GOTO precision?

[GrampyG]

Hi

I hope this is not a silly question but how precise should the image fall in the eyepiece view (26 mm) with a GOTO scope? Would one always expect the object to appear within the field or would it be occassionally slightly 'off'?

I've just received a Meade LS 6 and have spent some time 'setting it up ', e.g., training drives/making certain the scope/eclipse camera are centred.

I am new to this other than having very limited experience many years ago with an ETX125 so I hope this is not a silly question.

Many thanks

[DougM43]

I can only speak for my own go to scope, (meade lx 90) but once aligned, it's very accurate, both in finding objects and tracking, pretty much centres everything in a 20mm eyepiece.

[GrampyG]

Thank you for your reply.

I've been fiddling with the scope for a couple of weeks when the sky was clear. I have had problems with the automatic alignment system. It appears to align the 2 stars reasonably but then when asked to go to, say Jupiter, it is out of the field of the 26 mm eyepiece.

Last night, in freezing wind!, I started with a perfectly level tripod and calibrated the eclipse finder camera with the scope. I think this has improved things in that virtually everything I looked for in the 'Best of night' menu appeared within the eyepiece although in some cases distinctly not central.

If it clears tonight-and the wind drops-I'll give it another go but I do intend to persevere. I must admit that I am leaning towards an 8 inch scope as I really do enjoy (and did in the past) looking at the sky. Unfortunately I cannot find anybody in my locality who can help: the astronomy clubs are some distance away.

Many thanks for your input. It gives me a guide, and is very much appreciated.

[redmoo]

I used to have problems with my scope, although it wasnt the scope that was the problem, it was me! 

Completely different set-up mind (skywatcher goto), but one thing that caught me out a few times was the date and times that I was entering into the handset. The date has to be in American format and the time in 24h format, although usually the scope would go off on a little journey, pointing itself directly at the floor or something crazy!

How far out of the field are the objects you are looking for? Jupiter is a good example because that is easily visible without a scope. Could it be something as simple as the incorrect lat/long in the handset? 

With 2 star align on my HEQ5, it still doesnt go right to the center of the view, it is always a little to the side.

[michael8554]

As I understand it, the Autostar is pants at finding planets and the moon, as their positions are very dynamic, whereas stars are mostly "stationary".

You should get stars and DSO's in the FOV of a 26mm eyepiece if  your setup is good.

Michael

[ronin]

The LS6 is fairly good.

As with every goto it will have a start position and in general the closer to that the better.

Meades seem to have software that is based on the thing being level, so start with it as level as is reasonable.

It finds North itself.

As it has GPS it gets it's own Lat/Long and Time.

On Meades there is a Sync function that helps the accuracy, most useful at the initial targets.

Will say not sure it is on or applicable to the LS6 - try the manual.

Logically if you said goto M45 you would expect to see M45, but M45 is about a degree in size so what is the RA/Dec for M45. It is not a point as is a star, where does the scope point at ??

[GrampyG]

Thank you for your advice. The next suitable viewing session I will set the tripod up absolutely dead level and see what happens. I will also use the 'synch' feature as suggested.

[Northern Soul man]

Hi Ya Grampy and a very warm welcome to the SGL mate.  The GOTO functions (there a little different for me as I only have Celestron's) are always a little bit "here or there" but as Michael says - the Planets/Moon have a massive "range of movements against the "fixed" star positions - so much harder to predict - so I just tend to stick with the Auto 2 star align on my CG% EQ mount - just by leveling the mount with the tiny bubble level and "sighting" Polaris through the hole in the mount where the Polar scope should fit and centreing Polaris this way - Its just fine for me for visual and gives me a very accurate alignment - by the way I use a 40mm Plossl for my alignment routines and just "guess" when each star is in the centre of the FOV before moving onto the next alignment - just by using 2 stars the mount is really accurate, using 4 additional calibration stars, It only takes a minute or two - even the calibration stars are smack bang in the middle of the FOV.

Just going back to what I said about the GOTO's being a little here or there - I sent the scope from the Owl nebula in Ursa over to M35 the other night - the motors were whurring away, I walked away just to go and get another EP, the mount then stopped - so I thought that it would be at M35 - it was - but the actual tube assembly (I was using the 127 Mak) was pointing the wrong way up - with the dew shield pointing at the floor and the back of the scope where the diagonal and EP fits pointing sitting at the other end from where it should be!! - so on this occasion the GOTO had WENT - so a quick turn off and re - align only to hibernate and go indoors for a warm for a few hours - then back out - woke the mount up - sent it to Jupiter and it was a little Off, once centred, tracking was spot on for the next hour or two.

So the GOTO functions are great most of the time - but still need a little HUMAN intervention now and again.

Paul.

[ronin]

As Paul has mentioned Planets are a little different.

The stars and nebula have defined positions (RA+Dec) so the scope is at (a, and told to go to (x,y). The planets move and so are calculated. The scope then moves to the calculated position.

They sit "almost" on the ecliptic plane but there are variations and the movement is not constant, Mars, Jupiter, Saturn will start to move backwards at one time or another. It would probably help if the orbits were circular but they are elliptical just to throw more complexity into it all. And the processor in the scopes are not 32 or 64 bit machines with 4 Gb of memory. More likely 8bit and 4K of memory.

So you can easily find that going to a planet, or from one to a star the position can be out.

[Ouroboros]

'Usually in the eyepiece' just about sums up my GOTO experience with my AZ-EQ6. Seldom near the centre. More centred if moving between objects in the same part of the sky. More misaligned, even outside the field of view, if moving a long way to a new object. I just assume this is normal. Is it not?

It's not usually a pain because mostly I know what I'm looking at. It's only if I don't know which of the objects in the eyepiece is the one I'm looking for.

A trick that works there is to GOTO a nearby bright star, centre on that, and then GOTO the object I'm interested in. It's more likely to be near the centre of the FOV.

[GrampyG]

Many thanks to you all for your replies and especially your advice, knowledge and expertise.

Your input put my mind at rest in relation to the potential variable performance of goto's - all your experience and expertise is fully appreciated. It appears that there is certainly some variation. I just wish that there are some astronomers close to me.

Thank you all.

Oh, just had a 'senior moment' but don't tell the misses - it's a secret. Just picked up a CPC 800 from those great people at FLO in Exeter.

[Northern Soul man]

Many thanks to you all for your replies and especially your advice, knowledge and expertise.

Your input put my mind at rest in relation to the potential variable performance of goto's - all your experience and expertise is fully appreciated. It appears that there is certainly some variation. I just wish that there are some astronomers close to me.

Thank you all.

Oh, just had a 'senior moment' but don't tell the misses - it's a secret. Just picked up a CPC 800 from those great people at FLO in Exeter.

OOOOOOOOOH YOU LUCKY THING YOU!!! - Nice 1 Grampy, I think its like everything Tech nowadays - everything's not perfect even though the adverts say different - you will really enjoy the CPC - I think its got to the stage now where GOTO (in which ever form on whichever mount) is the norm for a lot of Astronomers - it would be really nice if we all had really dark skies but I have just learnt to live with the light pollution and just get on with it.

I couldn't believe when I got back to the scope the other night and it was pointing the wrong way around, the previous target was only a few degrees away from the next target - so not sure how the mount ended up like that and I've done a couple of Factory Resets since owning the mount - as have we all.

Your going to love the CPC - the range is very nice, for me, being mainly visual, it took me a long time and a lot of researching and searching on the net to settle on my current SCT and have never looked back.

Let us know and keep us informed of the trials and tribulations of the 800 - very nice scope.

Paul.

[Davey-T]

Couple of points, I think if you do a "sync to star" it overrides any previous alignment, generally I GOTO a star near where I'm planning on imaging then centre it and sync on it, after that it GOTOs pretty accurately, but mine is permanently mounted.

Dave

[GrampyG]

Hi Paul,

Thank you for your comment. I hope somebody doesn't notice that there is a new tripod in the garage. 'Dog' and 'house' come to mind!

I have tinkered with astronomy for many years - on and off - but in no way do I have the expertise of all those that have kindly replied in this forum. I have some experience of, let us say, 'equipment 'in that I have a small laboratory of my own for analytical purposes over the last 40 years. That is my profession - analyses.

I have read so many good reports of the CPC 800 that a mad moment decided that I would get one. Just now hoping that I get some good 'seeing'.

This forum has been so helpful, especially looking at all the past advice, that this is lead me to really get involved now with astronomy. As stated before I just wish there were astronomers much closer to me which could give me help and advice. But in this forum I now know where to get it.

Thank you all once again for all your help and advice-I can assure you it is much appreciated!

Regards

[Ouroboros]

Couple of points, I think if you do a "sync to star" it overrides any previous alignment, generally I GOTO a star near where I'm planning on imaging then centre it and sync on it, after that it GOTOs pretty accurately, but mine is permanently mounted.

Dave

Is this facility available on Skywatcher Synscan?

[jambouk]

Yes, it's called PAE: it's in the manual. I think, from memory, you press and hold escape, it names what it thinks should be in the centre of the fov, you press enter (if it names the right thing, else scroll through other near by things) and then it says "centre in fov" which you do with the handset, then press enter again and it re-maps that part of the sky with relation to the object. You can do it for 88 regions of the sky, but generally if you are staying in one wuarter of the sky doing it once per quarter will massively improve your goto accuracy. Next time you boot the mount and do a new star alignment, all the previous PAE entries you did will be wiped, and you'll have to do them again.

James

[Ouroboros]

Yes, got it! Pointing Accuracy Enhancement in the Advanced Functions section. Thanks.

[acey]

Can anyone quantify this? How many arcminutes error might one generally expect?

[YKSE]

Can anyone quantify this? How many arcminutes error might one generally expect?

Don't know how other mounts' precisions, my experience with EQ-3 mount is that it is about the square root of sums of error squares in DEC and RA, i.e. if DEC error is 10' and DA is 5' after alignment, I can expect error at about sqrt(10*10+5*5)=11'. I usually do 2-star alignment, avoiding meridian flip when choosing alignment stars, and trying to get both DEC and RA errors under 20' (difficult to get both under 10'), viewing objects without meridian flip will be surely in the 1 deg finder eyepiece.

The error is bigger with meridian flip, I usually redo the alignment.