PHD2 exposure times with multi star guiding - Forums

Topic starter

Oct 31, 2021

Hi folks,

for a while now, the multi star guiding functionality in PHD2 is available. Currently, I am using version 2.6.10dev2. The guide scope is a SVBony 60mm/240mm guide scope with an Altair GPCam3 178M or a ZWO AI 120MM mini. All this is mounted to my EQ6-R Pro. For all of my guiding sessions since the beginning of my astro photography journey, I keep the exposure times of the guiding cam in PHD2 (as suggested) within the 2-4 second range. For a while now, I use multi star guiding and it helped a lot to smooth out the guiding graph.

But some nights ago, I realized that setting the exposure time to about 1 seconds seems to improve the smoothness of the graph significantly. I tried several nights to compare the long vs the short exposure times and always ended up with better guiding if I set the shorter one. I also wondered about the speed of the data that is building up the graph. It seems, that it is twice the speed I set as exposure time. So if I set it to two seconds, there seems to be a new calculation every second. That is weired.

Is this behavior correct? Did you experience the same? The physics doesn't change, so I will think, that the 2-4 second range is still the better way to go. But the graphs are showing (only in my case?) the better results. If it happened occasionally, I wouldn't mind it. But it happened every time since I recognized it.

Is there someone who can explain this?

I am not too concerned about it, because I can't see any difference in my subs. Maybe it is there, but I haven't recognized it, yet.

Thank you

Christian

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Steven Rosen

Oct 31, 2021

I have always used 1.5 second expsures. When i started using the multi-star function, I did not change the expsure time. However, I did notice a signficant improvment in my guiding. The RMS error was about 20% lower and the grpah was smoother too. I find i need to recalibrate much less often as well.
I also use the guiding assistant which seems to improve the restuls a bit.

Concise

David Nozadze

Oct 31, 2021 · Edited: Oct 31, 2021

Hi Christian,

I think it depends, what is the maximum resolultion of your imaging train and the seeing conditions. If the total RMS is below your resolution threshold, then it does not really matter how smooth the graph is. Also, I've heard, that if the seeing is not perfect, a longer exposure time might be better, as the system will not be "chasing" every star fluctuation, caused by atmosphere turbulence.

I use ASI Air Pro (which uses a version of PHD2) and my best achievable reoslution threshold is around 1.9" in perfect conditions. Therefore, as long as my RMS is below 1.5", I am fine.

I usually shoot from suburbs, with city's heat upsetting the atmosphere quite strongly. I set my exposure at 3 seconds and this is the result I get quite consistently. Mount - EQ6R Pro, Guidecam - ASI120 MM Mini, Guidescope - ZWO 30mm F4.

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Sean van Drogen

Oct 31, 2021

Hi Christian,

For me when I switched to multistar guiding I lowered exposure to 1sec from 3sec. My total RMS went from around 1.11" to around 0.60".
Think the main effect of lowering the exposure was i got more frequent corrections but less drastic in either direction, it smoothed out the graph a lot for me.

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REVOL & ALEXIS LEREY

Nov 1, 2021

Je suis très interessé par cette discussion concernant vos réglages divers et la fonctionnalité "multi-étoiles" et vos résultats
Toutefois je note aucun commentaires concernant l'agressivité de réglage que vous utilisez
Personnellement je règle l'agressivité à 70% sur les deux axes DEC et RA
Quelles sont vos usages en cette matière?

Jean Pierre
Cavaillon - FRANCE

REVOL & ALEXIS LEREY

Nov 1, 2021

I am very interested in this discussion concerning your various settings and the "multi-star" functionality and your results.
However I do note any comments regarding the aggressiveness of the setting you are using
Personally I set the aggressiveness at 70% on the two axes DEC and RA
What are your uses in this matter?

Jean Pierre

Sean van Drogen

Nov 1, 2021

I am very interested in this discussion concerning your various settings and the "multi-star" functionality and your results.
However I do note any comments regarding the aggressiveness of the setting you are using
Personally I set the aggressiveness at 70% on the two axes DEC and RA
What are your uses in this matter?

Jean Pierre

Personally I have never touched the standard settings here 100 and 70 seems to work well enough for me

Christian Großmann

Topic starter

Nov 1, 2021

I am very interested in this discussion concerning your various settings and the "multi-star" functionality and your results.
However I do note any comments regarding the aggressiveness of the setting you are using
Personally I set the aggressiveness at 70% on the two axes DEC and RA
What are your uses in this matter?

Jean Pierre

I used to keep the standard settings or to use the guiding assistent tool to get the optimal values. I never put much effort into a really good polar alignment and this worked pretty well. At least I thought it worked well.

A while ago, I tried the Three point polar alignment plugin in Nina and that was, when things changed. With a real good polar alignment, I realized that the guiding assistent was not that helpful anymore. Changing some of the values can lead to a better smoothness of the graph. So, depending on the conditions, I change the agressiveness of the RA axis to a value between 60 and 70. Usually, I keep the DEC at 100%. However, if the DEC graph shows overcorrections and is always oscillating, I reduce the value to about 80%. With multi star guiding, I also reduce the threshold for the minimal mount movement to about 0.15 and this seem to work well in good conditions. If it's a bit windy (with my 8" Newt + dew cap) things may change a bit. Then I usually go for higher thresholds and longer calculation intervals, of course.

I think, I got a bit better in understanding, what the values do. I always check the resulting graph and sometimes correct my settings if it shows some changes during the night. I also check the subs and usually, the changes were visible very subtle.

Hope this is, what you're looking for...

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REVOL & ALEXIS LEREY

Nov 1, 2021

perfect
thank
jean pierre

[deleted]

Nov 5, 2021 · Edited: Nov 5, 2021

I believe this makes sense as multistar guiding is, by construction, less sensitive to seeing. Short exposures with single-star guiding is not a good idea as one is literally chasing seeing, but in multi-star mode seeing is averaged over several stars hence over several directions in the sky.

Tonight I found that 2sec exposures with multistar gives better results than 4sec exposures (which was my default value with single-star guiding).

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Andy Wray

Nov 5, 2021

I used multi-star guiding for the first time a couple of nights ago. I left my exposures at 2 secs and saw an improvement in guiding. Now down to 0.6 to 0.9 arc seconds which is close to the limit of my guiding camera and scope. (i.e. now down to 0.15 pixels or less … can't imagine PHD2 doing much better than that).

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Björn Hoffmann

Nov 7, 2021

I saw an improvement after reducing the exposure to 1sec. My CEM26 tends to run away sometimes, so quicker countermeasure helps a lot. But what I also saw: When I shoot during turbulent seeing condition (e.g from my balcony or very low near the horizon), 2sec is smoother. I guess 1sec is then disturbed by low frequency seeing such as temperature gradients from the appartements below mine.

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Jim Raskett

Nov 7, 2021

I too use 1-1.5 second exposures when seeing is fair-good. My mount seems to respond to errors more quickly without overcorrecting.

Last night the seeing must have been horrible. The HFD of my guide star was bouncing between 3.8-5.5. I increased the guide camera exposure to 3 seconds and it really smoothed things out. Later in the evening as the target rose in the sky, I reduced to 2 seconds.

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Dale Penkala

Nov 7, 2021

Hello Christian,
I’ve been using multi-star guiding for about 6mo’s now and I’ve found it very helpful. I have in the past tried anywhere’s from 1 - 4 second for guiding and I’ve ALWAYS come back to 1 second exposure rate. Maybe in windy situations I’ll go 1.5s but thats not very often.
I’m on the same page with @Christian Großmann with aggressiveness of RA & Dec. RA I fluctuate from 65-70%. Very rarely do I run 100% on Dec. That depending on conditions I’ll keep that in the 85-95% range. This is on my secondary rig which consists of a CGX mount.
I do adjust the MinMo on both axis’ at times but find the aggressiveness for me works about 80% of the time.

If I’m recalibrating I’ll run the guiding assistant afterwards and note the recommendations and then slowly move in that direction manually. For whatever reason I’ve found that in some cases it’s recommendation are not always a good option so thats why I’ll do it manually and I’ll make the adjustment usually in the axis that has the most error.

Dale

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REVOL & ALEXIS LEREY

Nov 11, 2021

bonjour à tous
Personnellement je débute dans le guidage
je me suis interessé aussi à l'examen des courbes et à l'évaluation de l'erreur périodique avec PHD2Log Viewer et PEC Prep pour l'évaluation de EP
Je bute sur un problème:
PHD2 enregistre sous une même date dans Log …..plusieurs enregistrements correspondants aux divers guidages effectués dans une même soirée
Lors de l'ouverture de PECPrep je n'obtiens qu'une seule courbe…ne sachant pas à quel guidage il se réfère
Comment séparer les différents enregistrements pour avoir des courbes différenciées pour chaque guidage

merci pour vos réponses
Jean Pierre

REVOL & ALEXIS LEREY

Nov 11, 2021

Hello everyone

Personally, I am new to guiding I was also interested in the examination of the curves and the evaluation of the periodic error with PHD2Log Viewer and PEC Prep for the evaluation of EP
I stumble on a problem:

PHD2 records under the same date in Log …..several records corresponding to the various guides carried out in the same evening
When opening PECPrep I only get one curve … not knowing which guidance it refers to

How to separate the different recordings to have differentiated curves for each guidance

Thank you for your answers

Jean Pierre

Dale Penkala

Dec 20, 2021

This is slightly off topic in the sense it talks about the “guiding assistant” but I watched this video on the Astro Imaging Channel with Bruce Waddington which is one of the guys that wrote PHD. While he talks about the GA, the video does talk about the Multi-star guiding and how the recommendations are calculated. Bruce even talks about why specific settings shouldn’t be adjusted if the mount is working correctly. I just found this useful to me and wanted to share it with you on this thread.

https://www.youtube.com/watch?v=LbhxX7PH2CE

Dale

DiscoDuck

IOTD Staff

Dec 21, 2021

Steven Rosen:
I have always used 1.5 second expsures. When i started using the multi-star function, I did not change the expsure time. However, I did notice a signficant improvment in my guiding. The RMS error was about 20% lower and the grpah was smoother too. I find i need to recalibrate much less often as well.
I also use the guiding assistant which seems to improve the restuls a bit.

It is worth noting I think that using multiple stars will automatically give a lower RMS value since you are averaging multiple sort-of-independent samples of the seeing. It doesn't necessarily mean your final image will be any sharper - but it can't do any harm as it will make it easier for PHD to spot and correct real errors that are not seeing. Just that the improvement to the final image will not be anywhere near as much as the reduced RMS would indicate.

This is very much the same as say increasing the exposure time. The RMS goes down (usually) because you are smoothing out the seeing sample over a longer period, but the image you take may or may not be any better.

RMS is a good proxy measure for the magnitude of the seeing over the night - but only comparable if you don't change parameters (such as exposure time or number of stars being sampled with multi-star guiding).

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