Sun, 09/23/2018 - 12:23
Hello! I've moved and have been performing upkeep on my 8" LX200, including cleaning the corrector plate. I needed new Landolt standards in order to reduced my variable star measurements.
I took some fields last night, but it is close to full moon. The fields were 90 degrees or so away from the full moon.
Is this OK? or would the full moon light adversely affect the landolt readings?
Thank you and best regards.
Mike
It really depends upon the quality of the atmosphere in your location: If it is dry and there is little in the way of aerosols to scatter light, then you are probably ok. If I were you, I'd look at the target to background ratios, comparing the latest measurements with some earlier ones taken under dark sky conditions. If the current set is not seriously degraded, then you are safe using these data.
(Where I live, in the deep VA suburbs of DC, any observing is ruled out within 3 days of the full moon because of our awful air quality. I have yet to observe here in noise limited conditions - it is always background limited. But if you live somewhere like AZ or NM then things are different for you.)
I'm sure Arne or some other more qualified photometrist will weigh in if I am wrong.
My own experience running Landolt standard fields every observing night: on average and with a bit of care, the moon makes very little difference in carefully measured Landolt standard magnitudes or transforms. This is with my rig: C14 OTA, Astrodon filters, SBIG 6303 CCD, and has been true in eastern Kansas (cirrus and haze common), and now in New Mexico as well.
My magnitude *scatter* (as std deviation) on moonlit nights is a bit elevated, so I do schedule a few more standard images each moonlit night than on moonless nights, say 30-35 rather than 20-25. But my magnitude *averages* just don't seem to depend on moon phase when measuring (1) via careful aperture photometry, (2) staying at least 40 degrees away from the moon (so that my short dew shield keeps direct moonlight off the front OTA plate), and (3) tending maybe as a precaution to run my front-plate dew heater a bit more often with bright moon (to prevent even a thin film of dew or frost which really hurts with brighter skies). When the sky is bright, you might want to prefer brighter standard stars and be ready to drop fainter standard stars as well.
Hi Mike,
Eric and Stephen have given good answers.
If you are talking about signal/noise, then of course a moonless night is best. A couple of days around full moon is a particularly bad period in this regard. The main affect is increased sky brightness, but there will also be a sky gradient across your field. There is a color change in the sky (bluer when the moon is up), but again, for bright objects, this is irrelevant.
If you are talking about whether the moon changes the actual coefficients, there is no effect. As long as your standard stars are bright (say, the 1985 Landolt standards), so that even increased sky doesn't add much noise, then your calculations will be the same as if the moon wasn't present. I often reserve moon-lit time for either bright variables or calculating coefficients.
Arne
Mike
Thank you!
Mike
Hello! Over about an hour, I imaged 12 Landolt fields over 6 hours of RA starting from astronomical twilight near Las Cruces, NM. My tracking has been a bit off (hopefully corrected soon), so I took eight 15-second images of each field in B,V and I and applied darks and flats. I summed/averaged the 8 images to get one final l image.
I used MPO Connections/PhotoRed for the reductions.
Using B-V, B transform = 0.114 with SD = 0.055; V = -0.022 with SD = 0.069; and I = 0.088 with SD = 0.042.
Do these look about in the right ball park? Thank you and best regards.
Mike
Hi Mike,
yep, those values are pretty typical. Good going!
Arne