Finder guiding issues with a Lodestar

[Ivor]

I have recently moved over to using a finder guider with my Lodestar for my guiding and my guiding is now in a bad way, previous I would happily guide at sub 3-4s throughout the night and now it is sub 8s. My googling suggested the new focal length is 200mm, PHD2 calculates the calibration step to be 1250ms but I'm not sure that's right.

I have copied the setting from phd2 below any help gratefully received; hopefully it won't be a month before I get to test any changes out

Screen output


Brain setting - Global


Brain setting - Guiding



Brain setting - Camera



Brain setting - Mount

[RobertM]

I've had good results with that combo.  Basically the further the guider moves for calibration the more resolution your guide movements will have.  Ideally the guide star should move half a image width in each direction though I have gotten away with half that or less - see what works for you.

Robert

[Rocket Pooch]

Ivor,

Unless you have a low PE mount and are doing passive guiding then the period you are using for the guiding subs is way to long.

I'd suggest you use 1 second subs and make sure the calibration moves the stars quite a bit so PHD can calculate the angles and movements correctly.

If you can use Drupal then you will suss this quickly.

Chris

[Ivor]

Thanks for the replies and the vote of confidence, the advantage of Drupal though is I don't have to wait weeks before I next test things out.

The length of the guiding sub is an interest topic I've had varying suggestions, Ian King told me from the start there was no point going lower than 1.5s with an EQ6 Pro as the components weren't good enough to support anything less. However I've had a number of recommendations to reduce this further.
The thing I'm kicking myself the most enthusiasm to play with my new toys I forgot to record my current settings. Actually whilst writing I just had a eureka moment I moved to phd2 and I still have phd installed to I have the setting there yes bring it on!!

From now on I'm going to be systematic and record the changes so I can revert back.

Oh to another clear night...

[The Thing]

I have recently moved over to using a finder guider with my Lodestar for my guiding and my guiding is now in a bad way, previous I would happily guide at sub 3-4s throughout the night and now it is sub 8s. My googling suggested the new focal length is 200mm, PHD2 calculates the calibration step to be 1250ms but I'm not sure that's right.

I have copied the setting from phd2 below any help gratefully received; hopefully it won't be a month before I get to test any changes out

Finder guiders are usually around 200mm. I would guess you have too long a focal length entered in PHD2, I would expect around 350ms depending on guide camera pixel size.
Your minimum move is way too small, seeing variation will make your guiding overwork. I find guiding improves if I use the default 0.15 or above, on a bad night I will use 0.40 and even then PHD is trying to guide out the seeing!

BTW My experience for getting the best guiding is to use zero hysteresis and a aggression around 80%.

[Ivor]

I'm still trying to improve my guiding, but as U2 would say "I still haven't found what I've looking for". I'm grateful for all the help I've received from numerous sources, what has been interesting is the variety in the recommendations I've been given which in some cases contradicts each other and in others appear to go against the known wisdom. Part of the problem here I think is a lot of people have married the finder guider with cameras that lend themselves to this method better than the Lodestar. My search of the internet has at last found a person (AndyUK from the Stargazerlounge) who got the Lodestar working this way, I've yet to try his suggestions and I need to get my head around his settings as they appear to go against my understanding of the parameters.
Below is the best graph to date from Saturday night, as you can see sub 2 arc secs, but not remotely smooth.



I've started to be more systematic about my approach to this problem, as "Push Here Dummy" just isn't working; I have been changing individual parameters by small increments and waiting 2 minutes to see the difference. Before trying the next approach I want to confirm the fundamentals and have the best possible methodology for the next clear night.

I'm using an EQ6 Pro which I understand should be capable of 1 arc sec guiding; this leads me to my first point of clarity:

1.        Seeing in Kent is limited to 1 -2 arc seconds so there is no point chasing below 1 arc second guiding as I'd only be chasing the seeing. Is this a correct statement?

My normal imaging is between F5 – F7 and the SW finder scope has an aperture of 50mm and FL of 180mm giving F3.6, FOV 1.96° x 1.51° and a resolution of 9.4 "/pixel which is significantly different to my old guide scope (a Bresser 70 now handed down to my son) which gave a resolution of 2.42 "/pixel at F10. My next point of clarity:

2.        Where I have been assured this is a viable option, a resolution of 9.4 "/pixel is surely going to make this very challenging to achieve, a lot of imaging nights are going to be lost trying to get this working is it really worth the effort?

Guiding setting

I use PHD2, below I've list the parameters I've been adjusting.

MIN MOVEMENT = 0.05

To calculate this I need to know the pixels per arc second which is 1/9.4 = 0.111. PHD2 needs sub-pixel movements so I've divided this number by 2 giving 0.05. This seems very small to me and surely on the edge of the limits of software and hardware.

CALIBRATION STEPS = 5200

This has been set based on the PHD2 calculation tool, I've watched the calibration process and there appears to be no issues, I'm not sure how to really decide if this needs adjusting.

EXPOSURE LENGTH = 2s

With my old F10 setup I had this set at 2.5s, by dropping down to F3.6 I could reduce this further. However I have been told previously the EQ6 can't respond to changes too quickly so an exposure of less than 2s wasn't going to make a difference.

3. Is there a way to judge whether the exposure is too long or too short?

MAX RA/DEC = 300

These parameter I think needs increasing and I'm also concerned this needs to be aligned to EQMOD. I've checked the PHD2 log file and there are a few times where the correction hits 300 so I'm inclined to double this. The EQMOD setting are as follows:



4. Should I change any of the EQMOD settings if I increase the MAX RA/DEC to 600?

RA AGGRESION = 70 HYTERESIS = 17

These are interactive settings which have to be the most debated variables and appear to be particular to an individual mount. The current settings provide the best results but my graph is still rubbish and so subject to further change. The problem here is everyone's setup is different with different scopes and mounts so you can't really just take someone else's numbers. Saying that AndyUK had success with 100/22 so I'll start my next test run at 90/17 and see what the adjustments result in.

5. Is there a way to calculate these variables other than systematic trial and error?

All thoughts and insights gratefully received

[MarkS]

Your exposure length of 2s is far too long for the EQ6.  By the time the exposure is taken and the mount correction is sent to the EQ6 it means a total delay of 2.5s or more in the feedback loop.  It is not a question of how quickly the EQ6 responds to changes (I don't understand what on earth they mean by that in any case) but it is a question of how much periodic error and mechanical "jitter" can take place within 2.5s and I can assure you it is quite a lot on the EQ6 - I've generated the PE graphs and you could do the same - it might also tell you if there are any obvious mechanical issues with your EQ6 i.e. is it a good or bad example.  Anyway, use an exposure of 1s or, on nights of good seeing, 0.5s.

Guiding of 1arcsec RMS in R.A. should be quite achievable under our skies on the EQ6 on an average night.

There is a commonly voiced argument about the risk of "chasing the seeing" when using short exposure times but this has to be balanced against the risk of falling behind the PE+Jitter curve.

I would leave hysteresis and aggression at their defaults until you have sorted out the major issues - I consider these settings as "fine-tuning".

The other thing is to guide on a bright but unsaturated star (but I guess you already do this).  The reason being that if you guide on a dim star then there is too much noise in PHD's calculation of the star centroid and if you guide on a saturated star it again mucks up PHD's centroid algorithm.

I happy to help this Saturday if you feel like bringing your equipment over (assuming clear night sky) - PM me if so.

Mark

[Ivor]

Thanks for the feedback Mark I will change the exposures down to 1s to see how I get on. I'd looked at PEC on my old setup and was planning to include it, however I made the move to this new setup beforehand and I didn't want to add another variable into the mix until I'd address the large  issues.

Equally,thanks for the offer of help on Saturday, I might well take you up on that let's see how the weather pans out.

[MarkS]

Don't play around with PEC i.e. correction of PE until the large issues are sorted.   What I meant was that it could be worthwhile graphing your PE to check for any underlying mechanical issue - it could be done as part of a diagnosis effort if you are still having issues.

In fact don't play around with PEC on the EQ6 at all.  PEC on the EQ6 can only correct the worm cycle but there are other period cycles generated by the rest of the gear train.  If you don't correct for those other cycles when creating a worm cycle PEC (which is very difficult indeed) then PEC will make your guiding worse in almost every case.

[Fay]

I always guide at 0.5 secs with NEQ6 pro & HEQ5Pro

[MarkS]

Looking closely at your chart, it would appear that you are already guiding at 1.5" RMS error in R.A.   So you haven't got that far to go to reach 1" RMS error.  To progress to sub 1" guiding would take a lot of stripping and tuning of the mount and may not make much difference anyway under most sky conditions and it also depends on where most of the errors are arising from i.e. which parts of the gear train.

[Ivor]

Unfortunately that graph paints a slightly better picture than reality, the erratic nature of the graph leads to big jumps, here is a sub from this session.



There's still a clear horizontal elongation of the stars.

I'm keeping a close eye on the weekend weather, as I looking likely to be calling your kind offer.

[MarkS]

Is horizontal the R.A. direction?

[Ivor]

Yes, I should have been clearer.

[Rocket Pooch]

Are your cables getting stuck?