  As we are heading towards the next solar cycle, presumably with a relatively
small amplitude, it is of significant interest to reconstruct and describe the
past grand minima on the basis of actual observations of the time. The Dalton
Minimum is often considered one of the grand minima captured in the coverage of
telescopic observations. Nevertheless, the reconstructions of the sunspot group
number vary significantly, and the existing butterfly diagrams have a large
data gap during the period. This is partially because most long-term
observations have remained unexplored in historical archives. Therefore, to
improve our understanding on the Dalton Minimum, we have located two series of
Thadd\"aus Derfflinger's observational records (a summary manuscript and
logbooks) as well as his Brander's 5.5-feet azimuthal-quadrant preserved in the
Kremsm\"unster Observatory. We have revised the existing Derfflinger's sunspot
group number with Waldmeier classification and eliminated all the existing
'spotless days' to remove contaminations from solar meridian observations. We
have reconstructed the butterfly diagram on the basis of his observations and
illustrated sunspot distributions in both solar hemispheres. Our article aims
to revise the trend of Derfflinger's sunspot group number and to bridge a data
gap of the existing butterfly diagrams around the Dalton Minimum. Our results
confirm that the Dalton Minimum is significantly different from the Maunder
Minimum, both in terms of cycle amplitudes and sunspot distributions.
Therefore, the Dalton Minimum is more likely a secular minimum in the long-term
solar activity, while further investigations for the observations at that time
are required.
