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  1. Hi Guys, Long story short: Is it worth doing 4 min exposures with ISO 100 using Canon 1300D (not yet moded..) on DSO. Or it's better to stick with default ISO 800 and 1min? @ISO800 1min is the top exposure I can use from my back garden in London even with a light pollution filter on.... Long Story: I "recently" managed to get all beginner's equipment and started to experiment with long exposures (I had DOB previously). "Recently" - almost 2 months ago, and during that period I managed to play with it 3 times only... London is like the Adams Family house, - constantly under the clouds... I have Canon 1300D and SW 130PDS guided via ZWO224 (not completely guided yet... as still learning ). Initially, I thought I will figure out the best ISO/exposure settings while experimenting... But 1 experiment per month... VERrrrY painful... and it would be the top rate masochism to enjoy experiments with such a frequency... So I have been googling, reading forums and even going through the astrobin/flickr pictures and checking ISO/Exposure parameters, checking all kind of data sheets with sensor data, but managed to find only for Canon 1200D like for example http://sensorgen.info/CanonEOS-1200D.html or http://www.photonstophotos.net/Charts/DXOPDR.htm and nothing for 1300D. If you have any link, you are welcome to share. From all data I have read for my range CANON ... usually, - ISO800 is recommended or even ISO1600!!! - Wow!!! Seriously??? I used ISO1600 with my DOB for 10sec exposures As far as I learned, - All DSLR cameras have the best Dynamic Range with the lowest ISO possible, with 1300D it's ISO100, so I would expect to get the best DSO colours and shadows using ISO 100, However, I was amazed founding out that Canons have the Highest Read Noise at low ISO settings... (I was not able to find any data for 1300D, but I guess data from 1200D sensor is more or less the same). It looks like a sweet spot would be ISO 400 which would be 1.5 - 2min exposures maximum... Please correct me if I am wrong. P.S. Last time I tried ISO800 and 1min exposures, - after stacking with all darks, flats and bias the result was very noisy. I even afraid thinking about the amount of noise with ISO100, but I will try it next night anyway... Curiosity ...
  2. I just upgraded to a 6D and checking in theory, what is the ideal ISO and frametime for the new setup. Looking at the read-noise characteristics, ISO should not be kept low - however, increasing the ISO kills the full well capacity, at ISO3200 I got only 2400e-(!), which quickly leads to DR problems. I feel ISO1600 could be a meaningful compromise between FW and read noise. Regarding noise and frametimes: I know, long subs are in general preferred. Looking at the overall S/N formula, in theory I'd loose only about 20% on the S/N if I shoot 10 subs instead of one long exposure thanks to the ~3e- read noise. Is my maths right? If the read noise is significantly below 5e-, can we really go in photon counting mode/save a lot on mounts? I made a quick excel plot on the story: practically ISO3200 delivers the same results using 10 frames as 5 frames shot at iso 800. I added 0.2e-/sec/pixel dark noise (~5°C cooled eos 6D value) and a signal, that generates 150electrons in a 2 minutes sub, I didn't add photon noise.
  3. Below is a comparison between single dark frames taken with the Nikon D7500 and D5300 with exposure durations varying from 1 sec to 240 sec ( my usual main light frame exposure ) all at ISO400. Firstly a graph of the standard deviation of the noise in the dark frames versus exposure time: The standard deviation of the noise is a fairly constant 2 ADU less for the D7500 compared to the D5300 ( pretty much the difference in the read noise between the two ) However, the difference is not just in absolute terms but also in the quality of the noise ... Below are the dark frames - ranging from 240 sec exposures at the top to 1 sec at the bottom: D7500 D5300 The D5300 dark frames clearly show the pattern in the read noise ( banding down the bottom ) and also have far more chrominance noise compared to the D7500. At 240 seconds ( the main exposure I have been using ) the difference is starkly different; the D7500 produces images with much lower noise that is significantly more even and random and hence more likely to be reduced during integration.
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