Context : Mass estimates of interstellar clouds from far-infrared and submillimetre mappings depend on the assumed dust absorption cross-section for radiation at those wavelengths .

Aims : The aim is to determine the far-IR dust absorption cross-section in a starless , dense core located in Corona Australis .
The value is needed the determining of the core mass and other physical properties .
It can also have a bearing on the evolutionary stage of the core .

Methods : We correlated near-infrared stellar H - K _ { s } colour excesses of background stars from NTT/SOFI with the far-IR optical depth map , \tau _ { FIR } , derived from Herschel 160 , 250 , 350 , and 500 \mu m data .
The Herschel maps were also used to construct a model for the cloud to examine the effect of temperature gradients on the estimated optical depths and dust absorption cross-sections .

Results : A linear correlation is seen between the colour H - K _ { s } and \tau _ { FIR } up to high extinctions ( A _ { V } \sim 25 ) .
The correlation translates to the average extinction ratio A _ { 250 \mu { m } } / A _ { J } = 0.0014 \pm 0.0002 , assuming a standard near-infrared extinction law and a dust emissivity index \beta = 2 .
Using an empirical N _ { H } / A _ { J } ratio we obtain an average absorption cross-section per H nucleus of \sigma ^ { H } _ { 250 \mu m } = ( 1.8 \pm 0.3 ) \times 10 ^ { -25 } cm ^ { 2 } H-atom ^ { -1 } , corresponding to a cross-section per unit mass of gas \kappa _ { 250 \mu { m } } ^ { g } = 0.08 \pm 0.01 cm ^ { 2 } g ^ { -1 } .
The cloud model however suggests that owing to the bias caused by temperature changes along the line-of-sight these values underestimate the true cross-sections by up to 40 % near the centre of the core .
Assuming that the model describes the effect of the temperature variation on \tau _ { FIR } correctly , we find that the relationship between H - K _ { s } and \tau _ { FIR } agrees with the recently determined relationship between \sigma ^ { H } and N _ { H } in Orion A .

Conclusions : The derived far-IR cross-section agrees with previous determinations in molecular clouds with moderate column densities , and is not particularly large compared with some other cold cores .
We suggest that this is connected to the core not beng very dense ( the central density is likely to be \sim 10 ^ { 5 } cm ^ { -3 } ) and judging from previous molecular line data , it appears to be at an early stage of chemical evolution .