Transforms: Can I Combine Landolt Fields and M67?

Mike:

As a chemist, I am a great believer in experimentation. Why not try and see what your coeffs look like with each option (M67, Landolt, combined?)

In any case, I suspect you use a spreadsheet  for transform calculations? If so, are there any values and or equations in your spreadsheet that involve comp location (RA/Dec) or airmass? Or, does it only include values of instrumental magnitude and standard magnitude for the comps?

This may answer your question?

Ken

Hello! Thank you for your note.

    I tried combining Landolts and M67 about 6 years ago. The M67 stars seemed to increase to scatter and error range of the final result, and, since I had a fair number of Landolts fields, I used those.

    However, M67 is so easy given the number of stars in the one field, so I am thinking about trying to add it again to my Transforms. But, I thought I would ask first to see what folks thought about combining Landolts and M67. Landolts are the gold standard while M67 is a close secondary? Best regards.

Mike

So you have tried before. Great!

M67 has lots of comps, many of which overlap depending on your aperture/image scale. So it is possible that you may have observed more scatter in the coefficients. However, you may also have had a much better number and range of colors with M67.This might lead to a more accurate if not precise coefficient? Defining the ends of the lines with more comps really helps with slope calculation.

One key question is how much different the coefficients were with the two standard fields (M67 vs Landolt)? Not precision but agreement?  Did you use just one landolt field or several? Yes, the Landolt standards are primary standards compared to AAVSO std fields. They do have smaller mag/color errors.

Have you tried to use TransformationGenerator? It speeds up the process significantly and allows you to easily see which comps may be inaacurate, and remove them. TG uses Landolt fields (singly) too. It is an amazing alternative to a manual spreadsheet! 

Ken

I can also suggest SA 98. Also, there are several additions to original Landolt works that contain more and fainter stars in/around SA fields. E.g. this one https://aas.aanda.org/articles/aas/abs/2000/16/h2151/h2151.html Seems that their photometry is a rather good one.

Tõnis

As long as you are including extinction variations, you can combine multiple fields together to get your transforms.  I usually do all-sky observations of Landolt fields, and include first and second-order extinction in the coefficient determination.  If you have good skies, that process reduces systematic error; a particular field (such as M67) might have a systematic error.

If you do not have good skies, or you don't want to take all night to get your transformation observations, then one of the standard clusters is your next best choice.  All of the stars are at basically the same airmass, so first-order extinction is eliminated.  The quality of the AAVSO standard clusters is quite high, but not at the level of Landolt fields (the gold standard).  The color range for a given cluster is also not as wide as for multiple Landolt fields.  For amateurs, one of the other issues in favor of clusters is that many of the Landolt stars are faint and require quite long exposure times.

So both methods work.

Arne

Thank you, everyone, for your guidance. Best regards.

Mike

Hello! I've been using Brian Warner's approach with his MPO Canopus software. It works well and takes into account first and second order coefficients.

I'll try both Landolts and M67; however, I won't mix the observations. I'll compare the transforms I obtain from each set of observations for my system. Best regards.

Mike