Lakeside Motorised Focuser/Focusmax

[narrowbandpaul]

Hi All

About to move in to the big bad world of autofocus, so was wanting a few things clarified if possible.

I understand the premise of focusmax: V Curve to characterise your system, calculate slopes and generate the position intercepts. Can I assume that your system is only characterised when the PID is zero or negligible. I.e you need a good v curve for good focus. I read an article by Neil Fleming who had Position Intercepts that agree to the 3rd SF.

Once this situation has been reached and you press focus what actually happens. Does the Focuser simply move to the calculated position and you're done, or is there more calculation involved?

How do you refocus during an imaging session, ie for a temp change (no temp compensation available). If focusing is just the position intercept from the V Curve then do you need to run another V Curve analysis to allow for a change of temp.

Currently we use a bahtinov mask and bahtinov grabber to assure focus as a close as possible. In terms of accuracy how does using the handset to move the motorised focuser with the bahtinov mask in place and using the software to tell you the pixel error compare to the standard fully automatic focussing with focusmax. The bahtinov mask is quick and easy to use and the motorised Focuser should allow for high precision in aligning the diffraction spikes.

Any helpful tips on motorised focusing would be great!

Cheers

Paul

[Martin-Devon]

Hi Paul, I think you'll soon see the benefits of using the Lakeside automated focuser. I use mine with Focusmax - took a bit of getting used to, since there's lots of variations on the settings etc and it's important to get these right otherwise it's not optimal. Once sorted though, Focusmax is absolutely seamless and just works, it's quick (1-2 minutes) and highly accurate. The first task is to run the V-curves, and to run around a dozen or so of these - if you use a set-up with and without a reducer, you'll need to run V-curves for each since the parameters will be different. This is the only time-consuming part, but once you have the V-curves saved, these are used by all subsequent focusing runs to get the outputs - so it's important not to cut corners here. On the "Systems" tab when you open up Focusmax, just make sure the "path" is set to the right V-curve package (ie., with reducer, or without reducer). To actually focus, it depends on what filter you are using. For LRGB it's very quick and easy since most of the bright stars in most field are fine for focusing, and you just need a 5-10 second (under "Setup" and then "Base Exp"). I prefer to select my star for focusing, so under the "Focus" tab, go to "Select", it will then do a 5 or 10 second download as you instructed and you just click on a decent bright star- then Focusmax just takes over and runs a focusing routine using HFD measurements and checking the slopes of the focusing curves against your V-max curves, the pinpoint focus (which is the slope, not the tip of the V-curve) is determined and your Lakeside moves automatically to this figure. It's fantastic to watch all this running. 

If you are using a narrowband filter, it can be a bit more fiddly. If you have a magnitude 4 or brighter star in your imaging field, then you are OK, and a 10 second exposure on "select"star will be OK. If you don't have a mag 4 star or thereabouts, then you can try to pick the brightest star (perhaps mag 5-6) and change the set-up to 20-30 second subs (all 2xbin) to get a decent exposure which Focusmax can work with. Usually this works, it just takes longer to run the focusing routine. Alternatively you can just swing to a mag 4 star in a nearby adjacent field (just get RA/DEC from Stellarium), swing to this, focus (will be quick now, since very bright star), then using Astrotortilla or similar, just swing back to your imaging field. I've done it both ways like this, and they both work OK.

During an actual imaging run, you can refocus whenever you want. Just stop the subs, run Focusmax, it takes just 1-2 minutes all automated, then restart your subs again. Alternatively you can go to the next step of automation such as using CCD Commander where you just programme it to refocus after every 2nd, 3rd or 4th sub, whatever you want, and it will refocus al night long like this, and then at the end of the run, turns off the CCD cooler and parks the mount. I'm getting to like CCD Commander! You'll find that CCD Commander automates the guiding as well, infact it automates everything.

The Lakeside/Focusmax set-up works superbly, but you will need to invest time to get the hang of it, and be prepared to find a few instances of optimizing things first, and having to figure a few things out Once you've got to grips with it though, you'll never look back, and will benefit from spot-on focusing.

Good luck,

Martin

[perfrej]

On a more technical level...

FocusMax first determines which side of focus it is on and adjusts to the side it is programmed for (outside or inside). It then moves the focuser to the position where HFD should be around 8" (tunable) and verifies that it is there. After that it shoots five exposures, measures the HFD of each and calculates what, according to the vcurve parameters, position the best focus would be at. Finally, it moves to the best position.

The advantage of focusing out of focus is that the seeing affects less. Measuring at the best focus position would yield a situation where the seeing will wobble the HFD back and forth by up to, say, 100 per cent.

There are, as described above, tons of parameters... You need to test and tweak. Once you are there, you'll never go back

/per

[BlueAstra]

I don't have a Lakeside, but I do use a temperature compenstated Baader Streeldrive on my Baader 3" focusser. One aspect I've found that interferes with the perfect operation descibed above is the adjustment of the Crayford tension, along with the effect of camera/filter assembly weight (in my case ~3kg). If the tension is not exactly right, and the weight of the camera assembly is in one direction, the incremental focal shifts are not the same in each direction (ie the focusser will move further in one direction than the other, per increment). It probably also induces some backlash. So a V curve is generated with the focusser moving in one direction, but when moving back to the optimum focus it doesn't get there because the distance moved per increment is different. I think this is all down to me not adjusting the focusser correctly, but I've tried lots of things and still not got it right.

[Ibbo!]

I rate my lakeside and focus max

sometimes it will throw an odd focus but that is usually down to a passing cloud.

I did have to fiddle with the settings so it does a long enough initail exposure when using NB filters.

Steve

[narrowbandpaul]

Hi All,

Thanks for the feedback. Had the briefest of opportunities to test out the focuser last night, between rain and cloud. Used it initially in manual mode with a bahtinov mask. So far so good. Now have very fine control of focuser movements. Next it will be V Curve time!

Cheers

Paul

[perfrej]

That sounds good!

On a side note; Crayford... Focuser slippage is about the only thing that will throw FocusMax off. Crayfords, basically, are evil things that should be avoided

/per

[narrowbandpaul]

Im hoping there will be no issues with the focuser, its a 3.5" feathertouch. Usually rock solid!

A further question if I may, again reading what Neil Fleming had to say, he usually uses around a dozen V curves. What is the point of this many? Does focusmax take an average? Also, when do you have a good enough V curve to achieve focus? Is it once both slopes agree on the position of best focus, ie the point of intersection is the same from both slopes of the V?

Cheers

Paul

[perfrej]

Many vcurves in order to even out the whole thing. It uses the average, probably with some intelligent filtering. Do between five and fifteen and you should be fine!

Feathertouch = no problemos!

/per

[Ibbo!]

IIRC he also suggests after the initial curves to run a series with smaller steps.

I have not done this as yet and still now get better focus than i did manually even with the C.

Steve

[Chris Rowland]

I've done autofocus using Sequence Generator Pro, it operates in the same sort of way as FM except that it gets the star size from a multitude of stars in the field.  Worth a look.  Not free but won't break the bank.

Chris

[perfrej]

Chris, the special thing about FocusMax is that it focuses OUT of focus and calculates where it should be for perfect focus from that result. All other software packages (that I am aware of) measures IN focus and are thus affected by the seeing.

/per

[Chris Rowland]

Have you actually used SGP?  Or read the manual?

Chris

[perfrej]

Indeed I have, Chris. It is a solid and good algorithm that runs a curve every time. It passes the point of focus and goes back to it. I am quite certain it is a good algorithm. The difference between that and FocusMax, good or bad, is that FocusMax, as I said, relies on a curve previously measured and does not need any step calculations or mask measurements. It also, and I think it is the only software that does it, takes repeated measurements at an approximate point at a larger HFD in order to reduce the effects of seeing. That's the main difference.

Personally, I like FocusMax very much and am impressed with the choice of algorithm. SImple as that

/per

[narrowbandpaul]

Hi again,

Have has the chance to practice a few v curves whilst dodging clouds and rain, so a full evaluation yet to be conducted.

Ran through the first light wizard and that was OK, then did some v curves. I was manually setting the start and end points and the step size. These ran fine though some stars I'm sure (I hope ) were saturated as the v curve had a pretty flat min fwhm. On the next clear night I will ensure that I'm only at 2/3 full well when approx focused. So I think I'm ok with the generation of V curves and selecting them to be included in the profile. The better v curves had a PID of about 5 steps (f4.5 scope).

This is a really stupid question but after some satisfactory v curves have been produced, you then want to focus. Do you hit the focus button on the focus tab? This selects the brightest star in the FOV but what if I wasn't using the brightest star. I could have used a different star.

There are a few values to set in the setup tab (I think) focus the near focus HFD. I used a value of 10 with 5 exposures. When I hit focus it ran the Focuser to about a HFD of 10 and took 5 exposures. Once the best focus had reached one value it moved to there and it was done. It gave a good focus anyway.

I'm not really sure what values to enter there. I have the manual but as I say I was fighting rain so had to be very swift. I don't get how the Focuser can move to a HFD of 10, take 5 exposures and some how calculate focus from that?!

The next properly clear night should see remaining questions cleared up. Although not fully evaluated yet, seeing your Focuser automatically generate v curves and the focus is a thing of beauty.

No plans to look back!

Any further advice any of you have is gratefully received

Cheers

Paul

[perfrej]

Do you have pinpoint installed? If so, the Acquirestar method of focusing is best because it chooses a star for you. The question about saturation is immaterial. Focusmax will neither complete vcurve nor focus if the star is saturated. And it uses just one star.

I suggest sticking to focusing around HFD 8 or so.

/per