Exposure expectations.

Home Forums Variable Stars VSS Campaign to observe U Leo Exposure expectations.

Paul Leyland

“Given this star is rather faint, at mag 17.3, it is a target for CCD observers. It will require long and unfiltered exposures to get a reasonable signal-to-noise ratio.”

It is rather tedious sitting here waiting for photons to arrive so I am trying to get ball-park estimates of what is likely to be achieved. As the autoguider on the 0.4m Dilworth / unfiltered CCD combination still doesn’t work I restrict each exposure to 30 seconds. The sky tonight is fairly dark but not exceptionally so. The SX 814 has a very low dark noise and so the major limit on SNR is sky glow. I have found that averaging 20 subs, for a total exposure time of 10 minutes, gives a SNR of 50 to 55 when U Leo is at altitude of 60 degrees. It will be worse at lower altitudes, of course, or if the Moon is above the horizon. A SNR of 50 corresponds to a precision of 0.02 magnitudes. The light curve in the VSS Circular suggests that the peak-to-peak amplitude is perhaps 0.1 magnitudes. A period of 3.2 hours corresponds to 192 minutes. Accordingly I can hope for at most 19 samples per period  at an amplitude precision of 20%. In practice it will be less because of download times and inevitable discarded images from poor tracking.

It is going to take a lot of heavy duty signal processing to get robust results from my observations alone. If I had perhaps ten times the data I should be able to get something useful. We need lots more observers, in other words.

One unexpected side benefit is that U Leo lies in a rich field of galaxies, including a bright face-on spiral known as 2MASX J10234921+1357083 (it appears on Jeremy’s finder chart roughly halfway between U Leo and the bright star in the lower left corner; an edge-on spiral of around 17th magnitude lies to its left).  Only a few hints of spiral arms have appeared on a large stack but a “pretty-picture” may yet be possible. The galaxy’s magnitude is g=15.9 and r=15.2 so it shows up on every single sub.  There are many more galaxies brighter than 20th magnitude within a few arc minutes of U Leo; they are starting to appear in my data as the stacks get deeper.

Added in edit: since typing the above text the SNR has almost doubled.  Presumably the sky has become notably brighter for some reason I do not understand.  Perhapsthe signal processing may be easier than feared.