Guiding vs. MLPT

[MarkS]

Hi All,

 

I notice that some of you are guiding the DDM85 mounts, whilst others do not. Has anyone done an in-depth comparison of guided performance vs. MLPT on a well set up system?

 

My reason for asking is that I have seen claims that stars will always be smaller and sharper with guiding, no matter how good the mount is. I find this hard to understand if the unguided mount is tracking to sub-pixel accuracy over the time of each exposure, which my mobile set-up certainly does - in fact I've measured my DDM60 to track within 0.75 arcsec over an hour.

 

I would dearly like to be enlightened on this issue.

 

Mark

 

 

[Jay]

Hi Mark,

 

I think the end result depends very much on what kind of (and how much) flexure your setup contains...

 

If the system would have 0 flex (which is impossible) the unguided system would be as good as the tracking accuracy. However, there will always be more or less flexure depending what kind of setup you have. That doesn't mean that an unguided imaging setup wouldn't be able to produce wonderful results if built rigid. 

 

Guiding with a separate guider will have similar flexure issues and they have to be dealt also before achieving good results. However, guiding with an OAG will automatically eliminate most of the flexure problems and should be able to produce good results quite easily.

 

Jay

Edited August 25, 2016 by Jay

[maxmirot]

My tests show I can do often do 20 mins or more unguided with MLPT. However, Sequence is necessary. Sequence  is pretty bare bones (does not do much of what I want).

 

Straight models get me 10 mins. They seem to need a model redo every few weeks even if the systems has not been touched. 

I still don't understand how to get the most out of my mount do to poor documentation and support.

 

I often find it easier and more reliable to guide.  I am sure there are many other users like my self could do better but there is no expert support.

I know I am not alone.

 

I am pretty tech savvy but these mounts are not easy to use.  There are big holes in the available knowledge and easy way to ask simple questions. 

 

Max

[maxmirot]

Mark what is you instrument?

 

Max

[MarkS]

Hi Max,

 

I'm using the ASA10N, usually at f/3.6, but often at f/6.8 and sometimes at f/2.8.

 

I have both a DDM85A and a DDM60. I have never tried to guide them. I always use Sequence and MLPT.

 

With my usual seeing conditions of about 3" - very rarely 2" - I think star size is determined by seeing and focus variations, not inadequate tracking of the mount, but I've never tried to do a comparison between guided and unguided.

 

Mark

[Waldemar]

Since correction with guiding is always done after the programm detected a miss, there will always be a delay of at least a second before the correction is effective, because of making the image, downloadtime and applying the correction

MLPT 'knows' exactly when a correction is needed, it will correct at the precise moment. and checks it's position 100x per sec so even with windgusts it is a lot preciser then guiding. Many say that working with encoders is the same as guiding, just done by the mount's software instead of by a dedicated guiding programm, but they never mention the time delay with 'normal' guiding...

So I assume MLPT is superior to tracking, I never made a coparrison though...

 

To get rid of atmospheric causes adaptive optics will be neccessary, I never used that, but it seems to work pretty good.

 

Waldemar

Edited August 26, 2016 by Waldemar

[GeorgeCarey]

I always use guiding. I accept that there will be a delay. I take 5 second guiding exposures so with downloads adding and processing a correction is made every 6 seconds.

However, if there is a wind gust Autoslew will correct it rapidly whether you are guiding or using MLPT.

[lukepower]

At the beginning I tried to do all with a pointing model or MLPT. While it was working quie well, there were enough situations where it didn't. Probabily it is due to some flexure in my setup which is not that easy to eliminate. I switched to guiding and since then I have always round stars, as it is used basically to compensate for those slow drifts. I did once a comparison of MLPT vs. guiding: The stars in the best MLPT-frame were slightly smaller than in the guided one.... I guess that the small corrections I apply are causing the star to wobble a bit more than what would be needed.

[GeorgeCarey]

Was the comparison on the same night? The seeing can have a big effect.

[lukepower]

Same night, same setup.

But again, honestly, the difference is probabily due to seeing here, too.. If I get with MLPT down to 1.5", with guiding I am not far from that

[MarkS]

Hi all,

 

Thanks for this discussion. What I understand from this is, with the ASA mounts, there is little difference between MLPT and guided performance provided that the system is well set up and aligned. There are reasons to expect that, if the MLPT track has been accurately determined, the very fast response of the mount/AS/ Sequence combination should give better tracking than guiding with its slow 'after the event' corrections.This is what I would expect but I have made no direct comparisons. I can vouch for the DDM's spectacular response to wind gusts - it is really interesting to see the short term current spikes as they happen.

 

As Waldemar says, adaptive optics is perhaps the only path to improving star FWHM for a given seeing condition.

 

Mark

[maxmirot]

You clearly get excellent performance with MLPT on the N10. I would assume that its should be better than chasing the seeing in most cases.

 

It too bad I don't like Sequence.

I can only pray that the ASA software gods have pity on those of us that use other automation programs and make it scripted.

 

Max

[lukepower]

Max,

 

this, regrettably, is an old situation. Once or twice a year we pray for it to happen, but it won't. Look at all the bugs which are still there both in Autoslew and Sequence 

I agree that MLPT would be better than chasing the seeing, on the other side guiding works very well with ASA mounts (especially if you lower the aggressiveness), so I personally have settled with that.