From: Milton Aupperle <milton@outcastsoft.com>

Date: August 29, 2010 12:04:03 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Re: My First (partial accidental) Variable Star

Hi Ron;

On 29-Aug-10, at 9:18 AM, cosmicrock2001 wrote:

This is a really neat bit of work Milton.  Given your SN error is slightly greater than the expected dip, its no surprise it doesn't clearly show up in the data.  I presume you used an uncooled TIS or similar 'video cam'.

Nope. I was using the Grasshopper 16 (14 actual) bit and cooled to 9°C, which was about 10 °C degrees below ambient that night. That was before I did my tune up up on the Grasshopper cooling system. For each 10°C drop, I get a reduction in background noise by about 50%.

Do you think a cooled ccd camera with better signal, less noise would do better under the conditions you had?

The issue that caused the high S/N was turbulence (which blurs the star erratically)  thin haze and a full moon. My sky background for 90 second exposures that night was 10,000 to 12,000 out of 65,535. With no moon, it should be about 1,000 to 2,000 out of 65,535. I was using a IDAS Light Pollution Reduction filter too on the C8 at 1100 mm focal length.

The S/N error is basically a comparison of signal to noise and since the moon and city illuminates the haze, this raises the background noise  and  that decreases the signal ratio. Also the size of the inner annulus you use for the star makes a difference too. To big and your brightness estimate is including Sky background which raises the variability. To small and your excluding the light from the star, especially if you live under the "Jest Stream" and turbulence moves things all over the place.

In indoor star bench mark tests, my S/N error is +/- 0.002 mags (+/- 2 millimags), which is pretty much raw CCD error and in the same ball park as SBig gets. Out on the scope on a darker night with moderate turbulence, I get errors in the +/-0.03 to +-0.015 mag range. The only way I can probably do any better is to likely add a Adaptive Optics unit which should reduce the FWHM error by about half.

And what source do you use to select your 'transit star' and others for comparison?   I haven't tried the diff photometry feature in AstroIIDC yet, but hope to in the future.

For finding exoplanets to image:

http://var2.astro.cz/ETD/index.php

is a phenomenal resource. It has forecasts for future events that is really good.

For selecting your comparison stars, I use Starry Night Pro 4.5 and then download the stars to around 19th magnitude. However what is missing in SNP is the ability to look at the star color ratio (B-V) to figure out what star colors your comparing too. You want to use stars of a similar B-V so that the air mass light drop doesn't cause more changes than your target actually has or you have to adjust the brightness for them afterwards. Many stars do not have B-V measured to 19th mag, so I don't know if this is a limitation of SNP or other software.

Exoplanet milli magnitude measuring is very tough to do. Asteroids or brighter variable stars only need 0.05 mag precision in most cases.

HTH..

Milton Aupperle