First test with OAG

[OK Apricot]

I toyed with the idea of getting an OAG for a couple weeks and decided to just do it, thinking if it fails I can sell it and go back to a smaller guide scope. 

So it was delivered yesterday and last night I managed to get out to try to set it all up. After a couple of hours trial and error I managed to find mutual focus between both cameras, shout out to the moon 🙌. The setup: Skywatcher 80ED>0.85xFR>ZWO OAG(ASI120MM-Mini)>ZWO filter drawer>ASI533MC-PRO, controlled by ASIAIR Plus on an EQ6-R Pro. 

After finding rough focus I slewed to my target, fine tuned focus and set it off for calibration and to take some test shots. Backspacing of camera looks pretty close, no obvious aberrations on the sub frames (but will find out later tonight when I process the data). I was a little disheartened to see figures up near 2"RMS. 

I calibrated close to celestial equator and meridian (Thor's Helmet). Mount was balanced, tried 1,2,3s exposures, tried gain between 75-100, changed aggression, duration of RA/DEC but the best I could get was 1.3"ish average. A couple things I noted was when a plane flew through the guider Fov it was not to be seen on my sub frame, I'm assuming this is normal seeing as the prism is not in the line of sight of the sensor, so doesn't see the same thing. Also that DEC was reporting the highest error up around 1", RA around 0.8".

I'm just wondering really whether this could've just been seeing, guiding showing higher error due to smaller image scale? Or could it be something else? I wasnt hoping for miracles to start with but was expecting a bit better than this seeing as the guiding software will show a good 0.4" on good nights with a guide scope. 

 

 

Edited February 8, 2023 by OK Apricot
Details

[Elp]

I've been using one the past few sessions and I think the seeing hasn't been brilliant, guiding varying from around 0.8 best to 2-3 RMS. But this is at 1000mm FL.

[michael8554]

Would need to see the PHD2 GuideLog to get the real story.

Michael

[Stefan73]

Did you start from a complete new profile and let it find some suggested values?  Always worth it when changing something as radical as moving to OAG from a guide scope.  Are you binning?  I use the  same OAG setup with my 130PDS and it's definitely better with binning 2x2 the guide camera. 

The larger DEC error might be bad polar alignment, the PHD2 Guidelog would tell you.

 

 

[OK Apricot]

So this is at 520mm. I'm not sure how to provide a log from ASIAIR? As far as polar alignment I got it to 4" accuracy according to that. For starters I'll try bin2 next time. 

[alacant]

4 hours ago, OK Apricot said:

was expecting a bit better

Hi

So the images with the oag were worse than with the separate guide telescope (SGT)? Along with the log, could you post an example frame from the SGT and from the oag?

The log files can be found in a folder called 'log' on the asiair internal card (or other usb device if you have set it up).

Not sure from where you're imaging, but Thor's Helmet is around 10º below the equator and so may be at a low enough altitude for calibration and guiding to be more affected by seeing than at higher altitudes. Try calibrating on the meridian but above the equator before slewing to your target?

Finally, although you almost certainly would with a reflector, I'm not sure if you will see much difference between using a SGT compared with an oag on a refractor at ~500mm focal length.
Cheers and HTH

Edited February 8, 2023 by alacant

[newbie alert]

Not too sure that there's anything much to benefit from using a OAG with a 80mm frac..

I use one with my esprit 80 as my camera has a internal filterwheel/oag

Rules for the oag setup is... Set your focal length of the guider as the full FL of the scope inc the reducer so I think a ed80 is 750mm X .85 of the reducer is 637.5....Setup your spacing needed for your telescope and imaging camera, this is dictated by the scope that you're using, ... Measure the distance from the prism to the imaging camera sensor,  and replicate this distance from the prism to guide camera sensor taking into account the backfocus distance of the guide cam.. both should now come to focus at the same time, fine tuning maybe needed to get round stars and not seagulls or rugby ball stars... Simples

 

With PhD, it's of no use to play with the min mo, aggressivness etc, simply run the guide assistant for at least a worm period of the mount and apply the settings.. 

Use a 3 sec loop of the guide exposure .. job done

Finally your pixel scale of your imaging setup is 1.22 so as long as your under that it's fine... Remember to look at arc sec per pixel and not pixels per pixel as most do too often... 

[ONIKKINEN]

How do the actual subs fare compared to previous guiding? That is the important part, not the prettiness of your guide graph. OAG guiding can look much worse than guidescope guiding because the guider is able to work more accurately - so report and attempt to fix more of the errors resulting in a seismograph. If your average FWHM values have gone down but guiding is worse, guiding is actually doing a better job.

Other than that, take 3s exposures or longer with an OAG. If the seeing is really bad you might want to increase to 4s or even 5s. The guide log will reveal anything there is to reveal if you find it.

[vlaiv]

Many people forget to update guide focal length in settings when switching to OAG (leave guide scope focal length).

This can lead to substantial error in RMS values (like doubling or tripling of the RMS).

Check if you have guide focal length correct.

Other than that - @ONIKKINEN is right - OAG does have higher precision in measuring star position - and that results in more jagged graph (guide scope with lower resolution "smooths" this graph).

[Starflyer]

Thor's Helmet is very low down, you'll have been imaging through a large mass of atmosphere, if you're imaging over any buildings or houses, even in the distance, then this will make the seeing even worse.

Apart from checking settings and running the guiding assistant, try a run at a higher altitude and see what you get.

[OK Apricot]

@alacant The subframes themselves were fine from what I could tell with the preview. The image through the guide camera were, I thought, acceptable - There was some astigmatism/aberrations present but nothing like oblong or crescent shaped like I've read they can be. I've attached a guide log from both OAG and guide scope. I will try calibrating further north next time. I've been getting excellent results with a separate guide scope but the idea of the OAG is to save space and weight, and in theory more accurate guiding. I'll be taking a small rig to Vegas later this year so space and weight saving are important.

@newbie alert As said above the idea of the OAG is to save space and weight on the mini rig I take abroad. I'm testing it on the 80ED because it's mostly galaxies I'd like to image for the next few months and this scope is the highest FL at my disposal! I didn't confirm in my original post, but the guide focal length was the first thing I'd changed before entering the ASIAIR. It has it down at 520mm with reducer after plate solving so I just copied that as my guide focal length. I didn't measure anything but to summarise, the process was this - Find focus with imaging camera and lock focuser, fiddle with OAG until focus achieved, double check focus of both cameras is close. The stars are perfectly acceptable with a little astigmatism present. If there is a guide assistant with the ASIAIR please could you point me in the right direction? According to Astro Tools CCD calculator the image scale of my 80ED/533 is 1.52"/px. Most of the time the guiding was slightly below that, but there were some excursions...

@ONIKKINEN The sub frames, from what I could tell from the preview, were pretty decent - no obvious trailing - No readily visible difference in subs from guide scope to OAG. I don't think I have the software to measure FWHM, but from eyes it all looks pretty close. I've attached guide logs from separate scope to OAG.

Thanks for the support so far chaps, much appreciated! So far the next things I'm going to try are to Bin2 and calibrate further north. I've attached guide logs - 07/02/23 is from last night with OAG, 23/11/22 with Evoguide 50ED. Same guide camera, same imaging scope, balance as close as I could get it. I don't know what to make of these logs so any education would be great! I've attached some snap shots for what it's worth. 

Thanks again guys

PHD2_GuideLog_2023-02-07_194639.txt PHD2_GuideLog_2022-11-22_203308.txt

Edited February 8, 2023 by OK Apricot

[alacant]

28 minutes ago, OK Apricot said:

The subframes themselves were fine

I'm not surprised. Good polar alignment with e.g. 0.57" RMS over over an hour with similar ra and dec deviations is fine. The guiding was excellent!

TBH, I'd simply reduce the calibration step to around 500 and fiddle no more.

Just make sure you stick to 520mm focal length for guidescope as you had entered 240mm for one of the sessions.

My conclusion is that if you're still not satisfied with your final images, then you've the luxury of being able to eliminate guiding issues from the equation. You have the infamous 122s error from the mount stepper teeth which last time I looked, I don't think the asiair can correct. On a native PHD2, you could set PPEC to 122s. But hey...

Cheers and HTH.

 

Edited February 8, 2023 by alacant

[OK Apricot]

@alacant the screenshot you've posted is when I was using the Evoguide 50ED - The log I posted for that was from Nov 2022 and I'm doubtful it's representative of last night. Please could you evaluate the log from last night with OAG? It's the first attachment if it helps?

[alacant]

Sorry. With the oag:
DEC is the limiting factor. Balance? But PLEASE look at the images rather than the numbers or graphs.

Resist switch at 520mm is going to find a lot more to correct than at 240mm, so you may want to increase the minimum move. Also don't forget to decrease calibration step.

In the end, the only way with sw mounts is to pull apart, clean, re-grease and adjust. It's an hour or so's job which will save you countless more fiddling with settings in software.

Cheers and HTH

 

 

Edited February 8, 2023 by alacant

[vlaiv]

@OK Apricot

I took a look at 2023 guide log (one with OAG), and here are couple of points that might make your guiding better:

1. There seems to be mechanical issue with DEC axis - probably quite a bit of backlash that can be tuned out mechanically.

Look at the graph above - once DEC drifts away (red line) - it takes many corrections to bring it back - and then it overshoots - and it takes many corrections again in other direction to bring it back. There are numerous sections like that in guide graph.

Mount is not responding properly to corrections.

2. RA guiding is too aggressive or some of parameters is improperly set:

now focus on blue line instead of red line. This time mount responds very fast - but overshoots almost all the time. Graph is zig-zagging above/below baseline a lot.

I would check following parameters:

RA Guide Speed = 13.5 a-s/s, Dec Guide Speed = 13.5 a-s/s

That is way too high. Sidereal is 15"/s and you are guiding at 13.5"/s - so that is 90% of sidereal. Mounts that are not mechanically "tight" don't like fast corrections - a lot of mass is being pushed / pulled quickly and that can lead to overshot a lot.

Try using very conservative guide speed of say 0.25% of sidereal - or about 3.75"/s in both axis.

Next thing is minmo. This parameter is impacted by focal length as it is set in pixels rather than arc seconds.

0.1px minmo translates to 0.15". If you have seeing issues - try raising this value. You don't want to react to every little shift in guide star position - those are most likely due to seeing.

Try putting that to say 0.2 - 0.25px in both axis.

Third is aggressiveness. You are guiding at 100% in DEC (which is ok at the moment as you have mechanical issue - but eventually leads to overshoot) - and 80% in RA.

Maybe bring that down a bit once you fix DEC - to say 60% if you guide short cycle of 2s.

[OK Apricot]

Great stuff, that's given me plenty for next time. I'll get to work as far as I can with the mount and report back. Thank you very much chaps 😊

[newbie alert]

4 hours ago, OK Apricot said:

@newbie alert As said above the idea of the OAG is to save space and weight on the mini rig I take abroad. I'm testing it on the 80ED because it's mostly galaxies I'd like to image for the next few months and this scope is the highest FL at my disposal! I didn't confirm in my original post, but the guide focal length was the first thing I'd changed before entering the ASIAIR. It has it down at 520mm with reducer after plate solving so I just copied that as my guide focal length. I didn't measure anything but to summarise, the process was this - Find focus with imaging camera and lock focuser, fiddle with OAG until focus achieved, double check focus of both cameras is close. The stars are perfectly acceptable with a little astigmatism present. If there is a guide assistant with the ASIAIR please could you point me in the right direction? According to Astro Tools CCD calculator the image scale of my 80ED/533 is 1.52"/px. Most of the time the guiding was slightly below that, but there were some excursions

Sorry , my mistake thought a ed80 was 750mm FL... Went on memory... So it should be 600mm FL and 510 with the reducer?

Sorry with the air you don't get the full PhD package, so no guide assistant? 

Is there the drift alignment tool? As I was going to suggest you pull that up to help with calibration on Dec 0 but again it may not be available with the air,  never used one..

[rickwayne]

Just a data point for you: I did see a measurable increase in guide performance (and, IIRC, better stars) when I went from a 162mm FL guidescope to an OAG on my refractor, (a mere 336mm FL itself). My mount was rather overloaded by then so I was grasping at every possible improvement.

I think the other folks have made excellent suggestions. I'll add that if you have the means to connect a "real" computer for testing, it would be worth your while to do so. You can run the full-up PHD interface and make use of the Guiding Assistant, examine the calibration graphs, etc.. And the GA can directly measure backlash.

Finally, your mount may become a bit more responsive if you decrease the moment arm. In RA, adding counterweights means the motor has to move more mass, true, but at a much shorter arm, hence the moment of inertia goes down. More weight but closer in == starts and stops quicker. Likewise, if you can move any weight away from the ends of the scope to be more centered over the DEC axis, that's a win too. And finally, pay attention to cable routing, if anything's hanging off the end of the scope that also increases the moment.

[Elp]

Mine went from 0.9-1.2 typical to 0.4-0.7 RMS at 360mm FL when I tested it back to back. A lot depends on the seeing, I tried low altitude recently and guiding was awful though the stars weren't too bad.

[OK Apricot]

Just checking in here to report from Monday night. I tried a couple of your suggestions and got some very acceptable guiding. I double checked over my mount but couldn't really feel any backlash, certainly not amounts that my grease monkey hands could deal with, so just decided the first things I would try would be non invasive things such as settings. Cam was binned x2, calibration was done more or less 20deg above the celestial equator, and I reduced the aggression on both by about 25%. Happy to say that for the most part it was reporting a total error between 0.5" and 0.6". Thanks a lot for your help guys, that will do me nicely!