From: "Terrence R. Redding, Ph.D." <tredding@mac.com>

Date: November 13, 2009 3:08:30 PM MST

To: Astro_IIDC@yahoogroups.com

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

Good evening Milton,

Thank you for responding.  My mind has been in hour long mutual events, where I control the exposure at the camera and track for long periods of time - last night up to an hour hoping to detect 0.2 changes in magnitude.  Here, as you say, much shorter exposures will be all that is needed.  I have a candidate star for comparison.  It is:

Name: Hoedus II

Catalogue number: HIP23767

Object type: Star

Bayer: Eta

Flamsteed/other: 10

HIP number: 23767

TYC number: TYC2899-2237-1

Apparent magnitude: 3.15

I measure it as 46' 34' from the target star and the DMK21 with Knight Owl focal reducer 0.5X on a small refractor can place both in the same field of view.

I find it tremendously helpful to prepare for a new event with a discussion here.  It forces me to think my way through the project and I know my results will reflect the greater expertise of the group.  In my discussions on the exoplanet group there has been some concern that the star light be confined to the same pixels to minimize the variance from pixel to pixel.  Thus I was thinking of guiding.  But with such short exposures it should not be an issue.  The nearby star is an issue if there is a good reason to use the 14" scope.  It will not be able to place the selected comparison star in the same field.  However, there are a number of visual mag 10 to 11 stars that are within the 14" narrower field.  Based on your comments I know the difference in magnitude is an issue.  May be you could comment on that - whether there is a practical limit to the difference in magnitude in comparison stars?  You mention the mag 6 star at 24', but even that is an issue.

So I appear to be left with the tradeoff between a significant magnitude shift in comparison stars, or using a much smaller aperture scope to work with more comparative stars.  I assume there will be a much higher signal to noise ratio with the smaller scope, and that will present its own set of problems.

Since this is a 30 day campaign I wonder how often I should collect data.  Two or three times a week seems to be the standard.  But I wonder if there is something more to be learned by doing doing a daily, or every few hour sample?  I am considering twice daily with the star an hour before zenith and an hour after zenith at each observation to attempt to hold the atmospheric variance to a minimum.  But any strategy you might recommend would be appreciated.

Tonight the transit is 1:53AM, and in a month it will be 11:55PM, so it would appear data collection would be at about 1 AM and 3 AM to begin with.  And end at about 11PM and 1AM.  But I would end up with a very nice set of 60 points over the 30 day period.

All thoughts greatly appreciated.

Terry - W6LMJ - 14.287 

Terrence R. Redding, Ph.D. 

Redding Observatory South, West Palm Beach, Florida

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

How do amateur astronomers learn?

American Association of Variable Star

Observers (AAVSO): RTN http://www.aavso.org/

On Nov 13, 2009, at 2:03 PM, Milton Aupperle wrote:

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.