Galaxy Zoo Competition

Matthew Emery

What Is Galaxy Zoo?

  • All sky galaxy surveys produce more images than can be handled just by experts
  • It’s 2011, better crowd-source it

Galaxy Zoo Snapshot

Galaxy Classification

  • Over 900000 galaxies classified in a few months
  • 37 categories arrived at by asking 11 questions
  • After 40-50 users see a galaxy, the answers are aggregated and weighted vote fractions in the decision tree

Galaxy Zoo Decision Tree

Competition Details


  • Ran from December 20th, 2013 to April 4th, 2014
  • 61,578 training images with vote fractions
  • 79,975 test images
  • 424 x 424 pixels
  • Prize of $16,000

Example Galaxy

Loss Function

  • This is a regression problem!
  • The answers to the first question should sum to 1
  • Answers to the next questions to sum to their parent probability

Example Galaxy

  • Note: We are measuring error against the what the crowd answered. A “good” model will have the same biases as the crowd

The Winning Solution

1st Place: Sander Dieleman

  • Also won first place in the 2014 National Data Science Bowl with a team
  • His approach was novel enough to get a paper in MNRAS and a job at DeepMind
  • Co-author of Lasagne
  • Second author on WaveNet, also on AlphaGo


  • Cropped 424x424 to 207x207
  • Downscaled to 3x to 69x69
  • Centering was done by Petrosian radius
  • Normalized in some images but not others
  • Keeping color significantly improved the model, despite it being artificial
  • Avoided cropping out the object interest by centering with SExtractor (Source Extractor)

Data Augmentation

Data Augmentation

  • Rotating uniformly from 0 to 360
  • Translating uniformly -4 pixels to 4 pixels
  • Scaling log-uniformly from 1.3^-1 to 1.3
  • Flip as a Bernoulli event with probability 0.5
  • Color perturbation using an equation in the ImageNet paper
  • This is all being done on the CPU while the GPU trains the network

Rotational Invariance

Data Augmentation Step 1: Rotate image 45 degrees and flip both images (4 images)

Step 2: Crop each 67x67 image into 4 overlapping 45x45 images (4x4=16 images)

Network Architectures

Best Model

Best Architecture

  • All 16 viewpoints are fed in at the same time to maximize parameter sharing
  • Best single model has 4 square convolutional layers (6-5-3-3)
  • More than 100 models were tested, 17 were included in the final ensemble
  • Maxout are like group-wise ReLUs
  • The 37 were scaled to their constraints


  • a network with two dense layers instead of three (just one maxout layer);
  • a network with one of the dense layers reduced in size and applied individually to each part (resulting in 16-way parameter sharing for this layer as well);
  • a network with a different filter size configuration: 8/4/3/3 instead of 6/5/3/3 (from bottom to top);
  • a network with centered and rescaled input images;
  • a network with a ReLU dense layer instead of maxout;
  • a network with 192 filters instead of 128 for the topmost convolutional layer;
  • a network with 256 filters instead of 128 for the topmost convolutional layer;
  • a network with norm constraint regularisation applied to the two maxout layers;
  • combinations of the above variations.


  • 67(?) hours of training for the best model on a GTX 680
  • Nesterov Momentum was used (16 “minibatches,” effectively 256 because of architecture)
  • Learning rate of 0.04 (updated to 0.004 after 18M samples, then 0.0004 after 23M)
  • Dropout was used during training to prevent overfitting


Model Averaging

  • Modeled across 17 architectures (they are all available on GitHub)
  • For each model, averaged predictions across 60 different transforms
  • 10 rotations x 3 rescalings x 2 reflections
  • It takes 4 hours to get a prediction from a single model

How Did He Do?

Example Galaxy

  • His single best network outperformed everything else

Things that Didn’t Work

  • Adding Gaussian Noise to the image
  • Changing gamma
  • Downsampling less
  • Adding shearing to preprocessing
  • RMSprop or adadelta


  1. I. Goodfellow, Y. Bengio, and A. Courville, Deep learning. 2016.
  2. S. Dieleman, K. W. Willett, and J. Dambre, “Rotation-invariant convolutional neural networks for galaxy morphology prediction,” Monthly Notices of the Royal Astronomical Society, vol. 450, no. 2, pp. 1441–1459, Apr. 2015.
  3. [1]“benanne/kaggle-galaxies,” GitHub. [Online]. Available: [Accessed: 16-Dec-2016].