In Search of pinpoint stars in 10-minute subs.

[chris_g]

Hello all,

I ran 5 10-minute subs on Vega to see how well things would turn out when the Sky Gods were nice a couple of months back. At first glance it all looks good but get into a deep pixel dive and you see the stars aren't quite pinpoint. They are all off slightly in the same direction, guiding...

Ok, how do I improve my guiding so I can run exposures 10 plus minutes?

6D, Z73, 5 10-minute subs, 3 darks, no flats, bias, ASI290, Orion 60mm multipurpose.

[Juno16]

Hi Chris,

Dang, the 1+hr and 2+hr graphics in the guide log look odd.

Really, I am no expert in PHD2 (or anything), but it looks like PHD2 is doing very little or nothing at all. Oh, it is definitely trying though! Tons and tons of RA and DEC corrections, but the mount looks to be completely un-responsive.

Your RA and DEC total rms error (arc/px) have similar values (good for round stars), but the total rms error is 4.4 on one run and 3.8 the other. Generally, values less than 1 are considered "good".

I did a rough calculation and it looks like your rig image scale is 3?

As a very rough guide, as long as your total rms error (arc/px) is less than your image scale, your stars would be round.

That would explain fatter stars and slight elongation.

Again, I am no expert, but the 8-9 minute RA excursions look like periodic error. Maybe your worm period is 8-9 seconds?
On your guiding graphs, PHD2 can't do anything to help control the P-E (normally, PHD2 can correct a good bit of periodic error).

My only guess is that something is off in the PHD2 setup. I'm sure that you have read all of the online documentation If not, here is the manual https://openphdguiding.org/manual/

As a test, you could create a completely new profile in PHD2. That would start everything off fresh and not effect your current profile. Enter all of your equipment details. Maybe something just got out of whack. BTW, did you have better total rms error values at one time and things started slipping?

As I am sure that you have already done this, but Calibrate PHD2 at a point near the intersection of the celestial equator and meridian. If you create a new profile, you must calibrate on the initial run.
Also, I always run the Guiding Assistant on each outing. It only takes a few minutes.

If this doesn't help, please consider the PHD2 Guiding Forum. They are very nice and helpful folks.

Just for comparison, here is a snip from my guide log.

[chris_g]

The only thing I do with PHD2 is make sure it doesn't complain about being too close or too far from the pole and it's between a certain RA when I run calibration. I usually have to calibrate every session, I'm always fiddling with the equipment between sessions and messing with the guide scopes.

Guiding assistant? What's that?

Looks like I need to learn more. I've also never done any PEC training , PHD2 is supposed to be able to compensate.

Thanks for the good info Jim. Will definitely follow up this post with what I find...

[Juno16]

The only thing I do with PHD2 is make sure it doesn't complain about being too close or too far from the pole and it's between a certain RA when I run calibration. I usually have to calibrate every session, I'm always fiddling with the equipment between sessions and messing with the guide scopes.

Guiding assistant? What's that?

Looks like I need to learn more. I've also never done any PEC training , PHD2 is supposed to be able to compensate.

Thanks for the good info Jim. Will definitely follow up this post with what I find...

Since you have done a bunch of fiddling, you might want to create a new profile and start fresh.

PHD2 will definitely help with periodic error. There is an RA guiding algorithm "Predictive PEC" that can help with correcting periodic error. Does EQMOD have PE training for your mount?

Please download the PHD2 Best Practices from here: https://openphdguiding.org/phd2-best-practices/
Best practices is a collection of very important recommendations, do's and don'ts.. It is really a great document to read even after you have been successfully guiding for awhile.

The good news is that your guiding issues are most likely related to PHD2 and once you get it down, you should have a marked improvement!

Good luck Chris and keep us posted!

[SkyHiker]

Are you sure you entered the right focal length - 60 mm? 10 arc seconds per pixel? I think that's the problem.

Your image looks quite good but maybe that is because of a small focal length. The statistics are quite horrible.

When there is trouble, it's always good to present the calibration log and to run open loop / no PPEC to check for mechanical trouble. If those look OK then you know you messed up the parameters.

[Juno16]

Good spot Henk! @SkyHiker

The fl might actually be 4x that. 60mm sounds like the aperture.

[JayTee]

When you are ready to get deeper into PHD2 you should join the Google Groups for PHD2. Here's a link.

https://groups.google.com/g/open-phd-guiding/

Refer to the top two stickies in the Guiding subforum also.

[chris_g]

I have some data for a project I'm working on from that same night, 5-minute subs and everything
looks fine, nice tight stars.

Ok, thanks folks, I've just spent this cloudy day reworking the cabling and guide camera for that
scope, had some cabling issues last session out that needed to be resolved.
No more cable snags. This scope in its current configuration for the foreseeable future with the
ASI120MM and the WO32mm. I mounted the scope on a D series dovetail to add some additional weight
up front so I could use the Vixen plate for the handle and the hub. No new cost to me! yay!

Everything gets a fresh profile next clear night, I haven't read anything
on PHD2 in the last 9 months, so I'll read. I'll get those 10 minutes subs out of it!

[chris_g]

Good article on guiding

https://github.com/indigo-astronomy/ind ... EPTIONS.md

I think this explains why I didn't see any issues at 5 minutes but did at 10 minutes...

Highlighting added

Several arc minutes of polar alignment error will lead to considerable field rotation

It depends, but generally it is not an issue. The bigger the field of view the worse the field rotation. Therefore let us assume that we have a relatively large field of view of 3° and a total polar alignment error of 5'. The math says that with 600s subs at the celestial equator the stars at the edge will be elongated by 0.35" and at 60° declination the elongation will be by 0.69". So with a good seeing of 1.5" it will be barely noticeable and the optical aberrations of the system at the edge of frame will have more noticeable effect.

With 1° field of view at the celestial equator the star elongation will be a mere 0.12" and at 60° declination will be only 0.23". Also halving the exposure time to 300s the numbers above are halved too.

Please note that telescope with diameter of 120 mm has theoretical airy disk size of approximately 1" and all the numbers above are well within this number. For larger telescopes the seeing is the limiting factor.

Do not be too obsessed with the polar alignment. In most of the cases polar alignment error of 5' is more than enough.

I've always gotten it to within 5 minutes and called it good. A closer PA would be needed, say within 2.5 minutes. Waiting for Clear Skies to test out my theory.

[chris_g]

Configuration, configuration and more configuration. I went reading and found what I was doing wrong with guiding, this time, calibration as far away from NCP as I could get and about 5 degrees off the celestial equator. I thought guiding within pixels was good, how about within arc seconds now. Thanks @SkyHiker, I put the aperture of the scope instead of the focal length, and it was the 60mm, not the 32mm, doh! That certainly didn't help.

So I switched out guide cameras for this rig, using the ASI120mm and the WO32, brand new profile, brand new library and calibration.

I'm excited, running a 10-minute plan now and a quick zoom shows tight round stars! Hope to be posting on that later on. Now to get the C8 configured!

Clear Skies,
Chris

[Juno16]

@chris_g

Hey Chris,

That looks great!

I was noticing your PHD2 image window. Are you not using multiple stars?
Brain, Guiding tab, “Use Multiple Stars “.
Using multiple stars might even give you a bit better guiding.

[chris_g]

@chris_g

Hey Chris,

That looks great!

I was noticing your PHD2 image window. Are you not using multiple stars?
Brain, Guiding tab, “Use Multiple Stars “.
Using multiple stars might even give you a bit better guiding.

I will tonight and see if it improves. Thanks for the suggestion, obviously there is more for me to learn!