Collimation with Cheshire on 200P - what am I doing wrong?

[Deisler]

Hi All,

Just received my 200P. Spent 3 hours on collimation following a few different tutorials (including those recommended here), but felt I might be doing something wrong as the image through cheshire is not completely centered, so maybe you guys can help me....

Here is what I did:

1) Put a white paper in the tube, then tilt the secondary mirror with the central screw to make it look like a circle;

2) use the three screws next to the central screw to make the three "clips" appear symmetrical. also, the little ring (at the centre of the primary) appears to be under the cheshire cross.

3) adjust the primary mirror so the support structure of the secondary mirror and secondary mirror itself appear symmetrical in Cheshire.

Am I missing anything?

The reason I suspected I did something wrong is:

1) The little white cicle (which I assume is the reflection of cheshire) is not completely centered in the little black circle (is that reflection of secondary?). How do I get this right? It is not too bad but I struggled to have fine tuning of this....

2) I cannot put all three clips in my view at the same time? I adjusted the secondary mirror so that three clips are at equal distance away from the center. Is that ok?

3) The white field, which I assume is the primary mirror, is not centered in Cheshire "view" (those very faint reflection on the Cheshire tube). I cannot adjust that whatsoever? Could it be misalignment of my focuser or Cheshire eyepiece?

4) One of the three adjusting screws for the primary mirror seems to be not working very well (I cannot tighten as much)? The other two work fine, and can compress against the rubber rings behind them. This one does not compress onto the rubber ring that much. Is it a problem?

I used my phone to take a phone through Cheshire. Apologies for poor quality.

Regards

Deisler

 

Edited November 11, 2019 by Deisler

[Adam J]

2 hours ago, Deisler said:

Hi All,

Just received my 200P. Spent 3 hours on collimation following a few different tutorials (including those recommended here), but felt I might be doing something wrong as the image through cheshire is not completely centered, so maybe you guys can help me....

Here is what I did:

1) Put a white paper in the tube, then tilt the secondary mirror with the central screw to make it look like a circle;

2) use the three screws next to the central screw to make the three "clips" appear symmetrical. also, the little ring (at the centre of the primary) appears to be under the cheshire cross.

3) adjust the primary mirror so the support structure of the secondary mirror and secondary mirror itself appear symmetrical in Cheshire.

Am I missing anything?

The reason I suspected I did something wrong is:

1) The little white cicle (which I assume is the reflection of cheshire) is not completely centered in the little black circle (is that reflection of secondary?). How do I get this right? It is not too bad but I struggled to have fine tuning of this....

2) I cannot put all three clips in my view at the same time? I adjusted the secondary mirror so that three clips are at equal distance away from the center. Is that ok?

3) The white field, which I assume is the primary mirror, is not centered in Cheshire "view" (those very faint reflection on the Cheshire tube). I cannot adjust that whatsoever? Could it be misalignment of my focuser or Cheshire eyepiece?

4) One of the three adjusting screws for the primary mirror seems to be not working very well (I cannot tighten as much)? The other two work fine, and can compress against the rubber rings behind them. This one does not compress onto the rubber ring that much. Is it a problem?

I used my phone to take a phone through Cheshire. Apologies for poor quality.

Regards

Deisler

 

Looks like the secondary mirror is rotated is this image. From the look and f the shadow and the location of the draw tube relative to the primary mirror reflection. 

[Alan64]

Yours is a 200mm f/5, my own is a 150mm f/5; here's the collimation-cap scene of my own...

At f/5, the secondary-mirror is off-set.  Note that the lighter circle is not centred within the black circle.  That's the off-setting, and it's correct and normal.  

Here's the cross-hair(sight-tube) scene of my 127mm f3.3 or f/4...

Note that the smaller circle is definitely not centred within the larger.  Faster, shorter Newtonians have off-set secondary-mirrors, and the off-setting occurs automatically as you collimate, thank goodness.

Both of those are well-collimated.  Use them as a guide if you wish.

[JOC]

If you check Astrobaby's guide http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/ you will find her perfect collimation still leaves the visual view of the mirrors slightly off centre as your's are - this is fine - I left mine off centre in the same way and had no issues. 

[Ricochet]

12 hours ago, Deisler said:

Put a white paper in the tube, then tilt the secondary mirror with the central screw to make it look like a circle;

This is wrong. The centre screw determines the "height" of the mirror up and down the tube. If you have twisted the screw to turn the mirror then it is most likely no longer facing the focuser properly. Once you have the height set using the centre screw you use the three outer screws to make the secondary appear circular. 

[Deisler]

35 minutes ago, Alan64 said:

Yours is a 200mm f/5, my own is a 150mm f/5; here's the collimation-cap scene of my own...

At f/5, the secondary-mirror is off-set.  Note that the lighter circle is not centred within the black circle.  That's the off-setting, and it's correct and normal.  

Here's the cross-hair(sight-tube) scene of my 127mm f3.3 or f/4...

Note that the smaller circle is definitely not centred within the larger.  Faster, shorter Newtonians have off-set secondary-mirrors, and the off-setting occurs automatically as you collimate, thank goodness.

Both of those are well-collimated.  Use them as a guide if you wish.

Thank you, Alan. This is very useful indeed. Learning new things everyday from you guys!

One thing I also struggled when I adjusted my secondary mirror was that I cannot make sure three clips are symmetrical in my view, as my cheshire seems to have too high magnification. I can only see one clip at a time at most. 

Does that mean for that purpose, I will have to use a collimation cap as you suggested?

Cheers

Deisler

 

[Deisler]

32 minutes ago, JOC said:

If you check Astrobaby's guide http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/ you will find her perfect collimation still leaves the visual view of the mirrors slightly off centre as your's are - this is fine - I left mine off centre in the same way and had no issues. 

Looking at his image through cheshire, it appears mine is not too dissimilar to his?

Only thing is my little small ring is slightly off-centred, although not by much? Will try to correct that.

Thank you. 

[Deisler]

25 minutes ago, Ricochet said:

This is wrong. The centre screw determines the "height" of the mirror up and down the tube. If you have twisted the screw to turn the mirror then it is most likely no longer facing the focuser properly. Once you have the height set using the centre screw you use the three outer screws to make the secondary appear circular. 

When I turned the centre screw, it also turned the mirror. So I had to hold the base of the secondary mirror to turn it back and make it appear circular. 

One thing I noticed - when I adjusted the 'height' of the 2nd mirror, it can only move left-right in my Cheshire view, but not up and down. 

Then I adjusted three outer screws to move the mirror to the centre of my view.

Maybe this is what I did wrong?

[JOC]

14 minutes ago, Deisler said:

mine is not too dissimilar to his

That's what I thought.  That is the most highly respected guide anywhere for collimating a telescope - anyone that follows every step to the letter no matter how daft it sounds should be successful.  In addition folks like you and I are lucky as the guide is written using the same scopes as we have.  Sometimes you have to loosen the centre screw of the secondary to get the outer three to move, also make sure your spider is central as described in the guide.  I find when I make the final tighten on the secondary screws it has a tendency to just give a final shift to the mirror, there seems no way around this on my own scope so now I assess what the final movement will be and account for this when I position the secondary.  I find a key thing is the bit about being able to see the clips that hold the primary.  Actually on my system the clips are literally just out of view, but despite this is is easy to tell when they are central as all mine are just about disappeared from view to the same degree.  Finally it is worth noting that there is an approved orientation for the Cheshire in the EP holder - you may or may not have caught up with this instruction, but it can make a difference if it is not correctly oriented.

[banjaxed]

7 minutes ago, JOC said:

  Finally it is worth noting that there is an approved orientation for the Cheshire in the EP holder - you may or may not have caught up with this instruction, but it can make a difference if it is not correctly oriented.

Could you please explain the approved orientation, thank you.

[Deisler]

18 minutes ago, JOC said:

also make sure your spider is central as described in the guide. 

Oh I thought, after reading a few other replies here, that the spider does not have to be central as it gives the secondary mirror the off-set it needs? Or I misunderstood where the off-set is from?

21 minutes ago, JOC said:

I find a key thing is the bit about being able to see the clips that hold the primary.  Actually on my system the clips are literally just out of view, but despite this is is easy to tell when they are central as all mine are just about disappeared from view to the same degree. 

I struggled to get the clips central as I can only see one or none, never three at the same time... As @Alan64 suggested, I might need collimation cap to do this.

23 minutes ago, JOC said:

Finally it is worth noting that there is an approved orientation for the Cheshire in the EP holder - you may or may not have caught up with this instruction, but it can make a difference if it is not correctly oriented.

I rotated my Cheshire just to get the cross orientated in the same way as the 'cross' of my secondary mirror. Is that correct?

 

Cheers,

Deisler

[JOC]

At the moment I'm falling short of finding the thread where it was mentioned.  However, IIRC there are marks/screws on the EP holder and marks on the Cheshire I think one on one item and two on the other, and the notion is to get the single mark between the two on the other object so the Cheshire is balanced at the midway point.   Currently annoyed I can't find the info quickly, but I'm sure the orientation of the Cheshire in the EP has a correct method - perhaps others will chip in if I can't find it.

[JOC]

1 minute ago, Deisler said:

Oh I thought, after reading a few other replies here, that the spider does not have to be central as it gives the secondary mirror the off-set it needs? Or I misunderstood where the off-set is from?

Follow Astrobaby - the guide starts off by getting the spider central IIRC.

1 minute ago, Deisler said:

I struggled to get the clips central as I can only see one or none, never three at the same time... As @Alan64 suggested, I might need collimation cap to do this.

You can do it without the collimation cap, as I suggested from my own experience.  It is possible to see when you have it right when all three clips just disappear out of view - the fit is that tight, that providing you can't see any it will be central

3 minutes ago, Deisler said:

I rotated my Cheshire just to get the cross orientated in the same way as the 'cross' of my secondary mirror. Is that correct?

See above - pity mine isn't out at the moment I'd be able to take a photo, but perhaps someone else will chip in.

[banjaxed]

16 minutes ago, JOC said:

At the moment I'm falling short of finding the thread where it was mentioned.  However, IIRC there are marks/screws on the EP holder and marks on the Cheshire I think one on one item and two on the other, and the notion is to get the single mark between the two on the other object so the Cheshire is balanced at the midway point.   Currently annoyed I can't find the info quickly, but I'm sure the orientation of the Cheshire in the EP has a correct method - perhaps others will chip in if I can't find it.

Thanks for your reply. After reading quite a few articles on collimation I cannot remember seeing any information on the orientation of the Cheshire. It will be interesting to find out what it is and how it affects the prceedure.

[joe aguiar]

I would have left it from factory.

It holds collamation good. After that I norm only do it once a year. If your only going from your living room to backyard it's not gonna mis align.

If you put it in your trunk of the car and drive hours even weekend with it then I can see rough roads making it un align.

Joejaguar 

[Alan64]

7 hours ago, Deisler said:

Thank you, Alan. This is very useful indeed. Learning new things everyday from you guys!

One thing I also struggled when I adjusted my secondary mirror was that I cannot make sure three clips are symmetrical in my view, as my cheshire seems to have too high magnification. I can only see one clip at a time at most. 

Does that mean for that purpose, I will have to use a collimation cap as you suggested?

Cheers

Deisler

 

First off, yours is a 200mm f/6 Newtonian-Dobson; my apologies for thinking that it was the 200P OTA, and at f/5.  The secondary off-setting for an f/6 will not be as "wild" as you saw within my images of an an f/5 and f/4, but there is an off-set nonetheless, just not as noticeable.  However, within your first image, it appears as an f/4 even, hence my thinking that it was an f/5.  The off-setting should not be that drastic for an f/6.  The small, lighter circle should appear more centred over the larger, black circle compared to the f/5 and f/4, although not close to being perfectly centred.  

Here's a mock-up I made with the scene from my 150mm f/5...

If I had a 150mm f/6, the scene would appear as that on the right.  The secondary-mirror(black circle) would be smaller, and the underside of the Cheshire or collimation-cap(smaller, grey circle) would be more centred within the black, but again, not perfectly centred.

You may not be able to see the primary-mirror's clips due to the focal-length of the telescope, and at 1200mm.  You can try to draw the Cheshire back away from the secondary-mirror, but then the tool may not be held within the drawtube as securely.  It may wobble, depending on how far you draw it away.  

A collimation-cap is, reputedly, easier to use over a Cheshire, but I do love my sight-tube with its cross-hairs.  I would suggest getting a cap...

https://www.firstlightoptics.com/other-collimation-tools/rigel-aline-collimation-cap.html  

You can also make one, and with the dust-cap from a 1.25" focusser, or from the 1.25" adaptor for the 2" focusser of your kit.  You would drill a 2mm hole precisely as possible in the centre of the cap, then line the underside with a circle of aluminum-foil, the dull or shiny side; try both.  This is my own collimation-cap, and that came with my 150mm f/5...

That's a 2mm-diameter peep-hole there.  You may find that easier to use, in the beginning.

Incidentally, the faster and shorter the Newtonian, the larger the secondary-mirror and the greater its off-setting.  The reason the mirror has to be larger with a shorter instrument is due to the fatter tip of the light-cone from the primary-mirror once it reaches the secondary-mirror...

See what happened when I shortened the tube.  Look at that monstrous tip of light heading towards that now too-tiny secondary-mirror.  You can't get past the ol' physics, else I would have a 200mm f/2, and for those bright, ultra low-power and ultra-wide views of the galaxy in Andromeda...

That's not happening, I'm afraid.

Know your Newtonian; its innards...

It's the secondary-assembly and -mirror that gives the most fits.  Master it; you will, in time.

[Ricochet]

23 hours ago, Deisler said:

When I turned the centre screw, it also turned the mirror. So I had to hold the base of the secondary mirror to turn it back and make it appear circular. 

One thing I noticed - when I adjusted the 'height' of the 2nd mirror, it can only move left-right in my Cheshire view, but not up and down. 

Then I adjusted three outer screws to move the mirror to the centre of my view.

Maybe this is what I did wrong?

That is correct. Really you should adjust the centre screw, which pulls on the mirror, and the outer screws, which push on the mirror, sequentially so that it does not rotate. As you have found it is sometimes easier to hold the secondary support with your hand, but you must be sure not to touch the mirror itself. 

You should get the secondary support centred as a starting point. The offset should be built into the support and not something you need to worry about. 

23 hours ago, Deisler said:

Thank you, Alan. This is very useful indeed. Learning new things everyday from you guys!

One thing I also struggled when I adjusted my secondary mirror was that I cannot make sure three clips are symmetrical in my view, as my cheshire seems to have too high magnification. I can only see one clip at a time at most. 

Does that mean for that purpose, I will have to use a collimation cap as you suggested?

Cheers

Deisler

 

Align the Cheshire cross with the mark on the primary mirror. 

22 hours ago, JOC said:

At the moment I'm falling short of finding the thread where it was mentioned.  However, IIRC there are marks/screws on the EP holder and marks on the Cheshire I think one on one item and two on the other, and the notion is to get the single mark between the two on the other object so the Cheshire is balanced at the midway point.   Currently annoyed I can't find the info quickly, but I'm sure the orientation of the Cheshire in the EP has a correct method - perhaps others will chip in if I can't find it.

Not true. If the orientation of the Cheshire makes a difference the Cheshire is faulty and should be binned. The only consideration is to angle the face so that it reflects a suitable amount of light. 

[JOC]

13 minutes ago, Ricochet said:

Not true. If the orientation of the Cheshire makes a difference the Cheshire is faulty and should be binned. The only consideration is to angle the face so that it reflects a suitable amount of light. 

Hi @Ricochet looks like you could be right and faulty memory on part - however, I have satisfied myself and found where the memory came from:  

Startlight 1's comment - 5th entry down.  It was when I'd tried to use a laser collimator which apparently did have a correct orientation.  Looks like you are good to go with a Cheshire no matter which way it's put in providing you've got enough light for it.  I do still think it needs to be an accurate central fit in the EP holder though.