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Nexstar 6" SLT factory issues and fixes - huge backlash - bad collimation - bad sideral tracking - focus motor wobble

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#1 Franchisse

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Posted 08 March 2021 - 10:57 AM

Greetings reader.

 

I feel sorry my first post here will be a bit long long. I found some help in other topics here. My goal is to summarize every tip and tricks regarding fixing the many issues I had with my stock Celstron Nexstar 6 SLT. Some issues I am still facing, others, fixed.

 

The equipment:

  • Nexstar 6 SLT
  • Starsense
  • Celestron focus motor V2
  • ZWO ASI290MC

Softwares:

  • CPWI
  • CFM
  • ASCOM platform 65

 

The issues from stock:

  1. Very bad stock collimation
  2. Poor tracking
  3. Huge backlash
  4. Broken quick finder
  5. Too sensitive focus knob (I know it's cheap mount)

Issues with aftermarket:

  1. Wobbly focus motor

 

Let's start with the collimation. To fix it, I pointed at Polaris, went out of focus and performed collimation. Made it easier with a laptop and my astro cam so I don't need to come and go each time I turned a screw. This was the easiest fix after all. There are plenty of material explaining how to collimate SCT OTA so I won't dwelve on it further.

 

Poor tracking, is still on going. I'm now sure it's not a mount level issue. As for weigh limit, that maybe the case since I have starsense, astro cam + focus motor at the back. But yesterday I noticed that when I lose connectivity with the HC through CPWI, the tracking magically become much much better. Very weird indeed. I'm using a 5m USB cable. I wonder if anybody witnessed such behavior with their SLT coupled with CPWI. I plan to use HC only next time and see if I have better tracking than with CPWI. For reference, I can't go past 4s exposure withtout seeing star drift. I read here that SLT mount perform usually good for 20 to 30s exposure. My mount is wayyy under that.

 

The backlash on this mount was crazy. In speed 4, I clocked at 121 seconds before the mount moves going from positive to negative (and vice-versa) Az. No backlash on Alt though. I stricly followed the procedure in Nexstar Guide II from Author Michael Swamson, withtout success. I found the answer, again, here on cloudy nights: https://www.cloudyni...problem-solved/ Indeed, my 2 screws were loose. So loose I could move by hand the gear about 2mm back and forth. I screwed it in properly. And voilà. No more huge backlash, only 1 sec. I need to tweak it a bit because it's a bit jumpy when changing direction. I'll follow the Nexstar Guide II once more. Also, it might play in my bad star tracking so I need it to be set perfectly for the sake of plugging all the wholes in understanding why my mount have such bad tracking performances. BEWARE: this procedure will void warranty.

 

The given quick finder had it's base broken, making it impossible to align to quicksearch. That's why I got a starsense, to fasten the alignment procedure but I got many crashes with CPWI which always make me redo the full alignment procedure. such a pain...

 

The focus knob on this OTA + mount is very very sensitive. Making reaching good focus really hard. I "fixed" that getting the aftermarket celestron focus motor. It's a nice device, only if Celestron made if easier to install with better quality plates, in my humble opinion.

 

Installing the focus motor is quite a challenge. If you follow strictly the celestron manual, you will fail. It's impossible for me to align // the focus motor with the focus motor plate. If I screw it in place, it tilts inward and makes my mirror very wobbly (and not just the normal shift) when the motor rotates. I just can't let this appliance loose. I don't like loose screws anywhere.

I partially fixed it following the instructions here, the going full clockwise untill you reach max was of great help and isn't in the celestron manual:  https://www.cloudyni...-focuser-on-c6/ + adding the original plate on top of the one provided with the motor focus providing support and thus limiting the inward tilt while tightening the screws. I find the wobble quite acceptable now.

 

So I'm still facing bad sideral tracking. I leveld the mount carefully during daylight. Balanced the OTA (and it was off indeed, it was going upward on itself. Too heavy on it's back, since the fork can be tilted by hand very easily) I just can't believe it's normal. Even with the aftermarkets + astro cam on it's back because sideral tracking is way under what speed the mount can achieve. And, again, when I had a bug the other night and lost connectivity to the HC with CPWI, the tracking was improved greatly. How weird is that! Still got to fine tune the Azimut backlash setting but I'll be quickly out of options if these few things left do not solve the issue. I tried positive and negative Go-To's withtout change to this behavior.

 

Any suggestion on how to fix the sideral performance is very welcome!

 

Clear skies to all!

 



#2 alphatripleplus

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Posted 08 March 2021 - 11:55 AM

Moving this from EAA to Celestron Computerized Telescopes for a better fit.



#3 Michael_Swanson

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Posted 08 March 2021 - 06:59 PM

Hi Franchisse,

 

The SLT mount is really only suited for visual use and some very light imaging.  And the 6" SCT is just too much weight for that mount for anything other than visual and will be straining the motors (and thus tracking).  Adding the focus motor, StarSense and a camera is just making a bad situation worse.

 

Regarding backlash, the SLT mount has significant backlash but yes, a loose motor makes it an unmanageable amount of backlash.  

 

It is unusual for a Celestron SCT to be poorly collimated from the factory.  That, plus the loose motor and broken finderscope mount makes me think this unit may have been an "open box" and someone with little knowledge of telescopes had used it before you got it?

 

As you are obviously interested in beginning astrophotography, I would recommend you return this and get the NexStar 6SE at a minimum.  If you think you are very serious about astrophotography, you might want to think about one of the German EQ mounts, or one of the higher end fork mounts.

 

Best regards,
Mike Swanson
https://www.NexStarSite.com



#4 Franchisse

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Posted 12 March 2021 - 07:52 AM

Hello Michael. Sadly, I acquired the scope too long ago to ask for a refund. I think. I can assure you the box wasn't opened. It had the regular celestron package + the transport protection packages.

 

Some 8" are sold with SLT mount so I figured, it should be ok although my resseller didn't mention the max weigh issue while I acquired all the aftermarkets at his store. I'll ask him if, maybe he'd be enclined to take back the mount if I get a SE mount but I doubt he'll agree. I've got nothing to lose so I'll ask him later today on the phone.

 

What is very strange, and makes me think the mount should be able to track, is that when I have the HC disconnect (unexpectedly) from CPWI (and unable to reconnect unless I unplug and plug back the usb on the HC), my tracking becomes much much greater, like, almost perfect.  Otherwise, I got downward and rightward drift. I get out of FOV in about 30 secs watching at Orion.

 

I'm using a 5m mini usb cable. I plan to try with a shorter cable and/or directly make my go-to's with the HC to see if's it's a CPWI issue, or else. I'm not sure what's the update rate of CPWI vs standalone HC.

 

I must say your books are great and were of great help. I also need to finish fine tune the backlash Az, just to be sure it's not taking part in the issue at hand.

 

Getting a better mount is a plan, but I don't have the money for it for now. These EQ motorized mounts are great but quite expensive!



#5 Michael_Swanson

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Posted 14 March 2021 - 07:48 PM

The SLT mount is not sold with the 8" SCT.  Maybe you have an SE mount?  Can you post a picture?

 

The drift after centering sounds like backlash so yes, fine tune the backlash compensation.

 

Best regards,
Mike Swanson
https://www.NexStarSite.com



#6 Franchisse

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Posted 21 March 2021 - 07:20 AM

Hello Michael.

I have a SLT mount 100%. And indeed, you're right. No 8" sold with SLT mount.

Nexstar 6 SLT
 
Next time I pupt my gear out, I'll take the time to fine tune backlash during daylight.
 
My understanding is that backlash will affect the centering only untill the gears catch up while I get continuous stardrift. Which is weird especially when you consider this weird behavior about perfect tracking as soon as CPWI unexpectedly lose connectivity with the hand controller.

 

Wish you a nice sunday.



#7 Michael_Swanson

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Posted 22 March 2021 - 12:04 AM

Yep, definitely an SLT.  So, back to my original reply:

 

The SLT mount is really only suited for visual use and some very light imaging.  And the 6" SCT is just too much weight for that mount for anything other than visual and will be straining the motors (and thus tracking).  Adding the focus motor, StarSense and a camera is just making a bad situation worse.

 

Best regards,
Mike Swanson
https://www.NexStarSite.com



#8 Franchisse

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Posted 22 March 2021 - 06:26 AM

****. So, it's unfixable.

 

I'm looking at Advanced VX mounts from Celestron which price is reasonable and compact enough to go on my nomad motorbike nights. But I suspect I'll have trouble putting my OTA + after market (especially starsense + finder) on this mount since there are no screws to place the finder and starsense on top in this new configuration.

 

Any thought on this?



#9 mlord

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Posted 22 March 2021 - 07:45 AM

Those accessories (StarSense AutoAlign and finder) don't need to be "on top".  The StarSense camera can go on a side if that works with the mount, and the factory red-dot finder can go pretty much anywhere with a small bit of 3M mounting tape.


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#10 Franchisse

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Posted 24 March 2021 - 03:00 AM

Hello mlord.

 

Right. Thanks!



#11 speedster

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Posted 24 March 2021 - 04:10 AM

You might check your tracking rate setting in CPWI.



#12 Franchisse

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Posted 11 April 2021 - 06:19 PM

Hello.

 

Clear sky tonight. Amazing actually. Long time since I've seen so many stars outhere. It was perfect so I put the Nexstar 6 out again.

 

A very bad session... again. Still stardrift beyond acceptable. No more backlash, I fixed that about 10 days ago. I've got very nice response from the mount. No bumpiness, smooth motions. But...Can't keep a star in the center for more than 2 seconds in tracking mode. Worse: Collimation didn't hold since last time and had to do it again. Seems like I have to collimate every time. Very weird. It's been 3 times in about a month and a half. I even saw collimation deviate during a 3 hours session. I always collimate on polaris, to avoir star drift issue. Centered etc.. to observe the airy disk, find the right screw and adjust untill it's concentric. Perhaps I should untight the screws instead of tightening. I'll try that.

 

Here's my first try at Orion after my first collimation about a month ago. That's when I noticed the collimation didn't hold. About 2,5s exposure. Stars aren't so round. Not a good picture but I'm still happy for a first try at EAAF.

orion mini

 

I checked tracking speed. As expected I was tracking in sideral speed. Sideral, lunar or solar, no improvment. Really feels like there is drag. Or maybe a faulty DC? Possible, but I bought it with the scope as my reseller advised. As Michael said, probably too heavy for the mount. I'll try again tomorrow(if the sky allows it) after removing weighy components such as starsense and ASI camera to go full visual and check if I still have stardrift. But I don't have much hope since I recall bad tracking even before I added aftermarkets. As I didn't perform a proper alignement, not even leveled the mount I thought, meh, it's normal.  Now I really think this OTA + mount have an (if not many) issue.

 

Even if I upgrade the mount, I'll still have a faulty scope that can't hold collimation for more than few hours...

 

It's unacceptable really. I hope I'll find an agreement: a refund as he has nothing in stock. Everything it still under warranty (1-year in august) The thing is, the SE mount doesn't have much more weigh capacity than the SLT. And out of stock untill october. The Evolution 6 should be refilled this month but cost twice as much as the 6"SLT but has increased weigh capacity (11kg). If he manages to put up a 20% discount maybe I'll add some bucks on the table.

 

Will give him a call during opening hours.

 

 

I'll update this topic with his answer!

 

Wish you all clear skies.



#13 mlord

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Posted 11 April 2021 - 07:10 PM

For "collimation":  you didn't say where on the planet you are located, or what the temperature was.  Important things to remember:

 

1) Let the scope acclimatise outside for 45min or so first.

2) Always, ALWAYS centre the object being used for collimation.  And recenter after every adjustment.  This is extremely necessary!!

3) Use an object high in the sky, rather than one close to the horizon.

 

For tracking, LEVEL the tripod.  Dead level.  Then set the OTA at the index mark, and check that it is indeed level.  If not, the tripod isn't level either, and tracking will be off.

 

If you've done all of that, then great.  But double check it next time out regardless.

 

Cheers


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#14 Franchisse

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Posted 12 April 2021 - 04:38 AM

Hello mlord.

 

Yes, I followed all those imperative guidelines regarding collimation. Polaris is a good choice. High in the sky and since it doesn't move (and I've got tracking issues) it allows me to be dead on center for the required amount of time (using astro cam and target in sharpcap). Recentering indeed everytime I turn a screw. Laptop next to it is very much appreciated so I can directly see the result on screen. I also take special precautions about the direction of focus (in/out). Best thing is, find the focuser value to observe the airy disk, and make a go-to <that value> on the focuser panel, collimate, then find the proper focus for observation and perform again a "go-to focus" to that new value. In other word, if you collimate going "in" you must focus for observation going in as well. And vice-versa.

 

I usually put out the scope hours before observing. So it gets in T°C properly. Prepare the cablings etc... Level it during daylight (much easier). On this OTA+mount there's no index mark. I perform level while pointing at the northern horizon, ready to start the align procedure in CPWI. To no change in tracking behavior.

 

Thanks again for your input mlord!


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