In ASTAP version 0.9.142 released today, I have added the possibility to force the star database. Below to number of matches for some of my images:
The two Gaia databases perform different but very similar. The UCAC4 U16 behaves a little poorer. Exposure time was between 50 and 200 seconds. The U16 is no longer provided since it was replaced by Gaia. In principle you could also use the TYC++ and TUC (UCAC4 and Tycho hybrid) but they are in 290-9, 290-10 format and I removed that support in ASTAP to save some code. So they will not work. The program goes for my images typical down to magnitude 12-13 so the Tycho with magnitude limit 12 will most likely not work well. ASTAP uses brute force for solving rather then index files as Astrometry.net and changing the database is easy.
You could try yourself if this performance is similar for your camera. Note you can select "show extended solver log" in tab alignment.
Creating a new star database is a huge task and will take 24 hours CPU time. I have still 60 gbytes files from the Gaia dr2 on my disk. In principle I could reasonable quick create a new version with a specific magnitude value calculated using the three magnitudes (G, B, R) provided. My computer will be one day crunching till it is ready.
The G16 (mag 16) contains also the Gaia blue en red difference. At the moment the only thing with this value is done is to colour the star annotation. In principle this could be used if the program detects a blue filter in the image header but it will be very exotic. There is no reason to image in other band then luminance for astrometric solving.
Probably the only thing I will do is to create a special G17 (mag 17) star database adding this blue-red difference as in the G16. Main purpose is for the HNSKY planetarium program to colour the stars up to magnitude 17. The penalty will be an 20% increase in file size. Note that Carte du Ciel can read/use these star database files. A G18 (mag 18) with colour info is also possible.
I'm have uploaded the u16 (mag16) database again at sourceforge for testing. It is a hybrid of Tycho2 and UCAC4 without abbreviations.
Edited by han59, 05 December 2018 - 09:47 AM.