Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Mounts

Pages: 1
Danzup77
professor emeritus


Reged: 09/12/11

RA/DEC vs GoTo
      #5589211 - 12/26/12 04:19 AM

I asked this before but for some reason it was moved to the astrophotography forum so allow me to attempt again lol...

long story short I was doing some AP with my C8 and CGEM and I noticed that my GoTos were spot on while my RA/DEC were low (they were just UP and out of the EP) so I kept having to bring them down into the EP. Anyone have a reason this would happen so I dont keep doing it lol...

Objects were about 40 deg-ish in the west and I was getting ra/dec from stellarium.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: RA/DEC vs GoTo new [Re: Danzup77]
      #5589225 - 12/26/12 05:19 AM

this is probably due to slack in the gear train.

make sure that, when you did the initial GoTo alignment, only use the up and right buttons for the final align on each star. This will ensure that all of the slack is taken up.


Post Extras: Print Post   Remind Me!   Notify Moderator  
cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: RA/DEC vs GoTo new [Re: orlyandico]
      #5589641 - 12/26/12 12:23 PM

This could be due to precession, the difference between the coordinates for now and those for J2000. The difference varies across the sky but can be 10 to 20 arc minutes.

Some scopes use J2000, some the current epoch. I don't know what Stellarium reports but you need to use the same as the mount.

Hope this helps,

Chris


Post Extras: Print Post   Remind Me!   Notify Moderator  
Danzup77
professor emeritus


Reged: 09/12/11

Re: RA/DEC vs GoTo new [Re: cn register 5]
      #5591771 - 12/27/12 08:30 PM

Orlyandico thanks for that info!

Chris, I think Stellarium displays both if I remember correctly. Can you elaborate a little more as to the difference? Thats something I know NOTHING about lol....


Post Extras: Print Post   Remind Me!   Notify Moderator  
cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: RA/DEC vs GoTo new [Re: Danzup77]
      #5592150 - 12/28/12 04:54 AM

Precession is the change of the direction of the Earth's rotational axis, this change causes the Ra and Dec coordinate system to change because it's based on the Earth's rotation axis. The effect is that the positions of objects change by about one arc minute a year.

Star positions in charts are usually converted to a particular date or epoch and the current one is usually the start of the year 2000, J2000.0.

The difference in position from 2000 to now will be of the order of 13 arc minutes and this is enough to move the position of an object by a noticeable amount.

Hope this helps, it's a very simple explanation and there are much fuller ones on the web.

Chris


Post Extras: Print Post   Remind Me!   Notify Moderator  
jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: RA/DEC vs GoTo new [Re: Danzup77]
      #5592467 - 12/28/12 11:06 AM

Sounds like either cone error or polar alignment error to me.

Use 3 calibration stars during alignment to correct cone error. Check your polar alignment too. For visual, centering Polaris in the polar scope is adequate. For imaging, you may want to drift align.

- Jim


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1


Extra information
21 registered and 45 anonymous users are browsing this forum.

Moderator:  Dave M, richard7, bilgebay, iceblaze 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 731

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics