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Exposure Length


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Hi All,

My question is in relation to the exposure times of subs. A few weeks ago, I took M27 @ ISO800 and went for 5min sub frames (if I remember correctly). The RAW images were completely overexposed (attached as jpeg just for the small file size), but does this matter so much as after processing using the Levels option (Gimp) I can stretch the histogram to bring out the image (2nd attachment).

Would it be better for shorter subs or using a lower ISO setting to keep the darker sky or would it be OK to carry on with longer exposures and just stretch these in the post processing? I am in a light polluted area and yes it would be better for me to travel somewhere darker but just for my learning experience I'm just in the back garden so far. Also just to note these are just lights I've taken, again I will get around to the darks & Bias frames in the near future (now the winter nights are coming) but I wondered as the data is over exposed am I loosing data by using the levels?

Many Thanks.


m_27 1st.jpg

m_27 2nd.jpg

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Hi Mike,

Not looking bad at all. What camera and mount are you using? You might find that iso1600 (depends on your camera) and shorter exposure times may help. I normally do 90sec exposures and get as many as possible as the more data the better the end result and normally aim for at least 30-40 per session depending on time and conditions. From what I gather there is a point in exposure time where you are just wasting your time.

Have you tried starttools or pi to process you images?



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Hi spill,

Thank you for that. You have answered my question and I will try shorter subs & plenty more of them. It's all trial & error I guess to get the best out of the conditions & equipment. Answering your question, I use a Canon 400D dslr (unmodded - so far), Skywatcher ED80 & guider, Celestron Advanced GT mount.

At the moment I'm stacking & processing in DSS & Gimp. I just want to learn AP at the bottom and then maybe in the future I will consider the more expensive software like PS, PI etc. It just seems a lot of money to purchase when I don't really know what I'm doing with Post processing. I will persevere & get there one day!

P.S. Before anyone mentions it, yes I have bought the book "Every photon counts" :icon_biggrin:

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