Getting Started

Hi @ allI would like to help to get on to the track of of eps auri. Therefore I just wondered if someone does have any experience with a C8 + DSI kind of equipment combination. Is the DSI suitable for that kind of task? What exposure time would you suggest as a first guideline or rough calculation? RegardsLars

Hi Lars,I use a C-8 with my PMT UBV photon counting unit. It works well. I have also experimented with both the DSI Pro and DSI Pro II with CCD Photometry. While they work well and I used them on a 12" LX-200, they will not work on a C-8 or any telescope for observing epsilon Aurigae. The reason is they are too sensitive and epsilon Aurigae is too bright. What I have found is buying a Mogg adapter and using the filter slide with photometric filters with a 50 mm camera lens works well. Let me know if you are interested in going that route and I can rovide more details. Foget about using the C-8, however.JeffCounting PhotonsHopkins Phoenix Observatory Phoenix, Arizona USAphxjeff@hposoft.com

It is worth trying with your existing equipment. De-focus a little bit to spread the light out over more pixels. Also, use a green filter which approximates the V band, or visual band filter. This may have come with your DSI or you may have a color unit, in which case you would have to extract the green pixels with software. Another option would be to get a green filter on front of it.One forum you might try for further advice is the DSLR forum which is addressing similar questions. Try a range of exposure times from 2 seconds on down to the shortest exposure available. I think that Brian Kloppenborg is possibly creating a tutorial about CCD based photometry that may help you out a lot. Once you get to the stage where you need to reduce data, you might check out a free software called Iris (www.astrosurf.com~buil/us/iris/iris.htm) Unfortunately, it may not work with mac....I hope you have good luck and use your equipment to make some observations of Eps Aur.Naomi

Hi Lars, I fully agree with Jeff, C8+DSI should be fartoo sensitive for a bright star like eps AUR. Let consider some numbers: I use a long focus tele-lens with a 50 mm clear pupil and a 450D. Any CCD or CMOSsensor saturate about at the same level ( ~900 e- per micron square of silicon) Ido apply a verylarge defocus and trail on ~200Bayer's cells that'sabout 30 x 30RGGB pixels !! At zenith under good sky condition I shall limit the exposure to6.2 seconds to avoid saturation at the lowest sensitivity (ISO100). To reach an overall integration of the scintillation of 4 minutes I have to accumulate 40 shots. With a C8 ( 200 mm pupil) you get 16timesthe photons I get. That means you will have to make exposure as short as 0.4 seconds and some total of about 600 shots to integrate the scintillation properly !In addition a DSI(if I remember well) is a monochrome CCD, without any filter, by the way the sensitivity would be about 3 times my RGB CMOS one... Eventually you should get to ~0.13 second exposure and near 2000 shots !! Probably not easy to process ? It's clear our usual processis not adapted... One possibility is maybeto process it like we do planetary imaging:stacking with RegiStax at high speed, and then making the photometry with IRIS.A new way to explore...Jeff's proposal is certainly less risky. Another issue is the narrow field on the sky you would reach withsuch smallCCD and a telescop, difficult to find the proper comparisonstars that are essentialtoachieve a good photometry. Yours truly, Roger

Hi Lars Where did you get the 4 min of total exposure time from ? To bet the scintilation noise donw to what level ? Did you make own experiments ? There is a series of papers on scintillation noise on ADS (with different large scopes, at different altitudes above sea level,at differentzenith angles, ... for different expo- sure times). Their formula states that one will effectivly need an indefinit exposure time for apertures getting smaller and smaller towards a pin holes. But they didn't take data for apertures in the range of small amateur scopes, camera lenses and smaller. So the scintillation noise behaviour with small apertures was not detected but just guessed and the formula does IMO notreflect the reality. The scintillation variationscan get 100% with very, very bad seeing where the stars might jump around or even disapear for a very short time. But thisis not the common case as most will not start observing under such bad conditions and they are actually rare. But if one looks at related atmospheric phe- nomena like the flying shadows during solar eclipses, one can see that the observable structures due to scintillation are not indefinite small nor of 100% contrast. Therefore I make the based guess that formula does not relect the reality well for very small a- pertures under good to common seeing conditions. I guess that a total exposure time of 30s to 1 min at max should be sufficient to beat the scintillation noise down below the 1 cmag level even for small aperture / stopped down camera lenses. There is of course nothing wrong with using camera lenses nor with defocusing to beable touse longer exposure times to not to have to take too many subframes to beat the scintillation noise. If there are suitable comp stars in the FOV, one can defocus as much that the star disks/rings of interest do not interfere with other stars and a save photometric measurement can still be made. Some photometry packages have a rejection algo- rithm implemented with the sky annulus, then its also save to havestars in the sky annulus. If there are no suitable comp stars in the FOV or if the star to measure is really very bright (e.g. Venus, Jupiter, or the next galactic SNe ;-) one can still switch to "PEP"- mode. First image the var (defocused up to half the detector hight) and later image the comp/chk stars seperatly. CS WR

Hi, This is Roger not Lars. Sorry I made a mistake due to the fact the message date is nearly unreadable on my screen, thecontrast of thefont being used by CS being far too low ! Lars in fact put that question one year ago ! I suspect he is no more looking at our discussions ? But ok, that subject is anyhowinteresting! I presently use an integration of 4 minutes with an aperture of 50 mm(DSLR and long focus lens) following my own experience after a lot of testing. I effectively started with about one minute but this soon revealed insufficient. In fact I ammaking experiments of various processing improvements(color correction , extinction... ) with the objective ofa couple ofm.mag, not c.mag only. Such experiments arededicated to DSLR specific solutions. I am not involved in PEP or similar devices. In addition I am not observing under observatory conditions butunder urbanconditions, as number of us. An integration ofone minute only clearlyshowednot enough in that context. With the lens I useand for bright stars (3~6 mag) the instrumental noise is negligible. The Standard Error for 4 minutesis thenessentially due to the scintillation. On best days I get SE = 0.001 mag, on most days: 0.002~0.005,on poor days: 0.01 (like today ! )I considersuch a reject.I think this shows 4 minutes is the optimum under mymean conditions, enabling observingas often as possible. I would not be surprised people observing under the superb sky of Arizona could get good results with muchless effort ! But belive me the sky of Burgondy (Dijon, France) is not that good (probably better formaking good wine !) For what concerns the use of a C8 + DSI, 1 minute of exposure would anyhow be something like 500 shots, even with my large defocusing (maybe not so obvious on a small DSI imager )... A lot to process ! Yours truly, Roger

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Thanks!
Blake