Newtonian Collimation.......again (Sorry)

[AdeKing]

I'm ashamed to admit that I've had an 8" Dob (Skywatcher 8" f/6) for several years and stopped using it for a number of reasons, but one of those reasons was that I've always had trouble nailing collimation.

Now that I've done a StarSense Explorer mod on it, I'm keen to sort collimation and put it back in to regular use.

So, I've been following the Astrobaby collimation guide and have got the collimation as close as I can, but I'm having trouble getting the circular reflection of the secondary mirror central when viewed through a Rigel collimation cap.

I have previously used a Catseye 2" Telecat and also a 1.25" Cheshire for collimation, but in my latest round of collimation I've found a Rigel Aline collimation cap easier to use.

I'd previously replaced the clamp on the end of the focuser drawtube with one of the Astro Essentials 2" compression adaptors and have realised that it didn't hold the original Skywatcher extension tube needed to reach focus with 2" eyepieces square to the drawtube, so I've now swapped the original spacer with a Revelation one of the same length that has parallel sides that fit into the drawtube rather than a dovetail-type fitting at the focuser end.  This means that the extension is now held square to the end of the drawtube.

I'm using a Howie Glatter Parallizer to hold 1.25" eyepieces in the drawtube, and this is in place when using the collimatrion cap.

This has helped a bit, but here is an overexposed image showing the view through the collimation cap to illustrate what I'm talking about.

To my eyes, the reflections show a good collimation, BUT the reflections are not central in the field of view through the Colli-Cap, they are too close to the top of the image.

I did replace the stock single speed focuser with one of the Skywatcher dual speed units some time ago, could I have introduced an error to the alignment when fitting the dual-speed focuser which might account for the reflections being shifted towards the top of the image?

This focuser also seems to have some grub screws on the focuser that I assume can be used to adjust the tilt of the focuser.

To clarify, I have used the adjusters on the end of the secondary Vanes to laterally shift the location of the mirror, but run out of adjustment before it is centred.  I have since re-adjusted the secondary by double-checking the distance from the inside edges of the OTA to the centre of the secondary mount adjustment screw (as per Astrobaby's guide) and this confirms that it is centrally mounted within the OTA.

If anyone can give me suggestions on what the problem might be and how I might fix it, that would be much appreciated.

Thanks in advance,

Ade

 

[cwis]

Could you rotate the secondary mirror down a little? Either that or tilt the focuser tube upwards (if you have that adjustment). 

[Pixies]

Hi,

You need to make sure that the secondary mirror is directly under the focuser (and rotated to face it) before you try and centre the image of the primary in it. In Astro baby's guide, that's "step 2" with the coloured paper. It's distance up/down the tube looks pretty near, but you need to 'lower' the secondary using the 3 adjusting screws.

A long Cheshire will help here, acting as a sight tube. You can use it to indicate how concentric you have the secondary. For example: 

[Mark at Beaufort]

Ade I am surprised you have asked this question because the 6" Newt I bought from you had perfect collimation - you must have done something right at some stage

When I collimate I always place a coloured piece of paper down the tube to stop the reflection of the primary. I then place another piece of different coloured paper opposite the focuser to determine that the secondary is underneath the focuser. Looking at your image it appears that the secondary is not totally in the centre. Measure the supporting veins to see that they are equal and then look again to see if the problem has been solved. You are pretty close to be honest.

Remove all the coloured paper and adjust the primary if necessary.

It looks like @Pixiesis showing a TS Concenter???  in the first photo above which explains the secondary position very well.

[Pixies]

Just now, Mark at Beaufort said:

It looks like @Pixiesis showing a TS Concenter???  in the first photo above which explains the secondary position very well.

Nope - just applying Mire de Collimation to the original image. It's a useful way of checking concentricity with an image from a collimation cap.

[Mark at Beaufort]

6 minutes ago, Pixies said:

Nope - just applying Mire de Collimation to the original image. It's a useful way of checking concentricity with an image from a collimation cap.

Pixies that is impressive

[jetstream]

"Now that your focuser is exactly square to the tube, it’s on to the secondary mirror.  This is where things get tricky.  Should you offset the diagonal, or should you not? "

http://umich.edu/~lowbrows/reflections/2003/dscobel.10.html

 

Is your diagonal offset Ade? and is your focuser square?

Edited November 17, 2020 by jetstream

[johninderby]

Most modern newts have the secondary mirror mounted offset on the holder so just centre the holder in the tube.

[Peter Drew]

If anyone is unclear why there can need to be an offset just draw a cone with a centre line.  Then cut through the cone with another line at 45 degrees to represent the diagonal mirror.  You will notice that where this line touches the edge of the cone, the top radius is smaller than the bottom radius.  Offsetting the mirror towards the wider radius makes sure that the full amount of the aperture is transmitted to the eyepiece.  If the diagonal mirror is oversize then offsetting is not important.    🙂

[Pixies]

Not wanting to take the OP's topic off-track. 

9 minutes ago, Peter Drew said:

If anyone is unclear why there can need to be an offset just draw a cone with a centre line.  Then cut through the cone with another line at 45 degrees to represent the diagonal mirror.  You will notice that where this line touches the edge of the cone, the top radius is smaller than the bottom radius.  Offsetting the mirror towards the wider radius makes sure that the full amount of the aperture is transmitted to the eyepiece.  If the diagonal mirror is oversize then offsetting is not important.    🙂

A Peter says, the issue is that when you intersect a cone, the ellipse is not centred over the axis of the cone. If you imagine a cone of light from the primary up towards the secondary, you will either need an oversized secondary mirror to capture the full area of the intersection, or an offset elliptical secondary. 

Follow this link and find the interactive diagram at the bottom of the page:

https://www.geogebra.org/m/NnxHw6gY

If you rotate the image so that you are looking down the axis of the cone, the elliptical intersection is offset.

the faster the scope (the shorter the cone) - the more the secondary is offset.

[Pixies]

But to make it clear to anyone wondering. The majority of modern Newtonian reflectors have this offset built in to the secondary mirror holder. You don't need to worry about it when collimating - just align the secondary concentrically within the focuser. As @johninderbysaid.

Edited November 17, 2020 by Pixies

[AdeKing]

3 hours ago, cwis said:

Could you rotate the secondary mirror down a little? Either that or tilt the focuser tube upwards (if you have that adjustment). 

Thanks, this is what I'm struggling to work out, I'm trying to work out which adjustment allows me to move "Up" and "Down" in this image.  This makes me wonder whether the focuser is centred properly in the focuser hole in the OTA.

2 hours ago, Pixies said:

You need to make sure that the secondary mirror is directly under the focuser (and rotated to face it) before you try and centre the image of the primary in it. In Astro baby's guide, that's "step 2" with the coloured paper. It's distance up/down the tube looks pretty near, but you need to 'lower' the secondary using the 3 adjusting screws.

A long Cheshire will help here, acting as a sight tube. You can use it to indicate how concentric you have the secondary. For example: 

This is where I'm struggling to find the correct adjustment sequence.

To help with orientation, in the image in my first post the fixed part of the secondary holder is to the Left of the image, so loosening the central screw on the secondary holder moves the secondary from left to right, so I can see from your first image that I need to adjust the central screw to move the secondary slightly to the left, but its the up and down adjustment that I'm struggling with.

I can improve matters slightly by adjusting two of the three screws on the secondary holder to tilt the secondary mirror "downwards" in my image above, BUT I then need to adjust these a second time to centre the reflection of the primary mirror in the next step of the process.

Astrobaby also states that after adjusting the secondary screws that you should check that the two faces of the secondary assembly stay parallel.  Doing the adjustment above causes one of the faces to tilt relative to the other if that makes sense, so it seems wrong.

2 hours ago, Mark at Beaufort said:

Ade I am surprised you have asked this question because the 6" Newt I bought from you had perfect collimation - you must have done something right at some stage

When I collimate I always place a coloured piece of paper down the tube to stop the reflection of the primary. I then place another piece of different coloured paper opposite the focuser to determine that the secondary is underneath the focuser. Looking at your image it appears that the secondary is not totally in the centre. Measure the supporting veins to see that they are equal and then look again to see if the problem has been solved. You are pretty close to be honest.

Remove all the coloured paper and adjust the primary if necessary.

It looks like @Pixiesis showing a TS Concenter???  in the first photo above which explains the secondary position very well.

Thanks for your comment Mark, this is what I can't work out. 

I had no problems whatsoever with collimating the 6" f/5 Newt you had from me, it was beautifully easy to collimate.  The 8" should be the same, but its proved to be an absolute pig to work with and I've had nothing but problems with it which makes me think that there is something amiss somewhere, but I'm struggling to work out what and where.

I have measured the length of the 4 secondary vanes and the centre of the adjustment screw is an equal distance from the inside edge of the OTA along all of the secondary assembly vanes.

46 minutes ago, Pixies said:

But to make it clear to anyone wondering. The majority of modern Newtonian reflectors have this offset built in to the secondary mirror holder. You don't need to worry about it when collimating - just align the secondary concentrically within the focuser. As @johninderbysaid.

I'm pretty sure that this is how the offset is taken into account in my Skywatcher 8" f/6 Dob, so I've been just centring the secondary holder until its centred in the OTA. 

As Mark said above, I didn't struggle with collimating a faster 6" f/5 that I sold to him, its just this particular OTA that is proving to be an issue which is why its been so frustrating.

The primary adjustment is also different from the 6" f/5 that I used to have, the 8" does not have springs under the primary mirror, only some rubber washers, so there is very little room for adjustment on the primary mirror and when the primary is aligned with the secondary as per my image at the top of the post, there is virtually no adjustment left in one of the three primary adjustment screws, but plenty of adjustment left in the other two screws.  This also suggests that something is amiss somewhere, though this may be unrelated to the issue I'm struggling with at the moment.

[jetstream]

The pursuit of perfect collimation can be a hard road to follow. Try taking a good, collimated laser and tighten the focuser screws watching if the laser dot moves around the primary a bit...

If you really want to check perfect closed path collimation use a 2 hole autocollimator.

These days I collimate simply and quickly and the results are vg, allowing very high mag. My autocollimator sits in the box now, after learning how to use it and knowing my scopes are capable of this level of collimation.

Ade, I find observing more rewarding than the pursuit of perfect collimation.

[jetstream]

7 minutes ago, AdeKing said:

only some rubber washers

The mirror cell must "float" IMHO

[Pixies]

Hi. As you say, the secondary holder and the assembly attached to the vanes should be roughly 'parallel'. You can normally check this by seeing that the 3 adjuster screws are close to even levels.

Another way is to wind them all out and tighten the central screw, bringing the secondary back up the tube. Bring it as far back as it can go, then do the opposite, but making sure you tighten the adjusters by equal amounts as the secondary moves back down the tube. That way, it should remain parallel.

Can you carefully get your phone inside the tube and take a pic of the secondary base?

[AdeKing]

On 17/11/2020 at 18:02, jetstream said:

The pursuit of perfect collimation can be a hard road to follow. Try taking a good, collimated laser and tighten the focuser screws watching if the laser dot moves around the primary a bit...

If you really want to check perfect closed path collimation use a 2 hole autocollimator.

These days I collimate simply and quickly and the results are vg, allowing very high mag. My autocollimator sits in the box now, after learning how to use it and knowing my scopes are capable of this level of collimation.

Ade, I find observing more rewarding than the pursuit of perfect collimation.

I agree wholeheartedly with you.

I'm not after absolutely perfect collimation as I have no interest in imaging, I'm strictly a visual only guy 😉. 

My issue is that it seems wrong that the circular reflection from the secondary is not central through the Cheshire or Collimation Cap and this just seems wrong to me, so I'm trying to work out where the problem lies.  And knowing that this is the situation is bugging me.

I did look at an autocollimator several years ago, but decided that I'd drive myself crazy with it, so I stepped away from that thought.

On 17/11/2020 at 19:15, Pixies said:

Hi. As you say, the secondary holder and the assembly attached to the vanes should be roughly 'parallel'. You can normally check this by seeing that the 3 adjuster screws are close to even levels.

Another way is to wind them all out and tighten the central screw, bringing the secondary back up the tube. Bring it as far back as it can go, then do the opposite, but making sure you tighten the adjusters by equal amounts as the secondary moves back down the tube. That way, it should remain parallel.

Can you carefully get your phone inside the tube and take a pic of the secondary base?

 

So a bit of a development today, though I can't call it progress.

Firstly, I loosened the four bolts attaching the focuser to the OTA to check for excessive play to see whether an error could have crept in there when I fitted the upgraded focuser.  However, this showed almost no lateral play in the position of the focuser base plate when the bolts were loose, so I duly tightened everything up.

As per @Mark at Beaufort suggestion, I placed white card down the OTA to block the reflection of the primary mirror and make things clearer.

Then, as per @Pixies suggestion above, I loosened all secondary adjusters off until they were level with the bottom of the secondary holder attached to the vanes, then tightened the central screw until the movable part of the secondary assembly was hard against the fixed part of the assembly between the vanes (hopefully this makes sense), so at that point everything should have been parallel.

Next I carefully turned the central screw until the reflection in the secondary was roughly centred in the view of the Cheshire and rotated the secondary until the reflection presented as a circle, then slowly and carefully tightened the three secondary adjusters by equal amounts until I could feel the slightest of pressure as they touched the top of the secondary mirror holder. 

I then made a minor tweak to one of the three adjusters to tilt the mirror assembly down towards the primary very slightly to get an even space all around the circular reflection through the Cheshire, but essentially all good so far.

At this point the image as seen through the Cheshire is shown below and this looks good to me, the reflection is circular in shape with an almost even gap between the edge of the reflection and the inside of the Cheshire sight tube and I thought this looked "Textbook".

Once the card baffle was removed from the OTA to reveal the primary reflection, I see the following.

 

At this stage, I move onto Stage 2 and use the three secondary adjuster screws to tilt the secondary mirror until I get a view of the whole primary mirror. 

This is where my problems begin.......  Once the secondary adjusters have been tweaked to show the whole of the primary mirror reflection then the circular secondary reflection is shifted "Up" in the image and I am back to where I was yesterday with the circular reflection shown by the secondary no longer being central in the Cheshire sight tube as shown in the image below (Apologies for the reflection of trees in the primary mirror).

At this stage I have stopped as I seem to be back at square one where I could continue to adjust the primary, but the reflection shown in the secondary is still not centred in the circular view through the Cheshire and logic suggests that it should be.

This is where I seem to be going wrong and can't work out what I'm doing wrong, or whether there is something wrong elsewhere.

On 17/11/2020 at 19:15, Pixies said:

Can you carefully get your phone inside the tube and take a pic of the secondary base?

I did try, but it is far too close to the secondary assembly to reach focus.

Hopefully this sheds a bit more light on the issue.

Edited November 19, 2020 by AdeKing
A few points clarified

[jetstream]

1 minute ago, AdeKing said:

This is where I seem to be going wrong and can't work out what I'm doing wrong, or whether there is something wrong elsewhere.

If you look through the sight tube as you rack the focuser out does the gap around the secondary change?

[AdeKing]

1 hour ago, jetstream said:

If you look through the sight tube as you rack the focuser out does the gap around the secondary change?

I'll double check tomorrow.

I know that racking out did give more "breathing space" around the circular secondary reflection as the gap widened, but I didn't pay attention to whether or not the drawtube was rocking and changing the relative proportions of the spaces if that makes sense.

I did dig out my Baader laser collimator to try your suggestion but wouldn't you know it, the batteries were dead, so need to remember to pick up some new ones.

[faulksy]

if you rotate your secondary this should bring your centre spot back to centre. then adjust your secondary screws to get your mirror clips back in view

[AdeKing]

32 minutes ago, faulksy said:

if you rotate your secondary this should bring your centre spot back to centre. then adjust your secondary screws to get your mirror clips back in view

Apologies @faulksy I'm not sure I understand. 

The secondary isn't centre-spotted, the black spot in the photo is the reflection of the sighting hole in the centre of the Cheshire.

Won't rotating the secondary at this stage make the secondary reflection elliptical rather than circular?

Sorry if I'm missing something.

[jetstream]

4 hours ago, AdeKing said:

I'll double check tomorrow.

I know that racking out did give more "breathing space" around the circular secondary reflection as the gap widened, but I didn't pay attention to whether or not the drawtube was rocking and changing the relative proportions of the spaces if that makes sense.

I did dig out my Baader laser collimator to try your suggestion but wouldn't you know it, the batteries were dead, so need to remember to pick up some new ones.

I mean does the gap get unequal as its racked out? just curious and i hope you get it sorted out.

[Pixies]

Hi,

Regarding the stage after removing the paper:

One thing to be aware of, is that with a Cheshire, you don't have to bother with trying to get the primary clips equally visible within the secondary. You are trying to centre the image of the primary within the secondary, so instead just adjust the secondary so that the doughnut (radioactive symbol in your case) on the primary is directly under the crosshairs in the Cheshire. That's the whole point of a Cheshire and isn't made clear in Astrobaby's guide.

I also think that you might get away with a slight rotation of the secondary - but try the above first.

[Ricochet]

If you can't get the secondary circular and pointing at the centre of the primary at the same time you need to adjust the spider veins. 

If the focuser is horizontal then you will have a secondary spider screw on the top of the telescope and one on the bottom. While looking through the Cheshire, tighten the one on the bottom and loosen the one on top so that the secondary moves "down". After adjusting the spider redo secondary collimation. 

The focuser probably isn't quite square due to the tube not being perfectly round. I thought the astro baby guide used to show how to check this but it isn't there now. As this requires removing the secondary I would just adjust the veins as above. 

[Starflyer]

If you replace the stock focuser you should definitely check it's square to the tube before collimating. This means removing the secondary and taking measurements to enable you to make a mark on the tube exactly opposite the centre of the focuser opening.  I can't find the procedure now, but it's documented somewhere on SGL.

Once you've marked your tube (Tipex is good for this), use your Cheshire in your focuser, still with the secondary removed. The crosshairs should exactly point at the mark you made on the tube opposite the focuser, if it doesn't then your focuser isn't square to the tube.

I have a feeling that this is your problem.  If your secondary mount is central to the tube and your secondary presents as a circle, but your secondary is not centred when viewed through your colli cap / Cheshire then it's a tilted focuser.

 

[AdeKing]

On 19/11/2020 at 21:46, Starflyer said:

If you replace the stock focuser you should definitely check it's square to the tube before collimating. This means removing the secondary and taking measurements to enable you to make a mark on the tube exactly opposite the centre of the focuser opening.  I can't find the procedure now, but it's documented somewhere on SGL.

Once you've marked your tube (Tipex is good for this), use your Cheshire in your focuser, still with the secondary removed. The crosshairs should exactly point at the mark you made on the tube opposite the focuser, if it doesn't then your focuser isn't square to the tube.

I have a feeling that this is your problem.  If your secondary mount is central to the tube and your secondary presents as a circle, but your secondary is not centred when viewed through your colli cap / Cheshire then it's a tilted focuser.

Thanks, I'm pretty sure that this is the case.

The secondary has been removed in preparation for squaring the focuser, but I've been pretty busy so haven't gotten around to doing anything about it yet.

The secondary holder has got three nice divots in the top so I'll be adding a steel washer and milk bottle washer when it's reassembled as well as replacing the secondary adjustment screws with  thumbscrews rather than the stock Allen grub screws.

The focuser is a Skywatcher low profile dual speed Crawford, but not the current version by the looks of it.

The tilt adjusters are a pain in the backside on the model I have because some wizard designer saw fit to put two sets of adjusters behind the focusing knobs so they're inaccessible to an Allen key without first removing the whole focusing knobs assembly, not just removing the knobs from the Crawford shaft either.

I notice this design flaw seems to have been corrected in the current design.

Hopefully I will find some time one evening this week to follow the alignment procedure in Vic Menard's book.

I'll post an update when I've had time to tweak and pull out my remaining hair no doubt.