Wed, 11/15/2023 - 21:45
Hi,
When using a colored dslr or cmos camera, when would it be appropriate to report magnitudes in the tricolor channels, or the CR and CV (and CBB notes) clear filters? Does having an IR/UV cut filter mean magnitudes should be in TR, TG and TB, and CR and CV if no IR/UV cut filters?
Thanks,
Raymund
These magnitudes should be reported as TG, TB, or TR unless you have obtained transformation coefficients for your set up. If so then you would use V,B,R for the transformed magnitudes. I don’t know of any circumstance where you would submit CV, CB, or CR for one of those cameras
Barbara
Raymund:
Clear filters have nothing to do with color (Bayer Array) cameras including DSLR or OSC that have internal tri-color filters in a Bayer Array. As Barbara reiterated, tri-color magnitudes from color cameras should be reported as TR, TG, or TB (Tri-color X) when reported as un-transformed magnitudes. IF you generate transformation coefficients for these filters in your camera, you should transform your un-transformed tri-color magnitudes to transformed photometric R, V, B magnitudes and report as such.
BTW. although TG filter magnitudes transform well to V, TB and TR filter magnitudes do not transform as well for targets of very blue or red colors, Therefore, your transformed B or R magnitudes may not be as accurate as desired. It may be more reliable not to transform TB or TR magnitudes although this is a conclusion of much continuing debate!
Clear (CV or CR) magnitudes are obtained with monochrome cameras using a Clear Filter, No Filter, Clear Blue Blocked Filter (CBB), or Luminance Filter. Clear filters may have either UV or IR cutoffs. As you have surmised, notes are helpful to clarify exactly what kind of Clear Filter was used. However, for efficiency in reporting and display in the LCG, they are all considered Clear V (CV) or Clear R (CR) filter magnitudes depending on which comp star magnitudes (V or R) were used as the reference for the magnitude calculation. They are reported as such (CV or CR) on the LCG.
Note that historically, CB has not been used since it experimentally does not provide reliable magnitudes that are close to photometric B magnitudes.
Clear magnitudes (CV or CR) are generally not transformed to photometric filter magnitudes (V or R) since their broad bandpass does not yield reliable transformed magnitudes.
HTH, Ken
Thanks, Barbara and Ken.
Just a follow up - is the FITS image from a colored camera that has not been debayered yet (i.e. still combined RGB) equivalent to a monochrome in terms of magnitude estimate?
And, would not having an IR cut filter still be accepted to report in tricolor channels, or maybe go the CR CV route (if combined RGB is equivalent to mono)?
Thanks,
Raymund
Raymund:
IMHO, even IF there were some logic as to considering a full tri-color image to be similar to a monochrome Clear filter image, it is just another way to increase (degrade) magnitude scatter in our light curves. The main objective of the AAVSO database is to gather 'reliable' photometric magnitude data. It only gets more difficult to obtain 'consistent' magnitudes with all the different camera and filter options that appear in the marketplace and are used by potential observers / members! Remember, our main magnitude data user is a professional astronomer/researcher. What do you think he/she is hoping for?
I don't think Barbara or I have raised any concerns about submitting tri-color data from a camera that has some IR cut filter. The RGB Bayer filters have a bandpass that excludes IR (most) anyway?
Again, IMHO, a Bayer full color Image is not considered the same as a clear monochrome image. If you feel otherwise, I would propose that you need to prove that with some comparative data.
Ken
Hi Ken,
I have no concern as well with tricolor channels using IR cut filter. My concern is the opposite - submitting tricolor without IR cut filter, for sensors with significant IR leak. The tricolor magnitudes would then be a lot brighter, esp. for red stars, compared to the V mag.
If the dataset of tricolors are a combination of those with and without IR cut filters, this may introduce the scatter of magnitudes due to IR leak, and may be hard for professionals to distinguish the ones with IR leak when using tricolor magnitudes. And since CV and CR are not suitable to be transformed to V and R, they would be treated as such - separately - particularly when it comes to red stars or those emitting a lot more in the IR.
I extracted TR TG and TB for red star DQ Vul, and the magnitudes are way brighter than the V mag - which lead me to believe that tricolors with and without IR cut when mixed in one dataset will cause scatter. I am leaning towards using CR and CV for those with IR leaks and TR TG TB for those with cut filters. Thus, my curiosity if there's a way to consider colored cmos data with IR leak to use CV and CR - to avoid scatter of the tricolor magnitudes. I am not sure if there's a way to transform tricolors to RVB if there's noticeable IR leak.
Raymund
Raymund,
Bayer color camera photometry should be reported as TB,TG,TR after extracting each channel and performing photometry on each color image. This is true regardless if there is an IR cutoff filter. If you want to add a note in the reported photometry that the camera has an IR cutoff that is ok but it should be reported as T(B,G,R) not CV,CB, or CR. Theoretically having IR cutoff should only affect the TR photometry and no significant impact on TG or TB.
Also, looking at your submitted photometry for DQ Vul, are you sure you have identified the correct target star in your images? It is a Mira star with a magnitude range of 14.5-17.6V per VSX. Your TG submitted magnitudes are around 11.8, about 3 magnitudes brighter than this Mira star at its brightest. So you may want confirm the location of the target star you are measuring.
Barbara
Hi Barbara,
I plate solved and centered the variable using Tycho Tracker, and checked the star using ASAS-SN. It's the same star, but the outer color in my star image is slightly purple (maybe owing to the IR leak influencing TB). I initially thought it was in outburst. I am using the IMX224 sensor and has IR leak, which could explain the brightening in TG and TB as well.
When I saw my tricolor data, that triggered me to maybe report in CV and/or CR to avoid scattering tricolor data.
Below is spectrum graph of the sensor showing IR leak, not just for TR
http://astronomy-imaging-camera.com/wp-content/uploads/QE-ASI224.jpg
Raymund