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Eq6-r pro poor accuracy with larger focal length?

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#1 Andrew_M12

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Posted 09 May 2021 - 12:04 AM

I purchased a EQ6R-r pro last month so i’m fairly new to it. When I set up the mount, polar align it and complete a 2 star alignment with a Canon rebel t4i and 200mm lens, I get perfect accuracy with any target.

However when I polar align and complete a 2 or 3 star alignment with my Nexstar 8” Schmidt Cassegrain (with a f/6.3 focal reducer) and a Canon Rebel t4i attached to the EQ6R-pro, every time it seems that I’m far from my target (only by about a finger width from my target)

Another thing to add, when I am finding alignment stars while using my 200mm focal length lens and canon rebel t4i, by the 2ed or 3rd star, my picked alignment star will usually will be centred perfectly on the canon rebel t4i’s screen display after being slewed to. While even after 2 alignment stars using my Nexstar 8” S Schmidt Cassegrain, I still need to correct the last alignment star to be centred in the Canon Rebel t4i’s display after being slewed to. 

Things to consider:

- my mount is always in home position facing North before I begin polar aligning
- I use the built in polar scope when polar aligning
- I am using the hand controller when finding alignment stars
- to centre the stars when using a 2 or 3 star alignment, I am using my Canon Rebel t4i’s screen display


Edited by Andrew_M12, 09 May 2021 - 12:28 AM.


#2 rgsalinger

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Posted 09 May 2021 - 12:59 AM

Read up on cone error. SCT's have mirrors that move around. You'll need to shim the scope to get better pointing - it's a one time exercise. 

 

Next realize that the field of view is much much tinier using the same camera with 10x focal length. This is why most people using camera use plate solving. Then you really don't care if the initial go to is off by a degree. It just doesn't matter. 

 

Finally, does "centered perfectly" mean that your software has crosshairs and the star is at their intersection? I ask that because my equipment won't perfectly center anything if I'm using the crosshair as a standard. If I'm just eyeballing things then yup objects are in the center but I would not use the adverb "perfectly". Just saying.  If not then you might want to calculate how far off your alignment star actually is. 

 

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#3 Andrew_M12

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Posted 09 May 2021 - 01:06 AM

Read up on cone error. SCT's have mirrors that move around. You'll need to shim the scope to get better pointing - it's a one time exercise.

Next realize that the field of view is much much tinier using the same camera with 10x focal length. This is why most people using camera use plate solving. Then you really don't care if the initial go to is off by a degree. It just doesn't matter.

Finally, does "centered perfectly" mean that your software has crosshairs and the star is at their intersection? I ask that because my equipment won't perfectly center anything if I'm using the crosshair as a standard. If I'm just eyeballing things then yup objects are in the center but I would not use the adverb "perfectly". Just saying. If not then you might want to calculate how far off your alignment star actually is.

Rgrds-Ross

Thanks Ross,

What I mean by “perfectly” centered is by the time I’m on my 2ed or 3rd alignment star, when the mount is finished slewing I never have to make a correction to have it in the field of view of the Canon’s display screen. It’s also pretty spot on to what I had originally aligned the previous stars to.

I also don’t use any softwares. Just the built in eyepiece of the mount to polar align and then the controller to pick a alignment method. I am considering using a plate solving method if could potentially help.

Hopefully that makes sense

Edited by Andrew_M12, 09 May 2021 - 01:45 AM.


#4 RichA

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Posted 09 May 2021 - 01:56 AM

Read up on cone error. SCT's have mirrors that move around. You'll need to shim the scope to get better pointing - it's a one time exercise. 

 

Next realize that the field of view is much much tinier using the same camera with 10x focal length. This is why most people using camera use plate solving. Then you really don't care if the initial go to is off by a degree. It just doesn't matter. 

 

Finally, does "centered perfectly" mean that your software has crosshairs and the star is at their intersection? I ask that because my equipment won't perfectly center anything if I'm using the crosshair as a standard. If I'm just eyeballing things then yup objects are in the center but I would not use the adverb "perfectly". Just saying.  If not then you might want to calculate how far off your alignment star actually is. 

 

Rgrds-Ross

All my U.S. made LX200's would achieve Goto accuracy of about five minutes with correct alignment.  When scopes don't achieve this it just means tolerances in the mechanical parts are poor or the alignment stars were too close to each other.



#5 rgsalinger

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Posted 09 May 2021 - 09:16 AM

You can get the same level of performance with an EQ6R if you use EQMOD and enough alignment stars. Since the OP is not using software he's limited to what the HC can provide. It's been too long since I used one of these to know if it supports (say) 6 alignment stars. 

 

Rgrds-Ross



#6 Eddie_42

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Posted 09 May 2021 - 10:24 AM

I don't have an SCT, nor to I use the paddle on my EQ6 - straight up peanut gallery opinion here.

 

I agree with the others. Some combination of mirror-flop (mechanical internals of an SCT), and the Field of View at play here too.  With the 200mm lens and that DSLR chip, the FOV is 6.4x4.2 degrees, with the 8" SCT reduced f/6.3, the FOV is 1x0.66 degree.   OP stated that the alignment is "within a finger width"....thats about 1 degree. 



#7 robodan

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Posted 16 May 2021 - 04:36 AM

I have an sct and EQ6-R PRO, and while I dont always get thr object dead centre, quite often it is, but can vary in what position the object is. I do a 1 star alighment and generally find its OK.

Celestron does a better job than skywatcher though.

Also how accurate is your polar alighment will make a difference, hense I can get by with a 1 star alighment. Also a 2 star alighment supports to correct cone error and works out how far off the pole you are. Try a 3 star alighment and check your polar alightment.

If your using the polar scope, not as good as ipolar, polemaster or sharpcap pro.

Edited by robodan, 16 May 2021 - 04:39 AM.



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