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Problem With Precession to Current Epoch

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#1 Tom Polakis

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Posted 08 November 2010 - 10:14 AM

Sorry if this has been answered, but I just noticed that my CGE mount is giving coordinates that are precessed to the current epoch rather than Equinox J2000. I understand that this is a desirable feature, but I couldn't figure out how to get it to display J2000.

The obvious application would be when you want to point the scope to the coordinates of something that's not in the database. If I tell the mount to go to coordinates I punch in, are those for the current epoch or J2000? I want them to be J2000, which is the standard, and will remain so for quite some time.

While precessing to the current epoch may result in better pointing, the display should show J2000, since it is the standard. Is there a way to make it do this? Thanks.

Tom

#2 Tel

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Posted 08 November 2010 - 11:02 AM

Hi Tom,

No doubt someone will come up with an appropriate technique as to how this might be done but to all intents and purposes it makes absolutely no difference whether you use Epoch J2000 or J "Now". It takes the Earth 26,000 years of "wobble" to make a full precession, thereby altering both DEC and RA co-ordinates, so you can understand that a decade, (from 2000 to 2010), will make next to no difference. I doubt even the GCE is accurate enough to notice such.

The bottom line therefore is that either epoch co-ordinates will do.

Hoping this helps,
Best regards,
Tel

#3 Tom Polakis

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Posted 08 November 2010 - 12:05 PM

Hi Tom,

No doubt someone will come up with an appropriate technique as to how this might be done but to all intents and purposes it makes absolutely no difference whether you use Epoch J2000 or J "Now". It takes the Earth 26,000 years of "wobble" to make a full precession, thereby altering both DEC and RA co-ordinates, so you can understand that a decade, (from 2000 to 2010), will make next to no difference. I doubt even the GCE is accurate enough to notice such.

The bottom line therefore is that either epoch co-ordinates will do.



Thanks, Tel.

In my case, I think it matters. Precession over a period of 11 years can approach 10 arcminutes. My really tiny chip at the prime focus of my C11 produces a small enough field where that can actually matter.

I agree that it's a small variable with respect to other forms of slop in pointing, but I would like to eliminate it.

Tom

#4 Tel

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Posted 08 November 2010 - 12:50 PM

Hi Tom,

I honestly think something is awry here: a difference of 10 arcmins would relate to a shift in distance of one third of the moon's diameter.

Here for example are the co-ordinates for Altair: the former Epoch J2000 the latter to J2010 i.e. "Of Date".

J2000: RA 19h:50m:47s / Dec +8^:52m:07s

J2010 (Of Date) RA 19h:51m:18s / Dec +8^53m:48s

Best regards,
Tel

#5 Tel

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Posted 08 November 2010 - 01:41 PM

Hi Tom,

Looking into the reasons for your CGE mount to be working on J "Now", I think it possibly due to it either being relatively new or if not that its HC has been upgraded to Version 4-18 which was designed, amongst other features, to work with J "Now" rather than as with previous versions, to J2000. It would therefore seem that you might only be able to revert to J2000 by downgrading your HC to Version 4-15 or 4-13. I am unsure though, as to whether this is possible.

Hoping this helps,

Best regards,
Tel

#6 Tom Polakis

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Posted 08 November 2010 - 01:52 PM

Hi Tom,

I honestly think something is awry here: a difference of 10 arcmins would relate to a shift in distance of one third of the moon's diameter.

Here for example are the co-ordinates for Altair: the former Epoch J2000 the latter to J2010 i.e. "Of Date".

J2000: RA 19h:50m:47s / Dec +8^:52m:07s

J2010 (Of Date) RA 19h:51m:18s / Dec +8^53m:48s



Tel,

In your example for Altair, its precession in declination is very small, only 1.7 arcminutes. But look at the precession in R.A., which is 31 seconds (of time). At the celestial equator, 31 seconds corresponds to 0.129 degrees, or 7.7 arcminutes.

So the angle of the discrepancy due to precession over a mere 10 years is nearly 8 arcminutes in 2010.0. I haven't checked how this error varies in other parts of the sky.

Tom

#7 Tel

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Posted 08 November 2010 - 03:33 PM

Hi Tom,

I understand what you are relating the 31secs to in terms of angular measurement:

i.e. 360 X 31 /60 X 60 X 24 = 0.129 degs or 7.74 arcmins

BUT I propose this calculation is unrelated.

I have to say that cosmology and astrophysics leaves me cold and there are others much better qualified than I in these matters, but surely RA and Dec are fixed co-ordinates, subject only to the minor fluctuations over centuries that Earth precession brings. I do see however that it appears Declination has more fluctuation with time than RA. (Not that I understand the reason why) !

Just as an afterthough, the last Epoch before J2000 was, I believe J1950. Thinking therefore about the 50 years between each Epoch: if precession had caused 7.74 arcmins shift in 10 years, each given RA co-ordinate would, by J2000 have shifted by 38.7 arcmins or more than half a moon ! This is clearly not the case.

Best regards,
Tel :confused:

#8 TonyDralle

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Posted 08 November 2010 - 04:41 PM

Without comment, I report the J1950 coordinates for Altair (from Becvar, Atlas of the Heavens-II, Catalog 1950 ): RA = 19h 48m 20.6s, Dec = 8° 44' 05" .

- Tony

#9 Tom Polakis

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Posted 08 November 2010 - 04:46 PM

Just as an afterthough, the last Epoch before J2000 was, I believe J1950. Thinking therefore about the 50 years between each Epoch: if precession had caused 7.74 arcmins shift in 10 years, each given RA co-ordinate would, by J2000 have shifted by 38.7 arcmins or more than half a moon ! This is clearly not the case.


Tel,

But it is. Divide 50 years (from 1950 through 2000) into a full 360-degree precession cycle of 26,000 years, and the fraction is 1 part in 520. Now divide 360 degrees by 520, and you get 0.69 degrees, or 42 arcminutes.

So, while 26,000 years is a long, long time, 50 years is very much a significant fraction of that period. And back to my original query, When you're dealing with CCD chips, even 11 years is significant.

Tom

#10 Tel

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Posted 08 November 2010 - 07:43 PM

Sorry Tom, I'm sure it's me being obtuse because I'm sure I'm confusing time (RA) with your angular measurements here which I need to address. Please correct me if I'm still not "seeing" this but let me try something else.

Taking some basic RA co-ordinates for Mintaka, (delta Orionis), which sits virtually on the celestial equator, I have the following values.

J2000 RA:5h 32m

J1950 RA:5h 29m

Now if RA 0h 00m, (angularly, 0deg 0m 0s), is the First Point of Aries, doesn't it follow that Mintaka must be 75.53 degrees distant from this according to J2000 ?

Likewise, for J1950, Mintaka would have been 75.48 degs distant from the First Point of Aries.

So in 50 years the change in this co-ordinate is 0.05 degs or 3 arcmins or 180 arcsecs.

If 180 arcsecs /50 years = 3.6 arcsecs per year, doesn't it follow that Mintaka's RA has changed angularly by 36 arcsecs over the ten years between J2000 and J-Now, or 36/60 = just 0.6 arcmins ?

Still :confused: :confused: :confused: ! ! !

Best regards,
Tel

#11 Tel

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Posted 08 November 2010 - 08:15 PM

Getting way past my bedtime Tom, (1:15am) but I came across this article.

See the section on Precession which might solve this calculation although the whole subject of Dec and RA shift with time appears much more complex.

Here's the link.

http://www.cv.nrao.e.../earth_rot.html

Best regards,
Tel

#12 Tom Polakis

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Posted 08 November 2010 - 10:33 PM

J2000 RA:5h 32m
J1950 RA:5h 29m


1 hour of R.A. at the celestial equator is 15 degrees. So 4 minutes of R.A. at the celestial equator is 1 degree. Given those values above, the star precessed by 0.75 degrees of R.A. in 50 years.

Now if RA 0h 00m, (angularly, 0deg 0m 0s), is the First Point of Aries, doesn't it follow that Mintaka must be 75.53 degrees distant from this according to J2000 ?

No, you are dividing 32 by 60, and getting 0.533. The problem is that this number only represents 0.533 hours, not degrees. If you want to get degrees, you take 0.53 hours and multiply it by 15 degrees per hour to get 8.00 degrees.

Now divide 29 by 60 to get 0.483, and multiply it by 15 to get 7.25 degrees.

The difference between 8.00 and 7.25 degrees is 0.75 degrees again.

The easiest way to look at this is that 4 minutes of R.A. at the equator is 1 degree.


I sure hope that folks haven't given up on this thread. The original query involved whether one could have a NexStar hand controller display in 2000.0 coordinates rather than coordinates of the present epoch.

Tom

#13 Tel

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Posted 09 November 2010 - 02:32 AM

Hi Tom,

Yes, finally understood. Thanks for your patience and understanding -- notwithstanding the sound maths lesson ! :bow:

As to your original question, I did a little further research on Mike Swanson's "Nexstar Site" and accordingly, if your HC is Version 4-15 or higher, (highest at the moment V4-19), then it will work on precession rather static J2000.

My past note presupposed you had a similar HC set-up to the SE series, ( most recent V4-18), which works on J-Now rather than J2000, but I guess the answer is the same. Unless you can downgrade to V4-13 if you do have a Version 4-15 (+), then you may not be able to change this. Here's Mike Swanson's webpage on the firmware progression. It's the best I can suggest.

www.nexstarsite.com/Firmware/CGEFirmware.htm

Best regards,
Tel

#14 Tel

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Posted 09 November 2010 - 05:04 AM

By the way, Tom, if you cannot change the CGE/HC set-up, I'm sure you're aware that Stellarium planetarium gives BOTH RA/Dec. J2000 and RA/Dec. "To Date" for its listed objects.

Best regards,
Tel

#15 Midnight Dan

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Posted 09 November 2010 - 07:30 AM

I sure hope that folks haven't given up on this thread.


Actually I'm finding this quite interesting - and learning a lot!

-Dan

#16 Peter9

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Posted 09 November 2010 - 07:43 AM

I sure hope that folks haven't given up on this thread.


Actually I'm finding this quite interesting - and learning a lot!

-Dan


Me Too. Don't understand much of it but found it fascinating to follow. Thanks to you both.

Regards. Peter.

#17 FebStars

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Posted 09 November 2010 - 08:56 AM

Thank you, Tel and Tom, for seeing this through with your questions. I did learn something here, not only about the subject, but the amiable way folks arrive at answers here. It's a joy to see that. :jump:

Tom

#18 Tom Polakis

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Posted 09 November 2010 - 09:09 AM

As to your original question, I did a little further research on Mike Swanson's "Nexstar Site" and accordingly, if your HC is Version 4-15 or higher, (highest at the moment V4-19), then it will work on precession rather static J2000.



Thanks, and it's what I suspected. I'm not very pleased to learn this, and I wonder how the engineers could possibly release a product that doesn't display equinox J2000 coordinates, no matter what's going on internally.

For example, one thing I may be doing with my CCD setup is follow-up work on asteroids. Ephemerides are always given in J2000. It shouldn't be the responsibility of the customer to precess coordinates to those on the hand control.

Tom

#19 Tel

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Posted 09 November 2010 - 11:11 AM

Hi Tom,

Now aware of where your interest lies; apart from the "majors", (Vesta, Ceres, Pallas, Juno), asteroid ephemerides are, as you probably realize, unavailble from Stellarium. However, both J2000 and J "Of Date" co-ordinates are readily accessible for many more of these objects from "Starry Night" planetarium which can be constantly updated via internet connection. This software program is however, as I suspect you know; relatively expensive ! It offers one route to what you possibly seek but there may of course be other asteroid displaying, freeware planetaria available if researched.

Hoping this helps a little more,
Best regards,
Tel

#20 Midnight Dan

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Posted 09 November 2010 - 11:21 AM

Just wanted to mention something. I placed a tech support question a few weeks ago to Celestron to ask whether their mounts use the J2000 epoch or JNow. The response was that they use J2000.

How did you determine that it's using JNow instead? I'm just trying to understand if the Celestron tech was wrong or if there's something else going on here.

-Dan

#21 Tel

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Posted 09 November 2010 - 11:32 AM

Hi Dan,

The Celestron tech. may well be right but not according to Mike Swanson's site. The SE HCs apparently work on precession as from Version 4-18, (which I read to be J "Of Date"), while the CGEs from Version 4-15; (current version for CGEs 4-19).

Check it out though if you wish, I might well be wrong in my interpretion of Mike's text ! :idea:

Best regards,
Tel

For J"Of Date" please also read J"Now".

#22 Sky Beast

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Posted 09 November 2010 - 11:39 AM

I just typed in Vesta, Ceres, Pallas, Juno in Cartes du Ciel and also found in the presented chart "(15)Eunomia". Also an asteroid. So it seems Cartes du Ciel has quite a few. It sits just above "(1)Ceres"

#23 mclewis1

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Posted 09 November 2010 - 11:42 AM

Mounts using the gem HC firmware (CG-5/CGEM/CGE/CGEPro) got JNow capabilities with the release of gem4.15 (released a couple of years ago). The Alt Az scopes (SE, CPC, NSi/GPS) got this capability with the release of NSX4.18 (last year).

Tom you can certainly downgrade your HC firmware to 4.13 to try out the J2000 coordinate capabilities. Another option if you don't want to touch your HC is to use a copy of NexRemote and configure it to whatever firmware level you like. This way you can trial any firmware level available without having to change anything in your HC.

#24 Tom Polakis

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Posted 09 November 2010 - 02:17 PM

Mounts using the gem HC firmware (CG-5/CGEM/CGE/CGEPro) got JNow capabilities with the release of gem4.15 (released a couple of years ago). The Alt Az scopes (SE, CPC, NSi/GPS) got this capability with the release of NSX4.18 (last year).

Tom you can certainly downgrade your HC firmware to 4.13 to try out the J2000 coordinate capabilities. Another option if you don't want to touch your HC is to use a copy of NexRemote and configure it to whatever firmware level you like. This way you can trial any firmware level available without having to change anything in your HC.



Thanks for the suggestions. I have a CGE mount for my C11 and an AS-GT mount for my TV101, and both have new firmware with this problem. I call it a problem, since there's no way that this is acceptable by any standard. If using the current epoch improves pointing accuracy, fine, but don't display the current epoch coordinates on the hand control!

As for how I know that it's using the current epoch rather than J2000, the test is very simple. Have it point to your favorite object, and then tell it to display the coordinates. You will notice that the displayed coordinates do not match those in a J2000.0 catalogue. There's a systematic error, and it matches the precession offset.

I don't think I'm just being anal in this case. I can think of plenty of Solar System objects that are not in the database, and I'd just like to key in current coordinates from an ephemeris. Likewise for variable stars and interesting deep-sky objects that are not in the hand control's database (or use a non-standard calalogue in the hand control).

Tom

#25 ftrobbie

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Posted 09 November 2010 - 04:16 PM

Tom

Not much to add to this, however when will JNOW be amended next. The advantage of the Epoch's was that it fixed the co-ordinates for fifty years although there must then be an algorithm in the mount/ handset to convert J2000 to whatever celestial time you are at. Perhaps that was the problem with that the firmware guys were having. Otherwise it would point in the wrong place. Or have I misunderstood RA and dec with the Epochs.

I am still at the "I am in awe stage at pretty common things" and just glad to get targets in the FOV and happy to nudge to centre it. I am sure one day when chasing objects with RA and dec it is going to annoy me. It's annoying and this is another example of moving away from simple standards to show how clever people are or are not.

My tuppence

Regards


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