Poor guiding with 5-min subs and OAG- help requested

[John Natale]

I finally managed to get my OAG working with PHD2 (thanks for your previous help). Now I am having an issue with poor guiding. I am using a C8Edge, Celestron OAG with asi 174mm guide camera and an EQ6 mount. My main imaging camera is an asi 294MC pro.

I tried 5-min subs last night for the first time on the Sunflower Galaxy, using the OAG. I looked at the guiding graph a couple times during my imaging and the rms was over 1. Is there anything I can do to improve my guiding? I did both a calibration and a guiding assistant run but these didn't seem to help. I attached my logs below and also my (very poor) processed image which shows a few egg-shaped stars at the top of the frame.

Maybe my polar alignment was poor? I did the alignment with Sharpcap's polar alignment tool, which usually works well, but I am not sure how good a job I did this time. Or am I expecting too much from a C8 taking 5-min subs??

Any help will be appreciated!

PHD2 Logs:

https://openphdguiding.org/logs/dl/PHD2_logs_t5DT.zip

Processed file:

https://drive.google.com/file/d/1iZDeHDHUNcvhhbZp48dRzmcQiLLhW5Pq/view?usp=sharing

[Brian Valente]

Hi John

I would characterize this as death by a thousand small cuts. There's not any one thing that sticks out as particularly problematic, but there's a lot of small things:

- the calibration looks pretty rough. it's usable but not ideal, orthogonality error is around 9°, nearing the threshold for a problem

- The seeing conditions look pretty rough as well for most of your run. There's a period where your guiding is quite good, around 0.44" total with ra and Dec nearly equal, between the green arrows

- your mount is generally pretty unresponsive. it could probably benefit from PPEC on the primary PE at 480 seconds, and more aggressive guiding. I would also enable Dec auto backlash compensation and start around 800ms. 

- It looks like you ran a continuous 5 hours, which means you are guiding into low altitudes and that is a lot of airmass to go through. It will produce less than ideal results and the kind of aberrations you are showing on your image. If you are using an osc camera, there are some stacking tricks in PixInsight you can use to improve that

- are you dithering at all? 

- although I can't say for sure, SCTs are notorious for needing good collimation and i'm not sure you have that on your C8. you'd have to take some single subs with an out of focus star to evaluate that. 

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[John Natale]

Thanks Brian. I'm not dithering, and my SCT was recently collimated (I think the collimation is very good, but you know how that goes, it could always be better!) The reason I don't dither is because I use Sharpcap for capture, which requires that a "sequence" be set up to include dithering, something I am not familiar with how to do, but want to learn at some point.

When you say:

" it could probably benefit from PPEC on the primary PE at 480 seconds, and more aggressive guiding. I would also enable Dec auto backlash compensation and start around 800ms." Can you please let me know how to change these settings in PHD2 (what menu items are these under, etc)? I am not familiar enough with the PHD2 program yet to know how to make these changes. I'd really appreciate it!

[Brian Valente]

Also keep in mind imaging past about 30° altitude is pretty rough. your FWHM is pretty high on those stars

this is in the advanced settings under algorithms. change the things indicated by the arrows

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[John Natale]

Thanks - I'll change those settings and try it again, hopefully soon. 

[mj.w...@gmail.com]

Hi John

1.  Your image has misshapen stars left of frame, suggesting Tilt.

But a Stacked image is not best for analysing star shape.

Look at a single short exposure, stretched if necessary, to reduce guiding and stacking artefacts.

2. There's a lot of Dec Backlash in some of your Cals, are you clearing Dec Backlash before Calibrating ?

3. You have a very challenging imaging pixel scale of 0.65arcsecs/pixel.

So is it realistic to expect your mount to guide to less than 0.65arcsec on each axis, to ensure round unbloated stars, I don't know ?

Your RMS guide figures were close to that on the final run, but there's those high Peak figures to contend with.

4. Your final Cal before guiding was near East, not at near south and near Dec = 0  "Dec = 41.9 deg,  Az = 78.9 deg"

Michael

Wiltshire UK

[John Natale]

Thanks Michael. I appreciate the assistance! Some questions for you. Sorry but I am still relatively new at this:

1. What is tilt, and how do I fix it?

2. How do I clear Dec backlash before calibrating?

3. I am assuming I need to calibrate pointing southward, correct? 

[mj.w...@gmail.com]

The most comprehensive answer to those questions John - read the PHD2 Instructions available via the Help menu in PHD2 !

Michael

Wiltshire UK

[mj.w...@gmail.com]

And Tilt is where the sensor ends up not at 90 degrees to the optical path.

Because of focuser sag, camera not attached by threaded connections, even due to the sensor not being tilted in it's body.

Detective work would involve comparing two short exposure images, with the camera rotated 90 degrees inbetween.

Michael

Wiltshire UK

[Brian Valente]

John

The two questions about calibration are handled when you use the calibration assistant. you should always use the calibration assistant (and consider upgrading to the latest dev release here https://openphdguiding.org/development-snapshots/ )

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