From: "cosmicrock2001" <ursamajor_1@mac.com>

Date: August 29, 2010 11:33:50 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: My First (partial accidental) Variable Star


Thanks for all the info, just one or two questions to followup - you mentioned the "size of the inner annulus of the star"...   so you are putting the star(s) slightly out of focus?  I think I've read this in other sources as a technique for photometry. 


Besides that I use AstroIIDC,  I'm particularly interested in your methods and results as we have similar poor seeing conditions and I've not found much info on doing comparitive photometry with poor or not great seeing, mostly just comments that it can be done. Are you using a standard suite of filters or just the V?  


Ron 


--- In Astro_IIDC@yahoogroups.com, Milton Aupperle <milton@...> wrote:


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