Skywatcher 190MN / fitting of a Moonlite focuser

[perfrej]

I read between the lines that you are worried about mirror flop. I solved that by adding some gasket material between the mirror holder assembly and where the three nylon screw adjustments sit (with a hole for the nylon screws, of course). After that I can build sky models that show promise, something I could not do with the scope as is from the factory. Also, collimation screws (primary) should be tightened as much as possible.

/per

[Freff]

Yes slop could be an issue, but I would have thought that Robert would have had that sorted by this time.

I was more interested in the results of different star field densities, I was getting inconsistent results (albeit small ones) with CCDInspector, although star shapes and flat field were ok. I therefore left the collimation alone with the results I've shown above. 

[olivdeso]

Hello, at our side in France, Joss has added 3 lateral screews. His model of SW190MN did not have the lateral nylon screews, the miror could move a lot when we released the presure from the lateral ruber pads.

Here is a link to the modifications made: (French astroforum, don't hesitate to ask me any translation if needed. The grey adhesive is a 1mm thick PTFE adhesive, cut from a 25mm ribon roll)

http://www.webastro.net/forum/showpost.php?p=1718776&postcount=24

You can see that the lateral screew is not exactly at the right position, it is too low  : it should be a little bit higher, at about 12mm from the miror backside plane. But this would require taller lateral parts made of some stiffer material. Also the 3 points of the miror cell are not well located, too close to the miror edge. They should better be located on a circle of  about 100 to 120mm diameter, which would decrease the miror error by about 4 times. But anyway, like this, the error is still lower than L/4, (about 90nm PTV on wave with horizontal miror according to plop) so it should do the job well enough, at least until we improve the miror cell...

Besides, I am still meditating about Adrian's concern : the location of the secondary miror vs drawtube while keeping everything well squared...This means the secondary miror angle should be precisely 45° and the primary as parallel as possible to the corrector. So I am thinking about how to measure everything and if the measurement precision make sense of course...

Then the second step would be to adjust the focuser position (adjusting both lateral position and tilt) in order to "see" the whole primary miror surface from the focuser...

[perfrej]

OK, folks. Drastic moves here. I am in the process of ironing out the final issues with a remote setup that gees to France in early December. Still, I just want this 190 to work so I put the Centaur tripod with the GM1000HPS on the balcony as an extra rig.

I think I am finally getting some performance out of this baby! Please ignore the fact that two of the primary mirror holders are a tad too tight. Here's a 10-minute sub, acquired unguided in Ha with my somewhat dated SBIG ST-8300M. It is just the single sub, no processing except a stretch (no BIAS, no flat), Melotte 15.

/per

[perfrej]

Ah, the image processing on the site is a mess. Here's a full jpeg: http://filer.frejvall.se/Mel15-Ha-ss-10m.jpg

/p

[ambro]

I was more interested in the results of different star field densities,

Field around Polaris. Previous shot was near Pleiades. Both images are from the same sesion.

I have to make longer exposures with better focus and guiding.

[opticalpath]

Looks good Per,  what was the latest change you made?

 

I also made some changes and, not perfect, but got a big improvement.   Here is my current status:  From last night ...... (Click image for full size) 

 

A single short 1x1 luminance exposure of a star field ; no flats/ darks/ bias, just a stretch:   http://universalconstant.com/test-101.jpg

and a single 10 minute Ha exposure (2x2); dark subtracted, no flats or bias, just a stretch:  http://universalconstant.com/Sh2-232-020.jpg

 

 

The big change I made was to remove the Moonlite focuser and remount the original SW focuser.  (I shimmed the loose-fitting drawtube extension to make it steadier).  The SW focuser sits higher up the OTA compared to the Moonlite so it's not necessary to lower the SM position to get it concentric with the drawtube, as I had to with the Moonlite.  I also re-collimated, combining the good suggestions of many people with a few of my own ideas.  In summary, these are the steps that worked best for me so far:

 

1. Remove primary cell and check physical centring of PM in the tube; verify centre marking ; replace primary.

2. Remove corrector assembly

3. Square focuser to OTA (laser and card templates or method of your choice)

4. Align primary mirror axis to centre-line of OTA (using '3 strings' method with camera, as described earlier)  ** I believe this step is crucial **  From here, change PM tilt as little as possible. 

5. Set initial position of the three secondary mirror tilt screws so that top of the mirror post is parallel to the corrector (i.e. an equal gap all round between SM post and central mounting boss in corrector)

6. Replace corrector assembly

7. With a cheshire/ sight-tube, adjust SM rotation and height. N.B. when raising or lowering SM, adjust 3 screws by equal amounts to preserve parallel orientation. Aim to get SM offset centre mark under Cheshire crosswires. 

8. Make initial adjustment of SM tilt in the usual way to centre the reflection of PM etc.  N.B. should only be small adjustments if you started with parallel orientation as above

9. Refine SM tilt adjustment using barlowed laser method described earlier by Olivier.

10. Refine PM tilt using barlowed laser method.  Required adjustments should be very small if step 4 was done.

 

I think that establishing 'squared' initial positions of PM and SM (steps 4 & 5) ..... and then making only very small subsequent tilt adjustments ..... is crucial.  It keeps the optical axis close to the centre of the corrector - which I believe is important for uniform coma correction.  If you don't do these steps, you can end up with what looks like a good collimation (and actually it would be fine for a Newtonian), but with some built in tilts that make the optical axis not aligned with the OTA centre line, and so not aligned with the centre of the corrector .... which can upset coma correction.  Anyway, that's what it seems like.

 

There's still a question about how SM offset is implemented.  Because the secondary is mounted geometrically centred in the OTA of the MN190 - i.e. without lateral offset - the offset is a compromise and there will be some misalignment of the primary axis to the corrector.  In theory, detaching the SM and remounting it with built-in lateral offset should yield optimal alignment with the corrector and provide the potential for best collimation.

 

 

Adrian

[perfrej]

Excellent!

That looks really good. I'm looking at my output right now and will post something in a while.

The 190MN is definitely worth the effort, at least for us DIY buffs

Have we established that the SM is indeed not offset from the factory? I will ask Grinde to peek at his today - it's stock.

/per

[perfrej]

Some test images. One stack of 18 Ha 10-minute subs. Cleans out the stars compared to the single sub.

This is my 190MN with stock focuser, SX wheel, Baader 36mm unmounted filter (7nm) and SBIG ST-8300M. Mount is the little 10Micron, GM1000HPS unguided with a not so good model (quick-and-dirty).

Full size single sub: http://filer.frejvall.se/HaTest_190_single.jpg

Full size stack: http://filer.frejvall.se/HaTest_190.jpg

Stars are still a bit triangular but that is easily fixed. The important thing is that collimation is moving in the right direction! Yeah!

/per

[perfrej]

A random single sub Ha:

Look like it is really moving in the right direction! 

/per

[Freff]

Adrian those images look excellent, appears you've nailed it.

[opticalpath]

Some test images. One stack of 18 Ha 10-minute subs. Cleans out the stars compared to the single sub.

 

This is my 190MN with stock focuser, SX wheel, Baader 36mm unmounted filter (7nm) and SBIG ST-8300M. Mount is the little 10Micron, GM1000HPS unguided with a not so good model (quick-and-dirty).

 

Full size single sub: http://filer.frejvall.se/HaTest_190_single.jpg

 

Full size stack: http://filer.frejvall.se/HaTest_190.jpg

 

Stars are still a bit triangular but that is easily fixed. The important thing is that collimation is moving in the right direction! Yeah!

 

/per

Excellent! .... we're getting there at last.

Adrian

[opticalpath]

..... Have we established that the SM is indeed not offset from the factory? I will ask Grinde to peek at his today - it's stock.

 

/per

It looks to me like the SM is mounted geometrically centred, Per, though it's quite hard to tell for sure just by looking. When I had the SM dismounted I made a rough check:

It's a poor photo but seems pretty close to geometrically centred.

 

When I look straight into the OTA, I can't see any lateral displacement of the SM outline;  it looks concentric/ centred in the tube.   By contrast, when I do the same with my Explore Scientific MN152, I can clearly see that the SM is not centred/ concentric and is physically offset a little in the direction away from the focuser.

 

 

The SM offset 'centre' marking is another matter.  Mine is definitely in the wrong place if it is supposed to be in the fully offset position; it's only about 1.6 mm from the geometric centre.  I have a friend with this scope and he tells me that his SM marking is 4mm from the geometric centre .... the correct full offset.  So there is some variation.   It would be very interesting to know where the mark is on Grinde's scope AND if it appears centred under the Cheshire cross-wires.

 

I am tempted to adjust my collimation once more, this time using the dot I marked in the correct place on my SM, rather than the (incorrect) factory mark I used for the current collimation.  But I don't want to upset what I've got. Perhaps it's time to stop fiddling and start imaging again!

 

Adrian

[opticalpath]

Tonight I noticed I have some elongated stars in one corner again .   Seeing is awful so I'll wait for better conditions to judge properly what is happening, but it seems like maybe something has moved.  I'll need to check the primary mirror side supports, I think. 

I don't have any side adjustment screws on my scope so I shimmed the posts that support the mirror clips to restrict sideways movement. Maybe that wasn't enough though and the PM shifted slightly.

 

Adrian

[opticalpath]

OK Adrian, here's the story...

 

My original is two-speed and almost as rigid as the Moonlite. Again, I am definitely not impressed with the Moonlite as it does have some slop even after a thorough tightening session. If I get this working I am going to glue the extension tube so that it really stays put. It is OK as it is now but not good ......

Per, I improved my SW focuser by shimming the pull-out section of the drawtube - I wrapped a couple of layers of thin plastic sheet (cut up A4 plastic section dividers) around the extension section making it a tighter fit in the outer drawtube. It's awkward: there is a little shoulder on the front edge of the outer drawtube (to prevent the inner tube coming right out) that makes it difficult to get the spacers in from the front. I did it from the back end of the drawtube, after unscrewing the ring that acts as an end-stop. It's fiddly but the extension piece feels more solid now after tightening the clamp.

A better permanent solution might be to insert three spacer rails 120 degrees apart to hold the extension section centred, then squeeze in some epoxy or gap-filing compound - i.e. glue it like you said.

Adrian

[perfrej]

Folks,

I took my bottom assembly out in order to fix the slightly over-tightened primary mirror. I figured then figured, what the hell, I'll just give it another go as it was a bit sloppy in the stock focuser.

The Moonlite... I looked at it, fiddled with the screws, and suddenly realized that I had managed to get it really rigid. I think I had the two tightening screws slightly out of whack with each other before, and this time I had slowly worked them equally in small increments. Hmmm... Crayford or not, it is definitely better than the Skywatcher one, that is for sure.

After taking the whole thing into my study, I took the top assembly off and measured the distance from the top of the tube to the upper inner edge of the draw tube. 61.3mm... I also used my Hotech laser and checked the "pointing" of the draw tube and made a note of that. Took it off and mounted the Moonlite. 61.1mm. So why shouldn't he Moonlite work?

It needed very little in terms of adjustments. Made a few runs of collimation with different mirror positions and again found that I have no way of getting the flat frame analysis any good. As my Catseye system should provide excellent collimation I decided to give it a go despite the uneven illumination.

Better than ever... I guess the Moonlite is OK (not a Feathertouch, but...) and that the uneven illumination is a mirror offset problem. I have a very slight tilt left that I eventually will correct when I have my Provence setup running in December. It will be a Christmas project, I think

/per

Full size: http://filer.frejvall.se/20131117.jpg

[opticalpath]

Looks pretty good, Per! 

I think we made some progress understanding the collimation of this scope, but still it feels to me like there's some unknown factor X at work that makes the process somewhat hit and miss.  Sometimes it turns out well .... other times not so well ..... but what did I do differently?  I don't really know.   I'm still looking for a repeatable procedure that *converges* reliably instead of hunting around for the sweet spot.

Good that you got your Moonlite working well.  I think with the tension screws, you have to tighten them just a little, then gently rock the drawtube to ensure that the flat has seated evenly on the drive spindle, before tightening them up equally to the desired tension.  I have some wear lines on mine now .... and I notice a couple of rough marks on the drawtube flat . 

Adrian

[perfrej]

My draw tube has taken a beating too. There are lines where the bearing have contact and there are some dents in the flat area. Aluminium... Pah!

/per

[Gunnar]

Looks good, Per!

[opticalpath]

An update on the novel method described earlier for using a barlowed laser to align the *secondary* mirror using a reflection of the secondary centre marking shadow on the primary.........

The method works but I think it is dependent on very accurately centring the secondary centre under the focuser draw-tube. Unfortunately it is very sensitive to small up-down centring error of the SM; any error in the up-down centring translates to the centre ring being illuminated by angled off-axis rays from the diverging light cone coming from the barlowed laser.  This results in a displacement of the shadow on the primary, and worse (if my geometry is right), the error is *magnified* very roughly in the ratio of the distances SM to PM and SM to laser  about 4 or 5 to 1 in my case.

Adrian

 

 

Olivier,

The method you described for aligning the *secondary* using a barlowed laser is a great discovery! A very nice way to do this adjustment.

Guys, if you haven't already .... you must try this; it's a really nice method. Here is what it looks like. This is a close-up photo of the centre of the primary mirror. You can see the laser spot is spread out to a wide patch by the barlow. Against that background you can clearly see the PM centre marking, the shadow of the factory centre mark on the secondary, and also a shadow of the black dot that I marked on the secondary in the (correct) offset position. It's very easy to use these shadows to adjust the secondary tilt. And just like using barlowed laser for primary adjustment, the method is not sensitive to axact alignment of the laser!  Thanks Olivier.
 
Adrian
 

[opticalpath]

Well, I bit the bullet and decided to remove the secondary mirror from its mounting post and reattach it with full offset.

As originally mounted, the SM was glued to the mounting post with zero offset; i.e. the centre line of the mounting post was directly above the geometric centre of the ellipse. Looking down on to the corrector, the circular outline of the SM looked exactly centred in the OTA confirming zero lateral offset built-in to the mounting. The factory 'centre' marking on the face of the SM was about 1.5mm from the geometric centre, whereas the theoretically correct offset optical centre for this size of mirror is 4.25mm from its geometric centre.

In case anyone is interested, here's how I did it:

I drew a paper template of the ellipse, made a small hole at the correct offset position, and used this to make a new offset centre dot mark with a pen, having removed the factory mark with acetone. The mirror was quite tough to remove from its mounting post. Don't know what kind of adhesive they used but I couldn't cut through it and had to pick away at it with the point of a scalpel to get it free. After cleaning up the surfaces I reattached the mirror with 3M VHB acrylic tape. Because of the 45 degree angle, it's tricky to reposition the mounting post accurately, ensuring that the post is directly above the new offset centre and the mirror is not rotated. To make this easier I made a jig by cutting a 32mm hole at 45 degrees through a block of wood. This holds the mounting post securely while the mirror sits of the surface of the block and can just be slid forward by the required 4.25mm from its marked original position.  I drew the outline of the ellipse on the block in its factory mounted position, and another outline in its target new offset position.  Then attached the adhesive tape, aligned the mirror with the offset outline and pushed it down on to the post.

 

I'm not sure how I would have reattached the mirror accurately without some kind of jig like this.  Even so it was tricky; the thickness of the adhesive tape pushes the mirror further away than the original thin adhesive layer so I had a couple of tries before getting it right.

The mirror mounted in its new offset position sits lower on the post and closer to the primary, so the SM now looks too low in the view through the drawtube. The mirror had to be moved up closer to the corrector in order to get a concentric outline through the sight tube. In fact it had to be moved up almost as far as it would go, with the three tilt adjustment screws almost fully withdrawn!

By the way, it's very worthwhile inserting a plastic disk (cut from a plastic milk carton) where the 3 screws press down on the mounting post. The screws then push, not directly on to the metal, but on to the plastic disk. The slight softness in the contact that this introduces makes the final fine adjustment of the three screws much easier. I don't remember where I read this tip, but I recommend it - it really works.

Now just waiting for improvement in the weather to try out the new setup.

Adrian

[olivdeso]

Hi MN190 adicts

Here in Paris, we finally found a process that seems to work well enough for deep sky imaging. (after many trial and errors of course and after reading the Vlad's webpage about the MN)

http://www.telescope-optics.net/Mak-Newton.htm

Firt of all, the key point to get orund star across the entire field, is to have the primary as parallel as possible to the meniscus (and centered of course)

So the colimation process will consist in 2 main steps:

1. square the primary

2. adjust the secondary tilt and focuser position to achieve a perfect on axis collimation 

Tools :

- Howie glatter laser with holographic circles pattern : the "attachment" with circle patterns is sufficient I guess together with the 1,25/2" laser

- TV x2 powermate

Her are the steps:

1. squaring the primary

-----------------------------

1.1 verify the secondary angular position and adjust if needed. At our side we had to remove the meniscus to be able to adjust the secondary angle. Once adjusted, the secondary should be seen as 

1.2 adjust roughly the focuser position in order it faces the secondary

1.3 use the laser to colimate the secondary. The 2" howie glatter gives a perfectly colimated beam. (turn the laser into the focuser to verify). adjust the 3 secondary tilting screews in order the beam points at the center of the primary.

1.4 screew the cilcles attachment to the laser, and plug the laser into a 2" barlow, here we used the x2 powermate.

We have to use a barlow here to get a wider pattern, in order to get circles until the edge of the primary (the circle attachement is rather made for cassegrains that have a convex secondary resulting in a wider spaning angle of the pattern)

* If you don't have any 2" barlow, just instert the Howie Glatter laser, it will still work if the secondary does not have any significant offset.

[olivdeso]

 

continuing...

 

without barlow the circles will be smaller: (2 times here)

 

 

1.5 using the barlow, you will get circles on all over the primary. Double check that the circles are well centered on the primary (the center beam may not be excactly at the center, so stay focused on the circles only)

 

1.6 point the tube to the ceiling : it will show nice circle patterns.

 

1.7 adjust the primary collimation screews in order to get the largest circle centered with the tube opening. (It should be also possible to use a transludent film put onto the meniscus.)

 

 

1.7bis alternate solution without barlow : since the secondary is factory centered (as far as we know), we can use the secondary shadow to center the circle:

 

 

the square grid pattern could also be used

 

 

Note : if you don't have any tools, it should be also possible to collimate the primary, by looking at the secondary reflexion onto the primary, seen from the front aperture of the telescope. Just stands few steps away from the front opening and adjust the primary screews until you get the secondary reflected image centered with the secondary itself, indicating the primary is collimated

 

Now the primary is collimated as quite parallel to the meniscus, assuming the menicus itself is well centered and squared. (Note that we have also added some lateral screews to held the primary well on its center position.)

 

IMPORTANT : once the primary is collimated, never touch it again. we will only adjust the secondary if further collimation is needed.

 

 

2. collimating the secondary and adjusting the focuser position.

------------------------------------------------------------------------------------------

 

2.1 use the laser without the barlow nor attachment, only a single sharp beam, and adjust the secondary collimation screews in order the beam returns to the center of the laser

 

 

2.2 observe the laser beam spot on the primary. It should be at the center of the primary. If it is not the case, adjust the focuser position a little bit and try again from step 2.1

 

2.3 once the focuser is at the right position and the secondary is collimated, the laser beam points to the center of the primary and returns to te center of the laser frame.

 

2.4 make a second iteration of the whole procedure, starting again from step 1. very small adjustment might be required.

 

So now you have the scope well collimated on the axis and also well corrected on the entire field thanks to the primary collimation made in priority.

 

Open points : 

 

- the secondary does not seem to have the right offset (either very small or none) -> so it might be still possible to find a better secondary/ focuser position. 

 

- here we used a laser for primary and focuser adjustments. On the field, we should have only to adjust the secondary at worst case and nothing else. The cats eye autocolimator, may be the right tool then, it is very precise.

 

- here we did not fully squared the focuser (we made it roughly using some 0,2mm washers in fact) . Depending on images, you might have to solve a little tilt issue if the focuser is not squared.

 The easiest for tilt issue analysis, is using a bahtinov mask and check the 4 corners:

       - center a mag 1 or mag 2 star and focus it using the bahtinov mask

       - move the star to each of the corners and note the de-focus (take a picture, count the pixels in paint, or use the bahtinov grabber software)

       - adjust the focuser tilt until you get the same de-focus in the 4 cornners (corresponding to the field curvature)

       - each time you add or remove a whasher or shim, make sure the secondary and focuser are still well collimated on axis using the laser, like in step 2

 

 

 

Olivier

[opticalpath]

Hi Olivier. This looks an interesting method. I agree - it seems critical to get the primary mirror 'axis' (not strictly an optical axis) exactly centred in the tube so it passes through the centre of the corrector and the mirror is parallel to the corrector. The circular holo. pattern projected on to a screen placed over the corrector sounds good.

I don't have the Glatter laser but anyway I think I have a couple of difficulties following your method exactly: 1) I replaced the focuser with a Moonlite ... which does not have slotted mounting holes, so no position adjustment is possible and to get alignment with the focuser the only option is to move the secondary mounting, and 2) I already removed the secondary from its mounting post and reattached it with the correct full offset, so it is no longer geometrically centred in the tube.

I found that the factory 'centre' mark on the secondary was offset about 1.6 mm from the geometric centre, but the correct offset for this scope should be 4.2 mm, so I removed the factory mark and made a new dot in the correct position to assist with alignment then removed and re-mounted the secondary. I immediately got better results after re-mounting the secondary with full offset. I think an important reason for this is that the correctly offset mirror produces the expected correct view through a Cheshire or collimation cap when collimation is good. Before I offset the secondary, I always found that getting the 'correct' view through the collimator did not guarantee well-corrected images!

Adrian

[jaymichael]

Thank you deeply to all the contributors of this thread.   I recently purchased one of these scopes used and it was completely out to lunch.  A couple long days rebuilding  with the knowledge found in this thread has me producing beautiful flat image frames.  Thanks heaps