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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 PeteM

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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.



#15 HowardK

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Posted 27 January 2015 - 06:29 AM

So ...

 

we know that with a CGE PRO it is necessary to enter JNow RA and DEC coordinates to get an object anywhere near the center of the FOV.

 

My problem is that i am working thru a double star catalogue that has J2000 coordinates...entering these into the CGE PRO's hand control never puts the star in the FOV..so how do I find out the JNow coordinates of an obscure double star?

 

The more popular stars have sao numbers, etc that i can look up in skysafari....this gives JNow coordinates...but there are plenty of double stars that are not listed in skysafari....and how about the Hickson 100 list of galaxy groups....the only catalogues of these amazing clusters show J2000 coordinates and none of these objects are listed in skysafari meaning i cannot easily find a Hickson group with my CGE PRO.

 

any ideas anyone?



#16 freestar8n

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Posted 27 January 2015 - 09:02 AM

Hi Howard-

 

I can see a few ways to do this.  One is just to go through the list of coordinates and convert from J2000 to JNow.  This can be done many ways - such as with a planetarium program or a web page that does the conversion.

 

Another is to use a planetarium program connected to the scope and do the goto's by clicking on the stars you are interested in using the planetarium program.  If it is set up properly it should do the JNow conversion and it will all work.

 

Finally there is a sort of a hack to use a 'sync' to make the mount think it is using j2000.  For that you would find a star near where you want to look and you would do a GoTo RA/Dec and enter the J2000 coordinates of the star.  The mount will be some ways off from the star, but just manually center it.  Then do a 'sync' and it should sync the star location to the j2000 value.  From then on, if you do goto's based on j2000 it should work pretty well in that area.

 

Frank



#17 HowardK

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Posted 27 January 2015 - 11:18 AM

Hi Howard-

 

I can see a few ways to do this.  One is just to go through the list of coordinates and convert from J2000 to JNow.  This can be done many ways - such as with a planetarium program or a web page that does the conversion.

 

Another is to use a planetarium program connected to the scope and do the goto's by clicking on the stars you are interested in using the planetarium program.  If it is set up properly it should do the JNow conversion and it will all work.

 

Finally there is a sort of a hack to use a 'sync' to make the mount think it is using j2000.  For that you would find a star near where you want to look and you would do a GoTo RA/Dec and enter the J2000 coordinates of the star.  The mount will be some ways off from the star, but just manually center it.  Then do a 'sync' and it should sync the star location to the j2000 value.  From then on, if you do goto's based on j2000 it should work pretty well in that area.

 

Frank

Hi Frank

 

is there a web page that converts J2000 coordinates to JNow?



#18 freestar8n

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Posted 27 January 2015 - 11:33 AM

I think there are several - just search on "precession JNow J2000 conversion" or something.

 

One is at http://www.robertmar...oordinates.html

 

I think that would work.

 

Do you have the whole list of stars in a text file with coordinates?  I could probably convert it for you if you want if the file has a fixed format that custom code can read.

 

Frank



#19 WadeH237

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Posted 27 January 2015 - 11:41 AM

Or you could just use the mount's Precise Goto feature.

 

You could do a Precise Goto to the double's coordinates and the mount would slew to a bright star nearby.  You then center that star and the mount will do an offset slew from there to your target.  It basically automates the sync process that Frank describes above.

 

-Wade



#20 HowardK

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Posted 27 January 2015 - 12:06 PM

I think there are several - just search on "precession JNow J2000 conversion" or something.

 

One is at http://www.robertmar...oordinates.html

 

I think that would work.

 

Do you have the whole list of stars in a text file with coordinates?  I could probably convert it for you if you want if the file has a fixed format that custom code can read.

 

Frank

Frank

 

i think the link you posted for the conversion may well work.

i tried it on NGC 190 ..inputed J2000 coords and it gave me JNow coords that are very close to skysafaris.

will try next time with scope.

 

planetarium program will not work...most of the objects i am observing with ra and dec coords are not in a planetarium's database.

 

thanks for the conversion offer but my star list is in sissy hass' double star book so not a text file.

am hopeful that the J2000/JNow conversion link will work.

howard



#21 HowardK

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Posted 27 January 2015 - 12:10 PM

Or you could just use the mount's Precise Goto feature.

 

You could do a Precise Goto to the double's coordinates and the mount would slew to a bright star nearby.  You then center that star and the mount will do an offset slew from there to your target.  It basically automates the sync process that Frank describes above.

 

-Wade

Wade...

 

am i right in saying that with the cge pro you can only do a precise goto to a listed object and not to inputted ra and dec coords?



#22 freestar8n

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Posted 27 January 2015 - 12:18 PM

If you do the conversion from J2000 to JNow - you may want to enter in a "now" date of 2017 or something - so that the values you write down will last longer - from 2015 to 2019 - and they would still be pretty good beyond that.

 

Frank



#23 HowardK

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Posted 27 January 2015 - 12:23 PM

If you do the conversion from J2000 to JNow - you may want to enter in a "now" date of 2017 or something - so that the values you write down will last longer - from 2015 to 2019 - and they would still be pretty good beyond that.

 

Frank

Not necessary

 

once viewed the double star is crossed off the list.

the Hickson cluster is captured with my Mallincam.

 

do not write down the JNow coords.

 

thanks for your help Frank



#24 WadeH237

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Posted 27 January 2015 - 04:38 PM

It's been a while since I did it, but I am pretty sure that you can do a Precise Goto to arbitrary coordinates.  I seem to remember doing exactly that for some double stars :)

 

-Wade



#25 HowardK

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Posted 28 January 2015 - 05:17 AM

Hmmm....

 

i will try next time out...but...I'm doubtful.

 

think the precise goto function needs a specific object in the database to work and not a set of ra/dec coords...

 

we"ll see....




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