Topics

Guiding issues Exos2Gt PMC8

JB1983
 

So a couple weeks ago I finally completed my rig.
Been out with it twice so far and I'm trying to find some insight as to why my guiding is so bad.

Exos2GT PMC8 mount, ASI120mm mini and using a 60mm f4 guidescope.

Polar alignment in sharpcap goes well.  PHD2 finds stars no problem.  Running POTH with stellarium and stellarium scope.
My imaging camera is a dslr, and I find it easier to unlock the clutches and move the scope by hand to do my 3 or 4 star alignments.

Everything is working well and communicating well.

60 second exposures, no star trailing.

The first time out I balanced the RA and Dec evenly. Had a total RMS of .75, but the graph was all over the place.
Tonight since inwas imaging to the west, I tried the easy bias balancing. Sold my counterweight up a pinch and my total RMS was over 3.00.  RA was good at .59, but the dec was at 3.92.

Currently using the default parameters in PHD2.

I'm just wondering, is there something special I'm not doing to make this mount guide correctly?   If not, I'll go out again here soon and try it all again.  Any help would be appreciated.
Thanks 

Mark Christensen
 

Hi, a couple of things:

1. Please sign your postings. Perhaps it is under the image? Anyway, it is a good idea to do sign all postings.
2. What units are you expressing you guiding error in? Pixels or Arc Sec (")? Please indicate that.
3. How long is your guiding exposure? If you took the defaults it is probably 2 or 3 seconds. If it is much shorter then you may be 'chasing seeing'.
4. How tight was your focus on the guide star? If it is too good then the guider may be jumping from one edge to another. I've seen that happen. Soften up the focus just a tad - the algorithm computes a centroid of the image of the guide star. You also don't want the image saturated.
4. Did you evaluate the DEC drift (DEC guiding OFF). If there is little drift (many guiding exposures to see if there is a trend) then it looks like you have (unavoidable) backlash in the DEC and very good polar alignment. That means when the guider sees the curve go on one side it may be overcorrecting in the other direction. Then the guider sees the overcorrection and tries to go back the other way, producing overcorrection in the other direction. That appears to be what is happening.

So I'd do the following next time:
A. Turn off DEC guiding and see if there is much drift - a definite trend line in one way or another.
B. If there is much DEC drift, say NORTH at a 1/2 pixel (about 2 arc sec) in a couple of guiding exposures. with your rig, then in the DEC Algorithm Tab set it to SOUTH only. If the drift is to the SOUTH, set the DEC Algorithm to North Only Similar to biasing the weight of the RA axis, this avoids the motor reversing which results in backlash.
C. If you see no meaningful DEC drift then your polar alignment may be too good.  Sounds backwards, right? If the polar alignment is really good with no DEC drift then the mount is probably being driven into oscillation by the guider. See note 4 above. Aside from buying a much more expensive mount with much less intrinsic backlash (there is always some, hence the trick of biasing the RA - it avoids the RA trying to reverse direction) this is often the only cure -introduce a tiny amount of polar drift and then set the DEC algorithm to only go in one direction.
D. It is possible the DEC gears are set too tight or loose. Since I don't have the PMC I can not advise you. With virtually every mount I have had or worked on (including a non-PMC G11, a CGEM, a AP800, a CG5-AST, and the older non-PMC  EXOS2GT) I've had to do some adjustments to improve guiding. But then I am imaging, usually, at 1100mm focal length with my G-11. With the CG5 at 750mm, with the EXOS-2GT at 400mm. With normal camera lenses (below 200mm) you should be fine not matter what.

You didn't say if your pictures were OK or not, by the way. That is what matters, not the PHD2 graph by itself. If you are using nominal camera lenses even 10 arc second of peak-to-peak (more relevant than RMS) would be tolerable.

Finally, make sure it you don't have cable drag in the DEC direction or a loose mounting of the camera to the guider. Those are unlikely to be the problem since you are only seeing the oscillations in one axis. But you never know.

Regards,

Mark Christensen


On Thursday, August 29, 2019, 8:15:09 AM UTC, JB1983 <capnjake83@...> wrote:


So a couple weeks ago I finally completed my rig.
Been out with it twice so far and I'm trying to find some insight as to why my guiding is so bad.

Exos2GT PMC8 mount, ASI120mm mini and using a 60mm f4 guidescope.

Polar alignment in sharpcap goes well.  PHD2 finds stars no problem.  Running POTH with stellarium and stellarium scope.
My imaging camera is a dslr, and I find it easier to unlock the clutches and move the scope by hand to do my 3 or 4 star alignments.

Everything is working well and communicating well.

60 second exposures, no star trailing.

The first time out I balanced the RA and Dec evenly. Had a total RMS of .75, but the graph was all over the place.
Tonight since inwas imaging to the west, I tried the easy bias balancing. Sold my counterweight up a pinch and my total RMS was over 3.00.  RA was good at .59, but the dec was at 3.92.

Currently using the default parameters in PHD2.

I'm just wondering, is there something special I'm not doing to make this mount guide correctly?   If not, I'll go out again here soon and try it all again.  Any help would be appreciated.
Thanks 

 

JB,
 
What Mark said.  
 
A couple of additional things: 
- My mount requires both an East-heavy bias and a camera-heavy bias - I'm assuming that you're using a refractor as well.  That's easy, low-hanging fruit that will not harm, and may help maintain gear mesh better.
 
- Differential flexure is a thing - do make sure that your guidescope is rigidly mounted to your imaging scope, and that the guide camera is held rigidly by its focuser.
 
- If PHD2 is having trouble making DEC corrections, that could be explained by play of one kind or another in the DEC power train.  I said power train rather than 'gears', because I found that I really needed to think of the pulleys, belt and gears as an integrated system. 
You can find cookbook instructions on adjustments here:  https://espmc-eight.groups.io/g/MOUNTS/files/EXOS2PMC8_TuningNotes_v1.pdf

- I don't think you should loosen the locks after you've un-parked your PA'd scope.  The PMC8 is trying to keep track of where the mount is pointing on the sphere above it, via motor counts.  There are no encoders, so if you move the scope manually, that mechanism is defeated.  I gave up on POTH's awkward buttons and went to plate solving to center and frame, but if that doesn't suit, there are folks here that can help with joystick control, and just a couple of days ago there was a member who tried APT and was happy with its buttons.
 
Hope this helps
 
- Bob  
 
--
Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras:  Nikon D5300, Altair GPCAM2-IMX224C
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer:  Thinkpad x230, Win7Pro/64

JB1983
 

Hey, thanks for the replies.
I'm guiding a 750mm focal length newtonian. And my images at 60 second exposures look great.

Someone had noticed that PHD2 isnt pulse guiding. I got an error when I first launched PHD2 saying my mount wasnt capable of pulse guiding so i restarted it, and it connected fine after but obviously wasnt working properly.

Ill try some of what's mentioned above.  I did balance with an east bias, weights slid up the shaft about an inch above perfect balance and the scope leaned all by itself slightly with the ra clutch unlocked. 

Guiding exposure is at 2 seconds.

Stars in the guidescope were slightly out of focus as I've heard stars too much in focus dont guide well

Mark Christensen
 

Captain Jake (I assume that is your name or at least your 'handle'),

With Newtonians it is often best to rotate the tube so the camera hangs over the DEC axis. That minimizes torques. Also, with many of the commercially sold 'imaging newts' the back focus is extreme, so the camera hanging off the side or the top (opposite the DEC axis) will generate a lot of off-axis torques in some orientations. That makes it tougher on the mount - it ain't just raw poundage that matters to a mount, it is the distribution of the masses as well.

With a EXOS2GT class mount (a mechanical derivative of the CG5s and LX75/85s  I believe) you should be able to image just dandy with a 750mm Newt, esp if it is a 6 incher. I used a CG5AST for years with a 6" f/5.

I guess the companies (usually GSO) that make these scopes think everybody has long 'imaging stacks' with filter wheels and rotators so they provide lots of back focus. With large, expensive observatory class instruments that makes sense. But I'd bet 90% of the people using imaging newts are using DSLRs.The other mistake most manufacturers make is putting much too small a diagonal in the system. Small diagonals are fine and are preferred for visual work esp. at high power. For imaging you want a large diagonal to fill the image plane. There are on-line applications that will help compute the diagonal size with that if you don't have the old books that go through it.

Mark Christensen
PS: The commonly available RCs have the same problem of excessive back focus. I recently helped a friend re-align his carbon fiber 8 inch. I swear the back focus took up 1/2 of the length of the whole system.
On Thursday, August 29, 2019, 2:31:16 PM UTC, JB1983 <capnjake83@...> wrote:


Hey, thanks for the replies.
I'm guiding a 750mm focal length newtonian. And my images at 60 second exposures look great.

Someone had noticed that PHD2 isnt pulse guiding. I got an error when I first launched PHD2 saying my mount wasnt capable of pulse guiding so i restarted it, and it connected fine after but obviously wasnt working properly.

Ill try some of what's mentioned above.  I did balance with an east bias, weights slid up the shaft about an inch above perfect balance and the scope leaned all by itself slightly with the ra clutch unlocked. 

Guiding exposure is at 2 seconds.

Stars in the guidescope were slightly out of focus as I've heard stars too much in focus dont guide well

JB1983
 

Thanks for the help Mark.

Camera is hanging down at the 6 O clock position on the scope, down towards the DEC.  

If anything, the balance on the RA was out as I balanced a bit east based.

Someone in another group I'm in had mentioned that PHD2 was not pulse guiding based on my graph, and that there may be a box that needs to be unchecked.

If it's clear tonight, I'm going to go at  this step by step, nice and slowly and see where there may be an issue.

There is one area I forgot about.

My guidescope focal length is 240mm.  But the focuser was slid all the way back to achieve focus, so i added a 50mm extension.

Should I be changing the focal length to 290mm in PHD2?  Focus should be achieved tight round the 240mm mark so the extension doesnt really change anything right ?

Mark Christensen
 

Captain Jake,

No, the focal length is not changed by the need to use the extension. I have to use one as well - I probably have the same guides scope on my 8 inch f/4 (used with a G-11), a 60mm f/4 objective with a non-rotating fine pitch (but only 10mm of throw) focuser. I guess they made them so you could use them as a lens with a DSLR so there is, like a lot of other gear, excessive back focus. Hopefully your extension is T-to-T thread - those are more robust (that is, flex less) than the 1.25 inch eyepiece extensions. Ask me how I know...

The operation for PhD2 in DEC is controlled by two sets of pulldown menus. Both are under the Advanced (Brain) tab and the Mount sub-tab. First you select the DEC Algorithm (NONE is a good place to start). Then below it are some global settings, one of which is DEC Mode. For that you can select OFF (no DEC correction), AUTO (which does whatever the Algorithm says to do) and NORTH and SOUTH. You'll need to read the Help and some of the tutorials to get a handle on what the various algorithms do.

NONE with Auto just issues whatever command direction in whatever duration the guiding error indicates based on the calibration. NONE with North or South will only move the DEC axis in one direction. That is helpful if you know you have a small amount of drift in one direction or another - it allows (with a small amount of balance bias) to avoid DEC backlash completely.  The others do more complicated things which somebody at some point thought would be useful.

Speaking of Calibration, you may or may not know that there should be at least 10 or so steps in the calibration in each direction. Otherwise the calibration values may be wonky. Watch the screen while it is doing it. You'll also get an idea of how much backlash you have in DEC.

Which reminds me, you should set the guide rate to something like 0.5X or so. That is set in the mount controller, not in PhD(2).

Good luck and keep trying!

Mark Christensen

On Thursday, August 29, 2019, 5:35:23 PM UTC, JB1983 <capnjake83@...> wrote:


Thanks for the help Mark.

Camera is hanging down at the 6 O clock position on the scope, down towards the DEC.  

If anything, the balance on the RA was out as I balanced a bit east based.

Someone in another group I'm in had mentioned that PHD2 was not pulse guiding based on my graph, and that there may be a box that needs to be unchecked.

If it's clear tonight, I'm going to go at  this step by step, nice and slowly and see where there may be an issue.

There is one area I forgot about.

My guidescope focal length is 240mm.  But the focuser was slid all the way back to achieve focus, so i added a 50mm extension.

Should I be changing the focal length to 290mm in PHD2?  Focus should be achieved tight round the 240mm mark so the extension doesnt really change anything right ?

JB1983
 

Thanks for the insight Robert. I'm using a Newtonian.  So by camera heavy, that would be back end heavy with my newt correct?  

Last night I balanced back heavy on scope by just a little bit and also did east bias on the RA.  The RA numbers looked good but the DEC was out to lunch.

But 2 nights before, I balanced bottom heavy on my counterweight by a little bit and the dec was perfectly balanced. Even though my graph that night looked just as bad, the total RMS was .75

Good news is that the weather now looks clear tonight. So I'm going to go spend 2 hours playing with polar alignment and balancing.  Try guiding and see what happens.  Missing one little thing along the way can lead to such a mess,  hoping it ends up being something small.

JB1983
 

Mark, how do you change the guide rate exactly?  To be honest, other than switching from wifi to wired, I've never tried touching any of the settings with the mount.

 

Captain Jake,
 
Yes, for you I guess it would be mirror-heavy.  The point of the exercise is to reduce the possibility of the scope rocking against its DEC gears as it approaches vertical and prior weight bias diminishes.  Jerry cued me in to that a while ago, and it has helped me.  
 
Re: "Something small"  Well, for me, it was 'peeling the onion'.  Fix a problem, then find another waiting behind it.  Could be mechanical, could be technique, could be conditions.  Anyway, if you peel enough layers, eventually you get there - and really, you don't sound like you're that far off.
 
Good luck tonight!
 
Q:  Which Newt are you using?  
 
- Bob
 
--
Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras:  Nikon D5300, Altair GPCAM2-IMX224C
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer:  Thinkpad x230, Win7Pro/64

JB1983
 

Robert,

Maybe my polar alignment wasnt accurate.  It did say excellent with a very low error.  

Scope is a Skywatcher 150PDS. I love it!   Got a great deal on it.  The pain in the tail is going to be doing star alignment with the laptop. 

A few people now have said not to unlock the clutches and manually move the scope when doing my 3 or 4 star alignment, buts it's kind of a pain seeing the dslr screen and slewing the scope at the same time with the laptop controls.

I may have to give APT a whirl.  I've heard scope control is quite a bit better.

I've been reading the same. Balance a bit back heavy on the scope and try to always be east heavy.  

Hopefully figure it out tonight.  Got 2 clear nights in a row here so I gooe to get the guiding sorted tonight and actually take some longer exposures tomorrow night 

 

Captain Jake,
 
If Sharpcap says that it's excellent, then it's probably excellent.  But it can degrade.  You'll find a discussion on tripod shift and (too briefly) mount head wobble in the tuning note I pointed you to.  Easily fixed if that's the case, but your initial post didn't sound like that was an issue.  
 
One or two layers of the onion at a time.  You'll get there. :-)
 
I wondered if it was a 150PDS.  Love my refractor, but if I was to go for more aperture/fl, that one would be high on the short list.  I'll be very interested to see how the EXOS2 handles it, once you get things settled down.
 
- Bob
 
--
Mounts: ES PMC-8 EXOS2
Scopes: SV 102EDT, ZWO 60/280 Guide
Cameras:  Nikon D5300, Altair GPCAM2-IMX224C
Software: ASCOM, CdC, AstroTortilla, BYN Pro, Sharpcap, PHD2
Computer:  Thinkpad x230, Win7Pro/64

Mark Christensen
 

I don’t have the PMC8 system – they came out with it after I was already equipped (some gear going back to 1960s). In most scopes hand controls it is under Utilities or Settings.

In the old EXOS-2GT (with DC motors, gearboxes, and encoders) it was under Utilities and was pre-set to 0.125X sidereal, which is way too low for most conditions. But it was easy to set to 0.5X. Some prefer 0.25X or 0.33X (in different mounts). Choosing the guide rate is a matter of experience with the mount+payload.

 

Mark C.

 

From: MAIN@ESPMC-Eight.groups.io [mailto:MAIN@ESPMC-Eight.groups.io] On Behalf Of JB1983
Sent: Thursday, August 29, 2019 4:45 PM
To: MAIN@ESPMC-Eight.groups.io
Subject: Re: [ESPMC-Eight] Guiding issues Exos2Gt PMC8

 

Mark, how do you change the guide rate exactly?  To be honest, other than switching from wifi to wired, I've never tried touching any of the settings with the mount.