Disappointed with guiding Mesu - logs now attached

[tooth_dr]

I was wondering if I can get some advice/opinions on this.

I have just started using a mesu 200 mk2, and initially started with a 500mm newt/finderscope guider and a 120MM.  I was getting around 0.4"DEC+RA -> 0.6"RMS total (peak to peak 1.0"RMS). This was 3-4 times better than the 2.23"/px imaging resolution.

I set up a 1200mm scope, which is similar in weight to the 500mm scope, in fact uses less weight to balance, but is obviously physically bigger.  I'm using a ZWO OAG with this, and a 178MC (binned 2x2).  I can barely achieve 1.0"RMS, with larger deviations, giving a peak to peak of 2.5"+RMS (on two separate nights, the second of which - last night - the stars are not round in 3min subs, however in the first night there were round).  The scope/camera has an imaging resolution of 0.93"/px.  Obviously this isn't an acceptable level of accuracy for the scope/camera combo.  

 

So I've never used an OAG before, and I appreciate that it is going to minimise differential flexure, but I'm disappointed in the performance.  Where am I going wrong?  Could it simply be poor seeing, or is there something else I'm doing wrong?  I think all the parameters are correct in PHD2.  Does it automatically adjust the graph scale for binned captures, as I'm using 2x2 to get better sensitivity, bringing the pixel size to a still ok 4.8px.

 

Any pointers welcome.  Here is a sample sub from last night.  I can attach PHD guidelogs from finderscope and OAG if this helps? I focused with a bhatinov mask, and it looked, but as you can see the quality of the sub is poor.  I didnt touch anything on the scope (except electronically focused) between the two subs.   I've never really paid attention to seeing, because I'm only ever imaging at a short FL, and even bad seeing wouldnt have much effect.  Is this just the norm when trying to image at a higher resolution?  I would have been better try to get RGB last night, but I guess this is a lack of experience shining through.

 

Best wishes

Adam.

 

180s luminance sub 30/03/20

 

600s h-alpha sub 29/03/20

Edited April 7, 2020 by tooth_dr

[david_taurus83]

Might sound silly but did you change the focal length to 1200mm in PHD? I tried binning my guide camera on my small setup before but it also gave questionable guiding results so I stuck with 1x1.

[tomato]

First off, you should be reassured that your mount can do better. I use a ZWO OAG with an ASI 120 mini on 1050 mm FL and can achieve 0.5" total RMS on a regular basis, better than 0.45" on a good stable night, and that's with two Esprit 150s hanging off a Mk 1 Mesu 200. I am seeing dec corrections all on the same side as well,  so my PA needs attention.

Your at an effective pixel size on the guide camera of 4.8 micron vs 3.75 for the 120, not sure if that is significant. Here is a 2 min raw sub, imaging at 0.47" per pixel. My guiding improved when I reduced the duration of the calibration steps, but to be fair, PHD warned me that it wasn't  seeing enough steps in the calibration.

 

 

 

 

[andrew s]

It looks to me that the change in arc sec guiding is just due to the longer focal length. The optical "leaver"to any error is over twice as long.

Regards Andrew 

[Laurin Dave]

Hi Adam..

Your numbers are not what you'd expect for a Mesu.. as per Tomato my Mesu/Esprit150 (1070mmm) OAG gives guiding better than 0.4"RMS on a good night and very rarely worse than 0.6" even when at low altitude and bad seeing..  even without guiding over 5 minutes or so its below 1" .   I'd run the PHD2 guiding assistant and see what it says about your drift rates and settings optimisation..  and check that focal length and pixel size are correct ..   I'd also check that everything is tight particularly the OAG on which, if its the old version like the one I have, the camera holder can rock on the OAG stalk as the holding screws are not perpendicular.... and balance...  especially if the guide corrections are all in the same direction..   Happy to look at your logs if you'd like..

 

Dave

[tomato]

36 minutes ago, Laurin Dave said:

Hi Adam..

Your numbers are not what you'd expect for a Mesu.. as per Tomato my Mesu/Esprit150 (1070mmm) OAG gives guiding better than 0.4"RMS on a good night and very rarely worse than 0.6" even when at low altitude and bad seeing..  even without guiding over 5 minutes or so its below 1" .   I'd run the PHD2 guiding assistant and see what it says about your drift rates and settings optimisation..  and check that focal length and pixel size are correct ..   I'd also check that everything is tight particularly the OAG on which, if its the old version like the one I have, the camera holder can rock on the OAG stalk as the holding screws are not perpendicular.... and balance...  especially if the guide corrections are all in the same direction..   Happy to look at your logs if you'd like..

 

Dave

Very good point on the OAG rigidity, I have the later version with the additional screws, I could easily see how floppy it could get without these.

[tooth_dr]

18 minutes ago, tomato said:

Very good point on the OAG rigidity, I have the later version with the additional screws, I could easily see how floppy it could get without these.

Thanks guys I’ll get back to you all later when the kids are in bed, it’s the middle ones 6th birthday today.

Its a new ZWO OAG from FLO. 

[Skipper Billy]

Could you easily attach your finder guider to the longer scope? That will tell you whether or not its your OAG or not?

 

[tooth_dr]

2 hours ago, Skipper Billy said:

Could you easily attach your finder guider to the longer scope? That will tell you whether or not its your OAG or not?

 

Hi David. Thanks for tip. I was in the process of doing just this, rigidly attaching a finderscope to the longer scope.  I can leave the oag in situ, and try out the finderscope guiding.  Clear skies will be the only hurdle 🤷‍♂️

[steppenwolf]

Hi Adam,

You're certainly not getting the best out of your new mount - with a focal length of 1070mm and an OAG, I become very disappointed if my tracking exceeds 0.3" RMS. What guide exposure length are you using currently?

As already suggested, check the integrity of the turret as on two different makes of OAG, I have found this to be a weak point and as well as adding and additional retaining screw to the wide face of one of them, I shim the turret retaining tube with kitchen foil to make it an interference fit to obviate differential flecure.

As an aside, I have found the conditions rather strange during this latest clear spell with tracking getting as high as a very disappointing 0.7" but tonight on M3 at about 33 degrees above the horizon I am getting RA 0.221" and Dec 0.178" which is more like it.

[tooth_dr]

1 hour ago, steppenwolf said:

Hi Adam,

You're certainly not getting the best out of your new mount - with a focal length of 1070mm and an OAG, I become very disappointed if my tracking exceeds 0.3" RMS. What guide exposure length are you using currently?

As already suggested, check the integrity of the turret as on two different makes of OAG, I have found this to be a weak point and as well as adding and additional retaining screw to the wide face of one of them, I shim the turret retaining tube with kitchen foil to make it an interference fit to obviate differential flecure.

As an aside, I have found the conditions rather strange during this latest clear spell with tracking getting as high as a very disappointing 0.7" but tonight on M3 at about 33 degrees above the horizon I am getting RA 0.221" and Dec 0.178" which is more like it.

Thanks Steve for the reply.

My tracking seems on par with my EQ6, which isn’t really what I wanted to find!

With the finderscope previously on the mesu I was getting 0.4-0.6 which i felt was ok.

With this OAG I suspect it must be an error with flexure, balance, wind, or something that has changed, maybe polar alignment is out a bit, but i don’t think so.

Ive been using anything from 2-4s exposures, no difference really.

I didn’t get my guidelogs this evening. Will get them tomorrow. Been making a way of fixing a guidescope solidly to the 10” to try that out also.  

Edited March 31, 2020 by tooth_dr

[tooth_dr]

7 hours ago, david_taurus83 said:

Might sound silly but did you change the focal length to 1200mm in PHD? I tried binning my guide camera on my small setup before but it also gave questionable guiding results so I stuck with 1x1.

I did change the figures. Checked it a few times!  I’ve never used the 178 colour for guiding before.  I’ve changed too many variables in one go here!

[tooth_dr]

7 hours ago, andrew s said:

It looks to me that the change in arc sec guiding is just due to the longer focal length. The optical "leaver"to any error is over twice as long.

Regards Andrew 

Hi Andrew. Sorry I’m not totally sure what this means?

I suspect if I had any telescope attached and was still using my finderscope to guide, I should in theory still be guiding at the original good guiding of 0.6”.  

[david_taurus83]

Just thinking about the binning again Adam, when I tried I'm sure my guiding RMS doubled and I couldn't figure out why? Does CMOS binning truly create a super 2x2 pixel and does PHD read it correctly? Have you tried 1x1 on the OAG?

[Laurin Dave]

13 hours ago, steppenwolf said:

Hi Adam,

You're certainly not getting the best out of your new mount - with a focal length of 1070mm and an OAG, I become very disappointed if my tracking exceeds 0.3" RMS. What guide exposure length are you using currently?

As already suggested, check the integrity of the turret as on two different makes of OAG, I have found this to be a weak point and as well as adding and additional retaining screw to the wide face of one of them, I shim the turret retaining tube with kitchen foil to make it an interference fit to obviate differential flecure.

As an aside, I have found the conditions rather strange during this latest clear spell with tracking getting as high as a very disappointing 0.7" but tonight on M3 at about 33 degrees above the horizon I am getting RA 0.221" and Dec 0.178" which is more like it.

Hi Steve..  please tell us how you achieve such excellent guiding ..  

Thanks 

Dave

[david_taurus83]

Just looked through the bumf on PHD and it makes reference to binning if the camera supports on chip (hardware) binning. CMOS cameras dont support this but I dont know if PHD has been updated since this was written.

https://openphdguiding.org/man-dev/Advanced_settings.htm

 

[andrew s]

12 hours ago, tooth_dr said:

Hi Andrew. Sorry I’m not totally sure what this means?

I suspect if I had any telescope attached and was still using my finderscope to guide, I should in theory still be guiding at the original good guiding of 0.6”.  

Sorry if I was unclear. What I had in mind was that if there were a mechanical error or limitation determining the accuracy of the guiding if would result in an arc second error proportional  to the focal length of the telescope where the arc second was expressed.

Probably an irrelevant speculation.

Regards Andrew 

[steppenwolf]

52 minutes ago, Laurin Dave said:

Hi Steve..  please tell us how you achieve such excellent guiding ..  

Thanks 

Dave

Well, a lot is down to the mount, of course, as the smaller the corrections required in the first place, the better.

I think that apart from ensuring the physical integrity of the mechanical parts - i.e. making certain that every connection is tight and in particular making sure that the OAG turret is absolutely solid (I go to some lengths to ensure that the USB cable is solidly supported so that gravity can have zero effect on the top of the guide camera) - the rest is down to settings. For my Mesu 200 (original design) I use the following settings:-

1. Guide camera (SX LodeStar original  version) a fraction de-focussed with regard to the imaging camera

2. Guide camera set to 2 x 2 binning

3. Guide camera set to 3 second exposures

4. Pulse guide rather than ST4

5. Guiding with MaxIm DL

[Laurin Dave]

Thanks Steve..  Much the same setup as you (imitation being the sincerest form of flattery) except I use PHD2, don't bin and haven't supported the guide camera USB cable..  so some things to do and experiment with

Dave

[gorann]

Interesting thread. I now run two obsys next to each other, one with a Mesu200 and a double Esprit rig (150 + 100 just like Dave but I have them side by side) and the other with my old EQ8 and a very heavy (40+ kg) Meade 14". I now have OAGs on both (the new ZWO on the EQ8 and a TS on the Mesu). Lodestar X2 on both. My main experience now is that I get very similar RMS values from the mounts and they follow each other tightly depending on sky conditions. So, around 0.4 " RMS on good nights and considerably worse on bad seeing nights. Recently moving from a ST80 guide scope to OAG for the Mesu made a big improvement - before that I had consistently better guiding on the EQ8 which was a bit of a disapointment, but now they are the same. I would have expected the Mesu to beat the EQ8 but I think that my atmosphere so far have not been able to tell them apart - still waiting for that magical night I did add that extra screw to the TS guider - in the original version the stalk was quite wobbly.

I do worry that the TS OAG somehow leaks in light since I have had some nights with an unexplainable gradient that I never seen before after I moved from guidescope to the TS OAG. Could that be an issue?

Edited April 1, 2020 by gorann

[tooth_dr]

23 hours ago, Laurin Dave said:

I'd also check that everything is tight particularly the OAG

 

18 hours ago, steppenwolf said:

As already suggested, check the integrity of the turret

 

22 hours ago, tomato said:

Very good point on the OAG rigidity, I have the later version with the additional screws

 

22 hours ago, Skipper Billy said:

That will tell you whether or not its your OAG or not?

Well another user error detected! There was a second grub screw which was hidden against the focuser. This was loose and allowing a fair bit of movement.  I tightened all up and there isn’t any movement now. I hope this was the problem!  I’ve highlighted the loose screw and a video of movement.

 

[Laurin Dave]

12 minutes ago, gorann said:

Interesting thread. I now run two obsys next to each other, one with a Mesu200 and a double Esprit rig (150 + 100 just like Dave but I have them side by side) and the other with my old EQ8 and a very heavy (40+ kg) Meade 14". I now have OAGs on both (the new ZWO on the EQ8 and a TS on the Mesu). Lodestar X2 on both. My main experience now is that I get very similar RMS values from the mounts and they follow each other tightly depending on sky conditions. So, around 0.4 " RMS on good nights and considerably worse on bad seeing nights. Recently moving from a ST80 guide scope to OAG for the Mesu made a big improvement - before that I had consistently better guiding on the EQ8 which was a bit of a disapointment, but now they are the same. I would have expected the Mesu to beat the EQ8 but I think that my atmosphere so far have not been able to tell them apart - still waiting for that magical night. I did add that extra screw to the TS guider - in the original version the stalk was quite wobbly.

I do worry that the TS OAG somehow leaks in light since I have had some nights with an unexplainable gradient that I never seen before after I moved from guidescope to the TS OAG. Could that be an issue?

Interesting Goran...  do you run the same guide cam exposure times on both mounts...    I also sometimes get a light leak from the OAG along the stalk..  all those LEDs I expect even though most taped over 

[Laurin Dave]

6 minutes ago, tooth_dr said:

 

 

 

Well another user error detected! There was a second grub screw which was hidden against the focuser. This was loose and allowing a fair bit of movement.  I tightened all up and there isn’t any movement now. I hope this was the problem!  I’ve highlighted the loose screw and a video of movement.

 

IMG_2664.MOV

Just like mine was.....   lets hope its the fix..  also pretty sure that as your guide cam doesn't bin on chip that your real numbers are twice as good as you think and that the stars are bad because of the wobbly OAG..  

[ollypenrice]

When using an OAG on Yves' 14 inch I made a strap to stop any movement. (It only rocked in one axis.)

 

I use the old Mesus so have a choice of just two guide speeds. I use slow. Could there be a relationship between the optimal guide speed and the pixel scale of the guide system?  I don't know the SiTech system and am just thinking aloud.

I would certainly run the guiding assistant as recommended above.

My guide RMS in arcsecs was unaffected by moving to ST80 guiders. It's usually in the 0.35" area on a stable night, this on two different mounts.

Olly

 

[steppenwolf]

1 hour ago, tooth_dr said:

Well another user error detected! There was a second grub screw which was hidden against the focuser. This was loose and allowing a fair bit of movement.  I tightened all up and there isn’t any movement now. I hope this was the problem!  I’ve highlighted the loose screw and a video of movement.

Well that'd do it!! Hopefully this will make an enormous difference.