My mount (AZ-EQ6) has been a reliable performer, but I feel that at the moment it is the weakest part of my imaging gear. Tonight, in spite of high humidity, tracking has been quite good, but at times it can be significantly worse and some nights I need to tweak balancing to make it work. I have attached a print screen with a few minutes worth of guiding.
Is it possible to refine AZ-EQ6 to track better, or is it about as good as it gets with a mass production mount?
I doubt you're going to get much better than that with a mount like this, Suavi. You're sampling at 1.09"/px if I'm not mistaken. I'm sampling at 1.11"/px and my RMS is usually 0.6" RMS or more, and with this I can get images with low 2" FWHM (sometimes high 1"). I'm pretty happy with that to be honest, given that it puts me squarely in undersampled territory I don't think you can ask for much more.
With regard to the tracking accuracy and the associated graphs, I'm not sure I understand the mechanics of it, but it seems the impact of periodic error diminishes the closer you get to the poles; it could be that the times you notice particularly good guiding are when you're closer to the poles? This definitely holds true for me. I saw 0.45" tonight which is about as low as I've ever seen it, and this was as I was crossing the meridian to the south. Earlier when pointing east I'd seen 0.72"
excellent results Suavi - what bit of 0.45 arcsecRMS did you want to improve/change?
I presume that you have already tuned the phd2 parameters to your mount, but some comments:
- you seem to have a very slight residual PE that might possibly be reduced by a bit more agression in RA and possibly smaller min motion
- you could maybe also try a shorter guide exposure and/or higher guide rate (eg 0.8 siderial) to tighten things up around the noticeable PE zone.
- I don't know how you have the mount supported, but a solid pier can help in reducing wind/turbulence vibrations
these are just a few things to try, but you really don't need to do anything - that is good tracking for any mount, mass produced or not. I assume that it is producing round stars.
Thank you Lee and Ray for your comments and advices. Will keep experimenting to see what works best. A pier sounds like a very good idea - but will need to wait with that until I have my own backyard
I am using a reducer so image scale is 1.33"/px. I have found that occasional adjustement of aggression and min movement throughout imaging session (same target) are needed with my equipment to get the best possible guiding. Also, additional calibration after a few hours can help too.
What sort of RMS are you getting when things are performing not-so-great? And what's the FWHM in the resulting images? You might find that you simply don't need anything better at your image scale. Having said that if you really do need to be constantly tweaking it, I can understand the attraction of acquiring a better mount (I assume that's where this is heading).
I definitely tweak the aggression and min move based on current seeing conditions. I don't usually do it throughout a session though, unless things are clearly not right. I usually just set it up at the beginning of the night and as long as the corrections look good I leave it be.
When things are not so great the RMS is around 0.7-0.8" and it causes noticable star elongation at 1.33"/px. Would like to have longer exposures with autoguider to reduce effects of seeing, but then moment to moment RA tracking errors of my mount can be too significant and need more frequent correcions. So I found 1.5 sec corrections to be a fine spot with my mount most of the time.
Yes, you are right, there is an attraction towards a better mount, but I am not in a rush
I use the same mount, guided using EQMOD, PHD2 and an ASI120MM. I typically use an OAG for longer FL imaging, but when recently doing some wide-angle shots with a Lens mounted on a guidescope (guiding at 1.61"/px), I was getting RMS of around 0.25-0.4" consistently. Side Note: It was complete overkill for my imaging scale, heh (11"/px)!
I think it's overall an excellent mount for its price - a sweet spot. (I have excellent PA [permanent setup] and an anchored tripod).
In general, a back of the envelope calculation shows that if you want to limit to an x% increase in FWHM in any direction, your RMS guiding error should be no more than about sqrt((1+x/100)^2-1)/2.4 times the seeing. So to limit to a 50% increase you'd need RMS guiding at about 0.5 times the seeing (1" in the above). 10% would require 0.2 times the seeing (0.4" in the above).
I'd say at 0.45" you're pretty good, Suavi!!
Personally, with my EQ6 (belt modded), I got the best I've ever got the other day, and that was still about 0.7" RMS. I'd be ecstatic to get a guiding graph like yours!! Not too sure why I can't get better.
Slawomir have you looked at PEC training at all? I've seen some improvements though the AutoPEC feature in EQMOD and I believe others (Barry?) have had similar success. Might be worth a try if you've not already.
In general, a back of the envelope calculation shows that if you want to limit to an x% increase in FWHM in any direction, your RMS guiding error should be no more than about sqrt((1+x/100)^2-1)/2.4 times the seeing. So to limit to a 50% increase you'd need RMS guiding at about 0.5 times the seeing (1" in the above). 10% would require 0.2 times the seeing (0.4" in the above).
I'd say at 0.45" you're pretty good, Suavi!!
Personally, with my EQ6 (belt modded), I got the best I've ever got the other day, and that was still about 0.7" RMS. I'd be ecstatic to get a guiding graph like yours!! Not too sure why I can't get better.
Thank you Paul for the link and your explanation.
My mount tracks okay, but I can see some degree of star elongation in some/most of my 15-minute subs. Some subs are nearly perfect though. I can live with slightly elongated stars, but what it means is that images are not as sharp as they could have been and probably overall acquisition time could be shorter with more precise guiding, because photons that should land only on one pixel are instead distributed over neighbouring pixels too, leading to a weaker signal. Saving for a better mount will need to take some years though...so in the meantime I will thoroughly enjoy the gear I currently have!
Quote:
Originally Posted by Eden
Slawomir have you looked at PEC training at all? I've seen some improvements though the AutoPEC feature in EQMOD and I believe others (Barry?) have had similar success. Might be worth a try if you've not already.
Hi Brett,
Thank you for pointing that out and yes, I tried PEC training but with mixed results. However, since I need to move my gear in and out of the house, I found PEC not being very practical in my case.
The best I have managed is 60min subs with only slight elongation. I have managed an aspect of 16 with 40 min subs but I do tend to get 10-12 as an average even on shorter subs. I do think that I am suffering of differential flexure more than anything in this case. Planning on upgrading mount as well but probably not for another 7 months (next financial year) at least.
As for PEC, if you park your mount at the end of every session you SHOULD be okay. I've never done PEC training myself so I am going to leave it at SHOULD
The best I have managed is 60min subs with only slight elongation. I have managed an aspect of 16 with 40 min subs but I do tend to get 10-12 as an average even on shorter subs. I do think that I am suffering of differential flexure more than anything in this case. Planning on upgrading mount as well but probably not for another 7 months (next financial year) at least.
As for PEC, if you park your mount at the end of every session you SHOULD be okay. I've never done PEC training myself so I am going to leave it at SHOULD
That's a great result Colin. With 60-min subs, field rotation might show up as well if your polar alignment is not spot on. Hopefully your mount upgrade will materialise very soon
I always park, but then I carry the gear in and out of the house (I carry the mount with the telescope on it...), so chances are things can move a bit when setting up/carrying the gear.
That's a great result Colin. With 60-min subs, field rotation might show up as well if your polar alignment is not spot on. Hopefully your mount upgrade will materialise very soon
I always park, but then I carry the gear in and out of the house (I carry the mount with the telescope on it...), so chances are things can move a bit when setting up/carrying the gear.
TSX said I was ~18 seconds away from SCP so not too bad :-)
My understanding is that it is only the movement of the gears that is of any importance and this shouldn't happen through movement. Meaning that you can move the telescope around freely when the clutches and lose and it should have not effect.
My mount tracks okay, but I can see some degree of star elongation in some/most of my 15-minute subs.
Presumably for those subs with elongation your guiding graph must be worse than the one you attached above, i.e. worse than 0.45" total RMS? What sort of FWHM are you getting?
Unless there's some sort of differential flexure (though I see you're using an OAG, right?)??
Presumably for those subs with elongation your guiding graph must be worse than the one you attached above, i.e. worse than 0.45" total RMS? What sort of FWHM are you getting?
Unless there's some sort of differential flexure (though I see you're using an OAG, right?)??
From my reply below
Quote:
Originally Posted by Slawomir
Hi Lee,
When things are not so great the RMS is around 0.7-0.8" and it causes noticable star elongation at 1.33"/px. Would like to have longer exposures with autoguider to reduce effects of seeing, but then moment to moment RA tracking errors of my mount can be too significant and need more frequent correcions. So I found 1.5 sec corrections to be a fine spot with my mount most of the time.
Quote:
Originally Posted by Atmos
TSX said I was ~18 seconds away from SCP so not too bad :-)
My understanding is that it is only the movement of the gears that is of any importance and this shouldn't happen through movement. Meaning that you can move the telescope around freely when the clutches and lose and it should have not effect.
Thanks Colin, I did not know that.
I feel my mixed results with PEC are because of:
1. My methodology has flaws
2. The RA errors vary from cycle to cycle in my mount thus it is impossible to have a well fitted single PE curve, even if PE curve is an average of several runs. So occasionally and inevitably PE corrections go the wrong way - enough to spoil a sub.
What's interesting is that most of your points in that original guiding graph, particularly in RA, are on one side of the axis only. Any idea why?? Mount out of balance (maybe deliberately of course)? But kind of invalidates the assumptions in all these simple calculations about what level of guiding accuracy is needed as they all assume a nice symmetric Gaussian! (but I wouldn't make too big a deal of it as it'll make your FWHM smaller than the calculation based on RMS would suggest).
BTW, you mention the fact that stars are elongated when it's not guiding well. Could it be argued that it's the FWHM that matters at the end of the day, e.g. you could get round stars when RA is a bit loose in guiding by deliberately making your dec guiding equally as bad! Ideally we want small FWHM in both axes, and that, I'd suggest, should be the goal rather than just trying to make them the same value and have round stars? OK, I'm being pedantic, and both approaches are trying to get to the same end, but I'd suggest we shouldn't be assessing "are the stars round" to see if our guiding is good, but "are the stars small"? In my case, it makes a difference, as I tend to have slightly rubbish guiding in BOTH dec and RA, so get round stars - though a tad wider than they should be! Just an idle thought on a late Friday arvo!
BTW, you mention the fact that stars are elongated when it's not guiding well. Could it be argued that it's the FWHM that matters at the end of the day, e.g. you could get round stars when RA is a bit loose in guiding by deliberately making your dec guiding equally as bad! Ideally we want small FWHM in both axes, and that, I'd suggest, should be the goal rather than just trying to make them the same value and have round stars? OK, I'm being pedantic, and both approaches are trying to get to the same end, but I'd suggest we shouldn't be assessing "are the stars round" to see if our guiding is good, but "are the stars small"? In my case, it makes a difference, as I tend to have slightly rubbish guiding in BOTH dec and RA, so get round stars - though a tad wider than they should be! Just an idle thought on a late Friday arvo!
totally agree. You can for example get nice round stars by defocusing them a bit, but that really isn't the point. Similarly, one often sees tracking performance assessed on the basis of how flat the phd2 graph is. You can get the graph as flat as you like if you make the exposures long enough (eg to cover a full worm cycle), but all that will mean is that the guiding system is not making any corrections - not that there is nothing to correct. I am quite happy to see phd2 jumping about all over the place if it ends up with better stars (ie, if in the process of following the seeing, it also corrects some mount errors).
Agree that minimising and equalising FWHM in both axes is the goal and this should be the basis for assessing guiding solutions.
(but I wouldn't make too big a deal of it as it'll make your FWHM smaller than the calculation based on RMS would suggest).
