# Helpful tips¶

This page summarises things that you **definitely should consider** when using
the ctools software. Make sure that you read at least this section.

- Always inspect your fit residuals
- Make sure your binning is sufficiently fine grained
- Only use energy dispersion when you really need it
- Only compute the Test Statistics when you really need it
- Fit extended source with a radial disk model
- Fix the spectral parameters of a source to compute an upper limit

Always inspect your fit residuals

**Never** trust the values of a ctlike model fit without having
inspected the fit residuals. Residuals in the region of your source of
interest should be flat.

Make sure your binning is sufficiently fine grained

If you use binned analysis, make sure that the bin size is sufficiently fine grained, since the model is sampled at the bin centres. A spatial binning of 0.02 degrees should be okay. Use at least 10 bins per decade for the energy binning, you may consider using more bins below 100 GeV when you analyse data near the energy threshold.

Only use energy dispersion when you really need it

You should only use the energy dispersion when you are sure that it impacts significantly the analysis results, or when you want to produce final results for a publication. Taking into account energy dispersion adds a further dimension to the analysis which considerably slows down the computations. In many cases the impact of the energy dispersion is small and can in first order be neglected.

Only compute the Test Statistics when you really need it

The Test Statistic value is useful for computing the detection significance of a source, but for many sources in your region of interest you may actually not be interested in that value. Computation of the Test Statistic value takes some extra time, so do not request the computation if you don’t really need it.

Fit extended sources with a radial disk model

The fit of extended sources is best done with a radial disk model since the
computations for this model are very fast (model type: `RadialDisk`

). Only
if the radial disk model is a bad representation of the spatial distribution
or to compute final values for a publication you should switch to other
extended models, such as the `RadialGaussian`

or `RadialShell`

models.
Note that the computations for elliptical models are even slower,
hence use these models only when you really need them, and prefer the
elliptical disk over the elliptical Gaussian model.

Fix the spectral parameters of a source to compute an upper limit

When computing an upper limit using ctulimit, make sure that the spectral parameters such as index, cutoff energy, etc. are all fixed. Only the prefactor or integrated flux should be left free. Computation of an upper limit requires to make a hypothesis on the underlying spectral energy density distribution, hence you have to fix the parameters of this hypothesis. Otherwise your upper limits are probably rather meaningless.