GoTo problems - any advice?

ceejay

I've had my Celestron NexStar 4SE since last Christmas and I'm really enjoying using it, but I'm having problems with getting the GoTo function to work well, so I'm looking for advice on what factors I should concentrate on to get it working right.

First the problem: once I have aligned the scope using the SkyAlign method using 3 stars and I then ask it to go to some feature, it slews towards the object but doesn't end up pointing exactly at the object. Sometimes it's within the viewfinder, other times it's not in the field of view at all - so not that useful really! I've tested it with objects like Jupiter, the Moon, stars I know like Betelguese, etc., and it just keeps missing.

Here's what I've done to try and make sure I'm setting the scope up correctly:

• Ensure the mount is level using the circular spirit level that came with the scope
• Ensure the longtitude and latitude are correct based on the GPS status app on my phone
• Ensure the date and time, timezone, and DST settings are correct
• Picked three stars that are roughly triangular arranged, i.e. not in a straight line, and also reasonably separated in the sky

I think the main problem I have is accurately aligning with the stars in the SkyAlign process. I've adjusted my finderscope as accurately as I can but there is always a difference between having the star in the centre of the finderscope and then in the eyepiece. The SkyAlign process always warns at the end that the finderscope is misaligned. Is this the root of the problem? I would have expected that if I get the stars in the centre of the eyepiece that's what's more important for alignment?

How sensitive is the process to aligning the star to the centre of the eyepiece anyway - if I'm off by a little bit would this cause the problem? Does it have to be exactly in the centre to get the GoTo to work properly?

Are there any other factors to consider? On my last session the batteries driving the mount seemed to be fading and that seemed to make the mount behave very oddly - trying to slew to the complete opposite side of the sky for example Would I be better off getting an AC adapter for it if possible?

What about firmware updates? Would they make any significant difference?

Any suggestions?

Plug

Here.

you'll find that the alignment issue is solved via a healthy set-up procedure, perhaps bettered if you perform a EQ North Wedge align but this takes a little longer than the 3 star Alt-Az align.

There may be a bit of help there for you.

ceejay

Thanks for that link. I did try an EQ North Wedge align once, and it didn't seem to make any difference

Some other comments in the first post there that reinforce my own thoughts:

Something I deduced from playing around with SkyAlign is that you MUST use the red dot finder to locate the stars and not the telescope itself. I suspect that this is because the SkyAlign software is based around naked eye objects

I had been advised that although the 4SE mount has a battery compartment for 8x AA batteries its GoTo system is usually erratic with battery power also that it will not function well on rechargeable batteries.

I found on first use the scope was missing its targets by a few degrees

The GoTo was slightly off but I am sure this was due to some fairly rough and ready alignment and levelling. Running a precise GoTo fix I am sure would have generated better results. The scope was off by a consistent amount each time suggesting that perhaps our time data was slightly wrong. Tracking over the limited time the scope was locked onto each target seemed fine further reinforcing my view that errors in the GoTo were most likely induced by a sloppy set-up.

From the 1 year on post:

After the flakey power was resolved the Nexstar hit alignment first time every time and both located objects easily and showed very little in the way of tracking errors

So I think it's worth trying an AC adapter to see if that makes any difference, plus seeing if I can get the finderscope better aligned with the eyepiece.

One of the follow-on comments offers some more useful advice:

Firstly, if you haven't got a reticule eyepiece to centre your selected object, defocus the scope so that the object becomes a large as possible because an extended circle is much easier to centre by eye.

Secondly, make a note of the direction the scope approaches objects; that is look, through the eyepiece, when the scope is doing it's final manoeuvre. Make a note which direction keys produce the same motion in the eyepiece and then use those keys to do your final approach to alignment objects.

So maybe a reticule eyepiece should be added to my shopping list too

dbran

Hi

I usually always found that when I have dodgy or erratic gotos it is power that is the problem.

A goto scope will eat ordinary batteries surprisingly quickly. Often times even though it seems to be slewing OK, because it is not giving enough power to work the encoders properly your gotos will be useless.

Also rechargable batteries are no good because they only give out 1.3 volts per cell instead of the 1.5 volts of non rechargable batteries. So straight away you are only going to get a reduced amount of power even when they are fully charged.

You would better off investing in a power tank or gel cell battery. One of those jump start batteries with a cigarette lighter outlet plug would be ideal. You can get them in halfords or maplins. Even a 7amh one will be able to run your scope all night long without any problems. Just remember to keep it fully charged and charge it up after any use. Even you can afford to get a bigger one do as you will be able to use it to run other equipment that you may get in the future ie dew heaters etc

Hope this helps

Kind Regards


dbran

Popoutman

With the Celestron goto scopes, it's important to always approach the target alignment object from the same direction that the scope performs itsfinal approach to selected objects - I think moving the scope up and to the right every time. If you overshoot, movethe scope back down and left and approach again.
There is some play in the gears, and doing the alignment this way ensures that the play is always the same - means it doesn't mess up the intenal calculations.

When centering, get the alignment object close to the centre with a low-magnification eyepiece, then swap to a high magnification eyepiece for the last bit. The centering is important to accuracy. If it helps to defocus the object until it nearly fills the field of view it may be easier to get concentric with the edge of the eyepiece view meaning it's in the center.
An eyepiece with a reticule or crosshairs would greatly help your accuracy.

ceejay

Thanks for all the tips.

I picked up a suitable AC adapter in Maplins which I'll try out as soon as I can. It'll do fine for use in the back garden, and I may invest in something more portable for use elsewhere.

ThatDrGuy

Hey
I also have a nexstar 4se. Same type of problems with slewing. To echo what others have said - level, plug in scope. Someone gave me an eye piece with crosshairs and a red light and it was very very useful. Also make sure its set to daylight savings time and that the seconds are right. Also if copying from car/phone GPS coordinates make sure you dont use decimalised - use minutes and seconds.