Synchronising UTC across my system.

[MikePaling]

In the next week or so I am hoping to start the initial set up my new DDM60 Pro. I have never used a computerised system before this and I am treading into uncharted waters here! After studying the manuals extensively I "think" that I know what to do for the most part .... but I have already hit a hurdle involving understanding the synchronisation of UTC across the whole system!

 

I am planning to control my DDM60 Pro with a dedicated stand alone laptop along with Carte Du Ceil planetarium software and of course Autoslew.

 

So ... a couple of very basic questions:

 

(1) Am I correct in thinking that the whole system (ie: laptop, mount & all software) the time is synchronised from the clock on the laptop?

 

(2) Is it best to set my laptop clock to UTC .... or can it stay on British Standard Time? If I leave my laptop on BST will Autoslew and Carte du Ceil convert my PC clock setting to UTC as required?

 

(3) I know that accurate UTC time can be obtained from a USB GPS receiver or can be re-set manually on my laptop. However, on the internet I have just come across a free configuration utility called "Atomic Clock Sync" which appears to enable me to set my laptop very accurately to UTC time. Does anybody know if this works? Is it worth downloading?

 

Any help on any of this would be very much appreciated.

 

Regards Mike

 

 

[w0mbat]

Hi Mike,

There is a function in the Autoslew menus called "Get time from GPS" or similar wording. I believe that function will give you the most accurate time synchronisation. Once you have selected that then each time you start Autoslew the PC time will automatically be synched to the GPS time. I believe that the GPS method will give better accuracy than any form of internet synchronisation. In order to work properly a GPS receiver must have very accurate time.

Regards

Ian

[tbaehnck]

Hi Ian,

 

are you sure that it always synchronize with the gps when you start autoslew?

I thought it will be synchronized  just one time if I click on it. There is no check mark or something similar.

 

Best regards 

Thomas

[RamaSpaceShip]

Hi Mike,

 

"Atomic Clock Sync" is just a configuration utility to set hidden values for the Windows time service.

 

The Windows time service uses NTP (Network Time Protocol) to adjust the system clock from a NTP time server. You need to choose a time server close to you like "uk.pool.ntp.org". NTP is used worldwide since decades, time servers are in sync with atomic clocks, and the protocol is robust and accurate. You have to sync before your astronomic operations begin, and not in the middle as this can confuse Autoslew.

 

 

Best regards.

 

Bernard

[Waldemar]

If you want to use GPS, you will have to obtain to ASA GPS stick.

 

Best regards,

Waldemar

[w0mbat]

Hi Thomas,

I have asked the same question but I never got an answer. My reason for believing it does work each time Autoslew starts is that if the GPS is not working or has not found sufficient satellites then an error appears in the Autoslew dialogue saying something like "unable to get accurate time because GPS cannot see enough satellites". The wording may be quite different to what I have written as I am going from fairly long memory but it is something like that.

Yet another example of something that should be in the documentation but is not.

Maybe what Waldemar says above is the explanation as I got the ASA GPS with my mount. However I was unaware that there was anything special about it but maybe there is or maybe different GPS sticks need to be read in a different way by software???? Who knows...I certainly don't.

 

As for using the internet time synchronisation protocol, I reckon there is an element of risk. The risk is that you never know if there has been a momentary variation in the transmission time across the net from the time server to your computer. I know that the service attempts to correct for transmission delay but that can only be totally accurate if transmission times are constant which, because of the way the internet works, they cannot be. Maybe I am being pedantic but I have seen this effect when I have time synched two computers one after the other and then found their clocks to be different by a number of seconds. This is probably more of a problem for me than others as I rely on satellite internet. Nevertheless it will be true of all internet connections to some extent. So my question is why rely on the internet method if you can get the time from a GPS as the GPS time MUST be very accurate or the GPS positions will be way out.

Ian

Edited August 22, 2016 by w0mbat

[Jay]

Hi Thomas, 

 

I'm pretty sure the GPS time is synchronized every time when starting the Autoslew. As Ian said, you'll get an error notification right away if the GPS is not working, and then it keeps checking the module every couple of minutes...

 

What comes to different GPS modules, I've always been using the basic GPS module without any problems ( http://usglobalsat.com/p-688-bu-353-s4.aspx#images/product/large/688.jpg).Can't comment about the accuracy because have never compared it to anything else...

 

Jay  

Edited August 23, 2016 by Jay

[robertp]

Hi,

 

I can't check it at my astro computer right now, so I have to rely on my not so reliable memory:

I'm quite sure that in the time-settings (mount - time setting or similar) there's an option that says something like "synch on each slew".

It's advisable to use a GPS and not fully rely on the PC clock as those clocks can deviate over time significantly.

I once had severe problems with tracking and after several tests and discussions with Philip Keller, I realized that the internal clock of the laptop I used at that time lost more than 30 seconds within an hour. In autoslew, one can measure that deviation and correct for it, but I would really recommend to use a GPS instead. To my understanding, it does not matter whether you use the one from ASA or any other.

 

Best regards,

 

Robert

[astrosirius]

Hi

 

Another option in case that you don't have a GPS is to install this kind of softwares, in my computer works perfectly with Windows 7 32 bits.

 

http://www.flexiblesoft.com/time-synchronization/absolute-time-corrector.html

 

Clear Skies

Luis.

[RamaSpaceShip]

Hi all,

 

 

It's advisable to use a GPS and not fully rely on the PC clock as those clocks can deviate over time significantly.

I once had severe problems with tracking and after several tests and discussions with Philip Keller, I realized that the internal clock of the laptop I used at that time lost more than 30 seconds within an hour. In autoslew, one can measure that deviation and correct for it, but I would really recommend to use a GPS instead.

The GPS (nor NTP) cannot do anything in this situation as it only allows for a one shot synchronisation (say before each slew), and not for continuous time getting during the exposures. Autoslew uses the system clock to keep the mount on target, and if the clock deviates significantly, then you have a big trouble that needs to be fixed. You can setup a NTP server on your lan to determine the deviation of your PC clock after one hour.

 

The choice of the GPS or NTP is up to you. Even on high latency internet lines (like satellite ones), the accurracy is sufficient, as a shift of a few seconds is completely corrected once a star is synched (it is a deterministic error).

 

Best regards.

 

Bernard

[maxmirot]

Buy the GPS it cost only 30 dollars to get the best info.

[Jay]

Hi all,

 

The GPS (nor NTP) cannot do anything in this situation as it only allows for a one shot synchronisation (say before each slew), and not for continuous time getting during the exposures. 

 

Is this a fact or an assumption ? ...very important thing.

 

I only know that if the GPS signal is not found when starting Autoslew, it will try connecting to the GPS module every couple of minutes. It would be odd if that procedure would stop after finding the GPS signal once  ...or only before every slew. Why would Autoslew be design in that way, especially when the time component is so critical ?

 

Jay

Edited August 25, 2016 by Jay

[RamaSpaceShip]

Hi Jay,

 

The GPS is not a clock, it only provides time reference from time to time. Getting time from a USB GPS takes several milliseconds while the loop that controls the ASA mount has a 5 milliseconds period or less. The accuracy of the GPS time cannot be easily better that a few milliseconds when Autoslew requires less than a millisecond. So no, it is not possible to use the GPS as a real time clock with a sub-millisecond resolution. This is a fact.

 

In addition GPS time has some jitter effects and needs several time values from satelites to be accurate: see http://catb.org/gpsd/gpsd-time-service-howto.html

 

The PC real time clock is accurate. Its deviation is about 1 second per day, which is good enough for astro-photography (it means 0.2 arc second RA error after a 20 minutes exposure). Getting time from the PC clock takes less than a microsecond. But as any device, it may (very rarely) be broken.

 

Autoslew needs to know what time it is before a slew, in order to determine the target position, and it uses the GPS for that purpose. Once this position is reached, it needs a real time clock to stay on the target, and it uses the PC real time clock for that. They are the best available devices for these different goals.

 

 

Best regards.

 

Bernard

[Jay]

Hi Jay,

 

The GPS is not a clock, it only provides time reference from time to time. Getting time from a USB GPS takes several milliseconds while the loop that controls the ASA mount has a 5 milliseconds period or less. The accuracy of the GPS time cannot be easily better that a few milliseconds when Autoslew requires less than a millisecond. So no, it is not possible to use the GPS as a real time clock with a sub-millisecond resolution. This is a fact.

 

In addition GPS time has some jitter effects and needs several time values from satelites to be accurate: see http://catb.org/gpsd/gpsd-time-service-howto.html

 

The PC real time clock is accurate. Its deviation is about 1 second per day, which is good enough for astro-photography (it means 0.2 arc second RA error after a 20 minutes exposure). Getting time from the PC clock takes less than a microsecond. But as any device, it may (very rarely) be broken.

 

Autoslew needs to know what time it is before a slew, in order to determine the target position, and it uses the GPS for that purpose. Once this position is reached, it needs a real time clock to stay on the target, and it uses the PC real time clock for that. They are the best available devices for these different goals.

 

 

Best regards.

 

Bernard

 

 

Thanks Bernard, and I agree with you that the GPS can't produce sub-millisecond time references. I was just wondering if the Autoslew only checks the time from GPS before every slew and not in between. Tracking one object without a slew can take several hours...

[MikePaling]

Hi everybody ...

Many thanks for all of the comments that have been sent in, so far, after I posted my original question.

 

After looking at what has been said I have made a few decisions ...

(1) I will be setting my laptop to UTC times (rather that British Summer Time ... BST) even though it seems that Autoslew and Carte du Ceil can make automatic adjustments for this off-set.

 

(2)I have decided to order a BU-353-S4 GPS receiver rather than the ASA GPS as it will do the job just as well. I have now made contact with somebody, outside of this forum, who uses the BU-353-S4 without any problems.

 

(3)Looking at the new version of Autoslew .... there is a button/icon which allows it to link to the GPS receiver ... at the minute I am assuming that this is a "one-off" procedure and does not continue automatically subsequently ..... unless I retrigger it again manually. I am also assuming that I can disconnect the GPS receiver, after this initial connection, until I decide to re-sync to UTC at a future date. How often would I need to re-sync???

 

Any comments about my last comment above (Item 3) would be much appreciated.

 

Regards

Mike

[Waldemar]

Hi Mike,

 

I bought the ASA GPS receiver 5 years ago and use it since then without any problems like time issues.

The stick is on top of my mount in a little USB 3 hub (Transcent), which is connected to the internal ASA hub again.

I never encountered any problem with it, so I thought those things just work constantly once switched on,  but maybe I am wrong.

Sometimes, I can see it looses a satalite connection, (i.e. it shows 6 sats instead of 7) which comes back on after a while, so to me it's just working all the time...

 

Waldemar

[RamaSpaceShip]

Hi Mike,

 

 

How often would I need to re-sync???

As the PC clock drift is about 1 second per day, which meens a RA error of 15 arc seconds (so about 15 pixels!!!) on a slew if you don't sync on a star, it seems to me that synchronising once a day is sufficient.

 

Don't forget that a sync on a star is able to compensate the time drift very precisely (the star movement is a very good clock ), and it is wise to do it after each clock sync.

 

Best regards.

 

Bernard

[MikePaling]

Hi Waldemar ...

I had been assuming the GPS receiver (whatever variety) was supposed to plug directly into one of the USB ports on my laptop .... reading your comments, just now, are you saying that it goes into one of the USB slots directly onto the DDM60 mount??? Or can it go in either?

 

Regards Mike

[Jay]

Hi Waldemar ...

I had been assuming the GPS receiver (whatever variety) was supposed to plug directly into one of the USB ports on my laptop .... reading your comments, just now, are you saying that it goes into one of the USB slots directly onto the DDM60 mount??? Or can it go in either?

 

Regards Mike

 

Mike,

 

The USB port/hub in the DDM60 mount is just a hub...and is connected to your computer via the USB cable. It doesn't matter where you put your GPS receiver...same result.

 

Jay 

[RamaSpaceShip]

I was just wondering if the Autoslew only checks the time from GPS before every slew and not in between. Tracking one object without a slew can take several hours...

You can only adjust the time between exposures, otherwise you create a step in the tracking that breaks the image. But Autoslew does not know when you start/end your exposures...

 

In fact, the little drift of the PC clock has a good side effect: it creates an automatic dithering every 100 minutes!!!

 

Best regards.

 

Bernard

[Jay]

You can only adjust the time between exposures, otherwise you create a step in the tracking that breaks the image. But Autoslew does not know when you start/end your exposures...

 

In fact, the little drift of the PC clock has a good side effect: it creates an automatic dithering every 100 minutes!!!

 

Best regards.

 

Bernard

 

I usually take 20 min exposures, so a little drift every 100 minutes would mean that I have every 5th image with double dithering

 

Best regards,

Jay 

Edited August 26, 2016 by Jay

[maxmirot]

This is incorrect.

 

Review Autoslew time settings. The GPS updates the clock as frequently as you wish not just slews, start and stops. I have Autoslew using the GPS to update every 3 minutes. This should be fine for most any exposure.

 

 

Max

[MikePaling]

Hi ...

 

How to do set a GPS to make automatic updates during a session? I can't find this anywhere :-(((

 

Regards Mike

[RamaSpaceShip]

This is incorrect.

 

Review Autoslew time settings. The GPS updates the clock as frequently as you wish not just slews, start and stops. I have Autoslew using the GPS to update every 3 minutes. This should be fine for most any exposure.

 

 

Max

Hi Max,

 

The fact that the PC clock is updated does not mean that Autoslew takes it into account between slews.

 

Anyway, a GPS update every 3 minutes is useless and may have more bad effects that good ones as most applications are not prepared for such clock changes and may react in a weird manner. Such clock updates are highly discouraged (when not completely forbidden) in real time systems, and a PC running Autoslew is one of them.

 

I don't know which benefits you expect from such a high frequency, and I am curious to see figures that show that this is helpful.

 

 

How to do set a GPS to make automatic updates during a session? I can't find this anywhere :-(((

Mike, I don't know as I don't use a GPS, I use NTP instead, and it is configured to update the clock once per day. But if you update your clock with the GPS at the beginning of your session, it is highly sufficient, as no updates during the session are necessary.

 

Best regards.

 

Bernard

[maxmirot]

I hope that Autoslew would not allow  time to shift rapidly during tracking. 

 

How time  updates from GPS or NTP are handled are unknown

No way to tell without asking Dr. Keller. 

 

I am sure it would be helpful have subsecond time accuracy during an exposure for tracking.

 

Syncing at the start on an exposure or session updates the time offset. After that you are relying on the system or other clock source.

Time drift is not desirable during an exposure.

 

 

Max

[maxmirot]

I take it back.

Bring up the time settings menu for Autoslew with GPS it specifically says " Sync Query during tracking for GPS.

You can input the time interval. default is three minutes.

This strongly suggests  the time reference  is update by Autoslew from the GPS during an exposure.

 

Max