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Autoguiding with a LX-200?

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#1 StuartT

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Posted 21 July 2021 - 05:39 AM

Forgive my ignorance, but I was just reading the spec of the LX-200 14" and it says it has an autoguiding port (presumably for astrophotography). Yet it has an AltAz fork mount.

 

I thought you couldn't really do AP with an altaz mount? Obviously I am mistaken, but would anyone be able to explain to me how that's done (i.e. how you avoid field rotation ruining your longer exposures)?

 

Thanks



#2 dcaponeii

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Posted 21 July 2021 - 06:58 AM

You don't avoid field rotation.  By keeping the individual exposures short you rely on the post-processing software to deal with the field rotation.  30s subs are pretty typical and you build up total exposure with hundreds of subs.  


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#3 LoveChina61

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Posted 21 July 2021 - 07:12 AM

Mount the scope on a wedge that is accurately polar aligned. That will enable you to shoot photos up to a few minutes of exposure time.

 

If you add on auto-guiding, then I was able to get the maximum 10 minute exposures that my sky conditions allowed for without any problem at all. Others have taken even longer exposures in better sky conditions.

 

Mike


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#4 bobzeq25

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Posted 21 July 2021 - 08:23 AM

There's a continuum of OK, better, and best.

 

You can image with an SCT on an alt-az mount.  You take a very large number of very short exposures, to minimize field rotation.

 

You can put the mount on a wedge.  Now you can take longer subs.  But, that inexpensive mount was never designed for astrophotography.  And, in particular, the various bits; the fork arms, the bearings, were not designed to work at that angle.  They will flex, and that will impact image quality.  On a 14 inch, that will be a serious issue (focal length magnifies tracking errors, a lot).  You'd not be getting the best the scope has to offer.

 

Or you can place the OTA on a good (and necessarily expensive) German Equatorial mount.  Since the designed is aimed at managing the loads well, it will do the best job, make the best images.  The solution favored by experienced imagers.

 

You can choose where you want to be on that continuum, it's a personal choice.  But don't fool yourself.  A fork mount that was designed as an alt az mount will not do as good a job as a GEM.  The mount is the most important part of a DSO AP setup.    People don't put SCTs on GEMs because they like the look.  They do it to make better images.


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#5 Peterson Engineering

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Posted 21 July 2021 - 03:25 PM

There's a continuum of OK, better, and best.

 

You can image with an SCT on an alt-az mount.  You take a very large number of very short exposures, to minimize field rotation.

 

You can put the mount on a wedge.  Now you can take longer subs.  But, that inexpensive mount was never designed for astrophotography.  And, in particular, the various bits; the fork arms, the bearings, were not designed to work at that angle.  They will flex, and that will impact image quality.  On a 14 inch, that will be a serious issue (focal length magnifies tracking errors, a lot).  You'd not be getting the best the scope has to offer.

 

Or you can place the OTA on a good (and necessarily expensive) German Equatorial mount.  Since the designed is aimed at managing the loads well, it will do the best job, make the best images.  The solution favored by experienced imagers.

 

You can choose where you want to be on that continuum, it's a personal choice.  But don't fool yourself.  A fork mount that was designed as an alt az mount will not do as good a job as a GEM.  The mount is the most important part of a DSO AP setup.    People don't put SCTs on GEMs because they like the look.  They do it to make better images.

Actually these scopes ARE designed for for AZ or Polar use.  AZ is better for visual and Polar is really the only way to go for imaging with anything but a planet-cam.  All mounts flex regardless of design.  Some more than others.  0.5mm or 3mm makes no difference.  The amount of flex may change slowly over several hours.  But the scope doesn't know about flexing and just tracks what it's pointed at.  My +42° observatory houses the very first 14" LX200GPS that Meade shipped and flex is not a problem.  The scope's performed flawlessly for visual, astrometry and imaging since 2004.


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#6 carolinaskies

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Posted 21 July 2021 - 07:28 PM

Forgive my ignorance, but I was just reading the spec of the LX-200 14" and it says it has an autoguiding port (presumably for astrophotography). Yet it has an AltAz fork mount.

 

I thought you couldn't really do AP with an altaz mount? Obviously I am mistaken, but would anyone be able to explain to me how that's done (i.e. how you avoid field rotation ruining your longer exposures)?

 

Thanks

The ST4 autoguiding port is a hold over from when a guide camera would be directly connected to the mount and used basic high level inputs to the RA & DEC like came from the hand control.  This was replaced with pulse guiding through programs like PHD2 which connect through the RS232 AUX port.  

In Alt-Az ST4 autoguiding results worsen the longer the exposure time is increased due to field rotation at the specific target location in the sky and the users physical location.  If you were to use Alt-Az at 90, -90 lattitude there would be little problem as the RA would have very little error and most correction would be in DEC and conversely at 0 lattitude there would be little problem as the DEC would have very little error and most correction would be in RA.   Away from these extremes both RA and DEC corrections increase. 

Meade developed a field derotator for it's 16" SCT LX200s to compensate(crudely) for field rotation, that's why they have an extra port for a field rotator on their panels.  Today Pxyis makes a fine rotator many Alt-Az mount users find quite reliable.  

These days exposure times have dropped significantly with the increase in sensitivity of imaging sensors.  In old film days and early CCD days often exposures were >10minutes or longer but now CMOS sensors have such good signal-noise ratios that even an F/10 telescope like the SCTs can image in a fraction of the time and with the advent of stacking the S-N ratio is improved further so exposure times can be cut even further.  This means that starting out even an Alt-Az mount can image a good number of targets successfully.  Long term better results are possible putting the fork mount onto a wedge and running periodic error correction routines as well as a guide camera with exposures still well under the old days of imaging.  


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#7 StuartT

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Posted 22 July 2021 - 01:53 AM

as usual, a great many informative and helpful replies. Thanks everyone for helping me on my learning curve!


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#8 LoveChina61

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Posted 23 July 2021 - 05:36 AM

get a focal reducer and you will able to take even shorter exposures and reduce the effects of any possible tracking errors as well.


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#9 StuartT

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Posted 29 July 2021 - 02:20 PM

get a focal reducer and you will able to take even shorter exposures and reduce the effects of any possible tracking errors as well.

sorry, I probably wasn't clear in my original post. I'm not asking because I have an LX-200 myself. I don't. I just happened to be browsing and saw that the LX-200 had an autoguiding port and (with my limited knowledge) this struck me as odd, because it's on an AltAz mount.

 

However, I have learned that AZ mounts can be used for AP under the right circumstances




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