GOTO with brightest star. How?

[Guest peepshow]

Could someone please explain how a GOTO tracking system, such as the Skymax synscan, can find and track stars etc when it is only aligned using the brightest star technique?

 

I can see how the two star set up will work but not a single.

How does the GOTO know how it is levelled for example?

Thanks.

 

[dawson]

The pre-requisits i suspect are that the operator has levelled the tripod and scope, starts with it pointing north [i'm not sure this is needed], and correctly enters the date, coordinates and time. All it then needs is the actual position of one star to syn the altitude and azimuth of that real star with the same star in its inbuilt astro globe, then you are away.

Simples.

James

[Brantuk]

Yup - if your tripod is level and the mount is accurately polar aligned - then it only needs one star to build it's night sky database inside the electronics. Of course the more accurate the PA the more accurate the goto will be. Also using more alignment stars (2 or 3) will increase the accuracy. The database is actually already there cos the star positions don't change (it's Earth that's spinning) - it just needs to align the grid using the time, position, and physical start point, for your setup - ie the first alignment star.

[Guest peepshow]

Thanks for quick replies.

Ah, so levelling and polar alignment are still required

for single stars.

That all seems rather bothersome in this computer age.

I suppose for another 100 quid :-) the system

might take care of levelling errors ?

I understand that one can use 3 stars !

I guess that alignment and levelling are not so important in that case?

[Daz Type-R]

Alignment and leveling are very important in all aspects of tracking, if one tripod leg is longer / shorter than the others, you will loose what you need to track.

[Guest Tweedledum]

You can increase the accuracy of a two star alignment by choosing stars at least 90 degrees apart.

[+RonC]

If I am planning to view just one area of the sky, then I do a one star alignment with the nearest star in that area, BUT my scope is on a pier (level) and Polar alignment is checked first! For a general nights viewing I do a 3 star alignment and still check Polar alignment!

[+Ibbo]

for a German Equatorial Mount it does not need to be level

it needs to be pointing at the North Celestial Pole(or South) which is about 2/3 of a degree from Polaris

 

 

on the other hand for a GoTo altaz mount level is important

 

As Ron says it is a good idea if you are not Polar Aligned to use a star in the neighbourhood of your target to sync on

 

Steve

Edited July 23, 2013 by Ibbo

[dawson]

I'd assumed the original question was about the alt-az synscan goto mount, not an equatorial mount and i think being level is important.

I can see the logic about not bothering to level an equatorial mount if it is polar aligned (as the main axis, the polar one, is now fixed), but i've not tried to use it unlevelled as everywhere talks about levelling. Also levelling allows home made marks on the setting circles to be used between setups. Ss far as i'm aware.

James

[+Ibbo]

I'd assumed the original question was about the alt-az synscan goto mount, not an equatorial mount and i think being level is important.

I can see the logic about not bothering to level an equatorial mount if it is polar aligned (as the main axis, the polar one, is now fixed), but i've not tried to use it unlevelled as everywhere talks about levelling. Also levelling allows home made marks on the setting circles to be used between setups. Ss far as i'm aware.

James

Thats what I said

 

Steve

[dawson]

No, that was what i said

Jd

[+Ibbo]

All I'm trying to pass on is that a EM does not have to be level to achieve proper polar alignment.

 

Steve

[dawson]

Steve, i can see how polar alignment can be achieved when the tripod isn't level, but will a one, two, or three star alignment work well if the tripod is very lop-sided, or once it's polar aligned the GOTO can cope with anything? I wanted to try setting up sometime with a lop sided tripod and seeing how it worked, but i've never got around to it.

James

[+Ibbo]

yes it will work

 

I never use a level when setting up my mounts, OK  the pier did have a spirit level waved at it but only so it looks right.

 

it is a myth that an EM needs to be level

 

it is a throwback to photographic tripods and the wedge mounted SCT's ( which are alt az mounts tilted  anyway)

 

as long as the polar axis is pointed at the NCP  the setting circles or goto system will have no difficulty working.

 

this always assumes that the pier/tripod is stable and does not fall over.

 

The software allows a certain amount of correction and the 3 star alignment with one star being on the opposite side of the meridian corrects(in some part) the cone error.

 

Another system allows for literally as many align points as you wish to take carry of the errors of the mount machining etc and refines the "pointing model"

 

 

Steve

Edited July 23, 2013 by Ibbo

[dawson]

Steve, so do you think it matters where the scope is pointing at the start of say a three star alignment? Some say it must start at the home position, but from what you are saying, if the mount is polar alined, and you show it where the three alignment stars are, then it will work everything else out for itself, irrespective of it's starting position.

James

[+Ibbo]

in some mount software (not mine) there are some slewing limits that you cannot disable, so you cannot slew into the mount or go over the meridian ( like my small mount) that makes it almost a law that you start at the home point.

 

 2 cases in point

 

1 when I get my large mount PAed again I will only do one star sync-in fact I cannot create a pointing model in the current  planetarium program that I use without getting pinpoint add on

 

2 same mount when I went to Kielder star party I had to put a brick under the N tripod leg as the alt is nominally set to 50 degs-it has been milled since so I think it would be OK at 60

 

 

HTH as I'm not sure I'm explaining this well in type

 

Steve

[dawson]

Yes. Thank you.

When i'm rich i'm just going to have my scope permanently mounted in my obsy so all this won't be an issue

I should get a lottery ticket for friday now.

James

[+Ibbo]

yes you should

it also means you are prepared even if you only get an hour or 2

[Perkil8r]

I never concern myself with leveling any more than by eye. Polar aligning though I am rather picky about even with guiding I still like i as close as I can get it.

[philjay]

This is what Ive been saying for years Steve. Ive seen so manynewcomeres wasting valuable dark sky time levelling and getting the North leg exactly aligned, then they tweak the head adjusters and ...... what was all that levelling about then????

 

Also I don't really approve of manufacturers sticking spirit levels on EQ tripod/heads, my CGE a case in point, all this does is compound the myth that it must be level. It does come in handy sometimes as I know roughly that when the level is OK Im somewhere near on the altitude adjuster.

Lets face it the eq mount is a dirty great big universal joint really so the RA can be pointed anywhere.

 

As for dead PA being important to good goto..... well it helps yes but isn't essential in my experience. When doing visual with the HEQ5 I just plonk it down, look through the polar scope and if polaris is somewhere in there then I don't muck around with the mount adjusters. I then go straight into a 3 star calibration and alls well with the goto. However tracking is affected because of the PA. Swings and round abouts

 

But for visual its OK and keeps the object in the fov for ages, even if the object does wander, theres buttons on the handset called left and right

[+RonC]

Well said Phil, fully agree!

[Daz Type-R]

Wow, you learn something everyday, thanks chaps.

[dawson]

Even Dion goes on about levelling the mount.

The home position cannot be accurately identified though i presume if the mount it not levelled. Dion talks about marking the setting circles with a pen to show where the home position is once the mount has been levelled by working out the precise positions of the RA and dec using a spirit level; these markings can't then be correct if the mount is subsequently mot levelled.

But how important is it to know exactly the home position. When i first set my mount up at wymeswold, phil just spun the scope round so it was roughly square and i started the alignment from that position and got excellent results.

Interesting to think levelling is a waste of time.

Good discussion, thanks all.

James

[+Ibbo]

Well he is wrong so was the guy who did the tutorial in Astro Now the other month

 

why waste the time and effort something that is not needed, they are overcomplicating things KISS is my motto

 

Why is the the home position so vital-you tell the mount where it is when you push the align-sync or whatever its called on your mount

 

 

I am more extreme than Phil when doing visual I don't look through the polar finder

 

 

 

Now when I'm imaging DSO's its is different I do take a bit of time PAing-but not all night

Edited July 23, 2013 by Ibbo

[Perkil8r]

I use a level on the scope it's self once PA'd so I can find the home position, only so it puts the scope closer to the target during the 3 or so star align. I don't waste time leveling the mount though. PA takes me about 5 mins including getting the home position sorted. I wouldn't want to waste any more time than that though.