DSLR Tutorial - Calibration: Beginner spreadsheet questions/comments
In working through the Finishing Analysis step and the spreadsheet, a number of questions arose in my mind that I found a little confusing. I'm referring here to the Calibration: Beginner tutorial - I've yet to progress to the Intermediate stage.
1. In the tutorial example of working the spreadsheet, you have chosen lambda Aur as the Check Star. Is there a particular reason for choosing this one? What criteria should one use to select a check star? What is the role of the Check Star (see further comments, below)? For a newcomer, this comes out of the blue without explanation.
2. After entering the instrumental magnitudes and the B-V values for the comparison stars (including the Check Star) one obtains the TC (Transformation Coefficient). This part of the spreadsheet works well. However, the last table in the spreadsheet I find confusing. In the worked example, you have entered here 4 image sets: the overall suite, then 3 sub-sets derived from this suite. I realize that the stdev() function in Excel requires at least 2 values upon which to work, so presumably this process was a means of overcoming this limitation. The point is, however, no explanation is given and the suite of images does not match the data set supplied in the beginner's tutorial, which has only 3 images. Another thing that worries me is the use of the complete suite + sub-sets of this suite: I can understand using sub-sets, but including the total set as another data point....? Also, if I remember my basic stats (from over 45 years ago), you cannot expect to get a reliable std dev from a sample set smaller than 6, more is better. Perhaps someone could advise me here. For the beginner's tutorial, though, shouldn't it be advised to do the measurement of instrumental magnitudes of both the check and variable stars in each of the 3 images supplied using the same set of flats, flat-darks, and darks used for the stacked image?
3. Finally, I realize that the spreadsheet is a first cut and is likely to be revised in the light of user comments. Here's some of mine:
To overcome the ugly #DIV!0 and similar error messages as you are processing, I would recommend that you change some cell formulae:
C37 =IF(ISBLANK(B37),"",(B37-$B$30*$C$12-$C$30)) (and so on down to C40)
E37 =IF(ISBLANK(D37),"",(D37-$B$30*$B$8-$C$30)) (and so on down to E40)
Then, you could set up permanently, the Average and StdDev fields as follows:
C41 =IF(COUNT(C37:C40)=0,"",AVERAGE(C37:C40)) and its E41 equivalent
C42 =IF(COUNT(C37:C40)<2,"",STDEV(C37:C40)) and its E42 equivalent
which would save you that clumsy description of how to set up these cells. I'm not an Excel expert, so perhaps others could make a contribution here. I haven't tested these formulae rigorously but they seem to work well if you enter data in a progressive fashion down and across the spreadsheet.
Just my tuppence worth. I hope it helps make a better tutorial.