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Equipment Discussions >> Mounts

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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Polar alignment 3 arc minutes acceptable? new [Re: shawnhar]
      #5949726 - 07/01/13 01:50 PM

Hi-

Your result may indeed be good - but it would help to know the actual fwhm in arc-seconds you obtained. Round is good - round and small, in arc-seconds, is even better.

I don't know what object that is, so I can't tell from context either. Is that a linear stretch?

Thanks,
Frank


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shawnhar
Post Laureate
*****

Reged: 06/25/10

Loc: Knoxville, TN
Re: Polar alignment 3 arc minutes acceptable? new [Re: freestar8n]
      #5949944 - 07/01/13 03:50 PM

Sorry, that is M16, the Eagle. Used a Canon XS dslr.
One mild curve in PS and a little noise reduction.
Astrometry.net wouldn't plate solve it.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Polar alignment 3 arc minutes acceptable? [Re: shawnhar]
      #5949962 - 07/01/13 04:00 PM

Quote:

Challenge accepted!
Now, would you please get rid of all the clouds and rain here so I can polar align each night using only the polar scope and report my findings.






Sorry I can't do that, same problem over here...
But it's easier for me since my new scope didn't arrive yet.
This year is really bad.


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Steve Mckean
member


Reged: 03/22/10

Loc: Bay Of Plenty, NZ
Re: Polar alignment 3 arc minutes acceptable? new [Re: Cliff Hipsher]
      #6087074 - 09/17/13 11:28 PM

Hi Guys, sorry if this has been covered elsewhere but I was wondering why manually adjusting the mount directly to the NCP or SCP using plate solving and direct camera imaging would not work, the plate solving would let you know exactly which star you looking at through the main OTA so as long as you can keep the cross hairs in the center using manual adjustments would that not work ? (For polar alignment I mean).

Cheers

Steve


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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #6087288 - 09/18/13 03:07 AM

Quote:

I remember when I was renting on GRAS (now iTelescopes). The typical pointing accuracy, even with the T-Point modeling, the fixed observatory, etc. etc. etc. was about 1' to 2'. The reason I know this is because as part of the workflow, after a slew, the GRAS software would plate-solve and display how far off the pointing was. And these were Paramount ME's.



That sounds like raw uncorrected pointing or a very poor pointing model. Maybe they weren't even enabling Tpoint pointing corrections at all. They're certainly not needed if the workflow includes plate solving and re-slewing as most do these days as a matter of course.

Also, they were probably using TheSky6 and Tpoint for Windows. TPoint for Windows was implemented using TPoint version 1 from Patrick Wallace. I'll bet their pointing model was fairly lousy because back then anything beyond a basic model had to be artfully hand crafted one term at a time, which was a PITA.

The TPoint Add On to TheSkyX is implemented using TPoint version 18. The new Tpoint supermodel feature automatically creates the pointing model for you, no artful hand crafting skills required. A new pointing model created by supermodel is likely to be far superior to any previously hand crafted one (especially when given a few hundred well distributed pointing samples to chew on).


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Phil Sherman
Pooh-Bah


Reged: 12/07/10

Loc: Cleveland, Ohio
Re: Polar alignment 3 arc minutes acceptable? new [Re: telfish]
      #6089315 - 09/19/13 02:01 AM

Quote:


Not yet! I have a guider I am going to add to the setup soon. It's a new pier and observatory I am now at the stage of aligning the mount before I start imaging.





If you're setting up the mount on a pier then you should be using your polarscope and/or any hand controller polar alignment routine for no more than a rough polar alignment. Once you've done this; you should be refining it using the drift method, preferably using your camera to measure the drift. Drift measurements are made along the celestial equator at the meridian (just W of the meridian) and close to either the E or W horizon. Close means around 30 degrees in altitude above the horizon. The meridian measurement is used to adjust azimuth while the horizon location is for altitude.

Set your tracking rate to sidereal, slew rate to 1x sidereal, exposure time to 70 seconds then start the exposure. Let the mount track for 5 seconds then slew E for 30 seconds, then slew W for 35 seconds. Look at the image. There will be stars (the 5 second tracked portion) with two trails forming a V shape. The size of the opening of the V at the star is an indication of how much drift occurred in 60 seconds. The relationship between the two trails will show you which way to adjust the mount. If the first adjustment makes the V wider, you moved the mount the wrong way. When the mount is polar aligned, the trails collapse into a line. At a minute per measurement, this technique is much much faster than using an eyepiece. It also makes it very easy to compare the current measurement with the last one.

If you're using an Atlas mount on a pier inside an observatory, you should be planning on running the mount with EQMOD. Everything to run the mount, including guiding is available as free downloads; EQMOD, ASCOM platform, CdC, PhD, and even AstroTortilla for plate solving. PhD can guide the Atlas using pulse guide, avoiding the need for an ST4 connection to the mount's guide port. This also allows webcams and any other camera that PhD can control to be used for guiding. You'll also want a motorized focuser that can be controlled from your computer. Orion has a relatively inexpensive geared DC motor unit that can be adapted to almost any focuser. Shoestring Astronomy makes a USB run controller for this motor that works with current Windows systems.

Phil


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Polar alignment 3 arc minutes acceptable? new [Re: Phil Sherman]
      #6089401 - 09/19/13 05:58 AM

the problem with the "V technique" is that it's not very accurate. the rule of thumb for declination drift is 1 arc-second in 5 minutes for 1 arc-minute of misalignment.

if your camera is 2"/pixel (a common size) in order to get a drift of 1 pixel (2") in 1 minute would require you to be out by 10 arc-minutes, which is a lot.

i have been doing this V-method for a long time and PEMPro polar alignment is bulletproof. Just leave it for 10-15 minutes for each of alt and az, and (assuming you didn't move the mount the wrong direction) you can get < 5' with one iteration of each Alt and Az.

also helps if you know how much (arc-minutes) the mount moves with every rotation of the adjuster bolts.


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Steve Mckean
member


Reged: 03/22/10

Loc: Bay Of Plenty, NZ
Re: Polar alignment 3 arc minutes acceptable? new [Re: orlyandico]
      #6090339 - 09/19/13 05:13 PM

Thanks for that, ill work on it.

Steve


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