SkySpy

Booked for three nights.  First time at this event and looking forward to meeting everyone.  I will be bringing a Skywatcher 250P GoTo Dobsonian, and our camper van with central heating! 

SkySpy

With reference to my previous post regarding alignment accuracy I mentioned that I had contacted Celestron themselves for feedback.  They have replied albeit without commenting on my real query which was to do with the time taken to do a slow motion adjustment at the end of a slew... however what information I have received has somewhat surprised me and I thought worth repeating here.   Celestron mentioned whether I had "started at the index point", to which I replied I did not know what an "index point" was!  The reply came:

The index mark is a starting position for alignment.  On the SLT it's not clearly marked, but it's when the optical tube is pointed at the horizon, so the telescope is flat and parallel with the ground.  Each time you align the telescope, start from this position.  This is a step that's often overlooked but is essential for a successful alignment. 

...in addition, they go on to say:

With each star (..you align to), you want to center it in the finder scope and press enter.  Then, you'll notice the scope moves much slower.  This is so you can center the star in the eyepiece.  First, use a low powered eyepiece like a 25mm to center the star.  If you want to be precise, you can then remove the 25mm, and place a higher powered eyepiece like a 10mm, and then re-center the star.  When the star is centered to your liking, press ALIGN to lock on.  Make sure you repeat this step for each of the 3 alignment stars.

So to summarize.. I did not know that leveling the scope was as essential as leveling the tripod (why is it not highlighted in the manual I wonder!),  and in addition I had missed the significance of having the scope slow down when you press ENTER on the HC.   I assume both of these things will significantly aid accurate alignment, but have yet to test them out.

I am still non the wiser about the slow-mo adjustment time taken by the scope which occurs after ALIGN is pressed, but the fact that they did not mention the time involved I can only presume means there is nothing unusual in the scope taking 30 seconds or more to finally stop and get a fix.  I am still going to time this precisely at some point but again, I have not been able to yet.

Hope the information will help others working on improving their GOTO alignment accuracy 

SkySpy

It does take some time to refine the position, but not 60 seconds.

Maybe collect some data on this, say measure the total time it takes to slew using GOTO from Deneb to Vega, repeat the test 10 times and see if it consistently takes the same length of time; then repeat for two stars further apart in bother RA and Dec, say Vega and Spica, and Vega and Castor. See if one axis is likely causing the problem. It may be that it just seems slow, but is normal. But without knowing the time it takes, it is difficult to comment on if this is abnormal; I can test it against my Skywatcher version once you have the data.

Again, it is undertaking a reasonably precise movement, and I am not an engineer, but for a £150 mount to be able to GOTO an object amongst a 42000 item database with an accuracy of under 30arcminutes and to be able to track it at sidereal rate... I think that is quite impressive, even if it does take 30 seconds to decide exactly where it thinks the object should be.

Get back to us with some data - maybe start a new thread as this is a bit off topic now.

James

Strangely enough it was calibrating between Deneb and Vega when I noticed the movement, but I was clicking time in my head using the old darkroom technique ("Kodak One..Kodak Two..etc) rather than

anything precise, but next time I will get the stopwatch out and also try a few wider spaced stars as you suggest.

If it merits it will post the results in a new thread and also feedback anything Celestron might comment on.  

SkySpy

Half a degree is reasonable, but I've had it tighter.

This mount is the most basic of GOTO mounts out there, and is relatively "cheap" so one is unlikely to be getting bang on GOTO accuracy; even with an HEQ5 or NEQ6 equatorial mounts people can be happy with half a degree accuracy.

The GOTO accuracy depends on just SOOOOO many things. All of which much be summarised in this thread - be worth taking the time to read the whole thread.

Is there much slop in either axis? If so, tighten the nuts (I'm sure discussed earlier in the thread).

Level the tripod before attaching the mount head.

Can you add stability to the tripod?

Ensure power supply is sound and constant.

Is the scope balanced as well as possible.

Try centring your alignment stars with the reticle eyepiece, then if you have one put a barlow on the reticle and centre again before accepting (effectivly turing the 20mm EP into a 10mm EP with a 2x Barlow); if you had a 12mm reticle EP I'd still add the Barlow and centre with effectively a 6mm EP.

Always end your mount adjustments using just the up and right arrows.

Do a two star alignment rather than one / brightest star.

Do not touch the tripod once the start alignment process has started or finished... 

There will be other things people advise, but I think these are the most important.

In my opinion, a GPS device is unnecessary as it is pretty easy to input the date and time (and doesn't matter greatly if these are out by an hour or so, or a day or so (the commonest error is to forget to enter the date in the American format)), and the precise latitude and longitude is equally unimportant (being up to a couple of degrees out in latitude or longitude makes little difference).

Keep at it, it will get easier and your accuracy will get better, but I think half a degree out is acceptible. If the object is likely to be out of the FoV with the 12mm EP, then I would use a lower power EP first when you've GOTO'd, re-cente, then swap for the higher mag EP, and use the PAE function to update the GOTO for that constellation.

Good luck.

James

James. Thanks  

All good feedback and advice.  I am doing most of what you suggest although I had not thought to add my Barlow to the reticule for even greater accuracy.

(Seems obvious now really!)

Also I have only recently learned about the wisdom of using the UP / RIGHT arrow approach for final adjustments.

Its a bit of a relief to know that my accuracy so far is sort of o.k, as with no benchmark I thought I was just missing something obvious, but it sounds from what you say that better results are merely down to greater refinement of set up and also (by inference) that given the nature of the tripod that anything better than half a degree and I haven't got too much to complain about!

One other thing I might mention here however is that when I command the telescope to slew to any object after alignment, I notice that the scope continues to refine it position for anything up to a minute before it finally settles on its fix.  The motion in the 20mm EP is so slow and gradual that I only noticed it with reference to the illuminated reticle but I have timed it over and over and it definitely continues to position itself long after I thought the scope was stopped. 

What is curious is that I have not heard anyone else mention this delay is final positioning so I even sent a query to Celestron themselves to ask if this was normal. (I am awaiting a response as yet).

SkySpy

SkySpy

This is a long term thread so apologies if this has been answered before, but I would like to know just how accurate can you expect a GOTO command to be, even if you get absolutely everything right (rare!).

I have a Celestron Nexstar 127SLT scope and I have been sacrificing productive observing time to test the actual accuracy I could achieve.   I have a StarGPS unit which auto updates the controller with 

bang on accurate date/time and location in the correct format, I use complete 360 degree precision leveling setup for the tripod, and I use a 20mm 70 deg FOV illuminated reticle for alignment.   

According to Stellarium my 70 degree FOV reticle should give an approximately 1 degree FOV in the eyepiece.  Using the illuminated reticle as a guide, the best accuracy I have been able to achieve (so far!)

is about 0.5 degrees of arc (i.e the target object is that far from the center of the eyepiece after slewing).  

0.5 degrees on my scope means using anything smaller than about a 12mm eyepiece and the object would be completely outside the field of view.  

Now the question is... is that acceptable or realistic?   Just how GOTO is well... GOTO?!!!

The manufacturers of GOTO scopes seem conveniently to give "no comment" on this... so I wonder what kind of real world accuracy other people are getting. 

SkySpy