Light pollution and exposure settings
Thanks for your website and this forum! <br/>
I bought your book, 50 best astrophotography targets for beginners. Its a nice read and gives useful information. I like the targets and might work my way through them. I bought this book to get an idea of the exposures for astrophotography. I started in astronomy and astrophotography in November 2017.<br/>
My problem is living in a high pollution area, Bortle 8 (clear outside app). Sky brightness 18.4. I have light intrusion from neighbours bedroom lights, 2 street lights LED white light nearby which shines partly into the back garden, in outer London, UK. <br/>
If I use your exposure settings, I would get an over exposed image. If I use less then I would probably not get enough useful data.<br/>
So with my setup 70mm ED Starwave refractor on a Eq3 pro synscan goto mount (basic equatorial mount from Skywatcher), no guiding, using Skytech CLS filter for Canon EOS-C (crop camera Canon 100D or 7D), I am getting reasonable images of M31 and Horsehead nebula near Alnitak star in Orion and others. The problem is I have overexposed some if the stars by choosing ISO 1600 or 3200 for 120seconds to 180 seconds.<br/>
Should I reduce the ISO or exposure time. At the moment, I have not connected the camera to a computer or laptop during imaging, so I can't assess the exposure or image well. The light pollution filter gives a strange colour cast and the LP makes it hard to assess the image target and framing. I could analyse it on a computer if I connect the camera to computer + software eg APT or Nebulosity etc. but am unsure if this will be too time consuming and waste time. I know I could waste more time by getting the exposure and ISO settings wrong !!<br/>
For example - on page 24 you quote ISO 800 and 300 secs for imaging Bodes galaxy M81 and M 82.<br/>
In fact I am getting reasonable images at ISO 1600 if I don't overexpose stars.Will I need hundreds of images stacked together to get good Andromeda, Horsehead, Bodes galaxy? or goto a dark sky site?<br/>
Here is a stacked image of 18 jpg files (120 sec exposure at ISO 1600 on Canon 100D with Skytech CLS filter); stacking in Sequator gave reasonable results; however, DSS gave bad results, and using .tiff files did not work - don't know why, I may need to run this through another time, maybe something wrong with the CR2 to tiff conversions or some other settings. Anyhow, here it is (reduced file size for the web).
Any suggestions would be appreciated!<br/>
The first thing I would recommend is to try and use a lower ISO and longer exposures if your mount can track that well. The reason is that the higher the ISO the lower the signal to noise ratio and lower the dynamic range.
To explain a little simpler.....
When you take an image, stack it and then stretch it, it is far easier to get a target to look good at lower ISO values than at higher ISO values. While every camera is different, all of them produce better images at lower ISO values (to a point). The trick is to have a mount that can get you long exposures with excellent tracking, and then use the longest exposures you can (600-800 seconds have worked excellent for me) and minimize the ISO setting to 800 or lower.
Of course everyone's options are different depending on their equipment and locations, but you get the idea.
Thanks Allan. I appreciate this. I will try ISO 800 and try again the M81, M31 and Horsehead regions.
No problem, be sure to post the results and let's see how it works for you.
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