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Nexstar goto mount slew speed

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#1 SouthernSky

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Posted 10 December 2013 - 01:21 AM

Hi,

I have a Nexstar SLT scope. I found the slew speed doesn't work below speed of 7. Although the alignment can still work if I change the slew speed manually, I wonder if there is a solution to this? I use rechargeable NiMH batteries. The default slew speed only work once(when I first test the scope) and for some strange reasons; it never work afterward. That's already out of warranty period.

Thanks all.

#2 RTLR 12

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Posted 10 December 2013 - 04:34 AM

Rechargeable batteries do not supply enough voltage for the mount to operate correctly. A rechargeable NiMH battery regardless of mAh capacity only supply 1.2 volts per unit. 8 AA NiMH batteries at 1.2 volts only supply the mount with 9.6 volts. If you must use AA batteries you need to use the standard 1.5 volt non rechargeable type. Even if using 1.5 volt AA batteries in your mount, they will only last for a few hours before the voltage drops to a level that will not supply the mount with enough power to function correctly. It is best to use a 12 volt auto jump pack or the like to power your mount.

Stan

#3 Tel

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Posted 10 December 2013 - 06:48 AM

Hi SouthernSky,

Welcome to CN and to this particular Celestron forum ! :bow::bow:

As Stan has informed and emphasised, it's imperative to have a good power supply available to these 'scopes as low voltage can produce all manner of inconsistent problems.

Regarding the slew speed or rate, you don't say specifically which SLT you have, but to my recollection, (based on the Schmidt-Cassegrain 'scopes of this series, e.g. the 4SE (Mak), 5SE, 6SE and 8SE, SCTs), when performing an Auto Two Star Align*, the 'scope will slew at Rate 9 to the appointed second star. It will then reduce this rate automatically to an arbitrary choice (?) of Rate 6.

However, Rate 6 being too fast for most operators to centre any alignment star/object in the eyepiece, one needs to manually adjust the slew rate to a more manageable, rate of, say 2, 3 or 4 depending on preference.

Additionally, once aligned successfully, I believe the same pattern is followed when slewing to celestial objects of your choice, in so much as the initial slew to the object is made at Rate 9 with Rate 6 being adopted after arrival at the object, but thereafter, should any centring of the object in the eyepiece be deemed necessary, the operator adjusts the rate manually to one, (i.e. Slew Rate 2,3,or 4), whichever is comfortable.


* Note : Other alignment options all require entirely manual slewing. However, the Slew Rate will in this instance still be automatically set initially to Rate 9 in order to minimise the time it takes for the 'scope to traverse the sky to the chosen alignment object. On arrival, I believe in this instance, that the Rate does not diminish to 6 but requires the operator to manually adjust for final alignment/viewing comfort.

Hoping I've not misinformed from mere recollection ! :idea:

Best regards,
Tel

#4 SouthernSky

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Posted 10 December 2013 - 11:47 AM

Dear Tel and Stan,

Thanks a lot for your informative answers, and I appreciate Tel's kind welcome. I am surprised this forum is full of knowledgeable people willing to help newbies like me. It's a privilege to be here.

Currently my slew speed 9 is actually just barely acceptable. At slew speed 7, its movement is barely noticeable. So it can be an issue with the low voltage. But I am a little afraid it might be the drive itself because I remember I used Alkaline battery once and it feel just the same. I will try it with Alkaline again and see if the drive come back to life.

Thanks

#5 Kon Dealer

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Posted 10 December 2013 - 01:39 PM

RTLR 12. I use NiMH rechargeable, but in a pack of 10 to give me 12V.
I simply attach the pack to the upright arm of my SE mount so cord wrap is not an issue.

last for about 4 hours and dirt cheap to run.

#6 RTLR 12

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Posted 10 December 2013 - 02:51 PM

Excellent!! As long as you have enough voltage the mount runs great.

I used 8 NiZn AA batteries that were rated at 1.5v and 2500 mAh. each. 8 of them would give me 2½ to 3 hours of power, but they just wouldn't hold up to the constant Charge/Discharge that I was putting them through. Just have to wait for the technology to catch up. They weren't that expensive, but they weren't that cheap either. I like the 10 pack idea much better.

Stan

#7 Tel

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Posted 10 December 2013 - 03:29 PM

Hi SouthernSky,

Whatever your thoughts and suspicions, I would at least be inclined to insert a fresh set of 8 non-rechargeable, 1.5V AA batteries into your mount's container and recheck the slew rates.

If these are not improved significantly over that which you are currently experiencing, then yes, suspect something more serious.

If though the slew rates on both axes are much improved, it still pays to remember that AA batteries are very limited in their power output versus operational time and as mentioned by both Stan and Kon Dealer, a more reliable and longer lasting source of energy to power your 'scope is thus highly desirable.

Best regards,
Tel

#8 Kon Dealer

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Posted 10 December 2013 - 06:16 PM

Tel is right-I keep 8 x 1.5V batteries in the mount, just in case the rechargeables pack up.
Can't happen much because they are still going strong after 13 months.

#9 SouthernSky

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Posted 11 December 2013 - 12:06 PM

Hi Tel,

You are right I should test it with Alkaline before voice my concerns. I just put a fresh set of Energizer to test. The good news is there is finally movement at rate 6, albeit very slowly. So there is improvement. Rate 9 also appears to be slightly faster. The bad news is, it still doesn't move at rate 5 and below. I will go star test when the sky is clear in my area to see if it improves the experience with alignment and tracking.

One other good news is I finally figure out the right battery and cable orientation to shut the battery cover completely. I always had the problem before.

I wonder if someone else had the same problem as me about the drive and any other way to improve it.

Best Regards.

#10 Tel

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Posted 11 December 2013 - 03:18 PM

Hi SouthernSky,

The slew rate for the SLT series appears, according to the manual, to be 4 degrees/sec. at Rate 9. In theory therefore, (and you can check this out): one full rotation in azimuth of your mount at Rate 9 should take 90 seconds.

Equally so, setting the 'scope to the horizontal and moving it to the perpendicular in altitude, should take 22.5 secs.

Note though that the given figures may not be correct/accurate.

For example, I have just checked out my own Nexstar 8i's slew rate 9 and found it to be 7 degrees/sec. in azimuth and not the specified, book, (manual), value of 6 degrees/sec.

By the same token, it only takes 10 seconds to move the mount arm from the horizontal to the vertical which gives a corresponding rate of 9 degree/sec.

Given the real possibility of anomalies, it would nevertheless be interesting to see how your SLT performs under the same tests.

Coming then to the subject of your Rate 6 and lower slew rates, Rate 6 will give an almost imperceptible movement of 0.5 degrees/sec. (in theory), when viewed from the outside of the 'scope. You will however be able to feel the movement with your hand.

But overall, are you sure your 'scope is not moving at the lower slew rates?

Have you tried setting it up in daylight, focusing it on a terrestrial object and then testing the low slew rates or, alternatively, without using an eyepiece, placing your hand on the tube while operating the directional buttons at these lower slew rates ?

Does this make any sense ? Perhaps you have already conducted such or similar tests ?

Best regards,
Tel

#11 SouthernSky

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Posted 12 December 2013 - 11:26 AM

Dear Tel,

Thanks a lot for the great tips for measuring the drive speed.

It took 125 seconds to go one full rotation in azimuth in Rate 9. Horizontal to vertical altitude took 30 seconds. So it's a bit off, doing about 3 degree/sec. Looking through the tube without eyepiece, I can see movements at Rate 5. Although there is drive sound at Rate 4, but there is no movement after pressing direction button for over one minute.

These don't really bother me as long as the drive track the Earth rotation correctly. I guess I will find out after clear sky for field testing.

Thanks.

#12 Tel

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Posted 12 December 2013 - 12:10 PM

Hi SouthernSky,

It's just possible that we're getting somewhere as a result of your testing.

You didn't say at any time which SLT you own. Accordingly; the single manual, covering the SLT Nexstar 60,80,102,114 and 130, states that the slew rate at maximum, (i.e. Slew Rate 9) is 4 degrees/sec. However, if you own a Nexstar 90 (Mak) or 127 (Mak), their specific manual refers to a maximum slew rate of 3 degrees/sec.

This is in fact what you have measured on both axes of your 'scope. Could you therefore advise as to which 'scope you are indeed operating ?

Best regards,
Tel

#13 SouthernSky

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Posted 12 December 2013 - 01:23 PM

Hi Tel,

I have a 102slt, so it is supposed to be 4 degree/sec.

Thanks.

#14 Tel

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Posted 13 December 2013 - 05:02 AM

Hi SouthernSky,

In theory, yes, but as I said above, my Nexstar 8i slew rates are not in keeping with its specification. Who knows, with the apparent availability of at least two drive motors, (one operating at 4 degs./sec., the other at 3 degs./sec.), which one may find itself in which mount ?

Given though, the impression that you are essentially trying to detect movement from the outside of your 'scope rather than look through it, if this is the case, I think a more definitive assessment is necessary.

If you have not already done so therefore, I would suggest you merely focus on a distant terrestrial object using the highest magnification EP you have available to you and then check each slew rate from say, 1-6 for movement. Above this, (i.e. 7-9), I gather you have basically detected movement already by simple, external observation.

Best regards,
Tel

#15 SouthernSky

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Posted 13 December 2013 - 09:35 AM

Dear Tel,

Many thanks for your advice. Actually, I detect the movement by looking through the telescope tube without eyepiece just as you suggested. There is no movement at rate 4 and below despite there is motor sound after press and hold one drive direction button for well over 2 minutes. I think somehow this particular motor of mine requires higher voltage than what the alkaline batteries provide.

It seems we can't get any clear sky in my location here for another week or so. When sky is clear, I will take it out for field test. Hopefully, the tracking can work correctly.

Sorry for taking so much of everyone's time. My lesson is that power tank is necessary. Also when you purchase a computerize telescope, it is recommended to test the drive speed at different settings according to the testing procedure provided by Tel. So you can get warranty service if something is not working properly.

Finally, despite of the little glitch, I still highly recommend the Nexstar series. The optical quality is superb and features are great for its price.

Clear Sky.

#16 Midnight Dan

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Posted 13 December 2013 - 10:08 AM

Hi SouthernSkies:

I detect the movement by looking through the telescope tube without eyepiece just as you suggested.


That doesn't quite seem like what was suggested:

Have you tried setting it up in daylight, focusing it on a terrestrial object and then testing the low slew rates or, alternatively, without using an eyepiece, placing your hand on the tube while operating the directional buttons at these lower slew rates ?


Tell suggests one of the following two:

1) Focus on a terrestrial object during the day (focusing requires an eyepiece)

- or -

2) Use your hand to detect the movement (does not require an eyepiece)

Looking through the scope *without* an eyepiece won't really help you detect movement.



If you say you hear the motor running but don't see any movement, seems to me that either it's moving very slowly and you can't see it, or it may be taking up the backlash in the gears and you need to set the anti-backlash settings in your handset menus.

Try Tel's suggestion again - look through the scope during the daytime *with* an eyepiece, focus on a distant target, and see if you detect the scope moving. You may have to wait a while after hitting the button (up to 30 seconds in some cases) before the scope starts to move. The longer you have to wait, the more backlash there is. If you notice a lot of backlash, you may want to set the anti-backlash settings to compensate.

-Dan

#17 SouthernSky

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Posted 22 December 2013 - 10:28 AM

Hi Dan, you are right I misread Tel's instruction between punctuations . It has been raining nonstop for past 2 weeks in my area and the view outside of my windows is just a nearby building. So I have to wait for the rain to stop for eyepiece test. Today there is a rain break(although cloudy), so I took it outside to test with 4mm high power ocular. It happens just as you described. It took around 20 to 30 seconds at lower speed settings to start movement. So it is indeed a backlash issue.

There are anti-backlash adjustments for positive and negative altitude and azimuth. I think the default settings are all zero. I tried to change some settings but the rain started again before long. So I have to wait for next rain break. I might try to set all values to 20 next time to check.

I am glad my drive is working properly. Perhaps get the right anti-backlash settings will get my drive to move faster. I will try and report back next.

On a side note, ever since I join Cloudynights recently, it has been cloudy nights every day in my area. ..... :confused:

Thanks all for the help.

#18 Midnight Dan

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Posted 22 December 2013 - 01:21 PM

Hi SoutherSky:

Not sure where you're located, but it seems like I've been sharing your weather! It's been since before Thanksgiving that I've had a view of the sky. This seems to happen every year around here. From November through January (sometimes even February) is a complete shutout. Very frustrating!

For your backlash adjustment, go outside when the weather permits during the day for testing. You'll need to do a mock alignment because backlash behaves different before and after alignment. Select the Auto two star alignment, use Polaris for you first star, and aim the scope North and upward at an angle that is approximately your latitude. Then accept whatever second star comes up on the menu, and accept the position that it slews to.

Now you can adjust for backlash. Move the scope by alternating between the left and right with the arrow keys at a speed of about 4. You want to set the azimuth backlash settings so that the scope stops immediately when you release the button, but minimize the startup delay when you press the button. Stopping is the most important part. If you end up with a delay of several seconds at startup to allow for an immediate stop, that's ok.

Do the same for the altitude axis. When you're done, you'll probably have the same number for the positive and negative setting for a given axis.

The other thing that's important for getting the scope moving correctly after a slew is to use the correct final approach movements, both when centering your alignment stars and when moving to an object with the arrow keys. You always want to move in the same final direction so that you take up any remaining slack in the gears.

-Dan

#19 SouthernSky

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Posted 22 December 2013 - 01:54 PM

Hi Dan, many thanks for your clear instructions. Looking forward to the next rain break to get the settings correct.

#20 SouthernSky

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Posted 01 January 2014 - 02:51 AM

Hi Dan,

We got a clear break today so I took the scope out for setting adjustments. After follow your instructions, the tracking and goto worked correctly. Although the slew speed is still around 3 degree/s, but it doesn't bother me. Many thanks to you and Tel.

One interesting thing I noticed is that while there is an option for "Betelguese" during 2-star alignment. There is no Betelguese as option for Named Star or Tour for GOTO function. But Rigel is there. I have to entered "Sao 113271" for the Goto to get to Betelguese. Maybe it's time for me to upgrade the controller firmware.

Although the sky is clear, but the seeing condition is bad. Must be some haze or turbulence up high in the sky. Even Betelguese doesn't appear bright and M42 shows no nebulosity. I hope for better conditions in the new year.

-Clear Sky.

#21 Tel

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Posted 01 January 2014 - 05:11 AM

Hi Southernsky,

Hmmm ? I can't, off hand, think of any good reason at all why you couldn't access Betelguese directly and only via its SAO number if it were visible at the time in your sky, but I don't believe a firmware upgrade will solve the problem.

All I can think of at this moment therefore, is to check both your 'scope's "Slew Limits" and "Filter Limits" and let us know the settings.

Assuming the above settings to be Slew Alt. 90 max./ 0 min., and (Filter) List Alt. the same: additionally, you might care to firmly establish your original findings by performing a mock (indoor simulated) alignment: entering last night's alignment time and date and then varying the time by, say, a couple of hours either side to see whether the "Named Star" feature eventually "logs" Betelgeuse. (?)

Best thoughts to date. Sorry if they are inadequate ! :idea:

Best regards,
Tel

#22 Midnight Dan

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Posted 01 January 2014 - 08:42 AM

Hi SouthernSky:

Glad you got the tracking and gotos working to your satisfaction!

For Betelgeuse, I agree with Tel. A firmware update probably won't help, and the only things I can think of are also the Slew and Filter limits.

-Dan

#23 SouthernSky

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Posted 03 January 2014 - 11:57 AM

Hi Tel and Dan,

My default slew and filter limit was from 0 to 65 degree. Since Betelguese was near the zenith, no wonder it doesn't show on the list. After I change it to 80 degree, it shows up on the Named Star list. Thanks a lot for the help.

#24 Midnight Dan

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Posted 03 January 2014 - 12:57 PM

That'll do it! :waytogo:

-Dan

#25 Tel

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Posted 03 January 2014 - 01:53 PM

No thanks necessary ! It's just good to hear that the problem was resolved. :waytogo:

Best regards,
Tel






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