From: Milton Aupperle <milton@outcastsoft.com>

Date: August 30, 2010 12:57:28 AM MDT

To: Astro_IIDC@yahoogroups.com

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

Hi Ron;

On 29-Aug-10, at 11:33 PM, cosmicrock2001 wrote:

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.

Nope. I've read about defocussing, but I don't do that (my seeing does that automatically) You can adjust the inner annulus around the star when you set up your differential analysis in Astro IIDC. If the seeing is "better" I use around a 1.2 times the FWHM. if it's worse or variable, I'll use 1.5 times the FWHM.

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?

So far, I have not experimented with using standard filters. I have just been shooting with the Luma filter. Even with a luma filter, my C8 needs about 90 second exposures for a mag 11 to 12 target. So with narrower filters, I'd need much more exposure time.

There isn't much info on people doing photometry with poor seeing. Even people in Arizona use Adaptive Optics and get their seeing down below 3 arc seconds, which is 2 times of what I get on a average / good night.

Bruce L. Gary has a really great description of all the steps needed for do milli magnitude exoplanet imaging:

http://brucegary.net/book_EOA/WebEOA/ExoplanetObservingAmateurs.html

I really like his introductory quote of :

"One last Philosophy I’ll mention is: WHEN YOU SEE SOMETHING YOU DON’T UNDERSTAND, WHILE OBSERVING OR DURING DATA ANALYSIS: STOP, DON’T PROCEED UNTIL YOU UNDERSTAND IT. This one is probably difficult to making a convincing case for unless you’ve ignored the advice and wasted time with fundamentally flawed data or analysis procedure. "

I think that can applied to anything to do with Astronomy.

HTH..

Milton Aupperle

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

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