Skywatcher 190MN / fitting of a Moonlite focuser

[opticalpath]

Remedy?

1. Mount the scope firmly.

2. Remove primary assembly

3. Add strings, top AND bottom

4. Place camera so that the strings top and bottom line up

5. Leave camera on tripo!

6. Re-install primary

7. Center primary by lining up center with top strings and camera

8. Tilt primary so that the outer parts of the strings line up with reflections

Would this improve it?

/per

Hm, yes, you're right Per.   I was assuming (always dangerous!) that the primary was already physically centred in the OTA and that the only adjustment required was the tilt of the mirror optical axis, not a possible sideways displacement.  If the primary is not laterally centred, this would end up compensating one error with another ...... which btw seems to be a pitfall at every stage of collimating this scope imo!

In practice, though, would I not as a first step anyway try to centre the mirror physically in the OTA - best I could - by measuring the mirror in its cell outside the OTA?  If it was easy to adjust the primary mirror laterally while it's mounted in the tube, the sequence you described would be a definite improvement, bu how could that be done?  The only mechanism to adjust lateral position seems to be packing pieces between the two rings of the mirror cell, so I guess centring would be trial and error, removing the primary cell, adjusting packing, replacing and test again.  It could be done with the scope solidly mounted back on its mount, but difficult for me due to height of my pier and lack of space in the observatory - would have to stand on stepladder outside the obsy with camera .... maybe not!

Adrian

[opticalpath]

I was regerring to your string theory above. I think it doesn't work ......

/p

It's a good theory ..... it just needs some extra dimensions to make it work.

Adrian

[opticalpath]

... First task is to get focuser movable up and down along the tube. The moonlite ends up too low down I thonk,

/p

Yes .... I moved the secondary down the tube to meet the (lower) Moonlite position ..... and that's when all my problems started

Have you come up with a good way to make the Moonlite position adjustable? I'd be very interested.  I thought of mounting it on two over-size intermediate plates between the focuser base and the curved mounting plate, with a sliding adjustment between them, but they would take up some back focus and I have very little left with my current camera configuration.

Adrian

[perfrej]

My primary has tvree small nylon tipped screws for centering in in the inner mirror assembly, and I have padded the inner assembly against tve outer. No sliding and shifting there.

As I am experiencing the same moonlite-induced problem, I am going to modify the focuser so that it can slide up and down.

/per

[opticalpath]

Don't think I have any side-adjustment screws for the primary, though I see some threaded holes that are in the right place for side screws.

Intrigued by your proposed focuser mod.  ..... not much metal around the mounting holes to widen them to slots.  Should it not be possible to move secondary down to meet Moonlite ... and still collimate properly?  Is there something special about distance of secondary from primary or from corrector??

Another idea on the string theory, avoiding need to remount scope solidly etc: make a second set of 3 strings to sit *above* the primary mirrror.  Cut a cardboard disk to a snug fit inside the OTA.  Mark 120 degree lines from centre to edge.  Cut out a large central circle to leave a semi-rigid ring that can be placed in the OTA and rested on one of the baffle rings (with a little careful bending to fit in).  Reach in and attach 3 strings across the 120 degree marks on the cardboard ring and rotate the ring till strings line up with OTA top strings.  Now have a double set of strings centred it the tube top and (near) bottom to line up and check centring of the primary mirror centre marking.

Adrian

[perfrej]

I think that maybe the primary can be centered out of tube. It's not like there's any room for sliding around (big assembly)...

/per

[ambro]

Hi Adrian,
First time I made centering focuser using circle of paper with small hole at center inside focuser tube. It help place laser at center (tightening by clampring isn't repeatable) inside focuser but I learned it is not good way to square focuser. Second time I had removed focuser and sticked sheat of paper inside hole for focuser. I made small hole at center of paper and mount focuser. Beam hit measured spot ( by wraping paper strip around the inside of the OTA...) at other end of tube through this small hole in paper. I noticed I have to add more shims.
I don't have spot on SM. I don't need this spot when I am using colimator from webcam. When using cheshire it help. If you center SM as seen in a focuser, it will automatically be correctly offset towards the primary. I think spot help only with offset  towards the primary mirror.  It won't help offset away from the focuser. Maybe it is elypse shape because spot reflection is circle shape at primary ?? Is it visible at primarry mirror ? Offset away from the focuser sholud be correct from factory. I don't know how to chceck offset away from focuser. I had measured hypotenuse only on mirror surface. Before I was trying collimate MN like Schmidt Newton (http://www.cloudynights.com/item.php?item_id=1794). When I centered reflection of tube end (with PM removed) at SM and then put primarry mirror i don't have to center reflection of PM at SM. It is already centered. In order to put PM center mark at focuser center now I am only using small adjustment of SM.
I don't know why You want move focuser up or down? I can do this with bader steel track, but I don't have reason. Moving SM up or down and tiling should be sufficient to center it at focuser and make it concentric. Whwn ilumination was not centered like in my first flat field analys I forgot that DSLR flip image and it was to high, not to low from chip center. I have to make test shots.

[opticalpath]

OK, my last idea on checking centring of primary and aligning of optical axis to OTA  (two sets of strings in front of the primary was impossible - too many reflections!) .....

I'm still not sure if this eliminates the possibility of a centring error plus compensating axial tilt error, though, so any opinions on this welcomed!

I made two aids: 

a card circle with centre cut-out and strings that is good snug fit inside tube (note small flat cut in circumference - tube is not exacly round: has a seam that makes a flat on one side)

and a paper viewing pinhole-screen with a couple of circles drawn to check alignment with open top of the OTA

Procedure is to reach down into the top of the (vertical) open OTA and place the string-ring-thing on the lowest or second lowest baffle, just above the primary.  If you look into the OTA and line up the strings and their reflections, they should both be centred on the primary centre marking.  Now place the pinhole mask on top of the OTA - pencil circles facing down -  and look through the pinhole from a few cm above it.  Shine a torch through the paper to illuminate the interior of OTA.

Now move the pinhole screen until the strings are centred over the mirror centre mark again and look for a small dark dot that is the reflection of the pinhole.  Move the screen carefully and find the position where the dark dot is right in centre of the mirror ring centre mark (and the strings also still centred hopefully).   Now look on the underside of the paper screen and notice where the drawn circles are in relation to the top of the OTA.  If they are concentric with the OTA, and the other items are centred, does that mean the mirror is laterally centred, as well as axially aligned?

I can't quite figure out if everything could still look right, but result from a combination of errors.  If it works, it's certainly easier than the other methods I tried.

Adrian

[Gunnar]

Hi.

I follow this with great interest.  

[opticalpath]

Hi.

I follow this with great interest.  

Hi Gunnar.

Now I have TWO Mak-Newts, so I really have to try to figure this out

Tell me, does the secondary on your MN190 have an offset centre marking?  The ellipse ring mark on mine looks like it marks the offset optical centre so it should be centred in the crosswires of a sight tube if it's correctly positioned.  Problem I have is that if I centre the mark in the sight tube, the circular outline of the secondary is not concentric with the end of the drawtube; it looks like the secondary has to be moved closer to the primary to get a concentric view.  Do you see this effect?

Maybe I need to just ignore the secondary marking ..... or calculate the correct offset and see if it's marked in the right place.

Adrian

[perfrej]

I think they ship with a center mark from the factory. It is drawn with a felt pen.

Squaring the focuser is indded a challange. The Moonlite can be adjusted, so that eliminates shimming, which is good.

Here's what I did...

First of all, I have a very good laser that has an expansion system that centers it totally in a 2" tube (Hotec). I trust it and have verified its accuracy. With the tube standing upright on a verified level surface, the laser makes a beam across the tube. With an accurate rule, I measure it to be the same distance from the top of the tube, both upon entry to the tube and on the opposing tube wall. Iterate that and it should be straight enough.

For horizontal adjustment I use a string that runs across the top between the corrector assembly mounting holes, parallel to the beam underneath. On the string is another string that hangs down some 20 cm in length with a weight at the end. Sliding it very close to the beam entrypoint, I adjust the focuser so that the beam hits the vertical hanging string. Move it over to the opposite end and check the accuracy. Make minute adjustments and your focuser is adjusted for orthogonality in the expected light path from the focuser.

I then proceeded to mount the corrector with the secondary. After that it is the Catseye collimation kit. Collimation is very good, but the focuser is in the wrong vertical position, proven by uneven illumination.

So, I lowered the primary as mitigation to the problem, saw that it got better and proceeded to lower it some more. That's when it got loose and fell on the primary. I now have a new secondary (without the mark), attached with an offset corresponding to another example of a 190MN that a friend has.

I still haven't licked the illumination issue completely, so I will soon do a focuser or tube mod or, as a filnal resort, use the standard focuser; which by the way is about as good as the Moonlite (not impressed with Moonlite at all).

Scope is coming down tomorrow or so.

I really believe in his scope and it is a gem in terms of optical quality versus price. Let's share our experiences and get them into good condition!

/per

[opticalpath]

Per, I like your string idea for horizontal adjustment of the focuser - I will try that.  The paper template method is not ideal because interior wall of the OTA has screws and plates intruding that make measuring difficult. 

Here is my tool for checking vertical alignment:

It's a carpenter's adjustable marking gauge with a paper pinhole taped to the handle end. I rest the adjustable block on the edge of the OTA at focuser side with stem pointing down and the flat edge of the stem resting on the inner wall of the tube near the focuser opening.  Then adjust the block until the beam of the (Hotech) laser passes through the pin hole as it exits the drawtube.  Then move the gauge to the opposite side of the tube and check if the laser spot hits the pinhole at the same height or not.

I agree with you - this scope is potentially a real winner for imaging, but many owners are frustrated by difficulty in nailing collimation, especially positioning of the secondary.  Do you believe that moving the secondary down to meet focuser axis would somehow give a different result to moving focuser up to meet the secondary position .... or are they exactly equivalent?   My head tells me it should not matter which way you achieve focuser-secondary alignment ..... but the results make me suspect that - for unknown reason - it's better to keep the secondary 'high' near the corrector and move the focuser up to meet it.  If that's true, do you know why?

Adrian

[ambro]

  Problem I have is that if I centre the mark in the sight tube, the circular outline of the secondary is not concentric with the end of the drawtube; it looks like the secondary has to be moved closer to the primary to get a concentric view.  Do you see this effect?

Maybe I need to just ignore the secondary marking ..... or calculate the correct offset and see if it's marked in the right place.

If You see perfect circle (secondarry is at 45 degree, corectly rotated)  and have to move SM close to the primary (screw out center screw, do it at horizontal position of OTA) to make it concentric, offset point is probably wrong. First try to make it concentring without primarry mirror (you have to make concentric reflection of tube end). Then screwin equal primary tilt screw, insert PM (you have to align primary to be at center of cell, use shims if it is too lose and focuser should be square to OTA). Reflection of primary will be almost at center of secondary without touching SM screws. Your secondarry will be close to corect position. Next try to tilt primary to align with corrector (string or other metod). Then make final aligment to center primary mark at focuser center by slighty tiling secondary. It is not final picture (I dont have it, but secondary is almost aligned):

I then proceeded to mount the corrector with the secondary. After that it is the Catseye collimation kit. Collimation is very good, but the focuser is in the wrong vertical position, proven by uneven illumination.

So, I lowered the primary as mitigation to the problem, saw that it got better and proceeded to lower it some more. That's when it got loose and fell on the primary. I now have a new secondary (without the mark), attached with an offset corresponding to another example of a 190MN that a friend has.

I still haven't licked the illumination issue completely, so I will soon do a focuser or tube mod or, as a filnal resort, use the standard focuser; which by the way is about as good as the Moonlite (not impressed with Moonlite at all).

/per

Have you measured offset of friend MN190 ?. I am not sure of my offset calculation from previous post. Could you post flat with uneven illumination?. I think you don't have to move focuser to make even illumination. (You can place primary mirror close or far to focuser by primary tilt screw. I think far is beter because spring works efficient, but length from OTA to focal plane is shorter)

You can enter your scope data to myNewton ( http://www.otterstedt.de/wiki/index.php/Benutzer:Heiner/myTelescope/de/Einf%C3%BChrung ) to check illumination and raytrace with diffrent position of focuser.

I think that maybe the primary can be centered out of tube. It's not like there's any room for sliding around (big assembly)...

/per

I think measure by caliper is sufficient for centering PM and Corrector.  (I have checked using level that OTA is not bend). Problem is how to tilt primary to be parallel with corrector (string etc).

[opticalpath]

Well, after all this further work on collimation, I remounted the scope this evening and took some test images. Result: worse than before -  bad coma on right side of the frame, and severe vignetting of bottom corners   .  Where to go from here?

At this latest collimation, the only thing that did not look quite right in the view through the sight tube was the offset centre marking on the secondary. This time I ignored the marking, suspecting it to be wrong, and positioned the secondary so it was concentric with the drawtube (as it should look.) That meant moving it towards the primary, compared to last collimation effort, so the secondary centre mark ended up looking high.  I then used only the secondary tilt adjusters to align the primary centre mark etc in the sight tube view.  At the end, as I said, it all looked pretty good except for the secondary centre mark.  But the resulting images were not an improvement on the previous collimation attempt.

Previous collimation effort, which was a good improvement on the past and much better than this latest attempt, was done a little differently: after aligning the primary axis with strings, I positioned the secondary by placing the secondary offset centre marking under the cheshire cross-wires.  That meant that the secondary was quite a bit closer to the corrector (i.e. not moved so far down) and the outline of the secondary was NOT concentric with the drawtube.  I ignored that and continued, centring the primary centre mark and reflections  this time by some further adjustments to the primary mirror tilt and not touching the secondary tilt, like a conventional Newtonian primary adjustment.  The result then was a nicely centred primary mark, secondary mark, peep hole, cheshire cross-wires and reflection of cross-wires. 

So, although it seemed right this time to ignore the secondary offset mark and to aim for a concentric outline, and it seemed sensible not to change the primary tilt after centring the mirror and aligning the axis with strings ..... in the end that gave a worse result !

I'll go back outside shortly and move the secondary back up towards the corrector, aligning on the offset centre mark and try to repeat the earlier procedure that turned out better.  I don't really understand why the earlier effort with a non-concentric view of the secondary outline worked out better.  I thought I was beginning to understand this scope but still it confounds me.

Adrian

[olivdeso]

Hello guys,

I am Olivier from France and new on this forum, thought I used to read it since a long time.

An astro collegue of mine, Joss, just aquired a SW MN190 and we were collimating it these days, keeping an eye on this very interesting post. Thank you very much to you guys for sharing your experiments and results, that helped us a lot.

Tonight we may have found something that could be interesting and I wanted to share it with you in case.

First of all we started the colimation process by adjusting the focuser position using a cheshire. It is the stock focuser, so we could adjust it in order to get the SM center marking just in front of the cheshire croshairs center.

(I found easier to close the tube and inject some light through the cheshire side opening to see the secondary miror center marking)

Then, I used a barlowed laser to allign the miror :

This is the most interesting point here : Using a barlowed laser (a x2 zeiss barlow at about x3 and a TS laser which is quite strong...that helps here...), the secondary miror center marking will be projected onto the primary miror on which we can see it's round shadow. Its size is about the same as the PM center marking, so it is easy to adjust the secondary miror tilt in a very precise way.

Finaly we collimated the primary miror still using the barlowed laser. Thus at the laser return target, we can see both the PM and SM center marking shadows that should be concentric if everything goes well.

We still have to test it on the field. We will share the results.

Now the main remaining concern, is the PM cell. It does not keep the PM aligned well enough.

Also I just I ran a quick and dirty plop analysis and found that moving the 3 points to a smaller diameter (80mm ideal, but 110mm is fine as well) would improve the PTV error by a x4 factor. Not so critical for deep sky imaging, but to be kept in mind in case of cell reworking.

clear skies

Olivier

[ambro]

Helo Olivier,

Thank You for sharing experience. Barlowed laser projection will be helpful to check offset away from focuser.

I don't have this central spot at SM. Could some one measure location of the spot and longer axis of spot eclipse?. This data is required to make paper template and paint spot on secondary.

[perfrej]

Second that.. My old chipped secondary has the marking centered on the short axis, and offset 2mm towards he corrector. Measured flat on the surface. Anyone else?

We also need the distance as per my drawing further up the thread so that it can be properly attached to the support. Please include mirror dimensions (major and minor axis) for the mirrored surface.

/per

[opticalpath]

So, after last night's disappointing results (following major collimation rework), I did a quick re-collimation in the observatory, starting  this time by re-positioning the secondary centre mark under the Cheshire cross-wires (instead of going for a concentric view).  Then followed normal procedure of adjusting secondary and primary mirror's tilt, ignoring the fact that I was upsetting my earlier carefully aligned primary-corrector axis.  Centring the secondary marking made the secondary outline NOT concentric with the end of the draw-tube - as noted before - but ignore that for now.  This was all done very late in cold dark  conditions, but the resulting images were better than my very careful earlier effort, with better coma correction, though still some uneven illumination.  What is going on?

Each time I try to improve collimation so everything looks 'right', the image results are worse - uneven coma correction and worse field illumination!   And it seems that to make the results better, I have to introduce some deliberate errors: move secondary to non-concentric position (closer to corrector than it 'should' be) and tilting the primary axis away from the centre line.  Very curious .... must be something else going on that I'm not understanding yet.

Adrian

[perfrej]

Here are a few tips from a guy (that would be me) who has spent countless hours on figuring out this beast.

1. Secondary tilt

Make sure that the secondary is symmetrically placed sideways (use a collimation cap). Adjust it to have equal distance to the tube on both sides.

2. Secondary rotation

Assuming you have your secondary more or less where it should be, start by getting its rotation right. I use a standard collimation laser for this. Adjust the secondary collimation screws so that the laser spot hits the primary half way out from the center towards or away from the focuser. It will most likely be off the imaginary line between the focuser and the opposite wall. Spin the whole secondary assemply in he corrector (it is felt lined) so that the spot hits the imaginary line. Then adjust it with the screws to hit the primary center spot.

3. Verify step 1

4. Proceed with the laser collimation

5. Use a Catseye collimation kit and perform a standard collimation (cheshire style)

I have found the rotation to be an important step!

/per

[ambro]

short axis = 63mm

long axis = 63 mm x 1,41 = 88.83

D is most important for me, then A or C

[perfrej]

Excellent! Folks, get to it! Take your scopes apart and measure

/per

[opticalpath]

Helo Olivier,

Thank You for sharing experience. Barlowed laser projection will be helpful to check offset away from focuser.

I don't have this central spot at SM. Could some one measure location of the spot and longer axis of spot eclipse?. This data is required to make paper template and paint spot on secondary.

Second that.. My old chipped secondary has the marking centered on the short axis, and offset 2mm towards he corrector. Measured flat on the surface. Anyone else?

We also need the distance as per my drawing further up the thread so that it can be properly attached to the support. Please include mirror dimensions (major and minor axis) for the mirrored surface.

/per

Robert, Per

I made a quick measurement of the secondary 'centre' mark when I had the scope dismantled yesterday. I don't claim great accuracy but it should be quite close:

major axis: 90 mm

minor axis: 63.6 mm

Position of centre marking on major axis: 43.5 / 46.5 mm from top/ bottom edges.  (So actually quite close - 1.5 mm -  to the geometric centre of the ellipse, as you can maybe see from the photo in my earlier post)

BUT hold on before you rush off to get a marker pen.  I think the mark is in the wrong position.

Using the approximate formula for lateral and longitudinal offsets:  Offset = minor axis / (4 * f/ratio)   gives  63.6 / (4 * 5.3), which is 3.0 mm.    That is 3.0 mm away from focuser, and 3.0 mm towards primary. 

So the offset from geometric centre on the face of the mirror should be 1.414 * 3.0 mm,  that is 4.42 mm.  NOT 1.5 mm from the geometric centre as marked on my mirror.

Please check if this looks correct, or if I missed something.  If it's right, it would explain why my view of the secondary is not concentric with the focuser when I centre the marking  (but still doesn't explain why using the (wrong) marking seems to produce better results   )

Per,

When I dismount my scope (again!)  in the next day or two, I'll remove the corrector assembly and measure the offset of the mounting stalk that you asked about.

Adrian

[opticalpath]

Here are a few tips from a guy (that would be me) who has spent countless hours on figuring out this beast.

1. Secondary tilt

Make sure that the secondary is symmetrically placed sideways (use a collimation cap). Adjust it to have equal distance to the tube on both sides.

2. Secondary rotation

Assuming you have your secondary more or less where it should be, start by getting its rotation right. I use a standard collimation laser for this. Adjust the secondary collimation screws so that the laser spot hits the primary half way out from the center towards or away from the focuser. It will most likely be off the imaginary line between the focuser and the opposite wall. Spin the whole secondary assemply in he corrector (it is felt lined) so that the spot hits the imaginary line. Then adjust it with the screws to hit the primary center spot.

3. Verify step 1

4. Proceed with the laser collimation

5. Use a Catseye collimation kit and perform a standard collimation (cheshire style)

I have found the rotation to be an important step!

/per

Thanks Per.  Could you explain a little more what you mean in Step 1 - maybe I understood it wrong?    The secondary mount is fixed, so no separate sideways adjustment is possible as with spider vanes on a Newt).  I can see that a little sideways movement happens when the secondary tilt screws are adjusted, but how can you use this to correct sideways misalignment without also introducing arbitrary tilt error?  Any  (unwanted) error introduced this way would be corrected in later stages of collimation, but that would undo the earlier sideways alignment correction.

I have so far resisted playing with secondary rotation.  I tried the T-template method (cloudynights) and it looks OK, but I think the method is prone to error so not ideal.  I guess I will have to get the pipe wrench out and tackle that locking ring! 

Adrian

[perfrej]

Just tilt it back and forth using TWO of the collimation screws until it is centered sideways (reference is to tube standing upright). Without this first step the rotation is difficult to get right. It may take an iteraion or two but the result is perfect.

I did not need to loosen my holder; it turned without the use of excessive force.

/per

[opticalpath]

Just tilt it back and forth using TWO of the collimation screws until it is centered sideways (reference is to tube standing upright). Without this first step the rotation is difficult to get right. It may take an iteraion or two but the result is perfect.

I did not need to loosen my holder; it turned without the use of excessive force.

/per

OK I'll try to improve the sideways alignment as you suggest - it does look a little out.  Still, suppose the tilt is already correct for other collimation steps .... then I would upset that by adjusting two of the screws for sideways alignment.  Seems I would have to choose beween tilt to align with primary centre spot or tilt to correct sideways mis-alignment, unless I misunderstand.   Anyway, I will try it ... there is definitely a small sideways error visible.

About secondary rotation: I think you got lucky!  No way I can rotate mine without loosening the knurled locking ring .... and it's very tight.  Some other owners reported the same and one even resorted to a pipe-wrench to loosen the rings - a scary thought.

Adrian