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How to determine if collimation is needed for a Ritchey Chretien

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#1 vishadow

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Posted 05 August 2020 - 09:52 AM

I've recently acquired an RC6 telescope and the stars seem to be fairly sharp when imaging. The diffraction spikes are quite apparent and the bahtinov mask seems to show centered spikes when focusing. I've seen several videos on the collimation procedure for these telescopes, but I don't know how to determine if it's at it's maximum sharpness. I have a laser collimator that I used for my 8" dob, but I suspect that its not perfectly centered. 



#2 Hesiod

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Posted 05 August 2020 - 10:35 AM

As with any other telescope: observe a bright star at best focus and high magnification (or take a picture of it)


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#3 bobzeq25

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Posted 05 August 2020 - 11:49 AM

Look at the diffraction spikes.  When I first got my scope it as so uncollimated that they were doubled.  These are pretty good.  Do not touch the primary unless you have to.  The mirrors interact, collimating the primary messes up the secondary.  If you do touch the primary, you'll need to sneak up on collimation, going back and forth.

 

Another thing.  The _optics_ in an inexpensive RC are often not centered, ie the optical center and the mechanical center don't coincide..  I started with a Cheshire, went to an artificial star, then real stars.  When I got to the point below, and went back to the Cheshire, it indicated mechanical misalignment.

 

I ignored it, of course.

 

Pleadies.jpg


Edited by bobzeq25, 05 August 2020 - 11:52 AM.

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#4 cuzimthedad

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Posted 05 August 2020 - 01:07 PM

Moved to Cats & Casses



#5 MitchAlsup

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Posted 05 August 2020 - 07:27 PM

There is a reason Takahashi sells telescopes that you put in the focuser of the Ritchey Chretien telescopes to check the optical centering of the secondary by using a reticle...........



#6 gwlee

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Posted 06 August 2020 - 07:16 PM

Do a “star test.”



#7 vishadow

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Posted 07 August 2020 - 11:01 AM

This is the best image that I've been able to capture so far. This was just under 1 hour of data. The image isn't tact sharp, but I don't know if that's just the result of not having enough integration time or slight misalignment with the mirrors. 

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#8 bobzeq25

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Posted 07 August 2020 - 02:13 PM

This is the best image that I've been able to capture so far. This was just under 1 hour of data. The image isn't tact sharp, but I don't know if that's just the result of not having enough integration time or slight misalignment with the mirrors. 

How do you focus?  I had chronic focus problems.  My first solution was making focus the very last thing I did before starting taking subs.  My second was to use the focus assistant in Sharpcap, checking it as the night progressed.  My third and best was autofocus.

 

I find poor focus to be, as a practical matter, indistinguishable from poor collimation.


Edited by bobzeq25, 07 August 2020 - 02:14 PM.

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#9 jgraham

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Posted 07 August 2020 - 02:36 PM

My experience has been that mis-collimation shows itself in the star shapes and the symmetry across the field. In an uncorrected RC field the stars in the center of the field should be circular and the stars in the corners should be slightly elongated and oriented towards the center of the image. Sensor tilt can cause some unique issues, but this will also show up and elongated stars. The sharpness of the image is more a matter of focus, seeing, and guiding. Keep in mind that the focus can shift as the temperature changes due to the thermal expansion coefficient of the telescope tube.

 

On my 8" RC I have worked very hard to tweak the collimation of the secondary. I'm the 3rd owner of this scope and it was way off when I received it. It's fine now. I'm also using a Baader Mk III MPCC, which does a great job cleaning up the edges of the field. Since I'm now confident that my secondary is aligned I'll be keeping my eyes open for on-axis astigmatism as a sign that the primary alignment needs to be corrected; which would also mean the the secondary alignment would also have corrected again. Once I have an aligned secondary (easy to check) with no on-axis astigmatism, both mirrors should be aligned, assuming that my sensor is centered and normal to the optical axis. Fortunately, I'm happy with the image quality across my APS-c sensor (Nikon D5300) and I have not had to touch the primary.

 

Enjoy your RC, they are a delight to use once you have them dialed in.


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#10 vishadow

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Posted 07 August 2020 - 02:51 PM

My experience has been that mis-collimation shows itself in the star shapes and the symmetry across the field. In an uncorrected RC field the stars in the center of the field should be circular and the stars in the corners should be slightly elongated and oriented towards the center of the image. Sensor tilt can cause some unique issues, but this will also show up and elongated stars. The sharpness of the image is more a matter of focus, seeing, and guiding. Keep in mind that the focus can shift as the temperature changes due to the thermal expansion coefficient of the telescope tube.

 

On my 8" RC I have worked very hard to tweak the collimation of the secondary. I'm the 3rd owner of this scope and it was way off when I received it. It's fine now. I'm also using a Baader Mk III MPCC, which does a great job cleaning up the edges of the field. Since I'm now confident that my secondary is aligned I'll be keeping my eyes open for on-axis astigmatism as a sign that the primary alignment needs to be corrected; which would also mean the the secondary alignment would also have corrected again. Once I have an aligned secondary (easy to check) with no on-axis astigmatism, both mirrors should be aligned, assuming that my sensor is centered and normal to the optical axis. Fortunately, I'm happy with the image quality across my APS-c sensor (Nikon D5300) and I have not had to touch the primary.

 

Enjoy your RC, they are a delight to use once you have them dialed in.

Great tip! The point about sharpness makes total sense. I'm very comfortable collimating my Dob, but I'm a bit antsy about messing up the RC6. From what I've seen, the RC8 seems to be a sweet spot with the focal length that I would like to have, but I'll need to wait to upgrade my mount. Additionally, I haven't tried using my Nikon on it yet (I have a D5500). So far, it's only been my ASI533. 

 

Thanks!




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