Collimation issue or otherwise?

[Stuart1971]

42 minutes ago, tooth_dr said:

Hello Stuart.  I'm using the tak CC, which is designed to work at 56mm.  I have a tak-nikon adapter than is supposed to be a direct fit, and means that the spacing is predetermined.  The sensor-flange distance of a Nikon is 46.5mm, and the CC looks to be 9.5mm, so this gives the proper 56mm back focus.  Looking at the above, if it is a back focus issue, then where do I need to go with it?

 

Hmmm, sounds like you have the spacing bang on...

Its just that in the images above you posted, it looked like the coma was running around the image, and I had a similar issue with a FF/FR on an SCT, where the stars were elongated in a circle around the image, and it was a spacing issue....

Does this help..

 

[tooth_dr]

 

Thats the image in CCDI

[sharkmelley]

3 hours ago, tooth_dr said:

Last night i took a few images. There is a lot of coma despite the scope looking fairly well collimated 🤷

L_0037.NEF 73.08 MB · 1 download

 

That's looking pretty bad.  We can be pretty sure the spacing of the flattener to the sensor is correct because you are using the correct adaptor.  But even the stars in the centre of the image are mis-shapen - they don't have circular symmetry but some kind of coma.  Even if the spacing were wrong this would not happen.  The collimation must be a long way off to cause this but I'm at a loss to understand why.

In the manual there is a section titled "Additional Collimation (Perfecting Collimation)" and it describes how to rotate the focuser, checking the crosshairs remain central.  What happens if you try this?  If your crosshairs are off-centre then your collimation will be off as well.

Mark

 

[tooth_dr]

1 hour ago, sharkmelley said:

That's looking pretty bad.  We can be pretty sure the spacing of the flattener to the sensor is correct because you are using the correct adaptor.  But even the stars in the centre of the image are mis-shapen - they don't have circular symmetry but some kind of coma.  Even if the spacing were wrong this would not happen.  The collimation must be a long way off to cause this but I'm at a loss to understand why.

In the manual there is a section titled "Additional Collimation (Perfecting Collimation)" and it describes how to rotate the focuser, checking the crosshairs remain central.  What happens if you try this?  If your crosshairs are off-centre then your collimation will be off as well.

Mark

 

Hi Mark, I did notice the misshapen central stars too, so there is something amiss for sure.  

The focuser cross hair does move when rotating, but I read elsewhere on CN that if you lock the focuser and dont move it then the collimation holds true for that particular position.  I'm more than happy to address this issue though, and see if it helps.  Do the grub screws on the side of the scope needs loosening when doing collimation.

Thanks for your ongoing assistance on this issue Mark.

 

Adam.

 

Ive added the text from the manual. What exactly am I adjusting here with respect to the cross hairs.

Edited September 29, 2020 by tooth_dr

[Allinthehead]

From the image you posted above collimation looks perfect. Is it possible you're pinching the mirror somehow? I had stars like this when i over tightened my mirror clips, I'm aware the 180 doesn't have mirror clips but could there be another way to pinch the 180 mirror.

[tooth_dr]

Just now, Allinthehead said:

From the image you posted above collimation looks perfect. Is it possible you're pinching the mirror somehow? I had stars like this when i over tightened my mirror clips, I'm aware the 180 doesn't have mirror clips but could there be another way to pinch the 180 mirror.

Hi Richard. I have the grub screws on the side of mirror some up tight. Perhaps this is causing it?

[Allinthehead]

1 minute ago, tooth_dr said:

Hi Richard. I have the grub screws on the side of mirror some up tight. Perhaps this is causing it?

Might be worth taking the primary off and check how the mirror is fixed in place, adjust if too tight. My 130 arrived pinched from factory but as i said they use different mechanisms.

[tooth_dr]

5 minutes ago, Allinthehead said:

Might be worth taking the primary off and check how the mirror is fixed in place, adjust if too tight. My 130 arrived pinched from factory but as i said they use different mechanisms.

I cant make it much worse I guess (famous last words!)

Ian King has replied to an email I sent to FLO - he believes it is still collimation, and that I have aligned the mirrors but with the primary mirror offset from the main mechanical axis of the scope.  This would make sense with the strange coma in the central portion.  

[Allinthehead]

Do you have the secondary mirror mark in the center of the crosshairs?

[tooth_dr]

3 minutes ago, Allinthehead said:

Do you have the secondary mirror mark in the center of the crosshairs?

[tooth_dr]

I also replaced the crosshairs with some of my wifes hair.

[Allinthehead]

16 minutes ago, tooth_dr said:

I also replaced the crosshairs with some of my wifes hair.

🤣 

I can't see the secondary dot in that image. Do you first align the secondary dot on the crosshairs? This ensures the secondary is aligned with the focuser 

[sharkmelley]

The manual's description of the "Additional Collimation" process is badly written, just like the rest of the manual.  What it means is that when you rotate the focuser, the crosshairs might describe a circle.  If so, then the centre of this circle (and not the actual crosshair position) is the reference point you need to use to line up the dots when you are collimating.

As for the mirror, you shouldn't overtighten the 3 grub screws that adjust the lateral position of the mirror cell.  Also there is the possibility the mirror is distorted as a result of overtightening something that bears directly on the mirror glass.  On the rear of the mirror cell there are 3 adjusters that are in contact (or should be in contact) with the rear surface of the mirror glass.  These should never be touched and the manual doesn't even mention them.  But maybe someone, somewhere at some time did make the mistake of touching them, so either they are too slack and don't support the weight of the glass or they are too tight and distort it.  However I wouldn't want to touch these without the benefit of an optical test bench.

Mark

[tooth_dr]

 

26 minutes ago, Allinthehead said:

I can't see the secondary dot in that image

Outlined in red here below.  Looking at that image again, the outer light circle doesnt look concentric.  What does that circle correspond to?

. 

 

[tooth_dr]

5 minutes ago, sharkmelley said:

The manual's description of the "Additional Collimation" process is badly written, just like the rest of the manual.  What it means is that when you rotate the focuser, the crosshairs might describe a circle.  If so, then the centre of this circle (and not the actual crosshair position) is the reference point you need to use to line up the dots when you are collimating.

Doesnt that make a bit of a mockery of the whole process of being super precise and lining up the secondary dot below the cross hairs, if in fact I have to line then up with my interpretation of the centre of a circle?

[sharkmelley]

Clarifying my earlier post, those adjusters that shouldn't be touched are the "vis d'appui" in this photo from a French website:

[sharkmelley]

7 minutes ago, tooth_dr said:

Doesnt that make a bit of a mockery of the whole process of being super precise and lining up the secondary dot below the cross hairs, if in fact I have to line then up with my interpretation of the centre of a circle?

Yes and no.  It simply admits the possibility of additional errors creeping it. For instance, one of your wife's hairs might go slightly slack without you realising it.  Using the "Additional Collimation" technique would prevent this causing a collimation error.

Mark

Edited September 29, 2020 by sharkmelley

[Davey-T]

Commiserations Adam, I've been trying to collimate the Sharpstar 150HNT for a while now, had to take the whole thing apart and rebuild it starting from the fact that the primary mirror wasn't central in the tube, nearly there hopefully, just waiting on some flocking material to put in front of the mirror to stop it flopping about in the cell.
Watched loads of how to videos but never works out quite like them, mind they're usually collimating 10" f/5 newt's 😂

Good luck

Dave

[tooth_dr]

1 hour ago, Davey-T said:

Commiserations Adam, I've been trying to collimate the Sharpstar 150HNT for a while now, had to take the whole thing apart and rebuild it starting from the fact that the primary mirror wasn't central in the tube, nearly there hopefully, just waiting on some flocking material to put in front of the mirror to stop it flopping about in the cell.
Watched loads of how to videos but never works out quite like them, mind they're usually collimating 10" f/5 newt's 😂

Good luck

Dave

Yes indeed my own 10” F5 is a dream to collimate now!

This is a disadvantage of buying a used scope. I think if this was new it wouldn’t have happened.

[Davey-T]

17 minutes ago, tooth_dr said:

This is a disadvantage of buying a used scope. I think if this was new it wouldn’t have happened.

Mine is new but didn't help 😂

Dave

[Xiga]

6 hours ago, tooth_dr said:

Doesnt that make a bit of a mockery of the whole process of being super precise and lining up the secondary dot below the cross hairs, if in fact I have to line then up with my interpretation of the centre of a circle?

Sorry to see you are having trouble with the Epsilon Adam. 

I can't offer much in the way of help i'm afraid, but the comment above reminded me of what i needed to do with the Polar Scope of my HEQ5-Pro when i first got it. It also needed aligned/collimated, as the central crosshair was not remaining fixed when i rotated the mount. I needed to adjust a few screws and then it was fine. I would take another look at the grub screws on your focuser, and see if it's possible to loosen and adjust them such that the crosshair doesn't move under rotation. Once done, perhaps it's then a case of carefully tightening all of the grub screws equally, to tighten it up so that there is no slop with in/out movement (which it sounds like you fixed before). If normal collimation, i.e centering the secondary in the focuser, followed by centering the primary, still isn't possible, then it's probably a good shout that the primary isn't sitting perfectly central. 

Of course, having never actually owned a Newt, feel free to take all of that with a large pinch of salt!

[tooth_dr]

9 hours ago, Xiga said:

Sorry to see you are having trouble with the Epsilon Adam. 

I can't offer much in the way of help i'm afraid, but the comment above reminded me of what i needed to do with the Polar Scope of my HEQ5-Pro when i first got it. It also needed aligned/collimated, as the central crosshair was not remaining fixed when i rotated the mount. I needed to adjust a few screws and then it was fine. I would take another look at the grub screws on your focuser, and see if it's possible to loosen and adjust them such that the crosshair doesn't move under rotation. Once done, perhaps it's then a case of carefully tightening all of the grub screws equally, to tighten it up so that there is no slop with in/out movement (which it sounds like you fixed before). If normal collimation, i.e centering the secondary in the focuser, followed by centering the primary, still isn't possible, then it's probably a good shout that the primary isn't sitting perfectly central. 

Of course, having never actually owned a Newt, feel free to take all of that with a large pinch of salt!

Hi Ciarán. There aren’t adjustment screws on the focuser like a polar scope.

I had a look at it again last night and I followed a different guide, and this is different to how I was reading the Tak manual so I’m hopeful  

 

 

[sharkmelley]

14 hours ago, tooth_dr said:

Hi Ciarán. There aren’t adjustment screws on the focuser like a polar scope.

I had a look at it again last night and I followed a different guide, and this is different to how I was reading the Tak manual so I’m hopeful  

 

Yes, that guide is correct and is easier to understand than the garbled instructions in the Tak Epsilon manual.

Mark

[tooth_dr]

9 hours ago, sharkmelley said:

Yes, that guide is correct and is easier to understand than the garbled instructions in the Tak Epsilon manual.

Mark

Mark,

I had 30 minutes of patchy cloud last night so I got a few images.  I still have some coma but I can see a significant improvement.  I will attach the image later if you would have time to look at it.

 

Thanks

Adam.  

[tooth_dr]

I decided to go back to the smaller 8mp CCD sensor as I wanted to image around the full moon in Ha.  The scope is performing well at that size of sensor with current collimation, so I might stick with it for the time being.