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Cge Pro pointing off....

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#1 HowardK

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Posted 30 July 2014 - 08:40 AM

Any ideas why my CGE PRO almost centers objects when chosen by Messier, NGC number or star name or double star name but when i dial in the RA and DEC coordinates the object is always off in RA?...not in the FOV

The coordinates are from sissy hass' double star book so are not dated

This after a good and proper 2+4 alignment

#2 mclewis1

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Posted 30 July 2014 - 10:48 AM

Decimal vs. minutes/seconds in the coordinates?

#3 WadeH237

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Posted 30 July 2014 - 11:23 AM

The manual coordinates that you are getting from a chart are probably in J2000 epoch. You hand controller probably uses coordinates in JNow.

If I remember correctly (and this one is really fuzzy, so I could be very wrong), the NexStar controller used to use J2000 coordinates many years ago. At some point, an update changed it so that it uses JNow. I never heard a reason, but I suspect that it's due to the increase in users connecting a computer to the mount and using software that uses JNow.

#4 mclewis1

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Posted 30 July 2014 - 01:45 PM

The differences in positions between J2000 and those calculated for 2014 (JNow) are tiny ... measurable but tiny. Even the differences back to B1950 coordinates (I think something in the area of a couple of arc minutes) are way below the +- pointing error in the goto capabilities of today's mounts.

Howard's an experienced user of the Celestron mount, he knows his stuff. Being outside of the entire fov on user inputted objects is a big error. I don't believe it's any sort of calculation error, the mount is being told to go to the wrong spot.

#5 WadeH237

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Posted 30 July 2014 - 03:21 PM

So I tried a little experiment.

As a simulation, I ran TheSkyX with a FOV indicator for my C14 with a Nagler 31 eyepiece. I then entered the J2000 coordinates for M11 and centered the field on that location.

When I told TheSkyX to interpret the coordinates as J2000, it was centered in the field. When I used the exact same coordinates and told TheySkyX to interpret them as JNow, it put M11 nearly all the way to the edge of the FOV...and nearly all of the shift was in the RA direction. With a narrower field of view, this would easily put the object outside of that field.

I maintain that my hypothesis very closely explains the behavior as described.

#6 mclewis1

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Posted 30 July 2014 - 05:02 PM

Wade,

That really interesting, it does seem to fit. But why that big a difference? Doesn't that eyepiece provide something like 40 arc/min fov and so a center to edge of about 20 arc/min?

A 20 arc/min shift is something like 20X what I thought it should be. All the detailed material I've read about the differences in various Epochs doesn't offer anything resembling simple calculations and all the simple explanations indicate the differences are small. I wonder if anyone has any reasonable math to explain what is actually happening?

#7 HowardK

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Posted 30 July 2014 - 06:24 PM

Thanks guys for your input here.

Next time out i'm going to input coordinates from skysafari...these are J2014 and differ from the ones in sissy's book..

Let's see what happens

#8 WadeH237

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Posted 30 July 2014 - 07:05 PM

TheSkyX puts the field size for the C14/31Nagler at 39 arc minutes in diameter.

The J2000 coordinates for M11, according to TSX, are:

RA: 18h 51m 06.00s
Dec: -6d 16' 00.00"

And for current are:

RA: 18h 51m 54.62s
Dec: -6d 14' 41.68"

Right ascension is expressed in clock seconds, not arc seconds, so you need to convert (this may be the source of your thinking that the error is very small). The sky movement rate is 15 arc seconds per second, so the difference in RA is 729.3 arc seconds, or about 12.2 arc minutes. You do need to adjust this for declination, which is why I picked an object near the celestial equator where that adjustment is 1x. At M11's declination, it would be a bit less than that 12.2 number, but still close to 25% of the diameter of the field.

I do more imaging than I do visual. As a part of that, my system plate solves each slew and records the accuracy. When I supply J2000 coordinates, I can pretty easily see that it misses the target by a non-negligible amount. It's no big deal in that case, because my imaging software does a correcting slew to place the target coordinates directly in the center of the field (to about 1.5 arc second accuracy on average).

After all this, I am very interested in Howard's test to see how it works entering JNow coordinates from Sky Safari. That will be the definitive way to see if this is the actual explanation.

#9 HowardK

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Posted 01 August 2014 - 07:09 AM

Tonight Wade

#10 HowardK

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Posted 02 August 2014 - 05:07 PM

Nope

Cge pro cannot point at an object using J2000 coordinates.

Only with J2014 coordinates does it put the object in the fov

#11 WadeH237

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Posted 04 August 2014 - 10:37 PM

So if I understand you correctly, you have confirmed that the hand controller interprets coordinates as JNow, and will miss the target if you enter J2000 coordinates - possibly by enough for the target to be out of the field of view.

 

Is this correct?



#12 HowardK

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Posted 05 August 2014 - 06:25 PM

So if I understand you correctly, you have confirmed that the hand controller interprets coordinates as JNow, and will miss the target if you enter J2000 coordinates - possibly by enough for the target to be out of the field of view.

 

Is this correct?

 

 

Yes...correct...

target out of the FOV with J2000

just off center with JNow



#13 WadeH237

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Posted 05 August 2014 - 06:37 PM

Thanks for confirming.  Hopefully this thread may help others who are seeing pointing errors with manually entering RA and dec.



#14 petemumbower

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Posted 06 August 2014 - 08:17 PM

This is good to know. Been kind of wondering why my CGE Pro is off once in a while. This explains it very well.








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