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Can Alt-Az Go-To System be Combined with Equatorial Platform?

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#1 msheald

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Posted 12 July 2019 - 08:07 AM

Hello! As I consider a large Dobsonian telescope, I'm wondering if an Alt-Az system can be combined with an equatorial platform.

 

   I am interested in a large aperture scope for photometry, so I need a reasonable FWHM but not perfectly round stars from corner to corner as astrophotographers need,

 

   Alt-Az Go-To systems are great to find the field; however, field rotation sets in as the exposure progresses. Programs like SGPro allow automatic adjustment of the field based on plate solving an image.

 

   What I have been thinking of is a system that runs on a traditional equatorial platform, but the Go-To is performed with the Go-To Alt-Az motors. Once the Go-To is performed, the Alt-Az motors would be switched off and tacking would be performed by the equatorial platform. The computer would have to do some fancy position reductions with each slew since the "Alt-Az" position would vary as the equatorial platform runs and, while the "Alt-Az" axes would remain perpendicular to one another, their orientation with respect to the earth's axis would be dynamic (though predictable). And, the scope's position would need to be reset each time the Equatorial platform resets.

 

    I'd hate to start such programming from scratch. Has anyone combined an equatorial platform and Alt-Az Go-To System? Thank you.

 

Mike

 

   



#2 Jeff Struve

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Posted 12 July 2019 - 08:13 AM

How long do exposures need to be to do photometry?



#3 msheald

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Posted 12 July 2019 - 08:24 AM

Hello! Most of my exposures with my 8" LX200 classic max out at 2 minutes. I have a few targets that require 5 minutes exposures.

 

    With a larger scope that allows deeper images, the ability for longer exposures would be helpful as well.

 

    I've ground/polished 4" and 12" mirrors. Before committing to anything larger as ATM, I'd want to test drive the drive system to make sure my idea would work since I may be out in left field with this idea. Best regards.

 

Mike



#4 billdan

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Posted 12 July 2019 - 08:24 AM

When you think about it, an Equatorial mount is an ALT/AZ mount with its Altitude axis tilted to be in line with the N/S Polar axis.

So you could just use a wedge that is at your latitude angle and bolt the DOB onto it so it doesn't fall off. You would also need to turn off the ALT motor when tracking. I don't know how this affects your GoTo accuracy though.



#5 Jeff Struve

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Posted 12 July 2019 - 08:31 AM

What size Dob are you considering?



#6 Pierre Lemay

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Posted 12 July 2019 - 08:45 AM

Alt-Az Go-To systems are great to find the field; however, field rotation sets in as the exposure progresses.

 

Once the Go-To is performed, the Alt-Az motors would be switched off and tacking would be performed by the equatorial platform.

Mike,

One thing many people wishing to do long exposure astrophotography don't realize about equatorial platforms is that any, inevitable, declination correction required to keep an object centerd during the exposure will only work, without field rotation, for objects located near the meridian (the imaginary line that crosses both poles and passes through the zenith). The more east or west of the meridian the scope is pointing, the more field rotation is introduced when declination correction is required.

 

For example, if you point any telescope mounted on a dual-axis platform looking just above the horizon exactly east or west, activating the "declination" correction will only result in a pure rotation of the tube around its optical axis. 


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#7 msheald

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Posted 12 July 2019 - 08:52 AM

Hello! If I would build this from scratch, I would look at 16 to 24 inch scope. If I do this, I would be interested in the slumped mirror designs that Me Bartels at others have worked with at f3. I would want the camera a prime focus with the focuser there, so I would have to make sure support at the secondary is excellent as well.

 

    At these focal length, the scope would be an excellent match for 9 micron chips. 13.5 micron chips would work excellently as well, though these are back illuminated and much more expensive.

 

    Current commercial alternatives would be something like a Celestron C14 and an appropriate heavy-duty mount (e.g., AP, Paramount, I-Optron, etc.) with 24 micron chip camera like FLI ML1001. However, that would lose the ATM enjoyment! Best regards.

 

Mike



#8 Jon Isaacs

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Posted 12 July 2019 - 09:04 AM

Mike,

One thing many people wishing to do long exposure astrophotography don't realize about equatorial platforms is that any, inevitable, declination correction required to keep an object centerd during the exposure will only work, without field rotation, for objects located near the meridian (the imaginary line that crosses both poles and passes through the zenith). The more east or west of the meridian the scope is pointing, the more field rotation is introduced when declination correction is required.

 

For example, if you point any telescope mounted on a dual-axis platform looking just above the horizon exactly east or west, activating the "declination" correction will only result in a pure rotation of the tube around its optical axis. 

 

Bob Brunck discusses the issues involved with imaging and autoguiding with a Tom O dual axis platform in this post:

 

https://www.cloudyni...form/?p=8533108

 

"I had also asked Craig Stark about his PHD guiding application with non orthogonal guiding.  He explained that the software actually creates correction vectors for the guiding corrections and the mount does not need to be perfectly aligned nor do the axis' need to be perfectly orthogonal to guide.  This is critical and why it is possible to use Tom's platforms with accurate tracking corrections.  "It should work ..."  That was a leap of faith I admit, but it did turn out to be possible."

 

It doesn't sound easy.

 

Jon



#9 tommm

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Posted 12 July 2019 - 09:30 AM

The firmware in the Goto system would have to account for the equatorial platform movement as digital setting circle firmware does, otherwise the skymap of the Goto system becomes inaccurate as the platform rotates.  I would think it would be easier to implement a system using high precision digital setting circles with a platform, and just push the scope to position.

 

I took some decent photos with 2 minute exposures using a platform in the distant past, but polar alignment and tracking speed have to be very accurate to avoid having to do pseudo-dec corrections using the platform south pad. I made a jig with turnbuckle to set the platform on to tweak platform alignment more accurately by sliding the front feet E-W using the turnbuckle.

 

It worked better to use a modified webcam (the old SC1 mod for longer exposures) and stack a few hundred 30 second exposures.  The platform was diy, with south pad adjustment by a rotating disk with offset axis (a "cam").

 

With slumping/grinding/polishing/figuring the mirror and building the scope with Goto and platform you're looking at an awfully large scope of work project!


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#10 ngc7319_20

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Posted 12 July 2019 - 09:31 AM

Hello! As I consider a large Dobsonian telescope, I'm wondering if an Alt-Az system can be combined with an equatorial platform.

 

   I am interested in a large aperture scope for photometry, so I need a reasonable FWHM but not perfectly round stars from corner to corner as astrophotographers need,

 

   Alt-Az Go-To systems are great to find the field; however, field rotation sets in as the exposure progresses. Programs like SGPro allow automatic adjustment of the field based on plate solving an image.

 

   What I have been thinking of is a system that runs on a traditional equatorial platform, but the Go-To is performed with the Go-To Alt-Az motors. Once the Go-To is performed, the Alt-Az motors would be switched off and tacking would be performed by the equatorial platform. The computer would have to do some fancy position reductions with each slew since the "Alt-Az" position would vary as the equatorial platform runs and, while the "Alt-Az" axes would remain perpendicular to one another, their orientation with respect to the earth's axis would be dynamic (though predictable). And, the scope's position would need to be reset each time the Equatorial platform resets.

 

    I'd hate to start such programming from scratch. Has anyone combined an equatorial platform and Alt-Az Go-To System? Thank you.

 

Mike

I am familiar with the Sky Commander system.  Yes, there are provisions for use with an equatorial platform.  You would set up the Sky Commander with the sidereal clock set to "off."  And there are some provisions to reset the platform without repeating the 2-star alignment.  I don't think you would need to turn off the Alt-Az motors -- they would simply be idle while tracking an object.  You can read the manual at

http://www.skyeng.co..._Manual_4v1.pdf

See p. 29.


Edited by ngc7319_20, 12 July 2019 - 10:37 AM.

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#11 msheald

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Posted 12 July 2019 - 12:09 PM

Thanks! I'll look at Sky Commander and see how it might interface with the stepper motors. Best regards.

 

Mike



#12 ngc7319_20

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Posted 12 July 2019 - 09:49 PM

Thanks! I'll look at Sky Commander and see how it might interface with the stepper motors. Best regards.

 

Mike

It may be useful to describe how this was implemented on StarMaster telescopes:  It used the Sky Commander and Sky Tracker together to provide goto and tracking.  I believe both were made by Sky Engineering, but Sky Commander is available publicly, whereas the Sky Commander was proprietary to StarMaster Telescopes which has since closed down.

 

Sky Commander has a database of object positions, a sidereal clock, and it connects to the Alt and Az encoders. The user would set it up by sighting two stars.  It generated Alt and Az error numbers (or something like that), and these data were sent to the Sky Tracker box which operated the two servo motors.  For large errors it would slew at high speed, and ramp the speed down as the error decreased.  For small errors it just moved at tracking speeds.  Sky Tracker had a hand box with four directional buttons, and four others that corresponded to manual slew, track, goto, and stop.

 

So you would need to either find a Sky Tracker setup, find something else with similar function, or make one. You could probably inquire to Sky Engineering to start with.  The other major system for Dobs is Argo-Navis + Servo Cat, which has similar functions to Sky Commander + Sky Tracker.  I believe the Servo Cat system can accept inputs from Sky Commander, so that might be one path for you.  And it is also possible that Argo-Navis will support EQ platforms, but I'm not familiar wit it.  There are probably other off-the-shelf servo controllers available for telescopes.  Maybe other people would know more about this than I...

 

Sky Commander:

http://www.skyeng.com/

 

Sky Tracker:

http://www.loptics.c...sky-tracker.htm

 

Argo-Navis:

https://www.wildcard...vations.com.au/

 

Servo Cat:

http://www.stellarca...s/servocat.html



#13 gregj888

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Posted 12 July 2019 - 10:07 PM

Add field derotation.  It's another axis, but probably the easiest for a larger scope.  

 

I've always planned a fork mount for m 20" but am actively reconsidering. The fork would have made things easier for it's initial use.  Changes there make Alt/Az equally viable and the mount much easier to construct.

 

Check the Alt/Az Yahoo group:

 https://groups.yahoo...Initiative/info

 

And the Sidereal Tech user sites have some good information as well as this forum. 



#14 hamishbarker

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Posted 14 July 2019 - 02:33 AM

The firmware in the Goto system would have to account for the equatorial platform movement as digital setting circle firmware does, otherwise the skymap of the Goto system becomes inaccurate as the platform rotates.  I would think it would be easier to implement a system using high precision digital setting circles with a platform, and just push the scope to position.

 

I took some decent photos with 2 minute exposures using a platform in the distant past, but polar alignment and tracking speed have to be very accurate to avoid having to do pseudo-dec corrections using the platform south pad. I made a jig with turnbuckle to set the platform on to tweak platform alignment more accurately by sliding the front feet E-W using the turnbuckle.

 

It worked better to use a modified webcam (the old SC1 mod for longer exposures) and stack a few hundred 30 second exposures.  The platform was diy, with south pad adjustment by a rotating disk with offset axis (a "cam").

 

With slumping/grinding/polishing/figuring the mirror and building the scope with Goto and platform you're looking at an awfully large scope of work project!

sky safari has an option for the mount to be an alt-az encoded telescope sitting on an equatorial table.


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