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Meridian Flip Calculator

Astrophotography DSO Equipment Imaging Mount
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#1 mfreedman

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Posted 02 April 2024 - 11:03 AM

I am looking for a Meridian Flip calculation that's accurate. I'd like to know when the Meridian Flip occurs so I can know how many images I can take before the Meridian Flip occurs. When a Meridian Flip occurs, my iOptron CEM60 mount never returns to the exact position to be able to continue imaging with a proper level of accuracy. Also, the Meridian Flip occurs that messes up PHD guiding, so I want to know exactly then the flip occurs so I can stop the autoguiding in PHD and stop the imaging in AstroArt7. I don't want to use Sequence Generator Pro or any other software to accomplish this, as I want to have my imaging session completely automated with AstroArt and just continue imaging.

 

My primary objective is to just have a calculator that estimates the time of Meridian Flip, and estimates the time that an object reaches the meridian. So, given a position in RA & DEC for a particular object, how many exposures do I have left before the object reaches the meridian?

 

I have compiled an Excel spreadsheet that I originally thought was correct, but now I don't think it's completely correct. I have attached it to this posting for your review. It is based on the Meridian Flip occurring at 180 degrees Azimuth and 360 degrees Azimuth, but I don't think that calculation is correct, as I have read about Hour Angle but I'm not positive I can get that information from my mount. My mount software allows me to see RA, DEC, Altitude, Azimuth, LST, pier position (East or West), and GPS coordinates.

 

I have looked at a multitude of postings on this subject and can't seem to find a resource that shows me an exact calculation.

 

Thanks in advance for any solutions you can offer.



#2 hyiger

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Posted 02 April 2024 - 11:17 AM

Meridian flip occurs when LST - object's RA + offset = 0 where the offset is typically some number of minutes before the meridian. So it really just depends on getting the LST calculated correctly. Have you tried using NINA which will compute this for you and will send the proper commands to PHD2 and your mount?


Edited by hyiger, 02 April 2024 - 11:17 AM.

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#3 starblue

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Posted 02 April 2024 - 11:20 AM

On my iOptron GEM45G you tell it how many minutes after the object reaches the meridian that you want to delay until the flip, up to a max delay of 1h 20m. SkySafari tells me when the object crosses the meridian. Add the 2 values together, and that's when it flips. What else do you need?

 

Ex: An object reaches the meridian at 2am. If you set the delay at 0, the mount will flip at 2am. If the delay is 60, it flips at 3am. You can also tell the mount to just stop then instead of flip.

 

Also, hour angle (HA) is the difference between the RA of the meridian and the RA of the object. If it takes 1 hour for an object to reach the meridian, its HA is 1h E, or -1h. On the meridian, its HA = 0. If it passed the meridian an hour ago, its HA = 1h W, or +1h.



#4 kathyastro

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Posted 02 April 2024 - 11:20 AM

The hour angle of the object tells you the time to meridian passage.  If you express the hour angle as a time between 00:00 and 23:59, then 24 - the hour angle is the time to meridian passage.

 

Meridian passage does indeed occur at an azimuth of 180 or 360 degrees, but the spherical calculations required to work out the azimuth are not worth it.  You are looking for a time, and hour angle already is a time.

 

The hour angle of an object is the local sidereal time (LST) minus the object's RA.  HA = LST - RA.  Since your mount gives you RA and LST, the calculation of HA is easy.



#5 DanMiller

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Posted 02 April 2024 - 11:47 AM

Meridian flip occurs when LST - object's RA + offset = 0 where the offset is typically some number of minutes before the meridian. So it really just depends on getting the LST calculated correctly. Have you tried using NINA which will compute this for you and will send the proper commands to PHD2 and your mount?

Also N.I.N.A. shows how long to the flip under the telescope tab.  I thought that there was a setting also for PHD2 to not guide when the meridian flip was occuring.  I could be wrong on that.

 

Dan



#6 hyiger

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Posted 02 April 2024 - 11:52 AM

I thought that there was a setting also for PHD2 to not guide when the meridian flip was occuring.  I could be wrong on that.

No, I think you're right. I'll check later. I've always used PHD2 as a service and let the client (NINA etc) deal with it. 



#7 F.Meiresonne

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Posted 02 April 2024 - 12:21 PM

APT tells you how much time before the object passes the meridian.

 

Secondly a CEM60 can continue after an object passes the meridian for about an hour.

 

Also CDC can tell you when the object passes the meridian.

 

I can't see the problem. A meridian flip it the least of my problems when i am imaging and i do it even manually. I do plate solving, so after the manual flip, i just plate solve, sync, check once more and go again...



#8 ngc2218

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Posted 02 April 2024 - 12:39 PM

I am looking for a Meridian Flip calculation that's accurate. I'd like to know when the Meridian Flip occurs so I can know how many images I can take before the Meridian Flip occurs. When a Meridian Flip occurs, my iOptron CEM60 mount never returns to the exact position to be able to continue imaging with a proper level of accuracy. Also, the Meridian Flip occurs that messes up PHD guiding, so I want to know exactly then the flip occurs so I can stop the autoguiding in PHD and stop the imaging in AstroArt7. I don't want to use Sequence Generator Pro or any other software to accomplish this, as I want to have my imaging session completely automated with AstroArt and just continue imaging.

 

My primary objective is to just have a calculator that estimates the time of Meridian Flip, and estimates the time that an object reaches the meridian. So, given a position in RA & DEC for a particular object, how many exposures do I have left before the object reaches the meridian?

 

I have compiled an Excel spreadsheet that I originally thought was correct, but now I don't think it's completely correct. I have attached it to this posting for your review. It is based on the Meridian Flip occurring at 180 degrees Azimuth and 360 degrees Azimuth, but I don't think that calculation is correct, as I have read about Hour Angle but I'm not positive I can get that information from my mount. My mount software allows me to see RA, DEC, Altitude, Azimuth, LST, pier position (East or West), and GPS coordinates.

 

I have looked at a multitude of postings on this subject and can't seem to find a resource that shows me an exact calculation.

 

Thanks in advance for any solutions you can offer.

I haven't use all image automation software, but I think most have this "problem" resolved. You don't need a calculator, you need automation software that does this for you.

 

It's normal behaviour for your mount to miss the target after a flip. You need to plate solve after a flip and recenter. 

This issue is resolved on better mounts that use absolute encoders, and land the target in centre after a meridian flip.

 

Also if your current software doesn't do this automatically why not try other software? NINA seems a popular option these days as free software. There are also better paid options, so pick the software that has the functionality you want be it free or paid


Edited by ngc2218, 02 April 2024 - 12:54 PM.


#9 mfreedman

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Posted 03 April 2024 - 11:18 PM

Thanks for the comments and suggestions.



#10 DuncanM

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Posted 09 April 2024 - 03:33 PM

I am looking for a Meridian Flip calculation that's accurate. I'd like to know when the Meridian Flip occurs so I can know how many images I can take before the Meridian Flip occurs. When a Meridian Flip occurs, my iOptron CEM60 mount never returns to the exact position to be able to continue imaging with a proper level of accuracy. Also, the Meridian Flip occurs that messes up PHD guiding, so I want to know exactly then the flip occurs so I can stop the autoguiding in PHD and stop the imaging in AstroArt7. I don't want to use Sequence Generator Pro or any other software to accomplish this, as I want to have my imaging session completely automated with AstroArt and just continue imaging.

 

My primary objective is to just have a calculator that estimates the time of Meridian Flip, and estimates the time that an object reaches the meridian. So, given a position in RA & DEC for a particular object, how many exposures do I have left before the object reaches the meridian?

 

I have compiled an Excel spreadsheet that I originally thought was correct, but now I don't think it's completely correct. I have attached it to this posting for your review. It is based on the Meridian Flip occurring at 180 degrees Azimuth and 360 degrees Azimuth, but I don't think that calculation is correct, as I have read about Hour Angle but I'm not positive I can get that information from my mount. My mount software allows me to see RA, DEC, Altitude, Azimuth, LST, pier position (East or West), and GPS coordinates.

 

I have looked at a multitude of postings on this subject and can't seem to find a resource that shows me an exact calculation.

 

Thanks in advance for any solutions you can offer.

AA8 SP6 GUI has implemented full meridian flip automation including a plate solve assisted recentering after the flip:

 

https://www.msb-astr...com/down_en.htm

Version 8.0 SP6 - April 2, 2024
--------------------------------

* New feature.  Automatic meridian flips, with autoguide and centering by plate solving

 

 

 

The star chart app in AA8 will give the time that the target object reaches the meridian. The decision as to when to do the flip can be set in your iOptron Commander software and/or in AA8. AA8 SP6 gives the option to delay the flip until the current exposure completes. There is a script  available on the AA8 forum that will allow for complete automation of multiple target imaging runs, including target centering via plate solving after a flip and re-acquisition of autoguiding/autofocus after the flip. AA8 has been always been able to fully automate flips via scripts.


Edited by DuncanM, 09 April 2024 - 03:40 PM.


#11 mfreedman

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Posted 11 April 2024 - 04:51 PM

DuncanM - thanks for the response - most helpful.




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