From: Terrence Redding <tredding@mac.com>
Date: November 13, 2009 8:44:58 PM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] Guiding - Observing Campaign on the Secondary Eclipse of zeta Aurigae
Milton, great message.
Okay on all. I wont worry about the single pixel issue and will try several comparative stars to see what kind of results I get. For some reason the website that is organizing the effort has a recommended star that is to far from the target star to be in the same field of view unless one is using binoculars. So, clearly your idea is superior for the kind of photometry I hope to do.
Thanks for the suggestions - they are greatly appreciated. Okay on the mag difference - I will try several to see what results I get.
There are at least three things very different about this project as compared to the normal occultations I have been doing. First is the duration. Most of my efforts deal with a variance in light levels lasting less than 30 seconds as an asteroid passes in front of a star. Or, the longer mutual events of the Galilean Moons. This event is called a campaign because it last 30 days.
Most of the occultations and photometric light curves I produce are associated with a change in magnitude of nearly one or more. The mutual events produce changes in the tenth of a magnitude range. This project will be the first to approach the level of sensitivity in the range of 0.02 change in magnitude. That is the same level sensitivity required by exoplanet work. The magjic must be in the comparative nature of differential photometry. It would be very neat to manage it with the DMK21 and Astro-IIDC.
I have clear skies tonight - so this will be the first effort.
Terry - W6LMJ
On Nov 13, 2009, at 5:46 PM, Milton Aupperle wrote:
Hi Terrence;
On 13-Nov-09, at 3:08 PM, Terrence R. Redding, Ph.D. wrote:
Good evening Milton,
I find it tremendously helpful to prepare for a new event with a discussion here. It forces me to think my way through the project and I know my results will reflect the greater expertise of the group. In my discussions on the exoplanet group there has been some concern that the star light be confined to the same pixels to minimize the variance from pixel to pixel. Thus I was thinking of guiding.. But with such short exposures it should not be an issue.
I would never do exoplanet hunting on a single pixel, that's just foolish because you can not guarantee the light is always on 1 pixel nor can you guarantee that pixel is a good one (hot pixel, cold pixel or even a stuck pixel that doesn't respond linearlily) . If your scope "wanders" or you get a burst of turbulence, the image drifts onto adjacent pixels which may have different characteristics that you have been sampling. It's better to spread it over multiple pixels so you can average out any pixel sensitivity variances.
The nearby star is an issue if there is a good reason to use the 14" scope. It will not be able to place the selected comparison star in the same field. However, there are a number of visual mag 10 to 11 stars that are within the 14" narrower field. Based on your comments I know the difference in magnitude is an issue. May be you could comment on that - whether there is a practical limit to the difference in magnitude in comparison stars? You mention the mag 6 star at 24', but even that is an issue.
That's only about 2 magnitudes fainter (6.25 times) than zeta Aurigae and you should be able to use it as a check star. See my graph for my Eros
http://www.outcastsoft.com/AstroImages/Eros_V2_20090919_MJA.png
was as much as 1.8 magnitudes fainter than Eros was, so that's in the similar ballpark ranges.
I would still us Hoedus as my Constant star.
So I appear to be left with the tradeoff between a significant magnitude shift in comparison stars, or using a much smaller aperture scope to work with more comparative stars. I assume there will be a much higher signal to noise ratio with the smaller scope, and that will present its own set of problems.
Why not shoot 100 images (likely a few seconds of elapsed time) and see what you get? It's not like you need to have a huge set up and shoot flats, darks get guiding like one needs to image Mag 15 asteroids.
Since this is a 30 day campaign I wonder how often I should collect data. Two or three times a week seems to be the standard. But I wonder if there is something more to be learned by doing doing a daily, or every few hour sample? I am considering twice daily with the star an hour before zenith and an hour after zenith at each observation to attempt to hold the atmospheric variance to a minimum. But any strategy you might recommend would be appreciated.
I don't know enough about the stars expected light curve behavior to venture a guess as to what's appropriate for sampling. You'll have to do some research.
I have found a better app for charting magnitudes that AppleWorks though:
http://www.appdonkey.com/appDetail.php?app=Plot
It allows me to flip the vertical (i.e. -ve mags are higher than +vew mag, which appleWorks won't do) among other things and works better with the graphing.
HTH..
Milton J. Aupperle
