Looking at the formula for your transformation coefficient, something is not quite right. What does your Delta V represent? In the usual procedure it is V - v where V is the catalogue mag of the star and v the instrumental mag. V - v is plotted on the Y axis against B - V on the X axis. The plot should be a straight line with a negative slope, i.e., the line slopes down to the right. The generic equation is V - v = a + m(B - V) and m, the slope, should be a negative number. Yours is positive. The transformation coefficient is 1/m which is typically close to zero for CCD cameras and some CMOS cameras. For Seestar I think it has been determined by others to be close to zero. Your slope is 0.173, and its inverse is 5.78. Can you explain exactly what you did and how you derived the formula?
The first thing I did, was to import images of M67 into ASTAP, then annotated all the stars with magnitudes from GAIA. Once that was done, I clicked the transformation button on the software. How it accomplished it, I don’t know. I relied on the software to provide me with the equation. I can look at the data again and see what I did wrong - it wouldn’t be the first time.
If you can tell me another method, I would appreciate it.
It is important to look for “Transformation” in the ASTAP user manual. I have used ASTAP only for astrometry, not for photometry, but if I read the manual correctly the automatic transformation routine looks at hundreds of Gaia stars in your image, determines the slope of a plot with ‘magnitude error’ on the Y axis and B-V (calculated from Gaia magnitudes) on the X axis. It reports the slope (a negative number in the example in the manual) in the AAVSO report.
I haven’t tried to read the manual exhaustively, but the ‘magnitude error’ on the Y axis is, I think, not clearly explained.
The above does not conform with standard AAVSO recommendations for determining transformation coefficients.
I believe you are correct regarding the method. Now ignoring ASTAP for the moment, I have been looking at how to determine transformation coefficients. I have looked in the DSLR guide, without luck on the proper method. Can you direct me to the proper source or recommend guide?
I found your guide while doing some searching. It is a bit complex One thing that I have run into, when entering the VPHOT, is that it asks me for a telescope. I looked at the drop down, and the SeeStar wasn’t there. I also looked to see if my equipment, under my profile, showed my SeeStar (and it does). While I understand each telescope is site specific, how do I enter my telescope in to the drop down list, so I can proceed further?
Maybe I am slow, but I didn’t see the way to accomplish this.
See that attached file for an explanation of transformation coefficients. It is a screenshot of a spreadsheet from a Microsoft Excel workbook. I can’t upload the workbook itself - I think the file type is not allowed on this Forum.
The column headings are:
Star (E Reg No.) IDs of the stars from the catalogue of standard stars from the E Regions.*
V Catalogue V magnitude
B-V Catalogue colour index
B Catalogue B mag (this column is not needed)
b Instrumental b magnitude
v Instrumental v magnitude
Menzies, J.W., Cousins, A.W.J., Banfield, R.M., and Laing, J.D. 1989, S. Arf. Astron. Obs. Circ. No. 13, 1.
If you study the description above and the spreadsheet itself, you should be able to get an idea of what transformation coefficients are about, and how you may determine them. You need an image of a field of standard stars on which you perform aperture photometry, information on the V and B-V of those stars, and the ability to calculate instrumental magnitudes.
I have been trying to get a handle on understanding the concept of the transformation coefficient. I can calculate the instrumental magnitudes from some standard fields like M50 and M67. I have a series of data that I have taken from a bunch of FITS when I was imaging them for my collection of Messier objects. I have several hundred FITS that I can use.
Now as I understand it, I have to get data from each of the filters and calculate those separately. However, the only filter that matches closely to standard filters is the green or TG.
As I understand it, the lowercase letters are the instrument values, and the Uppercase are known or reference values. In your spreadsheet, I see you have the b & v values - I am assuming that those values are the magnitudes measured with each filter?
I see how you use the spreadsheet to calculate the data (I think), however, the slope for the Tbv plot shows 0.995, while the coefficient in K1 is 1.005. I am not sure what I am missing. Would it be possible to email me the spreadsheet to me at kb0fhp@gmail.com?
At this point I am a bit confused, and need to study some more (and read the various papers) in more detail.
The b and v values in the spreadsheet are the instrumental magnitudes of the stars for the B and V filters respectively.
The coefficient Tbv is the reciprocal of the slope of b-v / B-V (cells K2 and L2).
The coefficient Tv_bv is the slope of V-v / B-V. (cells K3 and L3. What I wrote in my original post this morning was not correct - sorry). This is the coefficient you have been calculating. If you use this coefficient alone, you need the B-V of the variable star from a catalogue to determine transformed magnitudes. The B-V of some variables (e.g., those that change in temperature during their cycles) varies during the cycle - something to think about.
You wrote: “… the only filter that matches closely to standard filters is the green or TG.” That’s why transformation coefficients are necessary.
The coefficient Tv_bv also represents the error in a non-transformed magnitude if the B-V difference between the variable and comp star is 1 magnitude unit. The relationship is linear, so if you know your Tv_bv and the B-V of the variable and comp stars, you can calculate the approximate error (due just to the var / comp B-V difference) for a particular field.
I’ll send the Excel workbook to your email address.
Here is a recent light curve I did of AE Uma. I am still trying to refine my techniques and get the best sized stacking groups to achieve proper SNR. I am working on trying to get the transformation coefficients thru some fields of M67 that I imaged and extracted the various color channels of the SeeStar.
One thing that I have run into, when entering the VPHOT, is that it asks me for a telescope. I looked at the drop down, and the SeeStar wasn’t there. I also looked to see if my equipment, under my profile, showed my SeeStar (and it does). While I understand each telescope is site specific, how do I enter my telescope in to the drop down list, so I can proceed further?
