Guiding question

[Anthonyexmouth]

What would cause all my corrections to be in one quadrant if the circle?

[vlaiv]

Might be too low aggressiveness, or low min-mo.

Most of your hits are on one side of DEC axis (vertical axis is DEC and hits are below axis).

There are two (or maybe even more explanations) for this.

First is low aggressiveness. Guide system detects that DEC is not correct and issues correction - but due to decreased aggressiveness - correction is "too weak" to correct in single cycle so a bit is left to do in next guide cycle. However, due to polar misalignment - there is constant drift in DEC. If that drift is high - then it will "eat up" any correction in previous guide cycle and instead of giving final nudge in next cycle - guide system starts again - detects larger deviation, but issues weak correction with hope to again compensate in next cycle and so on....

If min mo is set too high to combat the seeing - then similar thing can happen.

Guide system detects error, but issues too weak correction. This correction brings mount position closer to wanted position - but this time mount lends in min mo zone. When mount is in min mo zone (minimum motion) - no correction will be issued if it is less then minimum desired motion, so guide system does nothing and subsequently drift (does not need to be high) pushes mount further of target, it goes out of min mo zone, correction is issued but weak - and mount again lends on same side but not quite at target.

Similar thing can happen with RA - but not completely. RA is subject to periodic error, so it does not have constant drift like polar alignment error.

However, it can have sort of constant but small drift, due to various reasons. First is inaccurate tracking speed. Even if you set to sidereal, it might not track at precisely that rate. That depends how accurate clock is in electronics of the mount system (it does not have real time clock and instead relies on quartz crystal - which might have some offset compared to designed frequency).

Another thing that can happen is difference between sidereal rates - there are several sidereal rates and apparent position of stars does not track perfect rate but instead tracks differently depending on part of the sky (atmospheric refraction and apparent position of stars thing).

Third thing is just large component of periodic error that is simply drift over duration of session. Worm wheel of RA turns once every day. That is about 1/4 of revolution per 6h session. It can be out of shape (ellipse) and that means it will have two "fast" and two "slow" segments - roughly 1/4 (for ellipse it is like that, but for egg shape it will won't be exactly 1/4, but still there will be long periods of faster and slower rotation).

In any case - it is similarly drifting due to one of above, but there is also periodic error component.

Same thing can happen as with DEC - if above combination makes correction less frequent than needed - it will mostly under correct, but sometimes due to periodic error it will switch to other side naturally - so you'll have more hits on one side of RA then on the other.

This is actually a good sign - it means that your mount is responding to proper tracking issues. If hits were equally distributed - then I would say that maybe it is all due to seeing to be random like that.

Maybe you don't want all hits to be on one side - but some imbalance is a good thing - so if frequency is higher in one quadrant - don't worry too much about it, but if all hits are mostly there - maybe increase aggressiveness and/or reduce min mo.

[Anthonyexmouth]

Thanks, guiding seems to be ok, stars are nice at the moment. Just never noticed it all bunched up in one quadrant before. My mount is on it's weight limit though, trying to be greedy and have a lunar imaging setup alongside my DSO. Tossing up whether a mount upgrade or going mono is my next purchase in the new year. 

 

[vlaiv]

Note the values of MnMo (min mo / min motion) in that screen shot.

It is set to 0.19. That value is in pixels rather than arc seconds (for some reason).

From same screen shot I can see that RA error is 0.17px = 0.72", so you are guiding at ~4.24"/px

If you set min mo to 0.19px - that is same as setting it to 0.8"

You are telling PHD2 not to issue correction if error is less than 0.8".

Maybe stars are round, but are they tight (as they can be)?

[Anthonyexmouth]

10 minutes ago, vlaiv said:

Note the values of MnMo (min mo / min motion) in that screen shot.

It is set to 0.19. That value is in pixels rather than arc seconds (for some reason).

From same screen shot I can see that RA error is 0.17px = 0.72", so you are guiding at ~4.24"/px

If you set min mo to 0.19px - that is same as setting it to 0.8"

You are telling PHD2 not to issue correction if error is less than 0.8".

Maybe stars are round, but are they tight (as they can be)?

If I set MnMo lower, how much should I reduce it by to start? What are the signs I've gone too low? Should I lower both Dec and RA the same?

these are the current numbers a bit lower as I think the last one was just after a dither. 

Edited October 10, 2022 by Anthonyexmouth

[vlaiv]

2 minutes ago, Anthonyexmouth said:

If I set MnMo lower, how much should I reduce it by to start? What are the signs I've gone too low? Should I lower both Dec and RA the same?

I'd go by what you expect your mount to be capable of doing.

You can't expect for example to guide at RMS 0.5" combined - if you won't even issue correction to anything below 0.8" per axis.

Maybe half of what you expect to achieve in terms of RMS?

I had my HEQ5 set to 0.3px on ~0.95"/px guiding setup - that is about 0.286" - and I was aiming for 0.5" RMS combined.

[newbie alert]

On 10/10/2022 at 22:25, Anthonyexmouth said:

 

 

If you look at the Dec trend line it doesn't touch the baseline much ,if at all and the RA is under the trend... That maybe linked to the star correction on one side and mainly in one quarter

[michael8554]

There are approx 50 guide pulses shown in the Target.

At say 2 secs exposure that's only about 2 minutes of guiding.

During that period guiding might have been correcting the first half of your PE curve.

And correcting a bit of Dec drift.

The Y-Axis on the Guide Graph is too coarse to see what's going on, try setting it to show +/-3 Arcsecs.

Michael