this particular case the 9000 second sub would have been bad, because the OP mentioned several of the 15 minute subs were lost to plane trails etc.

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In this particular case the 9000 second sub would have been bad, because the OP mentioned several of the 15 minute subs were lost to plane trails etc.

Yes I guess this is particular to areas near airports - whilst I'm not that close, Heathrow chuggs them out steadily so you can guarantee something will pass through the FOV :/ I suppose if you lived Brecons or Highlands with fewer air corridors you can get longer subs without the risk of 'UFOs'. I agree with Stuart, a lot of this seems to be what works in practice rather than theory. Different methods for different skies, LP, geographies etc. :)

Will

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Hi Stuart

Yes I agree that fitting the high dynamic range offered by a low ISO in to a 12 or 14 bit a RAW could lead to some quantising noise. I also agree that the limit hasn't been reached. Long subs and a high quality calibration with dithering would be a good start. I believe you posted an image with many hours so the DSLR has much use.

Paul

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If you want to know the limits of DSLR imaging, check out the work by Scott Rosen:

http://astronomersdoitinthedark.com/

I think that rather than being at the limit of the camera, you are at the limit of your skies!

Thank you very very much for posting that link Russell. That has totally restored my confidence in the capabilities of the DSLR. His pics are right up there. There are several things to note from his pics: he uses a wide selection of ISO values, from 100 to 1600. His exposure times are relatively short, in the 3 minute zone, though he is shooting at faster f stops than an 80ED. He's often using a lens rather than a scope. He shoots hours and hours of subs. He has a DARK SKY!!!! Olly was right all along - no amount of kit substitutes a clear dark sky...

So, back to the drawing board and more experimentation is required before I have a satisfactory, just set up and use it, set up...

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Sky is the limiting factor. :sad:

Don't mention planes........Gatwick inbound heavies and outbound holiday flights are murder here.

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I don't know why people get so hung up on planes , satelites and even the odd passing cloud... Given enough subs all of them dissapear if you use a suitable sigma reject stacking method...

Averaging will fade them . .

Peter...

Badly typed on my Galaxay S4 in Tapatalk4

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Assuming you've got more than one sub :-)

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ually very bad seeing due tio

Thank you very very much for posting that link Russell. That has totally restored my confidence in the capabilities of the DSLR. His pics are right up there. There are several things to note from his pics: he uses a wide selection of ISO values, from 100 to 1600. His exposure times are relatively short, in the 3 minute zone, though he is shooting at faster f stops than an 80ED. He's often using a lens rather than a scope. He shoots hours and hours of subs. He has a DARK SKY!!!! Olly was right all along - no amount of kit substitutes a clear dark sky...

So, back to the drawing board and more experimentation is required before I have a satisfactory, just set up and use it, set up...

Hi Gav,

Your observations are correct with regards to the usage of fast lens, in order to achieve the same exposure with an ED 80@ F6.4 ( 0.85X FF/FR ) the length of the esposures need to be increased by a factor of 3.35X ie: 1000s . This exposure length is suicidal  for a DSLR  under urban  skies with high LP and usually very bad seeing due to the turbulance in the atmosphere and the onset of thermal noise . He also imaged from a very dark site with only the natural skyglow to worry about. These captures although masterful and brillaint can not be taken as a yard stick as to what could be achieved by a DSLR or even a cooled OSC CCD under urban skies.

Regards,

A.G

Edited by lensman57

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F ratio is the photographic speed - so it is crucial to exposures. One minute of F4 is equal to 2 minutes of F5.6 (assuming focal length is constant so the pictures are the same).

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good point, I'd forgotten about BEOS. Like you I haven't used either APT or Nebulosity but I've heard good things about both (plus they will both control DSLR and CCD's which is a bonus).

Anyone able to give a quick thumbs up or thumbs down for APT, BEOS or Nebulosity?

Will

APT is excellent for DSLR control (with guider). Highly recommended by me.

ChrisH

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