Skywatcher 190MN / fitting of a Moonlite focuser

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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.

These are fantastic results, Tony; looks like you nailed it. So good to know it can be done!

I'd really like to follow the path you took as closely as possible: can I recap to make sure I understood it correctly?

1. Remove the secondary mirror and re-tape it in a new position on the mount with the correct offset (4.2 mm on the diagonal).

2. Square the focuser as accurately as possible in the OTA.

3. Measure the approximate downward movement of the secondary that will be required to align with the focuser axis.

4. Make a spacer to bridge some/ most of the gap between the SM holder and base to help ensure that starting position of SM holder is parallel to corrector.

5. Do a normal (classical bi-directional offset) collimation procedure .... BUT use the three SM tilt screws only in tandem to set correct 'height', not to adjust tilt; adjust SM tilt only by tilting the corrector plate (?)

Is that correct? A couple of things I was not sure I understood fully:

* Did you never use the SM tilt screws individually, only in tandem? You used a mechanical means (spacer) to get the focuser base parallel to the corrector and maintained that throughout, making any tilt adjustment of the secondary only by raising and shimming the corrector plate, never using the 3 screws?

* Also, you mentioned how critical the fist step was. What method did you use to square the focuser?

Many thanks

Adrian

[Freff]

Hi Adrian.

1.. I never removed the secondary mirror from the mount, that has remained as factory set.

2.. I found the dead centre of the focuser hole in the ota and transferred that to the opposite side with the use of a laser collimator. There is a simple method of doing this if you need it.

3.. No measurement made for this. (see 4) 

4.. It is not necessary to actually make a spacer, it just happened I had a 3mm piece of acrylic to hand. Simply use a vernier gauge to measure the gap around the holder so its square. It just give the holder a square starting position. With the use of a Cheshire move the SM up or down the ota with the three adjustment screws and centre screw,   turning each one the same amount then tighten the centre one . This will keep the SM holder reasonably square. This should put the secondary spot somewhere in the ball park.

5..  Do a normal (classical bi-directional offset) collimation procedure after using the corrector plate the get the tilt as close as possible, then use the adjusting screws for the fine adjustment.

I found if you use one screw for adjusting you must use another one to take up the slack, or slacken one and tighten another. The centre bolt should only be used for moving the holder forward or aft. 

No shims used on the corrector plate, just slacken of enough the move it, there should be no discernible movement laterally or back and forth. Use the nylon screws to keep the plate centre in the ota.

As all measurements are taken through the focuser so I think it is critical it is square on the ota. I found the dead centre of the focuser hole in the ota, and transposed that to the opposite side of the ota and used a white pencil to mark the spot. With the focuser attached and a laser in the eyepiece holder, adjusted the focuser until the laser spotted on the white mark. Tightened down the focuser making sure the laser did not move from the mark.

Hope this makes sense.

[opticalpath]

Hi Tony.
Thanks for all this information. Still a couple of things to clarify if I can trouble you a little longer:

< 1.. I never removed the secondary mirror from the mount, that has remained as factory set. >

This is a surprise. Per and I both measure the offset centre mark to be too close to the geometric centre - i.e. not offset nearly enough - as determined by the standard offset formula. I wonder if your secondary is marked in the same place.  Furthermore, the SM is mounted physically centred it the OTA - i.e zero lateral offset built in to the mounting (should be 3mm off centre according to formula.)

I find if I centre the SM marking under the cheshire cross-wires, the outline of the secondary is definitely not concentric with the drawtube - a misalignment you would expect if the mark is not in the correct offset optical centre, so no surprise really.  I was wondering if you observed the same non-concentric view within the circle of the draw-tube end?

< 5.. Do a normal (classical bi-directional offset) collimation procedure after using the corrector plate the get the tilt as close as possible, then use the adjusting screws for the fine adjustment. >

So you mean that you use the initial positioning of the secondary mount parallel to the corrector as a way of getting the starting point close, but after that use the three tilt adjustment screws in the normal way to finalise SM tilt?  I can see this would avoid the mistake of trying to correct a different error by skewing the SM without realising it.

< No shims used on the corrector plate, just slacken of enough the move it, there should be no discernible movement laterally or back and forth. Use the nylon screws to keep the plate centre in the ota. >

Are you slackening off the corrector plate mounting screws just to enable adjustment and accurate (lateral) centring of the corrector in the OTA, or for something else?

Thanks again.

Adrian

[Freff]

Adrian..  If I remember correctly the sm mark is on the centre of the mirror. I'll check it out after dinner and peep down the focuser and tell you what I see. 

2.. Yes. When I initially removed the corrector plate to install the moonlite focuser, I changed the stock screws in the sm adjuster with longer ones and left a 3mm gap as the starting point. With the focuser installed I replaced the corrector plate ensuring the space around it was concentric to the ota, again it was a simple case of measuring the gap with vernier gauge until it was centred. I then nipped up the nylon adjusters just enough to hold it. I replaced the outer ring loosely just for safety.

Looking then through the Cheshire I could see the sm was to far up the ota so I released the centre screw about 1 full turn then turned in each of the three screws 1/2 a turn each until they made contact with the adjuster base, I repeated this until the sm mark was under the cross hairs of the Cheshire.  I was very surprised how close everything was after this initial setting. HTH.

[perfrej]

OK, guys,

My 190MN is on the work-bench. I have detached the SM, re-attached it with proper offset, checked corrector centering and re-assembled the top. First collimation test shows a slight misalignment but with much less vignetting than before. The SM is a few mm further down the center screw compared to before. My SM was a little too offset

What size sensor are you guys using? I use an 8300-based camera.

OK, this CCD Inspector flat frame analysis is a tad off (ehum..) in collimation, but note the really low figures of illumination deterioration. Lowest figure is -7%, not counting the sneeze-bunnies... I'll tweak this over the coming days.

/per

[Freff]

Unfortunately, I can't see down the focuser enough to give an accurate answer. I do remember  that when I was researching collimating this scope some time ago. I measured the centre mark on the SM and it was centred as I expected it to be following others comments at the time.

I took a quick photo down the focuser with my phone while I was in the obsy. I'm not sure if it's of any use.

[opticalpath]

I'm using a QSI 583wsg ... so same 8300 sensor.

If Per gets a good improvement by remounting the SM with full offset, but Tony gets a really good flat, evenly illuminated frame with zero lateral offset - and by aligning on the geometric centre of the SM - where does that leave us?   I thought that correct lateral offset was perhaps the key to optimising this scope, but Tony's results with the factory setup tells a different story.   

The mist just got thicker again .

Adrian   

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Tony,

The CCDI analysis you posted was really impressive - a very flat field that promises a lot.  Can I ask:

- what camera (or what sensor size) you used for the image that was analysed? 

- have you got an example of a wide field image taken with the scope after you got it so well collimated?  It would be useful to confirm that the coma correction and star shape in the corners of the frame is as good in practice as the CCDI analysis suggests.

Thanks

Adrian

[Freff]

Hi Adrian

The camera was a 350D mod.

The image is the only one I have access to at the moment. I hope it's suitable.

[opticalpath]

Thanks, Tony ... that looks excellent, particularly considering the large sensor size it's covering.  Now we know what we're aiming for!

Adrian

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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

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

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I just tried Tony (Freff's) suggestion to start collimation with the secondary mounting exactly parallel to the corrector (by ensuring an even gap between the mounting post and the corrector central mounting boss), then to carefully maintain the parallel orientation as the secondary is moved closer to the primary to centre the offset SM marking.  By doing this I got a much better initial position - very concentric SM outline and centred marking.  This already looks much better than all my earlier collimation efforts.

Followed by Olivier's novel method for fine adjustment of SM tilt, I feel much more confident about the SM adjustment now.  I have not remounted the SM with correct offset yet .... maybe later. But I have marked the secondary with a dot in the correct offset position and am aligning on that, so this will be a test of a uni-directional offset .... but based on the correct optical centre.

One other small thing I found useful: My SM adjustment screws felt very difficult to tighten smoothly.  I think it's because the ends of the screws are just finished in the normal way with quite a sharp edge.  My screws had actually cut shallow circle marks on the top of the mounting post and I think the screws grab in the rough patch when tightened.  If you have this problem too, try this: I filed the rough edges of the screw ends flat then ground the ends to a slight dome shape.  That makes a smoother contact with the mounting post and now the screws work very smoothly right up to the fully tightened position.

Adrian  

[Freff]

Hi Adrian, there is a very useful sub on Youtube by Astronomyshed. He describes in detail how to get around the issue you are experiencing with the tight adjusting screws. He actually makes a disc that fits in the gap of the adjuster and alleviates the 'digging in' of the screws.

He also explains and demonstrates the use of the barlow and laser method.

I'm glad you got better results with the secondary alignment.

[perfrej]

I took off the Moonlite yesterday. I simply cannot get the thing collimated with it on. I think that the secondary ends up way too far down the tube, thus missing parts of the light cone from the primary.

Instead, I put the stock focuser back on and made a quick collimation run. More or less smack on and the focuser is a little bit up from its center position. I also mounted a Lakeside motor and cleaned everything up. It goes on the GM2000HPS tonight for a quick test run.

Now, looking at my slightly tightened stock focuser, there isn't much benefit in terms of flex from using the Moonlite. Why did I buy that thing? And a Crayford at that! (don't like 'em)

/per

[opticalpath]

Your experience sounds very like mine, Per. It seems like once you move the secondary down to meet the axis of the Moonlite focuser, something goes haywire and collimation ends in tears. You can have good coma correction, or an evenly illuminated field .... but not both!   It's like you can't mess with the spacing between secondary and corrector, but why should that be?  Question: does the (non-simplified) formula for offset amount contain a term for the distance between diagonal and point of focus?  Could that be a reason to why a lower position of SM causes problems?

I'll be very interested to see your first images with the old focuser back on and a higher-up position of the offset SM.

 

After following Tony's procedure, I was optimistic.  I got a much improved view through the cheshire: more concentric and better centred. Rotation adjustment certainly helped too, as you suggested.  I aligned on a new correctly placed offset mark on the secondary and took a few test images last night. Field illumination was much improved - very uniform - but coma correction was way off again   . 

As clouds rolled in I did a quick rough recollimation, centring this time back on the original factory mark on the SM, and grabbed an image through the thickening clouds.  Enough to tell me that the coma correction was  better  and field illumination was still good ..... so some progress at least. 

I'm sure that Tony's suggestion is an important step: i.e. to ensure that the (lowered) SM starting position is set carefully, with the top of the SM mounting post parallel to the corrector, and thereafter to make only small tilt adjustments.  If that's not done first, I think it's possible to get into a position where everything looks right through the sight tube, but in fact the secondary has been tilted far from the parallel position to compensate for misalignments elsewhere. 

I've dismounted the scope again and plan to do this:

- Reset the PM tilt so the starting position is once again axially aligned with the OTA (strings etc)

- Raise SM position to align the original factory centre mark with the (squared) focuser axis, and ensure the SM mounting base starts parallel to the corrector 

- Adjust SM tilt using the excellent barlowed laser method described by Olivier 

- Adjust PM tilt using conventional barlowed laser technique and confirm with sight-tube/ cheshire view

If any large tilt adjustments are required, go back and recheck PM and SM starting positions.

- Make test images.  Assuming good field illumination, use only focuser tilt adjustment - trial and error - to attempt to correct any problems with star shapes in corner(s)

- Recheck/ refine SM and PM tilt with barlowed laser

- Test image 

... See where that takes me.

Adrian   

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..... Now, looking at my slightly tightened stock focuser, there isn't much benefit in terms of flex from using the Moonlite. Why did I buy that thing? And a Crayford at that! (don't like 'em)

/per

Is your stock focuser the 2-speed version, Per, the one with that horrible sloppy extendable drawtube?  I could not get that telescoping - extendable drawtube to lock solidly: it's such a loose fit and only one locking screw.  It's the main reason I replaced it.  If that's the one you have, how did you fix the wobbly extending section?

Adrian

[perfrej]

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.

So, I tore off the Takahashi/QSI683 , got the 190MN with SBIG ST-8300M and SX wheel up there in about ten minutes. Fixed he cablig and balanced it. The plug for the SX wheel was very close to the top of the mount but did have clearance.

I then proceeded by clearing the model in the mount and had it slew to Vega. Said star ended up about 1/3 from the center of the CCD. Good. That means he optical axis is more parallel to the physical scope than it was with the Moonlite. Focused it manuall with the jog control in Focusmax and ran a vcurve. Slightly irregular movements of the focuser during that indicate that the focuser is a little too tightly adjusted.

A 50-point model was hen commenced during which I suddenly heard noise from the balcony and rushed out. The USB cable for the SX wheel had made contact. I do not know what had changed but it did. The GM2000 is strong so when I got it stopped, and had the scope down I saw that the focuser had a completely loose draw tube and the SX wheel a misplaced USB connector.

At that time I reverted to quickly putting the Tak back on as the sky was clear - a rare event these days. That went reasonably well with quick balance (to 0.0% indicated - nice touch) followed by a new vcurve and a 50-point model After about 30 minutes I was imaging 20-minute NB subs of Melotte 15, unguided as usual (love that mount).

The focuser has a broken screw. It is one of he four that holds the bearings against the draw tube, an M3 thread but with a 3.0mm short, unthreaded shaft. The SX wheel goes on the operating table today as the entire USB connector has been torn off he circuit board.

Luckily I still have something on the mount, but the GM1000 and the NEQ6 are empty 

More to come...

/per

[opticalpath]

.....The focuser has a broken screw. It is one of he four that holds the bearings against the draw tube, an M3 thread but with a 3.0mm short, unthreaded shaft. The SX wheel goes on the operating table today as the entire USB connector has been torn off he circuit board ......

This scope has got some scars!   Hope that focuser repair is just a straightforward replacement screw and no damage to the alu. body.

Adrian

[perfrej]

I may need to polish the draw-tube just a little bit. SX wheel is repaired and functional again

/per

[perfrej]

Ah!

Focuser repaired too. I have now iterated some and found reasonable flat frame status. I will star test this before going further. This is how it looks now. Not perfectly even but still very small difference between inner and outer areas.

/per

[opticalpath]

Looks promising, Per.  Hope the star test comes out good.

Can I ask: when you first got the scope, before focuser change and before SM change, did you look through a sight tube at the collimation .... do you remember how the view looked? In particular, did the SM look concentric in the drawtube and was the factory SM centre mark centred in the sight tube crosswires? 

If I centre the factory mark in the crosswires, the SM outline is not centred in the drawtube - it's too high.  If I make the outline concentric, the centre mark is not under the crosswires.  So far, we concluded that the reason for this is that the factory mark is in the wrong place - not offset far enough.  OK, this is with the lower mounted Moonlite, but I can't see any reason why the view would have been different through the stock focuser, with SM and focuser both mounted a little closer to the corrector.  But I can't remember how it looked before I started messing with it..... wondered if you can recall?

Adrian

[perfrej]

Adrian,

When I first got the scope I immediately took the focuser off The Moonlite was delivered with the wrong base plate, so I then had to wait a week for a new one. I then mounted the Moonlite, laser collimated and went to work.

I then ordered a Catseye collimation kit from the US as Jonas Grinde recommended that. It is great and it was my first look through a proper sight tube (my home-built one is not quite straight). So, in direct reply to your question; not a clue. Sorry.

After the mirror crash I had to buy a new secondary so there is no mark on that. I do have the original secondary though, and he mark on that is offset by pretty much 4.25 mm.

I guess we're both lost in "fiddle-first-think-later" land. 

/per

[opticalpath]

Another day of collimation .... another frustrating night of testing

 

But today I came across this very interesting thread:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6075098/page/5/view/collapsed/sb/5/o/all/fpart/1/vc/1

 

It's about similar collimation difficulties with the ES 152mm Mak-Newt and confirms some of what we have touched on here. (BTW, being a glutton for punishment, I have the ES Mak-Newt too!)

In particular, this thread discusses what happens to the collimation view when the diagonal is geometrically centred (or has insufficient lateral offset built-in.)  In that case, during collimation, making the SM outline appear concentric with the drawtube - i.e. a unilateral offset - *guarantees* that the optical axis will be tilted and will not pass through the centre of the corrector, which will compromise coma correction. So an apparently perfect collimation view may not produce uniform coma correction and a better collimation appearance would have the SM outline NON-concentric with the draw-tube. 

 

Furthermore, if we move the SM back up towards the corrector to reduce the induced tilt in order to improve coma correction, the SM outline will become increasingly non-concentric with the drawtube.  Eventually that will affect field illumination and we will see asymmetric vignetting effects.  But worse, in this scenario there is no 'signature' collimation view to help us find the sweet spot: focuser and PM axis alignment can be achieved in any of several height-positions of the SM, each of which looks OK in the sight-tube, but which has introduced more or less axial tilt.

 

I have felt from the beginning that I needed to move the SM closer to the corrector, away from the classic 'concentric outline' view, in order to get better coma correction, and that correct height positioning is hard to find - a matter of trial and error. This discussion provides some kind of explanation.  Changing the focuser and moving the SM height position is very likely to result in losing whatever was the factory good-compromise position.  Offsetting the SM on its mount should establish the signature concentric view for correct collimation so I keep my fingers crossed that Per's re-mounted offset SM will prove to be a good move. 

 

Adrian

[ambro]

My last results of collimation. Not best, but stars shapes are good.  I didn't have time to setup guider.  Telescope was not cooled completely and there was some clouds. Examples are from Canon APS-C sensor. I will use 2/3" Sony chip (ATIK 16HR). I think my current collimation will be sufficient.

Flat from image:

Curvatre map:

Flat on ceiling:

Clear Skys,

Robert

[Freff]

That's not bad at all Robert considering the conditions.

Can I suggest after taking the first test, as above, without changing anything slew to another part of the sky preferably with a good star field and repeat. Do at least three test and compare.