Sidereal time mismatch

[OneCosmos]

Hi,

I am using Pothub to connect to my dome and my EQ8 Pro and when I connect it complains that the sidereal time of the mount does't match the computer - to the tune of over 12 hours (-44,000 seconds!).

I found an old post online that said Pothub uses the sidereal time from the mount or, if the 'Quiet' checkox is checked it calculates sidereal time from the PC clock.

I have checked that box and so now Pothub has the correct sidereal time. EQMOD is still 12 hours wrong. Pothub has a button to update the scope, but it doesn't let me - just complains about latitude, longitude and Data/time changes.

I thought EQMOD also calculated sidereal time from the computer. I know the synscan hand controller metadata is stored in the Windows registry so perhaps at some point erroneous values for sidereal time found their way there and somehow were never overwritten. The hand controller does, however, show the correct sidereal time.

How can I force EQMOD to recalculate based on the PC time?

Cheers,

Chis

[OneCosmos]

I have an update on this.

I now believe I know why the problem occurred. I used to use a USB GPS unit and at some point it connected but didn't acquire data and just used some defaults. I must have not noticed and just clicked accept The latitude and longitude would have been zero but more importantly, the local sidereal time and GPS sidereal time together with a delta were generated. On that day, (or perhaps the last time it was working), that disparity was 4 minutes, (which equates to about 1 degree of longitude).

Ever since then the sidereal time calculated by EQMOD has been massively wrong - not just 4 minutes actually but 12 hours for some reason.

In POTH there is a checkbox called Quiet Scope which, if checked, means POTH calculates the sidereal time from the PC clock otherwise from EQMOD.

When I click quiet mode my LST is more or less right but has the 4 minute delta. EQMOD is completely wrong. For some reason I am unable to click the Update Scope button in POTH - I get some silly error like "Latitude change error, proceeding" and the same for longitude and date/time.

I have almost resolved the problem now but not quite. In EQMOD when you click the GPS button in site information rather than connecting to a GPS unit you can actually just type in to all those fields. Doing this I was able to set the local sidereal time in EQMOD to match the value POTH calculates from the PC clock - albeit still with the 4 minute error.

I tried adding 4 minutes to the GPS ST and specifying a delta but it didn't seem to use it.

Today I will buy a new GPS unit and just see if that fixes everything but I somehow thing I may not because the problem may be a setting once set and now always used in the registry somewhere. There is an offset perhaps being applied to calculation of sidereal time from the PC clock.


All very interesting but damned annoying!

Chris

[Occulta]

Chris

Before you make the new GPS purchase, download VisualGPS View here (http://www.visualgps.net/)

Works well and is a good GPS check.

Chris

[OneCosmos]

Further Update.

Thanks for the link for the VisualGPS. I have downloaded it and will take a look but getting the location information is no problem - using it with EQMOD has been the problem.

I have been trawling the Yahoo EQMOD forum and did a search on Local Sidereal Time and found a post which said you could just edit the eqmod.ini file which is stored in your profile in My Documents\Ascom. In that file there is a TimeDelta flag which you can set to 0 to hopefully eliminate the problem I am having whereby the calculated sidereal time by both POTH and EQMOD is wrong to the tune of 4 minutes.

Now, here's the interesting thing. On my laptop, that I used to use in the observatory I do have an eqmod.ini file but on the desktop I now use there isn't one. I even tried reinstalling EQASCOM v128k but it still has not produced such a file.

Anyone know why?

Chris

[AndrewJ]

Gday Chris

Have you tried using the file search routine in explorer to find it????
I couldnt find the files under documents in my W7 for a while
but after doing a search, i found the files under
users\<me>\AppData\Roaming\EQMOD

Also, an LST error of 4min is damn close to having the wrong
local day for the conversions.
ie maybe something isnt using a correct timezone????


Andrew

[Occulta]

Chris

Just had a look at my planetarium with EQMOD in simulator mode and both LST's were concurrent.

I then changed location using the options settings [simply changing E to W] and the LST was considerably different as it should be. I agree with Andrew's comment regarding a location mismatch.

This leads me to the GPS issue. If you haven't used the GPS for a while the almanac will be outdated. I suggest putting the unit outside and letting it acquire as many satellites as it can and then allowing it to run for at least 20 minutes to update the almanac.

Also check your location details in your planetarium and computer match.
You can also check your LST here (http://tycho.usno.navy.mil/sidereal.html)

Chris

[OneCosmos]

Thanks guys. I am now down to just 12 seconds out! Not perfect yet and I can't actually see why.

Essentially, because known file type extensions were not being shown it didn't find the ini file but %appdata%\eqmod did. I changed the TimeDelta to 0 and that corrected the LST to within 12 seconds.

I have ensured that the PC clock is correct against an internet clock (so accurate as a GPS would be) to the second, so it isn't obvious why there should be a 12 second offset. I have asked Chris Shillito on Yahoo group.

FYI I ma not using GPS at the moment because whilst my unit connects on a com port it doesn't seem to want to receive data.

Chris

[OneCosmos]

One further update. I connected the synscan controller and put in the same lat and long and used a LST app on the iPhone to set the time to the second and even that is out by 8 seconds. I guess there is likely to always be a slight delta because Brisbane, and my specific suburb and indeed garden will be not be precisely on the +10 time zone point?

Chris

[AndrewJ]

Gday Chris

Playing with time is fun :-)
Your timezone is mainly used when converting between UTC and local time, it doesnt care where "exactly" on the planet you are.
LST is based on the GST, and adjusted by your longitude.
ie, If you have obtained the correct UTC time, then the LST calcs only utilise your longitude, so how accurately is that entered???.
Also, ( just for fun ) can you retry your test tomorrow morning.
Up until about 11:00AM local, we are one UTC day ahead of England and the US. If your LST calcs are being done via the net, and they are using a UTC day of yesterday ( relative to us ), you may still get back the 4min error.
I got caught on something like that when morning tests failed, after lunch tests worked :-)

Andrew

[OneCosmos]

I'm not using the Internet because at the moment the desktop in the observatory has no access at all - that will change soon I hope.

"Accurate" UTC was set manually by setting the exact second whilst comparing it with an Internet clock from the mobile. Obviously it would be absolutely spot on to a fraction of a second, but it is accurate to within one second for sure.

Longitude is an interesting one too because in EQMOD you can be very precise - hours minutes seconds and fractions thereof, but in POTH it really doesn't seem to care about seconds.

I did try with my synscan hand controller which again only provides a means to type in hours and minutes for the lat and long and that was 8 seconds off proper LST too. I'm guessing total precision is not possible and, as you say may depend on the time of day and I am also guessing your absolute longitude which may not be exactly where you tell EQMOD you are simply because it doesn't allow you to be totally precise.

The only final thing to figure out now is why I can't use the Update Scope button in POTH when I know someone with exactly the same mount and software versions can.

Chris

[AndrewJ]

Gday Chris

Well 4 RA seconds is roughly 1 arcmin,
so the reduction in precision for entering longitude
could account for a +/- 4second error in LST straight away.

The rest is probably just down to latency in the actual processing of the time setting routines ( as well as the precision of the Hbx calcs etc ?????
All good fun

Andrew

[Occulta]

Chris

Instead of using POTH, try running the simulator from the telescope selection drop down and see if that works.

Also what planetarium program are you using??

Chris

[OneCosmos]

Can do but I am using POTH because I have a Sirius dome so I need to connect to the EQ6 ASCOM driver in that and then everything else can connect to POTH.

I only use Cartes Du Ciel, but frankly that isn't in the picture here because I really ever bother to use it to get coordinates to put in to Sequence Generator Pro and then I slew/plate solve to the objects from with that. When I was doing the LST tests CDC wasn't even connected. Nor was anything else for that matter.

Chris