From: "Milton Aupperle" <milton@outcastsoft.com>

Date: July 25, 2009 12:27:09 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: Photometry on an asteroid 790 Pretoria


Hi Terrence;


The main issue I have found with light pollution and accurate photometric work is inconsistent sky backgrounds. If your skies stay consistent you can likely do better than I have indicated. But not if the sky brightness (clouds, haze, smoke, water vapor or anything that will be lit up by the surface lights) changes erratically. With aperture photometry,  the background levels determine cut off levels for apertures and ranges for counting pixel brightness.


The shorter your exposure time, the less likely that the background becomes a factor too.


So you may want to experiment on some dark clear night and see what you get for results.


HTH..


Milton Aupperle


--- In Astro_IIDC@yahoogroups.com, Terrence Redding <tredding@...> wrote:


Very interesting.  I just spent an enjoyable hour on your website and  

reading about the cooler on your camera.  I have Mag 2 skies here in  

West Palm Beach most clear nights.  I check by walking out my front  

door and looking at Polaris.


I checked a few minutes ago and there was a band of clouds across its  

location - but blue below that - so I am hoping it will clear. I just  

checked and Vega and Deneb can be seen but not Polaris.  But the night  

is very young here and I suspect it will get much better.


You raise a few very good points.  I may never be able to do exoplanet  

work from here because of the city lights.  Though I was hoping with  

filters to do some work that is independent of city lights - we shall  

see.


Thanks for the help.  I plan to make it an early night and then get up  

at 4 AM for my first effort with Jupiter.  I am anxious to give it a go.


Cheers,


Terry - W6LMJ


Terrence R. Redding, Ph.D. RTN

http://olt.net/learningstyle/Site_2/Learning_Style_Research.html

How do amateur astronomers learn?





On Jul 24, 2009, at 7:05 PM, Milton Aupperle wrote:


Hi Terrence;



On 24-Jul-09, at 4:26 PM, Terrence Redding wrote:


It appears I can skip the DSLR color camera and simply use Astro- 

IIDC to gather a periodic set of frames of the asteroid for the  

highly sensitive photometric work.  I am currently bumping up  

against a .2 mag change limit with my occultation setup.  How  

sensitive might I get with the DMK 21?


The sensitivity is somewhat camera dependent (high inherent gains  

like your occultation setup make it very poor for accurate  

photometry) , but it's also sky dependent (turbulence and  

transparency) and how careful you are with tracking, darks and flats  

- which I mentioned in the Astro IIDC starting at  on page 52,  

starting with the line "To do reasonably accurate (< 0.1 magnitude  

variance)....". I also have a set of url links to organizations like  

AAVSO which discuss it in more detail too.


Eventually i have to be able to achive the .02 range for exoplanet  

work.  Might I do that with Astro-IIDC?


Yes, but I doubt if your going to get that low with an 8 bit  

uncooled camera. I gave mention of what I can achieve using a C8 and  

a Grasshopper 16 bit cooled camera an example as far as accuracy  

goes. At the dark site, I wasn't even doing Flat frames and was  

getting +/- 0.01 to 0.03 error magnitude accuracy just using a IR  

Cut filter, instead of one of the UVBI required filters.  If your  

skies are really dark and transparent, you can probably go to 0.02  

or lower, which is what I was getting last summer in my initial  

tests. The worse your light pollution and lower your transparency,  

the bigger your error factor is.


Since my last move, my skies have improved a lot (generally Mag 4 or  

lower), but there is barely 2 hours of real darkness during my  

summer (will be 14+ hours come this winter) so doing qualitative  

measurements is tough right now.


HTH..


Milton J. Aupperle