From: Milton Aupperle <milton@outcastsoft.com>

Date: November 13, 2009 12:03:15 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Guiding - Observing Campaign on the Secondary Eclipse of zeta Aurigae


Hi Terrence ;


Unless your mount is exceedingly poorly aligned and your RA motors are close to broken, you won't need guiding for a Mag ~ 3.7 star like Zeta Aurigae. You'll likely be shooting at 20 to 40 ms exposures to avoid saturating the detector above 80% (204 out of 255) range with your C14 and the DMK. You don't want the star to be completely white with pixel values of 255 or measuring the brightness will be poorly accurate.


The real challenge is that there are going to be no good near by comparison stars for doing accurate differential measurements within the narrow FOV of a C14 and the 640x480 DMK camera. At Prime focus on a C14 your field of view will be about  3.4 x 2.3 arc minutes and there are no stars brighter than mag 10 near it. And to pick up the Mag 10 star means you need to grossly over expose Zeta Aurigae and that ruins the measurement accuracy.


You get good accurate results if your comparison stars are within about 3 magnitudes (+/- 1.5 mag) of your target, and there are simply no stars even close to that magnitude in the FOV of the C14. The nearest Mag 6 star is Hip 23511 and it's 24 minutes away from Zeta Aurigae. The nearest Mag 3 star  (which would be ideal for comparison) is Hoedus II and it's 46 arc minutes from Zeta Aurigae.


If you stuck a 175 mm focal length lens on the DMK, you would have a FOV of about 68 minutes by 51 minutes and could fit Hip 23511, Hoedus II and Zeta Aurigae into the same FOV. That would give you one constant and a check star to work with. And with a 175 mm focal length, your guiding requirements are minimal. Even on a cheap alt azimuth tripod with no guiding, you can do exposures of up to 0.5 seconds before there would be 0.5 pixels of trailing visible at 175 mm focal lengths. It would also take 5 minutes before Zeta Aurigae drifted outside the FOV.


Defocusing the star slightly (no enough so the donut is visible with a CAT type scope) is better for photometric measuring, as then light is spread over more pixels which gives a better statistical averaging than a single pixel does. I've gotten good results using 2x2 binning too, as long as your stars do not saturate.


And keep the brightness / gain levels at minimum levels. Gain increases pixel variability and at high gains, you get poor accuracy. I've already posted numbers showing what the increases of gain does to pixel variation noise on August 31, 2009 to the Yahoo Groups (subject was "Re: Colour Jupiter from Ray") for a DMK21 camera. It's pretty scary once your past 600 levels.


HTH..


Milton J. Aupperle



On 13-Nov-09, at 10:38 AM, Terrence Redding wrote:



I have a few days of clear skies this weekend and thought I would try participating in an AAVSO campaign.  The campaign is described here:


http://www.citizensky.org/forum/mini-observing-campaign-secondary-eclipse-zeta-aurigae


This requires periodically collecting accurate photometry of the star over the next 30 days.  Has anyone done this with Astro-IIDC and a DMK21BF04 camera.  I assume I will need guiding and plan to use a DSI II with PHP for that.


All suggestions and comments greatly appreciated.