Huge difference in RA guiding on different sides of flip: Page 2

Huge difference in RA guiding on different sides of flip

ASIMount@ZWO

You're welcome.
For improving guiding performance, you can refer to this review
https://astronomy-imaging-camera.com/news/systematically-improving-guiding-quality-and-zwo-am5-mount-review.html

Kevin_A

One thing I did to ensure my tripod was stable and had a better centre of gravity without added upper mass was this additional lower counterweight bar. No chance of tipovers and better rigidity.

prastro

ASIMount@ZWO

Hello, here are results with Redcat51, which is much shorter FL and much lighter.

WoP and EoP imaging times are 1-2 hours and roughly the same on each side.

Please note that I have tried to equalise guiding parameters as much as possible. So all cases are 1.2 sec exposure, hysterisis, Aggression 70%/80%. The only thing which cannot be truly equalised is min-move as, the guidescale for RC6 is 0.87" and for Redcat is 6.45". Minmove in RA is 0.07" and in Redcat is 0.32".

So I would rather focus on the difference between EoP and WoP guiding in RA for a given target rather than the actual numbers.

As you can see

The guiding on both sides of meridian for Redcat is almost the same for a given target (green). Whereas for RC6, the difference is much more (highlighted in red)
Even for Redcat, there is still the 1100 sec harmonic, but it is much smaller, and can be easily guided out. The main harmonic is 430 sec or so (green). This is similar to what can be seen even for the RC6 when EoP.
Only in RC6, when WoP, the primary period is 1100 sec (red).

So is the weight of the RC6, or some weight distribution is causing this behaviour?

If this is correct, then the good news is that it can possibly be corrected with a counterweight - I will test and see.

The bad news - does this mean the weight limit of 13kg without counterweight and "no balance" feature of AM5 is restricted only to smaller and lighter scopes?

Request views please.

ASIMount@ZWO

Hi prastro,
We are investigating all your results.
Could you please send a copy of your guiding log to us?
Thank you for all your experiments!

prastro The bad news - does this mean the weight limit of 13kg without counterweight and "no balance" feature of AM5 is restricted only to smaller and lighter scopes?

There are differences between strain wave gear mounts and worm gear ones.
The payload capacity numbers are determined when considering the length of the arm of force is 20cm( from RA axis to the mass center of the payload),i.e. AM5 can hardly drive(especially in slew mode) a Sharpstar SCA260 plus a DSLR mounted on the upper dovetail plate even with a standard counterweight. The total weight is luckily within the recommended numbers but the length of the arm of force is way beyond 20cm.
Therefore we recommend a shorter focal lenth.

Again, we are analyzing your results and will reply to you ASAP.

prastro

ASIMount@ZWO
Thanks for the reply.

Regarding the moment, my scope CoG is about 23cm or so from centre of RA Axis. So torque is 8x9.8x0.23 = 18NM, which is still much less than capacity of 13x9.8x0.2=25NM. I appreciate the arm makes a difference but then 23cm and 20 cm are not that different actually.

Once again, the guiding is very good even with RC6 on EoP. Question is how to get it equally good on WoP :-)

I look forward to your analysis. To which email should I send the logs?

ASIMount@ZWO

prastro
bbs can upload files as well

prastro

ASIMount@ZWO
https://www.dropbox.com/s/yeu2m9gxh0pmwmq/AM5_RC6_RC51.zip?dl=0

RC6: GSO RC 6 inch at 1370mm fl, with and without dithering
RC51: Redcat 51: 250mm fl, with dithering

Please download asap as I am not sure how long link will work.

Thanks

ASIMount@ZWO

prastro

prastro https://www.dropbox.com/s/yeu2m9gxh0pmwmq/AM5_RC6_RC51.zip?dl=0

Received. We'll analyze them soon.
So far we go through all the guiding periods, most of them have exactly the same ~870s harmonics.

prastro

ASIMount@ZWO
No 1100sec? Can you tell me which software you are using?
I used PHD2 Log Viewer as follows:

In main Graph window, right click, "Exclude Frames Settling"
In main Graph window, right click, "Analyse Selected Frames, Raw RA"
In the analysis window that opens up, select "Frequency Analysis".

I get this for example for the first file (No Dithers), in the first session which is West of Pier.

Anyway, I look forward to the analysis once completed.

Thank you.

prastro

Some important results.

After it showed a good performance on WoP with the lighter scopes, I thought of using the counterweight and testing.
So, for example, if CW was 2 kg plus 0.5kg shaft weight, "net" weight of scope would be 5.5kg.

I have found good results. With higher weight, WoP and EoP are now both good. Also, the dominant period is now 430 or so sec, with the 1100 sec period becoming less important.

The difference is quite a bit, from 0.69 down to 0.46. Notice that EoP guiding stays similar around 0.45 or so.

This is over 3 nights, so I dont think it is guiding or seeing related, but I will continue to monitor. So far so good.

So the WoP loading does something in the gearbox?

Kevin_A

prastro i am finding that my results are better with a 10kg scope compared to a 4kg scope, both without counterweights however. I will have to check both sides of meridian to see my numbers. I thought it might be due to seeing but now I am wondering if that is not the case.

prastro

Kevin_A
So I don't really know how these gears work. The total weight on the shaft is obviously more with the CW (now over 10kg), but the net torque is different.

This could be specific to my mount or could be the nature of the system. Maybe the ZWO engineers can give some views.

Would be great if you could also check both sides.

ASIMount@ZWO

prastro
Hi prastro,
We've sent all the data to our strain wave gear manufacturers.
BTW, are your AM5 recently bought? (We adopted a new SW gear manufacturer this year.)
The behaviors of the strain wave gears are mostly determined by the harmonic generator. So, you can see period peaks in your analysis, like 108s/144s, 216s/288s, 324s/432s and 432s/576s(Yes, 432s occurred twice, hence the highest peak is mostly about ~432s) etc.

prastro

ASIMount@ZWO
Hello, I got the mount.in end October 2022. I suppose it would have been manufactured in Sep or early Oct 22.

prastro

ASIMount@ZWO When is a reply expected from your SWG manufacturers?

ASIMount@ZWO

prastro
Hi prastro,
We sent your analysis to them last week.So far, we haven't got results formally yet.
Generally speaking, harmonics larger than 432s are mostly from payload upside the RA gear and harmonics less than 432s are mostly from inside the RA gear.

w7ay

ASIMount@ZWO Generally speaking, harmonics larger than 432s are mostly from payload upside the RA gear

If not machined well, the flexible spline gear part of a strain wave gear will cause what looks like subharmonics (periods of greater than 430.82 sec in the case of a mount).

A bad flexible spline also causes a non-perfectly-concentric gear train, which in a mount causes coupling in the RA axis error and declination axis error. Usually, RA and declination should produce independent errors, but if that flexible spline has too much play, a large RA error will also cause a simultaneous declination error -- so the other clue to suspect it is the flexible spline is to look at the guide graph to see if there is any coupling of RA error and declination error.

The flexible spline is shown in red in this wiki page:

https://en.wikipedia.org/wiki/Strain_wave_gearing

Chen

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