Last updated 14 Sep 2017

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Stacking Techniques

(for difficult faint and fast moving objects)

Track & Stack is a technique mentioned on the Methods page that effectively strengthens the signal from an object by stacking many images together. If the target object is moving, then adjustment is automatically made to compensate for its motion, keeping the object in the same place and causing the stars to be trailed.

 

The software program Astrometrica includes powerful functionality to automatically track & stack multiple images of the same field of view very accurately and is an essential tool at Great Shefford, part of every observing session.

 

As an example of track & stack in action, images taken of the Amor minor planet 2004 SS early on 2004 September 19th demonstrate the basics. At the time the object was magnitude +18.1 and moving at 6.7"/minute in p.a. 21 (up and to the left). Each image was taken binned 2x2, has been enlarged by a factor of x2 and is 5' x5' in size with North up.

a. Individual Image
Single 28 second exposure with 2004 SS in the centre of frame. Note that it is of similar brightness to several other nearby stars.
b. Stacking
21 x 28 second exposures stacked together but without taking into account the motion of 2004 SS.

Note the minor planet now appears as a faint streak towards the bright star at top of frame, how the stars in a. of similar brightness are now much stronger and how stars too faint to show on a. are now easily visible

c. Track & Stack
Same 21 exposures used in b. but tracked on the motion of 2004 SS, keeping the minor planet in the same place at the centre of the frame and causing the stars to trail. 

Note that the faintest stars visible in b. are now smeared out so much that they are no longer visible, while 2004 SS is strongly recorded.

 

The 'Track' part of Track & Stack relies on knowing the motion of the object (i.e. the speed and direction it is moving in) reasonably precisely, even when the actual position of the object may not be that well known. This is almost always the case, even with newly discovered objects with only preliminary orbits calculated from just a few precise astrometric positions, the general speed and direction is often well determined even when the predicted RA & Dec coordinates may have large uncertainties..

 

Track & Stack allows many individual images of a moving object to be aligned so that each image is offset from the last by the precise motion of the moving object, this makes the background stars appear to trail and keeps the object as a point source, building up its strength (see image c. above). The more images that are stacked together the fainter the target object that can be recorded. For faint minor planets, often the object is not visible at all on individual frames (unlike 2004 SS in image a above) and is only revealed when many images are stacked together.

 

To obtain astrometric positions of a minor planet, normal procedure would be to take a number of images and simply stack them into three sets, so that three precise positions can be derived from the resulting digitally strengthened images.

 

However, there are occasions when variations on the normal method may help to locate faint fast moving objects more readily.

 

General techniques (for any faint object with large positional uncertainty)

NEOCP Uncertainty Maps & Variant Orbits 

Searching using NEOCP Variant Orbits (using the NEOCP to full effect for unconfirmed fast movers) 

Interlaced stacking (finding faint fast moving objects in rich star fields more easily)

Overlapping stacking (finding relatively bright moving objects in very crowded star fields) 

VFMO Extreme stacking (maximising exposure for faint very fast moving objects)

 

General Techniques

Take plenty of images.
  • To recognise an unconfirmed object with large positional uncertainty it needs to be made as obvious as possible, so the more total exposure time the better.
  • The magnitude listed on early search ephemerides may be incorrect  by one or more magnitudes, so if there is time, take more exposures than might be thought necessary for the quoted brightness to make sure the object is recorded.
Stack all available images into one big stack as well as three separate stacks.
  • When blinking the four sets together, the enhanced strength of image in the one big stack will often lead the eye to notice the object, which may subsequently be able to be made out faintly on the three 'normal' stacks that are used to identify the object by its motion.
  • A 'real' object should be in the same place on the stack of all images as it is in the smaller stack of images that starts with image 1. So, if 30 images are taken and stacked into four sets, set (a) being images 1-10, (b) images 11-20, (c) images 21-30 and (d) all images 1-30, then when blinking all four sets, look for any possible objects that are in the same place in stack (a) 1-10  and in stack (d) 1-30. If any are found then make sure they are also in stacks (b) and (c) in the appropriate positions. Finding an object in the 'all image stack (d) as well as stack (a) is NOT proof the object exists, it just gives a short cut to where to look for movement on the stacks (a), (b) and (c).

Be sure to match the exposure length to the expected speed of the object and your CCD's pixel size.

  • Even if you can't initially locate a faint object with a large positional uncertainty someone else may find it, allowing you to later re-examine your images and obtain useful astrometry. 

  • Too long an exposure for a fast moving object may lead to image trailing and cause poor astrometry, too short an exposure may mean the object is not recorded as strongly as possible, leading to either the object not being identified at all, or the signal/noise ratio being so low that astrometry measured may be quite inaccurate. (A fast and bright unconfirmed object may be revealed by exposing relatively long exposures so that the object is identified by trailing during the exposure. However, for fainter objects this is not an option and identification will need to be done via stacking so exposures need to be kept short enough to stop significant trailing).

  • When using different NEOCP variant orbits be sure to adjust exposure length according to the orbit being used - they can vary substantially. See the NEOCP  Uncertainty Maps & Variant Orbits page.

 

Overlapping Stacking

(coming soon)

VFMO Extreme Stacking

(Coming soon)


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