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Observing and Imaging Group
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There was clear skies a couple nights ago but I have been feeling unwell to go to Blackheath or out of London but I felt hungry to see something out there, so I opened my living room window facing East, plonked my Nexstar mount on a table on rickety floor boards and grabbed my small 80mm apo frac. Jupiter and its moons looked lovely, no great red spot in view, though. Then I tried to view deep space targets of which only M13, M92 globular clusters were prominently visible in the eyepiece. Then I thought, lets see how my very far from ideal setup copes with imaging the M13 with Canon 650d camera. I dont think I can recommend this being done frequently as to get a decent image in heavy light pollution on an Alt-Az, need to take hundreds of very short exposures and that means wearing out the shutter on the camera. How long is a shutter life on dslrs?
So my pic here comprises 1240 X 4sec frames with an equal mix of ISOs at 1600, 3200 and 6400. I wanted to also include ISO 800 but ran out of time or rather window space. Initially had 1500 frames but because I am using an Alt-Az mount, a few hundred frames were no good. I have to say DSS is amazing as it coped with stacking the various shifts of my images and the field rotation. I thought it will never cope but it did. I also took 50 dark frames at each ISO, so 150 dark frames in all.
Post processing wise, I had to manually adjust the RGB levels and then the normal levels and curves adjustments to pop the stars out as best as possible with surprisingly some colour in the stars upon increasing saturation, otherwise I would have just took the easy road and made the image monochrome. On closer zooming, I realise the crayford focuser drifted over time as I think I forgot to tighten the focuser so resulted in fatter stars.
But for the primitive capture setup, it aint half bad result for Living room/bedroom window astronomy 😀
I have a question about darks. Once DSS creates the master dark frames, can I bin the individual dark files? The master dark frame is all I need, right?
I did also use my 8" SCT at my window too that night and took a set of ISO 6400 images and sure enough, there is a lot my star resolution in the cluster but because of the high ISO, there is a lack of dynamic range to show up any colour in the stars.
Posted : 05/04/2017 2:59 am
Wow! Amazing, Tej!!
Posted : 05/04/2017 7:54 am
Hey Tej, you have very neatly demonstrated the main thing about Astrophotography. That is to just do it! I bet it was a bit more comfortable than sitting in a damp cold field!
A good result considering you broke all the rules! Looking at the image I think this could stand a lot more stretching.
So will you be now offering remote hosting?
Re your fears / questions. I don't know what Canon quote for shutter life but most Nikon SLR / DSLR's are rated for 150,000 actions so there's plenty of life left in your camera yet.
Dark frames are used to subtract noise. The more you take the better. You can throw away the individual frames and just keep a master. However what you have done is created a picture with a signal to noise ratio equivalent to a bit less than 50 frames overall. This is because if you took say 500 frames at ISO 1600 and stacked them you have a low noise result. If you then subtract a dark frame that is from 50 frames at ISO 1600, you have added back in the noise those 50 frames have. So although you have a lower noise result with 500 lights plus 50 darks, than 50 lights plus 50 darks, more darks is better. You should also have masters for each ISO. Don't combine them. The noise at ISO 6400 is really bad. I don't know how much worse it is relatively than at 1600 but its a lot that's for sure. Signal to Noise Ratio follows a square root rule so if you take 9 images then the S/N increases by 3 (square root of 9). This being the case, you need to take 9x as many frames (square of 3) to overcome the effect of adding one noisy frame. So 9 light frames cancels out the negative of adding only 1 dark at the same ISO. You can see that 50x9=450 so 500 frames only just deals with the effect of adding only 50 darks. You might find that by dropping all your ISO 6400 (and maybe also 3200) images that the noise level is so much lower it can be stretched more in processing without the same noise penalty. You certainly have enough data to experiment with things like this.
Posted : 05/04/2017 9:49 am
Topic starter
You might find that by dropping all your ISO 6400 (and maybe also 3200) images that the noise level is so much lower it can be stretched more in processing without the same noise penalty. You certainly have enough data to experiment with things like this.
Well, I done exactly as you advised and bloody hell, you're right (well of course you are, I just need to go through the motion in saying it). I stacked only the ISO 1600 frames of which there were 420 with a good DSS score and the relevant master dark (50 darks). And I was able to stretch the image a lot with so much more info in the cluster, so much so that I dared to crop it further to expand the view of the cluster. So this is my end result. Its not even a small difference, its a huge difference.
I would have been chuffed to bits if I had this image from a dark site. Now I cant wait to go to a dark site and image this in its full glory....in ISO 800 and 1600! Goodbye to ISO 3200 and ISO 6400, well unless I have very limited time.
Noted about the dark frames ratio, I'll keep in mind for future capture. Thanks
Oh and look forward to your imaging workshop tonight!
So will you be now offering remote hosting?
Yep, I'll draw up a price plan. Use of the rickety floorboard will be an optional extra!
Wow! Amazing, Tej!!
Thanks Andy but thanks to Rupert tips, here's a better version! But this is all naughty and I need to get out of the house, use my equatorial mount and do captures properly!
Posted : 06/04/2017 3:50 am
Topic starter
Well, that tip from Rupert made my new M13 image reach Flickr Explorer. Thanks Rupert, no more higher ISOs from me unless necessary!
Posted : 12/04/2017 1:22 pm
Well done Tej. I am convinced there is still loads more to come from it. The Sky is not that black so I reckon the cluster could be twice as large. Needs a Star Mask.
With a Canon, put it on ISO800 and leave it there. 1600 is very noisy. Better to stretch gently in processing than let the camera increase gain.
Posted : 12/04/2017 1:37 pm
