PHD2 HEQ5 guide log

[michael8554]

The results on both nights were marred by the conditions I'd say, with RA motions mirroring the dips in Star Mass.

You never got a good Calibration on either night, I guess time was pressing on the 6th.

Otherwise you might try binning the ASI 290MM to get more sensitivity, and to up the image scale to 0.86arcsecs/pixel, still fairly low.

That will hopefully reduce the Star Lost situations.

HFD is never great with an OAG but HFD over 7 could be improved to 5's.

Post again after a good night with a good Cal.

Michael

[vlaiv]

17 hours ago, knobby said:

Thanks Louise, target points very random, using oag so guide star fairly iffy but was round and value in PhD was 5.8 (best I could get it). Mount is nice and free so I'm fairly sure balance was good.

Noticed another thread where @vlaiv suggests that a heavy scope needs the belt tighter than the 2mm flex with light finger pressure? I've emailed Rowan to see what they suggest. Really appreciate your time helping 😊

Yes, my recommendation was due to the fact that I was experiencing rather fast ripple in tracking performance - 13.8s one, related to period of single tooth on motor gear. It was due to improper meshing between motor gear and belt.

I'm not seeing that in your guide logs - not yet anyway, it could turn out to be an issue after you get good guide results, but for now, if it's there, it is masked by much larger issues.

Here are some recommendations that I suggest you try:

1. Bin your camera pixels at least x3 or even x4. There is plenty of them and you don't need to go as low as 0.43"/px for guiding. My camera gives around 0.48"/px and I bin it at least x2 to get close to 1"/px (OAG and 1600mm FL). With x4 bin you'll be at 1.72"/px and that is good enough for best performance possibly offered by HEQ5 (which is around 0.5" RMS).

2. Use ASCOM driver for your 290 camera instead of native drivers - this will enable 16 bit readout mode and improve centroid precision and star SNR

3. You are using quite fast correction speeds. I think you would benefit from lowering those. You have it set at x0.9 sidereal (13.5"/s). Consider lowering it to something like x0.3-x0.4. HEQ5 type mount responds much better to slower/longer corrections than fast short ones.

4. Consider using dark library / dark calibration with PHD2

5. If you think that seeing is a problem - maybe experiment with guide exposures up to 4s long

[michael8554]

1 hour ago, vlaiv said:

3. You are using quite fast correction speeds. I think you would benefit from lowering those. You have it set at x0.9 sidereal (13.5"/s). Consider lowering it to something like x0.3-x0.4. HEQ5 type mount responds much better to slower/longer corrections than fast short ones.

Higher Guide Rate helps mounts with large Dec Backlash.

Although the mount has belt drive which removes the backlash due to the gearbox, the worm backlash could be better according to the Calibrations so far.

Michael

Edited July 8, 2020 by michael8554

[Thalestris24]

1 minute ago, michael8554 said:

Higher Guide Rate helps mounts with large Dec Backlash.

Although the mount has belt drive which removes the backlash due to the gearbox, the worm backlash doesn't look great according to the Calibrations so far.

Michael

Personally, as it's a brand new replacement mount and still appears to have the same issues as the original, I think I'd first look elsewhere for the source of the problems. I may be wrong but it just doesn't seem likely that two new belt-drive mounts would appear to have the same/similar problems.

Louise

[vlaiv]

10 minutes ago, michael8554 said:

Higher Guide Rate helps mounts with large Dec Backlash.

Although the mount has belt drive which removes the backlash due to the gearbox, the worm backlash doesn't look great according to the Calibrations so far.

Michael

My recommendation for lower guide speed is based on following:

- there is a setting in EQMod primarily kept for legacy reasons - minimum pulse duration. It is there because at the time, systems were not able to time properly intervals less than that. It is set at 50ms.

This limits minimum correction that one can make, depending on correction speed.

With 13.5"/s guide rate, that equates to 13.5"/s * 50ms = 0.675"

That is minimum correction that you can make. This alone gives you something like 1.3" of error in each guide cycle. If you are off the target by small amount - you will overshoot by 0.675" - error - in all likelihood that will be at least 0.4-0.5 for OAG as it can measure error down to 0.1". In most cases it will be due to seeing.

MinMo is set to 0.51 for both axis. This parameter is still set in fraction of a pixel, so that works out to be 0.2" or there about. Any error in that range will trigger correction - and likely overshoot - that will most likely happen due to seeing.

- Heavy scopes on lighter mount have quite a bit of inertia. Using high guide speed will result in them wanting to keep going further then they ought to by correction. For this reason - one must use slow correction speed so that energy in the system is low.

 

[michael8554]

1 hour ago, vlaiv said:

there is a setting in EQMod primarily kept for legacy reasons - minimum pulse duration. It is there because at the time, systems were not able to time properly intervals less than that. It is set at 50ms.

I wasn't aware of that setting, add that to the other setting that catches users out, the very low default Guide Rate.

Michael

[vlaiv]

2 minutes ago, michael8554 said:

I wasn't aware of that setting, add that to the other setting that catches users out, the very low default Guide Rate.

Michael

Here it is from their PDF documentation:

I'm quite certain that one can accurately measure much shorter periods of time, but this is what they say (and indeed, long time ago in Win95 and similar operating systems, when using simple timer component, one was limited to that sort of timer resolution), however things have changed and now one can measure in nano seconds (processor tick count).

[knobby]

Thanks for all the help / comments, I'll make a list and try these next clear night ... probably some time in August  🙂 

 

[michael8554]

Here's what the PHD2 Developers said:

Hi Michael

  I was aware of this kind of restriction, but I think the quote is slightly ambiguous because it suggests 50ms is a strict minimum value. 

 But I think 50ms is just the default value.  

There is a slider cotrol for this and at least on the version I’m running, it can be set as low as 20ms:

 

 

I think it’s unlikely to be a problem in practice.  An over-correction problem might occur when using a high guide speed like 0.9x sidereal and a very coarse guider image scale.  

For example, a guider image scale of 6 arc-sec/px and the lowest allowed min-move of 0.1px could result in guide commands as low as 44ms – still above the minimum limit above.  

We generally discourage a guiding image scale this coarse. 

 In the time I’ve been looking at PHD2 logs, I don’t think I’ve ever seen a problem with ongoing over-corrections using EQMOD.

[vlaiv]

13 minutes ago, michael8554 said:

Here's what the PHD2 Developers said:

I can see that pdf guide is outdated and that it is indeed possible to set this value lower - but default is still 50ms and it is still doing the same. It is rather obscure setting and most people have not heard about it - hence it will be set to 50ms for most if not all people anyway.

I'm sorry to say but logic that PHD2 Developer applied is flawed - let's just see what happens if we use that line of reasoning.

If we use 6"/px guide scale, which is according to the developer - too coarse anyway, and MinMo of 0.1 we will get 0.6" threshold of movement (if you want to guide at 0.5-0.7" RMS this is not acceptable, but let's go with it) - indeed at 13.5"/s it will take 44.444ms for correction to happen - and it won't be possible as minimum correction is 50ms.

We lower our minimum correction to 20ms and all is fine - no more error.

However, like developer mentioned, most people don't use such coarse guide resolution. Let's keep 20ms as minimum pulse width and use better guide resolution - let's say 2"/px?

We still have MinMo set at 0.1. Now we have 0.2" as our minimum correction and at guide speed of 13.5"/s that is 14.8ms - again below even lowest possible setting of 20ms. Using finer guide resolution just makes things worse with respect to this issue.

If you have lowest pulse duration and you want to do precision guiding with EQMOD, you need to lower correction speed. That is only sensible way to do it.

Manual further states:

Quote

Set the Rate to low and corrections will take longer so there is more chance that the guider will always be chasing the error rather than correcting it.

It's talking about setting guide rate "too low". Is this true?

Let's see on an example. If we set our correction speed to be fairly low - x0.3 sidereal or about 4.5"/s and need to make very large correction - 4" in a single cycle, can we do it? It turns out that we can.

Even with very short guide cycle of 1s (I advocate using at least 2-3s as a guide cycle to smooth out seeing influence and stabilize the mount), it will still have enough time to apply correction as it will take less than 1s to do correction of 4" at speed of 4.5"/s (in reality it can take even longer than this because camera exposure is stopped until correction is finished - it does not "fire" on every second like a clock - I just wanted to point out that correction is shorter than a guide cycle even for very large correction and one should not worry about chasing the error rather than correcting it).

[michael8554]

Sorry vliav, the way I read your reply you seem to be agreeing with the Developers that it's not likely to be a problem at 20mS ?

Michael

[vlaiv]

1 hour ago, michael8554 said:

Sorry vliav, the way I read your reply you seem to be agreeing with the Developers that it's not likely to be a problem at 20mS ?

Michael

It won't be a problem even at 50ms if you use low guide speed. It is high guide speed that will cause issues even at 20ms (you can't issue a pulse lasting less than that, but even at 2"/px guide resolution for MinMo of 0.1 you will need to go lower than 20ms - at 14.8ms to get good correction and not overshoot - with 20ms you'll overshoot by 25%. I'll let you do the math on 0.5"/px guide resolution).

[malc-c]

Late to the party on this occasion.  I've read through this thread, but can't seem to find info on how the mount is being controlled and how the guiding is set up, other than a comment that the OP is not using EQMOD.

Just want to add to the comments on backlash.  Belt mods do not remove backlash to these mounts.  There is still backlash in the worm and in any small tolerances in the pulleys.  When I was developing / experimenting with belt conversion years ago (before rowan engineering made a business from it) I opted to use EQMOD and went for a 4:1 ratio that helps with the harmonics for PE.  - But I feel we're digressing ...

The thing I can't understand is the fact that in this original image the calibration steps are not at 90 degrees to each other for RA and DEC

 

Are you still having the same problem ? - or does the calibration now gives you the correct L shaped tracks ?

What is bing used as a guide scope or is the OP using an off axis guider ?

If the OP is not using EQMOD, is this something they would consider?  Using an EQDIR cable -  It would certainly rule out whatever method is being used to interface the PC to the mount.

[knobby]

Very good questions Malc, I use an oag, lynx eqmod cable, PHD2 on laptop, pulse guide. The mount has been replaced and will now calibrate ok if I nudge North first. I prefer to use the Synscan app as I got used to it with my AZ-GTI. But the last 2 attempts were with EQ mod to rule out the Synscan app.

[malc-c]

Thanks for the summary of what you have and what you have tried...  Just to clarify things (not that I may be able to offer any more to the cause as other posters are more knowledgeable on PHD), are you still having the same issues as the first HEQ5, regardless if EQMOD or the Synsan app is used?

One observation I made is that with your scopes 1400mm (as per calibration data) focal length (presumable you are using a reducer as the C8 is normally over 2000mm f10), and an OAG, could the issue be that the calibration pulses are moving the star too much, or that it moves it out of the FOV of the OAG?.  Could it also be a factor that what has been suggested as poor seeing could be a combination of a faint star and f10 ratio giving a faint image of the star ?

My experience of guiding has been with short fast guide scopes like the ST80 and now the 9 x 50 finder, with no experience of OAG, so apologize if my comments are irrelevant, or my assumptions are wrong. 

[knobby]

Thanks Malc, I think it's a combination of things to be honest. Loss of confidence , poor weather / seeing, skill, expectation, mount limits.

It will calibrate pretty well on the replaced mount so long as I manually nudge North first.

Need a few nights of decent weather.

[Thalestris24]

12 minutes ago, knobby said:

Thanks Malc, I think it's a combination of things to be honest. Loss of confidence , poor weather / seeing, skill, expectation, mount limits.

It will calibrate pretty well on the replaced mount so long as I manually nudge North first.

Need a few nights of decent weather.

Hi Knobby

You shouldn't need to nudge a belt driven mount in order to calibrate - that probably indicates a likely balance or setup problem. You should be able to get a perfect L-shape calibration, especially with an oag (the RA and DEC axes can be assumed to be orthogonal and obviously should be with an oag).

Louise

[knobby]

9 minutes ago, Thalestris24 said:

Hi Knobby

You shouldn't need to nudge a belt driven mount in order to calibrate - that probably indicates a likely balance or setup problem. You should be able to get a perfect L-shape calibration, especially with an oag (the RA and DEC axes can be assumed to be orthogonal and obviously should be with an oag).

Louise

See what you've done 😁 there I was , prepared to accept the fact that I'm at the limit of the mount so living with it ! Now I'm going to be picky again 😂

[Thalestris24]

1 hour ago, knobby said:

See what you've done 😁 there I was , prepared to accept the fact that I'm at the limit of the mount so living with it ! Now I'm going to be picky again 😂

Might there be any nearby members that could give you a hand / fresh pair of eyes?

Louise

[knobby]

39 minutes ago, Thalestris24 said:

Might there be any nearby members that could give you a hand / fresh pair of eyes?

Louise

Maybe after COVID ... 😁

[Thalestris24]

27 minutes ago, knobby said:

Maybe after COVID ... 😁

By the way, I take it the mount is pointing close to North when you calibrate (always have DEC Compensation checked too)

Louise

[malc-c]

Have a look on YouTube for Dions guide to balancing the OTA. 

https://youtu.be/hGduG2jB9ec?t=131  and his recap https://youtu.be/uK2bXfVNoQU  They are old videos, but the information is current.  It deals with one issue that is often overlooked and that is the Centre of Gravity (CoG)

Basically with the mount as sweetly balanced, either with or without the bias for imaging it means that the guide system doesn't have to work harder or over compensates.  With belt modified mounts there are less items in the drivetrain to create backlash.  If the mount has been converted to a Rowan belt mode by the retailer, then the worm meshing and backlash should not need touching as they would (should) have set this up prior to selling the mount. 

My scope hadn't been used for nearly three years until recently when I gave it a full overhaul and set it all back up from scratch and then went through lots of fine tuning over several nights with PHD in order to get the best out of my finder guider.  Now the scope is performing really nice given the limitation of using a finder as a guide scope.

Edited July 17, 2020 by malc-c

[knobby]

8 hours ago, Thalestris24 said:

By the way, I take it the mount is pointing close to North when you calibrate (always have DEC Compensation checked too)

Louise

Ah, interesting ... I've always calibrated at around 30° Dec ... Sort of South South East and up towards celestial equator.

[knobby]

7 hours ago, malc-c said:

Have a look on YouTube for Dions guide to balancing the OTA. 

https://youtu.be/hGduG2jB9ec?t=131  and his recap https://youtu.be/uK2bXfVNoQU  They are old videos, but the information is current.  It deals with one issue that is often overlooked and that is the Centre of Gravity (CoG)

Basically with the mount as sweetly balanced, either with or without the bias for imaging it means that the guide system doesn't have to work harder or over compensates.  With belt modified mounts there are less items in the drivetrain to create backlash.  If the mount has been converted to a Rowan belt mode by the retailer, then the worm meshing and backlash should not need touching as they would (should) have set this up prior to selling the mount. 

My scope hadn't been used for nearly three years until recently when I gave it a full overhaul and set it all back up from scratch and then went through lots of fine tuning over several nights with PHD in order to get the best out of my finder guider.  Now the scope is performing really nice given the limitation of using a finder as a guide scope.

Cheers, I'll watch those later.

[vlaiv]

1 hour ago, knobby said:

Ah, interesting ... I've always calibrated at around 30° Dec ... Sort of South South East and up towards celestial equator.

Calibrate at around 0 DEC.