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NexStar 6SE autoguiding in AltAz?

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15 replies to this topic

#1 Todd1561

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Posted 24 August 2018 - 03:19 PM

Is it possible to do autoguiding with the NexStar while in regular AltAz mode?  My reason for wanting to do this is not to achieve long exposure but just to help keep my object centered better throughout a session.  I've fine tuned my backlash and have added extra reference points to my StarSense model but I'll still get the object drifting around the FOV.  It doesn't actually drift out of view, but drifts up and down gradually.  This is a problem because I'm using a small 1/3" CMOS chip and I wind up having to do a lot of cropping on an already small image to remove stacking artifacts.  My thinking is if I could just keep the object centered better I would at least solve that problem.  I'm still expecting to be limited to 10-20 second exposures, that's OK.

 

I realize nothing about the 6SE is well-suited for DSO work, but it's what I have and I want to try to work with it rather than throw money at the problem (different mount/OTA/camera/wedge).  I'm also not interested in a wedge because I don't want to deal with polar alignment.  I like the ease of just plopping the AltAz down and kicking off StarSense and I'm done in 2 minutes.

 

Let me know if anyone has done this or at least knows if the autoguide port is disabled in AltAz mode.  If this is doable can I just use the standard PHD2 software, or does that assume a GEM mount?

 

Thanks!


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#2 bobzeq25

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Posted 24 August 2018 - 03:38 PM

The time and money you would spend on an autoguider would be wasted.  You're trying to make that scope do something it really can't do.  You can't just put a camera on the back of a visual scope, and image DSOs.

 

It's just not intuitive, but here's the reason.  When your eyes see something they remember the pattern of what they're seeing in your brain.  So, if there's a small motion they compensate by moving the whole image.

 

Dumb .005mm pixels can't do that, and so they're completely intolerant of movement.  The long focal length of your scope magnifies tracking errors, and makes things worse.  Your eyes are better.  The camera doesn't work because it's better than your eyes, it works because it's different.

 

You can image planets and the Moon with the scope.  For DSOs you're better off with a camera and a lens.  This book shows you how.

 

http://www.astropix....bgda/index.html

 

Bottom line is that conventional astrophotography of DSOs is expensive and time consuming.  You might want to look into Electronically Assisted Astronomy, there's a forum for that.


Edited by bobzeq25, 24 August 2018 - 03:42 PM.


#3 photoracer18

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Posted 24 August 2018 - 03:48 PM

Your biggest issue is field rotation from using the mount in Alt-Az mode. Even if it was centered all the time the object rotates smearing detail. The more the magnification the worse the problem. The maximum length of exposure without seeing the rotation is directly connected to the magnification of the target. Also the control software expects the mount to be level even in Alt-Az mode so if its not I think it adds more wobble to tracking.

#4 Todd1561

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Posted 24 August 2018 - 03:58 PM

Your biggest issue is field rotation from using the mount in Alt-Az mode. Even if it was centered all the time the object rotates smearing detail. The more the magnification the worse the problem. The maximum length of exposure without seeing the rotation is directly connected to the magnification of the target. Also the control software expects the mount to be level even in Alt-Az mode so if its not I think it adds more wobble to tracking.

I concede field rotation is an issue, but it's not the biggest one.  I can watch the object drift around the FOV at a faster rate than field rotation.  By keeping my exposures around 10 seconds the software can fix this but it's having to accommodate poor tracking more than it is FR.  This leads to a lot of stacking artifacts, more than what would be created just by FR - this is what I'm trying to reduce.



#5 Cajundaddy

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Posted 24 August 2018 - 04:14 PM

Go for it!  I doubt you will get anything worthwhile being so far off the well beaten path of astrophotography but often our best life lessons are achieved through failure.  We pour in a bunch of time, effort, and money trying to reinvent the wheel and after 3 months, 6 months, 9 months the light goes on and we realize this is a dead end street.

 

Try it out and report back with your progress. 


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#6 ishorx

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Posted 24 August 2018 - 07:56 PM

the professional scopes are alt-az but whether amateur software can deal with the periodic error,field rotation and not being polar aligned plus seeing?



#7 groth12345

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Posted 23 April 2020 - 10:19 AM

The Nexstar 6SE is very capable of deepsky astrophotography in AltAz.  The only issue is that you will need to take a lot of short exposures (20-30 sec) and get rid of about 75% of them due to tracking and/or field rotation.  Therefore, to shoot M13, as shown here, it took about 20 min to produce this 5 min shot (10 x 30 sec @f/6.3).  I'm going to try the Evolution mount to see if it will eliminate some of the tracking errors but it will still leave the field rotation problem.  This can still be controlled somewhat by shooting the parts of the sky that aren't as affected by this.



#8 groth12345

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Posted 23 April 2020 - 10:24 AM

Sorry, here's the image

Attached Thumbnails

  • M13-small.jpg


#9 Don W

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Posted 23 April 2020 - 12:33 PM

The people in the EAA section would be puzzled to hear that you can't autoguide in Alt/Az mode. They do it all the time.



#10 rnyboy

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Posted 23 April 2020 - 02:17 PM

I see that this topic was started about a year and a half ago but I will chime in and say that if you want to use the "guide" port on the 6SE mount that the mount has to be mounted on a wedge because it will only guide in declination.  I don't want to say I'm 100% correct in stating that but I was recently looking into the idea of autoguiding my 6SE and came across the autoguiding limitations in several postings and pretty quickly dropped the idea when thinking about dealing with a wedge and the inherent limitations of the SE mount.  And as groth12345 stated, you're best bet is to stick to many sub 30s exposures on DSOs when doing alt/az tracking.  


Edited by rnyboy, 23 April 2020 - 02:18 PM.


#11 Don W

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Posted 23 April 2020 - 02:43 PM

And again, go to the EAA section. They autoguide with Celestron Alt/Az mounts on a regular basis.



#12 rnyboy

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Posted 23 April 2020 - 03:00 PM

Yes, but are they 6SE mounts?


Edited by rnyboy, 23 April 2020 - 03:34 PM.


#13 Don W

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Posted 23 April 2020 - 06:30 PM

Yes, they are using SE mounts. Doesn't  have to be 6SE. Many are using them with the 8" OTA. The mounts are the same. The autoguider doesn't know the difference between Alt/Az or RA/Dec. It only knows it needs to move the mount to stay aimed at the start it was aligned on.

 

Keep in mind this only works with short exposures, say under 20 seconds. They are stacking short exposures. You can't take long exposures due to field rotation.

 

I don't understand your statement about only guiding in declination. That's not how any sort of guiding whether manual or auto works. The guider will make corrections in RA or Dec in order to stay locked on a guide star.



#14 rnyboy

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Posted 24 April 2020 - 07:31 AM

Hi Don,

 

I brought it up because the little I could find about guiding a 6SE, yes I know same mount as the 8SE, was that it had to be on a wedge.  And a couple of posters said it was because the guide port only controls the declination in equatorial mode and it only has feed back to the alt motor moves and then depends on having things properly polar aligned.  I couldn't find anybody that stated that both alt and az movements can be controlled through the guide port while operating in alt/az even though that was my starting assumption.  I'm personally glad the scope can be guided in both axis while operating in alt/az for sub 30s exposures because then the only major issue is field rotation and one can do better crops if the object stays close to center of the FOV.    


Edited by rnyboy, 24 April 2020 - 07:51 AM.


#15 Jeff Lee

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Posted 24 April 2020 - 07:58 AM

Field rotation is taken care of by using SharpCap IN EAA .
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#16 rnyboy

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Posted 24 April 2020 - 11:34 AM

Hi Jeff.  I know but you lose info around the edges of the frame due to field rotation as it stacks.  That's what I meant about being able to crop reasonably well if the object stays centered.  My current best image of Bode's still has a dark wedge along the upper right of the cropped final image due to about 7 degrees of field rotation during the +20 minutes of capturing.  It stayed pretty well centered but once processed Bode's was nearly filling the vertical dimension of the 385mc frame when using a 0.5x reducer.




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