Poor mars - collimation issues?

[Mr niall 1,094]

Hi all

sorry  I can hear the groans already... I have a heritage 130p and I just can’t get to grips with it. I had some pleasing views of collinder 399 tonight and some nice widefield with my 20mm eyepiece but it all goes horrible wrong above about 50x.

Ive got a sneaky feeling that this scope is capable of quite a lot but I just can’t get it to deliver. I’m finding it a struggle to get any sort of focussing sweet spot, with flare and noticeable coma from about 50% from the centre of view.

planets are a disaster; and I know seeing is terrible at the mo but Mars showed nothing but a small tangerine tonight, no hint of anything vaguely resembling detail (and quite smudgy - sometimes almost looking like the view through poorly collimated binoculars...). Jupiter has shown bands just about but not in this lifetime or the next will I get enough focus to show GRS, similarly the Cassini division is a pipe dream. I’ve had all of these easily with my old Skymax 127 and that WAS slightly miscollimated.

range of cheaper eyepieces tried - up to about 175x, had a go with a BST  starguider (25mm) that I really couldn’t afford out of desperation but same again.

so the obvious answer is a collimation issue - thing is - I’ve been fiddling with this thing until I’ve lost my mind! Tried a cheap Cheshire and got nowhere so moved the Cheshire on, but now I’m just using a collimation cap; thought I had it cracked but same again tonight. I’ve read Astro baby, watched Orion UK videos, Gary Seronik and about a dozen others....

Using the Rigel Collimation cap I can get the centre of the primary aligned perfectly with the hole in the cap. I’m wondering whether I’ve repeatedly misunderstood the secondary collimation procedure... are there different symptoms for primary and secondary miscollimation? I don’t know - I’ve “started again” about a dozen times now and end up in the same place.

sorry for the rant, not easy to admit failure - especially when it’s something so routine.

thanks all.

[John 35,417]

What does your star test look like inside, at and outside focus at reasonably high power ?

Polaris is a good star to use.

As you go inside and outside of focus the diffraction rings expand around the dark shadow of the secondary and should be concentric with the shadow in the centre. The seeing probably won't make them precisely defined but you should get an idea if the collimation is there or therabouts.

[Mr niall 1,094]

Thanks I’ll try again with a star test but I’m finding when I’ve tried before that I’m not really getting clear diffraction rings, just an expanding smudge? The donut is pretty good in all honesty though and reasonable evenly defined. Although I did notice tonight that if the star was anything off dead centre it was “sliced off” at the side, which got me wondering about the secondary again.

[Ricochet 1,561]

The expanding smudge comment makes it sound like you're defocusing too much. You only need to be a touch our of focus to star test. You might also need to use a higher power eyepiece. 

Personally, I would keep an eye out for another Cheshire. They really are the tool for secondary collimation. 

What actually happens when it goes wrong above 50x? Which eyepieces are you using at these magnifications? I would think that even when miscollimated the scope should show decent views. 

[Mr niall 1,094]

17 minutes ago, Ricochet said:

The expanding smudge comment makes it sound like you're defocusing too much. You only need to be a touch our of focus to star test. You might also need to use a higher power eyepiece. 

Personally, I would keep an eye out for another Cheshire. They really are the tool for secondary collimation. 

What actually happens when it goes wrong above 50x? Which eyepieces are you using at these magnifications? I would think that even when miscollimated the scope should show decent views. 

Well it’s mostly an issue with focussing - most of the stars focus to a point where they kind of flare, with a bit of a tail on one side, imagine a tiny tiny comet or exacerbated edge coma - but uniformly throughout the fov. They just go through that phase and start to smudge so there’s nothing like a sharp focus. It’s not an issue with lower power eyepieces but still there just less pronounced. I’m using Opticstar wide angle 66 degree eyepieces. I think they’re just MA, and I’d expect coma at the edges, but this is something else... get the same thing with the supplied eyepieces. Have noticed a bit of seagull action too, and planets just get near focus then drift away again. Honestly my first scope was a celestron 70az so I know this isn’t an expectation vs reality thing. To be honest I was hard on my old Skymax and it’s blew this one out of the water: I’m not a conspiracy theorist, I’m 100% sure that I a m the problem I just don’t know why!

im pretty sure if someone with brains looked at it, they’d go “oh, you’re an idiot! Here’s why:”, but that just isn’t an option.

interesting that you say to use a Cheshire for secondary - Astrobaby says not to (I’m not challenging you, just interested in  your thinking; I’d more than happily buy another Cheshire if it helped!), they (she? Or he? I’m not sure) advise a collimation cap for secondary.

[Ricochet 1,561]

9 minutes ago, Mr niall said:

Well it’s mostly an issue with focussing - most of the stars focus to a point where they kind of flare, with a bit of a tail on one side, imagine a tiny tiny comet or exacerbated edge coma - but uniformly throughout the fov. They just go through that phase and start to smudge so there’s nothing like a sharp focus.

I would look at this "coma" closely. It is entirely possible that it really is coma. If it is then around the field of view the angle of the coma will be different. You should notice that they all point to the "centre", but if your telescope is not properly collimated the optical centre of the telescope and the centre of your field of view won't match up. In theory you can use this to adjust your primary mirror at night.

Another thing to look for is if the "coma" suddenly flips by 90° as you go from one side of focus to the other. If it does, the you've got some astigmatism in the system. 

Finally, your telescope has a single secondary vane, which is fairly thick. This should produce a diffraction spike at 90° to the angle of the vane across your scope. I would expect that this would only affect the brighter stars though.

28 minutes ago, Mr niall said:

I’m using Opticstar wide angle 66 degree eyepieces. I think they’re just MA, and I’d expect coma at the edges, but this is something else... get the same thing with the supplied eyepieces. Have noticed a bit of seagull action too, and planets just get near focus then drift away again.

To be honest, these are not great eyepieces and a fast focal ratio scope will push them a lot harder than your old Skymax would have been doing. I would concentrate on how things look in the centre of the field of view. The edges are likely to show eyepiece aberrations on top of anything from the telescope which will make diagnosis difficult. "Seagull action" will be entirely the fault of the eyepiece I would think.

32 minutes ago, Mr niall said:

im pretty sure if someone with brains looked at it, they’d go “oh, you’re an idiot! Here’s why:”, but that just isn’t an option.

That's a shame. Taking it down to your local astro society and getting a second pair of eyes to look at it could be a great help. 

36 minutes ago, Mr niall said:

interesting that you say to use a Cheshire for secondary - Astrobaby says not to (I’m not challenging you, just interested in  your thinking; I’d more than happily buy another Cheshire if it helped!), they (she? Or he? I’m not sure) advise a collimation cap for secondary.

Yes, I followed the Astrobaby advice to start with too but I am convinced the suggested tools are wrong. The tool I suggest for the secondary is actually a sight tube - a long tube with crosshairs at the bottom end - but as almost all "Cheshires" these days are a combined cheshire and sight tube that is what you want. For the first step of secondary collimation you have to centre the secondary under the focuser and angle it so that it appears circular. With a collimation cap the only reference you have for this is the bottom of your focuser, which usually appears to be a lot wider than the secondary mirror. This introduces quite a large margin of error in positioning your secondary. With a cheshire/sight tube the circle that you are comparing your secondary to is a lot smaller which reduces the margin of error considerably. In fact you can adjust the focuser position so that the circle made by the bottom of the sight tube and your secondary mirror are exactly the same size. This makes it easy to judge whether the secondary is centred and angled correctly. Note, however, that for some fast telescopes you may have to withdraw the cheshire from the focuser a bit or use an extension tube in order to see the whole secondary.

When it comes to the primary, this  is where I would actually favour a collimation cap. In order to position the primary, you have to centre the dot formed by the eye hole in your cap or cheshire in the doughnut at the centre of the primary mirror. With the combined cheshire and sight tube those crosshairs obscure the centre of the dougnut making positioning the dot difficult. With the collimation cap you have an unobstructed view.

[Mr niall 1,094]

8 hours ago, Ricochet said:

I would look at this "coma" closely. It is entirely possible that it really is coma. If it is then around the field of view the angle of the coma will be different. You should notice that they all point to the "centre", but if your telescope is not properly collimated the optical centre of the telescope and the centre of your field of view won't match up. In theory you can use this to adjust your primary mirror at night.

Another thing to look for is if the "coma" suddenly flips by 90° as you go from one side of focus to the other. If it does, the you've got some astigmatism in the system. 

Finally, your telescope has a single secondary vane, which is fairly thick. This should produce a diffraction spike at 90° to the angle of the vane across your scope. I would expect that this would only affect the brighter stars though.

To be honest, these are not great eyepieces and a fast focal ratio scope will push them a lot harder than your old Skymax would have been doing. I would concentrate on how things look in the centre of the field of view. The edges are likely to show eyepiece aberrations on top of anything from the telescope which will make diagnosis difficult. "Seagull action" will be entirely the fault of the eyepiece I would think.

That's a shame. Taking it down to your local astro society and getting a second pair of eyes to look at it could be a great help. 

Yes, I followed the Astrobaby advice to start with too but I am convinced the suggested tools are wrong. The tool I suggest for the secondary is actually a sight tube - a long tube with crosshairs at the bottom end - but as almost all "Cheshires" these days are a combined cheshire and sight tube that is what you want. For the first step of secondary collimation you have to centre the secondary under the focuser and angle it so that it appears circular. With a collimation cap the only reference you have for this is the bottom of your focuser, which usually appears to be a lot wider than the secondary mirror. This introduces quite a large margin of error in positioning your secondary. With a cheshire/sight tube the circle that you are comparing your secondary to is a lot smaller which reduces the margin of error considerably. In fact you can adjust the focuser position so that the circle made by the bottom of the sight tube and your secondary mirror are exactly the same size. This makes it easy to judge whether the secondary is centred and angled correctly. Note, however, that for some fast telescopes you may have to withdraw the cheshire from the focuser a bit or use an extension tube in order to see the whole secondary.

When it comes to the primary, this  is where I would actually favour a collimation cap. In order to position the primary, you have to centre the dot formed by the eye hole in your cap or cheshire in the doughnut at the centre of the primary mirror. With the combined cheshire and sight tube those crosshairs obscure the centre of the dougnut making positioning the dot difficult. With the collimation cap you have an unobstructed view.

Thanks a lot, that’s really useful. Next time I get some green squares I’ll get out and investigate. Your single diffraction spike theory is interesting and rings a bell actually.

ill invest in another Cheshire today!