Affiliation
Variable Stars South (VSS)
Mon, 02/10/2020 - 05:30
I usually calculate coefficients for b-v/B-V, B-b/B-V and V-v/B-V.
Recently during testing of a ZWO ASI1600MM camera, I noticed that the B-b/B-V transform consistently resulted in more accurate target V determinations than the V-v/B-V transform. The targets were standard stars from the E Regions.
For determination of target B, the results are not as consistent, but more often than not the B determination from B-b/B-V is as good as or more accurate than the determination from V-v/B-V.
I've tried but failed so far to find published comment on this.
Are the above results expected, or does anyone have any experience of such results?
Roy
Correction for the title of the previous post. Should have been B-b/B-V, not B-v/B-V.
Roy
Hello Roy,
I'd suspect V filter. How close it is to Johnson/Bessell V one (info from manufacturer or measured by yourself)?
Best wishes,
Tõnis
The filters are Astrodon 13mm Mounted Jounson/Cousins, edge blackened. The B filter is described as "Improved B" the V filter has no adjective. They are interference filters.
Your post implies that you are suspicious of my results. Do you have results which show clearly that there should be no difference between the two different transforms? I'm not talking about theory, but actual measurements.
Roy
My own experience tells that in general Astrodon V filter(s) and any sensors/cameras I have used produce good transformation coefficients. Very close to 1:1 relation for colours and very close to 0 for V magnitude. Things are much more "interesting" in B-filter, because that one covers the wavelength region where quantum efficiency of many sensors drop rapidly. Even when B-filter is perfect, camera QE * filter curve [* transparency of telescope] may result in significant deviation from standard system - and that is reflected in transformation coefficients.
I added a graph of recently measured Astrodon V filter (31mm version) transmission graph together with Johnson V filter curve from Synphot filter database.
Just out of curiocity, what kind of filter wheel are you using? I found that ZWO 8 position EFW has "infrared leak" that seems to be more like "positioning IR LED shining into the camera".
Best wishes,
Tõnis
Tonis, I use the ZWO EFWMini 5 position filter wheel (5x1.25" or 5x31mm).
Roy
I suspected that there is a NIR leak from (maybe?) constantly (or partly) on LED or so. At least a research-grade radiometric detector hinted that. That signal was there only when filter wheel was connected to computer.
Today I filmed my 8 position version of ZWO wheel and there are indeed two NIR LEDs well visible during the movement of the wheel. But apparently not when wheel is stopped. I will investigate that further, to find out from where that additional signal came that was detected as very low level light.
Best wishes,
Tõnis
Another typo. Should be Astrodon 31mm ...
Roy
Apologies. Mistake in spreadsheet formulae, now corrected.
Target B and V magnitudes from V-v/B-V and B-b/B-v transforms are essentially identical (same, or differ by only 0.001 mag) using the ZWO ASI1600MM camera.
Roy