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Glossary of Terms
« on: 15:29:34, 27 July, 2012 »
Whilst surfing round the world of Astro Imaging you are sure to come across a larger number of terms and acronyms, the meaning of which you are unsure.  So, in order to help those just starting out on that journey (and even those who've been at it for years) we've pulled together a Glossary of Terms  :thumbup:

Please note: this thread is locked and is for reference only !  If you have any other terms that you would like adding to the list please PM any member of the Mod/Admin team with the name of the term and the full wording you would like associated with that term.  Also, if you feel any of the descriptions are inaccurate/wrong then please again notify any member of the mod/admin team and most importantly don't just helpfully tell us "it is wrong"... please provide us with a clear explanation of what should be changed/added/deleted so that we can make the relevant changes directly without entering into debate.

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Re: anchors - terms
« Reply #1 on: 15:35:46, 27 July, 2012 »
Arc Minute - 1/60th of a degree

AO or A/O - Adaptive Optics or Active Optics - Guiding on steroids. In most A/O units a lense is moved in real time to correct for movements in the guide star utilising refraction. This differs from traditional guiding where corrections are sent to the mount over periods of ~0.5 to 5 seconds. At long focal lengths adaptive optics can be better than normal guiding.

Aperture - The bit that lets the Photons in hence large diameter scopes being referred to as "Light Buckets". Commonly quoted in inches though sadly mm is becoming increasings common.

Arc Second - 1/60th of an Arc Minute i.e. 1/3600th of a degree

Binning - Available in most image acquisition software/CCD combinations this effectively considers groups of pixels as one. this effectively increases the pixel size (to avoid oversampling) and at the same time increasing sensitivity and reduces imaging time. Logic would suggest that binning 2x2 so 4 pixels are summed would double the sensitivity but in reality, CCD electronics limit this performance gain quite considerably.

Pixels are grouped across & down, for example; x2 binning is 2 across & 2 down = 4 pixels; x4 binning is 4 across by 4 down = 16 pixels.

Degree - 1/360th of a full rotation.

Differential Flexure - When two scope are used (imaging and guiding) any movement or flexure between the two will lead to errors between the guide system and the imaging system. As we are imaging at scales of arc seconds, differential flexure need only to be very small to have a major impact and can be difficult to track down and remedy.

Dither - Small pseudo-random shifts in direction introduced between exposure of images. The main benefits of dithering include: aiding the removal of hot or bad pixels as, after registration, they appear to move with the dither whereas the image appears static. In addition, if the imaging software employs it, drizzling can be used to give a better resolution of an undersampled image.

Drizzle - Is the digital image processing method for the linear reconstruction of undersampled images and was developed for use with the Hubble images. The algorithm is formally knows as "Variable-Pixel Linear Reconstruction". In order to drizzle add a set of images, they must have been "dithered" by a number of known pixels to shift the image around the sensor. Registration/Stacking software can then employ the drizzle stacking algorithm if built in. The resulting image appears sharper and less pixelated than if not using this mechanism.

DSO - Deep Sky Object - An object that is outside of our Solar System.

DSS - Deep Sky Stacker - a freeware image stacking programme that is well featured and relatively easy to use.

Field Rotation - If a mount is not well polar aligned, even though it may track an object very well, there will be some residual rotation over time due to the error in alignment. This sometimes manifests itself in images as elongated stars in the corners of the image.

FL - Focal Length - the distance over which light is brought to focus by an imaging system.

FOV - Field Of View. How much of the sky a scope and camera combination can capture. Normally represented in arc minutes or even degrees.

f/ratio - The ratio of focal length to objective diameter. Normally represents as say f/7 this gives and indication of how "fast" a scope is. Smaller numbers will require less imaging time for the same end result. It is calculated by the Focal Length of scope divided by the scopes Aperture.

FWHM - Full Width at Half Maximum. A numerical value to help determine the best focus of a star. The smaller the number the better. Astro programs like Maxim DL have a readout mode which shows the value - to focus, use short exposures and watch the readout value of the FWHM. When the value reaches it's lowest, that is the best focus. It is best to see an average of the value as it will fluctuate due to the atmosphere or temperature.

Guiding - The Black Art of astroimaging. Guiding uses a second imaging device to "lock on" to a stars centroid and send correcting signals to the mount as this star moves. With good Polar Alignment, a stable mount and good guiding, long exposures are possible with very little translation of the objects being imaged. Guiding becomes more and more difficult as the scope focal length increases and FOV reduces.

Side by Side guiding - Where the two scopes (imaging and guiding) are mounted side by side. Has the advantage of reducing the lever arm of part of the weight of the payload thereby reducing the counterweight needed to achieve balance but the combination needs balancing around all axes which can be quite difficult to achieve.

Half Flux Diameter - An alternative focus measuring number used by programs such as FocusMax, which is considered an improvement to the FWHM (Full Width at Half Maximum).  The half-flux diameter is the diameter in pixels that contains half the energy in a star image.

Image Depth - Generally how well faint elements are clearly resolved. A good measure of an images depth is to consider the faintest star clearly visible.

Image Processing - We are trying to capture details of objects that are generally not visible to the naked eye. Images straight from a CCD or DSLR tend to be very dark with all the information within a very narrow part of the histogram. They may also not be well colour balanced and be lacking in sharpness and contrast. Image Processing is akin to developing a negative in that all the detail present is coaxed back. Common image processing software include Photoshop, GIMP and PixInsight. Image Processing is a huge topic in its own right but is an important aspect of AstroImaging.

Image Scale - How many arc seconds each pixel in the image represents. Calculated via the formula

        (pixel size in um / Focal length of scope in mm)  x 206.3

Mono (CCD) - Captures an image in monochrome or greyscale. Requires the use of coloured filters (RGB) to create a colour image.

Mount - The mechanical element of the imaging system that permits the telescope to be pointed at a target object. Mounts come in two flavours -

*Alt/Az - where the movement axes are vertical and horizontal

*German Equitorial - where the movements are in Declination and Right Ascension.

NCP - North Celestial Pole. The north celestial pole currently is within a degree of the bright star Polaris. It is the imaginary point to which the Earth's North axis points to and is therefore the point around which all objects in the Northern Hemisphere rotate.

NB - Narrow Band. The use of filters to restrict wavebands of light reaching the imaging device. Usually design to capture the emission line of common elements Ha (Hydrogen), OIII (Oxygen) and SII (Sulphur).  When processing an imaging using Narrow Band data the colours are typically "mapped" to the RGB channels where Ha = Red, OIII = Blue and SII = Green. For the Hubble Palette, R=Sulphur (wavelength is more "red"), Blue=OIII & Green=ha, but results in a very "green" tint as Ha is the dominant. Aesthetically, the "Green" is then translated to a "gold" colur & the rest towards a blue/cyan/teal colour.

OAG - Off Axis Guider - Guiding commonly uses a separate scope and camera to act as the guider. With an OAG a small mirror or prism in the light path of the main scope is "picked off" and sent to the guide camera. The main advantages are that differential flexure is eliminate, there is only one scope to carry on the mount and there is only a single scope to focus. The disadvantages are that the pick off unit is relatively small and so it only "sees" a small area of the sky. Finding a guide star in an OAG in some parts of the sky and with long focal length scopes can be a challenge.

OSC (CCD) - One Shot Colour. Includes a Bayer Matrix (normally RGB) in front of the CCD to enable a colour image to be taken directly.

OTA - Optical Tube Assembly.

Pinched Optics - Mechanical stress affects reflective and refractive imaging element properties. If one of these elements comes under local stress from the support system, this can translate into artefacts in the image.

RGB - Red, Green, Blue - The three primary colours which when combines create a "true" colour image as opposed to NB which create a "false" colour image.

Sampling - General wisdom and rules of thumb suggests that the image scale (above) should optimally be 1/2 of the seeing though some people suggest this should be 1/3. In the UK, at sea level, seeing varies from about 2" per pixel (very good) to 6" per pixel (poor) but more typically ~3" per pixel. This suggests an optimal image scale of ~1"-1.5" per pixel.

*Undersampled - Based on the above, if the image scale is greater than 1/2 the seeing it is considered under sampled i.e. the imaging system may be holding back the resolution of fine detail in the image. Another way of thinking about this is, there are too few pixels for the star - small stars will look "blocky" or pixelated. Increase in sensitivity at the expense of resolution.

*Oversampled - The converse of the above. In oversampled images the imaging system has a higher resolution than atmospheric conditions permit. A degree of oversampling is, not necessarily, a bad thing as it allows you to take advantage of any period of better than average seeing. So, another way of thinking is, too many pixels for a star - better resolution at the expense of sensitivity.

SNP - Starry Night Pro.

S/N Ratio - Signal to Noise Ratio - All imaging systems have a variety of noise sources. S/N is a measure of the ratio of the wanted to unwanted signal elements.

Stacking - The combination (via a variety of mathmatical methods) of a number of sub-exposures to effectively make one long exposure that approximates to the result of a single long exposure of a similar combined length.

Sub or Sub-Exposure - An individual exposure. The duration of the sub-frame will be dictated by the stability of the imaging system to maintain accurate tracking and the background brightness of the sky (Sky Limited Exposure).

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