How Accurate Is Your GoTo System?

[Shark Bait]

According to the Technical Specifications for my Sky Watcher Dobsonian 12" GoTo Telescope, the Pointing Accuracy is 'Up to 5 arc min'. When new, the accuracy was good to very good, depending on how well setup and alignment was carried out. It was never so far out to cause me to try to measure the difference.

I have now had the scope for just on one year. Over the past few months I have had the occasional session where the pointing accuracy has caused frustration. Last night I had my first chance to run a good test to quantify how much the scope misses the chosen target.

After a very careful setup to ensure the scope was level and the OTA was pointing at True North, I carried out the alignment routine. The result is that pointing accuracy is typically 30 arc minutes (0.5 of a degree) off target. This means that the target is always in the finder scopes field of view but rarely in the eyepieces fov. Some may not be happy with this, but to me this is acceptable as my scope is primarily for visual use. The tracking is fine as can be seen in these rough test images taken last night:

www.iceinspace.com.au/forum/showthread.php?t=100096 (http://www.iceinspace.com.au/forum/showthread.php?t=100096)

How accurate is your GoTo system and are you happy with its pointing accuracy?

[lepton3]

C11 on CGEM, with a f/6.3 focal reducer, the field of my Atik320E camera is about 10 arc min wide.

With 2-star align + 4-star calibration, the CGEM will reliably put a target within the middle half of the field, so say 5 arc mins.

If I do a Precise Goto, it will be within about 1 arc min.

-Ivan

[Shark Bait]

Thanks for the feedback Ivan. 5 arc min and down to 1 arc min for precise tracking is what I wish for. I am not all that familiar with EQ mounts but I assume that level of accuracy is considered the standard for long exposure astro photography.

[lepton3]

I need reasonable pointing accuracy because I'm usually using a filter and looking for a faint target (eg not visible until at least 30s exposure at 4x4 bin), so I can't aim by eye. And in any case, if I move the camera, I need to re-take the flats, so that is very inconvenient.

Tracking is a different matter. The mount can't really be used for photography without some form of autoguiding. I use a separate guide camera and 50mm guide scope for tracking, and this is satisfactory for my purposes (photometry). 280mm f/6.3 (1.76m f.l.) is not typically what people are using for astro photography.

-Ivan

[mozzie]

hi stu,
my lx200 14" has a f/l of 3556mm so pointing has to be good..and i find it is with 2 arc mins most of the time within 1arc...i do a manuel 2 star align and park the scope after each nights use.i only align every 3 months or so,very happy with it.

[pluto]

I have a SW EQ5 goto. If I have good polar alignment and do an accurate 3 star align then I find the target is always dead centre.
Maybe I'm just lucky :)

[tonybarry]

LX90-10" ACF. 0.5 focal reducer. Imaging at 1300mm. FOV about 17 x 13 arc min.

With two star align (all that Meade offers), I am in the FOV for targets about half the time, within 1 FOV most of the time.

With high precision pointing, I am in the FOV most of the time.

Regards,
Tony Barry

[The_bluester]

I have a CPC925 and given a decent alignment it usually puts goto targets well in the fov even at fairly high mag.

Given a good alignment it stays accurate all night, the best night I have had out of it was a star alignment on the first three objects visible as night fell and it was still tracking and making good gotos at 3am.

[Poita]

On a CGEM with a C9.25 if I did a multi star alignment, and the PA routine it would be on the QHY8 CCD every time. It is surprisingly acurate, but it is a newer one.

[Peter Ward]

Cold poinitng?

PME with a modest T-point model....about 15 arc sec.....which is more to do with the time-base accuracy.

Well.... you did ask :D

[Shark Bait]

Only 15 arc seconds! :thumbsup:

I can only assume that you are happy with that. Anytime you want to swap.......

[Shark Bait]

Thanks for all the replies. It is interesting to get a feel for what is considered accurate pointing.

It may not be fair to compare apples and oranges; or in this case precision EQ's with Sky Watcher Alt/Az Dobs, but I am starting to wonder if I should be content with an error of 30 arc minutes. :shrug:

[Astro_Bot]

It depends what you want to do. If you're happy using the finderscope to locate fainter objects, or only do that rarely, then it's probably fine.

You can get more accuracy, but it costs (money and setup time).

Me? I'm a lazy sod in a way - I'll trade off setup time and cost if it means the rest of the evening goes like clockwork.

Just for the record, with my EQ6 Pro, I get a typical 5-10 arcminutes without polar alignment, and <5 with PA.

[Terry B]

My Tak NJP is currently using a T point model of 40 stars. I slew anywhere in the sky and take a pic. This could be out by 15arcmins depending on how long it has been since I used the scope. I then sync to this image. All further slews will be within 5 arcmin. The target will be close to the centre of a 30arcmin wide frame.
Terry

[Peter Ward]

Therein is much truth.

Last last few percent is usless for many, but essential for some...and it costs.

If you don't expect a world-tour from the penny-ride and you'll be happy. :thumbsup:

[mozzie]

stu people that image would love to have it smack on like peter's 15 arc seconds wow!!!! for me 1 or 2 mins is fine at low power it is usually centre or very close,even at 200 odd times i can go from object to object and it is damn close to centre very happy...as for you doing your visual too and having to find things with finder scope isn't what you paid for 5 arc mins in your scopes specs is acceptable and placing objects close to centre low power..the distance of the moon out sounds like something maybe out of whack have you checked your goto system for any wear or movement.not familiar with your goto system but it sounds wrong to me.

[Shark Bait]

I will remove the Alt/Az covers and have a look for anything obvious. I do not want to void the warrantee, so I won't be pulling apart the drives. The specs do state that pointing accuracy is 'up' to 5 arc mins, which is an interesting choice of wording. It is possible that my expectations are a little high ...... I will do some more testing and see if I can achieve better performance.

[mozzie]

stu,5 arc mins is pretty good on low power eyepieces it should be mighty close to centre..meade are the same and with high precision they say 1 arc min..to me thats close enough to centre...
for visual use only-we know photography has to be better..
but 1/2 deg just ain't wright..

i do occasionally notice a star or ned,g/c that isn't quiet there in centre and this could be human error,as somebody has to put there R.A-DEC into equasion and errors are made!!!!!!!!!

[g__day]

Vixen atlux - raw with 3 stars aligned - under 3 arc minutes, via maxpoint or tpoint with 80 stars well distributed - about 30 arc seconds - very pleased!

[LewisM]

The worst my NEQ6 has been out was 3 arcmin. Pretty happy with it. That's me carrying it out of the garage, setting it down on pre-marked south line and going for it.

Always in FOV with 20mm eyepiece, usually centred.

[Steffen]

I usually get my HEQ5Pro to put things into the centre 50% of a 0.6 degree FOV, depending on setup accuracy.

If you don't use a GPS dongle pay close attention to the clock setting. Near the equator objects move at 15 arc minutes per minute (or 15 arc seconds per second), so being off by 30 seconds will shift things quite noticeably.

Cheers
Steffen.

[Shark Bait]

Hi Steffen,

Just to clarify, does this mean that you centre selected objects 50% of the time?

Edit: Sorry, disregard my query. It makes sense now.
I am careful to plug in accurate time, date, lat/long etc., but your comment has got me wondering...
I will try plugging in slightly different times to see if I can improve the pointing accuracy. It might be a waste of time and effort but it is worth a try.

Thanks,
Stu.

[Steffen]

Yes, I meant half a field radius into the field, or better.

Cheers
Steffen.