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DevonSkies

Frustrating GoTo problem with Nexstar 127 SLT

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HI,

OK, I bought a Celestron Nexstar 127 SLT Goto Mak about a week ago. Despite following all the setup instructions and carefully checking all the date/time/location data, I am having crazy problems getting the goto to work accurately.

I am able to successfully align the mount using either SkyAlign or two-star alignment. The alignment method doesn't seem to make any difference. What happens it, once I have aligned carefully to two well-spaced stars (e.g. Dubhe and Sirius), the scope can successfully be slewed back and forth between those two alignment stars and it hits them pretty much bang on (centred in the 25mm eyepiece). However, if I then try to Go-To to any other object it misses by a significant margin. Usually the scope is about half a degree off target (sometimes its just on the edge of the field of view). Sometimes (especially near zenith) it's even worse. The strange thing is, as I said, it will still slew accurately to the alignment stars. It's just everything else that is off!

I've checked and double checked the following: Latitude, Longitude (both taken from a handheld GPS), date (US format), time (Universal Time, Standard TIme (not Daylight saving)), levelled the tripod, I am running from a Powertank, I have aligned using the same direction as the goto approach, etc, etc. I've played with the goto approach direction, I've adjusted the backlash compensation, I've performed a factory reset and I've even updated the firmware. It's driving me crazy!

Surely the goto should be accurate enough to get a given target somewhere within a 25mm eyepiece? I don't expect it to be perfect, but if I didn't already know my way around the sky I'd never be able to locate anything with this scope as it stands. How accurate do other 127SLT owners find the goto? Does it usually locate any given object within the FOV of a 25mm eyepiece?

I'm starting to suspect my mount is faulty (as it happens my OTA is also faulty, but that's another story...), but I'd like to hear other people's opinions on this before I start to think about returning it.

Thanks!

Edited by DevonSkies

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You could try the initial aligning with a shorter eyepiece, or, realign on any star you slew to that's off center. It should become more accurate after realigning each of half a dozen or so well spread stars. Hth :)

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6 minutes ago, brantuk said:

You could try the initial aligning with a shorter eyepiece, or, realign on any star you slew to that's off center. It should become more accurate after realigning each of half a dozen or so well spread stars. Hth :)

Thanks. I've tried realigning to 'missed stars', and yes it fixes the alignment for that star, but then the original alignment star (the one it replaces) goes off kilter! As far as I understand, it only stores two alignment stars, so repeatedly realigning doesn't add any accuracy.

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Hi

I don't have your kit, but alignment problems are common to all mounts.

Your two alignment stars are a long way apart in Alt, but not so far apart in Az, so try say Dubhe and Aldebaran, see if it makes any difference.

Michael

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Yes I checked the manual and it stores two alignment stars only. So they need to be a bit more accurate - try realignment using a shorter eyepiece on each of the two stars. And realign on one or two well risen stars throughout the night to keep the goto on song as Earth rotates. Also check your approach settings and backlash settings and adjust if needed. :)

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Thanks for the replies. I've spent quite a bit of time trying different alignments and checking the various settings, and there's no improvement. The only success I did have was with using the "Precise Goto" feature. After aligning with the suggested nearby star (which the Goto would initially miss by a country mile!) the scope would then find the desired object very accurately. This was the only way I could get it to locate objects with any accuracy.

I have arranged to exchange the scope+mount for a replacement. Fingers crossed that the new one will be better. My current scope has a crooked corrector plate on the front - it looks like the retaining ring has been cross-threaded! *eek!* So who knows what else the QC might have missed!

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Yep - a crooked corrector plate will screw things up for you. Hope you get the new one soon and do let us know how you get on with it - good luck :)

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