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Modification of Sight Tube/Combination Tool To Match my Scope

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#1 MellonLake

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Posted 11 January 2019 - 03:41 PM

I just got my first telescope a couple months ago and have learned a lot about collimation.  I purchased a Celestron combo tool (Cheshire/sight tube) but it is way too long form my telescope (F4.7 10 in. dob).  I can't see the whole secondary/mirror clips in the tool.  I have now been thinking further about making a combo tool specifically for my scope by cutting down a long combo tool.  

 

I am going to use the paper tube method + collimation cap to try and figure out the correct length for the tube. I will attach a 1.5 in. paper tube to the collimation cap and determine how long a tube I need to see just see the 1) edge of the primary in the secondary, 2) edge of the secondary and 3) edge of the sight tube as closely spaced concentric rings.  I will then cut down the length of an inexpensive combination tool (Cheshire) to make it the correct length and then install new cross-hairs in the end of the tube.

 

Will this work?  Am I missing anything?



#2 Couder

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Posted 11 January 2019 - 04:04 PM

Unless I'm missing something, you only need to see the center of the mirror. Most people put a dot of some kind there, like a small piece of the sticky paper from a computer label.



#3 MellonLake

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Posted 11 January 2019 - 04:52 PM

Yep the dot on the primary is important for collimating the primary which is the last step in collimation.  However, to collimate the secondary, the secondary needs to centred (Step #1) and aligned with the focus tube axis (Step #2).  My understanding is the best way to do this is a sight tube (the sight tube portion of the combo tool).  By using a sight tube you can ensure that the secondary is well centred under the focuser and that the secondary is pointing squarely at the primary (i.e. seeing the mirror clips evenly at the periphery of the secondary).  Once the secondary is aligned you can then use the Cheshire component of the combo tool (or a laser collimator) to align the primary (Step #3 or pointing the primary squarely at the secondary).   

 

Phew... I think I have that right... I have spent way to much time learning collimation in the last 2 months!    



#4 airbleeder

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Posted 11 January 2019 - 07:30 PM

    Try extending the combo tool in the focuser and check concentricity. My 1.25" tool is too short, but I still need to extend it some to bring the primary reflection to size of secondary.



#5 jtsenghas

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Posted 11 January 2019 - 07:33 PM

If your secondary is properly sized for your scope you'll want the length of your sight tube to be approximately your focal ratio times the inside diameter of your tube. This way you'll only need to move it outwards a bit past focus to see the entire secondary for the centering process.

 

Alternatively  you can decide how far out you want your peephole when you use the tool to just show the entire secondary and use similar triangles to calculate the length required based on your secondary size. The ratio of the distance of the peephole to the secondary center divided by the secondary minor axis should be the same as the sight tube length divided by its inside diameter. 


Edited by jtsenghas, 11 January 2019 - 07:40 PM.


#6 jtsenghas

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Posted 11 January 2019 - 07:50 PM

You might simply want to make a second tool for the centering process.  If you leave your existing tool unmolested you won't have to duplicate the crosshair alignment and the longer tool will be a little more accurate for the collimation process.

 

A basin drain tube from the hardware store is typically 1 1/4" OD and will fit your focuser. If the inside diameter is 1 1/8" you'll want a length around 5.3". There are numerous ways you could make a peephole for it including adapting a medicine bottle or bottle cap to it with a precisely melted hole from a hot nail pushed on center.


Edited by jtsenghas, 11 January 2019 - 07:52 PM.

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#7 Vic Menard

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Posted 11 January 2019 - 07:54 PM

...Alternatively  you can decide how far out you want your peephole when you use the tool to just show the entire secondary and use similar triangles to calculate the length required based on your secondary size. The ratio of the distance of the peephole to the secondary center divided by the secondary minor axis should be the same as the sight tube length divided by its inside diameter. 

For all three circles (bottom edge of the sight tube, actual edge of the secondary mirror, and the reflected edge of the primary mirror) to be the same size, the pupil will need to be placed at the apex. You can find the math here:  http://www.vicmenard...rspectives.html   scroll down to, "Notes on matching a sight tube to your 'scope's focal length" Once you've determined the apex distance, you can determine the effective focal ratio of the cone, and apply this to the length of the sight tube (remember to use the clear aperture, not the outside diameter). If the fully illuminated image size is a point (or very small), the pupil position/apex will be located at (or near) the focal plane.

 

While positioning the pupil at the apex will also position the secondary mirror with a more precise offset, I prefer to position the pupil slightly inside the apex, allowing the three circles to be viewed as incremental steps. This still delivers good precision, and, in my opinion, improves the alignment read.



#8 MellonLake

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Posted 11 January 2019 - 09:45 PM

Thanks for the info all.  Vic your web page was very helpful.  I will go through the measurements and calculations before making my sight tube.  I think I will stick to it being a sight tube rather than a combo tool.   Thanks all


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