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Just an observation on field rotation

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#26 GSBass

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Posted 05 September 2021 - 01:58 PM

Thanks, was going to show you but I appear to have deleted it already , but I think maybe only 20% on this one, can be upwards of 50% near zenith though, when I did the Eagle the other night I framed it sideways to try to plan the crop in advance, bigger nebulas are more challenging, the 485c is a 4K chip but still small compared to nicer cameras…. But hey it’s only 400.00 lol


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#27 GSBass

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Posted 05 September 2021 - 02:00 PM

Thanks… I think my Az tracks bad enough that it does auto dither :)

Nice image GSBass! @steveincolo, you can even guide and dither in alt-az, and that helps a lot getting less of the "windshield wiper" effect one gets with noise trailing in circles. I did some tests with my faithful Celestron Evo or my AZ-GTi in alt-az mode, and the EAA results were quite enjoyable


Edited by GSBass, 05 September 2021 - 02:01 PM.

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#28 steveincolo

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Posted 05 September 2021 - 02:07 PM

Nice image GSBass! @steveincolo, you can even guide and dither in alt-az, and that helps a lot getting less of the "windshield wiper" effect one gets with noise trailing in circles. I did some tests with my faithful Celestron Evo or my AZ-GTi in alt-az mode, and the EAA results were quite enjoyable

Just what I need, an excuse to pick up some more gear!  Though I can probably cobble a guide scope setup with my 178MM and Stellarvue SV50 finder, or even use my TV85 as a guidescope on the AZ Mount Pro secondary scope saddle.  



#29 GSBass

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Posted 05 September 2021 - 02:08 PM

Btw, your mileage may vary but I have found trying to get the exposure as pretty as you can on screen while doing live stacking and then saving “as seen on screen” gives me a better base to work with in post processing, instead of stretching the image again I use a hdr function in Astrosurface to adjust levels and bring out the nebulosity without bloating the stars…… just a different workflow to try… it’s working for me better than just saving the stack normally 


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#30 GSBass

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Posted 05 September 2021 - 02:52 PM

Well unfortunately we can’t guide the Az pro… but it’s ok it does fine

Just what I need, an excuse to pick up some more gear!  Though I can probably cobble a guide scope setup with my 178MM and Stellarvue SV50 finder, or even use my TV85 as a guidescope on the AZ Mount Pro secondary scope saddle.  


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#31 steveincolo

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Posted 05 September 2021 - 03:05 PM

Well unfortunately we can’t guide the Az pro… but it’s ok it does fine

It might be possible with ASIAIR Pro as go-between, instead of using the typical ST4 cable guiding.   My hope is that the ASAIR Pro would issue guiding commands through the serial interface to the AZMP.  
 

https://telescopemou...-up-the-system/



#32 steveincolo

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Posted 05 September 2021 - 03:22 PM

Here’s something that basically looks like an old-school barn door mount, that you attach to an altaz mount.  It rotates the whole scope!

 

https://trackthestar...escope-rotator/


Edited by steveincolo, 05 September 2021 - 03:23 PM.


#33 GSBass

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Posted 05 September 2021 - 03:59 PM

My Az is a little squirrely , pressing the buttons to move the scope in various directions often causes a drift, it will stop in short order and track normally again but I think it’s just a result of being belt instead of gear driven…. I dunno… I just live with it since I know it was not really designed for photography, perhaps it just needs tension adjustment but it does track fine after I get it where I want it, let me know if you have any luck…. I just assumed it was not possible because of the design

It might be possible with ASIAIR Pro as go-between, instead of using the typical ST4 cable guiding.   My hope is that the ASAIR Pro would issue guiding commands through the serial interface to the AZMP.  
 

https://telescopemou...-up-the-system/


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#34 steveincolo

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Posted 05 September 2021 - 04:26 PM

My Az is a little squirrely , pressing the buttons to move the scope in various directions often causes a drift, it will stop in short order and track normally again but I think it’s just a result of being belt instead of gear driven…. I dunno… I just live with it since I know it was not really designed for photography, perhaps it just needs tension adjustment but it does track fine after I get it where I want it, let me know if you have any luck…. I just assumed it was not possible because of the design

Good point, it may not be designed for small adjustments.  



#35 Stargazer3236

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Posted 10 September 2021 - 12:18 PM

You will notice more field rotation at the north celestial pole and far less at the ecliptic or celestial equator. I was taking images of planetary nebula in Cepheus and Cass and noticed how much the field rotated in Sharpcap as opposed to imaging planetary nebula in Ophiuchus or Aquila.


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#36 steveincolo

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Posted 10 September 2021 - 12:38 PM

You will notice more field rotation at the north celestial pole and far less at the ecliptic or celestial equator. I was taking images of planetary nebula in Cepheus and Cass and noticed how much the field rotated in Sharpcap as opposed to imaging planetary nebula in Ophiuchus or Aquila.

That's consistent with the formula in the article Alien Observatory linked.  Also due east and due west have much less field rotation.  



#37 Ambart3561

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Posted 10 September 2021 - 06:16 PM

That's consistent with the formula in the article Alien Observatory linked.  Also due east and due west have much less field rotation.  

Interesting question and variety of responses. A few months ago I asked myself the same question and did a bit of internet hunting to see if there was a way to estimate exposure times for Alt/Az mounts. I found a good article by Niels Haagh and another on CaliforniaSkys.

 

https://telescopemou...lts/#stepbystep

https://www.californ...d-rotation.html

 

That led me to an equation to calculate estimated exposure time (similar to the reference from Pat - Thank You Pat!)

 

exposure time in seconds = Allowable field rotation in degrees * cos(Altitude) / (0.004178 * cos(Latitude) * cos(Azimuth))

[Note: 0.004178 is sidereal rotation rate in degrees/second]

 

And with that I made a graph that seems to summarize many comments made above.

 

This graph changes with observer latitude, I've shown an example for 43 degrees. I've also used an "Allowed Field Rotation" value empirically derived from Niels Haagh's data. I find the chart useful for choosing sub exposure times based on object position at time of observation. Its a ballpark estimator, not an exact science.

 

Est AltAz Exposure.jpg


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#38 steveincolo

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Posted Yesterday, 04:36 PM

Here's an example (I think) of the "windshield wiper" noise that Clouzot mentioned.  You have to click the image to see it clearly.

 

NGC 6888
 
I'm picking up a WO 32mm guidescope and ASI120MM mini.  This stuff is easy to setup with the ASIAIR Pro, so we'll see if my images clean up a bit.  I'll post back here with results.  

Edited by steveincolo, Yesterday, 04:37 PM.



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