Collimation confusion: all my tools disagree with eachother and none of them are right!

[pipnina]

I noticed two nights ago that my images had odd shaped stars in one corner, and a different shape in the opposite corner. I have managed to eliminate these issues before through good collimation however, so I know it's most likely not focuser sag.

Last night I tried to correct it by taking the camera out and putting my baader laser colli in. Problem with this is that if the laser isn't perfectly true to the focuser, the whole measurement is wrong. And the tolerances on all the bits are NOT good enough, even though I used that laser to get the good collimation I had during my iris nebula image only a few weeks ago.

So I collimate as best as I can with the laser again. I take an image, and the issue is unchanged. Ok, I put in my cheshire eyepiece, and it claims the collimation is WAAY off. So I collimate it to the cheshire eyepiece instead. I take another image, and it's actually WORSE!

So now I try with my cocenter. This is not really a primary alignment tool, but I found with my other telescope that if you shine a light down the aperture just right you can get a good feel for how the primary alignment looks. It thought it was perfect...

Then to make it worse, I tried a "reset" of the tensions and stresses of the adjustment knobs on the mirror cell... Only to find out that screwing in the adjustment knobs pushed the stud they drive, into the mirror, thereby pushing it off of the cork pads it was sealed to, making it unusable. I have re-sealed it today, and hope to retry it again tonight, but I just don't get how I am going to collimate this all properly.

 

I'm also really miffed that GSO, who made the mirror cell, decided on that stupid design when my much cheaper-per-inch skywatcher 130P-DS has a superior collimation control system, where the cell has the studs threadlocked in place, and the control knob pulls and pushes on the cell. I checked it only just today, and it makes a lot of sense that I'd find my old telescope so much easier to collimate...

I thought cheshire eyepieces were basically as foolproof as you could get, so I am now lost at how to get my F4 newt collimated well enough to achieve the round stars I have only rarely enjoyed with it thus far if anyone has any tips I'll gladly try them.

[markse68]

I set my 8" f4 using my ASI120mm and circle overlays to get the secondary in the right place and now each time I use  it I check collimation with a laser pointer in a Paralizer 2"-1.25" adapter, rotating the laser to compensate any miscollimation of it or the holder to tweak the secondary adjusters to hit the primary donut, then stick a barlow on the laser to adjust the primary - I made a target a bit like the concentre with engraved rings that is jammed into the filter threads of the paralizer so I can see the projected donut ring and centre it. Has been working well for a while now though I'm a bit surprised it almost always needs minor tweaks.

Ae you sure it wasn't senosr tilt though causing the initial issue you noticed?

Mark

Edited July 10, 2022 by markse68
added photos

[pipnina]

26 minutes ago, markse68 said:

I set my 8" f4 using my ASI120mm and circle overlays to get the secondary in the right place and now each time I use  it I check collimation with a laser pointer in a Paralizer 2"-1.25" adapter, rotating the laser to compensate any miscollimation of it or the holder to tweak the secondary adjusters to hit the primary donut, then stick a barlow on the laser to adjust the primary - I made a target a bit like the concentre with engraved rings that is jammed into the filter threads of the paralizer so I can see the projected donut ring and centre it. Has been working well for a while now though I'm a bit surprised it almost always needs minor tweaks.

Ae you sure it wasn't senosr tilt though causing the initial issue you noticed?

Mark

I have done my best to test for sensor tilt. I can't detect any but I don't have the full and proper jig to do it with. My best test was propping the baader coli laser on something so it was pointing at the camera's sensor, while the sensor was in a M42->1.25 adapter piece inside a 1.25" holder. I got the distinctive grid of dots and AR glass reflection, I tried my best to determine if the grid was rotating or not but it didn't seem to be.

I suppose the other way to test it is to flip the camera 90 degrees or 180 and take a second picture, and see if the pattern moves with it. However I managed to get round stars at all corners before, so can't that rule a tilted sensor out by itself?

The camera idea for secondary collimation is interesting. How do you get it to take a picture of the eye view down the focuser instead of the view a camera normally gets when at the focus position?

Sadly I don't have a barlow but I can try the rotation trick tonight too. Thanks!

[markse68]

I used the wideangle lens that came with the ASI120- it captures the insides of the focuser tube to align to. I have an Ocal now which is basically the same thing but a bit easier to use as you have a nicer software interface and ability to remote adjust the focus- nice little tool.

Rotating the camera should tell you if it's sensor tilt or collimation I'd have thought yes

Mark

[pipnina]

1 minute ago, markse68 said:

I used the wideangle lens that came with the ASI120- it captures the insides of the focuser tube to align to. I have an Ocal now which is basically the same thing but a bit easier to use as you have a nicer software interface and ability to remote adjust the focus- nice little tool.

Rotating the camera should tell you if it's sensor tilt or collimation I'd have thought yes

Mark

Ah, I have the 120 mini which doesn't come with that extra lens. I'll definitely give the laser rotation, and camera rotation a go however.

Now that it's getting dark, I'm going to see if my sealant has held the mirror as it should and re-attempt collimation

[ONIKKINEN]

How is the focuser in your scope? If its not rock solid 100% reproducible with every accessory you have, you will simply not have good reproducibility in collimation. I thought my focuser was OK until i fitted a baader diamond steeltrack to my scope and found out just how much better than OK a good focuser can be (in truth the focuser i had was kinda bad). The laser and cheshire you used are, i would imagine, different lengths so they are at different focuser positions or just at different distances from the primary. If you have any side-to-side movement when racking the focuser in and out you will also have different collimation readouts at different focus positions so you will again be unable to get a result that agrees with some other method. You could try to eliminate the sideways movement but that might not be possible.

I find that i need to collimate with the laser at the focal plane and the focuser at the same position as a camera would be, and also most importantly, through the coma corrector. Collimation without the coma corrector is irrelevant if the corrector sits in the focuser when imaging, so i collimate through that with a laser. Try setting up a bunch of extenders and spacers with your laser to make it be basically a camera replacement and have the laser originate from the focal plane and see if you start getting some reproducibility.

[pipnina]

19 hours ago, ONIKKINEN said:

How is the focuser in your scope? If its not rock solid 100% reproducible with every accessory you have, you will simply not have good reproducibility in collimation. I thought my focuser was OK until i fitted a baader diamond steeltrack to my scope and found out just how much better than OK a good focuser can be (in truth the focuser i had was kinda bad). The laser and cheshire you used are, i would imagine, different lengths so they are at different focuser positions or just at different distances from the primary. If you have any side-to-side movement when racking the focuser in and out you will also have different collimation readouts at different focus positions so you will again be unable to get a result that agrees with some other method. You could try to eliminate the sideways movement but that might not be possible.

I find that i need to collimate with the laser at the focal plane and the focuser at the same position as a camera would be, and also most importantly, through the coma corrector. Collimation without the coma corrector is irrelevant if the corrector sits in the focuser when imaging, so i collimate through that with a laser. Try setting up a bunch of extenders and spacers with your laser to make it be basically a camera replacement and have the laser originate from the focal plane and see if you start getting some reproducibility.

I'll give changing the focus position a go for sure! I do think I have some sort of tilt or sag going on, if it isn't collimation being the issue. This is because I see my pairs of diffraction spikes looking parallel when slightly defocused going up-down the telescope tube (relative to sensor/focuser), but not parallel (converge on one side, diverge on the other) on the axis going up/down relative to the focuser (i.e. if the focus lock and tension thumbscrews are down, and the side opposite that is up.)

It's unfortunately very hard to get the laser to the exact focal point as the camera however since the focal plane of the telescope is so far away from the secondary, so much so that I have to withdraw the TS-GPU CC that I use by about 2cm from the focuser in order to achieve focus, with the focuser already outstretched as much as possible! But I can try moving the focus position up and down with the laser and cheshire in place to see what effect it has.

[ONIKKINEN]

1 hour ago, pipnina said:

I'll give changing the focus position a go for sure! I do think I have some sort of tilt or sag going on, if it isn't collimation being the issue. This is because I see my pairs of diffraction spikes looking parallel when slightly defocused going up-down the telescope tube (relative to sensor/focuser), but not parallel (converge on one side, diverge on the other) on the axis going up/down relative to the focuser (i.e. if the focus lock and tension thumbscrews are down, and the side opposite that is up.)

It's unfortunately very hard to get the laser to the exact focal point as the camera however since the focal plane of the telescope is so far away from the secondary, so much so that I have to withdraw the TS-GPU CC that I use by about 2cm from the focuser in order to achieve focus, with the focuser already outstretched as much as possible! But I can try moving the focus position up and down with the laser and cheshire in place to see what effect it has.

Yep, sounds very familiar all of that. The diffraction spike thingy is for sure a sign that there is some collimation or tilt issue, but its rather difficult to tell which one since both will have the same effect of taking the scope out of collimation. By the way, you mention tension and focus lock screws, do these move the focuser drawtube by any amount when you use them? If they do, that's a big problem. You can check just by inserting a laser and watching whether it moves across the primary and if it does, you gotta figure out a way to put them in one setting where everything works and moves and then never touch them again. I didn't figure out a way to do that with the stock VX8 focuser.

Try building something like this for the laser issue:

Its just a regular cheap collimation laser with the middle target section roughly 55mm from the coma corrector lens when this is threaded to the coma corrector. Easy to use, just thread whatever you have connected to the CC off (and everything behind it, including camera of course) and thread this in place of the camera+adapters. Mark the focuser position at infinity and you can now collimate the scope with the coma corrector in place. The laser dot will probably blow up in size a bit when it goes through the CC and back, but that might actually make it easier to see the centering (i found that to be the case).

[pipnina]

1 hour ago, ONIKKINEN said:

Yep, sounds very familiar all of that. The diffraction spike thingy is for sure a sign that there is some collimation or tilt issue, but its rather difficult to tell which one since both will have the same effect of taking the scope out of collimation. By the way, you mention tension and focus lock screws, do these move the focuser drawtube by any amount when you use them? If they do, that's a big problem. You can check just by inserting a laser and watching whether it moves across the primary and if it does, you gotta figure out a way to put them in one setting where everything works and moves and then never touch them again. I didn't figure out a way to do that with the stock VX8 focuser.

Try building something like this for the laser issue:

 

Its just a regular cheap collimation laser with the middle target section roughly 55mm from the coma corrector lens when this is threaded to the coma corrector. Easy to use, just thread whatever you have connected to the CC off (and everything behind it, including camera of course) and thread this in place of the camera+adapters. Mark the focuser position at infinity and you can now collimate the scope with the coma corrector in place. The laser dot will probably blow up in size a bit when it goes through the CC and back, but that might actually make it easier to see the centering (i found that to be the case).

Ah! If I tighten the locking screw it does shift very slightly, and as I undo the tension screw and it begins to go loose it does shift again, but doesn't appear to move visibly once it has some tension on it.

The laser also shifts position as I tighten it into the 1.25" adapter (this is one with only one screw, but it does have the brass ring that it compresses instead of just driving the screws into the thing being held.

here's some sample images from tonight, maybe the star images can give a clue?

 

O3 FOCUS TEST.fits HA FOCUS TEST.fits SII FOCUS TEST.fits LUM FOCUS TEST.fits

[ONIKKINEN]

Top right corner looks maybe OK in the LUM file. Bottom left stars are seagull shaped which means you are way out of collimation. With F/4 way out is not nearly as much as you'd think it is, mind you and it really needs to be almost perfect every time. Tilt inspector in Siril below.

I'd say figure out the mechanical side of things first before tackling possible sensor tilt and collimation. If you try to collimate a scope that is not mechanically sound you will just end up going in circles and it will never seem right because it really isn't possible.

@alacant usually comments that none of the cheap newtonians available today are out-of-box compatible with the demands of astrophotography in terms of mechanical stability and yours is probably not different. One more common issue you could have if the stiffness of your tube, which if not stiff enough can bend/buckle under load ever so slightly and so cause the scope to go out of collimation. Fitting a longer dovetail bar and spreading the rings out further apart from each other (i think it was about 50cm) while also tying up the top of the rings together with another dovetail bar will significantly improve stiffness of the tube. Not seeing to the mechanical side of things first will lead you on a wild goose chase where tools dont agree with each other and results vary from night to night.

[pipnina]

9 hours ago, ONIKKINEN said:

Top right corner looks maybe OK in the LUM file. Bottom left stars are seagull shaped which means you are way out of collimation. With F/4 way out is not nearly as much as you'd think it is, mind you and it really needs to be almost perfect every time. Tilt inspector in Siril below.

 

I'd say figure out the mechanical side of things first before tackling possible sensor tilt and collimation. If you try to collimate a scope that is not mechanically sound you will just end up going in circles and it will never seem right because it really isn't possible.

@alacant usually comments that none of the cheap newtonians available today are out-of-box compatible with the demands of astrophotography in terms of mechanical stability and yours is probably not different. One more common issue you could have if the stiffness of your tube, which if not stiff enough can bend/buckle under load ever so slightly and so cause the scope to go out of collimation. Fitting a longer dovetail bar and spreading the rings out further apart from each other (i think it was about 50cm) while also tying up the top of the rings together with another dovetail bar will significantly improve stiffness of the tube. Not seeing to the mechanical side of things first will lead you on a wild goose chase where tools dont agree with each other and results vary from night to night.

Hmm, thinking about my setup when taking that image... I just put my telescope back to the position it was in.

It does seem like the weight of the camera/filter wheel/coma corrector would have been weighing roughly on the lower left part of the camera, but not quite the 45 degrees this image analysis would suggest.

I also took images last night of the bubble nebula, although those weren't focus tests and as such have much longer exposures, so I don't know if they'll be as easy to analyse accurately...

By eye, matters in this image don't seem as bad, but the objects are on opposite sides of the meridian, so one has the center of gravity of the camera pulling one way, and the other has it pulling in roughly the opposite direction in theory? What do you think?

NGC_7635_Light_002.fits

 

Edit: Alacant has stressed the importance of robust support, and no doubt at some point I will move things to a losmandy bar and puck, but the vixen style supplied with the scope is already rather long (skywatcher supplies 30cm I think, this one is much longer) and flipping the telescope from one side of the meridian to the other hasn't produced a shift in collimation on the laser since I followed one of Alacant's pieces of advice and removed the primary mirror clips and replaced them with neutral silicone sealant. So I am not sure tube flex can be blamed.

Edited July 11, 2022 by pipnina

[ONIKKINEN]

14 minutes ago, pipnina said:

Hmm, thinking about my setup when taking that image... I just put my telescope back to the position it was in.

It does seem like the weight of the camera/filter wheel/coma corrector would have been weighing roughly on the lower left part of the camera, but not quite the 45 degrees this image analysis would suggest.

I also took images last night of the bubble nebula, although those weren't focus tests and as such have much longer exposures, so I don't know if they'll be as easy to analyse accurately...

By eye, matters in this image don't seem as bad, but the objects are on opposite sides of the meridian, so one has the center of gravity of the camera pulling one way, and the other has it pulling in roughly the opposite direction in theory? What do you think?

NGC_7635_Light_002.fits 49.49 MB · 0 downloads

 

Edit: Alacant has stressed the importance of robust support, and no doubt at some point I will move things to a losmandy bar and puck, but the vixen style supplied with the scope is already rather long (skywatcher supplies 30cm I think, this one is much longer) and flipping the telescope from one side of the meridian to the other hasn't produced a shift in collimation on the laser since I followed one of Alacant's pieces of advice and removed the primary mirror clips and replaced them with neutral silicone sealant. So I am not sure tube flex can be blamed.

This one looks much better. I think this one had much better seeing also since your starsizes are much better compared to the focus test, but collimation looks better too for some reason. Top right corner appears to be the best according to Siril (pictured) and ASTAP image inspectors.

Star shapes are still a bit weird but that could also be in part due to guiding or something other than collimation (still guessing its mechanical, focuser most likely). But, i would say judging from the image that its still a bit out of collimation for some reason. But also i will admit that i would accept that as starsizes are pretty decent here and if binned x2 it would be hard to find something to complain about. Not sure how to proceed if i were you, but since your tube and mirror cell are good it leaves the sensor itself being tilted, focuser being unreliable or the mirror collimation itself as the causes. Or the collimation method you use is still somehow not quite ideal. Try the laser-through-CC method and see if something changes?

[wookie1965]

I only do visual when I had my 200p all I used was a Cheshire sight tube and a self centering adapter 2"-1.25". I bought 2 cheap lasers one went in the bin as it was uncollimatable the other was OK but not spot on. 

I think the self centering adapter is one of the best things going, with my Cheshire I could collimate in seconds even turning the Cheshire by 180° I still got the same results. 

I had some fantastic views through that I should never have sold it. 

 

[alacant]

On 09/07/2022 at 21:29, pipnina said:

GSO, who made the mirror cell, decided on that stupid design

GSO made the whole thing.

If you want to eliminate tilt, you simply have to pull it apart and fit proper springs and mirror supports. Lateral mirror movement and sagging on the inadequate springs makes the collimation change as the tube angle changes. The collimation is only correct at the angle at which you collimate it.

The tube also needs supporting. Pragmatism led us to this.

Cheers and HTH

Edited July 16, 2022 by alacant

[pipnina]

On 11/07/2022 at 22:39, ONIKKINEN said:

This one looks much better. I think this one had much better seeing also since your starsizes are much better compared to the focus test, but collimation looks better too for some reason. Top right corner appears to be the best according to Siril (pictured) and ASTAP image inspectors.

 

Star shapes are still a bit weird but that could also be in part due to guiding or something other than collimation (still guessing its mechanical, focuser most likely). But, i would say judging from the image that its still a bit out of collimation for some reason. But also i will admit that i would accept that as starsizes are pretty decent here and if binned x2 it would be hard to find something to complain about. Not sure how to proceed if i were you, but since your tube and mirror cell are good it leaves the sensor itself being tilted, focuser being unreliable or the mirror collimation itself as the causes. Or the collimation method you use is still somehow not quite ideal. Try the laser-through-CC method and see if something changes?

I gave the collimation a good re-do tonight as it was getting dark. I managed to pry the M48-M42 adapter from the CC (some of these threads get well and truly stuck!) so the 2-1.25 adapter that holds the laser would attach to the CC. It does make it a bit softer, but not a huge amount. I do think it made things a bit easier though so thanks!

As you can see I took a quick 5 second luminance image of a star-dense region and the FWHM measurements are MUCH better than before. But visually you can still see something isn't quite right when you look at the corners. Maybe I need to attempt barlow lasering to get it as bang on as possible?

Also I wish GSO made their mirror cell in the same way as skywatcher... My 130P-DS threadlocked studs into the mirror cell, then used thumbscrew nuts to pull the mirror or push it further away from the cell block, with three locking screws that drive into the cell. Never had any issues with this and it was SUPER easy and intuitive to make it work. This GSO one however seems like they put about 30 seconds of thought into it and went "good enough!". I have to both have enough tension on the locking screws to allow the adjustment screws to actually move the mirror, while also leaving it loose enough to move the adjustment screws without too much strain. It's just poor deisgn, and from an OTA that costs a lot more than the equivalently sized 200P-DS...

I think a focuser upgrade will be in my future too, I think it just makes sense to ensure it's all held properly since my kit is heavy and if it fits my corrector and collimator more snugly and consistently true as well, that can only make things easier. I found I had to double check my laser multiple passes over when rotating it 90, 180, 270 etc to check it was all in the right place... The focuser wasn't always keeping it quite the same way every time. When you changed your focuser, how easy was it and did you have to do any alignments with that, like shimming etc? Or do you just have to re-collimate?

Many thanks again!

[ONIKKINEN]

9 hours ago, pipnina said:

I gave the collimation a good re-do tonight as it was getting dark. I managed to pry the M48-M42 adapter from the CC (some of these threads get well and truly stuck!) so the 2-1.25 adapter that holds the laser would attach to the CC. It does make it a bit softer, but not a huge amount. I do think it made things a bit easier though so thanks!

As you can see I took a quick 5 second luminance image of a star-dense region and the FWHM measurements are MUCH better than before. But visually you can still see something isn't quite right when you look at the corners. Maybe I need to attempt barlow lasering to get it as bang on as possible?

Also I wish GSO made their mirror cell in the same way as skywatcher... My 130P-DS threadlocked studs into the mirror cell, then used thumbscrew nuts to pull the mirror or push it further away from the cell block, with three locking screws that drive into the cell. Never had any issues with this and it was SUPER easy and intuitive to make it work. This GSO one however seems like they put about 30 seconds of thought into it and went "good enough!". I have to both have enough tension on the locking screws to allow the adjustment screws to actually move the mirror, while also leaving it loose enough to move the adjustment screws without too much strain. It's just poor deisgn, and from an OTA that costs a lot more than the equivalently sized 200P-DS...

I think a focuser upgrade will be in my future too, I think it just makes sense to ensure it's all held properly since my kit is heavy and if it fits my corrector and collimator more snugly and consistently true as well, that can only make things easier. I found I had to double check my laser multiple passes over when rotating it 90, 180, 270 etc to check it was all in the right place... The focuser wasn't always keeping it quite the same way every time. When you changed your focuser, how easy was it and did you have to do any alignments with that, like shimming etc? Or do you just have to re-collimate?

Many thanks again!

That result looks great, i would be super happy with that collimation! That few % fwhm difference will not be noticeable in a final image.

I installed the Baader diamond steeltrack on my VX8 and had to drill 4 holes for it to fit. Also had to enlarge the drawtube hole on the OTA just a little bit to make sure it doesnt freeze on me. An afternoons work at best, not difficult to do at all.

It comes with a couple of shims in the box to make it sit flush on different sized tubes. It also has full adjustment in pitch and a little bit in yaw to do final adjustments on centering it on your tube. Bit pricey yes, but well worth the cost IMO, one of the better purchases i have made for sure.