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

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

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