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Farpoint Cheshire

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#1 The Ardent

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Posted 06 June 2020 - 06:29 PM

I recently tried the Farpoint Cheshire and in my 18” f/3.5 Dob, mainly out of curiosity. It appears very similar to the Cat’s Eye 2” Cheshire , a short metal plug with a peep hole.

I was able to use it at night by shining the flashlight on the primary and then making small turns on the primary. One thing stood out to me , there was no side entrance aperture.

My first Cheshire was the Tectron included in the 3 piece 1.25” set many years ago. This unit has the side aperture, and that’s how I learned to use a Cheshire, by shining a flashlight into this opening.

Later I went with the Glatter laser and have used that for years, even better with the TuBlug.

My question is: What’s the advantage of the Farpoint style Cheshire without side aperture?

#2 Justin Fuller

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Posted 06 June 2020 - 06:40 PM

I use the 2" Farpoint Cheshire (and Autocollimator) and I don't really think there is an advantage of using one vs. the other. They both get you very close to good collimation. Personally I like the solid build of the Farpoint over the other Cheshire type.

#3 ButterFly

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Posted 06 June 2020 - 07:42 PM

1) You don't have to hold a flashlight - you can clip one onto a spider.

 

2) You can size the reflective strip on the farpoint so that it's reflection off the primary is only slightly bigger than the the centerspot.  It's easier to align when there is a slight difference at the edge of the centerspot.

 

3) You don't have to worry about losing light reflecting of a rough reflective surface of a cheshire window when you shine the light directly on the primary.  You can get brighter images.


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#4 SteveG

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Posted 07 June 2020 - 01:23 PM

1) You don't have to hold a flashlight - you can clip one onto a spider.

 

2) You can size the reflective strip on the farpoint so that it's reflection off the primary is only slightly bigger than the the centerspot.  It's easier to align when there is a slight difference at the edge of the centerspot.

 

3) You don't have to worry about losing light reflecting of a rough reflective surface of a cheshire window when you shine the light directly on the primary.  You can get brighter images.

 

Good to know. I’m considering one of these. My problem is the hole produced by my Glub is too small for the ring etched in my mirror. I find I do better using a cheap collimating cap, but now I want a 2” tool for this. The Farpoint stuff looks really nice.



#5 The Ardent

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Posted 07 June 2020 - 02:43 PM

I also have the Astrosystems Lightpipe which allows passive illumination from all exterior angles

https://www.eyepiece..._p/11401005.htm
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#6 Cames

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Posted 07 June 2020 - 04:24 PM

Ray

 

Is your primary center spot a do-nut or a triangle,or hotspot?

 

The Farpoint Cheshire, how much larger does the reflection of the white circle on on the primary mirror appear than the center actual spot?  Does the reflected black center hole appear larger than the center spot?

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#7 The Ardent

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Posted 07 June 2020 - 06:17 PM

I’m at work tonight will have to look tomorrow. 



#8 Cames

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Posted 07 June 2020 - 08:22 PM

Ray

No need to go to the trouble.  You'd have to assemble and all that.  I just thought you could remember what the collimated picture of concentricity looked like.

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#9 25585

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Posted 08 June 2020 - 02:42 AM

Farpoint collimation video using their 2 inch kit. 


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#10 Cames

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Posted 08 June 2020 - 07:08 AM

Thanks 25585

I got it.

 

At Farpoint video timestamp 4:52

 

In the video, the gap surrounding the center spot of the 10 inch Dob looked pretty big.  I'm guessing the gap would appear even wider for a large dob with a longer focal length.  A larger center spot for the Cheshire might be helpful for a bigger dob.

 

So the answer to Ray's original question must take into account more than just the ease of illumination of the face of the Cheshire.

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C



#11 Vic Menard

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Posted 08 June 2020 - 12:58 PM

...At Farpoint video timestamp 4:52

 

In the video, the gap surrounding the center spot of the 10 inch Dob looked pretty big.  I'm guessing the gap would appear even wider for a large dob with a longer focal length.  A larger center spot for the Cheshire might be helpful for a bigger dob.

 

So the answer to Ray's original question must take into account more than just the ease of illumination of the face of the Cheshire.

I believe the FarPoint Cheshire ring is sized to match the (large) CatsEye HotSpot and the CatsEye triangle. Configured this way, the Cheshire/primary mirror center marker are "calibrated" for precision alignment. This is true close to the focal plane for any focal length. If you focus too far out, the primary mirror center marker will grow in size and lose its calibration. Similarly, if you're focused too far in, the primary mirror center marker will appear (slightly) smaller. In the video, it looked (to me) like the focuser was fully racked in...


Edited by Vic Menard, 08 June 2020 - 04:29 PM.

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#12 The Ardent

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Posted 08 June 2020 - 01:30 PM

Here is a daylight view thru the Farpoint Cheshire. 
 

No matter what tool I use if I want better collimation I use a star. Usually the tools are good enough. 

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#13 Cames

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Posted 08 June 2020 - 06:30 PM

Thanks for the photo, Ray

 

Didn't mean to put you to the trouble.  Vic Menard's explanation is helpful. Your triangle seems well-matched to the white circle and if you need a closer match in size you can make it shrink or grow with the focuser.  I like the way you've illuminated the components. Makes it nice and clear to perform adjustments.

 

Now, if the perfected collimation by Cheshire can be improved by using a star, perhaps the system is equiibrating temperature-wise.  I remember reading a Starman1 post saying that his truss poles shrink a bit as he is getting started and might need to tweak collimation a little bit during the first hour or two. If I understood correctly.

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C



#14 Jason D

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Posted 08 June 2020 - 07:00 PM

Here is a GIF file I created years ago to demonstrate Vic's point. The GIF file plays two superimposed photos. One for a racked-in focuser and the other for a racked-out focuser. The relative size of the center spot compared to the cheshire ring changes.

 

6026378-center_spot_size.gif

 

For the record, I am a fan of the Hotspot shape compared -- not so much the triangle shape smile.gif

 

Jason


Edited by Jason D, 09 June 2020 - 04:55 PM.

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#15 The Ardent

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Posted 08 June 2020 - 11:23 PM

This evening I was able to easily collimate at twilight. Racked the focuser until the triangle and circular face of the collimator had the best overlap. 
I checked with the autocollimator and it looked good. 
 

The collimation was quite acceptable.

 

I plan to continue to use the Glatter+TuBlug as my primary collimation tool. It’s good to experiment with other tools and devices in case of a dead battery or “left it at home” 

 

Here is a single iPhone image of Polaris taken thru a 12mm eyepiece giving about 150x , cropped. It’s been my experience that the prominent diffraction spikes indicate  ballpark acceptable collimation. I may be incorrect.

 

Unfortunately tonight was thick persistent cirrus clouds. 

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#16 Vic Menard

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Posted 09 June 2020 - 02:38 PM

...If you focus too far out, the primary mirror center marker will grow in size and lose its calibration. Similarly, if you're focused too far in, the primary mirror center marker will appear (slightly) smaller. In the video, it looked (to me) like the focuser was fully racked in...

It has been brought to my attention that the primary mirror center marker grows in size as you rack the focuser inward and diminishes in size as the focuser is racked outward (which makes perfect sense if you think about it, but it's the reverse of what I said above). I suspect I didn't really think about it because the video clearly shows the focuser pretty much fully racked in with a smaller triangular center marker. I guess it's possible that the FarPoint Cheshire/primary mirror center marker is not calibrated, but I now suspect the video might have been manipulated to simply show "centering" of a triangular center marker inside the bright Cheshire ring. 

 

And that's what happens when I jump to (the wrong) conclusions...  

 

(Good catch Richard!)



#17 Starman1

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Posted 09 June 2020 - 04:27 PM

I recently tried the Farpoint Cheshire and in my 18” f/3.5 Dob, mainly out of curiosity. It appears very similar to the Cat’s Eye 2” Cheshire , a short metal plug with a peep hole.

I was able to use it at night by shining the flashlight on the primary and then making small turns on the primary. One thing stood out to me , there was no side entrance aperture.

My first Cheshire was the Tectron included in the 3 piece 1.25” set many years ago. This unit has the side aperture, and that’s how I learned to use a Cheshire, by shining a flashlight into this opening.

Later I went with the Glatter laser and have used that for years, even better with the TuBlug.

My question is: What’s the advantage of the Farpoint style Cheshire without side aperture?

1) 2" aperture

2) Large center hole--calibrated to the larger center markers like the Hotspot™

3) Reflective surface to create the white ring requires very little light

4) shorter tool



#18 Starman1

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Posted 09 June 2020 - 04:29 PM

I also have the Astrosystems Lightpipe which allows passive illumination from all exterior angles

https://www.eyepiece..._p/11401005.htm

The 2" ones are now sized for the larger primary center markers, too.




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