Pixel scale

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Pixel scale

Postby ASTROM » Sat Dec 24, 2016 4:04 am

I am in the process of selecting a refractor for the a ZWO camera. I am confused about pixel scale. For example, articles prefer that you have a pixel scale of 2 arc-sec per pixel. The majority of the messier objects are very small, like M57 size is 1.4' by 1'. When I search the pictures and calculate the pixel scale is more like 0.6 arc-sec per pixel. Do I plan to have a focal length to get a better resolution of 0.6 arc-sec per pixel or go for the 2 arc-sec per pixel? I know you can add reducers and barlows but they don't cover the range from 0.6 arc-sec per pixel to 2 arc-sec per pixel. Thanks in advance for taking time to respond.
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Re: Pixel scale

Postby Merlin66 » Sat Dec 24, 2016 9:04 pm

Have a look at CCDCalc.

This will guide you on the two variables you need to consider.
The pixel plate scale and the overall field of view.
The choice of plate scale is heavily influenced by your seeing conditions. If you had excellent seeing at say 1 arc sec, then a plate scale of around 0.4 arc sec/ pixel will gather as much detail as available.

The FOV should be large enough to cover the more interesting objects you want to image.
It's a compromise........
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Re: Pixel scale

Postby ASTROM » Mon Dec 26, 2016 4:25 am

I did download the CCDCal program and find it useful. I am also reading an article from Craig Stark on signal to noise: Understanding it, measuring it, and improving it Part 4. Stan Moore page on pixel sampling is a good one to read. Thanks for your comments.
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Re: Pixel scale

Postby riklaunim » Tue Dec 27, 2016 5:28 pm

The bigger the resolution the way harder is to photograph DS objects. Very small DS objects require high resolution which is rarely used due to that difficulty level - and that's why DS imaging articles say about resolutions like 2 arcsec/pixel.

With current CMOS based cameras it's easier to photograph DS objects on high resolutions by lucky imaging - instead of few long exposed frames you take a lot of very short frames and stack that (rejecting bad ones too). This avoids seeing image distortion as well as tracking error. Bigger than usual aperture is also very much welcomed for this too.
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