Files in subdirectories are splitted by LINUX command "split" to a bunch of 1G files. File extesions like '.aa', '.ab','.ac', et al. are generated by the "split" command, which means it's one part of a big file. Each file's md5 checsum code could be found in correspoding ".md5" files. Sample commands to download files with "wget" wget -r -np -nH -R index.html http://www.lamost.org/dr8/v2.0/tar-split/lrs-fits/ wget -r -np -nH -R index.html http://www.lamost.org/dr8/v2.0/tar-split/lrs-png/ wget -r -np -nH -R index.html http://www.lamost.org/dr8/v2.0/tar-split/mrs-fits/ wget -r -np -nH -R index.html http://www.lamost.org/dr8/v2.0/tar-split/sky/ Files in "lrs-fits" directory are corresponding to LAMOST low resolution spectrum FITS files, after decompression it will be a "fits" directory. Files in "lrs-png" directory are corresponding to LAMOST low resolution spectrum thumbnail png files, after decompression it will be a "png" directory. Files in "mrs-fits" directory are corresponding to LAMOST medium resolution spectrum FITS files, after decompression it will be a "medfits" directory. Files in "sky" directory are corresponding to LAMOST sky FITS files, after decompression it will be a "sky" directory. Sample commands to decompress: cat lamost-dr8-v2-lrs-fits.tar.gz.* | tar -xzf - cat lamost-dr8-v2-lrs-png.tar.gz.* | tar -xzf - cat lamost-dr8-v2-mrs-fits.tar.gz.* | tar -xzf - cat lamost-dr8-v2-sky.tar.gz.* | tar -xzf - If you want to use LAMOST data, please follow the LAMOST data policy (http://www.lamost.org/lmusers/cms/article/view?id=1) and LAMOST publishing policy (http://www.lamost.org/lmusers/cms/article/view?id=2). The Standard acknowledgement is "Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences." Documents about low resolution data http://www.lamost.org/dr8/v2.0/doc/lr-data-production-description Documents about medium resolution data http://www.lamost.org/dr8/v2.0/doc/mr-data-production-description Please contact support@lamost.org if you find any problem.