Guider settings in MAXIM DL

[alan4908]

I've been guiding with dithering now with MAXIM DL for a couple of months and everything seems to be OK. By OK, I mean that I appear to get round stars with 10m sub frames and I can consistently guide throughout the night without loosing the guide star. All this is achieved with the MAXIM DL default settings, a 25 sec guider dither settle time between subframes, a guider exposure time of 2s (with autodarks) and the EQMOD pulse guiding rate set to 0.4 sidereal. In un-guided mode, I get round stars with a maximum of 3m subs.  For my particular set up (SW ED80 with x0.85 F/R and Trius SX 26C; ST80 with Loadstar X2 and NEQ6 mount controlled by EQMOD) I took this to be a good indication that everything is set up reasonably well. So, I initially decided to leave everything alone and get on with taking images and improving my imaging knowledge.

Through the summer, I've been reading the (rather large) MAXIM DL user manual and in particular the part on guiding. I discovered that in my version of MAXIM DL (v6), multi-star guiding has been implement, I also discovered that hidden away in the advanced setting guiding menu you also have a minimum and a maximum move times. So, I was wondering how I would go about calculating some initial values upon which to experiment.

Minimum move parameter (MAXIM DL default: 10mS) -  this is the minimum time that the guider asks the mount to move. Now in EQMOD I noticed that there is a setting that specifies the minimum pulse time during guiding (for my set up it is 40ms).  I presume this figure is determined by the minimum guide pulse that your mount can accurately respond to. So, the first question is what is the EQMOD recommended minimum guide parameter for a NEQ6 ? Since I don't know, I'll leave this at 40mS which in turn means that my minimum move parameter will be determined by this parameter eg 40mS.  So, if I set this parameter in MAXIM DL, how many arc seconds of movement does this correspond to with my system ? I presume that the answer to this question is determined by what you are setting the sidereal guide rate to. Given that the normal sideral rate corresponds to 15 arc seconds/s, a sidereal guide rate of 0.4 would give you 15 x 0.4 arc seconds/s. So, if I apply a minimum guide pulse of 40mS, then this would correspond to a movement of 15 x 0.4 x  0.04 = 0.24 arc second. For my guider optical system, pinpoint tells me I'm working at 4.42 arc seconds/pixel, so this would correspond to a guider pixel movement of 0.054 pixel.  If I assume that the average UK seeing is around 2 to 3 arc seconds then a minimum movement time of 40mS would correspond to about 10% of the average seeing. I presume that EQMOD would ignore all guiding pulses that are less than 40mS, so changing the MAXIM DL value to 40mS from the default of 10mS would have zero effect.

Maximum move parameter (MAXIM DL default: 2s): I understand that this is the maximum time that the guider is allowed to move.  So what value do I set this too, given that I don't want to chase the seeing ? Presumably, if I set this to (say) 25% of the seeing then this would reduce the probability of chasing the seeing by a factor of 4. So, if we assume that the average UK seeing is 2.5 arc seconds then this implies that the maximum move time should result in a movement of 2.5/4 = 0.625 arc seconds. So, the maximum move time will be: 15 x 0.4 x max move time = 0.625, which means the max move time = 0.625/(15 x 0.4) = 104mS.  So, at the moment with the default setting of 2s, I'm allowing MAXIM DL to move my mount by up to 15 x 0.4 x 2 = 12 arc seconds, which would appear to be excessive, moreover this default value would allow the guider to chase the seeing. 

Delay after correction [MAXIM DL default: 0.1s]: is a parameter that let's your mount settle after a change has been made. Since I don't know how I'd calculate a different initial value so, I'll leave this as it is.

Multistar guiding -  there doesn't seem to be much documentation about this feature but I presume it works on the principle that within the guider field of view if you simultaneously track multiple guide stars and then average out the result you will be reducing the chance of chasing the seeing since the seeing will be slightly different on each guide star, which sounds good. 

So, my conclusion at the moment,  is that I'm allowing my guider to chase the seeing, the only items that are slowing this down are my guider exposure time (2s) and the settle after correction (0.1s). Since the latter parameter is relatively insignificant and taking into account it takes about 1s for the guider image to download, I currently appear to be attempting to chase the seeing every 3s which doesn't sound optimum. What it does tend to suggest is that I should consider:

a) Turning on multi-star guiding (to reduce the probability of chasing the seeing)

Set a maximum move time of around 100mS (to reduce the probability of chasing the seeing by a factor of 4)

c)  Consider increasing the guider exposure time to say 5s (to reduce the probability of chasing the seeing)

Given the rather limited number of clear nights at the moment, I'd appreciate if someone more experienced that myself could comment on these "back of the envelope" recommendations and check my assumptions. On a more pragmatic level, I'd also like to understand the likely benefit from going from seemingly OK guiding to excellent guiding. Please be aware that I'm not implementing Periodic Error Correction on my mount at the moment, so perhaps that should be my priority. Given that my PEC errors are unknown, options a) and c) would seem to be the best options at the moment.

Alan

[alan4908]

Hello - is there any chance of a response of a response from someone experienced in MAXIM DL guiding or failing that guiding ?

Thanks

Alan

[opticalpath]

I'll comment on the Minimum Move and Maximum Move parameters.   

Maximum Move is not too critical.  It sets an upper limit for the guider response; i.e. how big a correction you want to allow and above which you DON'T want the guider to act. If typical corrections are 1 arcsec or less, say, you would not want the autoguider to try to correct for a sudden anomalous rogue shift of 10 arc sec caused by a gust of wind for example.  So you would set Max Move to a value that would represent perhaps 2x  or 3x the biggest 'normal' deflection that you would typically see in any guide cycle.   

Minimum Move is much more important in my experience.  If you set this too low, the autoguider will attempt to correct every tiny deflection and you will end up chasing the seeing.  Set it too high and the guider will allow too-large deflections to go uncorrected. The optimal setting depends very much on your setup and conditions.  It's pretty much trial and error, aiming ideally to have a good proportion of guide cycles with no correction applied at all, whilst not allowing unduly large deviations to build up.  As a starting point, watch the guide camera stats. or look at the guide graph with guiding OFF to get a feel for what is the amount of rapid movement occurring caused by seeing. Set the Minimum Movement parameter to about that value initially.  Err on the side of setting it high initially, especially in Dec, then watch the guiding stats/ graph during guiding.  Judge if the guider response looks too twitchy, or too lazy and adjust the parameter accordingly. Avoid very high Aggressiveness settings while you're doing this.

There is an interesting analysis here:

http://acp.dc3.com/McMillanAutoguiding11-2005.pdf

and a handy calculator here:

http://www.ccdware.com/resources/autoguidercalcv4.cfm

Adrian

[alan4908]

Adrian

Thanks for your response - that all makes sense. I'll read the article and also see what the calculator suggests for my system. 

On the basis of not chasing the seeing, I believe I'll need to increase the value from the current (default) value of 10ms to something that is above 40mS (given that my EQMOD minimum guide pulse is set at 40mS). Anyway, given that this seems too complex to analyse analytically (particular when you throw in the fact that I'm also dithering by 1.5pxls after every sub frame and I have unknown PEC) I think you suggestion of watching the guiding graph, taking the aggressiveness down whilst I adjust the minimum move parameter up seems sensible.  As a separate exercise, I'll also switch on multi-star guiding and increase the guide exposure to 6s since (as I mentioned above), I think this can only help with not chasing the seeing. 

On the basis of your advice, I'll leave the Maximum Move parameter alone for the time being. I presume that the maximum move within the guide cycle would be determined by either dithering (known) and PEC (unknown). So, at some point it would seem sensible to quantify my mounts PEC even if I decide not to apply it. 

Alan

[opticalpath]

Hi Alan.

On your last point, when looking at typical maximum deviation in a guide cycle, you should not count the displacement due to dithering.  When you dither, you are introducing a deliberate (arbitrarily chosen) displacement at the end of each exposure.  So at that moment you're not really seeing the mount's normal behaviour.  I think it's a good exercise to do a run with guiding set up and running (without dither), and then to disable guiding completely in one axis (say RA first).  Then observe the guiding graph for a while, and inspect the guide log.  Repeat,  this time with DEC axis guiding disabled.  What you see in the log/ graph tells you a lot about the mount's native *unguided* performance - how big are the swings in one guide cycle, how large are the seeing oscillations, is the PE slow and smoothly changing or rapid and spiky - and that can be a good starting point when thinking about guider settings.

You might find 6 sec a bit too long exposure for the guide camera and/or too long between corrections.  Most people seem to find 2-3 seconds about right.  It's important to have good S/N in the guide star; it should be bright ... but not saturated.

Adrian

[fwm891]

Agree here with Adrian - however it really does depend on the mount. I had an NEQ6 and that needed delays set in to allow for changes to take place and settling but my Avalon Linear needs instant responses and aggression set at a maximum even though they are both based on similar electronics.

Scope balance will also come into play. Some imager's actually apply a small imbalance to the scope/mount combination to keep gear in mesh. If their scopes are perfectly balanced then any slop in the gear meshing will cause the guider to hunt back-n-forth because small gaps between gears (necessary or they will bind) allow the scope to rock from one gear face to the other. The Avalon has an all belt drive system where backlash has been elimated (but still don't want to chase seeing...)

[alan4908]

Thanks Adrian and Francis.

Yes, I think I understand (almost) all of this.

Adrian: just to check my understanding of your dithering point : after I finish changing the various parameters and find the optimum minimum move time (with dithering turned off). Are you then suggesting turning dithering on and then keeping all the MAXIM DL parameters constant apart from the guider settling time ? in other words, the minimum move parameter should be set independently of any dithering parameters.  On the point made by Francis: I've currently set my guider settle time to 25s, since it seems to take that long for my NEQ6 to settle down after a dither.

Alan

[fwm891]

Alan, there are a number of things you will need to think about on settling times - one being filter changes, in Maxim it will tell you its changing filters and then a settling period, so with the amount of dither etc you may need to change your 25 second delay based on watching the error figures fed back from the guider. I set mine beyond the maximum for my mount and use 40s as a delay for dither/filter changes etc - it's a suck-it and see job

[Davey-T]

Hi Alan, you could join the Maxim Yahoo group, you may get some more hands on experience advice, I'm just about to try Maxim again V6.0, never did get guiding to work in V5.***

Dave

[opticalpath]

Adrian: just to check my understanding of your dithering point : after I finish changing the various parameters and find the optimum minimum move time (with dithering turned off). Are you then suggesting turning dithering on and then keeping all the MAXIM DL parameters constant apart from the guider settling time ? in other words, the minimum move parameter should be set independently of any dithering parameters.  On the point made by Francis: I've currently set my guider settle time to 25s, since it seems to take that long for my NEQ6 to settle down after a dither.

Alan

Alan,

Yes, minimum and maximum move parameters should be set independently of dithering parameters (or not dithering).  You want the settings to be optimised for the normal deviations and corrections you would expect during exposures.  The movement initiated by dither *between* exposures will not be limited by the Minimum Move value, if that's what you were concerned about.

(TBH, one thing I'd never thought about is whether it's necessary to keep *Maximum* move big enough to accommodate the (larger than normal) correction required after a dither.  However, it would be usual to set this parameter value corresponding to a maximum movement much larger than the dither amount so it shouldn't matter in practice anyway.)

If settling seems to take too long (and 25 sec seems pretty good to me btw), a few things to consider

-  Don't set the dither amount too high, just enough to ensure successive images are displaced by a pixel or so. The mount may take longer to settle after a larger shift.

-  It will usually take a several guide cycles to settle so, if you have relatively long guide exposures, settling may take longer.

-  If you have some backlash in DEC, recovery after a dither in DEC generally will take longer than the same in RA.

- You could try the alternative settling criterion (Guide/Options/ Settling Criteria) that uses distance of guide star from centre of the box as the indicator of being settled, rather than just a fixed time delay. 

Adrian

[alan4908]

Thanks for your help Adrian, that all makes sense. I shall give it a go.

Alan