Skywatcher 190MN / fitting of a Moonlite focuser

[opticalpath]

So in my haste I simply put some self adhesive chrome tape on the clips giving them a mirror effect. Eureka. it worked, to a fashion. It's not perfect but very near.

Where did you put the tape, Tony? I would have thought that reflective tape anywhere near the light path would be big trouble.

Adrian

[Freff]

On top of the clips Gunnar cut off.

[Gunnar]

Freff!

That was interesting!

At first I thought masking out the mirror clips. The idea was to make a mask made of cardboard or plastic, formed like a ring around outer edge of the mirror and over the mirror clips.

Gunnar

[Gunnar]

Hello,

During Friday / Saturday night it was finally time to try out the telescope after the modification. I dragged with me my stuff out to Sandvretens observatory, http://uaa.saaf.se/observatoriet.php to test it. In summary, there was no improvement. The lack of mirrorclips made ​​no major change. I started to take images of Vega and immediately saw the problem (the darker shade on the right side of Vega) remains on the brighter stars. In any case, I proceeded then to photograph pacman for the rest of the night. On the pacman picture it`s clearly that the error is across the entire image except perhaps in the center. It seems that the error rotates around the center, but starts a tiny bit off the more central parts of the image.

Every bright star has this dark defect in a direction towards the centre of the image!

What is causing this?

Some suggestions?

Attached picture Vega.

[perfrej]

I am in no way an expert on optics, but doesn't that speak mild coma? It would, given the design of the 190, most likely be caused by mis-alignment of the plate and the primary mirror or possibly the focuser. Just a guess...

Mine is assembled now and awaiting clear weather... Need a camera as well as I am not entirely at ease with my M26C and the SBIG is at SBIG for cooler repair. QSI, please ship my 683 ASAP!

/per

[perfrej]

Clear weather, please? Pretty please?

/per

[Steve25]

Hi /Per,

Very nice. Which motorised focuser system is this?, as it look very neat

Steve

[perfrej]

Thanks!

It's Lakeside's. Ian King sells them. I have one control box and several motors. Lakeside makes adapters for most focusers. I have one for the Takahashi FSQ106 as well.

/per

[Gunnar]

Looks nice Per!

[perfrej]

Need some help!

OK, so I have done the dreaded thing. The one thing that is of poor design on the 190, both the Orion and the SW models, is the secondary holder. It is made of aluminum and has a steel screw in it. It wiggles a bit durig collimation, which is expected behavior. The screw is too short so any adjustment of the holder vertically involves the risk of it falling as the threads in the aluminum part get worn. I have dropped it and am contemplating a new primary, something that cannot be purchased separately.

Orion can make a new mirror for me and a seconddary is not difficult to get (one coming tomorrow). I have assempled the remains and gotten it to better collimaton than ever...

Now, the problem, as it turns out, has, throughout the entire collimation nightmare been the secondary offset. Since I had to glue it back on I decided to finally calculate the offset. The mirror has been re-glued three times now, each time with a bigger offset and I'm getting there.

I used flat-field analysis to find the error. With the camera chip perpendicular to the tube the uneven illumination shows up as vertical (with the image in landscape). Further proof for it being so is given by the fact that after pefect laser collimation with a Hotec, the primary clips end up rather at the bottom of the secondary (past the bottom if you try to go the last mile for perfect illumination). So, obviously I need to offset the mirror some more and then get it up some in the tube. I'm getting used to the procedure. This is what I do:

1. Square focuser

I remove the corrector assembly and measure the desired spot, then use the Hotec to adjust the focuser (moonlites can be adjusted)

2. Adjust secondary height

I remove the primary assembly from the tube and use the Hotec. Looking up from the bottom you can see the beam hitting the secondary in order to center it.

3. Rotate secondary

This is actually a fairly tricky thing to do but I have found a simple procedure. Use the sight-tube so center the mirror horizontally (scope erect) or leave it be from step 2. If I use the sight-tube, I use the outer outline of the secondary, not the image reflected in it. You have to shine a flashlight on the opposite wall to see properly. Then, regardless of if you left it or used the sight-tube, with the laser in place, rotate the mirror holder until the dot falls on an imaginary line from the primary center mark towards the focuser center.

4. Complete secondary collimation

After the rotation, use the screws to center the dot in the primary. This will now only involve movement along the imagined line.

5. Complete primary collimation

As per the usual procedures.

I expect to have this baby collimated properly very soon (fingers crossed). Perhaps with a new primary, perhaps with the black marks covering the chips in the primary

So, can anyone who happens to take the top assembly off please measure the distance from the secondary holder to the lower edge of the secondary (in mm)? It's not in the manual...

Stay tuned!

/per

[perfrej]

First light after crash...

As you all well know, the weather has been total crapp in most parts of Europe. Now, all of a sudden, we have clear skies and I have a Catseye collimation kit that just arrived from the states. I still need to move the image center a little, perhaps by lowering the mirror a bit more - and you know where that got me the last time I tries...

So, just for test purposes, I shot M81/M82 last night with the 190MN/QSI 683 on the 10Micron GM1000HPS that I now have on the balcony. It looks like the crack in the primary mirror and the chipped edge of the secondary don't harm the images too much. The test is a little on the short side with 5-minute subs for R, G and B, and 3-minute subs for L, but it still gives a first impression of the scope. 18 each of L, R, G and B, pre-process in Neb3, registration, integration, individual stretch and RGB composition in PI, L addition in PS. Unguided, of course, and I forgot to do gradient removal...

/per

[ambro]

Helo,

I have second hand Skywatcher MN 190. I am trying to collimate this telescope. I have done:
1. Centering of the secondary mirror side-to-side as seen from the focuser (using collimation cap)
2. Centering primary reflection in secondary mirror (using collimation cap)
3. Tiled the primary mirror (using collimation cap and laser).

After collimation I have done flat frame analysis by CCDInspector.




I am thinking that fully iluminated field should be circle, not eliptical. Center of fully iluminated field is below chip center (APS-C chip and ICX-285AL, samples are from APS-C). I can move this up by tilling secondary, but then reflection of primarry mirror is shifted.


Could You give me any clue what is wrong with my collimation. I have done squaring the focuser. It is used telescope, I don't know what exactly previous owner have done. It is Maksuov Newton. I can change secondary offset only by lossenig screw and moving secondary holder up and down.  Maybe secondary mirror is too close to focuser ?. I can change this only by gluing SM to holder again.

This secondarry mirror should have center spot. My secondary mirror don't have center spot. Mayby secondarry mirror was replaced and wrong glued (It is glued by strong double-sided adhesive tape) . Is secondarry mirror glued by adhesive tape at factrory ?.

Robert

[perfrej]

The problem you are seeing is one that I have too, and I guess most people who fit a non-stock focuser will have. Had it been wit hthe original focuser I would have moved it up or down in order to center the illumination. As for the secondary, yes it is double-sided adhesive tape from the factory. If the mirror ends up shifted it means that you have to adjust it by lowering or raising it in the tube (center screw and collimation screws), and then meeting that change with a shift up or down of the focuser in order to keep everything perpendicular.

Now, a Moonlite doesn't have the up-and-down adjustment possibility, so you will, in that case, be left with a bit of a bummer, and will have to loosen the secondary and re-attaching it.

/per

[Gunnar]

Hi Robert

Yes as Per says, the secondary Is glued with adhesive tape from the factory. I have replaced the double-sided adhesive tape on my 190MN! My secondary-mirror Is now  glued in position with 3 blobs of silicone glue, not tested yet so cannot tell if there is an improvement there.

Regards Gunnar.

[ambro]

Hi,

Thank you for replys. My focuser is Baader Steel Track. I can move it dow or up. I am close to solve problem. I have measured offset and glued SM again, adjusted focuser (tilt only). I have added spacers to primarry  mirror cell. Ilumination is centered when I make flat on wall (tube is placed horizontal) .

If i made flat like before on ceiling illumination is bellow center of the chip.

I have checked focuer it is not tiled by heavy DSLR. I think it is still problem with primarry mirror cell. I have to add more spacers. Maybe primary mirror is moving or tube is bending.

Robert

[perfrej]

The primary always moves unless you "fix" it. You need to make sure the nylon screws that center the glass in the cell are just right, and you need to add spacers (about half a millimeter) at the three points where the inner metal part of the holder touches the outer one. The, the collimation screws need to be really tightened, so much that the o-rings acting as spring loading are more or less completely compressed. The, and only then, can this scope become a dynamite performer.

/per

[opticalpath]

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 figuring out why it's so hard to nail the collimation of this scope.  The key for me was finding this old posting on a forum, and if you haven't tried it, I recommend the step described here: 

http://www.telescopejunkies.com/forums/thread-view.asp?tid=782

(you have to register to view the images)

 

I think the key to the difficulty is this:  An ordinary Newtonian can be collimated well even if the primary optical axis is not centred on, or is at a slight angle to, the centre-line of the OTA; the secondary and/or focuser can still be adjusted to ensure alignment of the primary axis with the focuser axis ... which is the main requirement to collimate a Newt.  In other words, a Newt can tolerate and compensate for a slightly non-symmetric alignment of the primary mirror, or for a focuser that is not exactly square-on to the OTA.  As long as the axes are aligned to each other, collimation will be good and an on-axis star test will be good.  The only effect of any such asymmetry with the OTA axis is slightly uneven field illumination.

 

The Mak-Newt is different in one critical aspect: the optical axis of the primary mirror has to be aligned with the focuser, same as for the Newt, but it also must be aligned with the axis of the corrector lens!  Assuming the corrector is physically centred in the OTA, the only configuration that will meet that requirement is if the optical axis of the primary mirror runs exactly down the centre line of the OTA and through the centre of the corrector, which means that the focuser axis must also be square to the OTA.  The Mak-Newt will not tolerate small asymmetries in alignment of the optical elements with the tube.

 

I'm not an optical expert, but I think this is what is going on.  If we follow the usual (Newtonian) collimation procedure, we can easily end up with a slightly asymmetric orientation of the primary mirror axis.  It explains why everything can still look perfect through a Cheshire or collimation cap, or using a laser, and the on-axis star test looks great ..... and yet an imaging test shows stars that are somewhat mis-shapen in one or two corners, and the field is unevenly illuminated.   If we are not dead centre with the corrector, the on-axis image can be fine, but the corrector will not be doing its job of reducing coma evenly across the whole field. 

 

So, it is NOT the same as collimating a Newtonian and it seems that an extra first step of getting a preliminary alignment of the Primary mirror is important, as described in the link above.   I've just done this with my MN190 and got an immediate improvement in coma correction and evenness of illumination.  I'm still experimenting but initial results are very promising.  Having dismantled my scope, I also realise now that the circle marking on the secondary mirror is at the (physically offset) optical centre of the mirror and can be used to position the mirror correctly under the cross-wires of a sight tube. 

Hope this helps others wondering why it can seem so difficult to optimise collimation of this scope.

 

Adrian

[perfrej]

Good observation.

I have found that mine is a few arc-minutes off in orthogonality, and that is due to some misalignment of the optical path. I will definitely do the mirror centering the next time I take it down (soon).

Thanks for the pointer!

/per

[opticalpath]

After I did this thing with the strings to adjust the primary tilt (BTW, the six holes round the OTA for securing the corrector assembly are good for mounting the three strings) I reassembled the scope and checked the view through a Cheshire. It was already close to the 'target' picture for good collimation and I made just some small additional corrections of the primary tilt to get everything looking concentric as it should. A later imaging test was a big improvement with smaller FWHM, but still small coma in one corner.

So now I wonder if should try not to touch the primary at all, after the initial alignment-with-strings. Maybe instead I should try small adjustments of focuser tilt and secondary to complete the fine-tuning? I have dismounted the scope and plan to try this, now I'm not so fearful of dismantling the scope!

Adrian

[ambro]

I don't have nylon screws at promarry mirror cell and dont have screw at corrector level (like on this video http://stargazerslounge.com/uploads/monthly_03_2012/post-20861-133877753497.jpg ). Maybe my MN is older version?

Adjusting  primarry with strings is smart solution. I did it last time another way. I had detached primary mirror holder from corector plate, then mounted corector plate without SM and looked trough hole of center screw at primarry center mark to make PM tilt adjustment. Yesterday I used ATM 2" laser with cross. It fit corrector ring. Beam hit center mark at PM and I saw centered corss on reflection of secondary holder (without secondarry mirror).

My checklist for collimation MN is:.

1. Measure and center primarry mirror at its holder. Add spacers etc. Check with laser colimattor it didint shift when moving OTA.
2. Measure and square focuser with laser colimator (like on this video

).

Add shims if necessary. Baader Steel Track don't have tilt adjustment.
3. Measurre offset of secondarry mirror. It is (minor axis  [mm] / 4 x f ). 63 / 4 * 5.26 = 2.99 mm away from focuser and 2.99 mm towards the primary mirror. Delta (hypotenuse) is 2.99 x 1.41 =  4.2mm.  Mirror is 12mm thick and adhesive tape is about 1-1.5 mm. Support is at 45 degree angle. Mayor axis is about 89mm (63x1.41).  Center should be at 48.6mm from "bottom of secondarry mirror". Please correct me if am wrong with this calculation (I am not experienced user. It is my first telescope which I trying to collimate without help). Offset should be correct from factory.
4. Measure and center corrector plate. My corrector fit so tight I can't make any adjustment but it is concentric.
5. Align primarry mirror with corector plate (tube axis) with strings or other method. Don't change primarry mirror tilt from this point.
6. Align secondarry mirror inside focuser. First rotation, then offset towards the primary mirror and tilt to make it concentric with focuser. I prefere to use colimator from web cam (like on this video

)

or simple colimator from film cap.
7. Align primarry center mark only by tiling secondarry mirror. If previous step are corect it should be small adjustment. (In my case about 1/4 - 1/3 turn of screws). Round reflection of primarry mirror should be centered also.
8. Star test and make final adjustment. I haven't made this point yet and I don't have experience. Do you think I should use secondary or primary tilt adjustment ?. I think I should use primary mirror tilt adjustment (small adjustment, locking screw only if possible).

The weather yesterday was not good. I have to wait for better conditions or use artifical star. I made one from 9nm fiber cable pachcord and strong 3W LED. I can't focus it, because too small distance from OTA.

I have checked flat frame and illumination is now circle shape at sensor center. Ilumination doesn't move when I move OTA (horizontal and vertical position).

I think I will make thread on my focuser tube (like this http://www.teleskop-express.de/shop/product_info.php/info/p5078_TS-modification-service-for-2--Baader-Steeltrack-to-65mm-thread.html ). Clampring is poor connection for astrophotography. It is not repeatable and ilumination center move small amount according which screw tighten first.
 

[opticalpath]

Hi Robert. I'm watching your collimation plan with interest; I'm trying to find the right way to collimate this scope.

About the star test ....
I would say don't touch the secondary at this stage - only (small) primary tilt or focuser tilt adjustments. If a large primary adjustment is necessary at this stage, that would move the primary axis away from the centre of the corrector and would suggest that there's an earlier alignment error to go back and correct. After centring the primary axis (with strings etc) I think it would be desirable to make only very small later adjustments to primary tilt.

About secondary offset .....
Does your secondary have a mark like this ?

 

It looks like it's supposed to mark the offset optical centre and should therefore be possible to use it to correctly offset the secondary position by centring the circle marking in the crosshairs of a cheshire sight tube .   But when I do this, I get everything concentric, except primary reflection is right at the bottom edge of the secondary.  So I'm not sure if the circle really marks the correct offset.

By the way, I made a couple of developments of the 'three strings' method of aligning the primary axis.  This was to overcome the difficulty of keeping my eye steady as I checked alignment of strings, reflected strings and centre marking .... quite hard to keep still enough!

First is to make a 2-3mm pinhole in a sheet of white paper and look into the OTA through that.  The reflection of the pinhole appears as a dark dot and how this moves helps to judge better how things are lined up.

Second approach works really well: After fixing the 3 strings to top of OTA, set the OTA on the floor (or on the mount) at a low angle so you can look directly into from a distance of about 4 metres.  At that distance set up a camera and tripod, focus on the 3 strings, set the lens to f/16 so that the centre spot is in pretty good focus too. Now move the camera carefully until the strings target is aligned with the mirror central spot.  Take a picture and note where the (out of focus) reflected strings are in relation to the real strings.  Adjust primary tilt to bring the reflected strings into alignment.  It's quite sensitive, but taking photos from a tripod is easier than doing it by eye. 

Here's how it looks:

1. Initial view

2. After moving camera position to align strings with centre spot (reflected strings not quite aligned)

3. After adjusting primary tilt to align reflected strings

After doing this, I hope that any primary tilt adjustments during star test will be very small.  We shall see! 

Adrian

[perfrej]

I'm confused...

Assume the following:

Primary is 3mm off center.

You align the camera, obviously it is off the ideal optical axis.

Then you tilt the primary until it all lines up.

In my view you are off with your optical axis and it still lines up.

???

/per

[perfrej]

Remedy?

1. Mount the scope firmly.

2. Remove primary assembly

3. Add strings, top AND bottom

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

5. Leave camera on tripo!

6. Re-install primary

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

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

Would this improve it?

/per

[opticalpath]

Robert, about squaring the focuser ....

The method referred to in the youtube video is good, but assumes that the focuser drawtube axis is exactly centred in the circular cut out in the OTA that is used as the reference point for measuring .... which may not be true if the focuser was adjusted on slotted holes during assembly/ testing, or if you have changed the focuser.  I think it would be better to measure around the inside of the OTA from the outer sides of the drawtube (racked in so it's accessible from inside the OTA). 

Or better, make a pinhole near one end in the strip of paper and place it over the centre of the inner end of the drawtube (racked in to be level with the inner surface of OTA) so a laser beam passes through the hole and hits the other side.  Wrap the paper strip around the inside of the OTA and mark the point on the paper where the laser spot strikes opposite the focuser.  Now reverse the paper strip and wrap it round the OTA in the other direction, again lining up the hole so the laser beam passes through it.  Note the place where it strikes the paper strip on the other side.  Compare the two points marked on the strip to judge if the focuser drawtube axis is square to the OTA.

Adrian

[perfrej]

I was regerring to your string theory above. I think it doesn't work. Hence my suggestion to remove the primary for initial alignment. I am taking mine down soon... First task is to get focuser movable up and down along the tube. The moonlite ends up too low down I thonk,

/p