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I have a Canon EOS 1000d, which I modded back at the start of the year. I usually capture subs of about 3-5mins duration at ISO 800. However, I'm wanting to image the Deer Lick group with Stephan's Quintet, and I'm going to need to go right the way to 5 min subs for them.
I've noticed quite high noise levels at ISO 800, is this the optimal speed to use? Would it be better to capture subs at ISO 400, or would they then be really underexposed? Would I be better doing 3 min subs at ISO 1600?
I find ISO a little bit confusing, as far as I know it's a measure of how the camera's computer assigns brightness from the raw pixel values? ?
Thanks for looking!
I have completed a comparison of the level and pattern of read noise in bias frames between the Nikon D7500 and the D5300, please see the blog page ...
Telescope: Skywatcher ED80 Camera: Modified 700D Guiding: QHY5L-ii m + finderscope Image Capture: Astrophotography tool Processing: DSS + PS or Pixinsight Filters: Astronomik CLS clip on Over the last week I managed to get two nights with the telescope, both of which were spent capturing the iris nebula. On the first night I got about 1.5 hours and on the second night I managed a whopping 7.5 hours. I give thanks to the recently purchased 700d AC power adapter for that long stint. I live in a relatively light polluted area, but I can still manage to see the milky way on a clear night. Now, I usually image relatively easy images ... orions nebula, horsehead nebula, rosette, heart and soul, using the above setup as well as with the canon 200mm lens. The iris nebula on the other hand I wouldn't consider as easy a subject as the rest, given my FOV and the fact that most of the interesting artefacts around the nebula consist of VERY FAINT dust lanes. In any case, I decided I wanted to take the leap and just go for it.
A concern of mine was the colour of the stars; I wanted to make sure that I didn't clip any of them, and so I decided to expose my subs to accommodate this, which lead me to 240s subframes at ISO1600, dithered. This is what the histogram looked like for one sub:
And this is what a single sub looked like without any processing/calibration:
The stars seemed sharp enough and I had both the Iris in view as well as the ghost nebula (just about) keeping it company. My train of thought has always been that even if a single sub doesn't show the detail I'm after, stack enough of them and it'll soon appear. I went ahead and captured the images, got the corresponding flats, darks and bias frames and then ultimately started to process the images.
This is what the histogram looks like in PI before and after I apply the DEFAULT screentransferfunction curve:
And this is what the image looks like, without any post-processing other than Automatic Background Extraction and a 180 deg fast rotation:
There are a number of things that concern me about this image.
The colour motle/noise what ever you want to call it. There are blotches everywhere. The level of luminance noise which for 8+hours worth is simply disappointing. The lack of detail in the dust lanes. I come back to the title of this thread:
Where did I go wrong?
I apologise for the length of this post, I was just trying to be thorough.
The Sculptor or Silver Coin Galaxy ( NGC 253 ) in the Sculptor constellation - re-processed from the original subs
The Sculptor Galaxay ( NGC 253 ) ( Please click / tap on image to see larger and sharper )
The Silver Coin Galaxy ( NGC 253 ) in the Sculptor constellation ( aka the Silver Dollar or Sculptor Galaxy ).
Caroline Herschel in 1783 was the first to recordthis bright ‘nebula’ in an area of the southern sky that Nicolas de Lacaille had called the “Apparatus Sculptoris” or “the sculptor’s studio”.
Whilst relatively close to us compared to the billions of far more distant galaxies in the Universe, the great size of the “Sculptor Galaxy” and the huge distances involved are still hard to comprehend. To put this into some perspective, the light that is just now reaching one edge of the great disc left the opposite edge when the Earth was in the grip of last great Ice Age 70,000 years ago and the light we now see has been travelling towards us for over 11 million years.
NGC 253 - "Silver Coin" or "Sculptor" galaxy.
Catalogue location: RA 00 48 23, DEC -25 11 52.
From Image Solver:
Image centre: RA 00 47 33.4 Dec. -25 17 11.1
Focal length: 1008.5 mm
Field of view: 1d 19' 2.2" x 53' 0.3"
Skywatcher Quattro 10" f4 Newtonian telescope.
Skywatcher AZ Eq6 GT Mount.
Orion Short Tube 80mm guide scope & auto guider - PHD2.
Baader MPCC Mark 3 Coma Corrector & no filter.
Nikon D5300 (unmodified).
Field of view (deg) ~ 1.35 x 0.90.
20 x 180 sec ISO 800, long exp noise reduction on.
Pixinsight & Photoshop.
November 4, 2016
re-processed from original subs ( including Photometric Colour Calibration ) - September 9, 2017
Details of Sculptor Galaxy from SkySafari for the 4th of November 2016 around 10pm local time
Annotated version ( using Pixinsight ImageSolver and Annotation scripts )
Sculptor Galaxy ( NGC 253 ) - annotated
The cyan tags ( 35 or so ) relate to galaxies recorded in the Principal Galaxies Catalogue.
The white tags are brighter stars as recorded in the Tycho-2 Catalogue. The number beneath is the magnitude of the referenced star.
Link to previous version below ...