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GoTo Slew rates

Pete
 

When I started with OnStep I tried to make my telescopes slew as fast as they would go before something slipped or jammed.

I learned very quickly this was not a brilliant approach.  I have now got three telescopes running OnStep and the all work properly (okay I had to sell my CG-4 and buy a CG-5) which means to me that after a polar align and a 3 star align anything I aim at shows up in a 32mm eyepiece.

Now I am slewing at 2 degrees/second on my home built dob and my Vixen Porta Mount II and 1.5 degrees/second on the Celestron CG-5

What speeds are other slewing and on which mount please?  I am interested to know how you all have reconciled the desire for speed against the necessity for patience.

Thank you.
Pete

Khalid Baheyeldin
 

On Wed, Nov 27, 2019 at 09:33 PM, Pete wrote:
Now I am slewing at 2 degrees/second on my home built dob and my Vixen Porta Mount II and 1.5 degrees/second on the Celestron CG-5
I consider 2 deg/sec the minimum acceptable speed.
Anything below that, and it becomes noticeably slow, but still useable.
Perhaps 1.5 deg/sec is the low end of what is tolerable.

What speeds are other slewing and on which mount please?  I am interested to know how you all have reconciled the desire for speed against the necessity for patience.
There are lots of factors to consider here: the mechanical robustness, the overall reduction rate, the microstepping, ...etc.

My old non-OnStep Vixen GPDX was capable of 1200 X, which is 7.5 deg/sec. That is the fastest mount I ever used. You blink and the slew is done. Plenty fast.

On my OnStep Vixen SXD, I have a 180T worm wheel and with an 18:1 reduction in the 200 step motor, with 1/16 microsteps for tracking, and 1/4 microstep when slewing. This is one advantage the LV8729 and S109 have: you can use a different.

With the above, I am able to do close to 3 deg/sec,which is adequate. Not too slow, and what I consider above average.

Aisling Lightworks
 

If I run at 24V I can max out at 6.5° per second before I start getting stalling. At 12V it's 4.3°. I have 400 step nema 17 motors with a 540:1 reduction (180 tooth worm wheel × 3:1 pulley) 

Pete
 

@Khalid,  Thanks for your reply.  You are right, there are all sorts of factors that govern usable slew rates.  I was trying to find what others had found acceptable.  My home built 8" Dob now has a drive system that is enormously strong with 10mm belts and a 15 Newton meter motor.  However in the interests of accuracy I have slowed down the slew rate since I do not want microstep or (worse) tooth jumping.  Neither do I wish a stretched belt (yeah, done that!)

I would like to increase slew rates due to cold nights and my lack of patience... I shall do so slowly an try to find the optimum safe rate for each mount.  This may be mitigated further because I am considering buying a 9.25 SCT OTA and I am sure that will impose it's own limitations.

@Aisling Lightworks,  All my motors run at 24V.  I keep everything on the scope as a 12V input so I cannot make mistakes in the dark and can run off batteries or a 12V power converter.

I am currently slewing my CG-5 an 1.5 degrees and my home built 8" Dob at 2 degrees.  It is a dance of acceleration, balance and intertia that you need to get right in order to achieve high slew rates.  My CG-5 is now extremely accurate at 1.5 degrees
      Stepper-Steps Micro-Steps GR1 (Gearbox) GR2 (Worm/Wheel)  
  StepsPerDegreeAxis1 15360.0 400 32 3 144 (RA/Azm)
  StepsPerDegreeAxis2 15360.0 400 32 3 144 (Dec/Alt)

Given the above table from Howard's spreadsheet it is obvious to see why.

What mount do you have? 

tomofreno2000
 

On Wed, Nov 27, 2019 at 06:33 PM, Pete wrote:
I have now got three telescopes running OnStep and the all work properly (okay I had to sell my CG-4 and buy a CG-5) which means to me that after a polar align and a 3 star align anything I aim at shows up in a 32mm eyepiece.
Would mean more to me if you specified tfov of the scope rather than a particular ep since I don't know what tfov that ep gives on your scope(s). Tfov of my scope at my lowest power (102x) is 0.85 deg. Sometimes objects are in the fov after a Goto, but many times not on longer ones. Generally well within the 5 deg fov of the finder though. That's with a 3 star align, and I would guess some of the inaccuracy is due to my mount which is a large dob. I have a plan for testing this, but it's too cold to do it now. :^)

I currently use 3.5 deg/sec for Gotos, using 5160 drivers and about 900x reduction on both axes.  That results in about 25 sec for slewing 90 deg which is fast enough for me - gives me a chance to move the observing chair out of the way if I forgot. Slewing faster will be more difficult if you have lower steps per arcsec for AP. Mine is visual only, about 0.44 steps/arcsec, so the motors don't have to run too high rpm to achieve 3.5 deg/sec, and the 5160 uses spreadcycle for slews (QUIET mode), so plenty of torque. 

simingx@...
 

My setup (200 step motors, 2400:1 overall reduction) can slew at 3.2 deg/s at 24V, but the planetary gearheads are a bit too noisy for my liking. I therefore run them at 2.9 deg/s, then the noise and speed are acceptable. Gives me time to halt the motion too, if anything is in danger of running into something else. 

Pete
 

@simingx,  Thanks for the reply.  What mount are you using please?

Pete
 

@tomofreno2000,  Yes, sorry, I should have specified the tfov.  I was, and still am more interested in finding what other OnStep users are running their mount slew rates rather than an optical discussion.  I digressed rather in saying that object now show up in the eyepiece after a slew. That was really to illustrate that all three of my scopes are working with sufficient accuracy that with a wide field eyepiece I no longer need to resort to using the finder scope to begin the centering process.

3.5 degrees is quite a good rate.  WHich mount do you use please?  Thank you.

I am using DRV8825s on all my scopes today.  I used TMC2130s for a while but they got so hot that I reverted to the 8825s.  I too am visual only for the moment.

Khalid Baheyeldin
 

On Thu, Nov 28, 2019 at 04:17 PM, Pete wrote:
I am using DRV8825s on all my scopes today.  I used TMC2130s for a while but they got so hot that I reverted to the 8825s. 
If the DRV8825 work well, then the LV8729 should work better. They will be both, quieter and less 'rough'.
That is, if the current needed is within its specifications.

Pete
 

Thanks Khalid.  I bought some S109s but never put them into a scope.  I am just going to stick with the 8825s.  Funny thing.  I ran my 8" Dob with TMC2130s and a number of guys at my astronomy did not like the silence.  They liked the noise that my dob makes with the 8825 when it slewed.  Gotta give the audience what it likes!

I may soon be OnStepping (a verb???) a Losmandy G8 for a friend.  We shall see.

simingx@...
 

On Thu, Nov 28, 2019 at 01:09 PM, Pete wrote:
@simingx,  Thanks for the reply.  What mount are you using please?

 I built my own mount, it's in the "Showcase" section :)

Khalid Baheyeldin
 

On Thu, Nov 28, 2019 at 06:22 PM, Pete wrote:
Funny thing.  I ran my 8" Dob with TMC2130s and a number of guys at my astronomy did not like the silence.  They liked the noise that my dob makes with the 8825 when it slewed.  Gotta give the audience what it likes!
Silence is not a big thing for me. The noise bothers some people, but not me.

I did try the TMC2130 and they are noticeably quieter than the LV8729.
But I did not find any difference in accuracy.
And because the LV8729 runs much cooler, I switched back to them and have been running on them ever since.

tomofreno2000
 

On Thu, Nov 28, 2019 at 01:17 PM, Pete wrote:
WHich mount do you use please?
I guess you missed this in my post: "and I would guess some of the inaccuracy is due to my mount which is a large dob."
It is in Showcase under Dobsonians, but likely of little interest to you since it is a diy Dob rather than a manufactured GEM.  That's funny that the folks in your club prefer noise when your scope slews, but it does serve the function of an audible warning that the scope is moving.

Howard Dutton
 

On Fri, Nov 29, 2019 at 06:36 AM, tomofreno2000 wrote:
That's funny that the folks in your club prefer noise when your scope slews, but it does serve the function of an audible warning that the scope is moving.
I'd rather have silence.

Pete
 

That is what I thought but I am using 51:1 motors and they do make a really nice "Sci-F noise as the spin up and starts tracking.  I have said, ":Look, I can make this quiet.", and thyr ALL said, "No, we like the weird noise it makes/"  Whatever...