PHD2 - increasing total RMS error using Ioptron CEM40

[Legion Of Andromeda]

I bought a second hand cem40 two months ago in very good condition, aparrently it was checked over / serviced by Altair prior to sale

 

when i first started using it, i was getting 0.8 total RMS, which is fine for a GT81

but it has got steadily worse since then.... between 1.5 and 2 with a random erratic histogram

 

I'm using phd2 dev3 with multistar guiding

the scope , mount and tripod is very tight and stable as i can get it

No cable drag, all of it is through the mount (2 CABLES)

3d balance is spot on , and the axes are very free-moving

i run phd2 calibration every four sessions or so

I run the Guiding assistant every time, using its suggestions and i'm using predictive pec algorithm..other algos make little or no difference

framerate is anything between 1 and 3 seconds

pec playback is 'off' in the mount settings

ipolar is bang-on, but goes out very slightly after a slew

i dont use my scope if its windy

most of my imaging is at 75 degrees +++ lately (maybe thats an issue)

my guidescope is WO 50 x 200 and well focused

my guide camera is a ZWO mm120

my backlash seems almost non existant, ive set it up during the day and slewed back and forth on a distant object with no delay / backlash....that i can tell of

 

what can i do myself to try / check to improve matters?

my CW is towards the end of the bar, do i add another one and shift them both up? whilst i'm reducing inertia, i'm adding mass, so not sure if that helps?

is the CEM40 prone to flexure / wobbles?

can i improve my backlash? i'm happy to make the necessary adjustments, but lack the documents.

is my guidecam / scope any good?

what is odd, is that the reported backlash varies tremendously each time i run the Guiding assistant

any other ideas would be greatly appreciated

[masjstovel]

Hi, 
On all the "guide-help"-threads i've seen, the question that usually pops up very early:  "Can you upload your guidelogs?". I would adivise you to provide link to your logs, as people who are good at this can analyze it in PHD log viewer and hopefully provide help. 

Also i react to your comment "what is odd, is that the reported backlash varies tremendously each time i run the Guiding assistant"
If the variation is what you provide from your photos (41 - 159ms). I live with quite some backlash on my mount and GA says 1500 to 3500ms every time i run it. I still guide at 0.4-0.6 (0.3X in the best positions and if seeing is nice, and ~0.7X in the wrong spots, for example in south close to the horizon) - so a span of around 100ms i would not be concerned about.

What guidescope are you using (what is you pixel resolution?) 
About the weights: I'm absolutely no professional, but ive heard from others that its better to add weights and have them closer to center than using less weights and towards the end of the bar. 

Edited May 1, 2021 by masjstovel

[vlaiv]

2 hours ago, Legion Of Andromeda said:

most of my imaging is at 75 degrees +++ lately (maybe thats an issue)

my guidescope is WO 50 x 200 and well focused

my guide camera is a ZWO mm120

I think that here in lies the problem.

You are guiding at 3.87"/px

Centroid precision is about 1/16th of pixel or about 0.0625px - you simply can't get more precise measurement than that. That is ~0.24".

Guide assistant is calculating that to be your max drift rate as it gets that value each second it measures (it can't get better precision).

By the way 0.23"/s is rather high drift rate.

Imagine you have 15" P2P periodic error (as claimed by iOptron -/+ 7"). Worm period is 400s. This means that it takes 200s to go 15" so we have 15"/200s = 0.075"/s

Even if drift rate is twice that at 0.15" - 0.23" is still quite a bit higher than that - and that is your best result from guide assistant. Also notice that RA P2P is reported to be 8" in worst case and often less.

I think that you are limited by your guide system rather than mount.

You could try following:

1. Calibrate guider at DEC 0 regardless of where you guide (even at DEC 75°)

2. Use slower guide correction speed than x0.5 sidereal - if you can - set it to say x0.25 or x0.2 (not sure what mount supports)

3. Attach guide camera to main scope and run a guide session that way (don't forget to change FL in PHD2) - to see if additional resolution helps

4. Use 3-4s guide exposures rather than 1s or 1.5s

See if that changes things.

[Legion Of Andromeda]

1 hour ago, vlaiv said:

I think that here in lies the problem.

You are guiding at 3.87"/px

Centroid precision is about 1/16th of pixel or about 0.0625px - you simply can't get more precise measurement than that. That is ~0.24".

Guide assistant is calculating that to be your max drift rate as it gets that value each second it measures (it can't get better precision).

By the way 0.23"/s is rather high drift rate.

Imagine you have 15" P2P periodic error (as claimed by iOptron -/+ 7"). Worm period is 400s. This means that it takes 200s to go 15" so we have 15"/200s = 0.075"/s

Even if drift rate is twice that at 0.15" - 0.23" is still quite a bit higher than that - and that is your best result from guide assistant. Also notice that RA P2P is reported to be 8" in worst case and often less.

I think that you are limited by your guide system rather than mount.

You could try following:

1. Calibrate guider at DEC 0 regardless of where you guide (even at DEC 75°)

2. Use slower guide correction speed than x0.5 sidereal - if you can - set it to say x0.25 or x0.2 (not sure what mount supports)

3. Attach guide camera to main scope and run a guide session that way (don't forget to change FL in PHD2) - to see if additional resolution helps

4. Use 3-4s guide exposures rather than 1s or 1.5s

See if that changes things.

sorry, i meant 75 degree altitude, (50 degs in dec) so the scope is nearly vertical for m51 and 101, when i calibrate its usually towards the ecliptic / meridian, ...where do i change the guide correction speed?.... i will swap the camera over.... ive tried longer exposure times to no avail

However , since i made this post , i checked the belts which were both tight, but the gear meshing was loose...now waiting for some clear skies to test

[vlaiv]

31 minutes ago, Legion Of Andromeda said:

sorry, i meant 75 degree altitude, (50 degs in dec) so the scope is nearly vertical for m51 and 101, when i calibrate its usually towards the ecliptic / meridian

You should calibrate at DEC close to zero (ecliptic varies with season and swings between -23 and +23 degrees DEC).

This is because relative RA motion is affected by declination. If you image for example Polaris (northern star) - it will make a circle on your sensor around north celestial pole - and although it moves at same RA sidereal rate as all the other stars - it only covers very small number of pixels per hour. This is extreme example how DEC affects projection of RA onto pixels.

For highest calibration precision - you should therefore calibrate where motion of the star is greatest in number of pixels - to achieve good precision.

When you guide at high DEC - opposite thing happens if you don't have precise enough guide system.

If you have guide system that is only precise down to 0.06px and you have 4"/px resolution (rounded up). Then relative RA motion will be affected by cosine of DEC.

Say that you are imaging at dec 60° - cos(60°) is 1/2. To get actual error PHD2 needs to account for this 1/2 factor and 0.06px error becomes 0.25" and corrected by 1/2 factor now becomes 0.5". Your guide system has 0.5" error with every exposure that simply has nothing to do with seeing, mount tracking - anything - just precision of guider setup.

That is what I meant by high DEC and issue with guider precision. This might be issue that you are having regardless of what the actual cause of poor guiding is. If you've found that you have some meshing issues with gears - yes, correct that first and later look how much difference the longer focal length makes to guide performance (maybe you'll be able to guide at 0.5" RMS instead of 0.8" RMS when you fix everything).

 

[vlaiv]

1 hour ago, Legion Of Andromeda said:

where do i change the guide correction speed?.

Should be somewhere in mount settings of your driver.

 

[Legion Of Andromeda]

2 hours ago, vlaiv said:

You should calibrate at DEC close to zero (ecliptic varies with season and swings between -23 and +23 degrees DEC).

This is because relative RA motion is affected by declination. If you image for example Polaris (northern star) - it will make a circle on your sensor around north celestial pole - and although it moves at same RA sidereal rate as all the other stars - it only covers very small number of pixels per hour. This is extreme example how DEC affects projection of RA onto pixels.

For highest calibration precision - you should therefore calibrate where motion of the star is greatest in number of pixels - to achieve good precision.

When you guide at high DEC - opposite thing happens if you don't have precise enough guide system.

If you have guide system that is only precise down to 0.06px and you have 4"/px resolution (rounded up). Then relative RA motion will be affected by cosine of DEC.

Say that you are imaging at dec 60° - cos(60°) is 1/2. To get actual error PHD2 needs to account for this 1/2 factor and 0.06px error becomes 0.25" and corrected by 1/2 factor now becomes 0.5". Your guide system has 0.5" error with every exposure that simply has nothing to do with seeing, mount tracking - anything - just precision of guider setup.

That is what I meant by high DEC and issue with guider precision. This might be issue that you are having regardless of what the actual cause of poor guiding is. If you've found that you have some meshing issues with gears - yes, correct that first and later look how much difference the longer focal length makes to guide performance (maybe you'll be able to guide at 0.5" RMS instead of 0.8" RMS when you fix everything).

 

sorry thats my poor terminolgy, i meant the equatorial. ....after sorting out the meshing. it started badly see pic below, but i'm currently getting 0.9 in poor seeing looking east and i havent recalibrated with the lowered the guiding rate yet because there are clouds due south now

 

[Newforestgimp]

On 01/05/2021 at 23:45, vlaiv said:

Should be somewhere in mount settings of your driver.

 

Sorry to resurrect an old thread, but did this guide speed change in commander help ? Did you adjust it in the screen shot provided ?

Andy

[vlaiv]

3 minutes ago, Newforestgimp said:

Sorry to resurrect an old thread, but did this guide speed change in commander help ? Did you adjust it in the screen shot provided ?

Andy

I would not know. I found that screen shot online to try to help - I believe it is screen shot of CEM40 drivers where it allows for guide speed adjustment - but I don't know for sure since I never operated that mount.

@Legion Of Andromeda should be able to answer that but as far as I can tell - he/she have not visited SGL since May 5th

[Newforestgimp]

28 minutes ago, vlaiv said:

I would not know. I found that screen shot online to try to help - I believe it is screen shot of CEM40 drivers where it allows for guide speed adjustment - but I don't know for sure since I never operated that mount.

@Legion Of Andromeda should be able to answer that but as far as I can tell - he/she have not visited SGL since May 5th

Sorry Vlaiv, I thought I was replying to the original poster. 👍

Andy