Hi All

I have been able to generate some B,V,R transformation coefficients for the S50. I outline the steps briefly as follows:

1. Coefficients are based on images of the standard field for NGC 3532. This is quite a crowded field with a large number of standard stars so I reduced the number to avoid stars close together, etc.

2. The hardest bit of the process has being trying to work out the Bayer pattern such that the correct B, G and R channels can be extracted. I have found out that it is a **BGGR** pattern. **Do not crop your image prior to extracting the B, G and R channels. **Cropping the image will impact the Bayer pattern assumed in the channel extraction. I use AstroArt 8 software for the extraction as it is very simple and requires no scripts to be developed. AstroArt 8 is a very good piece of software for astrometry, photometry and image processing in general.

3. Once the B, G and R images have been extracted and plate solved, I just follow the process of preparing data through Vphot for analysis in Transform Generator (TG).

The results are as detailed below:

Tbv= 1.819

Tb_bv= 0.457

Tbr= 1.652

Tb_br= 0.299

Tv_bv= 0.020

Tvr= 1.445

Tv_vr= 0.044

Tr_vr= -0.273

[Error]

Tbv= 0.022

Tb_bv= 0.006

Tbr= 0.011

Tb_br= 0.004

Tv_bv= 0.008

Tvr= 0.017

Tv_vr= 0.015

Tr_vr= 0.008

[R Squared Values]

Tbv= 0.990

Tb_bv= 0.988

Tbr= 0.997

Tb_br= 0.987

Tv_bv= 0.082

Tvr= 0.991

Tv_vr= 0.117

Tr_vr= 0.951

The errors are generally small and the "goodness of fit" is high. I have used these in successfully transforming some observations already made with the Seestar.

I hope this helps anyone else interested in determining coefficients for their Seestar.

Regards

Andrew

Andrew's results raise a point of interest relevant to those observers who do not use transformation coefficients, and for those who do, perhaps a point for discussion.

Tv_bv is 0.020. If non-transformed photometry is performed using the Seestar's G filter, this low coefficient means that, for a target-comp pair that differ in B-V colour index by 1.0 mag units, the error due only to the B-V difference will theoretically be 0.02 V mag. The relationship is linear, so the error for a B-V difference of 0.5 will theoretically be only 0.01 V mag.

Therefore, if the target-comp B-V difference could be kept to less than 0.5, the accuracy of a non-transformed V mag could theoretically be less than 0.01.

Given the possibility for such good results, could transformation coefficients be dispensed with? Would using them give more accurate results?

The answer to the first question, in my opinion, is yes, provided that: (1) the B-V for the comp and target are within or close to the B-V range of the stars used to determine the TCs, (2) the variable does not have a peculiar spectrum and (3) the observer was interested only in V mags.

Using non-transformed magnitudes in the above circumstances is particularly useful if the aim of the observations is to obtain time series for short period variables (for timing maxima or minima), since the complications inherent in trying to transform B and V magnitudes for stars changing rapidly in brightness (and perhaps in colour) could be avoided.

The answer to the second question would require experiment.

Of course, if it were necessary to use 'relatively large' target-comp B-V differences and to achieve high accuracy, it would be better to use transformed magnitudes.

Roy