Thu, 10/06/2022 - 13:20
I'm currently changing from DSLR to CMOS cameras and adjusting my procedures.
In the DSLR observing manual it says that the number of pixels in the annulus, at a minimum, should be equal to the number of pixels in the measurement aperture and preferably more. In the most recent CCD/CMOS photometry guide it says there should be 10x as many pixels. I know that 10x is covered by 'preferably more' but 10x is a lot more. Have I read this wrong?
Cheers
Steve
Steve,
I'm not surprised you are confused.
Steve Howell recommends the numbers of pixels in the annulus be 3X those in the measuring aperture (Handbook of CCD Astronomy, page 107 of the 2nd edition). This assumes you have good SNR's for your target and comps, The situation is different if you are working with images with low SNR's in the stars.
For new observers I think picking targets and learning methods that give good SNR's in the stars is more important than getting lots of pixels in the annulus.
Phil
…
Thanks Phil, excellent.
Steve
It would be worth testing this on your own system. We did this in the current VPhot CHOICE course. The test I ran on one image showed that the the overall noise decreased linearly to a minimum when the ratio of the annulus area to the aperture area was 14. Error then flattened out with further increasing annulus area (the error actually showed a VERY slight increase).
When I get time, I'll repeat the exercise to document the accuracy of the check star magnitude estimates, which were not recorded in the test.
Phil mentioned Howell's recommendation of a ratio of 3. It is therefore interesting that AstroimageJ, which can set aperture and annulus radii for you, has a default setting for this ratio of about 3.9.
Roy
I ran the test on one image of the SY Cnc field with the 7 closest AAVSO comps. The 126 star was chosen as the Check. Annulus / Aperture area ratios were 6, 9.8, 14, 18.8, 24, 29.8 (corresponding to annulus widths of 4, 6, 8, 10, 12 and 14 pixels).
As before, the overall error fell linearly to a minimum of 0.009 at an annulus / aperture area ratio of 14. Check star delta mags (check estimated V mag minus check catalogue) were -0.003, 0.006, 0.007, 0.010, 0.004, 0.007.
Check delta mag was 0.007 at the annulus / aperture area ratio (14) at which the check mag estimate error was minimum.
Of course the presence of other stars, which may lie within a larger annulus, could affect such results.
Conclusion: It may be worth testing for optimum annulus / aperture area ratio (with respect to overall error), and using the result obtained, subject to tests of accuracy.
Roy