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Equipment Discussions >> Mounts

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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: AVX Calibration Stars - Function? new [Re: freestar8n]
      #5967401 - 07/12/13 01:22 PM

Quote:

Yes - the index marks correspond to the switch positions - and as such they are assumed to be slightly offset from the true 0/90 location and not perfect. That offset has to be calibrated very accurately - for a gem or fork mount - if the pointing is to be accurate.




That's true only if by "pointing" you mean the slew to the first alignment star. It has no effect at all on pointing accuracy after alignment; once aligned, pointing accuracy depends on the alignment. Any arbitrary initial orientation can be chosen. The only difference will be the amount of correction required to center the first alignment star because the mount bases that slew on the presumption that the initial position was on the marks (or switches).


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freestar8n
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Reged: 10/12/07

Re: AVX Calibration Stars - Function? new [Re: jrcrilly]
      #5967412 - 07/12/13 01:26 PM

I completely disagree. The dec. offset is very different from the RA offset - which is what I assume you are referring to.

I think it is understandable to expect that the dec. offset is a trivial issue and somehow the cone value is critical - but in fact the dec. offset is a very important and elusive value that affects pointing across the sky - all the time - and not just for the initial stars.

Frank


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: AVX Calibration Stars - Function? new [Re: freestar8n]
      #5967465 - 07/12/13 01:55 PM

Quote:

I completely disagree. The dec. offset is very different from the RA offset - which is what I assume you are referring to.




No need to assume. I was responding to your post, which specifically referred to the index marks (as seen in the portion I quoted). I specifically referred to them also.


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freestar8n
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Re: AVX Calibration Stars - Function? new [Re: jrcrilly]
      #5967482 - 07/12/13 02:08 PM

The "dec. index offset" I am referring to is the deviation in either the switch location or the index mark - from the true 0/90 degree dec. location. So whatever reference you use - it is assumed to be imperfect - and is corrected for by the calibration stars. For RA, any error doesn't matter too much - but for Dec., it matters a lot - because RA errors are independent of dec., while errors in Dec. are coupled to errors in RA. If that subtlety is not clear then none of this will make sense.

So - I am not referring to either the index marks or the switch positions - but the departure of them from the true 0/90 values. That is an issue that is trivial in RA and only affects initial pointing - but is elusive in Dec. - and affects all pointing - and not just to the initial stars.

Frank


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: AVX Calibration Stars - Function? new [Re: cn register 5]
      #5967498 - 07/12/13 02:19 PM

Quote:

The Dec offset is one of the things that is calculated as part of the alignment process, along with Ra offset, cone error and the two polar align errors



RA offset is not calculated as part of the 2+4 alignment process.


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: AVX Calibration Stars - Function? new [Re: Alph]
      #5967552 - 07/12/13 02:41 PM

Wrong. Try it.

Chris


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: AVX Calibration Stars - Function? new [Re: cn register 5]
      #5967615 - 07/12/13 03:11 PM

Quote:

Wrong. Try it.

Chris



Wong what? Read this


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: AVX Calibration Stars - Function? new [Re: Alph]
      #5967633 - 07/12/13 03:17 PM

If you had read the thread you would see that we are talking about calibration stars, not the calibrate function.

Chris


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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: AVX Calibration Stars - Function? new [Re: Alph]
      #5967686 - 07/12/13 03:42 PM

Quote:

Wong what? Read this




Yup. Just what I said.


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freestar8n
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Reged: 10/12/07

Re: AVX Calibration Stars - Function? new [Re: jrcrilly]
      #5967725 - 07/12/13 04:03 PM

I was talking about the dec. offset. Dec. is not discussed anywhere in the referenced link - so your comment has no relevance to the point I was making - and to the importance of calibration stars to all-sky pointing.

Frank


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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: AVX Calibration Stars - Function? new [Re: freestar8n]
      #5967778 - 07/12/13 04:45 PM

Simple test. Set the dec index mark as accurately as you can. Do a 2 star alignment with no calibration stars. Take note of pointing accuracy.

Next turn the mount off and set the mount so the dec index mark is off. Do the 2 star alignment with no calibration stars again taking note of the pointing accuracy.

If there is a great difference in pointing accuracy you are correct. If there is no difference you are wrong.

I always thought both the offset in ra and dec from perfect were corrected for in the sky model by the 2 initial alignment stars.

Do you any "evidence" to show this is incorrect?


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freestar8n
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Reged: 10/12/07

Re: AVX Calibration Stars - Function? [Re: Stew57]
      #5967792 - 07/12/13 04:54 PM

Sure - set the dec. offset away by a small value like 1 degree - since it doesn't really matter anyway. Then do a 2-star alignment, then go around the sky and point at objects. Might as well offset it by 10 degrees since it only affects pointing to the first two stars - according to some theories. Since people think cone is so important - go ahead and leave it at the exact value it thinks it is.

But just don't add any calibration stars, since the mount will then try to figure out what the real dec. offset is - and correct for the error based on where the stars really are.

Frank


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