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side by side telescopes - how to drift align

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#1 sferlix

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Posted 17 June 2019 - 10:39 AM

Hello,

 

this is my 1st post on this forum. I hope I'm using the proper section to ask for support.

 

I've an EQ8 carrying 3 telescopes, mounted in 2+1 config, side by side.

 

Side1: C925HD

Side2: WO132FLT + APO quadruplet 100/580 (on top)

 

In order to guide, I use 2 different OAGs, one per side.

Side 1: QHY174

Side 2: QHY178 (OAG is on the WO132FLT).

 

here is the issue.  The guide with the C925HD is fine, but when guiding with the WO132FLT, the PHD2 graph is not that fine.. Well, on both cases, it's within 1 arcsec, but I was expecting much less variations on the WO132FLT. 

 

I was thinking that maybe this is because I've adjusted the mount with the drift method using the C925HD.

 

I would need to improve the guiding on the WO.

Any suggestion ? what is your opinion ?

 

thanks in advance.

francesco



#2 Phil Sherman

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Posted 17 June 2019 - 11:06 AM

I doubt you can improve things very much. You did your drift alignment with a 2350mm scope and are guiding with a 924mm scope. The pixel sizes of the guide cameras will effect guiding accuracy but with a 2.5x focal length difference, you won't see better guiding with the shorter fl scope. 

 

If we assume perfect alignment, then the mount's polar alignment is identical for all three scopes which leaves PE and atmospheric refraction as the only things guiding needs to correct. Again, the longer focal length scope's OAG will have the most seneitivity to these guiding errors.



#3 spokeshave

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Posted 17 June 2019 - 11:45 AM

The guide with the C925HD is fine, but when guiding with the WO132FLT, the PHD2 graph is not that fine.. Well, on both cases, it's within 1 arcsec, but I was expecting much less variations on the WO132FLT. 

Forgive me if I'm misunderstanding things, but why would you expect better guiding when using the WO132FLT? Regardless of what scope you are guiding with, the mount is swing the whole load and guiding should therefore be roughly the same for each scope.

 

Tim



#4 Spoonsize

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Posted 17 June 2019 - 12:00 PM

Seems the biggest, hardest to control issue would be the difficulty in obtaining an accurate polar allign with both scopes at the same time. OR, perhaps the OP drift alligns each time the OAG is switched from one to the other. No? Yes?

#5 spokeshave

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Posted 17 June 2019 - 12:10 PM

Polar alignment aligns the RA axis of rotation of the mount and should be independent of the payload. If one scope is polar aligned, then the other should be as well. If there is a difference in polar alignment between the scopes, it is possible that there is flexure that causes drift during the PA process.

 

Tim


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#6 sferlix

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Posted 17 June 2019 - 12:28 PM

In theory, if both telescopes axis are really parallel, there should be no difference. Guide should be the same.

But it seems this is not the case.
Indeed, the guide is good for the C925HD but is systematically not that good with the 132FLT.


I would try to run drift align on the 132FLT and check if it's better.
Otherwise I should find a way to make the scopes really parallel...
Right ?


What if I align using the polemaster ?


Francesco

#7 OldManSky

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Posted 17 June 2019 - 12:55 PM

Seems the biggest, hardest to control issue would be the difficulty in obtaining an accurate polar allign with both scopes at the same time. OR, perhaps the OP drift alligns each time the OAG is switched from one to the other. No? Yes?

Um, no.

It's the mount that's polar aligned, not a particular telescope.  And one of the nice things about the drift method is that it works independently of the scope being aligned precisely with the mount.  You can have significant cone error, and drift aligning will still work just fine.

 

If you're seeing significant guiding differences, it's more likely due to the focal length or the PHD2 settings varying between the OAGs.  Or perhaps some flexure in the WO132 mounting.  Or you're using the 132 OAG to guide, and that's not accounting for mirror flop in the C9.25.

 

How are the shots taken using the 132 when using the 132 OAG to guide?  


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#8 sferlix

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Posted 17 June 2019 - 01:17 PM

Forgive me if I'm misunderstanding things, but why would you expect better guiding when using the WO132FLT? Regardless of what scope you are guiding with, the mount is swing the whole load and guiding should therefore be roughly the same for each scope.

 

Tim

 

Hello

 

Guiding on the C925HD is smoother and peak to peak is less when compared with guiding on the WO132FLT.

I attach some images (same evening, in the same few minutes)  . Please note how good is just under the label "c925" on the picture.

 

Thanks

Francesco

Attached Thumbnails

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  • received_379523459353955.jpeg


#9 sferlix

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Posted 17 June 2019 - 01:30 PM

I doubt you can improve things very much. You did your drift alignment with a 2350mm scope and are guiding with a 924mm scope. The pixel sizes of the guide cameras will effect guiding accuracy but with a 2.5x focal length difference, you won't see better guiding with the shorter fl scope. 

 

If we assume perfect alignment, then the mount's polar alignment is identical for all three scopes which leaves PE and atmospheric refraction as the only things guiding needs to correct. Again, the longer focal length scope's OAG will have the most seneitivity to these guiding errors.

Thank you Phil.

Could I get some improvements if I have same guiding cameras, so same pixel size?

Thank you



#10 Chuckwagon

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Posted 17 June 2019 - 03:37 PM

It isn't the pixel size so much as the guide image resolution (arc-sec per pixel) that matters.  With the cameras and scopes listed, my calculations indicate that your guiding image scale for the C9.25 is around .51 arc-sec per pixel, and for the WO132FLT it's around .53.  That's pretty similar, so changing cameras doesn't seem like it would be needed.

 

There are many variables that affect guiding, so it's not at all surprising to see differences, even from the same scope and camera on two separate runs in different regions of the sky.  Even changing seeing conditions during the night can cause alterations.  So don't worry too much if they don't match.

 

However, if you really want to test the two setups against each other you can.  To do so, eliminate the OAGs for the first part of the test, and connect directly to the scope.  Create a PHD2 profile for each camera/scope.  Find a star to guide on near the meridian and celestial equator.  Then get both scopes pointed at the same star, and focused equally well.  Guide with one profile for 10-15 minutes.  Switch to the other profile and guide on the same star for the same time.  Then find a different star further back to the east near where you started before and repeat the test but reversed, using the 2nd profile first.  Once that test is done you can reintroduce the OAGs.  Then repeat the test.  You'll then be able to compare results for each camera/scope combo with and without OAG.  This will give you as close to the same test conditions for each camera scope combination as you can get.  It would be interesting to see any differences.



#11 sferlix

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Posted 17 June 2019 - 03:42 PM

Thank you Charles.

Should I run calibration when switching between scopes?



#12 Chuckwagon

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Posted 17 June 2019 - 04:26 PM

Thank you Charles.

Should I run calibration when switching between scopes?

It might not be required, since the image scales are so similar, but considering how quickly the calibration runs, I'd probably do it.  Calibrate and guide on the same star to make it easier.  :)


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