Hi Thomas,
Thinking about your images returning 'centre accuracies' of less than 1 arcsecond when you plate-solve - wouldn't you agree that this level of accuracy is probably well within the limits imposed on your image data simply due to 'seeing' conditions?
Another thing to consider is the 'resolution' of your images - what sort of 'pixel-scale' are you working with (you would get this from the results of the plate-solve itself)?
You really need to know all of this information if you expect to be able to decide whether one catalogue sourve is better than another, or whether one plate-solve process is better than another. You have to be able to clearly define all of the information that has been used to perform any of the calculations.
This reminds me of when I used to manage and run an electronic calibration laboratory. If we set out to offer a 'Certificate of Calibration' for some device, then that certificate had to show 'traceability' all the way back to some internationally defined 'reference'. Tke for example a sophisticated electronic frequency counter - with an eight-digit display, and which we use to measure a frequency of 1000000Hz. We have only used 7 of the 8-digit capability of this meter so, let's assume that we can even read down to 0.1Hz. Now our display could return a displayed measurement of 1000000.0Hz.
But, what would it mean if that is exactly what we observed when we made the measurement?
Was our meter 'perfectly' calibrated? Well, that would depend on how accurate our 'source' was (equivalent to the catalogues you are trying to plate-solve against). If the frequency source was wrong, but our meter was also wrong - yet we still see the 'perfect' display, how can we possibly know that we have 'two' errors? Simply put, we can't - it is impossible.
In my case the 'source' itself had to be calibrated - once a year - assuming that it's previous performance did not indicate that even more regular testing was required. Our test conditions also had to be 'perfect' (within tightly controlled limits for temperature and humidity), just to give us the best chance of maintaining a frequency source that was reliable. So, we had to send our 'calibration reference source' away for 'calibration' itself. And the lab that tested it for us had to send their source off for calibration as well - and so on, all the way up the chain to an internationally accepted 'reference standard'. Typically, for frequency sources, this would be a Caesium Beam Frequency Standard, an 'atomic clock, if you will - but, even these have to be 'calibrated' (and, back in the 1980's I was one of only a very small handful of people specially trained to set up such atomic clocks !!).
So, your reference star charts would have to be 'traceable' in as much as the position data they contain for the objects that plate-solving processes work with. And, as I suggested at the rtart of this reply, it is highly doubtful that anyone other than a full-blown specialist photometric observatory would have the required equipment, and specialist knowledge, to be able to call into question the positional data of a ctalogue.
We, as 'normal users' simply have to accept whatever solution is returned to us by whatever process we have used to plate-solve against whatever star catalogue we have selected.
If you have acquired an image that contains some object of special interest, whose position appears to either not to be contained in any catalogue that you have access to, or whose position appears to be significantly different to that contained in the catalougues, then you would be best to submit that image (with whatever positional data that you have calculated) to an authoritative body who can examine your data with higher orders of accuracy.
I hope I haven't discouraged you - but 'measurement accuracy' is a special area of interest to me. I spent my whole electronics career trying to explain this to colleagues (and trainee delegates). I have had seasoned expedition-adventurers, trekking their way 'solo' across the frozen deserts Antartica, contact me over the HF radio, in tears, because they couldn't understand or explain the innacuracies that they were observing between their GPS receivers, their daily log and their magnetic compasses. Some of these seasoned veterans even had to be rescued (from themselves) before they perished, or were otherwise lost forever.
So, please take my comments as positive, and don't try to over-analyse data that may simply 'not be there'. Of course. if I have missed the point in your query, or if others also want to jump in and comment (or even question my thoughts on the subject), I am always happy to see a thread develop and flourish!!
