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Is it possible to manually change the StarSense to OTA center calibration values?

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#1 rnyboy



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Posted 02 December 2020 - 10:58 AM

Pretty much ever since I've had my 6SE it's had a bias for putting objects high or just out of the FOV after a CPWI goto such that I now look at it as something of a quirk in my mount's personality.  This same same bias continued with the StarSense calibration even after centering at f/25 using SharpCap's reticule with a final "up and right" mount movement for centering.  Backlash values are set properly, scope is slightly back heavy, and objects are oriented such that an up and right mount movement shifts the object being viewed down and left.  So I was wondering if it is possible to easily change the two values that are used to center StarSense to the C6 OTA via the HC or with software?  Figuring a few empirical changes in the alt value would eventually produce a mostly unbiased centering on the FOV of my 385mc.


As a work around I'm thinking that doing a few StarSense center calibrations with the object intentionally low would do the same thing but is more of a PITA than simply iteratively making some slight changes in the value currently stored in memory somewhere.


My guess at the moment as to my topic question is probably a "No!" because Celestron tends to store things in the HC or mount thus making them inaccessible to user changes without the user doing some kind of manual adjusting through the scope or mount instead of being allowed to directly change any data values.  

#2 PolyWogg


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Posted 02 December 2020 - 03:12 PM

I find it interesting that you are consistently "off" in altitude, have adjusted backlash, etc, but say that when you go RIGHT and UP vs. DOWN and LEFT. The goal is for your scope to move RIGHT and UP, to take the tension out of the movements...how it works on your HC or in the EP is irrelevant. Your HC settings can throw that off anyway. I suggest double checking that your scope at SPEED 3 is going RIGHT and UP when you press RIGHT and UP. Since it is fine for azimuth, it is likely only an issue with your final "UP" to check. 

On my 8SE, the default setting for azimuth was INVERTED -- so when I thought I was going RIGHT and UP (according to full reset to defaults), my 8SE was actually going RIGHT and DOWN. Sooo, for five years, I chased a gremlin with inconsistent alignments. It was always a little off vertically but not consistently. We changed the setting to normal, it worked fine. Alternatively, you could try going RIGHT and DOWN and see if your result is better. Or, as I said, check that your scope goes up and right at speed 3 (your likely final adjustment speed -- or at whatever speed you do use).


Let us know your results, I suspect you're going overkill on the other settings...



#3 rnyboy



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Posted 02 December 2020 - 05:51 PM

Yes, it goes right and up at 4 or 5, which are the typical speeds I use when actually viewing an object, when using the up and right slew buttons and my azimuth has been correct from the start.  I've used far off pine trees to have my imaging sensor show the correct orientation of objects viewed and the object moves in the proper direction relative to the how the front of the scope is moving.  Nothing is off in relation to how the scope moves and how an object moves in the FOV.


This last weekend I was operating at f/3.2, which is very close to a 0.5 arcmin FOV in the vertical on my 385mc, and every goto put the object around 1/4 of the way down from the top edge of the sensor's FOV.  Not always exactly at the same place but depended on where I went but it was always closer to the upper edge of the sensor FOV.  I think that 0.5 arcmin FOV on my sensor would be close to a 25mm EP if using the scope at native f/10 for visual use with one's eyeball.


Having an easy way to offset that vertical bias would be quite nice to have.  My suspicion is that it's simply a fairly consistent error when center calibrating the StarSense to the scope.  That's why I'll try doing the center calibration by putting a star about 1/4 up from the bottom edge of the image at some point and see if that brings the object closer to imaging center after gotos.  Since I pretty much know an object out of the FOV of the sensor is most likely high I just start moving the scope up until I see the object come into view.  A fairly minor issue overall but it would be nice to have things start out closer to the FOV center more often.

Edited by rnyboy, 02 December 2020 - 05:59 PM.

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#4 speedster


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Posted 02 December 2020 - 06:56 PM

If you intentionally introduce up error in the east, then down error in the west?  Doesn't sound like the geometry will work with fudging the SS calibration.  Have you tried adding alignment points?  In CPWI, point anywhere and click "add alignment point" (or some click like that) and then SS plate solves and adds a point.  Try about a dozen points and see if that doesn't put your goto's where you want them.

#5 Noah4x4



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Posted 03 December 2020 - 03:27 AM

The challenge is simply that it is more difficult with any small sensor cameras or narrow FOV eyepieces and small aperture scopes. It is all a about available field of view. The larger the FOV, inevitably the less likely you will miss.


At f/2 on Hyperstar I can't miss my target as the FOV is huge. At f/6.3 I get good GoTo results with my large sensor ASI294mc provided I have carefully calibrated Starsense and added additional alignment points. But with my tiny sensor ASI224mc it is far more likely that I miss and my target might be just out of FOV. Your ASI385mc is a similar small sensor camera. This is perhaps like comparing the FOV of a 25mm eyepiece with 12mm eyepiece. You need to get multiple targets into FOV and then calibrate and add additional alignment points. Not easy. I don't think fudging this by false centering calibration will work.


What I do now is plug two Hand Controllers into my Evolution. I connect my Nexstar + HC to my computer using cable and it will enter Boot loader mode when I connect and align with CPWI and hence cease functioning. However, my Starsense HC Direction Buttons still work. That means that if my target is not in camera/screen FOV I can use these direction buttons at the scope and my 9x50 RACI illuminated finderscope to pull it back into range before confirming calibration or adding additional alignment points in CPWI. This will tighten up accuracy for a narrower FOV camera with less frustration. 


This is simply employing visual finderscope  techniques with a camera. If you have only one HC, a Game-Pad might suffice. Sadly, this trick is harder with a Nexstar SE model as it doesn't have multiple AUX ports. Having calibrated and added additional alignment points in CPWI, you can then revert to one (or no) HC. The challenge is getting the targets into initial FOV. Having a decent finderscope and some control at the scope is a huge bonus. 


Plate solving via Sharpcap can also assist. 

Edited by Noah4x4, 03 December 2020 - 03:31 AM.

#6 rnyboy



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Posted 03 December 2020 - 08:29 AM

I totally agree with Noah4x4, small FOVs are tough.  But this is much more like a bias.  It's not randomly centering around the center of my FOV, it's randomly centering around a point that is high in my FOV.  So should be correctable I would think.


One thing I just thought of is that I've been using the same StarSense center calibration since around late July/early August and I was operating at f/25 for the planets and had two filters, 2.5x Barlow, and ZWO ADC hanging off the back.  BUT, it was still centering above the sensor's FOV.  The other night I was operating at f/3.2 and didn't rebalance but I did add a extra small vial containing lead shot over the focus motor, so not terribly far from the mount pivot point, for a rough balance because the center of mass of the f/3.2 optic setup shifts towards the scope but has about the same mass as the f/25 setup.  It might be that I need to redo a center calibration every time I change the optics??


Another thing I've done, starting early this year, was to fill three roughly 3/4" x 3" plastic vials with #8 lead shot and with a silicone rubber band I can hold them in place on top of the focus motor housing to help with OTA balancing.  I use one, two, or all three as necessary to have a slightly back heavy balance because I have a rather long heated dew shield I need to compensate for.  My OTA is as far forward as it can go in the mount so I can hit zenith when operating at f/5 and a few other optical stack setups I use so I add or subtract those weights as needed.

Edited by rnyboy, 03 December 2020 - 08:30 AM.

#7 SanjeevJoshi


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Posted 03 December 2020 - 01:31 PM

Since my standard set up is now based on a mini pc at the mount, connected wifi to another laptop, i am going to be moving away from Starsense as well, although I love it.


I am getting a 50 mm guide scope and will be using a CMOS cam on it.   Calibrate it one time to the OTA FoV center.  Use that set up to platesolve, sync with CPWI, correct mount, center (in OTA FoV) and sync.   This is all of one click in Sharpcap Pro.  This should work very well for small FoVs whether visual or camera based.


No Starsense, no finders, no calibration.  Time and equipment savings.

Edited by SanjeevJoshi, 03 December 2020 - 01:34 PM.

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