after a dissapointing view of jupiter and the moon, thanks to my pine trees which i threatened with a chainsaw, I decided to give sombrero another shot. I spent close to 2 hours capturing data and used the best frames which had no visible star trails.
15 x 30sec subs @ISO 1600 RAW
15 x 30sec subs @ISO 3200 RAW
30 Darks @ ISO1600 & ISO3200 (total)
30 BIAS @ ISO1600 & ISO3200 (total)
Finished in Lightroom
I am happy with the results, the end result has alot more detail in it than my first attempt. But there seems to be some digital noise and i am unsure on how to process more of the noise out of the picture?
Im expecting my OAG and QHY-5 on friday, very excited!!!!!!!!! just keep your eyes out for the "OAG Troubles" post on friday night lol
I would greatly appreciate any pointers, or comments on how to improve the quality
A Screen layer, an Overlay layer, both with layer masks to increase contrast and detail a little, then a median filter to remove some noise pixels and median substitute them.
I tried a highpass sharpen too, but the jpeg artifacts made it worse - I'd need your raw data to bring out more details.
James,
From my very limited experience, high ISO will create noise. If your able to guide - longer exposures at a lower ISO say around 800 will produce more data for better results.
Saying that I like your second attempt, well done.
James,
From my very limited experience, high ISO will create noise. If your able to guide - longer exposures at a lower ISO say around 800 will produce more data for better results.
Saying that I like your second attempt, well done.
Clear skies
Rod
Agree - ALL my imaging is done at 500 ISO or 800 ISO. 500 for the brighter stuff, 800 for the dimmer stuff. I WILL push it to 1600 if the object is INCREDIBLY faint, but then you get all sorts of noise issues - I have found even with a full set of flats, dark flats, bias and darks that at 1600 the stacking programs still leave a lot of residual noise that it cannot differentiate from stars - so, if shooting 1600, I will turn in-camera long exposure noise reduction ON, and live with fewer subs. This helps further in post-shot noise reduction in DSS.
ALL my subs are either 5 or 10 minutes, NOTHING shorter. I need to with a 4" refractor
Also, possibly consider PRE-PROCESSING some subs - put them into a RAW editor and edit as far as possible, especially noise reduction (Photoshops "Reduce Noise" filter in CS5 and later works really well, as does Nik's DFine plugin), THEN stack them - you'll be amazed at the resulting differences.
And don't forget the golden rule - NEVER STACK JPEG's - absolute waste of time. JPEG's are VERY lossy files in terms of detail mapping etc. Stack RAW, TIFF or FIT only (strangely I have had issues stacking TIFF's - cannot explain why - get rotational effects, whereas the exact same RAW's stacked do NOT)
Well done James, especially given they are unguided subs!
As others have said, you will see less residual noise after dark subtraction and stacking if you can use lower ISO values. The longer sub exposures that are required at lower ISO are not really practical with your current setup with no guiding, but once you have guiding working nicely you should give it a go. I have read that ISO800 is often considered a good compromise between camera sensitivity and noise. Its certainly the setting that I use most frequently.
Re the suggestion to *pre-process* RAW subs...well I don't agree with that...
Why? One of the reasons to stack sub exposures is to improve the signal:noise ratio. So applying a noise reduction process before the image is stacked and 'stretched' will mean that the signal is not as well differentiated from the background noise as it should be, and so both noise AND signal will be reduced (bad). It also means that when your astro software (eg DSS) subtracts the dark files, the dark noise in your light images files will no longer correspond to the dark noise in the dark file, so in the end you're not subtracting like from like. Pre-processing the RAW sub may also affect any gradients in the image, which means that your flat subtraction may not work to reduce gradients of vignetting properly either. IMHO it's much better to apply any noise reduction steps once you have a stacked image. And besides, why spend all that time to pre-process perhaps 50, 100 or even 200 sub frames when you can just work on a single final stacked image.
Same thing goes for using 'in-camera noise reduction'. unless you take a separate set of darks, flats and bias frames all with in-camera noise reduction turned ON, it's apples and oranges.
Agree Richard, but I have found that SOMETIMES DSS won't fully "denoise" an image, despite how many sets of DFB I apply, and I have overcome that by using in-camera noise reduction ON and THEN applying the DFB. Works for me anyway
I did find doing GX work that PRE-processing DID improve the stack result, quite nticeably. From NO dust lane visible with a no-preprocessing DSS stack to VISIBLE dust lanes, and better star colour in the arms (from NIL blue showing to blue showing)
Thanks all for your comments, Because of the short exposures i thought by upping the ISO would help bring out the detail, maybe my assumption was some of the reason behind the noise in the image?. I have ordered a Off Axis Guider with a QHY-5 which should be here on friday, Im hoping the next image i post of sombrero will be a lot better.
I like targeting the same object over and over again to help me improve my techniques in AP,my theory is, if i dont see an improvement, then i must be doing it wrong! . Im not expecting magazine quality, just a quality that i can take to a photo lab to be re-sized and framed for my wall that i can look at and say, "hey i did that"
Nice work, I am in the same boat as you right now, need to step up to guiding to get longer stubs and beat down the noise.
The reason high ISO doesn't help is because you cannot change the sensitivity of your sensor, quite literally you are just changing gain - the conversion of number of photons to the brightness of a pixel in your image.
The noise comes in because photons arrive at the pixels in a Poisson distribution, so for N photons you will have a standard deviation of square-root of N. Thus low N values have low signal to noise ratio. Doubling ISO also doubles noise and so cannot bring out more detail.
There are a few caveats to this, but it is in general why you need longer stubs or bigger scopes.
Paradoxically, if you are using a canon DSLR, going to higher iso can reduce the noise adding during the sensors reading process (called read noise).
