Latitude and longitude with EQ mounts

[jambouk]

How important is it to get these even remotely accurate when using a GOTO handset like the Skywatcher Synscan on an EQ mount?

My [very] limited understanding is that they help the handset work out what stars will be visible to the observer given the time and date also input by the observer, to help make useful suggestions when undertaking a star alignment. But once the star alignment is done, the mount knows where the celestial pole is (assuming it is roughly polar aligned), and following a star alignment the handset has mapped the sky. Does it then need to rely on the latitude and longitude for anything further?

James

[Steve Ward]

As you may have seen in numerous threads ... the more accurate the data that you give the mount the more accurate the Go-to set-up procedure will be , start introducing errors at stage 1 and it will go steadily downhill from there ... 

[Gazabone]

They don't need to be fantastically accurate. Using Starsense, I can't seem to find a way of telling it my exact coordinates and have tell it a city. The nearest one to me is 50 odd miles up the road but the goto seems pretty accurate

[jambouk]

Thanks both. I appreciate everyone talks about the need for this data to be reasonably accurate, but why? What part of "its" algorithms use this data other than to suggest what stars might be visible to the observer?

Gazabone - although you are coping without perfect coordinates, there are various free app for phones which will give you GPS coordinates, but if no signal and you know where you are going to be setting up (roughly) there are numerous websites which convert a dropped pin on a map to lat and longitude.

James

[Owmuchonomy]

How important is it to get these even remotely accurate when using a GOTO handset like the Skywatcher Synscan on an EQ mount?

My [very] limited understanding is that they help the handset work out what stars will be visible to the observer given the time and date also input by the observer, to help make useful suggestions when undertaking a star alignment. But once the star alignment is done, the mount knows where the celestial pole is (assuming it is roughly polar aligned), and following a star alignment the handset has mapped the sky. Does it then need to rely on the latitude and longitude for anything further?

James

I see what you are saying when you align each session. In my situation though it is critical. This is because after each session I 'park' my scope and as requested by the handset, power down. Next session, as long as I put in a very accurate time and Lat/Long (actually it remembers the latter) then it will go straight to my requested target. No star alignment is required.

[jambouk]

Owmm, yes, I appreciate that, but if on your first set up you put in the totally wrong lat and longitude, but still managed to do a successful star alignment, then parked it, and next time used the same incorrect lat and longitude but entered the correct date and time, would the GOTO still work?

James

[Tinker1947]

i would go with the more accurate the better scheme after all you only do it once, so unless you move home its sorted out until the mount dies....

[jambouk]

Tinker, agreed.

But what I am trying to ascertain is how the latitude and longitude data is used in the GOTO algorithms

James

[Owmuchonomy]

Owmm, yes, I appreciate that, but if on your first set up you put in the totally wrong lat and longitude, but still managed to do a successful star alignment, then parked it, and next time used the same incorrect lat and longitude but entered the correct date and time, would the GOTO still work?

James

No I doubt it would at all. I will try it for you if you are really concerned but I know it won't work.

[D4N]

I would have thought that the star alignment is only to tell it where the RA / DEC axis are currently set and correct for altitude not to correct for lat long errors.

Sent from my iPad using Tapatalk

[Viewmaster]

The stars will always be in the same relative position to each other no matter where you are.

But the handset needs to know the lat/long of your location because these coordinates tell the system where the

stars are in the sky at any time.  

If I am in London, say at midnight, any particular star or set of stars will not be in the same position

in the sky as someone, say in Glasgow at midnight. The lat/long has changed, not the stars.

Hence the lat/long is required.......approx 70 miles gives about 1 degree difference

 in the stars.

Once these 2  coordinates are entered into the handset it remembers them, so you no need to

worry about them again.............just have fun.

[Tinker1947]

Tinker, agreed.

But what I am trying to ascertain is how the latitude and longitude data is used in the GOTO algorithms

James

Sorry i am just a simple woodworker, now if you want to know how to make a "half blind dovetail"  i might be-able to help.......

[BritAngler]

If you have Google Earth, putting the mouse pointer over a location will give its latitude and longitude (and height above sea-level).

[dph1nm]

Must be needed to calculate the position of the Moon ...

NigelM

[jambouk]

Thanks all. I don't want to appear ungrateful (i'm not) but you've all missed what I'm trying to get at.

I know how to find my lat/long and how ones position the the earth relates to what will be visible at that particular date and time.

What I am trying to ascertain is once a star alignment has been sucessfully undertaken, and assuming the mount is polar aligned, does the GOTO algorithm ever again use the lat/long data, or does it then just use the sky map which the star alignment has picked and just keep rotating that around the presumed celestial pole?

I might just set my mount up and have a go.

James

[jambouk]

Must be needed to calculate the position of the Moon ...

NigelM

If the star alignment has taught the mount where three stars are, say, then it knows their RA and dec coordinates and could slew to them. The database will effectly have the RA and dec for the moon at any given moment so could just slew to those coordinates without using latitude and longitude data... But this is what I'm trying to find out. Does it do that, or does it access the lat/long data after the star alignment has been undertaken.

James

[Viewmaster]

Set everything up in London, alignment, time, lat/long and  then take the whole lot to Glasgow.

In Glasgow do a good PA and set up the date and time correctly and then try to find the moon or anything else.

Every sky object will be approx 10 degrees out.

QED.

[jambouk]

Right, i've set my mount up and experimented.

I pointed the RA roughly north. I entered the lat and long for new york, but entered the date, time, elevation for where i am (in wales at the moment). Starting from the home position i told it to do a three star alignment. The first star it suggested was vega. It slewed to where it thought vega should be if i was in new york (i checked roughly with skysafari and it was correct); i slewed the RA on by 5 hours to account for the time difference between wales BST and New york (EST), where the mount was pointing for the new position of vega correlated with where skysafari thought vega should be from my location in wales, just above the northern horizon. Enter. Then it suggested deneb and slewed to where deneb should be in wales (2 hours east of vega and a bit different in dec). Enter. Then it suggested capella and again slewed to where it thought capella should be as viewed from wales right now. Enter.

Then the handset unsurprisingly said "Warning... RA offset >45 degrees". The next screen said "Please check time and location settings". The next screen said "alignment failed". Pressed enter a few times and go into the main menu.

Using GOTO it then perfectly (as far as i could tell) would slew to anything in its database and point to it as if i were viewing it now from wales. So this answers the first bit of the question for me. Once a star alignment is done, whether the handset says that alignment was sucessful or whether it failed, it uses that alignment and it does not appear to use the latitude and longitude again when making GOTO manoeuvres, it just uses its star map you've aligned it to and the rotation of that around the celestial pole.

So then i told it to park in the home position. It did, went straight to the home position and told me i could turn it off. So i did.

Re booted 30 seconds later, selected start from home position, left the lat and long as new york, it had kept the date and i re entered the current time. All seemed good. Then told it to GOTO vega... Started slewing but stopped when it reached vega as seen from new york! Interesting. So when starting from a parked position, it does again draw on the lat and longitude data, which seems odd as i'd have thought it would have used the star map again and just rotated the last one it remembered around the celestial pole to the appropriate point in time.

So thank you all for your comments, i think these tests answer my question.

James

[jambouk]

Viewmaster, i totally agree with that, but that is not the question i was asking, but thank you. I think i've got to the bottom of what i was trying to ascertain (above) and i apologise if this wasn't clear to everyone else

I think my take home message is, as long as you are not parking the scope, it doesn't matter what latitude and longitude you input into the handset, as long as you can undertake an accurate star alignment and centre each star as accurately as possible (may have to turn the advanced suggestion filters off first else it will suggest stars to align on which may not be visible to you), the GOTO will subsequently work whether the handset says the alignment was successful or not. I suspect along the same lines, the actual date and time are unimportant, unless it has algorithms inside which is taking into account the drift over many hundreds of years of the various celestial objects in the night sky, as it will just track at sidereal time irrespective.

James

[jambouk]

No, even after turning off the handset advanced filter option, it only presents you stars on which to align too, which it thinks are visible above the horizon given your co-ordinates and date/time.

But again, it wouldn't matter if you were a few weeks out and/or a few hours out as long as you weren't parking it, as long as you centred the named star(s) accurately when aligning.

James