brooke@pacific.net said: > The telescope has around arc second pointing capability so I think he needs > a hundredth of a second or slightly better. That doesn't sound right. What's the field of view of the telescope? > I've found some NMEA software that will set a PC clock, but it expects the > data on a COM port. Don't know how it might work with USB data. I don't use Windows, but most likely it will magically show up as COMx when you plug it in. >> It is not clear whether the message is fixed of if it will reply >> to queries. Normally, NMEA devices send a clump of "sentences" each second. Usually, you can turn off the ones you don't want. -- These are my opinions, not necessarily my employer's. I hate spam.

Hal Murray wrote: > brooke@pacific.net said: > >> The telescope has around arc second pointing capability so I think he needs >> a hundredth of a second or slightly better. >> > That doesn't sound right. What's the field of view of the telescope? > > Worst case (declination = 0), 1 arc second of pointing is equivalent to about 1/15 (~67ms) second of time. The telescope field of view isn't really relevant (as long as its greater than 1 arcsec) if one needs to maintain 1 arc second pointing. In this case a timing accuracy of 10ms isn't overly conservative. If one merely needs to ensure that the telescope can acquire the object of interest so that it is well within the field of view of an eyepiece, then 1 arc second pointing isnt necessary. However if one needs to place the object of interest directly on a spectrometer slit or fibre then a somewhat lower pointing error is desirable. To achieve 1 arc second pointing accuracy usually requires correcting for such effects as Tube flexure The orthogonality error between axes drive train eccentricities The value of dUT1. etc One also requires high resolution encoders (or equivalent on each axis). Alternative a stellar compass system like those used to gage the orientation of spacecraft can be employed. Despite what Meade would have one believe the application of such devices to telescope pointing did not originate with them. > >> I've found some NMEA software that will set a PC clock, but it expects the >> data on a COM port. Don't know how it might work with USB data. >> > I don't use Windows, but most likely it will magically show up as COMx when > you plug it in. > > > >>> It is not clear whether the message is fixed of if it will reply >>> to queries. >>> > Normally, NMEA devices send a clump of "sentences" each second. Usually, you > can turn off the ones you don't want. > > > Bruce

Hi Hal: FOV is 0.2 min x 0.3 min @0.47arcsec/pixel when using 1x1 binning. The seeing doesn't often justify 1x1 binning. Mount is the Paramount ME. http://www.bisque.com/help/paramountme/performance_specifications.htm There's a software package called TPoint where you manually point to known stars and that data is used to fit a model of the common mount errors and so correct them. For example if the RA and DEC axis are not at exactly 90 degrees. After doing many dozens of stars this correction (and the PEC correction) allow making images without a guide star for say 10 minutes, something that's impossible on a Meade or Celestron type setup. The scope is a Planewave 17": http://www.planewave.com/index.php?page=1&id0=0&id=1 Camera is SBIG STL-11000M http://www.sbig.com/sbwhtmls/online.htm Have Fun, Brooke Clarke http://www.PRC68.com Hal Murray wrote: > brooke@pacific.net said: > >> The telescope has around arc second pointing capability so I think he needs >> a hundredth of a second or slightly better. >> > That doesn't sound right. What's the field of view of the telescope? > > > >> I've found some NMEA software that will set a PC clock, but it expects the >> data on a COM port. Don't know how it might work with USB data. >> > I don't use Windows, but most likely it will magically show up as COMx when > you plug it in. > > > >>> It is not clear whether the message is fixed of if it will reply >>> to queries. >>> > Normally, NMEA devices send a clump of "sentences" each second. Usually, you > can turn off the ones you don't want. > > > >

OK, after dropping the amount of coin on the mount, scope, and camera, (my guess in the neighborhood of $40,000 US) a few hundred bucks for a timing solution shouldn't be too bad. Would an external NTP box, perhaps a Soekris Net4501 driven by a Thunderbolt or another GPS receiver with PPS work? My Windows boxes typically track my Soekris NTP server within a few ms, my Macs to about 1 ms, and my FreeBSD boxes to about 30 microseconds. Ralph On Nov 28, 2010, at 7:50 PM, Brooke Clarke wrote: > Hi Hal: > > FOV is 0.2 min x 0.3 min @0.47arcsec/pixel when using 1x1 binning. The seeing doesn't often justify 1x1 binning. > > Mount is the Paramount ME. > http://www.bisque.com/help/paramountme/performance_specifications.htm > There's a software package called TPoint where you manually point to known stars and that data is used to fit a model of the common mount errors and so correct them. For example if the RA and DEC axis are not at exactly 90 degrees. After doing many dozens of stars this correction (and the PEC correction) allow making images without a guide star for say 10 minutes, something that's impossible on a Meade or Celestron type setup. > > The scope is a Planewave 17": > http://www.planewave.com/index.php?page=1&id0=0&id=1 > > Camera is SBIG STL-11000M > http://www.sbig.com/sbwhtmls/online.htm > > Have Fun, > > Brooke Clarke > http://www.PRC68.com > > > Hal Murray wrote: >> brooke@pacific.net said: >> >>> The telescope has around arc second pointing capability so I think he needs >>> a hundredth of a second or slightly better. >>> >> That doesn't sound right. What's the field of view of the telescope? >> >> >> >>> I've found some NMEA software that will set a PC clock, but it expects the >>> data on a COM port. Don't know how it might work with USB data. >>> >> I don't use Windows, but most likely it will magically show up as COMx when >> you plug it in. >> >> >> >>>> It is not clear whether the message is fixed of if it will reply >>>> to queries. >>>> >> Normally, NMEA devices send a clump of "sentences" each second. Usually, you >> can turn off the ones you don't want. >> >> >> >> > <23Apr2010Pauls.jpg>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.

I'm new on this list but This is something I know a little about. Pointing a telescope acuratly is more then just knowing the time. The air refracts, or bends the line of sight depending on the angle from zenith and no matter how strong the structure of the telescope sags under it's own weight. I'd not bring this up except that I know there are people here concerned about very small errors. The solution is a closed loop system. You measure the location of a star on the image plain periodically and compute it's motion and then send small correction signals to the motors driving the scope to drive the star's apparent velocity to zero. In general if things are set up well you can measure a star's location to about 1/10th of a pixel because the star is blurred and you can fit a function. There is more to it but you get the point, you only need to know the time well enough to center an object in the field of view. Typically 15 arc seconds of very good. For that you need to know the time to just better than one second. I can get that without owning a GPS receiver using NTP servers on the Internet. For other uses you might need much better time. For example high speed photometry (occupations) knowing current time to 10 ms would be good Some people will record audio from WWV simultaneous with the optical data. If you already have NTP as 1 ms youhave what you need already On Sun, Nov 28, 2010 at 6:37 PM, Ralph Smith <ralph@ralphsmith.org> wrote: > OK, after dropping the amount of coin on the mount, scope, and camera, (my guess in the neighborhood of $40,000 US) a few hundred bucks for a timing solution shouldn't be too bad. Would an external NTP box, perhaps a Soekris Net4501 driven by a Thunderbolt or another GPS receiver with PPS work? My Windows boxes typically track my Soekris NTP server within a few ms, my Macs to about 1 ms, and my FreeBSD boxes to about 30 microseconds. > > Ralph > > On Nov 28, 2010, at 7:50 PM, Brooke Clarke wrote: > >> Hi Hal: >> >> FOV is 0.2 min x 0.3 min @0.47arcsec/pixel when using 1x1 binning.  The seeing doesn't often justify 1x1 binning. >> >> Mount is the Paramount ME. >> http://www.bisque.com/help/paramountme/performance_specifications.htm >> There's a software package called TPoint where you manually point to known stars and that data is used to fit a model of the common mount errors and so correct them.  For example if the RA and DEC axis are not at exactly 90 degrees.  After doing many dozens of stars this correction (and the PEC correction) allow making images without a guide star for say 10 minutes, something that's impossible on a Meade or Celestron type setup. >> >> The scope is a Planewave 17": >> http://www.planewave.com/index.php?page=1&id0=0&id=1 >> >> Camera is SBIG  STL-11000M >> http://www.sbig.com/sbwhtmls/online.htm >> >> Have Fun, >> >> Brooke Clarke >> http://www.PRC68.com >> >> >> Hal Murray wrote: >>> brooke@pacific.net  said: >>> >>>> The telescope has around arc second pointing capability so I think he  needs >>>> a hundredth of a second or slightly better. >>>> >>> That doesn't sound right.  What's the field of view of the telescope? >>> >>> >>> >>>> I've found some NMEA software that will set a PC clock, but it expects  the >>>> data on a COM port.  Don't know how it might work with USB data. >>>> >>> I don't use Windows, but most likely it will magically show up as COMx when >>> you plug it in. >>> >>> >>> >>>>> It is not clear whether the message is fixed of if it will reply >>>>> to queries. >>>>> >>> Normally, NMEA devices send a clump of "sentences" each second.  Usually, you >>> can turn off the ones you don't want. >>> >>> >>> >>> >> <23Apr2010Pauls.jpg>_______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- ===== Chris Albertson Redondo Beach, California