Tue, 03/30/2021 - 19:47
I'm just processing my first set of images of SA32 in order to calculate the transform coefficients. I'm using a DSLR on an 200mm Newtonian with a 1000mm focal length. I'm following the procedure given in the CCD photometry catalogue.
My graph of b-v versus B-V although reasonably straight has a negative slope so that Tbv is negative. Should that be possible? Is it back to the drawing board?
The other graph of B-b versus B-V is quite straight and has a positive slope.
Cheers
Steve
Make sure your plot shows B-V standard magnitudes on the X axis with the values increasing from left to right, and b-v instrumental magnitudes on the Y axis with values increasing from bottom to top.
Phil
Thanks Phil, I'm pretty sure I've plotted them correctly and in any case swapping the axes around would still give a negative slope. I'd better check my work again.
Steve
Steve, it occurred to me that the following may help, although of course different equipment may give different results.
With my DSLR during a recent transformation coefficient run, for a range of B-V values from about 0.14 to 1.45, my b-v values ranged from about 0.14 to 0.67.
Roy
Thanks Roy, I think my problem is that the spread of b-v values is very small so what I see as a straight line is an illusion. It's just a bunch of close values spread over a y axis. I need to check everything again.
Steve
Basically, Tbv should be close to +1.0, as it says that the instrumental color is similar to the standard color. A negative slope would indicate anti-correlation: what you measure as a red star is actually a blue star, and vice-versa.
My guess is that you have swapped instrumental magnitudes when creating your instrumental color, so that you are creating (v-b) instead of (b-v).
Arne
Thanks Arne, I think you are right but it's not the only problem. There's hardly any spread on the values of b-v. I've got to check my work all the way through.
Steve
Tbv is close to one with monochrome CCD or CMOS cameras and V and B filters.
With my Canon EOS 500D (Rebel Ti, I think in the USA) DSLR Tbv is currently close to 2.5.
Steve is using a DSLR camera.
Roy
I found my problem but I'm not sure why it was wrong. If someone could explain I'd be grateful.
I'm using MaximDL and the way I did it initially was like this -
Calibrate all images
Stack all images
Split the stacked image into B and G images
Do the photometry
After looking at the AAVSO Photometry software guide I realised I was doing things in the wrong order. So then I did it like this -
Calibrate each image
Split each image into B and G files
Stack all the B images
Stack all the G images
Do the photometry on the two stacked images B and G
So now I get a nice straight line and Tbv comes out at 2.06
I don't understand why the first method was wrong. I looked at the R, G1, G2 and B output files from the first (wrong) method. Then I picked two stars which had widely different B-V values. Then I measured the intensity of the two stars in the unsplit stacked image. Then I measured the intensity of the same stars in each of the R, G1,G2 and B images separately. I found that if I added the R, G1, G2 and B intensities I got a value which was the same as the intensity of the unsplit image. Which is what you'd expect. However I discovered that the individual intensities (R, G1, G2, B) were almost exactly 1/4 of the total intensity. This was for both stars even though they had very different B-V values.
Steve
I have been working on this same issue for a bit
I do not understand why the photometry guide list the instrumental b and v values as negative magnitude?
I also have a negative slope for my coeffecient graph when I recreate the spread sheet
If I eliminate the negative b and v instrumental mags then slope is positive
bryan
Instrumental…
Hi Bryan,
Instrumental magnitude is defined as
Instrumental Magnitude = -2.5 log10(star ADU)
So it usually comes out negative. That equation actually comes from the AAVSO DSLR photometry guide. As it happens I use MaximDL and it uses a slightly different definition of instrumental magnitude as follows
v=-2.5 log(ADU/t) + 25
where t is the exposure time. The main difference is that it adds 25. I assume that that is just to make sure of a positive magnitude but I don't think it matters because we are always subtracting one magnitude from another so the 25 disappears.
I now get a reasonable result. My main problem was that I was stacking before extracting the colours. You have to extract each colour from the images and then stack the green images, stack the blue images and so on. In my case the Tbv and Tb-bv charts have positive slopes and the Tv-bv has a negative slope which is the same as the AAVSO CCD Photometry guide.
Steve