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Cheshire crosshairs

Pete D

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Pete D

Pete D

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When trying to collimate my 200P using a Cheshire collimator I cannot get the crosshairs on the Cheshire to line up with the centre doughnut when all 3 clips are in view no matter what I try. The crosshairs are always too high when you look through the Cheshire and the secondary mirror seems too low in the focuser. I can get the primary mirror correctly lined up but the crosshairs are always out. Will this effect my viewing, any advice would be appreciate thanks. I have read the focuser might need shimming out, but want to be sure before I try anything like this.

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Jason D

Jason D

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When trying to collimate my 200P using a Cheshire collimator I cannot get the crosshairs on the Cheshire to line up with the centre doughnut when all 3 clips are in view no matter what I try. The crosshairs are always too high when you look through the Cheshire and the secondary mirror seems too low in the focuser. I can get the primary mirror correctly lined up but the crosshairs are always out. Will this effect my viewing, any advice would be appreciate thanks. I have read the focuser might need shimming out, but want to be sure before I try anything like this.

You might need to move the secondary mirror up/down the tube using the spider vanes assembly central bolt. Here is what you need to do:

1- Align the cross-hairs of the cheshire with the primary mirror center spot.  

2- Examine the primary mirror reflection with respect to the secondary mirror. If the reflection is too low (shifted towards the bottom of the tube) then you need to move the secondary mirror lower -- and vice versa.

3- After moving the secondary mirror using the central bolt, you need to repeat steps 1&2. That is, never assess the position of the primary mirror reflection with respect to the secondary mirror until you have aligned the cross-hairs of the cheshire with primary mirror center spot again.

Think of the secondary mirror as a window to the primary mirror (see attached animation -- click on the image to see the animation). As long as you keep the cheshire cross-hairs aligned with the primary mirror center spot, the primary reflection position will remain stationary with respect to the focuser. What you need to do is to move the window (secondary mirror) to get the whole primary mirror reflection into view . Again, only assess the primary mirror reflection once you have completed the cross-hairs alignment with the center spot. This is an iterative process that will take time; however, you shouldn't go this process again for sometime.

Jason

post-5330-0-47295700-1382385360.gif

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Pete D

Pete D

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Thanks for all your comments. As your animation shows the secondary is in the centre of the tube but mine is towards the bottom of the tube and I can`t seem to adjust it higher. Will moving the central bolt achieve this.

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Jason D

Jason D

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No that adjusts the inward and outward travel of the mirror in the OTA the way you are describing it just sounds like the spider vanes in on plane are not right making the mirror down towards one side of the focuser.

We need to agree on what the terms "up" and "down" reference. Typically, these reference the tube axis. "down" indicates the direction towards the primary mirror -- not towards the ground.

Jason

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Jason D

Jason D

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 and I can`t seem to adjust it higher.

Why? Did the central bolt run out of room and you can no longer twist it clock-wise?

If yes, then you might need to adjust your focuser -- which requires more effort.

Jason

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Pete D

Pete D

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I have checked the spider vanes and they are spot on. I can adjust the secondary mirror to move side by side with the tilt screws but then when you adjust it to see the 3 clips and centre the crosshairs the secondary is always more to one side.

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wookie1965

wookie1965

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We need to agree on what the terms "up" and "down" reference. Typically, these reference the tube axis. "down" indicates the direction towards the primary mirror -- not towards the ground.

Jason

If you read the ops first post i think what i have said is true he has lined his secondary up but it is low in the focuser i.e top to bottom not towards the primary.

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wookie1965

wookie1965

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I have checked the spider vanes and they are spot on. I can adjust the secondary mirror to move side by side with the tilt screws but then when you adjust it to see the 3 clips and centre the crosshairs the secondary is always more to one side.

Do you mean more towards the primary or vertically.

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Moonshane

Moonshane

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your last comment suggests you have unequal adjustment in the three secondary adjusters. try to look from the side with a mirror. hopefully you'll see that one is longer than the others and they are not (as they should be) all of equal length.

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wookie1965

wookie1965

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your last comment suggests you have unequal adjustment in the three secondary adjusters. try to look from the side with a mirror. hopefully you'll see that one is longer than the others and they are not (as they should be) all of equal length.

As Moonshane has said check your secondary so it looks straight not twisted like these

post-20428-0-85478500-1382390503.gif

post-20428-0-34738500-1382390504.gif

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JED-E3

JED-E3

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Assuming it's the wire crosshairs that won't line up, I would bet the focuser is out of alignment. Try moving the focuser assembly side to side or up down by hand whilst looking through the cheshire and you'll see how flimsy the tube is.

If this does turn out to be the problem, The stock skywatcher focuser can be adjusted for alignment without using shims. 

I had to make this adjustment on my 200p dob, most collimation guides don't seem to mention it.

James.

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Moonshane

Moonshane

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bear in mind that the secondary vanes are correctly adjusted (they don't have to be all the same length as the tune might not be quite round but the ones across from each other should be the same length) when they are directly opposite each other (i.e. they form a line if you ignore the central hub) and all should be at 90 degrees to each other.

If you read the notes I added to the thread, for the next stage it makes sense to block the primary mirror. this was you cannot see the clips or the donut and they are not needed for this step.

then remove the paper and get the primary donut aligned with the cross hairs of the cheshire - these will be fuzzy.

then align the primary donut with the dot in the centre of the cheshire.

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Moonshane

Moonshane

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Assuming it's the wire crosshairs that won't line up, I would bet the focuser is out of alignment. Try moving the focuser assembly side to side or up down by hand whilst looking through the cheshire and you'll see how flimsy the tube is.

If this does turn out to be the problem, The stock skywatcher focuser can be adjusted for alignment without using shims. 

I had to make this adjustment on my 200p dob, most collimation guides don't seem to mention it.

James.

you can do this of course but it's not necessary as there is enough adjustment in the system to accommodate a few degrees of focuser misalignment. the focuser can only go up and down in one plane assuming it has no slop. if the secondary faces the same plane (i.e. is correctly adjusted) then it is not an issue.

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