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RASA 8 first light / AVX question

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#1 SanjeevJoshi

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Posted 22 November 2020 - 02:36 AM

My AVX mount certainly had to do some exercising - literally with my first backyard outing with RASA 8.   Had to add 6 pounds to the standard weight to balance my set up.  This will put it to 17 pounds if I am estimating correctly.

 

1.  Whats the best way to add weight? Get a 4 pound, 2 pound, and one pound?  Mix and match as I go?

 

2.  Sort of getting to the AVX limit which is 30 pounds I believe.   I am probably 36 pounds.  AVX handled the go to and tracking just great for EAA.   At one point, the tube was below the weight horizontally.   I was a bit nervous and watching closely but AVX was steadfast!  So I am inclined to just use it as I am.   Any downside?  Could this possibly just wear out the AVX quickly?

 

 

Config:  RASA 8 + Celestron LPR, ZWO ASI 294 MC Pro to 25 mm backfocus (6.5 + 11 + 7.5mm extension), AVX with Nexstar+ Remote, Manual Focus (set once, done)

Other:  Pegasus Automatic Dew Control (5 channel) with external humidity / temp sensor, Beelink U57 i5 8 GB Ram 256 GB SSD, Polemaster for precise polar,

Power:  3 x 12v 3A, plus Celestron Lithium Power Tank Pro  (camera, mini computer, Pegasus, RASA fan)

Software:   CPWI with ASCOM platform and ASCOM drivers, Astrotortilla, Sharpcap Pro, Windows Remote Desktop from Home laptop.

 

Workflow:  precise polar, CPWI go to  (no align or calibration), Platesolve, slew correction, and mount sync with Sharpcap Pro.

 

Pic below is simple live stack from RASA first light.

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Edited by SanjeevJoshi, 22 November 2020 - 02:37 AM.


#2 robbieg147

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Posted 22 November 2020 - 03:59 AM

Hi Sanjeev I also have your mount but I would not go above 15lbs myself, at 36lbs I would say you a way overloading the AVX?

 

I believe it's best to have the counterweight as close to the mount as possible to reduce the lever arm, there are several threads discussing this on here.

 

Nice image!



#3 SanjeevJoshi

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Posted 22 November 2020 - 09:53 AM

I am corrected - the 30 pound limit on AVX is the scope? Not including the counter weight.   I asked in equipment thread and got good guidance.

 

https://www.celestro...load-capacities
 

I just need to buy proper weights so AVX can continue exercising and do the marvelous job it does :)



#4 robbieg147

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Posted 22 November 2020 - 01:40 PM

The 30lbs for the AVX would not include the original counter weight, but if you add another I would include this in with the payload?

 

To be honest I would not put 30lbs on my AVX even for visual, and I would never leave it loaded up the minute I stop imaging I get the scope off. 



#5 TelescopeGreg

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Posted 22 November 2020 - 05:57 PM

Every AVX has its own personality, so your particular instance will probably be different.  But my AVX seems to start complaining when the weight above gets above about 20 lbs, and I need to be really careful on setup (balance-wise) above about 25.  But, more important than the total weight, what seems to matter most is the distribution of it.  Just like when an ice skater spins easier when they pull their arms and legs in, the mount is really happiest when the load is low and short.  Putting guide scopes on stilts above the OTA, or heavy finders or guiders out on the Finder Shoe at the head or tail of the scope really hurts.  I even noticed an improvement by pulling the main (and rather massive) OTA focuser in shorter, and adding an equivalently long extension tube between it and the camera.  Unfortunately, that introduced vignetting, so I have to live with the slightly worse guiding of the other configuration.  Also, adding more counterweights, and moving them up the shaft, will be better than less total weight out at the end.

 

Good luck!


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#6 SanjeevJoshi

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Posted 22 November 2020 - 07:26 PM

hi yes, my AVX (bought new three months ago) has been chirpy since I replaced the stock combo saddle (an apology for a saddle) with ADM combo.   It looks eager to take on more :)    Its very smooth, no grinding or slipping noise anytime.  Tracking is awesome compared with my Alt Az Evo mount, my bar is low for EAA.

 

Yes to be sure will watch the weight.  I need 6 lb but ordered 7.5 lb weight so i can ride it 2 to 3 inches off the bottom.



#7 TelescopeGreg

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Posted 22 November 2020 - 08:02 PM

Yes to be sure will watch the weight.  I need 6 lb but ordered 7.5 lb weight so i can ride it 2 to 3 inches off the bottom.

A 12lb weight nearer the top of the shaft would actually be better.  Moment of Inertia is the key, not the weight itself.



#8 SanjeevJoshi

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Posted 22 November 2020 - 09:26 PM

A 12lb weight nearer the top of the shaft would actually be better.  Moment of Inertia is the key, not the weight itself.

 

hmm i need to research this some more then.   I am calculating the moment force  (weight x distance), not the moment of inertia (which is motion related).   With moment of force, i am likely not gaining more than 4 more inches for adding 5 more pounds if my calculations are correct.  Not sure how much MOI matters if I am not slewing from one target to another (introducing some stress) depending on slew speed.



#9 TelescopeGreg

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Posted 24 November 2020 - 03:03 AM

hmm i need to research this some more then.   I am calculating the moment force  (weight x distance), not the moment of inertia (which is motion related).   With moment of force, i am likely not gaining more than 4 more inches for adding 5 more pounds if my calculations are correct.  Not sure how much MOI matters if I am not slewing from one target to another (introducing some stress) depending on slew speed.

 

You're always slewing, perhaps microscopically.  That's what guiding is. You're trying to make tiny, precise changes to the movement of the mount in real time, and do it with a limited amount of force.  The lower the inertia the rotisserie motor has to overcome, the better the result will be.

 

The key is that it goes by the distance squared, which is why moving the mass up the counterweight rod (closer to the pivot) has such a big effect.


Edited by TelescopeGreg, 24 November 2020 - 03:23 AM.


#10 SanjeevJoshi

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Posted 24 November 2020 - 10:06 AM

Thanks for the info Greg.  I am not convinced yet it will matter for EAA and AVX type set up, so will continue to research -  I appreciate your input.   Once a body is moving microscopically and in my case in a EQ axis, the impact will come when there is a change of some kind - like a mount stop or slew to a new target.  Conceivably there could a small impact due to static friction when starting from full stop.  I am assuming impact of dynamic friction is minimal in these set ups.

 

another calc -  if my working weight is say 19 pounds, best set up is a counterweight of 19 pounds, same distance from center.   If I am balancing where ever it is with 18.5 pounds that’s pretty close.  If I go up to 23 pounds, MOI impact will be the other way - not causing by OTA but by the weights.


Edited by SanjeevJoshi, 24 November 2020 - 10:30 AM.


#11 TelescopeGreg

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Posted 24 November 2020 - 03:02 PM

Thanks for the info Greg.  I am not convinced yet it will matter for EAA and AVX type set up, so will continue to research -  I appreciate your input.   Once a body is moving microscopically and in my case in a EQ axis, the impact will come when there is a change of some kind - like a mount stop or slew to a new target.  Conceivably there could a small impact due to static friction when starting from full stop.  I am assuming impact of dynamic friction is minimal in these set ups.

 

another calc -  if my working weight is say 19 pounds, best set up is a counterweight of 19 pounds, same distance from center.   If I am balancing where ever it is with 18.5 pounds that’s pretty close.  If I go up to 23 pounds, MOI impact will be the other way - not causing by OTA but by the weights.

I'm not well versed in the ways of EAA, specifically the length of sub exposures.  But overall the shorter the exposure the less sensitive the system is to lumpy tracking.  If they're short enough, and the imaging train's pixel scale large enough, then, yes, it won't matter.  Planetary AP, with its super quick exposures, is certainly in that situation.

 

But do consider that we're talking microns here.  No ball bearing is exactly round, nor perfectly smooth, so even when the mount is spinning on its own, there will be always be some bumps and dips in its arc.  Think driving down a cobble stone street.  The AVX in particular often has a small 11 second oscillation in RA, due to the pinion gear not being exactly centered on its shaft.  It's large enough (at least on my mount), that any reasonably long exposure is going to capture it.  PHD2 can definitely detect it, and will issue pulses to the mount to correct it.  That's the sort of thing we're trying to let it do.

 

The big picture here is that a pixel is only a few microns wide, and captures light from about an arc-second worth of the sky.  That's about the size of a coin being held by someone standing a couple of miles away.  Not the person, just the coin.  And that person is moving.  The mount and guider are trying to track that coin so that its light stays on that same pixel for tens of seconds to several minutes at a time, and then repeats that task reliably as you acquire the image over the course of an evening.  Microns matter, and the easier we make it for the guider system to control that movement, the better.

 

To your weights question, the closer to the pivot point you can put the mass, be it the scope and attachments or the counterweights, the better.  But there's no advantage in symmetry in terms of same weight on both sides.  Distance squared.  Twice the weight at half the distance balances the same, but it has half of the inertia (2x weight times 1/2 squared distance), making it twice as easy (half as hard) for the motors to correct the bumps and dips discussed earlier.

 

It's really noticeable.  I went from an 8" Newtonian to a 5" refractor this year.  The weight wasn't much different, but the lower profile of the refractor moved the weights up the shaft a couple of inches.  Guiding improved dramatically.  I went from tossing a quarter of my 20 second subs to not losing any, and my exposures can now extend to multiple minutes and still have round stars.  Those longer exposures enable me to lower the ISO / Gain of the camera, lowering the noise.  Win all the way around.

 

Good luck and clear skies!



#12 SanjeevJoshi

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Posted 24 November 2020 - 05:49 PM

Thanks Greg, will keep an eye.  Right now with 15 to 20 second exposures (max), i am dropping zero frames out of several thousand but still, good to know.




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