What standard clusters, observable from the southern hemisphere temperate zone, are now availble via VPhot for calculating transforms?
Specifically M67 is visible part of the year from the south - e.g. available right now up through most of May at latitude -32 degrees.
I think what Phil is asking is for which standard fields does VPhot have photometry. The answer is that it has whatever standard fields that VSP has. For any image you can ask for comp stars or standard field stars. So the fields that Ken listed above should all be available.
This is for the April CCD Photometry-2 class. There may be one or two students from down under who are not able to use M67 because of trees or other obstructions. It transits in Melbourne (Lat. about -38) at an altitude of ~40 degress if I did the arithmetic correctly. It looks like M11 (~20 degrees higher) would be a good choice for them if they observe it in the early morning hours.
Thank you all.
Thanks Terry. I wasn't aware that the Landolt fields were available via VSP. For people using the spreadsheet method it seems this would be a good possibility.
In April NGC 3532 is high in the southern sky and easy to find. It provides many standards at a variety of FoVs. Gordon used two of my 2015 images to check it out and got the same transformation coefficient for V-I that I got from measuring 336 Cousins standards over 20 nights!
Attached are B-V and V-I transformations I did for this cluster just over a week ago using 19 of the brighter stars.
I see this is now supported in TG v.6. It might be better for people not wanting to stay up (or get up) in the wee hours for M11. I'll take another look at this regarding the color spread availble for those with FOV's in the 15-20 arcmin range.
I completed CCD Photometry II course last November and can help your students with BVI images of NGC 3532 if they have problems getting their own. My 2015 images (VI bands only) were taken using an Orion Starshoot G3. To use them in VPHOT the FITS header needs to be edited but Gordon discovered the changes that need to be made and has succesfully used them. I can make the changes if required.
My more recent BVI NGC 3532 (and NGC 1252) images were taken with a ZWO camera (see transform in previous post) and I will check the FITS header to see whether it needs editing for use in VPHOT sometime in the few weeks. As I already have spreadsheets with standard stars in NGC 3532, it is also an easy exercise for your students to measure the brighter stars (~23) using software such as IRIS, Astroart, etc. and plot the line of best fit in Excel.
Happy for you to contact me directly by email.
Thank you very much for the offer to help with images. I won't know the total composition of the course until it actually starts at the beginning of April. For now, this is only a potential problem, but I will keep your kind offer in my mind if images are needed.
Phil, Gordon, Ken, George,
I got a photometrically 'reasonable' night on 14 February as evidenced by measurements of extinction star through air masses ranging from 2.4 to 1.2 (see first figure). During this time I measured NGC 1252, NGC 3532 and a few Cousins E-region standards in BVRI (unfortunately some images of the Cousins standards turned out to be saturated).
Plots for the color indices are given in the second figure (blue crosses for NGC 1252, solid grey circles for NGC 3532 and green Xs for Cousins standards). The slope of the linear fits were 1.062 for B-V, 0.943 for V-R and 1.042 for V-I (transformation coefficients reciprocal of these).
All instrumental magnitudes were corrected for extinction (all-sky photometry approach).
These plots show good agreement between standard values for NGC 1252 and 3532 and agreement with the few measurements of Cousins E-region standards. I believe the AAVSO standard values for these 2 clusters can be used with confidence.