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Skywatcher 190MN / fitting of a Moonlite focuser


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Like many MN190 owners, I've had a frustrating time with collimation, especially after replacing the focuser (with a Moonlite).  After a lot of research and some experimenting, I think I'm finally fig

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,

Now I took the opportunity to replace the secondary mirror holder center screw to prevent the secondary mirror to be able to come off in the event of too much adjustment on the three collimating screw

Posted Images

You mean rotate the corrector along with the secondary?   Usual advice is not to do so as the corrector may have been adjusted to optimal rotational position at the factory (if they actually do that in the SW factory.)

I think I'll try to make a tool to fit the pattern on the locking ring, using Polymorph mouldable plastic - excellent stuff for one-off jobs like this. 

I just brought the scope indoors so will do more measurements on the secondary + holder today.

Adrian

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The corrector plate locking ring can be very tight (or loose if lucky) to be turned with your fingers. You can't get enough purchase on it.

I suggested in this post http://stargazerslounge.com/topic/141012-skywatcher-190mn-fitting-of-a-moonlite-focuser/page-2#entry1632954 to use pipe grips. There should be nothing scary in using them if you are careful and take a simple precaution and use a cardboard protector. The grips can be adjusted to just fit the ring, so there is no crushing force and with some tape on the jaws to avoid scratching. It will only take a small amount of force to slacken because of the grips leverage. HTH

I used longer screws than the stock ones in the secondary adjuster to allow any extra movement if needed with the Moonlite focuser, and to avoid it falling off if I went to far. If I get a chance I'll post some CCDInspector results after my first collimation attempt following the Moonlite fit. Unfortunately, I haven't kept the final ones.

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I made some measurements of the secondary as requested by Robert and Per.  I borrowed Robert's diagram to annotate.  A couple of things to be aware of:

 

1. Some of the measurements are awkward to do because of the angles, but I hope these are enough to work out the rest.  In particular, the one that Per asked for (23.0 mm ? in the diagram) is hard to measure directly because the layer of adhesive tape means that the edge of the mounting stalk does not actually contact the back of the mirror so the point of contact has to be inferred.  For this dimension, it would be safer to calculate it from the distance marked 16.6 in the diagram .... which was easier to measure accurately. Or just mount the mirror so the indicated dimension is 16.6.

 

2. The offset centre spot position is where I measured it on my mirror ..... but I think it is in the wrong place!  The calculated correct offset for this scope means it should be 4.24 mm from the geometric centre on the face of the mirror, so I would suggest not to use the centre spot position in the diagram ..... unless someone can justify why it is in this place so close to the geometric centre.

 

(PER, was your original secondary marked ..... where was the mark?)

 

Adrian

 

secondary_dims.jpg

Edited by opticalpath
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The corrector plate locking ring can be very tight (or loose if lucky) to be turned with your fingers. You can't get enough purchase on it.

I suggested in this post http://stargazerslounge.com/topic/141012-skywatcher-190mn-fitting-of-a-moonlite-focuser/page-2#entry1632954 to use pipe grips. There should be nothing scary in using them if you are careful and take a simple precaution and use a cardboard protector. The grips can be adjusted to just fit the ring, so there is no crushing force and with some tape on the jaws to avoid scratching. It will only take a small amount of force to slacken because of the grips leverage. HTH

I used longer screws than the stock ones in the secondary adjuster to allow any extra movement if needed with the Moonlite focuser, and to avoid it falling off if I went to far. If I get a chance I'll post some CCDInspector results after my first collimation attempt following the Moonlite fit. Unfortunately, I haven't kept the final ones.

Tony, I assume that you lowered the position of the secondary to line up with the Moonlite focuser axis.  Did you use an offset centre marking on the secondary to line up on, or did you just move it until you got an outline concentric with the end of the draw tube?    I'm interested to know because the offset marking on my secondary seems to be not offset nearly enough and if I use it, the view is not concentric.  Strangely, though, I get a better collimation if I centre the mark and have a non-concentric view.

Adrian

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Adrian I used the offset centre marking to line up with the primary and the Cheshire cross hairs. The secondary was also concentric although it appeared egg shaped. I know that sounds strange but it's common on fast scopes.

I'll see if I can fin some examples.

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There is a good example of the offset after collimation, and the primary and secondary after alignment, on Astrobaby web site. This clearly shows the same results I get using this method.

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More on secondary mounting measurements ....  Please read before using dimesions I provided in the earlier post !  Per, I don't think you should mount your secondary using the dimensions I provided.  Read on to see why.

I just noticed that the measurements I made of the secondary show something odd about the way it's mounted. What I noticed was that my secondary mirror is mounted with very little physical offset away from the focuser.  This picture gives an idea of what I mean.  Sorry about the quality; I'll do this more carefully later.

SM_attachment.jpg

Even this crude picture shows that the centre mounting bolt is pretty much lined up with the *geometric* centre of the secondary ellipse.  Earlier I calculated that the correct offset for this scope and mirror is 3 mm lateral and 3 mm axial (or 4.24mm on the mirror face).  That means that the geometric centre of the ellipse should be located 3mm away from the centre axis of the OTA and corrector (and 3mm down from the focuser axis.  My secondary definitely does not meet that condition: the mounting is providing very little if any physical offset in the direction away from the away from the focuser. 

Up to now, I've been worrying about offset along the axis of the OTA and where to position the secondary in the up-down direction.  Now I realise I have to worry about the offset (or lack of it!) away from the focuser!  Are all MN190s built this way, or is it just mine that did not have the correct offset built in whe the mirror was glued to the post?  If anyone else has their scope in bits, I would really like to know if they're all like this. 

Lack of offset in a fast Newtonian is a BAD THING.  In a Mak-Newt that has a corrector lens axis to worry about too ..... it's a very bad thing.  There is maybe an explanation.   The so-called 'New Model' collimation (search on cloudynights) proposes that it's not necessary to offset away from the focuser.  If the secondary is offset sufficiently in the axial direction towards the primary, correct alignment and good collimation is still possible; the lack of lateral offset can be compensated by slight tilt of the primary away from the OTA axis, and a small adjustment of secondary tilt.  This method is sometimes labelled Unidirectional Secondary Offset, as opposed to the classical Bidirectional Secondary Offset. It's generally thought to be a good enough alternative for Newts. and saves the need to de-centre the secondary mounting.  But I think it brings problems for the Mak-Newt because of the non-alignment of corrector and primary that it results in.  I came across this in a cloudynights discussion:

Alas, if using the "new Method" of collimation (Unidirectional Secondary Offset), the optical axis is tilted slightly toward the focuser and the angle of reflection at the secondary is greater than 90 degrees.
That's fine, because superb collimation can be achieved. But, there are consequences:

 

1) The image of the secondary mirror will not be exactly round. It will appear ever-so-slightly shortened in the up tube/down tube direction. If you make the secondary mirror's reflective surface coincident with the inside edge of a sght tube, you will see this. It's tiny, but it's there.
2) The screws on the outside of the secondary shroud will not appear equally visible. That shroud will be slightly tipped relative to the optical centerline, making screws on the long edge of the shroud more visible than the screws on the short edge of the shroud. Admittedly, the effect is fairly small, but it's real, and it's visible. It's because the sides of the shroud are not in the same line as the optical axis.

To eliminate #1 and #2 above, classical offset (bi-directional secondary offset) needs to be employed. Then the sides of the secondary shroud will be parallel to the optical axis and the secondary will appear exactly round in outline in the sight tube.

And the larger the amount of offset, the greater the degree of #1 and #2 above as seen through collimation tools.

Again, it is not a great amount, and near-perfect collimation can be achieved. But the "New Model" (UDO) of collimation does have this issue
.

This may finally explain some of the unexpected behaviour I'm seeing during collimation and I'm very interested to hear if others see the same absence of mechanical offset in the SM mounting too.  Is this a design compromise in the scope?   It makes me wonder about perhaps detaching the secondary and reattaching it with the correct offset built in.  I'll wait to see what Per does :smiley: .

Adrian
 

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Now this is really going somewhere! Good work, Adrian. I am taking mine down from the balcony today!

Now, my center spot is offset by just a few mm (original), just like yours.

I agree that the secondary bidirectional offset has to be old school in order for the scope to work. I can deduce that my scope's resulting optical axis is not completely along the tube as I always get a rather large orthogonality error in the modeling of my mount when I use the 190.

The mount modeling is very accurate, so I will know when I am getting this right.

On telescope-optics.net I cannot find anything that contradicts the use of bidirectional offset in a Maksutov-Newton design. If yo uguys haven't checked out telescope-optics.net yet, DO! Here's his calculation of offsets ;)

BO.PNG

Anyway, I'll run through the numbers, re-clue my secondary and line everything up according to spec. My usual procedure is the to collimate and take flats repeatedly.

Will revert!

/per

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I found these articles give useful explanations of the different collimation models:

http://www.vicmenard.com/telescopes/addendum-to-perspectives.html

http://www.cloudynights.com/item.php?item_id=2677

Per/  a couple of questions:

* What value does the full formula give for the x- and y- offsets for our scope (f/5.3 and 80mm major axis SM)?  The value I quoted was calculated from the simple approximation formula. 

* What is the best material/ method to use to attach the secondary mirror to its mounting plate (glue, tape, silicone ....)  to avoid catastrophes?  If I do this, I want to be able to sleep at night!

Adrian

Edited by opticalpath
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The offsets are calculated only in one dimension. The secondary should go n mm away from the focuser and n mm down the tube. It is the minor axis that is used in the calculation, not the major. According to the site, the approximation formula is really good enough, so 3mm is our offset, translating to 4.25 along the surface of the mirror.

I think the thin double acting tape method must be good enough. It is used at the factory and I have changed my secondary to a 64mm one (what I could get hold of) and attached it with a very similar glue.

One more note on the subject is that when your scope is collimated, the secondary should end up at 45 degree angle to the scope. This can be chacked by taking the top assembly off after collimation and looking at the distance between the two parts of the secondary holder. Should be even around. That, of course, assumes that the secondary is glues straight on the holder :)

/per

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I found these articles give useful explanations of the different collimation models:

http://www.vicmenard.com/telescopes/addendum-to-perspectives.html

http://www.cloudynights.com/item.php?item_id=2677

 

Per/  a couple of questions:

 

* What value does the full formula give for the x- and y- offsets for our scope (f/5.3 and 80mm major axis SM)?  The value I quoted was calculated from the simple approximation formula. 

 

* What is the best material/ method to use to attach the secondary mirror to its mounting plate (glue, tape, silicone ....)  to avoid catastrophes?  If I do this, I want to be able to sleep at night!

 

Adrian

Correction: major axis of SM is 90 mm, not 80 mm

Adrian

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The offsets are calculated only in one dimension. The secondary should go n mm away from the focuser and n mm down the tube. It is the minor axis that is used in the calculation, not the major. According to the site, the approximation formula is really good enough, so 3mm is our offset, translating to 4.25 along the surface of the mirror.

 

I think the thin double acting tape method must be good enough. It is used at the factory and I have changed my secondary to a 64mm one (what I could get hold of) and attached it with a very similar glue.

 

One more note on the subject is that when your scope is collimated, the secondary should end up at 45 degree angle to the scope. This can be chacked by taking the top assembly off after collimation and looking at the distance between the two parts of the secondary holder. Should be even around. That, of course, assumes that the secondary is glues straight on the holder :)

 

/per

Yes, that's what I meant by x- and y- offsets. Terminology is confusing here - I think I'll refer to lateral and axial offset amount(s). The approximate formula result is 3 mm towards primary and 3 mm away from focuser ( equivalent to 4.24 mm on the diagonal.)

About the double-sided tape .... In DIY use, I found that some does not stay stuck quite as well as others! In this application, it has to withstand continuous slight tension from weight of the mirror and not be affected by big swings in temperature and humidity. I noticed that the tape layer on my SM is about 1 mm thick; I think it's the type that has a thin layer of foam sandwiched between two layers of adhesive film. The type of tape that leaves only a very thin film of adhesive when you peel off the backing may not be so suitable for this application: I like the idea of a slighly flexible layer to cope with expansion/ contraction.

What do you think? It would be good to find a brand that has been tried and tested by ATMs. You know the cost of a misjudgement here!

Am I being paranoid?

Adrian

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I would highly recommend using 3M 4611F Acrylic Foam tape to attach your secondary. It is a high grab tape with very strong adhesion. Check out the specs.

Make sure both surfaces are clean, I used isopropanol alcohol. I have used this tape to fix rear view mirrors to windscreens, and they have never come adrift. My Land Rover is wetter on the inside than out, :rolleyes: so the damp, frost etc has no effect. HTH

Edited by Freff
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I have a roll of slightly less than one millimeter thick, foam-like, double-sided adhesive that seems very good. It hasn't come off yet ;)

I have no idea of where I got it or what it is...

/per

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Per it sounds similar to the 3M tape which is .75mm x 19mm x 3m. This tape is used in one situation where it holds huge sheets of steel together whilst being powder coated. It's not cheap, but for piece of mind.....

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The tape's very strong bond looks reassuring.  The attachment also has to apply no stress to the glass and I would guess that might happen with a large patch of tape compared to three blobs of silicone adhesive. Hmm. 

I'm stressing too much over this I think!

Adrian

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Adrian.. I think you maybe.

The tape is foam based so there is a very small amount of flexibility. I have never liked silicon for this type of bonding, it doesn't seem to get into the micro porous surface of metal like chemical based tape or glues.

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Hello Gents,
In my previous post my offset calculation is the same like yours 2.99 x 1,41 = 4,2mm.
My current offset is 4.2mm at telescope. I have almost even illuminated field. (I used double side adhesive tape).

I have made more accurate calculation with (equations from http://www.telescope-optics.net for Newton) Results bellow:

post-17243-0-71946800-1383218845_thumb.p


H is distance from SM to focal plane and S is distance from primary to secondary , f=S+H. I don't have telescope at home. I dont know H value but I aproximate it is about 230 mm. It is simple to measure S (distance from PM to SM = focuser axis. It's mean offset should be about 3.7 mm along mirrored surface. If it is true, my offset is now only 0.5mm too big. I don't know if I was so accurate. My current flat looks good.
I am not sure data and equations for this calculation. Probably PM is biger than corrector (>190mm), native focal lenght of mirror is slightly longer than 1000mm, it is MN. I think good way to know correct value would be measure offset in right working scope (Per friend scope for example :) ). I asked Orion USA (Orion MN 190 version is almost like Skywatcher) about offset. They said it is old product and not produced from years. I don't speak English well. Maybe someone form UK could phone Orion USA Support.

Offset is responsible for even iluminated field:

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4242330/page/0/view/collapsed/sb/5/o/all/vc/1

Quote from http://www.telescope-optics.net/newtonian.htm

"Offsetting Newtonian diagonal flat doesn't influence image quality, but evens up field illumination and makes collimation easier, by centering the flat in the focuser opening (for near-minimum size diagonal flat, offsetting may also prevent a small light loss at the field center)."

Wrong offset is not directly responsible for oblong stars at corners.

I haven't made test shots. Yesterday weather was verry good but wife ..... Maybe tomorrow :).


 

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According to Pinpoint, my scope comes out at 997 to 998 mm of focal length. The corrector is smaller than the primary mirror, a feature that is a part of the edge coma combatting of the design.

I am going to figure out how to get the mirror on straight and re-glue mine with the proper offset. After that, centering it in the focuser should do the trick!

We are almost there and the mist grows thinner for every iteration here. Good thread!

/per

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When I first fitted my Moonlite focuser I collimated as http://www.cloudynights.com/item.php?item_id=2677 in Adrians post.

The most critical step I found was making sure the focuser was completely square to the tube. After fitting new longer bolts to the secondary I used a spacer to adjust the gap between the SM holder and the base. This ensured the SM was reasonably square to the corrector plate. Any up or down in the tube was done by adjusting the three screws exactly the same amount,

During the initial collimating I used the corrector plate ( slightly slackened) to adjust the tilt. A Cheshire was used for checking concentric alignment. Here are a few Images of the first analysis with CCDInspector. Unfortunately, the trial period ended before I could save the final one, which were better again after a more thorough collimation. 

post-7131-0-96886500-1383222994_thumb.jp

post-7131-0-40599300-1383223007_thumb.jp

post-7131-0-73186000-1383223019_thumb.jp

post-7131-0-13030000-1383223033_thumb.jp

post-7131-0-80092700-1383223043_thumb.jp

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The last analysis showed a 0.0 in both axis, and the two X in the flat frame on top of each other. Unfortunately, I don't have those results. But as you see it can be collimated. :cool:

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Hello Gents,

In my previous post my offset calculation is the same like yours 2.99 x 1,41 = 4,2mm .......

...... Wrong offset is not directly responsible for oblong stars at corners.

I think it's possible that wrong offset can affect off-axis stars, because it causes the primary mirror optical axis to be tilted slightly to achieve collimation. If the PM optical axis is not aligned with the OTA axis, it does not pass through the centre of the corrector lens. My guess is that the corrector then would not correct coma evenly across the field and that could cause bad-shaped stars at some place in the frame.

Adrian

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