Focusmax and focusing stars in H-alpha - help!

[Martin-Devon]

I'm still trying to optimize the automation of my focusing. I have a Lakeside focuser fitted to my scope, which works very well, so the mechanics are sound. So far I've used this manually to optimize the FWMH value for stars in my FOV to get the best focus. However, for a while now I've been trying to get Focusmax sorted out (connected to Maxim DL5) but have had mixed success. With a luminence filter in place I can get reasonable V-curves, although every now & then it plays up so I still suspect I don't have the settings quite right.

My biggest issue though is trying to get Focusmax to work properly with a H-alpha filter. The stars are much weaker, understandably, but even when I increase exposure times to 4 seconds in 2xbin mode the V-curve routine still fails. As the focuser goes outwards to start it's incremental step-by-step movements inwards Focusmax always seem to "lose" the star. I've tried shallower start/end figures but this undermines a lot of what Focusmax is built on, apparently long repeated V-curve slopes are what it has to get it's teeth into. If I increase the exposure time more, then a 50 step V-curve will take an age to run.

I'd appreciate any tips/advice from others that use Focusmax to optimise focus on stars through a H-alpha filter. At the moment Iseem to have hit a brick wall...

Thanks,

Martin

[perfrej]

Martin,

The first point to make is that it doesn't matter which filter you do your vcurve with. Do it with L for speed. The curve is the same for each and every filter you can think of as it is a relative pointing table with slope parameters only.

For focusing you should use the Acquirestar function. It looks up a proper star to focus on and slews to it. Return slew is optional. The problem comes when you want to run both NB filters and LRGB with the same settings. Acquirestar has a range of magnitudes it searches for and they need to be different for NB (brighter stars). Unfortunately, CCD Autopilot doesn't let you specify that.

Another factor to consider is what the starting exposure time should be. CCD Autopilot lets you set that for each filter, which is good.

When I run CCD Autopilot I decide on NB or LRGB and simply change the magnitude parameters in Focusmax prior to each session. I am sure it can be scripted.

ACP 7 is supposed to have support for all Focusmax parameters set from filter choice but I have yet to confirm this.

So, do try Acquirestar!

/per

[ncjunk]

What magnitude star are you using? It should be around 4-7, if i remember correctly. This usually gives me good focus on all filters with a 2.5sec sexposure, can't remember what the ha is in comparison but it does focus.

You need at least 12 good v-curves and more is better, then you can remove the worst.

Why is the star going out of the square? Does it start off in the centre?

If it starts off in the centre but disappears,quite quickly then check your polar alignment is ok and you have no mechanical movement or slop anywhere.

I know you said the focuser is mechanically sound but you need to double check for any movement or slack in the system (anywhere in the system not just the focuser).

My stars stay center when focus max does it's stuff. Also check your number of steps and max/min positions aren't too high of low. I stick to a range of values around focus that means focus max won't lose the star at the min or max value i put in. Once you get used to it all you get to know roughly where focus is and can start it off near focus.

...kkids screaming...gotta go!..

Sent from my GT-P5110 using Tapatalk 2

[Martin-Devon]

Per & Neil, many thanks for this quick feedback - already there are some very useful pointers in there.

I didn't realise that I could do my V-curves using a Lum filter and then switch to quickly getting prime focus with a H-alpha filter. I assumed (wrongly it seems) that since I had a different filter in place I'd have to complete a fresh V-curve as I hopped from broad to narrowband. Certainly when I do manual FWMH focusing there is sizeable shift in the focus between Lum and H-alpha, but from what Per is saying this is irrelevant to Focusmax - this makes things much easier now. I will look into the AcquireStar feature as you suggest Per.

Neil, I'm pretty certain that I have good polar alignment and no slop in my set-up, I can get nice round stars for 10 minutes unguided with my Mach-1 and manually the Lakeside goes in/out sharply to focus. However - with the H-alpha V-curve runs, which like I say often fail at the moment, the 4 second downloads on my chosen star do annoyingly keep changing; one moment it's a nice small box with the star in, then the box is 3 times bigger, then the box moves across the screen to a different position - why it's doing this I have no idea, since I don't see any of this nonsense with Lum driven V-curves. This is why I think I just don't have the settings right for H-alpha Focusmax runs at the moment. I've also seen others say to focus on a very bright star for H-alpha runs in Focusmax, so maybe I'm just not giving it a bright enough star. Most times I don't know the actual magnitude of the stars in my FOV, I just chose one of the bigger brighter ones (magnitude unknown), but from the feedback this is probably wrong, for H-alpha I may need to slew to a brighter star nearby, optimize Focusmax, then swing back to my imaging target - probably wrongly I'm trying to do it all within the FOV of my framed imaging target at the moment.

Thanks again,

Martin

[ncjunk]

Sounds like the stars not brght enough to me. I use acp which always slews to a specific brightness of star and uses that, so theres no failure. When i do it manually i search for a nearby star on carte du cieles although luminance is much more forgiving.

Sent from my GT-I9003 using Tapatalk 2

[Lars]

I was about to start a new thread regarding a similar problem and not wanting to hijack this thread but maybe some experienced FocusMax user here can give me some advice : Since I have a moving mirror (with some mirror flop/shift) and a small FoV, the focus star will inevitably disappear out of the box when FocusMax tries to make a V-curve. FocusMax will then increase the box until it finds the star again. It then continues the focusing routine and starts reducing the box/FoV. I guess it does that in order to improve the focusing. But when the focusing starts again and the mirror moves, the focus star will soon disappear from the box and the problem starts all over again. Is there any way to set a fixed FoV for the box? /Lars

[perfrej]

Maybe fix the mirror flop?

/per

[Lars]

Well, fixing the mirror shift comes with a cost that is not neglible. I have already bought second-hand mirror locks, but I will also need an external focuser with a motor that can handle heavy loads and that doesn't come for free. Until then it seems I'll continue with the Bahtinov mask. I have an old C9.25 that has neglible mirror shift but the C14 with its heavy mirror has a lot of shift visible even with a reducer that puts it at almost the same focal length as the 9.25. /Lars

[ncjunk]

I went with the external focuser and crayfrd on the c11. I plan on fixing the mirror in place to stop the flop and use the crayford.

Sent from my GT-I9003 using Tapatalk 2

[Martin-Devon]

OK let me update on my attempts to get Focusmax sorted. I concentrated on running multiple Lum V-curves last night, they work very well now, nice symmetrical V and the PID (position intersection difference) values almost matched perfectly. I have the AutoFocus box ticked as well, and it comes up with a focuser figure almost identical tothe PID value. However, and here's the problem, the PID/focus values look OK, and my Lakeside focuser shows these same values on it's screen - but when I now download a 5 second full screen sub in Maxim the stars look bloated and it's very clearly out of focus! I then resorted to manual focus tweaking the Lakeside about 150 "ticks" to get nice sharp stars with the lowest FWMH figures and now they look perfect. So - WHY am I getting this discrepancy, the V-curve Focusmax supposedly optimised figure that comes with perfect V-curve is clearly way out from what the true (manually determined) position should be. This is driving me nuts!

I think I've sorted the H-alpha focusing out - as Neil suggested, I just needed a much brighter star, now V-curves run just fine. As above though the PID and Autofocus positions of supposed optimized focus yet again gives a full screen download of bloated out-of-focus stars....., so the same problem as above really for Luminence. I'm clearly doing something fundamentally wrong, but what is it? Please - any thoughts, tips would help!

Martin

[Lars]

I went with the external focuser and crayfrd on the c11. I plan on fixing the mirror in place to stop the flop and use the crayford.

Thanks for the input Neil. I guess I have to go that route as well if I want to use FocusMax.

/Lars

[Martin-Devon]

Tonight (if the haze stay away) I get the chance to try out AcquireStar in Focusmax as recommended by Per. Somewhat annoyingly although Maxim 5DL Pro has inbuilt PinPoint for plate solving, it's only the Light Edition and thius will not link up to Focusmax - you have to pay to get the full PinPoint package for this. There is a 60 day free trial though for that so I've taken this up, let's just hope that it all behaves tonight!

Martin

[ncjunk]

Thanks for the input Neil. I guess I have to go that route as well if I want to use FocusMax.

/Lars

Just to clarify.

Only the external crayford is needed.

My focusmax is working well with the crayford as i'm not moving the mirror. The mirror flop for me is bad for the pointing as it suddenly moves a couple of degrees out during a slew so i'm going to fix it in place.

Sent from my GT-I9003 using Tapatalk 2

[perfrej]

I would like to see a text file with the log window content in FocusMax. Post?

/p

[perfrej]

Also, a clarification. The V-curve does not contain any information of the focuser position. It only tells FocusMax how many steps it needs to move the focuser if the HFD is, for instance 8.2.

For this to work you CANNOT have any mirror flop and the slack of the focuser needs to be managed. You fix the latter by making sure that FocusMax approaches every position it goes to from the same side. There is a check box for that feature.

/p

[x6gas]

Have you managed to get this working now, Martin, and solved the problem above?

I'm considering an autofocus system too and the LakesideAstro unit / FocusMax combo looks attractive price wise...

Ian

[Gina]

I plan to set up autofocus too, by adding the ASCOM bit to my Arduino sketch and using some suitable software. Is Focusmax considered the best?

[ncjunk]

Yes focus max is pretty much the best and free. Works great when you have figured out what you have to do!

Sent from my GT-P5110 using Tapatalk 2

[perfrej]

Focusmax is the best. What sets it apart from most other software is the fact that it measures focus out of focus, usually at around 4 or more times the FWHM of focus. It works like this:

You have a vcurve that tells Focusmax the distance to move the focuser in order to get perfect focus from a certain measured FWHM. As it does the measurements way out of focus, atmospheric disturbances produce a proportionally smaller impact on the measurements.

If it was to measure FWHM and then change the focuser position, re-measure and iterate that it would take forever and a "flickering" of the seeing of, say, 1" would make it very difficult as the flickering itself would conbstitute 30% of the width it is trying to measure.

Brilliant!

/p

[Martin-Devon]

Ian, yes I've now managed to get Focusmax working reliably, and owe thanks to Per and Neil for their help with this. There were 2 things I was doing wrong, firstly picking a star to focus on that was too faint - this was an issue especially with the H-alpha filter, you need a really bright star for this and secondly I had the focus depth either side of the true focus set far too high and after some trial and error I can now set it optimally to get a good V-curve every time with out any funny stuff going on. I found that 750 focus units either side of my optimal focus worked well for my set up. Having finally got to grips with Focusmax I now need to move onto the key next step which is to automate the focusing, and for this I'm looking into 2 pieces of software that can do this, CCD AutoPilot and CCD Commander. Both include filter offsets, which are very handy since it means that you can always use the current FOV where your target is and a luminence filter, so any fairly bright star in the FOV will suffice to do the autofocus, then if you are using a H-alpha filter it uses the offset figure to adjust the focus accordingly and give prime focus - without this you would have to slew the scope outside of your current imaging FOV to find a star bright enough for the H-alpha. There are inbuilt Wizards to run and determine the offset values for you as well to make it even easier.

The weather here in Devon is now awful, and looks set to stay that way for the next week at least. When it clears though I'm going to try out the free trials of both softwares to see how I get on with automated focus runs using Focusmax.

Martin

[perfrej]

Excellent, Martin!

My advice for the automation is CCD Autopilot. It gives you truly full automation of everything but needs to be looked after every day. ACP, on the other hand, will run on its own for years without any attention from you. The latter, of course, comes at a price

Currently, I do not have a set up runing but as soon as it gets dark I can point you in the right direction when it comes to CCD AP.

Filter offset... Hmmm... I have considered this but have such good experience with separate focusing for each filter that I do not want to change. I may, however, due to the fact that changing the Acquirestar magnitude from a script is currently not possible with Focusmax. I have requested that feature, though! If push comes to shove, I will use offsets for my Provence obs.

Now, focusing without selecting a star (i.e. not using Acquirestar) is not om my list of things I would like to do. I really like the fact that you always focus on something that will work and definitely isn't a double star. Stick with Acquirestar! Of course, if you have pointing accuracy like a Meade ETX-125 then you have to rethink, but you don't It will put you right back on target every time after focus.

/p

[perfrej]

Ha! Prayers answered! Steve already put it in 3.7.0.84 (a beta release). Goodbye filter offsets, hello Acquirestar!

/per

[Gina]

Very interesting I'm planning to get automatic focussing sorted out myself eventually - I have a few other things to sort out first. My current method of focussing is simply to approach focus from one side, graduallu reducing FWHM until it starts increasing again and then going back a bit. This is pretty tedious and also very prone to the seeing and FHWM jiggling up and down so the Focusmax method is very interesting and would clearly produce better results than my manual method.

[perfrej]

Gina,

You will be amazed at how accurate focus you get with Focusmax. There is no way manual focusing can deliver the same level of accuracy and predictability.

/p

[ncjunk]

I love the theory behind focusmax f taking the two gradients either side of focus and calculating the intersection point which is the point of best focus....very elegant.

Sent from my GT-I9003 using Tapatalk 2