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Who do I trust - Cap or Cheshire?

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7 replies to this topic

#1 dark_night

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Posted 01 March 2021 - 11:30 PM

I am new to the basics and have a 8" F5.9 Newtonian reflector. Till now i had been using the collimation cap which came with the scope to collimate. Not that I got good at it, but I got comfortable enough based on what I saw through the pin hole and star test that my scope is in decent state.

But then ... I got a long 1.25" metal cheshire with a cross hair. The view through the Cheshire seems completly wrong and the secondary does not seem circular and rather an oval and I don't even see the 3 primary clips. Don't think it's due to the focuser slop because I tried moving the chesier which does not change things much.

Now I don't know if I should trust the cheshire and I have been doing something wrong and should look to adjust the secondary.

How do I know if either of the instruments is bad or damaged ?

Edited by dark_night, 01 March 2021 - 11:33 PM.


#2 mehdymo

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Posted 01 March 2021 - 11:34 PM

Cheshire has an aluminum piece inside that has a hole to look through. Sometime that aluminum part is not coaxial with the body and gives wrong result. The best way to test it is to do a star test and see which one is more accurate. I guess the simple collimation cap is more accurate in your case.



#3 radiofm74

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Posted 02 March 2021 - 06:06 AM

… but don't rule out the Cheshire just yet. True, they may be off, but the chances of your secondary not being well aligned are probably higher. 

 

I thought I had a "bad Cheshire" for exactly the same reason as you. Then other CN members encouraged me to work harder at my collimation, and it turned out that the Cheshire was actually well-functioning, and right in its assessment that my secondary was off.

 

A Cheshire is simply a lot more demanding than the collimation cap.

 

Here's the link to the thread that helped me get my secondary reasonably well aligned: https://www.cloudyni...condary-mirror/

Astroman's collimation guide has become a sort of Bible for me.


Edited by radiofm74, 02 March 2021 - 06:09 AM.


#4 dark_night

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Posted 02 March 2021 - 07:00 AM

thanks for the replies. Its tempting to not trust the Cheshire :)

I will give it another shot over the weekend. But just wanted to confirm that cheshire will be the right tool to use to centre and align the secondary.



#5 Vic Menard

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Posted 02 March 2021 - 10:02 AM

thanks for the replies. Its tempting to not trust the Cheshire smile.gif

I will give it another shot over the weekend. But just wanted to confirm that cheshire will be the right tool to use to centre and align the secondary.

For what it's worth--the collimation cap and a Cheshire eyepiece are both primary mirror alignment tools and both should show the same alignment. For the collimation cap, the corrected primary mirror tilt alignment will show the primary mirror center marker/donut surrounding and concentric with the collimation cap pupil. For a Cheshire eyepiece the primary mirror center marker/donut will be centered and concentric with the bright Cheshire ring. Both tools magnify any residual primary mirror tilt error 2X. This crititcal alignment centers the "sweet spot" in the eyepiece field of view.

 

The part of your "Cheshire" that doesn't seem to agree with your collimation cap is the sight tube portion of your Cheshire/sight tube combination tool. The sight tube cross hairs show you where the focuser is pointing (adjusted using the secondary mirror tilt screws), and the bottom edge of the sight tube shows your secondary mirror placement (adjusted by rotating or moving the secondary mirror closer to or farther from the primary mirror end of the tube assembly). These two connected alignments are separate from the primary mirror tilt alignment, and should be completed before making the final primary mirror tilt adjustment.

 

The cross hair alignment to the the primary mirror center marker/donut delivers optimal focus ensuring the primary mirror focal plane will be perpendicular to the focuser/eyepiece axis. Together with the primary mirror tilt adjustment, these two alignments deliver image performance and "snap" focus.

 

Getting the secondary mirror correctly centered under the focuser delivers centered and balanced field illumination in the eyepiece. Of the three alignments, this one has the least impact on image performance. Unfortunately, many beginners will adjust their secondary mirror so they can see all of the primary mirror clips at the expense of the more important cross hair alignment. The visibility of the primary mirror clips by itself (without a correct cross hair alignment) is NOT a valid indicator of good alignment!

 

Getting the secondary mirror correctly centered involves the alignment of three circles: the bottom edge of the sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror. When all three circles are concentric, the secondary mirror placement will be correct and the cross hair alignment will also be correct (or very close). 

 

If your sight tube is too long (or if it can't be inserted deep enough into the focuser drawtube/2- to 1.25-inch adapter), you may not be able to see all three circles. At f/6, as long as you can see the actual edge of the secondary mirror (it helps to boost the visibility of the secondary mirror edge if you put a sheet of white paper behind the secondary mirror against the inside edge of the tube wall opposite the focuser) and the cross hair alignment, you should be able to get your collimation sorted out.

 

Use this diagram for reference: the outermost light blue circle is the bottom edge of the sight tube and blue cross hairs are the sight tube cross hairs, the green circle is the actual edge of the secondary mirror, the red circle is the reflected edge of the primary mirror, the yellow circle is the outer edge of the bright Cheshire ring, the small circle in the middle is the primary mirror center marker/donut, and the black dot in the center is the Cheshire pupil (the violet circle surrounding the bright Cheshire ring is the silhouette reflection of the secondary mirror, which appears offset toward the primary mirror end of the tube assembly--ALL of the other circles are concentric). Also note that the spider vanes will probably not coincide with the sight tube cross hairs--so don't use them as an alignment reference!

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#6 SteveG

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Posted 02 March 2021 - 01:18 PM

I am new to the basics and have a 8" F5.9 Newtonian reflector. Till now i had been using the collimation cap which came with the scope to collimate. Not that I got good at it, but I got comfortable enough based on what I saw through the pin hole and star test that my scope is in decent state.

But then ... I got a long 1.25" metal cheshire with a cross hair. The view through the Cheshire seems completly wrong and the secondary does not seem circular and rather an oval and I don't even see the 3 primary clips. Don't think it's due to the focuser slop because I tried moving the chesier which does not change things much.

Now I don't know if I should trust the cheshire and I have been doing something wrong and should look to adjust the secondary.

How do I know if either of the instruments is bad or damaged ?

In short and as others have noted:

Your Cheshire is actually a combination Site-tube / Cheshire.

 

Step 1 - Center Secondary under focuser (site tube)

Step 2 - Align focuser axis (secondary tilt and rotation) (crosshairs in site tube)

Step 3 - Align Primary axis (pupil in combo tool, or collimating cap)

 

Your secondary was likely never centered properly (step 1), but your primary axis was good because you used your collimating cap (step 3). The combo tool will help you with the first (2) steps, where the collimating cap is only good for step #3.

 

I recommend using the combo tool for the first (2) steps, then finish the primary using your collimating cap, which is easier to read IMO.


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#7 MellonLake

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Posted 02 March 2021 - 03:25 PM

Often the pupil of Cheshire (Combo Tools) are too large as well. For this reason I prefer the smaller pupil of the collimation cap for Step #3 (the Pupil is the size of the dark shadow that gets centred in the mirror marker.  As Steve G says the Combo tool ((Cheshire with crosshairs that can work as a sight tub)) is better for Steps #1 and #2.   

 

 

Rob    


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#8 Vic Menard

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Posted 02 March 2021 - 03:46 PM

Step 1 - Center Secondary under focuser (site tube)

Step 2 - Align focuser axis (secondary tilt and rotation) (crosshairs in site tube)

Step 3 - Align Primary axis (pupil in combo tool, or collimating cap)

If I may add, you can start on step 1 or step 2--regardless which step you start with, you'll likely have to repeat these two steps two or three times, maybe more, until you get both corrected (and always end with aligning the focuser axis). I also prefer to separate the adjustments for each alignment--so I use:

 

Step 1 - Center the secondary under the focuser using the bottom edge of the sight tube and the reflected edge of the primary mirror as reference circles. Use only rotation and/or position closer to or farther from the primary mirror end of the tube assembly--do NOT use the tilt adjustment.

 

Step 2 - Align the focuser axis using the sight tube crosshairs. Use only secondary mirror tilt adjustment to bring the primary mirror center marker/donut into alignment with the sight tube cross hairs--do NOT use rotation and/or position closer to or farther from the primary mirror end of the tube assembly. 

 

By separating the adjustments to accomplish the specific alignments, repetition will systematically reduce both errors until both are corrected. If you don't separate the adjustments, you can get caught up in an endless loop that never resolves.

 

When step 1 and step 2 are complete, ALWAYS finish with step 3.

 

For the record, the common cold of secondary mirror misalignment is a combined tilt/rotation error (both secondary mirror placement and focuser axis involved). Once you recognize it, and know how to correct it, Newtonian collimation will seem easy. I suggest you read the first 6 posts (and play the animations) in this discussion:  https://www.cloudyni...ment/?p=2742900


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