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Question Avalon Linear Guiding Optimisation

  • Hugues
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27 Apr 2020 21:42 #245 by Hugues
Replied by Hugues on topic Avalon Linear Guiding Optimisation
Thank you for answer michel.

I've not pay to much attention to the guiding for now, because i never experienced trail stars.
My stars are mostly round but as i said, i'm searching reason for an excentricity between 0.5 and 0.6 in my image. It's barely visible but it's not round. I think that we cannot see excentricity bellow 0.45.
That's why i'm digging in those guiding numbers.

I've reduced the guiding speed to 0.15, 0.25 in my last test.

I'm binning x2 the lodestar. I get a better SNR and also it's reduce the effect of the interlaced mode present in bin1 with the lodestar.
I've a focal length of 530 with the FSQ106, and i maybe got it wrong here ?
I know that i'm undersampled here.

Another thing/fact is that the lodestar did not have square pixel.
Pixel size: 8.2µm x 8.4µm
Maybe the combinaison of not square pixel and bin2 impact the RA guiding ?
But i never heard of that before...
Maybe i should try to turn the guider by 90° ?

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03 Jun 2020 13:00 #302 by ken.self
Replied by ken.self on topic Avalon Linear Guiding Optimisation
You could try using the Z-filter algorithm in PHD2. This lets you use shorter guide exposures without "chasing the seeing". I wrote it as an alternative to LowPass2 after I looked into the code and saw some issues with the algorithm. I had in mind the Avalon mounts which prefer shorter guide intervals. If you are using 1s exposures you could try an ExpFactor of 4x. This has the effect of smoothing the seeing effects equivalent to taking 4s guiding exposures but adjusting the mount every second. Note that when you use shorter exposures your RMS error increases as the seeing error is increased. Having said that, I also experience an elongated scatter plot with my M-Uno but it has never really bothered me.
Also, the slightly rectangular pixels should have no material effect on guiding.

-- Ken
Avalon M-Uno; GSO RC8; ASI1600; Optec focuser; Aaeon UP/Ubuntu/INDI
The following user(s) said Thank You: Daan

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29 Jun 2020 08:51 #339 by Michel
Replied by Michel on topic Avalon Linear Guiding Optimisation

ken.self wrote: You could try using the Z-filter algorithm in PHD2. This lets you use shorter guide exposures without "chasing the seeing". I wrote it as an alternative to LowPass2 after I looked into the code and saw some issues with the algorithm. I had in mind the Avalon mounts which prefer shorter guide intervals. If you are using 1s exposures you could try an ExpFactor of 4x. This has the effect of smoothing the seeing effects equivalent to taking 4s guiding exposures but adjusting the mount every second. Note that when you use shorter exposures your RMS error increases as the seeing error is increased. Having said that, I also experience an elongated scatter plot with my M-Uno but it has never really bothered me.
Also, the slightly rectangular pixels should have no material effect on guiding.


How much error are you getting with the z filter? and on which RA/DEC guidespeed?

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30 Jun 2020 08:55 #341 by ken.self
Replied by ken.self on topic Avalon Linear Guiding Optimisation
Just had a better look at your first post and you are way undersampled. PHD2 can resolve down to about 0.1px at best and anything under 0.2px is a good result. The difference between your RA and Dec RMS is around 0.05px so basically invisible to PHD2.
In that situation, the algorithm choice is largely irrelevant. So you should try to improve your guiding pixel scale and reduce your exposure times. If you then find that you are chasing the seeing then try Z-filter.
Note that you will always see more oscillation with shorter exposures as the filtering effect of longer exposures is reduced. Halving exposure time leads to a 40% increase in RMS.
Also, the RMS value is not necessarily an indication of guiding accuracy. With a perfectly guided mount the RMS is just an indication of the seeing. However, differences in RA and Dec RMS is indicative of imperfect guiding.
Not really relevant ion this forum but you should always attach your PHD2 guide log when looking for analysis of guiding. Screen shots are useless for analysis.

-- Ken
Avalon M-Uno; GSO RC8; ASI1600; Optec focuser; Aaeon UP/Ubuntu/INDI
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19 Jul 2020 19:50 #346 by Hugues
Replied by Hugues on topic Avalon Linear Guiding Optimisation
Thank you ken, for your comment on my little issue.
I will try to guide without binning, and have a look at this z-filter algo.

I've found since i wrote my fist post that my avalon is very very susceptible to the cable weight.
I first need to find a good way to route my cable. I need a way to support the cable near the polar finder.

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