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Old 23-06-2019, 10:49 AM
Startrek (Martin)
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Astrophotography- Image Scale or Pixel Scale

Can anyone advise the correct terminology for determining your arc sec per pixel value for astrophotography when checking your telescope / camera angular resolution
Formula
P= ( pixel size in microns x 206.3 )/ focal length in mm
Lots of sites use “Image Scale” and others use “Pixel Scale”
Which one is technically correct ??
A bit confusing for beginner and intermediate astrophotographers me included
Thanks in advance
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Old 23-06-2019, 03:51 PM
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Ryderscope (Rodney)
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I’m not sure that there is a correct term here Martin as I have seen both terms used interchangeably. As long as you are comfortable with what it means that should be ok.
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Old 24-06-2019, 12:07 PM
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The_bluester (Paul)
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They are used more or less interchangeably but I prefer Image scale as a term to pixel scale.
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Old 24-06-2019, 12:26 PM
JA
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Originally Posted by Startrek View Post
Can anyone advise the correct terminology for determining your arc sec per pixel value for astrophotography when checking your telescope / camera angular resolution
Formula
P= ( pixel size in microns x 206.3 )/ focal length in mm
Lots of sites use “Image Scale” and others use “Pixel Scale”

Which one is technically correct ??
A bit confusing for beginner and intermediate astrophotographers me included
Thanks in advance
Hi ST,

I find it more intuitive to do it from first principles:

Image Scale = (Field of view in Image Horizontally* expressed in arcseconds)/(Number of Pixels in Image Horizontally*)

* - I've used horizontally, but vertically would also yield the same answer.

Best
JA
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Old 24-06-2019, 03:41 PM
Startrek (Martin)
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JA
I was referring to Image Scale or Pixel Scale as the angular resolution of your image train ( telescope and camera ) expressed in arc sec per pixel
IS or PS = camera pixel size in microns x 206.3 / telescope focal length
General rule of thumb, < 1.0 arc sec per pixel means your oversampling and > 2 arc sec per pixel means your undersampling

Are you referring to FOV of your image train ( telescope and camera ) expressed in arc minutes
FOV = width of camera sensor in mm x 3460 / telescope focal length in mm

Eg: M8 lagoon nebula 90 arc minutes horizontal by 40 arc minutes vertical

Sorry but I don’t quite get what your referring to , excuse my ignorance

ST
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Old 24-06-2019, 07:21 PM
JA
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Quote:
Originally Posted by Startrek View Post
JA
I was referring to Image Scale or Pixel Scale as the angular resolution of your image train ( telescope and camera ) expressed in arc sec per pixel
IS or PS = camera pixel size in microns x 206.3 / telescope focal length
General rule of thumb, < 1.0 arc sec per pixel means your oversampling and > 2 arc sec per pixel means your undersampling

Are you referring to FOV of your image train ( telescope and camera ) expressed in arc minutes
FOV = width of camera sensor in mm x 3460 / telescope focal length in mm

Eg: M8 lagoon nebula 90 arc minutes horizontal by 40 arc minutes vertical

Sorry but I don’t quite get what your referring to , excuse my ignorance

ST
Hi ST,

I'm referring to the angle subtended by each pixel in the recorded image, (the image scale, some call it the pixel scale), it's just that I find it more intuitive to use a first principles approach, i.e: for the image as a whole in either dimension horizontally or vertically, what angle of view is seen by the imaging sensor at the focal plane, expressed in arcseconds DIVIDED by the number of pixels that represent that.

I'm a DSLR guy and compute the image scale for all my lenses / telescopes /sensor combinations and tabulate these for comparison/reference (see excerpt attached, incomplete in some cells, hopefully not too many typos. there is a Nikon D600 Image scale column and a Nikon D800 Image scale column in RED).

As an example:
Take a Nikon D800 coupled to a Nikkor 400mm f2.8

The Nikon D800 has a Full Frame ~36 (35.9)mm Horizontal x ~24mm Vertical sized sensor with 7360 pixels Horizontally and 4912 pixels vertically in landscape orientation.

When used as intended on Full-frame cameras the Nikkor 400mm f2.8 has a 5.2 degree Horizontal angle of view and a 3.4 degree Vertical angle of view. If you need to you can get /compute these angles from manufacturer's data, although they unhelpfully use diagonal angle, but you can work back to the Horizontal & Vertical angles using the sensor 3:2 ratio in a right angle triangle OR use the Scantips FOV calculator ( I will link later).

OK now use these data to calculate the Image scale for the example:

Horizontal FOV = 5.2 degrees = 18720 arcseconds (5.2x3600)
Horizontal pixels = 7360 pixels,

therefore
Image scale = 18720 / 7360 = 2.54 arcseconds/pixel

Now try the same calculation your way (using The Nikon D800 with 4.88* µm pixels,) coupled to a 400mm Focal length and see what you get..........


Best
JA

* Based on 35.9mm (35900 µm) wide sensor of 7360 pixels = 4.88 µm sized pixels
Attached Thumbnails
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Last edited by JA; 24-06-2019 at 07:58 PM.
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Old 24-06-2019, 07:58 PM
Startrek (Martin)
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Understand what your trying to demonstrate

Gee that’s a mouth full

The equation I use with the Nikon D800 gives 2.52 arc sec per pixel
Very close to yours

As this is well over 2 arc sec per pixel that means your under sampled according to Nyquist, so stars will be blotchy or blocky ?

Is that correct ?
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Old 25-06-2019, 12:30 PM
JA
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Quote:
Originally Posted by Startrek View Post
Understand what your trying to demonstrate

Gee that’s a mouth full

The equation I use with the Nikon D800 gives 2.52 arc sec per pixel
Very close to yours

As this is well over 2 arc sec per pixel that means your under sampled according to Nyquist, so stars will be blotchy or blocky ?

Is that correct ?
In relation to the example I gave (Nikon D800 + Nikkoe 400mm f2.8), it was really just to illustrate how to calculate the image scale, from the field of view and sensor pixel specs. As to the star quality issue: A 400mm lens on full-frame yields a widefield image at 5.2 degrees wide. If you look at anything insanely closely / at high enough magnification you will always see something, but at the image scale we are talking about: around 2.5 arcseconds/pixel, the seeing + guiding errors+ any optical abberations will also conspire to effect the image.

If you venture on to Astrobin and look at various images, for which the pixel scale is often quoted, you will find many astounding images (pixel peeped or not) at that sort of image scale.

Best
JA
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