what makes an image HI RES

[Daniel-K]

i constantly see people putting click here for HI RES. 

i understand that HI RES is of better quality but how do you get it. ive produced some ok images but when i zoom in the image starts to degrade. how do get a image to become hi res.

[Uranium235]

Its a bit difficult to explain, but in my mind I see it as an image shot at a fairly good sampling rate (~1" p/p or just below) - which would be derived from the pixel size and focal length of the telescope used. However, the image also needs to be clean (ie: lots and lots of data) so when it is viewed at pixel scale (100%) there is no noise present.

However, most CCDs lack the pixel count to cover large objects at a long focal length in just one frame - so you would end up doing a mosaic to increase the image scale (overall size).

Having said that, resolution isnt the be-all and end-all of a good image. Having lots of clean data is more important than resolution IMO.

[Ruud]

If you make a 2x2 mosaic and reduce it by 50% you'll have doubled the resolution (per dimension).

[michael8554]

Hi Daniel

Not sure where you've seen this HI RES thing, but people often post a LO RES image as a teaser in the body of a topic, and when you click on the image you then get to see a HI RES image.

The HI RES image could be at the resolution your camera produces eg 10.1 Megapixels, or 3888 x 2592 pixels.

You then Resize this in Photoshop etc to eg 1800 x 1200 pixels, that's your LO RES image.

That's my understanding.

Michael

[ollypenrice]

In photography 'to resolve' means to distinuish fine detail. (For example to separate a double star into two points of light rather than one.)

At the capture stage the term refers to the image scale in arcseconds per pixel. Broadly speaking, a fine pixel scale (eg 0.5 arcsecs per pixel) can be had by using a long focal length and/or using smaller pixels. A coarse pixel scale (eg 3.5 "P/P) can be had by using a shorter focal length and/or larger pixels. Of course this isn't hard and fast because you might obtain 0.5"P/P with large pixels and a very long focal length. But the two players (and there are only two) are pixel size and focal length. (The number of megapixels does not, in itself, determine the resolution. This mis-use of the term is an unwelcome import from camera advertisements!)

At the presentation stage a high display resolution involves allocating one camera pixel to one screen pixel. Such a display is called 'full size' or '100%.' When an image is downsized tp fit on a screen, or on a page, or on just part of a page, then this is a low display resolution. So for display resolution we deal in absolute terms: one camera pixel for one screen pixel defines 'full resolution.'

The definition of resolution at the capture stage is subjective, with no hard and fast numerical values, but the range I gave (0.5 to 3.5 arcsecs P/P) would probably cover most amateur telescopic deep sky astrophotographers. Widefield camera lens imagers might go well below this.  In reality there is a relationship between the size of the target and the pixel scale. When Tom O'Donoghue and I image, say, M33 at 3.5"P/P we'd say that our image was of low resolution. When we make a 33 panel mosaic of most of the Orion constellation using the same gear we would call that very high resolution.

Planetary and lunar imagers can greatly exceed the resolution possible in deep sky imaging because they can catch fleeting moments of excellent 'seeing' (freedom from atmosphric turbulence) since they use sub second exposures on bright targets.

'Resolution' is one of my pet hates as a term because it is often used in a completely incoherent way!

Olly

[MattJenko]

People put hi res because the image they share on here will be a jpg which has been compressed and the image may also have been downscaled by the author to make it fit nicely in a monitor screen. In this sense, the hi res simply means the native resolution the image was taken at, not the downsampled interweb version.