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40 arcseconds of periodic error on my EQ6 mount

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#1 ComputerPhysics

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Posted 03 April 2011 - 07:55 AM

Hi,

I am looking for some advice about my EQ6 mount. It is the basic model, without GoTo and without PEC. It lets me doing astrophotography taking short exposures with short focal lengths.

Recently I took several shots to M42 and after stacking without movement correction I got a picture of the periodic error of my EQ6 mount.

After measuring the distance between peaks and valleys I found it yields 40 arcseconds of periodic error, and it takes 6 minutes to complete this distance from peak to valley.

Just wanted to know if that values are what you should expect from an EQ6 mount or if they are much worse. I would also want to know about alternatives to this mount with better periodic error values and similar price. Thanks for any help or tips you could offer me.

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#2 Tapio

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Posted 03 April 2011 - 08:07 AM

I think that 40 " is pretty much average what can be expected for an older EQ6.
The new EQ6 Pro/Atlas is a little better (maybe 10-20").
You could upgrade your mount with newer motors and goto to Pro/Atlas class.
But to get better in that price range there aren't too many alternatives - maybe HEQ5 Pro/Sirius and Vixen mounts (but with less load capacity).
But to get round stars you have to autoguide anyway.

#3 Asimov

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Posted 03 April 2011 - 09:45 AM

I have the same mount & that looks about standard. I only planet image but at 8960mm FL (C11 working at F32) you can certainly see these peak to valleys happening in real time on the laptop screen in a 6 min Saturn AVI. It's interesting, but I now equate (or convert) arc seconds of PE into pixels on a screen..

I also DSO image with a 400D and an ED80 & my simple answer is to manual guide with a guide scope. Works well for me as I'm not forking out for a GOTO mount anytime soon.

#4 Starhawk

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Posted 03 April 2011 - 09:52 AM

Is this image a purposely done star trail? If that error is 40", you were drifting close to 40" every 6 minutes. Work on getting a better drift alignment.

Are you sure you are measuring the 40 arcseconds correctly? I know there are the calculators online, but it's good to check. You can check by imaging a double star with known spacing to get a distance calibration.

-Rich

#5 Duncan Kitchin

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Posted 03 April 2011 - 11:46 AM

That sounds about right - I see about that with mine (which is an older model - I've had it maybe 5 years - but has goto)

Regards & Clear Skies
Duncan

#6 ComputerPhysics

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Posted 03 April 2011 - 11:54 AM

Thanks a lot for your replies. It is a relief to find out that 40 arcseconds is a regular periodic error for this mount. And in reply to Starhawk's question, this image is not a purposely done star trail, but a regular set of subframes in astrophotography. Correctly stacked delivers a final steady image of M42, but that is not the case, because I have applied a software called Startrails in order to get the tracking error pattern.

The way I have measured the 40 arcsecond value is by calculating firstly the arcseconds per pixel value, done by getting a plate solved of a frame and dividing the width spaing value in arcminutes by the width in pixels of a frame. I think this is quite accurate.

The north-south drift you can appreciate in the image is due to a not accurate polar alignment, but this drift has needed 30 minutes of time of exposition, because this is actually a sequence of 62 shots of 30 seconds each.

#7 rmollise

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Posted 03 April 2011 - 05:28 PM

Hi,

I am looking for some advice about my EQ6 mount. It is the basic model, without GoTo and without PEC. It lets me doing astrophotography taking short exposures with short focal lengths.

Recently I took several shots to M42 and after stacking without movement correction I got a picture of the periodic error of my EQ6 mount.

After measuring the distance between peaks and valleys I found it yields 40 arcseconds of periodic error, and it takes 6 minutes to complete this distance from peak to valley.

Just wanted to know if that values are what you should expect from an EQ6 mount or if they are much worse. I would also want to know about alternatives to this mount with better periodic error values and similar price. Thanks for any help or tips you could offer me.


The original EQ-6 came in at 30-arc seconds-plus usually. You value is about what I see with the early non-go-to mounts.

#8 stevethatsmyname

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Posted 03 April 2011 - 09:50 PM

I have around the same time period of HEQ5 and it is much worse. Probably about 3 arcminutes peak to valley. I am currently working on completely re-doing the drive mechanism and hopefully that will help a little. It will also hopefully make autoguiding better and allow for slightly faster slews (16x sidereal rate at the fastest is annoying sometimes)

#9 Starhawk

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Posted 04 April 2011 - 08:08 AM

Seriously, I get a plot like yours when I purposely put a mount out of polar alignment for star trails. Look at Starizona's instructions for drift alignment (very sensitive method). It won't fix the periodic error, but that much polar alignment error has got to be causing image rotation.

-Rich

#10 shams42

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Posted 04 April 2011 - 12:47 PM

That's about how much my Atlas has as well. It guides out at shorter focal lengths. I'm fine imaging at 800mm, but I wouldn't try anything much longer than that.

#11 ComputerPhysics

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Posted 05 April 2011 - 06:29 AM

Thanks all for your comments. Tonight I have tweaked a bit more my polar alignment and now north-south drift is almost gone, but obviously periodic error is still there as an RA oscillation, limiting my exposures to 30 seconds at 500mm focal length.

I guess PEC or guiding are at this point the only way out...

#12 Starhawk

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Posted 05 April 2011 - 08:10 AM

Yep, it's PEC and guiding time. Or put an f/2 hyperstar setup on it.

-Rich

#13 Gregk

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Posted 05 April 2011 - 10:40 AM

Could you go into how you set up this test and how you measured the PE

Thanks

Greg

#14 Falcon-

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Posted 05 April 2011 - 12:01 PM

I could be way off-base on this as I have not used an EQ6, but since your PE fluctuations are in RA and seems to be over a very short time period it has me wondering... How do you balance your scope?

I know from my own lowly CG-5 and from a few other reports on the forums here that if the mount is balanced EXACTLY perfectly or if it is balanced slightly west heavy then what can happen is the RA axis gears, rather then meshing continuously/smoothly, can be bumped ahead slightly and then sits waiting for the motor to catch up, where it then gets bumped slightly again, etc. That would created a sawtooth pattern something like yours with size of up to as much as the backlash in RA.

You might try balancing slightly East heavy to keep the gears continuously meshed/pushing and see if that changes anything.

#15 stevethatsmyname

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Posted 06 April 2011 - 05:20 AM

I could be way off-base on this as I have not used an EQ6, but since your PE fluctuations are in RA and seems to be over a very short time period it has me wondering... How do you balance your scope?

I know from my own lowly CG-5 and from a few other reports on the forums here that if the mount is balanced EXACTLY perfectly or if it is balanced slightly west heavy then what can happen is the RA axis gears, rather then meshing continuously/smoothly, can be bumped ahead slightly and then sits waiting for the motor to catch up, where it then gets bumped slightly again, etc. That would created a sawtooth pattern something like yours with size of up to as much as the backlash in RA.

You might try balancing slightly East heavy to keep the gears continuously meshed/pushing and see if that changes anything.


hi falcon. I think what he did here was do a SUPER-LONG exposure. so those aren't super short bumps, but long 6-minute bumps. I think they are just normal periodic error.

#16 ComputerPhysics

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Posted 06 April 2011 - 05:24 AM

I am balancing as perfectly as possible, not East not West. But I am going to take seriously your advice and give a try to balancing slightly against the RA movement. Let see what happens ... Thanks!

#17 ComputerPhysics

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Posted 06 April 2011 - 05:55 AM

I took a bunch of 30 seconds shots aiming M42 nebula as regular, in order to stack them later. Polar alignment was also regular, using EQ6 polar scope, probably not perfect.

I think M42 is a good target to measure periodic error in RA movement due to its near 0 degrees declination.

In order to show the drift, I stacked the shots without drift correction using a free software called startrails that gets the brightest pixels per shot, obtaining that way the best startrail you can achieve.

To measure the length of the drift, I requested a single shot solved plate from astrometry.net. They provide an exact width in arcminutes of the field. Then, I divide the width field by the width in pixels of my DSLR camera sensor, obtaining the resolution per pixel in arcseconds (a number close to 1 arcsecond/pixel for a 1,200 mm effective focal length telescope).

Then I measure the height of RA drift pattern with my regular post-processing free software, Fitswork4, and multiply that value by the previous resolution.

That is the way I have found that my EQ6 mount drifts around 40 arcseconds in RA movement. Dividing total exposure by the number of cycles (top peak to lower peak) I got the elapsed time needed to fulfill the periodic error: around 6 minutes long.

#18 shams42

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Posted 06 April 2011 - 11:13 AM

I think the worm gear period of the EQ6 is 8 minutes. You may need to add the image download time to your calculations.

You can measure your PE very easily if you have a webcam using the free programs PERecorder and PECPrep, both part of the EQMOD suite.






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