The dust extinction in spiral disks can be estimated from the counts of background field galaxies , provided the deleterious effects of confusion introduced by structure in the image of the foreground spiral disk can be calibrated . ( 31 ) developed a method for this calibration , the “ Synthetic Field Method ” ( SFM ) , and applied this concept to a HST/WFPC2 image of NGC4536 .
The SFM estimates the total extinction through the disk without the necessity of assumptions about the distribution of absorbers or the disk light .
The poor statistics , however , result in a large error in individual measurements .
We report on improvements to and automation of the Synthetic Field Method which render it suitable for application to large archival datasets .
To illustrate the strengths and weaknesses of this new method , the results on NGC 1365 , a SBb , and NGC 4536 , a SABbc , are presented .
The extinction estimate for NGC1365 is A _ { I } = 0.6 _ { -0.7 } ^ { +0.6 } at 0.45 R _ { 25 } and for NGC4536 it is A _ { I } = 1.6 _ { -1.3 } ^ { +1.0 } at 0.75 R _ { 25 } .
The results for NGC4536 are compared with those of ( 31 ) .
The automation is found to limit the maximum depth to which field galaxies can be found .
Taking this into account , our results agree with those of ( 31 ) .
We conclude that this method can only give an inaccurate measure of extinction for a field covering a small solid angle .
An improved measurement of disk extinction can be done by averaging the results over a series of HST fields , thereby improving the statistics .
This can be achieved with the automated method , trading some completeness limit for speed .
The results from this set of fields are reported in a companion paper ( 40 ) .