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MN190 Collimation woes

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#1 coenie777

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Posted 05 March 2017 - 01:05 PM

This question relates to my MN190 but I guess the technical principals at play here may be the same for all Newts.

 

In trying to tweak my MN190 to show round stars all the way to the edge (I have them round up to around 90% of the edge at this time), I need some advice please.

 

I build myself a 2" Cheshire sightube and tried my best to get the cross hairs exactly square. They are within 0.5mm I would say.

 

I started off by trying to get the marking on the secondary mirror centred on the Cheshire. Here I need the first bit of advice;

 

  • For fear of extending the secondary mirror down too much and risking a mirror drop, I also loosened the focuser and moved it up and down the tube.
  • I got the crosshair close to the marking, but it was lower than the horizontal crosshair. To explain this, if I moved the focused up and down the tube after loosening the screws, the centre dot would move left and right of the vertical cross hair. The position relative to the horizontal line is not affected by this. Moving the focused only moved the mark left and right (if I do not twist it laterally off course).
  • I then figured out that by tilting the focuser on one side, I can actually get that horizontal offset aligned. I added some padding to the one side of the focused and got the dot centred. So I raised the lower fixture point, tilting the focuser to one side.

First question is if tilting the focuser by adding padding is a good idea or not?

 

Second question is if this scope should actually be able to have the dot centred without having to tilt the focuser.

 

Once i got the dot centred, it looked like the secondary image circle under the focuser were not 100% following the roundness of the Cheshire. To my eyes it looks like the mirror needed to be dropped further down the tube.

I can see all three primary clips and they actually seem to extinquish  at the same time. It is just that the circle looks to be slightly shifted towards the upper end of the tube (towards the corrector lens).

Before trying to drop the secondary down in order to try and make up for the above, would this be recommended? If the centre dot of the secondary is under the crosshairs now, can it still be a case of the secondary being too high up the tube? Surely if I drop it down now, it would move the dot away from the crosshair centre?

 

Clouds are going to not see me being able to image anytime this week so I hope someone can share some experience or maybe advice that I can try in the mean time.

 

I am fairly confident that the secondary twist is correct as I used a laser and got the dot actually on the primary centre dot, taking care while tightening the lock down nut to not twist it again.



#2 Starman1

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Posted 06 March 2017 - 06:51 PM

If the secondary is centered between the tube walls, you do NOT want to center it under the focuser.

This results in having the optical axis tilted slightly toward the focuser.

While OK for a newtonian, it is ESSENTIAL to have the optical axis centered on the corrector in your scope.

So the secondary either must be Bi-directionally offset (down the tube to center under the focuser AND away from the focuser.

(maybe this is built into the secondary holder?)

OR, adjusted so the secondary is centered between the tube walls and the center of the optical axis hits the exact center of the secondary with a 90° reflection thereafter.

The appearance of the reflected image of the primary will appear to be low on the secondary, not centered, if this is the case, and illumination around the edge of the field

will not be uniform.

 

I've only critically looked at one commercial Mak-newt, and its secondary had the bi-directional offset built in.  When you looked at the front of the scope, it was obvious the center of the secondary holder

was not centered under the corrector.

 

So, is the secondary offset away from the focuser, or is it centered under the corrector.  That will determine how to proceed.


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#3 ram812

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Posted 06 March 2017 - 08:48 PM

Starman has it, but a star test will definitely show your adjustments (When the weather clearsfrown.gif !) It's a great test. Ralph



#4 Astrowl

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Posted 07 March 2017 - 05:09 AM

Hi Coenie,

 

In my opinion, there are two reasons that can explain this.

First, maybe the less possible regarding your comment, a small twist of primary. If it is the case stars on the edge should tilt the same direction.

Second, maybe the most possible, the primary support is not well designed on the MN190 (it is a well known problem). There are 6 screws on the bottom of the tube, 3 to push the mirror (to collimate) and 3 to pull (lock the mirror).

When the pull screws are tightined too hard, they generate pressure mechanical constraints on the mirror that can create coma on the edge. It is difficult to find the correct balance to do not apply too much pressure on the primary mirror. One of my friend has rebuilt the primary support to correct this issue.

On my side, I have not yet changed anything on my MN190, because I have not really noticed this problem.

 

Just one question Coenie, do you know where I can get the user guide for the MN190 ?

 

Thanks



#5 Jeff B

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Posted 07 March 2017 - 12:36 PM

If your focuser can translate along the tube that's good.

 

This is what I do for my Intes Mak-Newts:

 

1. I loosen the secondary holder and pull the holder flush with the back of the meniscus and a rough rotation as seen through the focuser.

2. I then move the focuser forward or back along the tube and make sure it's axially centered over the secondary using a sight tube.

3. Again, using the sight tube, I then rotate the secondary so that it's "tangentionally" centered.

4. I now use the sight tube and the focuser tilt adjustments to center the focuser over the diagonal using the inside edge of the sight tube as my guide.  This can actually be really difficult as the stray light baffling Intes incorporates under the focuser is typically VERY effective making it difficult see the inside edges of the sight tube and exact outside edges of the secondary.  You might have to flood it with bright light.

5. If I made any adjustments during 4, I tweak the secondary rotation if needed.  I've typically not needed to tweak the axial position of the focuser.

6. Using a good laser, I tweak the secondary screws so that the beam hits the center spot on the primary.

7. Using a barlow I tweak the primary until the shadow of the center spot falls evenly over the laser's aperture.

 

So this is basically the same procedure I use for my newts except for step one, which I find very important and the fact that I can't reach in from the front to adjust the secondary's rotation.  I instead use the secondary holder.

 

This procedure works really well for me.

 

Good Luck!

 

Jeff


Edited by Jeff B, 07 March 2017 - 12:36 PM.


#6 coenie777

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Posted 07 March 2017 - 01:14 PM

Thanks for the feedback guys.

 

 

If the secondary is centered between the tube walls, you do NOT want to center it under the focuser.

Starman, since this is a MakNewt the secondary is fixed between the tube walls. Am I understanding you correctly that you are refering to the position of the secondary relative to the tube sides if I viewed it from the front of the tube? If I am not to centre it under the focuser, how much do I leave it up or down the tube?

 

 

So, is the secondary offset away from the focuser, or is it centered under the corrector.  That will determine how to proceed.

I measured from the edge of the tube diagonally between two screws on the tube ring and I got the same measurement to the edge of the cap that goes onto the secondary fixture. It measures 96mm from the edge everywhere. Would this mean the secondary is centred between the tube walls or am I missing the point somehow? How do I determine if the secondary is offset from the focuser without dismantling the corrector?

 

I will definitely try a star test ram, as you say, as soon as weather permits confused1.gif

 

Watchever, here is a link to the Orion MN190 which is apparently the closest you will get to a SW manual for this scope.

 

http://www.telescope...29370_07-09.pdf

 

I checked the stars in the corners and they are not pointed in the same direction but rather seem to radiat from the centre. You may have a point there with regards to the small allen nuts thay you lock the primary with. I tighten mine by quite a margin, usually having to re-do collimation slightly thereafter. I will try and bring off some tension there and see of maybe this resolves anything.



#7 Starman1

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Posted 07 March 2017 - 02:41 PM

If you look at the side of the secondary at an angle through the corrector, ignoring the holder on the corrector, you can see whether or not the edges of the secondary seem to correspond equally with the secondary holder all the way around.

If the secondary appears to be slightly closer to the tube wall opposite the focuser, you use one method of alignment.

If the secondary appears to be exactly centered between the tube walls you use another method.

 

To wit:

If the secondary appears to be slightly closer to the wall of the tube opposite the focuser, then you can use a sight tube/cheshire to center the secondary under the focuser and collimate with the normal tools.

Simple.  Exactly like a standard newtonian.

 

If the secondary appears to be centered in the tube, then you will NOT center the secondary under the focuser.

You will try to have the sight tube crosshairs or laser hit the secondary mirror on its geometric center.

This can be accomplished with a small dot made by a sharpie dead center on the secondary reflective surface or by ignoring the centering of the 

secondary mirror under the focuser and just collimating by ignoring that parameter.

When the scope is collimated, if you look in the focuser through a sight tube, when the crosshairs are lined up or the laser hits the center of the primary

then the reflection of the primary mirror in the secondary will appear to be slightly offset away from the center of the secondary mirror, toward its lower end.

The amount the primary reflection will appear offset toward the primary will be small--a few millimeters--but it will be obvious.

 

Now, a commercial Mak-newt is probably designed to have the secondary offset away from the focuser, regardless  of the centering of the plate on the corrector.

You should be able to tell by where the collimation screws are.  If the center of the collimation screws triangle is dead center on the holder on the corrector plate, it is probably not

offset.  In which case, it is best to ignore the lowering of the secondary to center it under the focuser.

 

If the secondary IS centered in the tube, AND you have centered it under the focuser, then collimation will result in a tilting of the optical axis toward the focuser.

That is NOT a problem in a newtonian, but in a Mak-Newt it is because the optical axis will no longer coincide with the center of the corrector and the correction of the lens will be

off-center, introducing misshapen stars, astigmatism, and poor images in general.

 

P.S. Just read the manual and the collimation instructions are worthless because you will NOT be able to get the images to look like the instructions.

AND, I note the collimation screws are centered, so it is unlikely the secondary holder is offset in the tube.  That is too bad, because it means your images will have to have

their edge-of-field stars adjusted by trial and error.  I assume you will make the primary holder modifications previously mentioned in the thread.

 

What I would do is this: Remove the secondary and corrector as one unit from the front of the scope and place a tiny Sharpie dot in the dead geometric center of the secondary's reflective surface.

(that is an equal distance from both ends of the ellipse in both minor and major axes).

Replace it in the tube and raise the secondary toward the corrector until a laser or sight tube simultaneously hits the center dot on BOTH mirrors.

This will assure the optical axis coincides with the center of the corrector.  You will not see the reflected image of the primary to be centered in the secondary's reflective surface through the focuser (it will seem a bit offset toward the bottom), but you will get as perfect a star image as the scope is capable of in your images.


Edited by Starman1, 07 March 2017 - 02:51 PM.


#8 coenie777

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Posted 07 March 2017 - 02:56 PM

Thanks for the reply Jeff

 

 

1. I loosen the secondary holder and pull the holder flush with the back of the meniscus and a rough rotation as seen through the focuser.

Please explain to me how this is done. Do you loosen the three adjustment screws and then pull back the whole assembly towards the corrector by tighthening the central screw? If you then moved the focuser laterally to centre the secondary and your movement is anadequate to get there, would you then drop the secondary again by loosening the centre screw?

 

 

2. I then move the focuser forward or back along the tube and make sure it's axially centered over the secondary using a sight tube.

Could you do this by centering the sight tube's crosshairs over the centre spot of the secondary?

 

 

3. Again, using the sight tube, I then rotate the secondary so that it's "tangentionally" centered.

Please excuse my lack off technical knowledge but could you perhaps explain how you would gauge the above from what you see? Has it to do with how round you

 

 

6. Using a good laser, I tweak the secondary screws so that the beam hits the center spot on the primary.

If I do not have a good laser, would I then use the combination tool and bring the primary mirror centre dot in line with the cross hairs and secondary dot?

 

I find that despite having read for three years about collimation and having access to really cool resources on the subject, I still catch myself not procedurally following the steps. From your explanation, it also is clear that there are steps where after you have done the adjustment you need to go back to a previous step and re-check. I will study all my collimation resources again as well and ensure that I do not skip steps. Thanks a lot for your valuable advice so far.



#9 coenie777

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Posted 07 March 2017 - 03:13 PM

Thanks Starman! From your description it sounds like the secondary is indeed centred in the MN190. Maybe a more knowledgeable MN190 owner can confirm?

 

The three set screws are definitely centred on the central screw from what I can see and which is nicely illustrated in this photo: (I do not have a permanent setup and my MN190 lives in a custom crate. So I need to haul the lot out every time to check anything. I found it quicker to just use images from the internet to illustrate rather than taking my own each time)

 

https://stargazerslo...33877605228.jpg

 

The secondary is centre marked with a nice round sticker so you can get really accurate as you can see the cross hairs in the middle of it. Here is a nice picture from a site showing this:

 

http://i.imgur.com/9toWrC0.jpg

 

I will make some modifications to the primary cell at a later stage. For now I have this suggestion on my A list of things to try next, thanks for all your help:

 

 

Replace it in the tube and raise the secondary toward the corrector until a laser or sight tube simultaneously hits the center dot on BOTH mirrors.



#10 Vic Menard

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Posted 07 March 2017 - 06:05 PM

How do I determine if the secondary is offset from the focuser without dismantling the corrector?

If the specifications of your MN190 are the same as those in the pdf, your offset is about 3mm (almost 1/8-inch).

 

If your secondary mirror is centered relative to the meniscus corrector, you will need to move the secondary slightly closer to the corrector instead of centered under the focuser. You'll know the secondary placement is correct when the reflections inside the primary mirror reflection are all concentric. Jason's graphic illustrates what's concentric (and what isn't) when the secondary mirror placement is "centered" (not offset).

Attached Thumbnails

  • centered.JPG


#11 Vic Menard

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Posted 07 March 2017 - 06:12 PM

Just to clarify, to get a "centered" secondary mirror placement, your secondary mirror will need to be offset from "centered under the focuser" about 3mm closer to the meniscus.



#12 Starman1

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Posted 07 March 2017 - 06:18 PM

Jason's illustration perfectly illustrates what I meant by "You will not see the reflected image of the primary to be centered in the secondary's reflective surface through the focuser (it will seem a bit offset toward the bottom)".

In this case the lower end (bottom) of the secondary is to the right of the image.

 

Thanks, Vic.



#13 Jeff B

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Posted 08 March 2017 - 09:57 AM

Great discussion and it has prompted me to go back and look at my two MNs, an Intes MN76 and APM/MW 8" F6.

 

For both, the exterior "holders"  with the adjustment screws are centered on the meniscus.  However, the secondaries and their holders do indeed have a very small radial offset, downward, towards the tube wall opposite their focusers.  Cool, and this explains why the collimation method I use works so well.

 

Conie, your physical and adjustment configurations may be different than my MN's, which might explain the need for a center dot on your secondary when mine have none.  If you do have a small radial offset in you secondary, it should be readily visible as your secondary is larger than the interior holder.....or you could just take it apart an measure it.

 

Good, informative thread.

 

Jeff


Edited by Jeff B, 08 March 2017 - 09:58 AM.


#14 coenie777

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Posted 08 March 2017 - 03:23 PM

Thanks for some really great additional pointers. Vic, thanks for clarifying the offset you refered to. It makes perfect sense now and I hope I will be able to look down the drawtube next time and see the whole picture, not just what I am convincing myself I am seeing.

 

I did manage a quick shot tonight before the clouds appeared. Unfortunately after polar alignment and fixing the focus on my guide scope I only managed one 60 sec exposure.

 

I did try and get intra and extra focal shorts to gauge the collimation. Below are the shots zoomed to 200% and overlayed with Als. Unfortunately I did not manage to have the star equally sized intra and extra focal. I may also have over exposed (Sirius)? Any comments are welcome. I also include a CCDI Curvature analysis. This was based on the 60 sec image. I also attach the image for review if required. From the image you will see that the lower left and right appear to me out of alignment.

 

If possible and just out of interest sake, if I want to look for the impact of what CCDI call X or Y axis tilt, where in this image would I see this? Is that the lower left and right being manifested? Where would I see the impact of curvature?

 

 

Extra focal:

 

Extra_zpszr25nmsr.jpg

 

Intra focal:

 

Intra_zps6y3gfttt.jpg

 

One 60 sec starfield exposure measured in CCDI:

 

CCDI_zpsh1wjmtmk.jpg

 

Image used for the above CCDI analysis:

 

Single__0014_ISO800_60s__30C_zpsfqekb4vq

 

 



 



How do I determine if the secondary is offset from the focuser without dismantling the corrector?

If the specifications of your MN190 are the same as those in the pdf, your offset is about 3mm (almost 1/8-inch).

 

Would you mind sharing with me how you calculate the offset Vic? I made a note of that and will keep that in mind next time I am adjusting things.



#15 Starman1

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Posted 08 March 2017 - 03:43 PM

Offset = (secondary size)/(4*focal ratio).



#16 glend

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Posted 08 March 2017 - 03:45 PM

You should never mess with the secondary offset on the MN190. The offset is factory set, and even if you remove the corrector for cleaning, and have hopefully used witness marks to properly relocated precise position, you do not move the secondary with the centre lock screw. This is exactly what messed up so many folks that tried to do early focuser upgrades - before there was a properly designed focuser adaptor for this scope. Moonlight now have a proper adaptor available, i know this because i designed it and provided the information, drawings and measurements to Ron. Mak-newt focusers must be very close to the front of the tube due to the secondary position, which cannot be changed. The MN190 secondary is in fact already centre spotted in the factory and thus is used to setup the optical path. I can't speak about the Orion version of this scope, which had the larger secondary. If you have purchased a scope that has been tampered with by someone else, and messed up, then yes you may have to put the secondary back in the correct position. In that case use the secondary centre spot applied at the factory and centre the focuser centre over it, this should get you very close. 


Edited by glend, 08 March 2017 - 03:52 PM.


#17 coenie777

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Posted 08 March 2017 - 04:07 PM

Offset = (secondary size)/(4*focal ratio).

Thanks Don!

 

Thanks for the bad news Glen lol.gif I am the person to blame for messing with the secondary. And I do not even have a nice Moonlight to show for it.

 

If the factory could align the scope then what would hold me back? Do they use special laser equipment or something like that? Honest question.

 

I have the Skywatcher MN190 and my secondary is spotted. If I did not touch the meniscus to date (luckily) should I not be able to follow the factory's steps in some way with the spotted secondary and get things sorted again?



#18 Starman1

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Posted 08 March 2017 - 04:50 PM

If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary when you adjust its tilt, then raise it in the tube toward the corrector until it does. If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center.

It doesn't sound like you have to lower it.

When you're done, the beam will hit the center of both mirrors simultaneously.

THEN collimate the primary mirror with a cheshire or barlowed laser.



#19 Vic Menard

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Posted 08 March 2017 - 06:06 PM

...If the factory could align the scope then what would hold me back? Do they use special laser equipment or something like that? Honest question.

I have the Skywatcher MN190 and my secondary is spotted. If I did not touch the meniscus to date (luckily) should I not be able to follow the factory's steps in some way with the spotted secondary and get things sorted again?

I collimated a poorly performing Intes Mak/Newt at the Peach State Star Gaze several years ago. The owner had installed a Clement focuser (with an adapter) to accommodate his imaging equipment. The focuser/secondary geometry was way out of alignment--thankfully both the Clement focuser and the adapter had leveling screws! 

 

It's important that the secondary mirror placement is corrected to ensure that the optical axis is perpendicular and central to the meniscus. The mounting for the secondary mirror on the Intes Mak/Newt was physically centered in the meniscus corrector, and the secondary mirror was offset on the mounting away from the focuser ("classic" offset). Once the focuser/secondary mirror geometry was sorted out, the rest was easy. The subsequent imaging performance was what you would expect from a Mak/Newt--outstanding.

 

(Tools used: CatsEye TeleCat combo tool, Glatter laser with 1mm aperture stop.)


Edited by Vic Menard, 08 March 2017 - 06:08 PM.


#20 coenie777

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Posted 09 March 2017 - 10:51 AM

If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary when you adjust its tilt, then raise it in the tube toward the corrector until it does. If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center.

It doesn't sound like you have to lower it.

When you're done, the beam will hit the center of both mirrors simultaneously.

THEN collimate the primary mirror with a cheshire or barlowed laser.

Thanks Don, the above sounds like a sure fire solution to get the secondary sorted. I will try this, albeit with a cheapy laser which I will collimate before the attempt. I plan to document this and will post my results. If this will resolve the secondary issue I can assure you that many MN190 owners would be forever greatfull towards you.

 

I really need a CatsEye and Glatter.



#21 Astrowl

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Posted 09 March 2017 - 11:32 AM

Hi Coenie on my MN190 there is a 4th screws next the 3 secondary collimation screws to change the secondary offset.



#22 Starman1

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Posted 09 March 2017 - 01:13 PM

 

If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary when you adjust its tilt, then raise it in the tube toward the corrector until it does. If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center.

It doesn't sound like you have to lower it.

When you're done, the beam will hit the center of both mirrors simultaneously.

THEN collimate the primary mirror with a cheshire or barlowed laser.

Thanks Don, the above sounds like a sure fire solution to get the secondary sorted. I will try this, albeit with a cheapy laser which I will collimate before the attempt. I plan to document this and will post my results. If this will resolve the secondary issue I can assure you that many MN190 owners would be forever greatfull towards you.

 

I really need a CatsEye and Glatter.

 

Argh, my explanation above had a typo!!

it should have read: "If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary, and when you adjust secondary tilt to place the laser beam in the center of the primary it no longer hits the center of the secondary, then raise the secondary in the tube toward the corrector until it does. I.e., If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center of the secondary at the same time it hits the center of the primary.  When you're in the right position, both centers will be struck simultaneously."



#23 coenie777

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Posted 09 March 2017 - 02:17 PM

Hi Coenie on my MN190 there is a 4th screws next the 3 secondary collimation screws to change the secondary offset.

I know of this fourth screw and was led to believe it is for locking rotation on the secondary in some way. If you say it can be used to adjust the offset, how do you go about that? Mine seem to be either locked down or unlocked. It does not seem to adjust anything if you tighten it more or less.


Edited by coenie777, 09 March 2017 - 02:27 PM.


#24 Starman1

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Posted 09 March 2017 - 02:21 PM

The center screw is a pull.  The 3 screws push.

By loosening the center and tightening the 3 screws, the secondary is moved down, away from the corrector.

By loosening the 3 screws and tightening the centerbolt, the secondary is moved up, toward the corrector.



#25 coenie777

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Posted 09 March 2017 - 02:40 PM

 

 

If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary when you adjust its tilt, then raise it in the tube toward the corrector until it does. If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center.

It doesn't sound like you have to lower it.

When you're done, the beam will hit the center of both mirrors simultaneously.

THEN collimate the primary mirror with a cheshire or barlowed laser.

Thanks Don, the above sounds like a sure fire solution to get the secondary sorted. I will try this, albeit with a cheapy laser which I will collimate before the attempt. I plan to document this and will post my results. If this will resolve the secondary issue I can assure you that many MN190 owners would be forever greatfull towards you.

 

I really need a CatsEye and Glatter.

 

Argh, my explanation above had a typo!!

it should have read: "If you adjust the secondary so that a laser beam hits its center, but the beam does not hit the center of the primary, and when you adjust secondary tilt to place the laser beam in the center of the primary it no longer hits the center of the secondary, then raise the secondary in the tube toward the corrector until it does. I.e., If adjusting the tilt to hit the center of the primary results in moving the laser beam away from the center of the secondary, then raise it until it does hit the center of the secondary at the same time it hits the center of the primary.  When you're in the right position, both centers will be struck simultaneously."

 

Don, thanks for the correction.

 

I just want to check that I understand you correctly. From another post I got the tip to pull the secondary up towards the minuscus as much as possible.

  • I will then ensure that the three tilting screws are all loose so the mirror is free hanging against the centre bolt.
  • I now will place the laser in the focuser and since I adjusted my focuser position previously, I will slide it towards the minuscus as much as the guide rails would allow.
  • In doing so I am trying to get the laser to hit the centre spot of the secondary.
  • Once I hit that centre spot on the secondary by either moving the focuser towards the top of the tube or the secondary down (if I run out of movement on the focuser), I will check to see if I hit the primary centre spot.
  • If I do not, I adjust tilt on the secondary until I hit the primary's centre spot.
  • I now check the secondary again. If the laser no longer hits the secondary's centre spot, I will raise the secondary towards the minuscus until the laser hits the secondary's centre spot.
  • I guess I will then alternate between putting the laser back on the primary spot, adjust secondary etc, until I hit both centre spots at once.
  • Would it be sufficient to then finish off with a barlowed laser collimation on the primary?



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