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Old 25-09-2015, 10:10 AM
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Octane (Humayun)
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V-curves once hockey sticks, now proper Vs

Hey all,

For the last few years, whenever I have tried performing V-curves in FocusMax, I have always ended up with skewed curves which resemble hockey sticks.

This is with a relatively heavy setup (but, probably quite modest considering what today's payloads are like) of a RoboFocus focusing an FSQ-106N (4" focuser), CAA, SBIG STL-FW8 and SBIG STL-11000M.

I had figured that it was likely better for the focuser to work against gravity and push the camera/filterwheel inwards, rather than let the pull of gravity influence the V-curve run.

It used to annoy me as I could never find anywhere on the web where other people were having these same issues, but, I didn't concern myself too much with it as I was getting perfect focus (FWHM often around 1.5 (or lower) for luminance).

I had reason to pull apart my gear the other day to install an AO-L (which was a waste of time as I've run out of inwards focus travel -- need to mortgage the house and have PreciseParts make a thinner adapter between the CAA and AO-L, replacing the chunky Feldstein #6 adapter (CAA to FW8/STL) I've been using for the last few years).

Anyway, once I'd removed the AO-L and installed the camera and filterwheel again, I thought I might redo my V-curves. However, this time, set the final move direction to outwards, rather than what I'd been using previously.

Miraculously, I had /perfect/ V-curves with the apex of the V splitting the X-axis precisely!

I am still facing an issue (which I've had since I installed the RoboFocus) where my centre focus position keeps shifting. For instance, after the first V-curve's autofocus run, my focus was at 4040 counts. On the next run, that shifted down to 4037. This progressively moves downwards. I wish I knew the cause of this problem so that I could reliably use filter offsets instead of wasting 10-20 minutes every hour performing refocus routines. The Baader Planetarium 2" filters I use are meant to be parfocal, but, at f/5, I believe there are some slight differences between each filter. Probably not enough to really worry about, but, I'm a stickler for this kind of thing.

Could this issue have anything to do with backlash settings? They're currently set to the default of 20 in the RoboFocus software. Do I set that to 0 and then change the backlash settings within FocusMax itself?

Anyway, thought I'd share this as it brought a massive smile to my face seeing the V-curve perfectly split the X-axes for the first time in over 4 years!

If anyone has suggestions as to my other issue, I'm all ears!

H
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Old 25-09-2015, 01:08 PM
SpaceNoob (Chris)
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I forced my backlash setting as 0, now there's no visible "drift". This was done after various tests to see where the curve was reaching focus position. 0 seemed to be the most suitable, +/- from 0, I could see a visible drift in the v-curves. Note that I'm using an FT 3.5" focuser on the FSQ so your backlash settings may be different.
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Old 25-09-2015, 02:06 PM
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Octane (Humayun)
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Thanks, Chris!

I will set my backlash to a large amount (50, say) and perform a V-curve to see where the X-axis is crossed. Then, if that is adverse, I'll work my way down in lots of 10, and see how I go.

Cheers!

H
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Old 25-09-2015, 03:31 PM
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Not sure if this is matching your question but the focus position of an FSQ will constantly shift with temperature change. So you would not expect it to remain the same. FSQ is well known to be sensitive to temperature changes. 1C change requires a refocus. Also the step sizes on Robofocus are rather large.

Greg.
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Old 25-09-2015, 03:38 PM
SpaceNoob (Chris)
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Quote:
Originally Posted by gregbradley View Post
Not sure if this is matching your question but the focus position of an FSQ will constantly shift with temperature change. So you would not expect it to remain the same. FSQ is well known to be sensitive to temperature changes. 1C change requires a refocus. Also the step sizes on Robofocus are rather large.

Greg.
True, but it won't shift a lot compared to an incorrect backlash offset over a quick 30 point v-curve test. Each point will end up incorrectly off-set and the drift will be heavily amplified over the 30 points (30x offset error). The temp shift in comparison would be negligible, especially with 9micron pixels given the sampling. I just kept adjusting until the trough was exactly centered and that happened to be 0.
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Old 26-09-2015, 05:16 AM
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I think both Starlight Instruments and Starizona (Microtouch) have moved away from using motors on the fine focus shafts of FT focusers because it seems they do slip. Now the motors attach directly to the coarse focusing shaft and there is no slip. I'm not sure if this might effect your focuser. It might be the root of the problem of focus repeatability

Peter
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Old 26-09-2015, 05:37 AM
SpaceNoob (Chris)
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Quote:
Originally Posted by PRejto View Post
I think both Starlight Instruments and Starizona (Microtouch) have moved away from using motors on the fine focus shafts of FT focusers because it seems they do slip. Now the motors attach directly to the coarse focusing shaft and there is no slip. I'm not sure if this might effect your focuser. It might be the root of the problem of focus repeatability

Peter
I use the coarse focusing shaft for that exact reason.
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Old 26-09-2015, 01:45 PM
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Octane (Humayun)
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I don't have a fine focuser on mine; the RF goes straight on to the shaft via a bushed coupler.

H
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Old 26-09-2015, 04:52 PM
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Do you have the Tak bushed coupling or a generic one? They used to supply a generic one and it was the wrong sized hole and it would wobble a bit as it turned if it wasn't the specific bush for a particular scope.

Greg.
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Old 26-09-2015, 06:50 PM
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I don't have back lash set on my focusmax settings. And I think I did 40 sample points on the V curves when I did them. I set up around 20 V curves to get a good average. It seems to do an efficient job. I did note that until I did over 25 points the bottom of the V curve was as you described H.

Have you just checked all the grub screws etc for being tight. I found during summer it was fine but in the first winter I had several instances of what you are describing until I tightened things up in the winter months. Now there is no issue.
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Old 29-09-2015, 09:05 AM
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Octane (Humayun)
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Hi all,

Setting my backlash compensation to 0 in both RoboFocus and FocusMax is finally giving me consistent results!

Now, I might even tackle temperature compensation and see how that pans out.

And, yep, I grabbed the little Allen key and tightened the little grub screws some more. A couple of them required a tiny little bit of torque to get them snug.

I'm a happy camper.

H
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Old 04-05-2016, 01:57 AM
carlosgib (Charles)
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Robofocus/FocusMax issue

Hi. I have read with interest your note in the forum going back many moons, it was the one about your focus point ****fing back with every run.

I have the same problem, being trying to make the Focusmax work now for something I am almost at a stage to throw the towel. I have a TaK106ed, with the Robofocus.

Last night I did a manual focus to around 2, and started the V Curve routing. Having 35mm of drawtube extension with 4400 steps, put on 'half width' column, the first run I managed to get a v shape, but after that the V curve moved to the left, and the focus step started to move down.

So, I question if you do not mind, did you managed to solve your problem.

If you did do you mind sharing your solution.

KR & Thanks

Charles
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