Strange collimation behaviour

[Albenn]

I have been quite happily collimating my Heritage 130p scope for a couple of weeks now using my deluxe Cheshire/Sight tube combination tool from FLO. Now, as I can only just about see the edge of the primary mirror clips through this tube due to its length when getting the secondary aligned, I decided to make a collimating cap from a black 35mm film cannister to give me a better FOV to see the primary mirror clips.

Anyway, I drilled the cap dead centre with a 2mm drill bit and cut the cannister in two about 2/3rds up so with the lid on it sits nice and central in the focuser. So far so good. However, when I looked through this to check the reflection of the primary I found that it's reflection was well off in the secondary as though the secondary mirror had been twisted along the major axisror!

So, I retried with the Cheshire tool again and all is well. I don't know if the Cheshire tool is flexing the focuser mount on the Heritage 130 or what it is that's going on to make the two views so very different but it's driving me mad trying to decide which is giving the correct view.

The Cheshire is inserted all the way in so that the wide part sits on top of the focuser rim so I assume it will be aligned to the focuser tube.

When I look through the Chesire I can see that the reflection of the bottom of it is off centre and toward the edge of the secondary mirror furthest away from me.

BTW, a there are no issues when the scope is being used and a star test proved okay.

Does anyone have any idea as to what may be happening here?

[steelfixer]

Hi Alan

It could be your home made cap is not sitting square in the focuser tube or the hole you put in it is not exactly square to the tube or any number of things.

If it star test ok after using the Cheshire then stick with it.

[Albenn]

Thanks for the feedback Steelfixer, I double checked the hole in the home made cap and it is definitely central. The hole itself is round and has no burrs and I even tried just using the film cannister lid on it's own without the half cannister tube but the results were the same!

[Moonshane]

I suspect this is eye placement possibly re the canister. if you move your position does the position of the reflections change? Just do your best and get everything looking round and centralised (put a piece of paper down the tube to block the primary reflection in the secondary).

Then put your cheshire in and centralise the cheshire cross hairs with the donut.

then collimate the primary.

[LukeSkywatcher]

Do a simple star test. Point the scope at any bright star and defocus the EP. If you see what looks like a polo mint then you are fine.

[Albenn]

I too first suspected a parallax issue but if I change eye placement over the cap or Cheshire peepholes I just lose the view through them as the holes are so small.

I have been through the collimation procedure from scratch and am doing everything the way I have always done it.

Secondary rounded and centred in the x,y planes in the wire cross hairs.

Secondary rotated/tilted to show equal primary clip exposure with the paper primary centre circle also on the wire crosshairs.

I then adjust the primary to align with the reflection of the cheshire.

As a beginner, I thought I was doing well and had everything boxed off until I tried the collimator cap which has now made me really unsure if I had it correct in the first place. The star test is satisfactory but can this test appear okay even if collimated incorrectly?

[LukeSkywatcher]

The Heritage should have been extremely well collimated when you bought it.

Did you go messing with it just for the sake of it?

I've had mine a couple of years and only collimated it once. It was perfect when it arrived from the UK, but me being me, tweaked a few knobs here and there (as per AB's guide) and its still fine.

Mind you when i move my scopes around the place, i treat them like a paramedic would with a suspected spinal injury victim.

[Albenn]

Do a simple star test. Point the scope at any bright star and defocus the EP. If you see what looks like a polo mint then you are fine.

Paul, I was busy writing my previous reply and as you can see I have star tested previously and all is well. I should add that no changes were made from the last collimation check except for a very minor primary adjustment which I believe is normal for a flex

This really is a strange one this!

[LukeSkywatcher]

Stick a camera to the EP tube and take a quick snap of the situation. That usually helps resolve collimation issues because we see exactly what you see.

Oh yeah and post the pic here. The Chesire laser collimator may actually need collimating itself. I have heard before that they can be out of line and this makes a perfectly collimated scope seem out also.

Something to check. Also it may be worth checking if the spider arm on the secondary is tightly fit. Mine when it arrived was loose.

I'm sure you have checked that already though.

I'm just tossing ideas your way, as collimation is something i dont worry about too much unless the views i am getting are terrible and thats usually down to bad "seeing".

[Albenn]

The Heritage should have been extremely well collimated when you bought it.

Did you go messing with it just for the sake of it?

I've had mine a couple of years and only collimated it once. It was perfect when it arrived from the UK, but me being me, tweaked a few knobs here and there (as per AB's guide) and its still fine.

Mind you when i move my scopes around the place, i treat them like a paramedic would with a suspected spinal injury victim.

Paul, I made no changes other than those mentioned and like you I found that the scope collimation was almost perfect when purchased. I believe if it isn't broken then leave it alone. My reference to the collimation procedure was merely a check-list of the procedure.

I too treat the scope carefully and it hasn't been knocked. Let's face it, its not that often I can get outside to use it with the cloudy weather:D

[Albenn]

Stick a camera to the EP tube and take a quick snap of the situation. That usually helps resolve collimation issues because we see exactly what you see.

Oh yeah and post the pic here. The Chesire laser collimator may actually need collimating itself. I have heard before that they can be out of line and this makes a perfectly collimated scope seem out also.

Something to check.

Paul, I do not use a laser. It is a Cheshire combination tool.

Will try and post a pic.

[LukeSkywatcher]

Sorry. I'm cofusing the Chesire with the Hotec laser thingie. I thought the Cheshire was also a laser collimator. I'll bow out now from this because i'm wasting time and ink. Others can help more then i can.

Do post a pic if you can as it will really help.

[Albenn]

Okay, I have managed to get a pic of the view down the focuser but don't know if it will be of any use, but here goes...

[LukeSkywatcher]

That pic should help a lot. Something is not quite right (i'm stating the obvious i know). Assuming the secondary mirror is aligned (as it seems to be relative to the shield behind it). This to me leaves the only option that the primary is out of alignment. It all seems to be pointed towards the 9-11 o' clock position.

[Albenn]

Agreed. But when I view through the Cheshire everything looks perfect! I don't know how square I got the camera on to the focuser so there may be a bit of error in the photo.

The primary looks central with an even gap around it when I look down to the bottom of the tube

[Moonshane]

step one - align the secondary to the focuser

when using the collimation cap, ignore all alther information other than the position of the secondary mirror (i.e. ignore the reflection of the primary with the mirror clips) in the focuser - the reflective surface should be round and central all round in the focuser. use a sheet of paper to block the primary reflection as this makes it a lot easier.

step two - align the secondary to the primary

using the Cheshire align the Cheshire cross hairs with the central donut. this can be fiddly but persevere with it. refer again to the alignment of the focuser / secondary (step one) and adjust if necessary.

when both are aligned properly move to step three

step three alignment of primary to secondary

using the cheshire adjust two of the three primary bolts to ensure the dot is inside the donut

thankfully after the optics are aligned, step one and two rarely need adjusting (just checking) with the easier and quicker step three being the one that needs doing almost every time I observe.

hope this helps.

[Albenn]

step one - align the secondary to the focuser

when using the collimation cap, ignore all alther information other than the position of the secondary mirror (i.e. ignore the reflection of the primary with the mirror clips) in the focuser - the reflective surface should be round and central all round in the focuser. use a sheet of paper to block the primary reflection as this makes it a lot easier.

step two - align the secondary to the primary

using the Cheshire align the Cheshire cross hairs with the central donut. this can be fiddly but persevere with it. refer again to the alignment of the focuser / secondary (step one) and adjust if necessary.

when both are aligned properly move to step three

step three alignment of primary to secondary

using the cheshire adjust two of the three primary bolts to ensure the dot is inside the donut

thankfully after the optics are aligned, step one and two rarely need adjusting (just checking) with the easier and quicker step three being the one that needs doing almost every time I observe.

hope this helps.

Thanks Moonshane. Those are the steps I have been following all along.

I have just made the hole in the home made collimating cap a tiny bit larger and this seems to have done the trick! The secondary and also the primary mirror dot are now both showing central to this cap and also the Cheshire tool wire cross hairs.

The primary required a tiny tweak to get the Cheshire reflection centred and all seems well but I have to do this anyway whenever I expand the tube. I'll check it out later on if the sky clears.

One thing that still puzzles me though: All the pictures and diagrams I see show the reflection of the bottom of the Cheshire tube as bang in the centre of the secondary, but mine persists in being toward the edge of the secondary furthest away from the viewing position.

The secondary mirror holder is ever so slightly off centre (1mm or so) in the horizontal plane but is set about 4mm down from centre in the vertical plane ie away from the focuser line of site. I wonder if this vertical offset is normal?

It may be that this is just how this scope collimates? I dunno.

[Moonshane]

great stuff, sounds like you got it sorted. I am sure that you are getting good collimation if you are getting sharp lunar and planetary images.

[LukeSkywatcher]

Glad you got it sorted Alan.

[Albenn]

Update.

I retried collimation from scratch 3 times and each time the primary mirror was as shown as in the photo I had previously posted. However, a star test last night and my first light of Saturn (report posted in the planetary observation section) proved the collimation was fine.

However, this misalignment was driving me mad I just couldn't understand why! I then decided to look down the centre of the ota and realised the the secondary holder was not pointing down the ota centre and consequently the centre of the primary! Ah-ha! So, I twisted the stalk until it was looking at the primary centre and re-collimated from scratch. Result!

It seems pretty logical now what was happening with the primary reflection but at the time it just didn't register.

HTH anyone else having this sort of issue with their Heritage.

[bombassa]

Good work solving your problem.

I have a few issues when collimating this thing. Like you I use a collicap and a cheshire because I can never see all the primary mirror clips at once when using the cheshire-sight tube. I think I read somewhere that this problem can be fixed by shortening the sight tube? But I would be sorry to ruin my cheshire for my future GIGANT telescope *in my dreams at least*.

The cheshire is long and heavy and I *think* it flexes my focuser more than even my heaviest eyepiece so I try to support the sight tube somewhat during collimation.

I think de-centering the secondary is difficult since it is fixed on a single spiderwane only so I never worry (much) about that. Rotating the secondary on the other hand is much too easy. I usually manage to do it just by banging the scope against the door on my way out. Usually after collimating naturally...

Regards,

[Albenn]

Hi Bombassa.

I wouldn't shorten the Cheshire tool. I too thought about doing that at one point but figured that these things were designed by people who knew what they were doing, so reckoned it would be best to leave it alone. You'd lose the cross hairs for a start.

I too found that there was a little bit of play in the focuser which I thought wouldn't allow me to obtain 'perfect' collimation. Then I figured that, as in most things in life, perfection wasn't achievable - especially in a budget scope, so I decided to collimate the best I could and just accept the scopes shortcomings re the focuser.

I reckoned that if I left the focuser at the last 'in focus' position when using my 8mm ep and collimated from there, then it would probably be good enough for the lower magnification ep's.

To eliminate as much 'tilt' in the Cheshire as possible, I insert it fully and gently push the Cheshire down to maintain contact with the rim of the focuser to prevent it tilting then gently 'snug' up the ep screws.

I have to admit that I have not noticed any flexing of the focuser itself induced by the weight of the Cheshire and think that this could possibly be the result of the Cheshire not being fully inserted as above and thus being allowed to pivot. I really don't think you would be able to prevent any 'flexing' of the plastic focuser mount by hand anyway unless you built a jig to keep it from moving.

At first, I too struggled to see all the primary clips but found that when the mirror was centred, rotated and tilted correctly, that there was enough of the clips visible in the sight tube to fine tune the alignment of the primary clips.

You mention that banging the scope on the doorway caused the secondary to rotate. Did you mean that this caused the primary mirror to go out of alignment? If so, then I would expect it would knock it out of alignment. Still, that's the easy bit to fix!

I have to say, that even when my scope exhibited the collimation pattern as shown in the photo I posted, star tests showed that this was more than acceptable and the views I got from the scope were good.

I think one can get too wrapped up in trying to achieve the 'perfect' collimation patterns as shown in the 'idealised' diagrams and believe that, with all other things being equal, that as long as you are methodical and follow the steps as outlined in the guides, then the resulting collimation for 'your' scope will be more than adequate for viewing.

Just my views based on my limited experience of collimating my Heritage 130. Hope this helps.

[AuntyCis]

Think I just started a new thread about the same 'lose' secondary problem! I have just got a Heritage 130 and came across this while collimating (have been viewing this thread with interest for comparison!) . If it is a common problem it would explain your strange alignment issue.

Guess it cant be too bad, ' cos I also got a nice look at Saturn last night :-)

Cis x

[LukeSkywatcher]

Update.

I retried collimation from scratch 3 times and each time the primary mirror was as shown as in the photo I had previously posted. However, a star test last night and my first light of Saturn (report posted in the planetary observation section) proved the collimation was fine.

However, this misalignment was driving me mad I just couldn't understand why! I then decided to look down the centre of the ota and realised the the secondary holder was not pointing down the ota centre and consequently the centre of the primary! Ah-ha! So, I twisted the stalk until it was looking at the primary centre and re-collimated from scratch. Result!

It seems pretty logical now what was happening with the primary reflection but at the time it just didn't register.

HTH anyone else having this sort of issue with their Heritage.

Told you so. I did ask if the spider vane on the secondary was tight. Mine was loose when i got the scope.

[Albenn]

I know Paul, I will do 50 lines in order to teach me to check what others have already found!

But seriously, the secondary holder looked okay at first glance as the flat end of the cylinder where the collimation screws are adjusted looked level with the edges of the plastic frame, so I never bothered to double-check your suggestion. Strange thing with the stalk on mine though that it was tight as hell and took an adjustable spanner and some effort to rotate it! I then had to use a little clear nail varnish to lock it when I was happy with the adjustment.

Got to admit you guy's are so helpful and I'm learning so much. Thanks for all your help.