April 2009 - Utah-Astronomy - mailman.xmission.com

Re: [Utah-astronomy] Omega Centauri (NGC 5139)
by daniel turner 01 Apr '09

01 Apr '09

Jim: How early in the year depends on who late you want to stay up. How late depends on how deep into the evening twilight your willing to try. Since Spica is at about the same Right Ascension as Omega Cen I try for it visually every time Spica is on the meridian to the due south. I've seen it visually as early as February and as late as Memorial Day at latitude 41 North. DT --- On Wed, 4/1/09, Jim Gibson <jimgibson00(a)yahoo.com> wrote: > From: Jim Gibson <jimgibson00(a)yahoo.com> > Subject: [Utah-astronomy] Omega Centauri (NGC 5139) > To: "Utah Astronomy" <utah-astronomy(a)mailman.xmission.com> > Date: Wednesday, April 1, 2009, 6:07 AM > It looks like Omega Centauri NGC 5139 > will reach it max Alt for the most part of April of about 2* > 13.5’. Does anyone have better data? > > Jim > > > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy(a)mailman.xmission.com > http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy > Visit the Photo Gallery: http://www.slas.us/gallery2/main.php > Visit the Wiki: http://www.utahastronomy.com >

1 0

Solar limbo
by Dale Hooper 01 Apr '09

01 Apr '09

How low can it go (not an April Fool's joke) http://science.nasa.gov/headlines/y2009/01apr_deepsolarminimum.htm?list1611…

1 0

Journey into a black hole
by Chris 01 Apr '09

01 Apr '09

Movies, animated GIFs, brief explanations: http://jilawww.colorado.edu/~ajsh/insidebh/schw.html

1 0

Re: [Utah-astronomy] Utah-astronomy] Surface Brightness
by Jim Gibson 01 Apr '09

01 Apr '09

Dang! Kurt, you are a rare find. Your wrote: > B the above equation is not the same as surface brightness expressed in magnitudes per square arcsec.< When I saw my outcome of 20 for surface brightness and Berry was saying 4*10 to the minus 6 I knew I was talking apples and oranges; but I didn’t know how to convert apples to oranges. >From your suggestion I did down load Covington’s calculator and played with it. I am really in tune with you on this comment, > reviewing the math does help to gain some understanding of how things work.< and that is how I feel. I like to see and feel the relationships as expressed in math to see how things work. So, Covington’s calculator is nice, but you have already got me creating my own calculator, and that is just to see how things work. I can use Covington’s calculator as a check though. His calculator says that I need 25 minutes of exposure time for M1. I can mess with my calculator with the formulae you provided till I get in the ball park. Speaking of ball park, Yogi once said, “theory and practice are the same in theory, but not in practice.” Thanks Kurt Clear sky images Jim

1 0

Re: [Utah-astronomy] Utah-astronomy] Surface Brightness
by Canopus56 31 Mar '09

31 Mar '09

Jim wrote; > Now that I have the surface brightness, all I have to do is stick > it into this formula: E = (Pdesired/ Pmax)*(N^2/SB) Where B is the > surface brightness you gave me and S is the camera ISO, N is the > focal ratio of the scope, and Pmax is the bit depth of my camera 4096 > and with this information it will give me the approximate integration > time for what ever object I want to image. . . . . > E = (Pdesired/ Pmax)*(N^2/SB) Since giving a first reply, I checked out page 122 in Berry's HAIP 2d. With the equation you referenced, I now see where you are trying to go. You want to have an estimate of the first integration test exposure for a CCD camera using the photographer's exposure time equation: E = N^2 / ( S*B ) where N is the focal ratio and S is the ISO film speed and E is the exposure time in seconds. For most CCD chips, the equivalent ISO speed is around 1000 to 1200. B the above equation is not the same as surface brightness expressed in magnitudes per square arcsec. In the photographic brightness system, surface brightness (B) is measured in a somewhat outdated English measure of candelas per square foot or footcandles. That is the units which appear in the table on page 122 Berry's HAIP (2d), e.g. 35E-6 or 3.5E-5 for M1. The term (P desired / P max) is added by Berry to adapt the photographer's basic equation for the desired well capacity exposure on CCD chip. Before getting further into this, I have spent some time in the past looking into whether there was a good algorithm to estimate the time of the first exposure for use with CCD cameras. Even after polling high-end imagers on various newsgroups, the answer was that nobody really does it, since taking a first integration exposure on a CCD camera is so quick and easy. Since you can quickly look up the amount of well-capacity that is exposure from your first test exposure and the response of CCD chips is linear, estimating the second desireable exposure test time is a simple ratio computation. E2 / WC_Desired = E1 / WC_1 E2 = ( E1 * WC_Desired ) / WC_1 There are commercial products that will do initial estimate for you. Others in the group here can post there preferences. (There was a digital camera exposure software that has been mentioned in Utah Astro several times, but I did not note down the name.) Here are some online calculators: Covington's Exposure Calculator (win app download) http://www.covingtoninnovations.com/astro/astrosoft.html Mike Roger's Online Implementation of Covington's Calculator http://www.rphotoz.com/astrophoto/expcalcs.html Starzonia Exposure Calculators http://starizona.com/acb/ccd/calc_ideal.aspx When I did this a few years ago, I considered this a "dry well" exploration, but reviewing the math does help to gain some understanding of how things work. The missing component in your system of equations is to convert surface brightness in mpsas to surface brightness in the photographic system. Covington's _Astrophotography for Amateurs_ Appendix A gives this as: B ~ 2.512^(9.0-m") where m" is the surface brightness of the object in magnitudes per square arcsec. For the Crab Nebula, Clark gives 20.6 mpsas as M1's surface brightness, which gives about 2.3E-5 or 0.0000228 - in the ball park of Berry's 3.5E-5 footcandles. Covington's book includes Appendix B which gives typical values for photographic B and mpsas for a variety of planetary objects, the Moon, DSOs and urban to rural skies. His tables also suggest exposure times. In addition to the photographer's basic photographic equation - that measures surface brightness in footcandles - there is also the old EV-LV system, which fell out out of favor with automatic exposure meters built into SLRs. For example, a typical rural dark sky with a ZLM of 6.0 mags has an mpsas of 20.7, a photographic surface brightness of 0.00001 footcandles and an Exposure Value (EV) of about -15. None of the above gives a particularly accurate method of predicting initial exposure time. Early this year, I started looking at how professional astronomers use radiance (W m-2 str-1) or spectral radiance (W m-2 str-1 micron-1 ) of DSOs to estimate an exposure time for CCD cameras and spectrographs. Unfortunately, I got stumped at the lack of a catalogue that gives radiance values for commonly observed DSOs. Hope the above helps and is not more confusing than the light it sheds. Clear Skies - Kurt

1 0