PHD2 - RA oscillating

[masjstovel]

Hi,

I have an RA guiding problem thats bothering me. 

It oscillates up and down ~2 arcseconds for every correction. Ive tried fiddling about with the agressiveness and min/mo settings etc, but it doesnt change. I use 0.9 guidespeed in EQMOD. I've tried changing RA only to 0.5 and upwards, but doesnt seem to better itself.  Its a beltmodded EQ6. Ive done the Guiding assistant many times and tried applying the suggested settings. Got a screendump and log file attached. The logfile in its whole isnt the best, but the photo shows the problem ive had for many sessions over time now.  Can provide other logfiles if necessary. The yellow (upper line) is starmass, and the white is SNR. I have had the problem for a long time now, so i dont believe its just related to seeing.

Can anyone help?

https://openphdguiding.org/logs/dl/PHD2_logs_cp6m.zip

Edited August 26, 2020 by masjstovel

[wimvb]

I’m not an expert, but my guess is that with a scale of 5”/pixel, you are getting decent values. The rms in ra and dec is only a quarter of a pixel. To see if you really have a problem, can you change the pixel scale? Different guide scope, different camera. If you use a dedicated astro camera, use your imaging camera and scope to test guiding. This should  help you decide if it’s the mount, or the guide setup that is to blame.

[kens]

As @wimvb  says you are getting close to the limits of guiding precision at that pixel scale.

But you do have a spike in the frequency response at 10.2s which is the gear tooth mesh period. Try adjusting the belt tension.

[vlaiv]

What @kens said - this is down to teeth meshing with belt on RA axis. If you open up your mount and take magnifying glass and observe how belt meshes with gear on motor shaft when the mount is tracking - you will see that it goes "tooth to tooth first" instead of tooth coming against trough.

I had that on my HEQ5 after belt mod - it was 13.6s (tooth period for HEQ5) and I fixed it by adding more belt tension until there was proper meshing.

[masjstovel]

@wimvb Thanksm. Probably your right. I have been stable at 0.75" though, still with the oscillating. 

@kens & @vlaiv Thanks! Where can i see this 10.2 number? So i presume it needs tightening. What is "thight" in this matter? I pushed my fingers in between the motor and the housing with what i would call "moderate force". How much can i push this before it is too tight and makes the gears grind? Would hand-tight do with some more force, or should i use the crowbar methos with a screwdriver etc? 

What guidescope would you recommend? 

Edited August 26, 2020 by masjstovel

[wimvb]

12 hours ago, masjstovel said:

Its a beltmodded EQ6

 

1 hour ago, masjstovel said:

What is "thight" in this matter?

You have to increase the distance from the motor pulley to the worm gear pulley.this stretches the belt a little, making it tighter. In a belt modded HEQ5, I did this by loosening the motor  hex bolts just a little, and pushing the motor away from the worm pulley, at the same time tightening the bolts again. Mounts which have belt drives, have belt tightening screws in place.

The 10.2 s peak shows when you do frequency analysis of the ra guide data. Use phd log viewer for this.

[kens]

I analysed the guide log with PHD Log Viewer https://openphdguiding.org/phd2-log-viewer/

@wimvb beat me to it.

It has an option to graph the frequency response of your RA axis both with and without guiding pulses. This shows a strong response at 10.2 seconds.

 

 

 

[masjstovel]

13 minutes ago, wimvb said:

 

You have to increase the distance from the motor pulley to the worm gear pulley.this stretches the belt a little, making it tighter. In a belt modded HEQ5, I did this by loosening the motor  hex bolts just a little, and pushing the motor away from the worm pulley, at the same time tightening the bolts again. Mounts which have belt drives, have belt tightening screws in place.

The 10.2 s peak shows when you do frequency analysis of the ra guide data. Use phd log viewer for this.

Thank you. I think i was unclear. I did the beltmod myself, but what i mean is.. What is "tight". I thought i tightened the belts enough when installing the belts, but maybe not after all.

Like how tight? I understand its hard to describe, but handtight or tight with use of tools? How much force? 

 

@kens Ah thank you. I have PHD Log viewer, but Im not good in analyzing it, unfortunately Frequency response means the you can see the highest deviation from 0" arcminutes? 

[kens]

The graph of way your mount behaves, guided or unguided, can be broken down into a series of sine waves of different frequencies, amplitudes and phases. So the frequency response is the graph of amplitudes against frequency (or period which is 1 divided by frequency). What I look for is spikes at significant periods.

You can see on the frequency response from your guided mount there are two spikes at periods of 480s and 10.2s. The 480s spike corrresponds to the fundamental frequency of your mount - the time it takes the worm to rotate once. It's possible this could be reduced e.g. by increasing aggression (currently 0.5 which is lower than the default of 0.7)

The spike at 10.2s is as discussed but is harder to guide out. The smaller spike at 122s corresponds (to my memory) to the time it takes the motor gear to make one revolution.

 

[wimvb]

1 hour ago, kens said:

The smaller spike at 122s corresponds (to my memory) to the time it takes the motor gear to make one revolution.

I would say so, the pulley has 12 teeth. 12 * 10.2 = 122.4

[vlaiv]

Belt is a bit elastic.

I tightened my by hands only but I had to pull rather hard on motor to get it there.

Try magnifying glass approach - look at gear/belt meshing to confirm that it is tooth - tooth meshing and then tighten the belt (by hand - put some tension on it) and see if you've made a difference to meshing. There should be some difference but if you are not getting clear meshing - add more tension.

[masjstovel]

@vlaiv, Its different on EQ6 than HEQ5, as the belts are "hidden" underneath the motors. I used my phone and pushed downside the motor when i installed the belts aligning to  the cogs and still i didnt get a great view.
I will try to push it with greater force and see. 

@kens thanks alot. Ive seen the graph before and wondered what to get out from it but that actually explains it for me! 

Will try PEC to im thinking. Is it best done trough EQMOD panel or via PHD2 ?
Do i understand correct out from what you guys are saying, that i need a bigger guidescope?

[vlaiv]

PEC won't solve this I'm afraid - it is too quick and it is not harmonic of base worm period. It can help you with guiding / tracking in general.

If I'm correct and you are using 4.77"/px guide resolution (orion mini + ASI120mm) then you should be ok, guiding resolution wise for RMS of about 0.7"-0.9".

In another words - once you get below 1"/px with your current scope - you can't really be sure if you are actually guiding at 0.9" RMS or is that figure reported due to guide system resolution being at the limit.

For EQ6 belt modded mount, if you want to be sure about your guide figures, I would say you need something like 3"/px guide resolution (that is good for up to 0.4-0.5" RMS and mount will generally do 0.6-0.7" RMS at best - so you should be fine at those figures with guide resolution).

[masjstovel]

Thanx  alot @vlaiv! I will look for a guidescope that suits! I did now tighten the RA motor. Its tight with sausagefingers between motor and casing, so i used a flat screwdriver in between and pushed it lightly further out. There was some room made so i think you guys might be correct in what youre saying. Just afraid it will be too tight and make the motors grind, but we will see. I felt it was a "handtight pressure" i made with the screwdriver. 

 

As for PEC I meant in general as it peaks 20+++ in a worm period. This is better in EQmod than in PHD2? Does EQmod take in consideration the guiding corrections or should i guide uncorrected when recording EQmod PEC?

[masjstovel]

No grinding! Puh!

[vlaiv]

You can either record it in EQMod - that will be "automatic mode". Just hit "record" while you are guiding and record for at least 4-5 worm periods (that would be around 40 minutes since your worm period is ~ 480 or 8 minutes).

Another way to do it is to start PHD2 and tell it not to output guide commands - there is option to disable guide output, but have PHD2 logging turned on. Then do about an hour of logging and just tracking guide star (at declination 0 for best resolution) and I believe you need to hit the "time stamp" button once on EQMod control panel during this time (this will serve to synchronize PEC file later).

After that - you need to load PHD2 log file into software called PECPREP and create PEC curve there for your mount. You do that by basically filtering out all frequencies that are not harmonics of worm period - and leaving just a few significant harmonics. This software has list of all significant periods for your mount that you can check.

There are tutorials on how to do this on youtube.  I think I prefer later method as it gives more control, but its much more involved - you need like couple of hours to get all done. Simple pec recording in EQMod is much easier to do.

[vlaiv]

3 minutes ago, masjstovel said:

No grinding! Puh!

Nice.

Does your mount have PPEC feature or encoders?

You need to check your manual, but issue with EQMod PEC is that you always need to park your mount after you are done unless there are encoders present. This is because synchronization must be kept between where computer thinks the mount is and actual gear positions - otherwise PEC will "correct in the wrong place".

For this reason I was not able to use both PEC in EQMod and my hand controller - as it is simple syntrek version and does not have park to home feature. EQMod does have park to home, but in case of crash or power failure or anything that powers down the mount or computer while mount is operating and not parked - will void pec and new one must be recorded.

PPEC can be loaded in mount itself and won't be lost, but I have not done that so I don't know exact procedure.

[masjstovel]

Thanks again @vlaiv.
There are no encoders and no PPEC. So does it matter if i use "Park to home position" in EQmod, or unclutch-clutch? 

I feel i need some more experience to do the PHD2 version of PEC. 
Will the PEC upload itself automatically when i start a new session in EQmod? 
I assume i can "delete" or disable the PEC curve from EQmod if not wanting to use it, or changing to PHD2 PEC ?
 

[vlaiv]

32 minutes ago, masjstovel said:

Thanks again @vlaiv.
There are no encoders and no PPEC. So does it matter if i use "Park to home position" in EQmod, or unclutch-clutch? 

I feel i need some more experience to do the PHD2 version of PEC. 
Will the PEC upload itself automatically when i start a new session in EQmod? 
I assume i can "delete" or disable the PEC curve from EQmod if not wanting to use it, or changing to PHD2 PEC ?
 

You have to park to home position - steppers need to do it since this has to do with stepper - mount synchronization and not scope/mount.

EQ6/HEQ5 maintain their position information by counting stepper ticks - it always starts at zero ticks when mount is powered up (no encoders), so it needs to be "reset" to this home position every time as this "returns stepper ticks to 0 position" as well (in reality its not the motors that count the steps - it is firmware - each time it tells stepper to do a step it either increases or decreases a counter - depending on direction of movement, it knows that each micro step is equal to certain number of arc seconds - something like 0.2" or similar - depends on the mount, so it can calculate exactly where the mount is pointing from tick count multiplied with single step angle for each axis).

PEC will be enabled in EQMod once you record it, but there is option to disable it or choose another PEC file, so there should be no problems if you loose synchronization for ever reason - you just either record a new one or tell EQMod to use different PEC file - one made by PECPREP. Look at above image that I posted - it has all the controls related to PEC - stop / pause, load, unload and so on ... screen shot is from EQMod manual in pdf format so have a look there as well.

Just a word of caution - if you now record PEC in EQMOD and decide later to switch to PHD2/PECPREP version - remember to turn off PEC when you record your log without guiding for PECPREP.

[masjstovel]

Lovely! Thanks alot @vlaiv, @kens and @wimvb! Looking forward to check RA-performance after this. First un-PEC'ed then with PEC. Will post the results if interested!

[masjstovel]

Guys... Im speechless! 
Thank you so much! I'm truly amazed you could figure that it was a too loose belt that did it! 
Theres been alot of blood and sweat and tons of frustration for 8 months or so since i did the beltmod and now finally i start seeing some fruits grow. 
Again, thanks alot! 

 

Edited August 27, 2020 by masjstovel

[masjstovel]

@vlaiv
I've been looking at the SW EvoGuide 50 ED as guidescope. This has 242mm focal length (50mm = f/4.8) and would give me a resolution of 3.2"/pixel with the ASI120MM mini.
Maybe the way to go? Or any objections before i order?

[vlaiv]

22 minutes ago, masjstovel said:

@vlaiv
I've been looking at the SW EvoGuide 50 ED as guidescope. This has 242mm focal length (50mm = f/4.8) and would give me a resolution of 3.2"/pixel with the ASI120MM mini.
Maybe the way to go? Or any objections before i order?

No objections, just a suggestion, or rather a question: did you consider OAG instead?

It can be cost effective solution and probably provides best guiding results. Only problem that I could see is the fact that you have newtonian scope and those have issues with back focus. Not sure if you could spare additional 15-20mm of you light path behind coma corrector.

I personally use this: https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html

and with addition of this for example:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

You have complete solution that is guaranteed to provide good guiding resolution and very precise guiding. It is also half the price of EvoGuide 50ED.

Back to original question - yes, EvoGuide 50ED looks like excellent little guide scope to provide you with needed guide resolution.

 

[masjstovel]

Thanks @vlaiv, good to know.
Well i didn't really consider OAG yet. There's some downsides to it i've heard, like vignetting, and as you say backfocus, especially on a newtonian.
Also, besides weight - what errors do you eliminate with OAG in relation to a guidecope? movement in the "guiding-train" (guidescope/camera/adaptors) only? 

I have my point of focus at ~85mm from tube. I've just changed to Baader Diamond Steeltrack focuser. It is minimal height 81mm with Clicklock clamp attached (which i love).
It has 40mm drawtube travel. I dont know if this is better or worse in relation to the stock PDS focuser?
I dont understand the backfocus issue with OAG- is it that you cant get far enough IN or that you cant get far enough OUT? 

Edited September 6, 2020 by masjstovel

[vlaiv]

48 minutes ago, masjstovel said:

Thanks @vlaiv, good to know.
Well i didn't really consider OAG yet. There's some downsides to it i've heard, like vignetting, and as you say backfocus, especially on a newtonian.
Also, besides weight - what errors do you eliminate with OAG in relation to a guidecope? movement in the "guiding-train" (guidescope/camera/adaptors) only? 

I have my point of focus at ~85mm from tube. I've just changed to Baader Diamond Steeltrack focuser. It is minimal height 81mm with Clicklock clamp attached (which i love).
It has 40mm drawtube travel. I dont know if this is better or worse in relation to the stock PDS focuser?
I dont understand the backfocus issue with OAG- is it that you cant get far enough IN or that you cant get far enough OUT? 

Well, most people that complain about OAG say they have issues with guide stars, but I haven't had any. I use OAG (one I linked with 16mm thickness) on 1624mm of focal length and find guide stars always (or at least - so far).

You improve your guiding with OAG because you eliminate any differential flexure, mirror flop and such - what main camera sees - OAG sees for the most part (it sees same thing only to the side of main FOV). Also - you use full focal length of your main imaging telescope - so guide precision is always sufficient.

Problem with newtonian scopes is mostly due to coma correctors - as most have 55mm working distance. If you have mono camera and need filters - you need to "pack" OAG, filter wheel, all the needed "adapters" (M48/T2 and other "converters" that you may need) together with camera back focus into those 55mm - which sometimes is not easy thing to do.

For example, let's say you have CC with 55mm back focus and M48 threaded connection. OAG 16mm screws into that, and then you need T2 camera side thread for that (it adds no optical path). Then you need filter wheel - manual basic filter wheel is something like 23mm, you already have 11mm T2 female / female extension (which you will need) and 6.5mm of camera back focus.

16 + 23 + 11 + 6.5 = 56.5

See the problem? You need really thin components - like thinner filter wheel / filter drawer instead, or shorter T2 female / female extension (then one included with camera). Other coma correctors require longer distance and there you have more room to fit it all, so it depends on what sort of CC you have, or alternatively - you can go for really short 9mm OAG.

In any case, focus will be at the same point as it is now, but you will have to replace some spacers with OAG+spacers combination if there is enough space.

I used OAG 16 with my TS80 F/6 refractor without issues - it uses x0.79 FF/FR with has working distance of about 61mm and I managed to pack OAG, T2 ring and Filter drawer (don't know exact length of it but it is <=18mm). I think I needed to use 5mm extension as well.