Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Help on DEC and RA drive readings

Meade mount
  • Please log in to reply
10 replies to this topic

#1 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 25 May 2020 - 07:20 PM

Hello,

 

I hope that you are well and staying healthy.

 

Here’s my story.

 

I bought an LX200 ACF - 10" F/10 around 2005. At the time I thought that it would be an intuitive instrument to use and I confess that I really didn’t understand how to use it correctly. I never could get it to align properly. As a result of my stumbling around learning the scope there were many times the cords got wrapped in the mount and, to my shame,  the worm drive gears were likely clicking away.

 

The scope was stored for years for various reasons. When I got it out of storage in 2017 the DEC function would not work at all and I replaced the entire DEC motor assembly (parts ordered from Meade). Still no luck aligning the scope.

 

Fast forward to 2020:

 

I ‘properly’ rediscovered my love for astronomy last year, and have spent a lot of time figuring out how to set up, level and align the LX200 using all the resources on the web. I had some limited alignment success in ALT/AZ mounting and wanted to try basic astrophotography, so I put the LX200 on a Meade X-Wedge.

 

Currently, the mount runs smoothly in RA and DEC at all rates. The direction of movement is correct. No noise or weirdness.

 

I’m struggling to understand the DEC and RA readings I am seeing on the Autostar II and that’s why I think I may have a problem with the RA and DEC drive encoders or calibration.

 

As I understand it, on the Meade X-Wedge in Polar Home configuration, with the OTA parallel to the forks, the DEC should read 90 degrees. My Autostar II reads 0 degrees. When I replaced the DEC drive did I reverse a connector?

 

Also, with the forks orthogonal to the X-wedge I believe that I should see a RA value of zero. My RA only reads zero with the forks canted 45 degrees or so. Plus, the viewfinder and Telrad are on the top side of the mount when they should be on the bottom (or hard to use side) in Polar Home mode.

 

Attempts to polar align the LX200 lead to wild pointing errors, or stars selected that are below the horizon.

 

I don’t see any instructions in the Meade downloadable manuals on recalibrating the DEC or RA encoders. I am running the 4.2i firmware on the Autostar II.

 

Any advice would be much appreciated. Here are some pictures of what I am seeing.

 

Many thanks in advance,

 

Paul

Groton, MA

Attached Thumbnails

  • IMG_2207.jpg


#2 OzAndrewJ

OzAndrewJ

    Gemini

  • *****
  • Posts: 3,397
  • Joined: 30 Nov 2010

Posted 25 May 2020 - 07:44 PM

Gday Paul

The mount has no idea where it is when it boots up, but if polar, it defaults to DEC = 0 and HA = 0.

When you do a polar align, the Hbx tells you to put the mount in polar home

ie OTA pointing at the pole with DEC axle horizontal and finder underneath

( ie DEC = +/-90 and HA = 0 )

When you hit enter to start the align, the mount internally resets its datums to suit this position.

After that, it should work and report positions "relatively" accurately.

 

Andrew Johansen Melbourne Australia



#3 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 25 May 2020 - 08:45 PM

Ah, thank you so much Andrew. That helps a lot.

 

I thought that the RA and DEC encoders were "absolute" i.e. they always magically knew their position relative to the mount. Sort of like an industrial robot.

 

If that is not the case then I don't think that I have an issue. 

 

If I understand your post correctly, then the DEC and RA values on the Autostar II won't ever reflect real celestial pointing until the polar alignment procedure is initiated. The RA and DEC values are relative to the actual alignment.

 

If that is correct, then a big sigh of relief from Groton, MA USA!

 

Thanks again Andrew

 

Paul



#4 OzAndrewJ

OzAndrewJ

    Gemini

  • *****
  • Posts: 3,397
  • Joined: 30 Nov 2010

Posted 25 May 2020 - 09:08 PM

Gday Paul

 

I thought that the RA and DEC encoders were "absolute"

Nope, they are simple "relative" quadrature encoders mounted on the motors.

On booting, they set to zero, and report relative to that until told otherwise,

irrespective of where the mount physically was when booting.

 

 

If I understand your post correctly, then the DEC and RA values on the Autostar II won't ever reflect real celestial pointing until the polar alignment procedure is initiated.

Not 100% ( but close )

As noted, the mount "assumes" it is in a set position when booting.

( For polar, this is HA=DEC=0 )

For the alignment process to work, you move it to a set position

and the mount does an internal synch, based on assuming you put it in home.

What you can do ( tho its not suggested you do this normally )

is set the mount up on the wedge with the OTA pointing to HA=DEC=0.

( ie where the mount "thinks" it is on booting )

Now boot the mount and set the date/time etc correctly.

The mount will start tracking and it will "assume" it is pointing correctly.

From then, you can simply do a goto to a star nearby, centre and synch, and you are aligned.

This can affect some other internal calcs, but it will work for a quicky setup.

 

Again, on booting, the mount has no real idea where it is,

it requires you to follow a process to confirm where it is pointing,

so it can set its encoder datums for the session.

Doing a proper align is best, but you can cheat :-)

 

Andrew Johansen Melbourne Australia



#5 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 25 May 2020 - 09:45 PM

OK, brilliant. I understand now. many thanks for taking the time to educate me. Will try both of those methods!

 

Best regards

 

Paul

Groton, MA



#6 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 26 May 2020 - 09:42 PM

Hi Andrew et al,

 

So I did a rough polar alignment tonight but the scope still acts weird. I've checked all the date, time, DST, site, UTC offset etc.

 

I put the mount and OTA in polar home position and did the one star alignment. It didn't display "taking GPS fix..." Is that because I specified an observing site on the Autostar? I am running 4.2i.  Anyway...

 

I was able to get the LX200 to slew to some good pointing stars like Vega, Castor etc. I would then come back to Polaris. The pointing was very rough at first but it was somewhat close to the target. Probably because I haven't trained the mount yet.

 

When I wanted to slew to Rigel and then back to Vega it wouldn't go back to either and told me that they hadn't risen yet, despite being clearly visible.

 

I slewed the mount back to polar home as a test and DEC was spot on at just under 90 degrees but the RA read 15:25:03 in exactly the same place it had polar aligned.

 

I thought that once I had accomplished the polar alignment from Polar Home and returned to it the RA would have read 00:00:00?

 

I'm completely puzzled and might take the scope off the wedge for now to see if it is equally daffy in ALT-AZ.

 

Any suggestions are welcome.

 

Paul



#7 nitegeezer

nitegeezer

    Galactic Ghost

  • *****
  • Posts: 7,043
  • Joined: 27 Nov 2007

Posted 26 May 2020 - 10:00 PM

There is a setting in the Telescope menu to control the GPS. IIRC, it can be off, on at power up, or on when needed. Normally I would leave mine on at power up, but currently it is off because I have not updated the firmware since the GPS rollover issue.

From your description, the mount does not know time correctly.

#8 OzAndrewJ

OzAndrewJ

    Gemini

  • *****
  • Posts: 3,397
  • Joined: 30 Nov 2010

Posted 26 May 2020 - 10:02 PM

Gday Paul

 

Your GPS should have fixed.

If the date/time and LST were correct, and you didnt manually enter any data during boot, it must have fixed.

 

As to getting "Not risen yet", that normally comes down to a date/time error

so i would double check the reported LST from the mount against a web based app for your site.

 

As to getting it to slew back to home

Polar home means HA(Hour Angle) =0 not RA=0.

In polar home, RA = LST, hence why using the reported LST is a good cross check

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 26 May 2020 - 10:03 PM.


#9 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 29 May 2020 - 01:34 PM

Thanks Andrew and Chuck for your help.

 

I agree that is seems my LST is off and I think I suspect why. Let me ask:

 

I manually set my Date, Location and LST to the second using a iPhone app. I set DST to ON and GMT offset to -4.

 

The LST seems to read correctly.

 

Still I seem to have a timing issue with the Autostar II telling me that stars I can plainly see overhead have not yet risen.

 

Perhaps this is the issue: If my GMT offset in Massachusetts is -5 in the winter and -4 in the summer do I:

 

A) Set GMT offset to be -5 and Daylight Savings Time ON

or

B) Set GMT offset to be -4 and Daylight Savings Time ON

 

Many thanks!

 

Paul



#10 nitegeezer

nitegeezer

    Galactic Ghost

  • *****
  • Posts: 7,043
  • Joined: 27 Nov 2007

Posted 29 May 2020 - 02:14 PM

Set your GMT offset to -5. In the spring you set DST ON and in the fall turn it back OFF. Now enter the time and date so the controller knows how to properly adjust between local time and GMT.

#11 TBM850

TBM850

    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 25 May 2020
  • Loc: Groton, MA USA

Posted 29 May 2020 - 04:07 PM

Thanks Chuck for the help. I have been doing -4 and DST ON which would result in a one hour error. 

 

Best regards

 

Paul




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: Meade, mount



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics