I've been acquiring data for instrumental magnitudes of LPVs for a bit over a year now in both V and B bands when possible. For the most part my results have been in line with other observers, and the experience has allowed me to develop techniques and an understanding of why and what I'm doing...it's been a lot of fun also. So, I reached the point where transforming my data is the next step to be tackled. I've read a bit and have downloaded Transformation Generator and the required Python software to use it.
My initial questions have to do with selecting a standard star field for my equipment and acquiring the initial data. I use a 102mm f/7 ED refractor, 714mm focal length, an Atik 314L+ mono camera which yeilds a FOV of 43'x32' (.72x.54 degrees), scale is 1.86 arc/sec per pixel.
It seems M67 would be the natural target, but at this time of year it is not available. My next logical choice would be NGC 7790 in Cas., which is in my sky most of the night. NGC 7790 seems somewhat compact though for my focal length. Would NGC 7790 be acceptable for my setup? If not, what might be usable in Transformation Generator that is visible in this season?
How are useful exposures determined for transformations? Just avoid saturation of as many target stars as possible?
Some discussion on what has worked in field selection for your intruments would be helpful...
NGC7790 will work fine. Field is about 20'.
Yes, try a few different exposures. Say 30-180s. This does take a little guess work. I suspect you know what exposure to use for different magnitudes on your scope. Check the NGC7790 chart to see what the magnitude range is. Your B exposure should probably be about 2x the V exposure due to efficiency of B filter being lower. Check which exposures get you a good set of stars with good snr (>100). Use them all. Don't worry about culling them first. TG will allow you to ignore the bad stars by unchecking them.
Take multiple images (8?) in each filter you have. Stack, perhaps in two groups of 4 on a night, so you can get two sets of coeffs on TG per night. Take the set on another night or two. Use TG to average them.
Have fun with TG! HTH.
While NGC 7790 is a neat area for transformations, it has two problems - it is surprisingly crowded (and that hits you because of the plate scale) and it's faint for small telescopes. When I'm using a 600 mm reflector with back-illuminated CCD, I'm taking 180 and 300 second exposures to expose fainter stars well.
However, there is an option which, because of something (slightly more complex analysis?), is generally not encouraged in AAVSO forums. Namely - there is an excellent selection of Landolt standards, both bright (V = 8-12) and faint ones over wide range of colours. There are equatorial and "dec +50" areas. For wide(ish) field of view, they are really very good ones (sparse and large fields!) and besides that - they are The Standards (I don't want to say anything bad about M67 or NGC7790 and other well established secondary ones!). And IMHO Landolt fields are supported also by Transform Generator.
Of course, observing just one Landolt field probably is usually not enough (typically because of colour range) and when observing more than one field, facing with extinction + airmass questions is unavoidable. But nowadays that is really nothing difficult, even calculating individual airmass values for all the observed standards is trivial...
In addition, there is a list of Stetson secondary (cluster) standards, that have been considered very good ones. Unfortunately, they tend to be on the faint side.